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Full text of "Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center Towers. Federal Building and Fire Safety Investigation of the World Trade Center Disaster (NIST NCSTAR 1-2)"

NIST NCSTAR 1-2 

Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster 

Baseline Structural Performance and 
Aircraft Impact Damage Analysis of 
the World Trade Center Towers 



Fahim Sadek 



Nisr 



National Institute of Standards and Technology • Technology Administration • U.S. Deportment of Commerce 



NIST NCSTAR 1-2 

Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster 

Baseline Structural Performance and 
Aircraft Impact Damage Analysis of 
the World Trade Center Towers 



Fahim Sadek 

Building and Fire Researcti Laboratory 

National Institute of Standards and Technology 



September 2005 









sfim 






U.S. Department of Commerce 
Carlos M. Gutierrez, Secretary 

Technology Administration 

Michelle O'Neill, Acting Under Secretary for Technology 

National Institute of Standards and Technology 
William Jeffrey, Director 



Disclaimer No. 1 

Certain commercial entities, equipment, products, or materials are identified in this document in order to describe a 
procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is 
not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or 
equipment are necessarily the best available for the purpose. Nor does such identification imply a finding of fault or 
negligence by the National Institute of Standards and Technology. 

Disclaimer No. 2 

The policy of NIST is to use the International System of Units (metric units) in all publications. In this document, 
however, units are presented in metric units or the inch-pound system, whichever is prevalent in the discipline. 

Disclaimer No. 3 

Pursuant to section 7 of the National Construction Safety Team Act, the NIST Director has determined that certain 
evidence received by NIST in the course of this Investigation is "voluntarily provided safety-related information" that is 
"not directly related to the building failure being investigated" and that "disclosure of that information would inhibit the 
voluntary provision of that type of information" (15 USC 7306c). 

In addition, a substantial portion of the evidence collected by NIST in the course of the Investigation has been 
provided to NIST under nondisclosure agreements. 

Disclaimer No. 4 

NIST takes no position as to whether the design or construction of a WTC building was compliant with any code 
since, due to the destruction of the WTC buildings, NIST could not verify the actual (or as-built) construction, the 
properties and condition of the materials used, or changes to the original construction made over the life of the 
buildings. In addition, NIST could not verify the interpretations of codes used by applicable authorities in determining 
compliance when implementing building codes. Where an Investigation report states whether a system was 
designed or installed as required by a code provision, NIST has documentary or anecdotal evidence indicating 
whether the requirement was met, or NIST has independently conducted tests or analyses indicating whether the 
requirement was met. 

Use in Legal Proceedings 

No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the 
National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter 
mentioned in such report (15 USC 281a; as amended by P.L. 107-231). 



National Institute of Standards and Technology National Construction Safety Team Act Report 1-2 
Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-2, 458 pages (September 2005) 
CODEN: NSPUE2 



U.S. GOVERNMENT PRINTING OFFICE 
WASHINGTON: 2005 



For sale by the Superintendent of Documents, U.S. Government Printing Office 
Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250 
Mail: Stop SSOP, Washington, DC 20402-0001 



Abstract 



The baseline structural performance and aircraft impact damage analysis of the National Institute of 
Standards and Technology (NIST) Investigation of the World Trade Center (WTC) disaster had two 
primary tasks: (1) to develop reference structural models of the WTC towers and use these models to 
establish the baseline performance of each of the towers under gravity and wind loads, and (2) to estimate 
the damage to the towers due to aircraft impacts and establish the initial conditions for the fire dynamics 
modeling and the thermal-structural response and collapse initiation analysis. This report provides the 
technical approach, methodology, and results related to both tasks. 

For the first task, the baseline performance of the WTC towers under gravity and wind loads was 
estabhshed in order to assess the towers' abihty to withstand those loads safely and to evaluate the reserve 
capacity of the towers to withstand unanticipated events. The baseline performance study provides a 
measure of the behavior of the towers under design loading conditions, specifically: (1) total and inter- 
story drift (the sway of the building under design wind loads), (2) floor deflections under gravity loads, 
(3) the stress demand-to-capacity ratio for primary structural components of the towers such as exterior 
walls, core columns, and floor framing, (4) performance of exterior walls under wind loading, including 
distribution of axial stresses and presence of tensile forces, (5) performance of connections between 
exterior columns, and (6) resistance of the towers to shear sliding and overturning at the foundation level. 

Wind loads were a governing factor in the design of the structural components that made up the frame- 
tube steel framing system. Wind load capacity was also a key factor in determining the overall strength 
of the towers and was important in determining not only the ability of the towers to withstand winds but 
also the reserve capacity of the towers to withstand unanticipated events such as major fire or impact 
damage. Accurate estimation of the wind load on tall buildings is a challenging task, given that wind 
engineering is still an evolving technology. For example, estimates of the wind- induced response 
presented in two recent independent studies of the WTC towers differed from each other by about 
40 percent. In this study, NIST developed refined estimates of wind effects by critically assessing 
information obtained from the Cermak Peterka Peterson, Inc. (CPP) and Rowan Williams Davis and 
Irwin, Inc. (RWDI) reports and by bringing to bear state-of-the-art considerations. Furthermore, the 
available prescriptive codes specify wind loads on tall buildings that are significantly lower than wind 
tunnel-based loads. This case study provided an opportunity to assess effectively current design practices 
and various code provisions on wind loads. 

For the purpose of establishing the baseline performance of the towers, various wind loads were 
considered in this study, including wind loads used in the original WTC design, wind loads based on two 
recent wind tunnel studies conducted in 2002 by CPP and RWDI for insurance litigation concerning the 
towers, and refined wind load estimates developed by NIST. 

In order to develop the reference models and conduct the baseline performance analyses, the following 
steps were undertaken: 

• Develop structural databases for the primary structural components of the WTC 1 and WTC 2 
towers from the original computer printouts of the structural design documents. 



NIST NCSTAR 1-2, WTC Investigation ill 



• 



Abstract 



Develop reference structural analysis models that captured the intended behavior of each of 
the two towers using the generated databases. These reference models were used to establish 
the baseline performance of the towers and also served as a reference for more detailed 
models for aircraft impact damage analysis and thermal-structural response and collapse 
initiation analysis. The models included: (1) two global models (one for each tower) of the 
major structural components and systems of the towers, and (2) floor models of a typical 
truss-framed floor and a typical beam- framed floor. 

Develop estimates of design gravity (dead and live loads) and wind loads on each of the two 
towers for implementation into the reference structural models. The following three loading 
cases were considered: 

- Original WTC design loads case. Loads included dead and live loads as in original 
WTC design, in conjunction with original WTC design wind loads. 

- State-of-the-practice case. Loads included dead loads; current New York City Building 
Code (NYCBC 2001) live loads; and wind loads from the RWDI wind tunnel study, 
scaled in accordance with NYCBC 2001 wind speed. 

- Refined NIST estimate case. Loads included dead loads; live loads from the American 
Society of Civil Engineers (ASCE) 7-02 Standard (a national standard); and refined wind 
loads developed by NIST. 

• Perform structural analyses to establish the baseline performance of each of the two towers 
under design gravity and wind loads. 

For the second task related to aircraft impact, the aircraft impact damage to the exterior of the 
WTC towers could be visibly identified from the video and photographic records. However, no visible 
information could be obtained for the extent of damage to the interior of the towers, including the 
structural system (floors and core columns), partition walls, and interior building contents. Such 
information was needed for the subsequent fire dynamics simulations and post-impact structural analyses. 
In addition, for the fire dynamics modeling, the dispersion of the jet fuel and the location of combustible 
aircraft debris were required. The estimate of the extent of damage to the fireproofmg on the structural 
steel in the towers due to impact was essential for the thermal and structural analyses. The aircraft impact 
damage analyses were the primary tool by which most of the information on the tower damage could be 
estimated. 

The focus of the analysis was to analyze the aircraft impacts into each of the WTC towers to provide the 
following: (1) estimates of probable damage to structural systems, including exterior walls, fioor 
systems, and interior core columns; (2) estimates of the aircraft fuel dispersion during the impact; and (3) 
estimates of debris damage to the building nonstructural contents, including partitions and workstations. 
The results were to be used to estimate the damage to fireproofmg based on the predicted path of the 
debris field inside the towers. This analysis thus estimated the condition of the two WTC towers 
immediately following the aircraft impacts and established the initial conditions for the fire dynamics 
modeling and the thermal-structural response and collapse initiation analysis. The impact analyses were 
conducted at various levels of complexity including: (1) the component level, (2) the subassembly level, 
and (3) the global level to estimate the probable damage to the towers due to aircraft impact. 

iv NIST NCSTAR 1-2, WTC Investigation 



Abstract 



In order to estimate the aircraft impact damage to the WTC towers, the following steps were undertaken: 

• Constitutive relationships were developed to describe the behavior and failure of the 
materials under the dynamic impact conditions of the aircraft. These materials included the 
various grades of steels used in the exterior walls, core columns, and floor trusses of the 
towers, weldment metal, bolts, reinforced concrete, aircraft materials, and nonstructural 
contents. 

• Global impact models were developed for the towers and aircraft. The tower models 
included the primary structural components of the towers in the impact zone, including 
exterior walls, floor systems, core columns, and connections, along with nonstructural 
building contents. A refined finite element mesh was used for the areas in the path of the 
aircraft, and a coarser mesh was used elsewhere. The aircraft model included the aircraft 
engines, wings, fuselage, the empennage, and landing gear, as well as nonstructural 
components of the aircraft. The aircraft model also included a representation of the fuel, 
using the smooth particle hydrodynamics approach. 

• Component and subassembly impact analyses were conducted to support the development of 
the global impact models. The primary objectives of these analyses were to (1) develop an 
understanding of the interactive failure phenomenon of the aircraft and tower components, 
and (2) develop the simulation techniques required for the global analysis of the aircraft 
impacts into the WTC towers, including variations in mesh density and numerical tools for 
modeling fluid-structure interaction for fuel impact and dispersion. The component and 
subassembly analyses were used to determine model simplifications for reducing the overall 
model size while maintaining fidelity in the global analyses. 

• Initial conditions were estimated for the impact of the aircraft into the WTC towers. These 
included the aircraft speed at impact, aircraft orientation and trajectory, and impact location 
of the aircraft nose. The estimates also included the uncertainties associated with these 
parameters. This step utilized the videos and photographs that captured the impact event and 
subsequent damage to the exterior of the towers. 

• Sensitivity analyses were conducted at the component and subassembly levels to assess the 
effect of uncertainties on the level of damage to the towers due to impact and to determine the 
most influential parameters that affect the damage estimates. The analyses were used to 
reduce the number of parameters that would be varied in the global impact simulations. 

• Analyses of aircraft impact into WTC 1 and WTC 2 were conducted using the global tower 
and aircraft models. The analysis results included the estimation of the structural damage that 
degraded their strength and the condition and position of nonstructural contents such as 
partitions, workstations, aircraft fuel, and other debris that influenced the behavior of the 
subsequent fires in the towers. The global analyses included, for each tower, a "base case" 
based on reasonable initial estimates of all input parameters. They also provided a range of 
damage estimates based on variations of the most influential parameters. This range included 
more severe and less severe damage cases. 



NISTNCSTAR 1-2, WTC Investigation 



Abstract 



• Approximate analyses were conducted to provide guidance to the global finite element 
impact analyses. These included: (1) analysis of the overall aircraft impact forces and 
assessment of the relative importance of the airframe strength and weight distribution, 
(2) evaluation of the potential effects of the energy in the rotating engine components on the 
calculated engine impact response, (3) influence of the static preloads in the towers on the 
calculated impact damage and residual strength predictions, and (4) analysis of the load 
characteristics required to damage core columns compared to the potential loading from 
impact of aircraft components. 

Keywords: Aircraft impact, finite element analysis, floor system, load, model, structural, truss, wind 
loads. World Trade Center. 



NIST NCSTAR 1-2, WTC Investigation 



Table of Contents 



Abstract iii 

List of Figures xiii 

List of Tables xxi 

List of Acronyms and Abbreviations xxiii 

Preface xxvii 

Acknowledgments xxxvii 

Executive Summary xxxix 

Chapter 1 

Introduction 1 

1.1 Background 1 

1.2 Reference Models and Baseline Performance Analysis 1 

1.3 Aircraft Impact Damage Analysis 4 

Chapter 2 

Development of Reference Structural Models 9 

2.1 Introduction 9 

2.2 Development of Structural Databases 10 

2.3 Global Models of the Towers 11 

2.3.1 Exterior Wall Modeling 14 

2.3.2 Core Columns Modeling 22 

2.3.3 Hat Truss Modeling 22 

2.3.4 Flexible and Rigid Floor Diaphragm Modeling 24 

2.3.5 Boundary Conditions 26 

2.3.6 Results of Modal Analysis 26 

2.4 Typical Truss-Framed Floor Model — Floor 96 of WTC 1 29 

2.4.1 Primary Trusses 31 

2.4.2 Bridging Trusses 32 

2.4.3 Concrete Slab and Metal Deck 33 

2.4.4 Viscoelastic Dampers 34 

2.4.5 Strap Anchors 34 

2.5 Typical Beam-Framed Floor Model — Floor 75 of WTC 2 34 

2.5.1 Composite Beams 35 

NIST NCSTAR 1-2, WTC Investigation vii 



Table of Contents 



2.5.2 Horizontal Trusses 35 

2.5.3 Concrete Slab and Metal Deck 36 

2.5.4 Viscoelastic Dampers 36 

2.6 Review of the Structural Databases and Reference Models of the Towers 37 

2.6.1 Structural Databases 37 

2.6.2 Reference Structural Models 37 

2.7 Summary 38 

2.8 References 39 

Ch^}tBr3 

Wind Loads on the WTC Towers 41 

3.1 Introduction 41 

3.2 Original WTC Design Wind Loads 43 

3.3 State-of-the-Practice Wind Loads 44 

3.4 Refined NIST Estimate of Wind Effects 45 

3.4.1 Summary Comparison by Weidlinger Associates, Inc., of CPP and RWDI Estimates 46 

3.4.2 Review of CPP Estimates 46 

3.4.3 Review of RWDI Estimates 47 

3.4.4 Comments by Third Party Reviewer (Skidmore, Owings & Merrill LLP - SOM) - 
Appendix D 49 

3.4.5 Summary 50 

3.5 Comparisons of Wind Loads, Wind Speeds, and Practices 50 

3.5.1 Wind Loads 50 

3.5.2 Wind Speeds 53 

3.5.3 Wind Engineering Practices Pertaining to Tall Buildings 55 

3.6 References 57 

Chapter 4 

Baseline Performance of the WTC Towers 59 

4.1 Introduction 59 

4.2 Baseline Performance of the Global Models 59 

4.2.1 Analysis Methodology 59 

4.2.2 Total and Inter-Story Drift 62 

4.2.3 Demand/Capacity Ratios 63 

4.2.4 Exterior Columns Axial Loads and Stresses 77 

4.2.5 Exterior Columns Splice Connection 84 



viii NIST NCSTAR 1-2, WTC Investigation 



Table of Contents 



4.2.6 Resistance of the Towers to Shear Shding and Overturning Moment 84 

4.3 Basehne Performance of the Typical Floor Models 85 

4.3.1 Typical Truss-Framed Floor 85 

4.3.2 Typical Beam-Framed Floor 89 

4.4 Review of Baseline Performance Analyses 90 

4.5 Summary 90 

4.6 References 92 

Chester 5 

Development of Tower and Aircraft Impact Models 93 

5.1 Introduction 93 

5.2 Development of Tower Impact Models 94 

5.2.1 Exterior Wall Model Development 95 

5.2.2 Core Columns and Floors Model Development 99 

5.2.3 Truss Floor Model Development 102 

5.2.4 Interior Contents Model Development 105 

5.2.5 Global Impact Models Assembly 107 

5.2.6 Tower Material Constitutive Models 110 

5.3 Development of Aircraft Model 1 16 

5.3.1 Airframe Model Development 121 

5.3.2 Wing Section Component Model Development 125 

5.3.3 Engine Model Development 126 

5.3.4 Aircraft Material Constitutive Models 130 

5.4 Component and Subassembly Level Analyses 131 

5.4.1 Exterior Column Impacted with an Empty Wing 132 

5.4.2 Bolted Connection Modeling 133 

5.4.3 Floor Assembly Component Analysis 134 

5.4.4 Modeling of Aircraft Wing Section Impact with Fuel 138 

5.4.5 Engine Impacts Subassembly Analyses 145 

5.5 Summary 148 

5.6 References 150 

Chapter 6 

Aircraft Impact Initial Conditions 151 

6.1 Introduction 151 

6.2 Motion Analysis Methodology 152 

NIST NCSTAR 1-2, WTC Investigation ix 



Table of Contents 



6.2.1 Videos Used in the Analysis 152 

6.2.2 Complex Motion Analysis 153 

6.2.3 Simplified Motion Analysis 156 

6.3 Refinement of Aircraft Impact Conditions 158 

6.4 Comparison with Previous Estimates of Aircraft Impact Initial Conditions 164 

6.5 Summary 166 

6.6 References 166 

Chapter 7 

Aircraft Impact Damage Results 167 

7.1 Introduction 167 

7.2 Analysis Methodology, Assumptions, and Limitations 167 

7.3 WTC 1 Base Case Impact Analysis - CASE A 171 

7.3.1 Impact Response 173 

7.3.2 Tower Structural Damage 178 

7.3.3 Fuel and Debris Distributions 190 

7.4 WTC 1 More Severe Impact Analysis - CASE B 196 

7.4.1 Impact Response 197 

7.4.2 Tower Structural Damage 202 

7.4.3 Fuel and Debris Distribution 212 

7.5 WTC 1 Less Severe Impact Analysis - Brief Description 217 

7.6 WTC 2 Base Case Impact Analysis - CASE C 217 

7.6.1 Impact Response 218 

7.6.2 Tower Structural Damage 224 

7.6.3 Fuel and Debris Distributions 235 

7.7 WTC 2 More Severe Impact Analysis - CASE D 241 

7.7.1 Impact Response 242 

7.7.2 Tower Structural Damage 247 

7.7.3 Fuel and Debris Distributions 257 

7.8 WTC 2 Less Severe Impact Analysis - Brief Description 263 

7.9 Comparison Between WTC 1 and WTC 2 263 

7.9.1 Exterior Wall Damage 264 

7.9.2 Core Column Damage 264 

7.9.3 Floor Truss Damage 267 

7.10 Comparison with Observables 267 

X NIST NCSTAR 1-2, WTC Investigation 



Table of Contents 



7.10.1 Comparison with Observables on WTC 1 268 

7.10.2 Comparison with Observables on WTC 2 277 

7.10.3 Summary 291 

7.11 Comparison with Previous Studies 291 

7.1 1.1 Comparison of Exterior Wall Damage 291 

7.1 1.2 Comparison of Core Column Damage 295 

7.12 Summary 296 

7.13 References 297 

Chapter 8 

Findings 299 

8.1 Baseline Performance Analysis 299 

8.1.1 Wind Loads on the World Trade Center Towers 299 

8.1.2 Baseline Performance of the Global Tower Models 300 

8.1.3 Baseline Performance of the Typical Floor Models 301 

8.2 Aircraft Impact Damage Analysis 302 

8.2.1 Safety of the WTC Towers in Aircraft Collision 302 

8.2.2 Preliminary Impact Analyses (Component and Subassembly Levels) 302 

8.2.3 Aircraft Impact Damage Results 302 

AppendxA 

Salient Points with Regard to the Structural Design of the World Trade Center Tower ..305 

AppencfxB 

Estimation of Sectorial Extreme Wind Speeds 309 

AppendxC 

Wind Tunnel Testing and the Sector-by-Sector Approach to Wind 

Directionality Effects 321 

AppendxD 

SOM Project 2, Progress Report No. 3, WTC Wind Load Estimates 329 

AppendxE 

Still Images of the Video Records Used in Chapter 6 339 



NISTNCSTAR 1-2, WTC Investigation 



Table of Contents 



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xii NIST NCSTAR 1-2, WTC Investigation 



List of Figures 



Figure P-1. The eight projects in the federal building and fire safety investigation of the WTC 

disaster xxix 



Figure 2-1. Rendered isometric views of the WTC 1 global model 12 

Figure 2-2. Frame view of the WTC 2 model: (a) exterior wall elevation, and (b) interior section 13 

Figure 2-3. Frame view and rendered view of the WTC 1 model (foundation to floor 9) 15 

Figure 2^. Exterior wall tree panel (taken from Drawing Book 2, page 2-AB2-2) 16 

Figure 2-5. Frame and rendered view of an exterior wall tree 17 

Figure 2-6. Typical WTC tower exterior wall panel 18 

Figure 2-7. (a), (b) Shell element, and (c) frame element models of a typical exterior wall panel 19 

Figure 2-8. Selection of column and spandrel rigidity of typical exterior wall panel 20 

Figure 2-9. Shell element and frame models of typical exterior wall corner panel 21 

Figure 2-10. As-modeled plan of the WTC 1 hat truss 23 

Figure 2-11. Rendered 3-D model of the WTC 1 hat truss 23 

Figure 2-12. Deflection of typical beam-framed floor model due to lateral loading (exaggerated 

scale) 25 

Figure 2-13. Deflection of equivalent floor model due to lateral loading (exaggerated scale) 25 

Figure 2-14. Deflections of the north and south faces of the floor for the detailed and equivalent 

floor models 26 

Figure 2-15. Displacement of floor 70 of WTC 2 after impact based on video analysis (NIST 

NCSTAR1-5A) 29 

Figure 2-16. Typical floor truss framing zones 30 

Figure 2-17. Typical truss-framed floor model (floor 96 of WTC 1), slab not shown 31 

Figure 2-18. Typical primary truss cross-section, as-designed and as-modeled 32 

Figure 2-19. Typical bridging truss cross-section, as-designed and as-modeled 33 

Figure 2-20. Strap anchors modeling, slab not shown 34 

Figure 2-21. Typical beam-framed floor model (floor 75 of WTC 2) 35 

Figure 2-22. Horizontal truss modeling, slab not shown 36 

Figure 4-1. Cumulative drift diagrams for WTC 1 under the three wind loading cases 64 

Figure 4—2. Inter-story drift diagrams for WTC 1 under the three wind loading cases 65 

Figure 4-3. DCRs for the exterior walls of WTC 1 under original design case, (a) north elevation, 

(b) east elevation, (c) south elevation, and (d) west elevation 71 

NIST NCSTAR 1-2, WTC Investigation xiii 



List of Figures 



Figure 4^. RCRs for WTC 1 under original design loads below floor 9, (a) north elevation 72 

Figure 4-5. DCRs for WTC 1 core columns under original design loads, (a) 500 line, and (b) 600 

line 74 

Figure 4-6. Distribution of normal stresses in the exterior walls of WTC 1 due to original 

WTC wind loads only at (a) floor B6, and (b) floor 39 78 

Figure 4-7. Three-dimensional distribution of normal stresses in the exterior walls of WTC 1 due 

to original WTC wind loads only at floors B6 and 39 80 

Figure 4-8. Tension force distribution (kip) in the exterior wall columns of WTC 1 under original 
design dead and wind loads (no live loads included), (a) 100 face (north), and (b) 200 
face (east) 82 

Figure 4-9. DCRs for the typical beam-framed floor under original WTC design criteria loading 89 

Figure 4-10. Beam-framed floor member groups 90 



Figure 5-1. User interface for exterior panel generator 95 

Figure 5-2. Impact face of the WTC 1 global model, floors 91-101 96 

Figure 5-3. Impact face of the WTC 2 global model, floors 75-86 97 

Figure 5-4. Model of the spandrel splice plate connection 98 

Figure 5-5. Placement of spandrel splice plates in the exterior wall model 98 

Figure 5-6. Model of the WTC 1 core columns and connections, floors 95-97 99 

Figure 5-7. Detail of wide flange core columns splices 100 

Figure 5-8. Detail of box column-to-wide flange core columns connection 100 

Figure 5-9. Model of the core of floor 96 of WTC 1 (with and without floor slab) 101 

Figure 5-10. Model detail of core column and beam connections 102 

Figure 5-11. Model of the WTC 1 core, floors 94-98 102 

Figure 5-12. Model of a truss floor segment 103 

Figure 5-13. Simplified far field truss floor model 104 

Figure 5-14. Truss floor connection detail at exterior wall 104 

Figure 5-15. Truss floor connection detail at core perimeter 105 

Figure 5-16. Detailed model of floor 96 of WTC 1 105 

Figure 5-17. Model of floor 96 of WTC 1, including interior contents 106 

Figure 5-18. Global impact model of the WTC 1 tower 107 

Figure 5-19. Interior structures and contents of the WTC 1 global impact model 108 

Figure 5-20. Nonstructural building contents in the WTC 1 global impact model 108 

Figure 5-21. Global impact model of the WTC 2 tower 109 

Figure 5-22. Interior structures and contents of the WTC 2 global impact model 110 

Figure 5-23. Nonstructural building contents in the WTC 2 global impact model 110 

xiv NIST NCSTAR 1-2, WTC Investigation 



List of Figures 



Figure 5-24. Finite element models of the ASTM 370 rectangular tensile specimen Ill 

Figure 5-25. Tabular true stress-strain constitutive model curves for the tower steels 112 

Figure 5-26. Comparison of rate effects model and test data 113 

Figure 5-27. Finite element analysis of the unconfined compression test 114 

Figure 5-28. Comparison of the calculated unconfined compression behavior with concrete 

compression test data 115 

Figure 5-29. Tabular concrete strain rate effects curve 116 

Figure 5-30. Finite element model of the Boeing 767-200ER 119 

Figure 5-31. Boeing 767-200ER with fuel load at time of impact 120 

Figure 5-32. Boeing 767-200ER model wing deflections 120 

Figure 5-33. Empennage model of the 767-200ER aircraft 121 

Figure 5-34. Retracted landing gear components for the 767-200ER aircraft model 122 

Figure 5-35. Underside of the 767 airframe model (skin removed) showing retracted landing gear, 

engine, and ULDs 122 

Figure 5-36. Complete wing structures for the 767 aircraft model 123 

Figure 5-37. Model of fuselage interior frame and stringer construction 124 

Figure 5-38. Integration of the fuselage and wing structures 124 

Figure 5-39. Wing section model for component level and subassembly analyses 126 

Figure 5-40. Pratt & Whitney PW4000 turbofan engine 127 

Figure 5^1. PW4000 engine cross-sectional geometry and simplification 128 

Figure 5^2. Pratt & Whitney PW4000 turbofan engine model 129 

Figure 5^3. True stress-strain curves developed for various aircraft aluminum alloys 130 

Figure 5^4. Tabular stress-strain curves developed for various aircraft aluminum alloys 131 

Figure 5^5. Exterior column response comparison, showing contours of the displacement 

magnitude (in.) 132 

Figure 5-46. Modeling of exterior column bolted connection 133 

Figure 5^7. Failure comparison of exterior column bolted connection treatments 134 

Figure 5-48. Detailed model of the truss floor system 135 

Figure 5-49. Simplified model of the truss floor system 136 

Figure 5-50. Constitutive behavior for the combined concrete and metal decking 136 

Figure 5-51. Floor assembly impact response with brick element concrete slab 137 

Figure 5-52. Floor assembly impact response with shell element concrete slab 137 

Figure 5-53. SPH and ALE fuel models in the small wing segment 138 

Figure 5-54. Wing segment, fuel, and exterior panel configuration 139 

Figure 5-55. Impact response of a wing section laden with fuel modeled using ALE approach 140 

Figure 5-56. Impact response of a wing section laden with fuel modeled using SPH approach 141 

NIST NCSTAR 1-2, WTC Investigation xv 



List of Figures 



Figure 5-57. Exterior panels after impact with a wing segment with fuel 142 

Figure 5-58. Top view of structural damage and fuel dispersion at 0.04 s 143 

Figure 5-59. Side view of structural damage and fuel dispersion at 0.04 s 144 

Figure 5-60. Tower subassembly model 145 

Figure 5-61. Response of the subassembly model to engine impact 146 

Figure 5-62. Subassembly-engine impact and breakup response (side view) 147 

Figure 5-63. Speed history for the engine subassembly impact analysis 148 

Figure 6-1. Definition of the aircraft impact parameters 152 

Figure 6-2. Complex motion analysis to measure object motions using multiple cameras 154 

Figure 6-3. Simplified motion analysis procedure to determine aircraft speed 156 

Figure 6-4. Estimated impact locations of aircraft components superimposed on the damaged face 

ofWTC 1 159 

Figure 6-5. Orientation and trajectory of AA 11 that matched the impact pattern (vertical 

approach angle = 10.6°, lateral approach angle = 0°) 159 

Figure 6-6. Estimated impact locations of aircraft components superimposed on the damaged face 

ofWTC2 161 

Figure 6-7. Orientation and Trajectory of UAL 175 from Video Analysis 161 

Figure 6-8. Orientation and trajectory of UAL 175 that matches the impact pattern (vertical 

approach angle = 6°, lateral approach angle = 13°) 162 

Figure 6-9. Orientation and trajectory of UAL 175 that matches the impact pattern (vertical 

approach angle = 6°, lateral approach angle = 17°) 163 

Figure 6-10. Projected trajectory of the starboard engine of UAL 175 with an initial lateral 

approach angle of 13° 164 

Figure 7-1. WTC 1 global impact model 172 

Figure 7-2. WTC 1 base case global impact analysis (side view) 174 

Figure 7-3. WTC 1 base case global impact analysis (plan view) 176 

Figure 7-4. Normalized aircraft momentum for the WTC 1 base case impact 178 

Figure 7-5. Base case impact damage to the WTC 1 exterior wall 181 

Figure 7-6. Base case impact damage to the WTC 1 core columns 182 

Figure 7-7. Classification of damage levels in core columns 183 

Figure 7-8. Base case impact damage to the core beams of floors 95 and 96 of WTC 1 184 

Figure 7-9. Base case impact damage to the WTC 1 floor trusses (front view) 185 

Figure 7-10. Base case impact damage to the trusses on floors 95 and 96 of WTC 1 (plan view) 186 

Figure 7-11. Base Case impact damage to the slabs on floors 95 and 96 of WTC 1 (plan view) 187 



xvi NIST NCSTAR 1-2, WTC Investigation 



List of Figures 



Figure 7-12. Summary of the floor-by-floor structural damage to the floors and columns of WTC 1 

(base case) 188 

Figure 7-13. Cumulative structural damage to the floors and columns of WTC 1 (base case) 189 

Figure 7-14. Calculated fuel distribution in the base case WTC 1 analysis 192 

Figure 7-15. Plan view of calculated WTC 1 building, fuel, and aircraft debris distribution for the 

base case 193 

Figure 7-16. Calculated floor 95 contents and fuel distribution (base case) 194 

Figure 7-17. Calculated floor 96 contents and fuel distribution (base case) 195 

Figure 7-18. WTC 1 more severe global impact analysis (side view) 198 

Figure 7-19. WTC 1 more severe global impact analysis (plan view) 200 

Figure 7-20. More severe impact damage to the WTC 1 exterior wall 203 

Figure 7-21. More severe impact response of the WTC 1 core columns 204 

Figure 7-22. More severe impact damage to the core beams of floors 95 and 96 of WTC 1 205 

Figure 7-23. More severe impact damage to the WTC 1 floor trusses (front view) 207 

Figure 7-24. More severe impact damage to the trusses on floors 95 and 96 of WTC 1 (plan view) 208 

Figure 7-25. More severe impact damage to the slabs on floors 95 and 96 of WTC 1 (plan view) 209 

Figure 7-26. Summary of the floor-by-floor structural damage to the floors and columns of WTC 1 

(more severe case) 210 

Figure 7-27. Cumulative structural damage to the floors and columns of WTC 1 (more severe 

case) 211 

Figure 7-28. Calculated fuel distribution in the more severe WTC 1 analysis 213 

Figure 7-29. Plan view of calculated WTC 1 building, fuel, and aircraft debris distribution for the 

more severe case 214 

Figure 7-30. Calculated more severe WTC 1 impact response of floor 95 contents 215 

Figure 7-31. Calculated more severe WTC 1 impact response of floor 96 contents 216 

Figure 7-32. WTC 2 global impact model 218 

Figure 7-33. WTC 2 base case global impact analysis (side view) 220 

Figure 7-34. WTC 2 base case global impact analysis (plan view) 222 

Figure 7-35. Normalized aircraft momentum for the WTC 2 base case impact 224 

Figure 7-36. Base case impact damage to the WTC 2 exterior wall 226 

Figure 7-37. Base case impact damage to the WTC 2 core columns 227 

Figure 7-38. Base case impact damage to the core beams of floors 80 and 81 of WTC 2 228 

Figure 7-39. Base case impact damage to the WTC 2 floor trusses (front view) 229 

Figure 7-40. Base case impact damage to the trusses on floors 80 and 81 of WTC 2 (plan view) 230 

Figure 7^1. Base case impact damage to the slabs on floors 80 and 81 of WTC 2 (plan view) 231 



NIST NCSTAR 1-2, WTC Investigation xvii 



List of Figures 



Figure 7-42. Summary of the floor-by-floor structural damage to the floors and columns of WTC 2 

(base case) 233 

Figure 7-43. Cumulative structural damage to the floors and columns of WTC 2 (base case) 235 

Figure 7-44. Calculated fuel distribution in the base case WTC 2 analysis 237 

Figure 7-45. Plan view of calculated WTC 2 building, fuel, and aircraft debris distribution for the 

base case 238 

Figure 7-46. Calculated floor 80 contents, and fuel distribution (base case) 239 

Figure 7-47. Calculated floor 81 contents and fuel distribution (base case) 240 

Figure 7-48. WTC 2 more severe global impact analysis (side view) 243 

Figure 7-49. WTC 2 more severe global impact analysis (plan view) 245 

Figure 7-50. More severe impact damage to the WTC 2 exterior wall 248 

Figure 7-51. More severe impact damage to the WTC 2 core columns 249 

Figure 7-52. More severe impact damage to the core beams of floors 80 and 81 of WTC 2 250 

Figure 7-53. More severe impact damage to the WTC 2 floor trusses (front view) 252 

Figure 7-54. More severe impact damage to the trusses on floors 80 and 81 of WTC 2 (plan view) 253 

Figure 7-55. More severe impact damage to the WTC 2 floor slab (plan view) 254 

Figure 7-56. Summary of the floor-by-floor structural damage to the floors and columns of WTC 2 

(more severe case) 255 

Figure 7-57. Cumulative structural damage to the floors and columns of WTC 2 (more severe 

case) 257 

Figure 7-58. Calculated fuel distribution in the more severe WTC 2 analysis 259 

Figure 7-59. Plan view of calculated more WTC 2 building, fuel, and aircraft debris distribution 

for the more severe case 260 

Figure 7-60. Calculated floor 80 contents and fuel distribution (more severe case) 261 

Figure 7-61. Calculated floor 81 contents and fuel distribution (more severe case) 262 

Figure 7-62. Comparison of base case impact damage to the exterior walls of WTC 1 and WTC 2 265 

Figure 7-63. Comparison of base case impact damage to the core columns of WTC 1 and WTC 2 266 

Figure 7-64. Comparison of base case impact damage to floor trusses of WTC 1 and WTC 2 267 

Figure 7-65. Comparison of observable and calculated base case impact damage to the north wall 

of WTC 1 269 

Figure 7-66. Base case aircraft debris distribution in WTC 1 270 

Figure 7-67. More severe damage aircraft debris distribution in WTC 1 271 

Figure 7-68. Damage to the south face of WTC 1 from the more severe damage global analysis 272 

Figure 7-69. Landing gear found at the corner of West and Rector Streets 273 

Figure 7-70. Landing gear found embedded in exterior panel knocked free from WTC 1 274 

Figure 7-71. Base case stairwell disruption in WTC 1 275 



xviii NIST NCSTAR 1-2, WTC Investigation 



List of Figures 



Figure 7-72. Observed and calculated WTC 1 damage (front view) 276 

Figure 7-73. Comparison of observable and calculated base case impact damage to the south wall 

of WTC 2 277 

Figure 7-74. Impact damage to the northeast corner of the exterior wall of WTC 2 278 

Figure 7-75. Documented damage to the northeast corner of floor 81 of WTC 2 279 

Figure 7-76. Base case response on the northeast corner of floor 81 of WTC 2 280 

Figure 7-77. Base case stairwell disruption on floor 78 in WTC 2 281 

Figure 7-78. Base case damage aircraft debris distribution in WTC 2 282 

Figure 7-79. Aircraft debris distribution in the more severe WTC 2 impact 283 

Figure 7-80. Starboard engine fragment trajectory in the base case global analysis of WTC 2 285 

Figure 7-81. Speed of the aft portion of the starboard engine 286 

Figure 7-82. Calculated and observed engine damage 287 

Figure 7-83. Starboard engine impact with the south face of WTC 2 in the base case global 

analysis 288 

Figure 7-84. Projected debris path for the WTC 2 north face cold spot 290 

Figure 7-85. Base case WTC 2 impact orientation and trajectory (vertical approach angle = 

6° lateral approach angle = 13°) 290 

Figure 7-86. Comparison of impact damage to the north wall of WTC 1 293 

Figure 7-87. Comparison of impact damage to the south wall of WTC 2 294 



NIST NCSTAR 1-2, WTC Investigation xix 



List of Figures 



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XX NIST NCSTAR 1-2, WTC Investigation 



List of Tables 



Table P-1 . Federal building and fire safety investigation of the WTC disaster xxviii 

Table P-2. Public meetings and briefings of the WTC Investigation xxxi 

Table 2-1. Approximate size of the reference structural models (rounded) 14 

Table 2-2. Lateral displacement (in.) for the shell and frame models of typical exterior wall 

panel with varied column and spandrel rigidities 21 

Table 2-3. Calculated first six periods and frequencies without P-A effects 27 

Table 2-4. Calculated first six periods and frequencies with P-A effects 27 

Table 2-5. Comparison of measured and calculated natural frequencies and periods for WTC 1 28 

Table 3-1. Approximate maximum base moments for WTC 2 induced by ASCE 7-98 standard 

wind loads 46 

Table 3-2. Comparison of wind load estimates for WTC 1 based on various sources 52 

Table 3-3. Comparison of wind load estimates for WTC 2 based on various sources 53 

Table 3-4. Base shears and base moments due to wind loads based on various building codes 53 

Table 3-5. Comparison between various design wind speeds 55 

Table 3-6. Comparison between the various wind studies 55 

Table 4-1. Total drift for WTC 1 and WTC 2 under the three loading cases 62 

Table 4-2. Statistics of DCRs for WTC 1 under original design load case 68 

Table 4-3. Statistics of DCRs for WTC 1 under the lower estimate, state-of-the practice case 69 

Table 4-4. Statistics of DCRs for WTC 1 under the refined NIST estimate case 70 

Table 4-5. Statistics of DCRs for WTC 1 under the refined NIST estimate case using LRFD and 

ASD 11 

Table 4-6. Maximum calculated DCRs for exterior wall column splices for WTC 1 under 

original design dead and wind load case 84 

Table 4-7. Summary of maximum deflections for typical truss-framed floor under dead and live 

loads for areas outside of core 86 

Table 4-8. DCR statistics for the typical truss-framed floor under the original design load case 87 

Table 4-9. DCR statistics for floor the typical truss-framed floor under the ASCE 7-02 loading 

case 88 

Table 4-10. DCR statistics for the typical beam-framed floor under the original design loading 

case 90 



NISTNCSTAR 1-2, WTC Investigation 



XXI 



List of Tables 



Table 5-1. Summary of the size of the global impact tower models 95 

Table 5-2. Boeing 767-200ER aircraft model parameters 118 

Table 5-3. Density scale factors and weights for aircraft components 125 

Table 5-4. Boeing 767 Engine Comparison 126 

Table 5-5. Engine model parameters 129 

Table 5-6. Exterior column component analyses comparison 133 

Table 5-7. Truss floor assembly component analyses comparison 134 

Table 6-1. Videos used for the analysis of aircraft impact initial conditions 153 

Table 6-2. Measured UAL 175 impact speeds using the simplified analysis methodology 157 

Table 6-3. Summary of measured aircraft impact conditions from video analysis 158 

Table 6-4. Aircraft impact locations on the WTC towers 162 

Table 6-5. Summary of refined aircraft impact conditions 162 

Table 6-6. AA 11 (WTC 1) aircraft impact analysis comparison 165 

Table 6-7. UAL 175 (WTC 2) aircraft impact analysis comparison 165 

Table 7-1. Summary of core column damage for the base case WTC 1 impact 183 

Table 7-2. Fuel and aircraft debris distribution for the base case WTC 1 impact 196 

Table 7-3. Input parameters for the more and less severe WTC 1 impact analysis 197 

Table 7-4. Summary of core column damage for the more severe WTC 1 impact 205 

Table 7-5. Fuel and aircraft debris distribution for the more severe WTC 1 impact 212 

Table 7-6. Summary of core column damage for the base case WTC 2 impact 228 

Table 7-7. Fuel and aircraft debris distribution for the base case WTC 2 impact 241 

Table 7-8. Input parameters for the more severe WTC 2 impact analysis 242 

Table 7-9. Summary of core column damage for the more severe WTC 2 impact 251 

Table 7-10. Fuel and aircraft debris distribution for the more severe WTC 2 impact 258 

Table 7-11. Comparison of damage to core columns from various studies 296 



xxii 



NIST NCSTAR 1-2, WTC Investigation 



List of Acronyms and Abbreviations 



Acronyms 

AA American Airlines 

AISC American Institute of Steel Construction 

ALE Arbitrary-Lagrangian-Eulerian 

ARA Applied Research Associates, Incorporated 

ASCE American Society of Civil Engineers 

ASD Allowable Stress Design 

ASTM American Society for Testing and Materials 

CDL Construction dead load 

C.O.V. coefficient of variation 

CPP Cermak Peterka Peterson, Inc. 

CPU central processing unit 

CSU Colorado State University 

DCR demand/capacity ratio 

DL dead load 

E-W East-West direction 

FBI Federal Bureau of Investigation 

FEMA Federal Emergency Management Agency 

HAZ heat affected zone 

HFFB heat-frequency force-balance 

LERA Leslie E. Robertson Associates, RLLP 

LL live load 

LRFD Load and Resistance Factor Design 

MIT Massachusetts Institute of Technology 

NIST National Institute of Standards and Technology 

N-S North-South direction 

NOAA National Oceanic Atmospheric Administration 

NTSC National Television System Committee 

NYCBC New York City Building Code 



NISTNCSTAR 1-2, WTC Investigation 



xxin 



List of Acronyms and Abbreviations 



PAL Phase Alternating Line 

PANYNJ Port Authority of New York and New Jersey 

RWDI Rowan Williams Davies and Irwin, Inc. 

SDL superimposed dead load 

SOM Skidmore, Owings, and Merrill 

SPH Smoothed Particle Hydrodynamics 

UAL United Airlines 

ULD unit load device 

WAI Weidhnger Associates, Inc. 

WSHJ Worthington, Skilling, Helle & Jackson 

WTC World Trade Center 

WTC 1 World Trade Center 1 (North Tower) 

WTC 2 World Trade Center 2 (South Tower) 

WTC 7 World Trade Center 7 



Abbreviations 



± 

2D 

3D 

fps 

ft 

ft^ 

GHz 

h 

in. 

kip 

ksi 

lb 

m 

min 

mph 

pcf 



plus or minus 

two dimensional 

three dimensional 

foot per second 

foot 

square foot 

gigahertz 

hour 

inch 

a stress equal to 1 ,000 pounds 

1 ,000 pounds per square inch 

pound 

meter 

minute 

miles per hour 

pounds per cubic foot 



XXIV 



NIST NCSTAR 1-2, WTC Investigation 



List of Acronyms and Abbreviations 



psf 


pounds per square foot 


psi 


pounds per square inch 


s 


second 


ms 


millisecond 



NISTNCSTAR 1-2, WTC Investigation 



List of Acronyms and Abbreviations 



This page intentionally left blank. 



xxvi NIST NCSTAR 1-2, WTC Investigation 



Preface 



Immediately following the terrorist attack on the World Trade Center (WTC) on September 11, 2001, the 
Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers began 
planning a building performance study of the disaster. The week of October 7, as soon as the rescue and 
search efforts ceased, the Building Performance Study Team went to the site and began its assessment. 
This was to be a brief effort, as the study team consisted of experts who largely volunteered their time 
away from their other professional commitments. The Building Performance Study Team issued its 
report in May 2002, fulfilling its goal "to determine probable failure mechanisms and to identify areas of 
future investigation that could lead to practical measures for improving the damage resistance of buildings 
against such unforeseen events." 

On August 21, 2002, with funding from the U.S. Congress through FEMA, the National Institute of 
Standards and Technology (NIST) announced its building and fire safety investigation of the WTC 
disaster. On October 1, 2002, the National Construction Safety Team Act (Public Law 107-23 1), was 
signed into law. The NIST WTC Investigation was conducted under the authority of the National 
Construction Safety Team Act. 

The goals of the investigation of the WTC disaster were: 

• To investigate the building construction, the materials used, and the technical conditions that 
contributed to the outcome of the WTC disaster. 

• To serve as the basis for: 

- Improvements in the way buildings are designed, constructed, maintained, and used; 

- Improved tools and guidance for industry and safety officials; 

- Recommended revisions to current codes, standards, and practices; and 

- Improved public safety. 

The specific objectives were: 

1. Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the 
aircraft and why and how WTC 7 collapsed; 

2. Determine why the injuries and fatalities were so high or low depending on location, 
including all technical aspects of fire protection, occupant behavior, evacuation, and 
emergency response; 

3. Determine what procedures and practices were used in the design, construction, operation, 
and maintenance of WTC 1, 2, and 7; and 

4. Identify, as specifically as possible, areas in current building and fire codes, standards, and 
practices that warrant revision. 



NIST NCSTAR 1-2, WTC Investigation xxvii 



Preface 



NIST is a nonregulatory agency of the U.S. Department of Commerce's Technology Administration. The 
purpose of NIST investigations is to improve the safety and structural integrity of buildings in the United 
States, and the focus is on fact finding. NIST investigative teams are authorized to assess building 
performance and emergency response and evacuation procedures in the wake of any building failure that 
has resulted in substantial loss of hfe or that posed significant potential of substantial loss of life. NIST 
does not have the statutory authority to make findings of fault nor negligence by individuals or 
organizations. Further, no part of any report resulting from a NIST investigation into a building failure or 
from an investigation under the National Construction Safety Team Act may be used in any suit or action 
for damages arising out of any matter mentioned in such report (15 USC 281a, as amended by Public 
Law 107-231). 

Organization of the Investigation 

The National Construction Safety Team for this Investigation, appointed by the then NIST Director, 
Dr. Arden L. Bement, Jr., was led by Dr. S. Shyam Sunder. Dr. William L. Grosshandler served as 
Associate Lead Investigator, Mr. Stephen A. Cauffman served as Program Manager for Administration, 
and Mr. Harold E. Nelson served on the team as a private sector expert. The Investigation included eight 
interdependent projects whose leaders comprised the remainder of the team. A detailed description of 
each of these eight projects is available at http://wtc.nist.gov. The purpose of each project is summarized 
in Table P-1, and the key interdependencies among the projects are illustrated in Fig. P-1. 



Table P-1. Federal building and fire safety investigation of the WTC disaster. 



Technical Area and Project Leader 


Project Purpose 


Analysis of Building and Fire Codes and 
Practices; Project Leaders: Dr. H. S. Lew 
and Mr. Richard W. Bukowski 


Document and analyze the code provisions, procedures, and 
practices used in the design, construction, operation, and 
maintenance of the structural, passive fire protection, and 
emergency access and evacuation systems of WTC 1, 2, and 7. 


Baseline Structural Performance and 
Aircraft Impact Damage Analysis; Project 
Leader: Dr. Fahim H. Sadek 


Analyze the baseline performance of WTC 1 and WTC 2 under 
design, service, and abnormal loads, and aircraft impact damage on 
the structural, fire protection, and egress systems. 


Mechanical and Metallurgical Analysis of 
Structural Steel; Project Leader: Dr. Frank 
W. Gayle 


Determine and analyze the mechanical and metallurgical properties 
and quality of steel, weldments, and connections from steel 
recovered from WTC 1, 2, and 7. 


Investigation of Active Fire Protection 
Systems; Project Leader: Dr. David 
D. Evans; Dr. William Grosshandler 


Investigate the performance of the active fire protection systems in 
WTC 1 , 2, and 7 and their role in fire control, emergency response, 
and fate of occupants and responders. 


Reconstruction of Thermal and Tenability 
Environment; Project Leader: Dr. Richard 
G. Gann 


Reconstruct the time-evolving temperature, thermal environment, 
and smoke movement in WTC 1 , 2, and 7 for use in evaluating the 
structural performance of the buildings and behavior and fate of 
occupants and responders. 


Structural Fire Response and Collapse 
Analysis; Project Leaders: Dr. John 
L. Gross and Dr. Therese P. McAllister 


Analyze the response of the WTC towers to fires with and without 
aircraft damage, the response of WTC 7 in fires, the performance 
of composite steel-trussed floor systems, and determine the most 
probable structural collapse sequence for WTC 1, 2, and 7. 


Occupant Behavior, Egress, and Emergency 
Communications; Project Leader: Mr. Jason 
D. Averill 


Analyze the behavior and fate of occupants and responders, both 
those who survived and those who did not, and the performance of 
the evacuation system. 


Emergency Response Technologies and 
Guidelines; Project Leader: Mr. J. Randall 
Lawson 


Document the activities of the emergency responders from the time 
of the terrorist attacks on WTC 1 and WTC 2 until the collapse of 
WTC 7, including practices followed and technologies used. 



xxvin 



NIST NCSTAR 1-2, WTC Investigation 



Preface 



WTC Building 
Performance Study 
Recommendations 

Government, 
Industry, 
Professional, 
Academic Inp 

I 
Public Inputs 



Video/ 

Photographic 

Records 

Oral History Data 

Emergency 

Response 

Records 

Recovered 
Structural Steel/ 



Nisr 



NIST WTC Investigation Projects 



i / Analysis of '^^ 
\ I Steel J 


^^^^^1 Structural \ 
I Collapse J 






/^ Baseline 


^ / 






( Performance 


A / 




^H 


I & Impact 
\^ Damage , 


-^ / Thermal and \ 
( Tenability J 
V Environment / 




1 


/ Analysis of \ , 
( Codes and j / 
\ Practices / 


Active Fire \ V 

Protection j ^-^ ~~^^ 

""^ -^ I Emergency | 

I Response j 


Evacuation 










1 



Figure P-1. The eight projects in the federal building and fire safety 
investigation of the WTC disaster. 



National Construction Safety Team Advisory Committee 

The NIST Director also established an advisory committee as mandated under the National Construction 
Safety Team Act. The initial members of the committee were appointed following a public solicitation. 
These were: 



Paul Fitzgerald, Executive Vice President (retired) FM Global, National Construction Safety 
Team Advisory Committee Chair 

John Barsom, President, Barsom Consulting, Ltd. 

John Bryan, Professor Emeritus, University of Maryland 

David Collins, President, The Preview Group, Inc. 

Glenn Corbett, Professor, John Jay College of Criminal Justice 

Philip DiNenno, President, Hughes Associates, Inc. 



NISTNCSTAR 1-2, WTC Investigation 



Preface 



• Robert Hanson, Professor Emeritus, University of Michigan 

• Charles Thornton, Co-Chairman and Managing Principal, The Thomton-Tomasetti Group, 

Inc. 

• Kathleen Tierney, Director, Natural Hazards Research and Applications Information Center, 
University of Colorado at Boulder 

• Forman Williams, Director, Center for Energy Research, University of California at San 
Diego 

This National Construction Safety Team Advisory Committee provided technical advice during the 
Investigation and commentary on drafts of the Investigation reports prior to their public release. NIST 
has benefited from the work of many people in the preparation of these reports, including the National 
Construction Safety Team Advisory Committee. The content of the reports and recommendations, 
however, are solely the responsibility of NIST. 

Public Outreach 

During the course of this Investigation, NIST held public briefings and meetings (listed in Table P-2) to 
solicit input from the public, present prehminary findings, and obtain comments on the direction and 
progress of the Investigation from the public and the Advisory Committee. 

NIST maintained a publicly accessible Web site during this Investigation at http://wtc.nist.gov. The site 
contained extensive information on the background and progress of the Investigation. 

NIST's WTC Public-Private Response Plan 

The collapse of the WTC buildings has led to broad reexamination of how tall buildings are designed, 
constructed, maintained, and used, especially with regard to major events such as fires, natural disasters, 
and terrorist attacks. Reflecting the enhanced interest in effecting necessary change, NIST, with support 
from Congress and the Administration, has put in place a program, the goal of which is to develop and 
implement the standards, technology, and practices needed for cost-effective improvements to the safety 
and security of buildings and building occupants, including evacuation, emergency response procedures, 
and threat mitigation. 

The strategy to meet this goal is a three-part NIST-led public-private response program that includes: 

• A federal building and fire safety investigation to study the most probable factors that 
contributed to post-aircraft impact collapse of the WTC towers and the 47-story WTC 7 
building, and the associated evacuation and emergency response experience. 

• A research and development (R&D) program to (a) facilitate the implementation of 
recommendations resulting from the WTC Investigation, and (b) provide the technical basis 
for cost-effective improvements to national building and fire codes, standards, and practices 
that enhance the safety of buildings, their occupants, and emergency responders. 



NIST NCSTAR 1-2, WTC Investigation 



Preface 



Table P-2. Public meetings and briefings of the WTC Investigation. 


Date 


Location 


Principal Agenda 


June 24, 2002 


New York City, NY 


Public meeting: Public comments on the Draft Plan for the 
pending WTC Investigation. 


August 21, 2002 


Gaithersburg, MD 


Media briefing announcing the formal start of the Investigation. 


December 9, 2002 


Washington, DC 


Media briefing on release of the Public Update and NIST request 
for photographs and videos. 


April 8, 2003 


New York City, NY 


Joint public forum with Columbia University on first-person 
interviews. 


April 29-30, 2003 


Gaithersburg, MD 


NCST Advisory Committee meeting on plan for and progress on 
WTC Investigation with a public comment session. 


May 7, 2003 


New York City, NY 


Media briefing on release of May 2003 Progress Report. 


August 26-27, 2003 


Gaithersburg, MD 


NCST Advisory Committee meeting on status of the WTC 
investigation with a public comment session. 


September 17, 2003 


New York City, NY 


Media and public briefing on initiation of first-person data 
collection projects. 


December 2-3, 2003 


Gaithersburg, MD 


NCST Advisory Committee meeting on status and initial results 
and release of the Public Update with a public comment session. 


February 12,2004 


New York City, NY 


Public meeting on progress and preliminary findings with public 
comments on issues to be considered in formulating final 
recommendations. 


June 18,2004 


New York City, NY 


Media/pubhc briefing on release of June 2004 Progress Report. 


June 22-23, 2004 


Gaithersburg, MD 


NCST Advisory Committee meeting on the status of and 
preliminary findings from the WTC Investigation with a public 
comment session. 


August 24, 2004 


Northbrook, IL 


Pubhc viewing of standard fire resistance test of WTC floor 
system at Underwriters Laboratories, Inc. 


October 19-20,2004 


Gaithersburg, MD 


NCST Advisory Committee meeting on status and near complete 
set of preliminary findings with a pubhc comment session. 


November 22, 2004 


Gaithersburg, MD 


NCST Advisory Committee discussion on draft annual report to 
Congress, a public comment session, and a closed session to 
discuss pre-draft recommendations for WTC Investigation. 


April 5, 2005 


New York City, NY 


Media and public briefing on release of the probable collapse 
sequence for the WTC towers and draft reports for the projects on 
codes and practices, evacuation, and emergency response. 


June 23, 2005 


New York City, NY 


Media and public briefing on release of all draft reports for the 
WTC towers and draft recommendations for public comment. 


September 12-13, 
2005 


Gaithersburg, MD 


NCST Advisory Committee meeting on disposition of pubhc 
comments and update to draft reports for the WTC towers. 


September 13-15, 
2005 


Gaithersburg, MD 


WTC Technical Conference for stakeholders and technical 
community for dissemination of findings and recommendations 
and opportunity for public to make technical comments. 



• A dissemination and technical assistance program (DTAP) to (a) engage leaders of the 
construction and building community in ensuring timely adoption and widespread use of 
proposed changes to practices, standards, and codes resulting from the WTC Investigation 
and the R&D program, and (b) provide practical guidance and tools to better prepare facility 
owners, contractors, architects, engineers, emergency responders, and regulatory authorities 
to respond to future disasters. 

The desired outcomes are to make buildings, occupants, and first responders safer in future disaster 
events. 



NISTNCSTAR 1-2, WTC Investigation 



Preface 



National Construction Safety Team Reports on the WTC Investigation 

A final report on the collapse of the WTC towers is being issued as NIST NCSTAR 1. A companion 
report on the collapse of WTC 7 is being issued as NIST NCSTAR 1 A. The present report is one of a set 
that provides more detailed documentation of the Investigation findings and the means by which these 
technical results were achieved. As such, it is part of the archival record of this Investigation. The titles 
of the full set of Investigation publications are: 

NIST (National Institute of Standards and Technology). 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Final Report on the Collapse of the World Trade 
Center Towers. NIST NCSTAR 1. Gaithersburg, MD, September. 

NIST (National Institute of Standards and Technology). 2006. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Final Report on the Collapse of World Trade Center 7. 
NIST NCSTAR lA. Gaithersburg, MD. 

Lew, H. S., R. W. Bukowski, and N. J. Carino. 2005. Federal Building and Fire Safety Investigation of 
the World Trade Center Disaster: Design, Construction, and Maintenance of Structural and Life Safety 
Systems. NIST NCSTAR 1-1. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Design and Construction of Structural Systems. 
NIST NCSTAR 1-1 A. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 

Ghosh, S. K., and X. Liang. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Comparison of Building Code Structural Requirements. NIST 
NCSTAR 1-lB. National Institute of Standards and Technology. Gaithersburg, MD, September. 

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Maintenance and Modifications to Structural 
Systems. NIST NCSTAR 1-lC. National Institute of Standards and Technology. Gaithersburg, 
MD, September. 

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Fire Protection and Life Safety Provisions Applied to the Design and 
Construction of World Trade Center 1, 2, and 7 and Post-Construction Provisions Applied after 
Occupancy. NIST NCSTAR 1-lD. National Institute of Standards and Technology. Gaithersburg, 
MD, September. 

Razza, J. C, and R. A. Grill. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Comparison of Codes, Standards, and Practices in Use at the Time of the 
Design and Construction of World Trade Center 1, 2, and 7. NIST NCSTAR 1-lE. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 

Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Comparison of the 1968 and Current (2003) New 



xxxii NIST NCSTAR 1-2, WTC Investigation 



Preface 



York City Building Code Provisions. NIST NCSTAR 1-lF. National Institute of Standards and 
Technology. Gaithersburg, MD, September. 

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Amendments to the Fire Protection and Life Safety Provisions of the New 
York City Building Code by Local Laws Adopted While World Trade Center 1, 2, and 7 Were in 
Use. NIST NCSTAR 1-lG. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Post-Construction Modifications to Fire Protection and Life Safety Systems 
of World Trade Center 1 and 2. NIST NCSTAR 1-lH. National Institute of Standards and 
Technology. Gaithersburg, MD, September. 

Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safety Investigation 
of the World Trade Center Disaster: Post-Construction Modifications to Fire Protection, Life 
Safety, and Structural Systems of World Trade Center 7. NIST NCSTAR l-ll. National Institute of 
Standards and Technology. Gaithersburg, MD, September. 

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Design, Installation, and Operation of Fuel System for Emergency Power in 
World Trade Center 7. NIST NCSTAR 1-1 J. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 

Sadek, F. 2005. Federal Building and Fire Safety Investigation of the World Trade Center Disaster: 
Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center 
Towers. NIST NCSTAR 1-2. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 

Faschan, W. J., and R. B. Garlock. 2005. Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster: Reference Structural Models and Baseline Performance Analysis of 
the World Trade Center Towers. NIST NCSTAR 1-2A. National Institute of Standards and 
Technology. Gaithersburg, MD, September. 

Kirkpatrick, S. W., R. T. Bocchieri, F. Sadek, R. A. MacNeill, S. Holmes, B. D. Peterson, 
R. W. Cilke, C. Navarro. 2005. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Analysis of Aircraft Impacts into the World Trade Center Towers, NIST 
NCSTAR 1-2B. National Institute of Standards and Technology. Gaithersburg, MD, September. 

Gayle, F. W., R. J. Fields, W. E. Luecke, S. W. Banovic, T. Foecke, C. N. McCowan, T. A. Siewert, and 
J. D. McColskey. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: Mechanical and Metallurgical Analysis of Structural Steel. NIST NCSTAR 1-3. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 

Luecke, W. E., T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Contemporaneous Structural Steel 
Specifications. NIST Special Publication 1-3 A. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 



NIST NCSTAR 1-2, WTC Investigation xxxiii 



Preface 



Banovic, S. W. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: Steel Inventory and Identification. NIST NCSTAR 1-3B. National Institute of Standards 
and Technology. Gaithersburg, MD, September. 

Banovic, S. W., and T. Foecke. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Damage and Failure Modes of Structural Steel Components. NIST 
NCSTAR 1-3C. National Institute of Standards and Technology. Gaithersburg, MD, September. 

Luecke, W. E., J. D. McColskey, C. N. McCowan, S. W. Banovic, R. J. Fields, T. Foecke, 
T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Mechanical Properties of Structural Steels. NIST NCSTAR 1-3D. 
National Institute of Standards and Technology. Gaithersburg, MD, September. 

Banovic, S. W., C. N. McCowan, and W. E. Luecke. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Physical Properties of Structural Steels. NIST 
NCSTAR 1-3E. National Institute of Standards and Technology. Gaithersburg, MD, September. 

Evans, D. D., R. D. Peacock, E. D. Kuligowski, W. S. Dols, and W. L. Grosshandler. 2005. Federal 
Building and Fire Safety Investigation of the World Trade Center Disaster: Active Fire Protection 
Systems. NIST NCSTAR 1-4. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 

Kuligowski, E. D., D. D. Evans, and R. D. Peacock. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Post-Construction Fires Prior to September 11, 
2001. NIST NCSTAR 1-4A. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 

Hopkins, M., J. Schoenrock, and E. Budnick. 2005. Federal Building and Fire Safety Investigation 
of the World Trade Center Disaster: Fire Suppression Systems. NIST NCSTAR 1-4B. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 

Keough, R. J., and R. A. Grill. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Fire Alarm Systems. NIST NCSTAR 1-4C. National Institute of Standards 
and Technology. Gaithersburg, MD, September. 

Ferreira, M. J., and S. M. Strege. 2005. Federal Building and Fire Safety Investigation of the 
World Trade Center Disaster: Smoke Management Systems. NIST NCSTAR 1-4D. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 

Gann, R. G., A. Hamins, K. B. McGrattan, G. W. Mulholland, H. E. Nelson, T. J. Ohlemiller, 
W. M. Pitts, and K. R. Prasad. 2005. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Reconstruction of the Fires in the World Trade Center Towers. NIST NCSTAR 1-5. 
National Institute of Standards and Technology. Gaithersburg, MD, September. 

Pitts, W. M., K. M. Butler, and V. Junker. 2005. Federal Building and Fire Safety Investigation of 
the World Trade Center Disaster: Visual Evidence, Damage Estimates, and Timeline Analysis. 
NIST NCSTAR 1-5 A. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 



xxxiv NIST NCSTAR 1-2, WTC Investigation 



Preface 



Hamins, A., A. Maranghides, K. B. McGrattan, E. Johnsson, T. J. Ohlemiller, M. Donnelly, 
J. Yang, G. MulhoUand, K. R. Prasad, S. Kukuck, R. Anleitner and T. McAllister. 2005. Federal 
Building and Fire Safety Investigation of the World Trade Center Disaster: Experiments and 
Modeling of Structural Steel Elements Exposed to Fire. NIST NCSTAR 1-5B. National Institute of 
Standards and Technology. Gaithersburg, MD, September. 

Ohlemiller, T. J., G. W. MulhoUand, A. Maranghides, J. J. Filliben, and R. G. Gann. 2005. Federal 
Building and Fire Safety Investigation of the World Trade Center Disaster: Fire Tests of Single 
Office Workstations. NIST NCSTAR 1-5C. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 

Gann, R. G., M. A. Riley, J. M. Repp, A. S. Whittaker, A. M. Reinhorn, and P. A. Hough. 2005. 
Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Reaction of 
Ceiling Tile Systems to Shocks. NIST NCSTAR 1-5D. National Institute of Standards and 
Technology. Gaithersburg, MD, September. 

Hamins, A., A. Maranghides, K. B. McGrattan, T. J. Ohlemiller, and R. Anleitner. 2005. Federal 
Building and Fire Safety Investigation of the World Trade Center Disaster: Experiments and 
Modeling of Multiple Workstations Burning in a Compartment. NIST NCSTAR 1-5E. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 

McGrattan, K. B., C. Bouldin, and G. Forney. 2005. Federal Building and Fire Safety 
Investigation of the World Trade Center Disaster: Computer Simulation of the Fires in the World 
Trade Center Towers. NIST NCSTAR 1-5F. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 

Prasad, K. R., and H. R. Baum. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Fire Structure Interface and Thermal Response of the World Trade Center 
Towers. NIST NCSTAR 1-5G. National Institute of Standards and Technology. Gaithersburg, 
MD, September. 

Gross, J. L., and T. McAllister. 2005. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Structural Fire Response and Probable Collapse Sequence of the World Trade Center 
Towers. NIST NCSTAR 1-6. National Institute of Standards and Technology. Gaithersburg, MD, 
September. 

Carino, N. J., M. A. Starnes, J. L. Gross, J. C. Yang, S. Kukuck, K. R. Prasad, and R. W. Bukowski. 
2005. Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Passive 
Fire Protection. NIST NCSTAR 1-6A. National Institute of Standards and Technology. 
Gaithersburg, MD, September. 

Gross, J., F. Hervey, M. Izydorek, J. Mammoser, and J. Treadway. 2005. Federal Building and 
Fire Safety Investigation of the World Trade Center Disaster: Fire Resistance Tests of Floor Truss 
Systems. NIST NCSTAR 1-6B. National Institute of Standards and Technology. Gaithersburg, 
MD, September. 

Zarghamee, M. S., S. Bolourchi, D. W. Eggers, O. O. Erbay, F. W. Kan, Y. Kitane, A. A. Liepins, 
M. Mudlock, W. I. Naguib, R. P. Ojdrovic, A. T. Sarawit, P. R Barrett, J. L. Gross, and 



NIST NCSTAR 1-2, WTC Investigation 



Preface 



T. P. McAllister. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: Component, Connection, and Subsystem Structural Analysis. NIST NCSTAR 1-6C. 
National Institute of Standards and Technology. Gaithersburg, MD, September. 

Zarghamee, M. S., Y. Kitane, O. O. Erbay, T. P. McAllister, and J. L. Gross. 2005. Federal 
Building and Fire Safety Investigation of the World Trade Center Disaster: Global Structural 
Analysis of the Response of the World Trade Center Towers to Impact Damage and Fire. NIST 
NCSTAR 1-6D. National Institute of Standards and Technology. Gaithersburg, MD, September. 

McAllister, T., R. W. Bukowski, R. G. Gann, J. L. Gross, K. B. McGrattan, H. E. Nelson, L. Phan, 
W. M. Pitts, K. R. Prasad, F. Sadek. 2006. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: Structural Fire Response and Probable Collapse Sequence of World Trade 
Center 7. (Provisional). NIST NCSTAR 1-6E. National Institute of Standards and Technology. 
Gaithersburg, MD. 

Gilsanz, R., V. Arbitrio, C. Anders, D. Chlebus, K. Ezzeldin, W. Guo, P. Moloney, A. Montalva, 
J. Oh, K. Rubenacker. 2006. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Structural Analysis of the Response of World Trade Center 7 to Debris Damage 
and Fire. (Provisional). NIST NCSTAR 1-6F. National Institute of Standards and Technology. 
Gaithersburg, MD. 

Kim, W. 2006. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: Analysis of September 11, 2001, Seismogram Data. (Provisional). NIST NCSTAR 1-6G. 
National Institute of Standards and Technology. Gaithersburg, MD. 

Nelson, K. 2006. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: The Con Ed Substation in World Trade Center 7. (Provisional). NIST NCSTAR 1-6H. 
National Institute of Standards and Technology. Gaithersburg, MD. 

Averill, J. D., D. S. Mileti, R. D. Peacock, E. D. Kuligowski, N. Groner, G. Proulx, P. A. Reneke, and 
H. E. Nelson. 2005. Federal Building and Fire Safety Investigation of the World Trade Center Disaster: 
Occupant Behavior, Egress, and Emergency Communication. NIST NCSTAR 1-7. National Institute of 
Standards and Technology. Gaithersburg, MD, September. 

Fahy, R., and G. Proulx. 2005. Federal Building and Fire Safety Investigation of the World Trade 
Center Disaster: Analysis of Published Accounts of the World Trade Center Evacuation. NIST 
NCSTAR 1-7A. National Institute of Standards and Technology. Gaithersburg, MD, September. 

Zmud, J. 2005. Federal Building and Fire Safety Investigation of the World Trade Center 
Disaster: Technical Documentation for Survey Administration. NIST NCSTAR 1-7B. National 
Institute of Standards and Technology. Gaithersburg, MD, September. 

Lawson, J. R., and R. L. Vettori. 2005. Federal Building and Fire Safety Investigation of the World 
Trade Center Disaster: The Emergency Response Operations. NIST NCSTAR 1-8. National Institute of 
Standards and Technology. Gaithersburg, MD, September. 



xxxvi NIST NCSTAR 1-2, WTC Investigation 



Acknowledgments 



The analyses presented in this report were conducted in collaboration with four contractors: 

• A team of experts from Leslie E. Robertson Associates, whose work included the 
development of the structural databases, the reference structural models, and the baseline 
performance analysis of the World Trade Center (WTC) towers. The team was led by Mr. 
William J. Faschan and Mr. Richard B. Garlock. 

• A team of experts from Skidmore, Owings, and Merrill, who provided the third-party review 
of the structural databases, the reference structural models, the baseline performance analysis, 
and the refined NIST estimate of the wind loads on the towers. The team included Mr. 
William F. Baker, Mr. John J. Zils, and Mr. Robert C. Sinn. 

• A team of experts from Apphed Research Associates, whose work included the analysis of 
aircraft impacts into the WTC towers. The team was led by Dr. Steven W. Kirkpatrick with 
major contributions from Dr. Robert T. Bocchieri. 

• Dr. David M. Parks, who provided expertise in the area of computational mechanics for the 
aircraft impact analysis. 

In addition. Dr. Shankar Nair of Teng & Associates provided help with the baseline analysis study. 
Professor Daniele Veneziano of MIT provided help with the uncertainty analyses, and Professor Kaspar 
Willam of the University of Colorado provided help with the constitutive modeling for aircraft impact. 

The following individuals from the National Institute of Standards and Technology (NIST) made 
contributions to this report: 



• 



• 



Dr. Emil Simiu provided the wind engineering expertise required for the development of 
Chapter 3 "Wind Loads on the WTC Towers" of this report, of which he was the primary 
author. He was also co-author of Appendix B "Estimation of Sectorial Extreme Wind 
Speeds." 

Dr. Michael A. Riley and Dr. William P. Fritz assisted with preliminary stability analyses of 
the towers that were reported in the Interim Report of June 2004. In addition. Dr. Fritz 
participated in the wind study and was the primary author of Appendix B. 

The mechanical and metallurgical analysis of structural steel team (Dr. Frank W. Gayle, 
Dr. Richard J. Fields, Dr. William E. Luecke, Mr. J. David McColskey, Dr. Tim J. Foecke, 
Dr. Stephen W. Banovic, and Dr. Thomas A. Siewert) provided the mechanical 
characteristics of the tower steels that were used in the constitutive models for the aircraft 
impact simulations. In addition. Dr. Foecke conducted the comparison of the calculated and 
observed damage to the exterior walls of the towers and provided the images presented in 
Figures 7-65 and 7-73. 



NIST NCSTAR 1-2, WTC Investigation xxxvii 



Acknowledgments 



Dr. Therese P. McAllister and Dr. John L. Gross contributed to the interpretation of the 
impact simulation results and provided the link between the impact analysis and the 
subsequent fire-structural analyses. They also helped with the review of certain parts of this 
report. 

Dr. Wilham M. Pitts provided assistance in the identification of videos and photographs 
relevant to this project, and in the interpretation of the video and photographic data collected 
by NIST. 



• 



Dr. James Filliben provided guidance in the performance of the uncertainty analyses. He 
contributed to the methodologies that allowed a large reduction in the total number of 
analyses required. 

NIST acknowledges the parties to an insurance litigation concerning the WTC towers for voluntarily 
making available to NIST the Cermak Peterka Peterson, Inc. report and the Rowan Williams Davis and 
Irwin, Inc. (RWDI) reports containing their estimates of wind loads on the towers. NIST also 
acknowledges Dr. Najib Abboud of Weidlinger and Associates, Inc. and Dr. Peter Irwin of RWDI for 
their cooperation in providing answers to NIST questions concerning the RWDI reports. 

The author also acknowledges Dr. Raymond Daddazio and Mr. David K. Vaughan of Weidlinger and 
Associates, Inc., and Professor Tomasz Wierzbicki of Massachusetts Institute of Technology for early 
discussions on aircraft impact analysis. 



xxxviii NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



E.l INTRODUCTION 

The National Institute of Standards and Technology (NIST) investigation into the collapse of the World 
Trade Center (WTC) towers included eight interdependent projects. The Baseline Structural 
Performance and Aircraft Impact Damage Analysis project had two primary tasks. These were: 

1. To develop reference structural models of the towers and use these models to establish 
the basehne performance of the two towers under gravity and wind loads. 

2 To estimate the damage to the towers due to aircraft impacts and establish the initial 
conditions for the fire dynamics modeling and thermal-structural response and collapse 
initiation analysis. 

For the first task, the baseline performance of the WTC towers under gravity and wind loads was 
estabhshed in order to assess the towers' ability to withstand those loads safely and to evaluate the reserve 
capacity of the towers to withstand unanticipated events. The baseline performance study provided a 
measure of the behavior of the towers under design loading conditions, specifically: (1) total and inter- 
story drift (the sway of the building under design wind loads), (2) floor deflections under gravity loads, 
(3) the stress demand-to-capacity ratio for primary structural components of the towers such as exterior 
walls, core columns, and floor framing, (4) performance of exterior walls under wind loading, including 
distribution of axial stresses and presence of tensile forces, (5) performance of connections between 
exterior columns, and (6) resistance of the towers to shear sliding and overturning at the foundation level. 

This task included the development of reference structural models that captured the intended behavior of 
the towers under design loading conditions. These reference models were used to establish the baseline 
performance of the towers and also served as a reference for more detailed models for aircraft impact 
damage analysis and the thermal-structural response and collapse initiation analysis. The models 
included: (1) two global models (one for each tower) of the major structural components and systems of 
the towers, and (2) floor models of a typical truss-framed floor and a typical beam- framed floor. In the 
towers, tenant floors were typical truss-framed floors, while the mechanical floors (floors 7, 41, 75, and 
108) and near mechanical floors (floors 9, 43, 77, 107, 110, and roof) of both towers were typical beam- 
framed floors. 

For the second task, the aircraft impact damage to the exterior of the WTC towers could be visibly 
identified from the video and photographic records collected. However, no visible information could be 
obtained for the extent of damage to the interior of the towers, including the structural system (floors and 
core columns), partition walls, and interior building contents. Such information was needed for the 
subsequent fire dynamics simulations and post-impact structural analyses. In addition, for the fire 
dynamics modeling, the dispersion of the jet fuel and the location of combustible aircraft debris were 
required. The estimate of the extent of damage to the fireproofmg on the structural steel in the towers due 
to impact was essential for the thermal and structural analyses. The aircraft impact damage analyses were 
the primary tool by which most of the information about the tower damage could be estimated. 



NISTNCSTAR 1-2, WTC Investigation 



Executive Summary 



The focus of this task was to analyze the aircraft impacts into each of the WTC towers to provide the 
following: (1) estimates of probable damage to structural systems, including exterior walls, floor 
systems, and interior core columns; (2) estimates of the aircraft fuel dispersion during the impact; and (3) 
estimates of debris damage to the building nonstructural contents, including partitions and workstations. 
The analysis results were used to estimate the damage to fireproofmg based on the estimated path of the 
debris field inside the towers. This analysis thus estimated the condition of the two WTC towers 
immediately following the aircraft impacts and established the initial conditions for the fire dynamics 
modeling and the thermal-structural response and collapse initiation analysis. 



E.2 



DEVELOPMENT OF REFERENCE STRUCTURAL MODELS 



The reference structural models were developed to capture the intended behavior of the WTC towers 
under design loading conditions. The models were used: (1) to establish the baseline performance of the 
towers under design gravity and wind loads and (2) as a reference for more detailed models used in other 
phases of the NIST investigation, including aircraft impact analysis and thermal-structural response and 
collapse initiation analysis. The reference models included the following: 

• Two global models of the primary structural components and systems for each of the two 
towers. 

• Two models, one of a typical truss-framed floor (tenant floor) and one of a typical beam- 
framed floor (mechanical level), within the impact and fire regions. 

All reference models were linearly elastic and three-dimensional, and were developed using the 
Computers and Structures, Inc. SAP2000 software. SAP2000 is a commercial finite element software 
package that is customarily used for the analysis and design of structures. A summary of the size of the 
global and floor models of the towers is presented in Table E-1. 

Table E-1. Approximate size of the reference structural models (rounded). 



Model 


Number of 
Joints 


Degrees of 
Freedom 


Number of 
Frame Elements 


Number of 
Shell Elements 


Total Number 
of Elements 


WTC 1 global moder 


53,700 


218,700 


73,900 


10,000 


83,900 


WTC 2 global model' 


51,200 


200,000 


73,700 


4,800 


78,500 


Typical truss-framed model 


28,100 


166,000 


27,700 


14,800 


42,500 


Typical beam-framed model 


6,500 


35,700 


7,500 


4,600 


12,100 



a. Model does not include floors except for flexible diaphragms at 17 floors as explained later. 

The models were developed by Leslie E. Robertson Associates (LERA), the firm responsible for the 
original structural engineering of the WTC towers, under contract to NIST. The models were reviewed 
by independent parties to ensure objectivity. The review process included a third-party review by the 
firm of Skidmore, Owings, and Merrill (SOM), under contract to NIST, and an in-house review by NIST. 

For the global models of the towers, the large amount of data required to construct the models dictated 
that a database of the primary structural components of the towers be developed from the original 



xl 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



computer printouts of the structural design documents. The various databases, developed in Microsoft 
Excel format, were linked together using the relational database technique. The relational databases, 
developed using Microsoft Access, were generated in a format suitable for the development of the global 
finite element models of the towers. 

E.2.1 Global Models of the Towers 

Three-dimensional models of the 1 10-story above-grade structure and 6-story below-grade structure 
within the footprint of each of the two towers were developed. The global models for the towers 
consisted of all primary structural elements in the towers, including exterior walls (exterior columns, 
spandrel beams, and bracings in the basement floors), core columns, hat trusses, and rigid and flexible 
diaphragms representing the floor systems. 

For the development of the global models, each tower was divided into several sub-models that included: 

• Exterior walls, which in turn was divided into 

- Exterior wall, foundation to floor 4 

- Exterior wall trees (floors 4 to 9) 

- Exterior wall, floors 9 to 106 

- Exterior wall, floors 107 to 110 

• Core columns 

• Hat truss 

After these sub-models were assembled into a unified model, rigid and flexible diaphragms representing 
the floor systems, boundary conditions, gravity and wind loads, and masses were added to the unified 
model. Isometric views of the complete WTC 1 model showing exterior walls, core columns, bracings, 
hat trusses, and flexible floor diaphragms are shown in Figure E-1. 

The global models were developed primarily using prismatic and non-prismatic frame (beam) elements. 
Shell elements were used only to represent the flexible floor diaphragms. For the development of beam 
element representations of the exterior wall panels, detailed shell element models of the panels were 
developed and used to calibrate the behavior of the beam element model under gravity and lateral loads 
(Figure E-2). Similarly, the detailed floor models were used to calibrate the response of a simplified shell 
element representation of the floor systems for use as flexible diaphragms in the global models. 



NIST NCSTAR 1-2, WTC Investigation xli 



Executive Summary 





Figure E-1. Rendered isometric views of the WTC 1 global model. 



xlii 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



To validate the global models, the natural periods of WTC 1 calculated from the model were compared 
with those measured on the tower based on analyzing acceleration records obtained from accelerometers 
installed atop WTC 1 . Table E-2 presents a comparison of the calculated first three natural frequencies 
and periods against measured frequencies and periods for WTC 1 . The measurements were taken during 
the period from 1978 through 1994 for wind speeds ranging from 1 1.5 mph to 41 mph. The table 
indicates longer periods measured at larger wind speeds. The natural periods and frequencies predicted in 
the original design are also presented in the table. The table shows a good agreement between the 
calculated and measured periods. Thus, Table E-2 indicates that the reference global model provided a 
reasonable representation of the actual structure. 













- 








lLj 








A 



A 



A 



A 



Figure E-2. Shell element and frame element models of an exterior wall panel. 



NISTNCSTAR 1-2, WTC Investigation 



xliii 



Executive Summary 



Table E-2. Comparison of measured and calculated natural periods for WTC 1 


Data Source/ 
Event Date 


Wind Speed & 
Direction 


Frequency (HZ) 


Period (s) 


Direction of Motion 


Direction of Motion 


N-S 


E-W 


Torsion 


N-S 


E-W 


Torsion 


Historical Data 


October 1 1 , 1 978 


ll.Smph, E/SE 


0.098 


0.105 


0.211 


10.2 


9.5 


4.7 


January 24, 1979 


33 mph, E/SE 


0.089 


0.093 


0.203 


11.2 


10.8 


4.9 


March 21, 1980 


41 mph, E/SE 


0.085 


0.092 


0.201 


11.8 


10.9 


5.0 


December 11, 1992 


- 


0.087 


0.092 


- 


11.5 


10.9 


- 




February 2, 1993' 


20 mph, NW 


0.085 


0.093 


0.204 


11.8 


10.8 


4.9 


March 13, 1993^ 


32 mph, NW 


0.085 


0.094 


0.199 


11.8 


10.6 


5.0 


March 10, 1994' 


14 mph, W 


0.094 


0.094 


0.196 


10.6 


10.6 


5.1 


December 25, 1994^ 


N 


0.081 


0.091 


- 


12.3 


11.0 


- 


Average of Measured Data 


Average 


- 


0.088 


0.094 


0.202 


11.4 


10.6 


4.9 1 


Orginal Design - Predicted Values 


Theoretical Value 


- 


0.084 


0.096 


- 


11.9 


10.4 


- 


Reference Global Model 


LERA/NIST-WTC 1 
without P-Delta 




0.088 


0.093 


0.192 


11.4 


10.7 


5.2 


LERA/NIST-WTC 1 
with P-Delta 




0.083 


0.088 


0.189 


12.1 


11.3 


5.3 



Notes: 

'Reported frequency value is the average of the SW corner, NE corner, and center core frequency measurements. 

^Reported frequency is based on center core data only. 



E.2.2 



Typical Truss-Framed Floor Model - Floor 96 of WTC 1 



The model of the typical truss-framed floor contained all primary structural members of the floor system, 
including the primary trusses, bridging trusses, spandrel beams, columns above and below the floor level, 
concrete slabs, dampers, strap anchors, and beams in the core. The model was developed primarily using 
frame elements with the exception of the floor slabs, which were modeled using shell elements with 
typical element sizes of 20 in. An isometric view of the model is shown in Figure E-3. 



xliv 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 




Figure E-3. Typical truss-framed floor model (floor 96 of WTC 1), slab not shown. 

E.2.3 Typical Beam-Framed Floor Model - Floor 75 of WTC 2 

The model of the typical beam- framed floor contained all primary structural members of the floor system, 
including the primary composite beams, horizontal trusses, spandrel beams, columns above and below the 
floor level, concrete slab, dampers, and beams in the core. Similar to the typical trussed-frame model, 
this model was developed primarily using frame elements with the exception of the floor slabs, which 
were modeled using shell elements with typical element sizes of 40 in. An isometric view of the model is 
shown in Figure E-4. 



NISTNCSTAR 1-2, WTC Investigation 



xlv 



Executive Summary 




E.3 



Figure E-4. Typical beam-framed floor model (floor 75 of WTC 2). 



WIND LOADS ON THE WTC TOWERS 



Wind loads were a governing factor in the design of the components of the WTC towers' perimeter 
frame-tube system. Their study was required for evaluating: (1) the baseline performance of the towers 
under design loading conditions, (2) the towers' reserve capacity to withstand unanticipated events such 
as a major fire or impact damage, and (3) design practices, procedures, and codes. Accurate estimation of 
the wind loads on tall buildings is challenging, since wind engineering is still an evolving technology. 

Wind estimates for the WTC towers considered in this study included: (1) wind loads used in the original 
WTC design, (2) wind loads based on two recent wind tunnel studies conducted in 2002 by Cermak 
Peterka Peterson, Inc. (CPP) and Rowan Williams Davis and Irwin, Inc. (RWDI), and (3) refined wind 
loads estimated by NIST by critically assessing information obtained from the CPP and RWDI reports 
and using state-of-the-art knowledge. These estimates are summarized below. 



E.3.1 



Original WTC Wind Design Loads 



Wind loads were determined for the original design of the WTC towers through the development and 
implementation of a boundary-layer wind-tunnel study, which simulated the mean and fluctuating 
(turbulence) properties of the wind from ground to gradient height by using the knowledge and techniques 
available in the 1960s. The wind tunnel tests were conducted at Colorado State University (CSU) and the 
National Physical Laboratory (NPL), United Kingdom. Aeroelastic tests at CSU were conducted at a 
scale of 1 :500, while the aeroelastic models at NPL were conducted with a scale of 1 :400. Results from 
the tests conducted at NPL were in good qualitative and quantitative agreement with those obtained from 
the CSU tests. Wind tunnel data were collected for each tower for wind approaching from 24 wind 



xlvi 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



directions in 15 degree increments. The wind effects were estimated as the summation of static and 
dynamic components based on results obtained from the wind tunnel tests. The most severe wind effects 
were determined from diagrams of wind-induced shear forces and overturning moments. 

E.3.2 State-of-the-Practice Wind Loads 

For the WTC towers, two wind tunnel tests and wind engineering studies based thereon were conducted 
in 2002 by independent laboratories as part of insurance litigation unrelated to the NIST investigation. 
The tests and studies were conducted by CPP and RWDI. The results of both studies were made available 
to NIST. 

The CPP wind tunnel tests modeled the terrain surrounding the WTC towers over an area with a radius of 
about 2,300 ft. The tests used a high-frequency force-balance (HFFB) model and an aeroelastic model of 
the south tower only. The test scale was 1 :400, and testing was conducted for 36 wind directions at 
10 degree intervals. The wind-induced loads and responses were determined by combining the wind 
tunnel test data with recorded directional wind speeds and unspecified hurricane wind speed data. The 
recorded wind speeds were obtained at the three major airports in the New York area over about 25 years. 
The directional wind tunnel and wind speed data were combined by using the sector-by-sector approach. 
Wind effects corresponding to a damping ratio of 2.5 percent were provided for WTC 2 only for nominal 
50 year and 720 year mean recurrence intervals. The wind-induced effects were provided as peak shear 
forces and bending moments for two orthogonal directions and peak torsional moments. The peak 
components were not applied to the structural model of the tower simultaneously, but were combined by 
using the full peak load in one direction and "companion point-in-time" loads in the other direction and in 
torsion. 

The RWDI wind tunnel tests modeled the terrain surrounding the WTC towers over an area with a radius 
of about 4,000 ft. The tests used an HFFB model for both towers and an aeroelastic model for the north 
tower only. The test scale was 1:500, and testing was conducted for 36 wind directions at 10 degree 
intervals. Corrections were made to account for the effects on the flow of the presence of the building 
model (i.e., of wind tunnel blockage). Predictions of the full-scale wind effects and responses were 
obtained by combining the wind tunnel test data with a statistical model of winds for New York City, 
based on surface wind measurements taken at three airports between 1948 and 1995 and proprietary 
simulated hurricane winds. The directional wind tunnel and wind speed data were combined by using an 
out-crossing approach developed by RWDI. Two sets of wind effects on the towers were developed by 
scaling the wind loads to the design wind speeds provided in the New York City Building Code 
(NYCBC) 2001 and the American Society of Civil Engineers (ASCE) 7-98 Standard. The wind effects 
were provided for a damping ratio of 2.5 percent. The wind-induced effects were provided as peak shear 
forces and bending moments for two orthogonal directions and peak torsional moments. These peak 
components were combined using the "principle of companion loads," entailing combination factors 
based on engineering judgment and in-house experience. 

E.3.3 Refined NIST Estimates of Wind Effects 

NIST completed an independent analysis to estimate the wind loads that would be appropriate for use in 
designing the towers. The analysis was based on results provided by CPP and RWDI, with refinements 
that drew on the state of the art in wind engineering. The objective of this analysis was not to assess the 



NIST NCSTAR 1-2, WTC Investigation xlvii 



Executive Summary 



adequacy of the original design wind loads, but rather to better understand and assess the effects of 
successive changes in standards, codes, and practices. The analysis yielded refined estimates of wind 
effects for the north and south WTC towers. These estimates made use of independent extreme wind 
climatological estimates developed by NIST, based on airport wind speed data obtained from the National 
Climatic Data Center and on the NIST hurricane wind speed database. The estimates of wind effects 
relied primarily on RWDI results, since no results for WTC 1 were available from CPP. However, the 
estimates took into account a comparative assessment of the RWDI and CPP results for WTC 2. 

A summary comparison between CPP and RWDI estimates of maximum base moments and shear forces 
on WTC 2 indicated that the CPP estimates were larger by about 40 percent. The critical base moments 
from both studies occurred for a wind direction of about 210 degrees. This agreement suggested that a 
comparison between those results was warranted in some detail for that wind direction. An independent 
estimate by NIST of the 720 yr, 3 s peak gust speed for that direction was 99.8 mph, while the CPP 
estimate was about 1 17.5 mph. The CPP results were therefore multiplied by a factor of approximately 
(99.8/117.5)^=1/1.386. 

In addition, the CPP results were modified to account for the use by CPP of the sector-by-sector approach 
to integrating aerodynamic data and extreme-wind climatological data. The sector-by-sector approach is 
not valid from a physical point of view. A study by NIST concluded that the sector-by-sector approach 
underestimated the wind effects corresponding to a specified mean recurrence interval. According to 
preliminary estimates, it was assumed that the underestimation was approximately 1 5 percent. Therefore, 
the CPP results, modified via multiphcation by the factor 1/1.386, were further modified via 
multiplication by the factor 1.15. The reduction factor applied to the estimated CPP effects was therefore 
about 1/1.205. To within the limitations inherent in the information available for this investigation, and to 
within the approximations noted, these reduced values are reasonable estimates of the actual responses of 
interest. 

According to the conclusion concerning the modified CPP results, the RWDI results underestimated the 
towers' response. This conclusion was consistent with the fact that RWDI assumed wind profiles in 
hurricanes to be flatter than wind profiles in non-hurricane winds. According to state-of-the-art 
information on wind profiles at high elevations, hurricane profiles do not differ substantially from non- 
hurricane wind profiles, and an unconventional model such as the relatively flat hurricane profile model 
used by RWDI is not supported by measurements in the atmosphere. Based on the assumption that 
hurricane wind profiles are relatively flat, RWDI used a ratio of approximately 1 . 1 between tower 
responses to 88 mph and 80 mph wind speeds. However, in view of the current state of the art, according 
to which hurricane and non-hurricane profiles are substantially similar, a ratio of about (88/80)^=1.21 is 
more appropriate than the ratios of approximately 1 . 1 used by RWDI. 

In addition, the weighting of hurricane wind speeds in proportion to their squares, as used by RWDI in 
the out-crossing method to integrate aerodynamic and chmatological data, did not appear to be warranted. 
In the standard Peaks-Over-Threshold approach applied to extreme wind speeds, all data above a 
threshold are affected by the weighting factor 1 , while all the data below the threshold are weighted by 
the factor zero. No justification was provided in the RWDI report for the weighting procedure based on 
squares of speeds, nor did RWDI list any pertinent reference. 

Based on this analysis, the refined NIST estimates, consistent with the design wind speed in the 

ASCE 7-98 and ASCE 7-02 Standards, were estimated by using the RWDI results multiplied by a factor 

xlviii NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



equal to the ratio of the modified CPP estimates to the corresponding RWDI estimates. This factor was 
found to be about 1.15. The factor 1.15 was recommended for baseline analysis. However, the actual 
factor could be anywhere between, say, 1.10 and 1.20. It would have been desirable to perform more 
elaborate calculations providing more comprehensive and precise results than those presented in this 
document. However, in the absence of sufficiently transparent and detailed information in the CPP and 
RWDI reports, this was not practicable. 

E.3.4 Comparison of Wind Loads 

Table E-3 provides a summary of the wind-induced base shears and base moments for WTC 2, while 
Table E-4 presents a summary of design base shears and base moments based on the prescriptive 
provisions in various building codes at the time of the design. The tables indicate the following: 



• 



The original design wind load estimates exceeded in all cases those established by NYCBC (a 
prescriptive code) prior to 1968, when the WTC towers were designed, and up to and 
including 2001. The design values were also higher than those required by other prescriptive 
building codes of the time (the 1964 New York State Code, the 1965 BOCA Basic Building 
Code, and the 1967 Chicago Municipal Code). However, the prescriptive approach in these 
codes is oversimplified. These codes are therefore not appropriate for super-tall building 
design. This was confirmed by the fact that wind effects obtained from three separate wind- 
tunnel-based studies were in all cases higher than those based on the prescriptive codes. 

The two orthogonal base shear and base moment components used in the original design were 
in the majority of cases smaller than the CPP, RWDI, and NIST estimates. However, the 
most unfavorable combined peaks from the original design were larger than, or smaller by at 
most 15 percent than estimates based on the CPP, RWDI, and NIST estimates. This is due to 
the conservative procedure used to combine the loads in the original design. (For example, 
NIST estimates were higher by about 15 percent than the most unfavorable original design 
wind loads for WTC 1 and lower by about 5 percent than the most unfavorable original 
design loads for WTC 2.) 

The estimated wind-induced loads on the towers varied by as much as 40 percent between the 
wind tunnel/climatological studies conducted in 2002 by CPP and RWDI. The primary 
reasons for these differences appear to lie in the different approaches used in those studies to 
estimate extreme wind speeds, to estimate wind profiles, to integrate aerodynamic, dynamic, 
and extreme wind climatological information, and to combine wind effects in two orthogonal 
directions and in torsion. Such differences highlight the limitations associated with the state 
of the practice in wind engineering for tall buildings and the need for the development of 
consensus standards in the field of wind tunnel testing and wind effects estimation. Among 
the issues that need to be considered are: estimation methods for combining directional wind 
loads, integrating climatological (wind) and aerodynamic (wind tunnel) data; protocols for 
conducting the wind tunnel tests; and profiles of hurricane and non-hurricane winds. 



NIST NCSTAR 1-2, WTC Investigation xlix 



Executive Summary 



Table E-3. Comparison of wind load estimates for WTC 2 based 


on various sources. 


Source 


Year 


Base Shear 10^ kip 


Base Moment 10^ kip- ft 


N-S 


E-W 


Most 

unfavorable 

combined 

peak 


About 

N-S 


About 
E-W 


Most 

unfavorable 

combined 

peak 


NYC Building Code 


1938 


5.3 


5.3 




4.2 


4.2 




NYC Building Code 


1968 to 
date 


9.3 


9.3 




7.6 


7.6 




RWDI / NYC Building 
Code 


2002 


9.7 


11.1 


12.3 


10.1 


9.2 


11.3 


RWDI / ASCE 7-98 


2002 


10.6 


12.2 


13.5 


11.1 


10.1 


12.4 


CPP / NYC Building 
Code 


2002 


None 


None 


None 


None 


None 


None 


CPP /ASCE 7-98' 


2002 


15.1 


15.3 


17.1 


15.5 


14.0 


17.0 


NIST / third-party SOM 
review 


2004 


12.2 


14.0 


15.5 


12.8 


11.6 


14.3 


Original WTC Design 


1960s 


13.1 


10.1 


16.5 


8.8 


12.6 


15.4 



a. Using ASCE 7-98 sections 6.5.4.1 and 6.6. 



Table E-4. Base shears and base moments due to wind loads based on various 

building codes. 



Building Code 


1938 
NYC Code 


1968 to date 
NYC Code 


1964 
NY State Code 


1965 
BOCA/BBC 


1967 

Chicago 

Municipal Code 


Base Shear 
(lO^kip) 


5.3 


9.3 


9.5 


9.8 


8.7 


Base Moment 
(lO^kip-ft) 


4.2 


7.7 


7.6 


8.5 


7.5 



E.4 BASELINE PERFORMANCE OF THE WTC TOWERS 

E.4.1 Baseline Performance of the Global Models 

The reference global models were analyzed under gravity and wind loads to establish the baseline 
performance of the towers. Three loading cases were considered for this analysis. They included: 

• Original WTC design loads case. Loads were as follows: dead and live loads as in original 
WTC design, used in conjunction with original WTC design wind loads (Section E.3.1). 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



• State-of-the-practice case. Loads were as follows: dead loads as in original design; 
NYCBC 2001 live loads; and wind loads from RWDI wind tunnel study (Section E.3.2), 
scaled in accordance with NYCBC 2001 wind speed. This wind load was considered to be a 
lower estimate state-of-the-practice case, as the CPP wind tunnel study produced larger wind 
loads. 

• Refined NIST estimate case. Loads were as follows: dead loads as in original design; live 
loads from ASCE 7-02; and refined wind loads developed by NIST (Section E.3.3). 

The following is a summary of the results. 

Total and Inter-Story Drift 

The calculated total drift of both WTC 1 and WTC 2 induced by the three loading cases is presented in 
Table E-5. The table lists calculated total drift values at the top of the tower, in absolute terms and as a 
fraction of the building height, H, from the foundation level to the roof (referred to in the table as the drift 
ratio). According to LERA, limiting total building drift under wind loads was not part of the original 
WTC design criteria. Instead, inter-story drifts were determined and compared to the capability of the 
architectural building systems, such as the partitions and the exterior cladding, to accommodate these 
inter-story drifts. Accordingly, there are no historical project-specific data available to which the total 
drifts may be compared. 



Tab 


e E-5. Total drift for WTC 1 and WTC 2 


under the three loading cases. 


Loading 

Case 


WTC 1 


WTC 2 


E-W 


N-S 


E-W 


N-S 


Total 
Drift (in.) 


Drift 
Ratio 


Total 
Drift (in.) 


Drift 
Ratio 


Total 
Drift (in.) 


Drift 
Ratio 


Total 
Drift (in.) 


Drift 
Ratio 


Original 
design case 


56.6 


H/304 


55.7 


H/309 


51.2 


H/335 


65.3 


H/263 


SOP case 


56.8 


H/303 


68.1 


H/253 


59.7 


H/287 


56.1 


H/306 


Refined 
NIST case 


70.6 


H/244 


83.9 


H/205 


75.6 


H/227 


71.0 


H/242 



Under the original WTC design loads, the cumulative drifts at the top of the WTC towers ranged from 
H/263 to H/335. For the lower estimate state-of-the-practice case, those drifts ranged from H/253 to 
H/306. The drifts obtained from the refined NIST estimate case were about 25 percent larger than those 
from the state-of-the practice case. Under design loading conditions, the maximum inter-story drift was 
as high as h/230 and h/200 for WTC 1 and WTC 2, respectively, where h is the story height. Maximum 
inter-story drifts under the state-of-the practice case were about h/184 and h/200 for WTC 1 and WTC 2, 
respectively. For the refined NIST estimate case, these inter-story drifts were about 25 percent larger than 
those from the state-of-the practice case. 



NISTNCSTAR 1-2, WTC Investigation 



Executive Summary 



Currently no building codes specify a drift limit for wind design. The commentary to Section B.1.2 of the 
ASCE 7-02 Standard indicates that drift limits in common usage for building design are on the order of 
1/400 to 1/600 of the building (for total drift) or story (for inter-story drift) height to minimize damage to 
cladding and nonstructural walls and partitions. Structural engineers often use in their practice the 
criterion that total drift ratios should not exceed H/400 to H/500 for serviceability considerations and to 
enhance overall safety and stability (including second order, nonlinear P-A effects). Reducing the drift of 
the WTC towers to the range of H/400 to H/500 would entail enhancing the stiffness and/or damping 
characteristics of the towers. For inter-story drifts, structural engineers often use in their practice an inter- 
story drift limit in the range of h/300 to h/400. This is primarily done for serviceability considerations. 
Similar to total drift, inter-story drifts of the towers were larger than what is generally used in practice. 

Demand/Capacity Ratios (OCRs) 

DCRs were based on the allowable stress design procedure and were estimated using the American 
Institute of Steel Construction (AISC) Specifications (1989). Figure E-5 shows the distribution of DCRs 
for the four exterior walls of WTC 1 under the original design load case, while Figure E-6 shows DCRs 
for the WTC 1 core columns on lines 600 and 900. The results of the baseline analyses indicated that, for 
both towers, the DCRs estimated from the original WTC design load case were in general close to those 
obtained from the lower estimate state-of-the practice case. For both cases, a fraction of structural 
components had DCRs larger than 1.0. These were mainly observed in both towers at (1) the exterior 
walls at the columns around the corners, where the hat truss connected to the exterior walls, and below 
floor 9; and (2) the core columns on the 600 line between floors 80 and 106 and at core perimeter 
columns 901 and 908 for much of their height. 

While it is a normal design practice to achieve a DCR less than unity, the safety of the WTC towers on 
September 11, 2001, was most likely not affected by the fraction of members for which the demand 
exceeded capacity due to the following: (1) the inherent factor of safety in the allowable stress design 
method, (2) the load redistribution capability of ductile steel structures, and (3) on the day of the attack, 
the towers were subjected to in-service live loads (a fraction of the design live loads) and minimal wind 
loads. 

The DCRs obtained for the refined NIST estimate case were higher than those from the original 
WTC design and the lower-estimate state-of-the-practice load cases, owing to the following reasons: 
(1) the NIST estimated wind loads were higher than those used in the state-of-the-practice case by about 
25 percent, and (2) the original WTC design and the state-of-the-practice cases used NYCBC load 
combinations, which result in lower DCRs than the ASCE 7-02 load combinations used for the refined 
NIST case. 



lii NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 








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1.00 



Figure E-5. OCRs for the exterior walls of WTC 1 under original design case, (a) north 
elevation, (b) east elevation, (c) south elevation, and (d) west elevation. 



NISTNCSTAR 1-2, WTC Investigation 



liii 



Executive Summary 



106 FL 


105 FL 


104 FL 


103 FL 


102 FL 


101 FL 


100 FL 


99 FL 


98 FL 


97 FL 


96 FL 


95 FL 


94 FL 


93 FL 


92 FL 


91 FL 


90 FL 


39 FL 


88 FL 


87 FL 


86 FL 


85 FL 


84 FL 


83 FL 


82 FL 


81 FL 


80 FL 


79 FL 


78 FL 


77 FL 


76 FL 


75 FL 


74 FL 


73 FL 


72 FL 


71 FL 


70 FL 


69 FL 


68 FL 


67 FL 


66 FL 


65 FL 


64 FL 


63 FL 


62 FL 


61 FL 


60 FL 


59 FL 


58 FL 


57 FL 


56 FL 


55 FL 


54 FL 


53 FL 


52 FL 


51 FL 


50 FL 


49 FL 


48 FL 


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46 FL 


45 FL 


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43 FL 


42 FL 


41 FL 


40 FL 


39 FL 


38 FL 


37 FL 


36 FL 


35 FL 


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30 FL 


29 FL 


28 FL 


27 FL 


26 FL 


25 FL 


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22 FL 


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19 FL 


18 FL 


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16 FL 


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11 FL 


10 FL 


09 FL 


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07 FL 


06 FL 


05 FL 


04 FL 


03 FL 


02 FL 


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B1 FL 


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TOWER A, DCR of CORE COLUMN 
600's COLUMN NUMBER 


601 


602 


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605 


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84 





88 


n 


87 





87 


0.91 





83 


0.80 





85 





84 





87 


n 


86 





86 


0.90 





89 


0.82 





89 





87 





89 


n 


88 





89 


1 07 





90 


82 





89 





87 





89 





88 





89 


^^1 


92 


84 





92 





90 





92 


90 





91 


^^1 


90 


81 





89 





86 





88 


87 





88 


0.99 


91 


83 





90 





88 





89 


88 





89 


1 01 





93 


84 





92 





89 





91 


89 





91 


1 03 





90 


0.81 





86 





90 





87 





87 





87 


1 05 





91 


0.82 





87 





91 





89 





88 





88 


^7 
1 05 





93 


0.83 





88 





93 





90 





89 





89 





90 


0.81 





86 





90 





87 





86 





86 





92 


82 





87 





92 





88 





87 





87 


^7 
1 04 





93 


83 





88 





93 





89 





88 





88 





87 


81 





86 





90 





91 





86 





83 





88 





8? 





87 





91 





9? 





87 





84 


1 06 





89 





83 





88 





93 





93 





88 





85 


1 08 





87 





81 





86 





90 





90 





86 





83 


1 04 





89 





8? 





87 





91 





91 





87 





83 


1 05 





90 





83 





88 





92 





93 





88 





84 


1 07 





88 





81 





86 





93 





90 





84 





82 


1 03 





89 





82 





87 





94 





91 





85 





83 


1 05 





91 





83 





88 





96 





92 





86 





84 


1 07 





83 





77 





81 





86 





84 





80 





75 





92 





85 





78 





83 





89 





86 





81 





77 





95 





64 





56 





60 





65 





65 





61 





58 





70 





66 





56 





63 





68 





68 





64 





61 





72 





85 





77 





83 





88 





90 





84 





81 





92 





85 





76 





82 





88 





90 





84 





81 





92 





84 





77 





a? 





88 





88 





84 





80 





90 





85 





78 





83 





90 





89 





85 





81 





91 


c 


37 


c 


79 


c 


34 


c 


91 


c 


90 


c 


86 


c 


82 


c 


93 


c 


35 


c 


78 


c 


31 


c 


88 


c 


37 


c 


84 


c 


81 


c 


91 


r 


36 


c 


79 


c 


3? 


f 


89 


f 


38 


(. 


85 


c 


82 


c 


92 





88 


c 


80 


c 


3? 





91 


r 


90 





86 


c 


8? 


c. 


93 


c 


34 


c 


77 


c 


31 


c 


89 


c 


37 


c 


83 


c 


80 


c 


92 


c 


35 


c 


77 


c 


32 


c 


90 


c 


38 


c 


84 


c 


81 


c 


93 


c 


36 


c 


78 


c 


33 


c 


91 


c 


39 


c 


85 


c 


82 


c 


94 


c 


'.? 


r 




c. 


3? 


f 


89 


r 


37 


r 


■14 


f 


81 


r 


97 





83 


r 


78 





a? 





90 





88 


n 


85 


f 


8? 





94 


c 


34 


c 


78 


c 


33 


c 


91 


c 


39 


c 


86 


c 


83 


c 


95 


c 


33 


c 


77 


c 


32 


c 


89 


c 


58 


c 


83 


c 


80 


c 


93 


r 


34 


c 


78 


c 


3? 


f 


90 


r. 


38 


f 


84 


c 


81 


c. 


94 





85 


c 


79 


c 


83 





91 


f 


89 





85 


c 


8? 


c. 


95 


c 


32 


c 


78 


c 


31 


c 


89 


c 


38 


82 


c 


80 


c 


94 


c 


33 


c 


78 


c 


31 


c 


90 


c 


39 


82 


c 


81 


c 


95 


r 


34 


r 


79 


r 


3? 


f 




r 




r 




f 


8? 


r 


96 





8? 


r 


78 





81 





9? 





88 


n 


8? 


f 


81 





94 


c 


33 


c 


79 


c 


32 


c 


93 


c 


39 


c 


83 


c 


82 


c 


96 


c 


34 


c 


79 


c 


33 


c 


94 


c 


90 


c 


83 


c 


83 


c 


97 


c 


32 


c 


78 


c 


31 


c 


92 


c 


58 


c 


82 


c 


80 


c 


93 


r 


33 


c 


79 


c. 


3? 


f 


93 


f 


39 


c 


a? 


c 


80 


c 


94 


c 


34 


c 


79 


c 


33 


c 


94 


c 


90 


83 


c 


81 


c 


95 


c 


31 


77 


c 


30 


c 


93 


c 


90 


81 


c 


80 


c 


93 


c 


32 


78 


c 


31 


c 


93 


c 


91 


c 


82 


[ 


80 


c 


94 


r 


33 


79 


r 


3? 


f 


94 


r 


9? 


r 


8? 


f 


81 


r 


95 





8? 


78 





81 





93 





90 


n 


8? 


f 


79 





94 


c 


33 


C 


78 


c 


32 


[ 


94 


c 


91 


c 


82 


c 


79 


c 


95 


c 


34 


c 


30 


c 


33 


c 


95 


c 


92 


c 


84 


c 


80 


c 


96 


c 


92 


c 


76 


c 


90 


1 04 


1 


01 


c 


97 


c 


87 




r 


9? 


c 


31 


r 


90 


1 04 


1 


V 


f 


98 


c 


87 


1 04 


c 


57 


c 


53 


c 


56 


c 


62 


c 


50 


f 


60 


[ 


55 


c 


51 


c 


59 


c 


55 


c 


57 


c 


63 


c 


52 


c 


62 


[ 


56 


c 


52 


r 


-^9 


r 


55 


r 


58 


c 


54 


r 


53 


r 


53 


f 


57 


r 


5? 





•^7 


r 


54 





56 





6? 





61 





61 





55 





60 


c 


56 


c 


31 


c 


54 


c 


71 


c 


70 


c 


69 


c 


63 


c 


59 


c 


59 


c 


52 


c 


56 


c 


74 


c 


72 


c 


71 


c 


64 


c 


72 


c 


30 


c 


79 


c 


52 


c 


92 


c 


90 


c 


38 


c 


79 


c 


38 


r 


79 


c 


78 


r 


31 


f 


91 


r 


59 


f 


H7 


c 


79 


r 


38 





81 


c 


79 


r 


3? 


f 


93 


f 


90 





88 


c 


80 





39 


c 


77 


75 


c 


31 


c 


89 


c 


57 


c 


87 


c 


78 


c 


34 


c 


79 


83 


c 


39 


c 


99 


c 


97 


c 


95 


[ 


86 


c 


95 





80 


83 





90 


1 


00 





98 





96 





87 





96 



106 FL 


105 FL 


104 FL 


103 FL 


102 FL 


101 FL 


100 FL 


99 FL 


98 FL 


97 FL 


96 FL 


95 FL 


94 FL 


93 FL 


92 FL 


91 FL 


90 FL 


89 FL 


88 FL 


87 FL 


86 FL 


85 FL 




83 FL 


82 FL 


81 FL 


80 FL 


79 FL 


78 FL 


77 FL 


76 FL 




74 FL 


73 FL 


72 FL 


71 FL 


70 FL 


69 FL 


68 FL 


67 FL 


66 FL 


65 FL 


64 FL 


63 FL 


62 FL 


61 FL 


60 FL 


59 FL 


58 FL 


57 FL 


56 FL 


55 FL 


54 FL 


53 FL 


52 FL 


51 FL 


50 FL 


49 FL 


48 FL 


47 FL 


46 FL 


45 FL 


44 FL 


43 FL 


42 FL 


41 FL 


40 FL 


39 FL 


38 FL 


37 FL 


36 FL 


35 FL 


34 FL 


33 FL 


32 FL 


31 FL 


30 FL 


29 FL 


28 FL 


27 FL 


26 FL 


25 FL 


24 FL 


23 FL 


22 FL 


21 FL 


20 FL 


19 FL 


18 FL 


17 FL 


16 FL 


15 FL 


14 FL 


13 FL 


12 FL 


11 FL 


10 FL 




08 FL 


07 FL 


06 FL 


05 FL 


04 FL 


03 FL 


02 FL 


01 FL 




B2FL 


B3FL 


B4FL 


B5FL 



TOWER A, DCR of CORE COLUMN 
900's COLUMN NUMBER 


901 


902 


903 


904 


905 


906 


907 


908 




0.76 


r. 


98 


r. 


97 0.80 C 


.-1. ^^^ 1 


0.76 


0.82 


1 


05 


1 


02 1 0.71 1 C 


91 ^^H 1 Oft 


0.80 


0.83 





95 





94 ^1^ r 


75 


0.96 


0.97 


0.80 


0.92 


1 


01 





98 ^^H r 


80 


1.01 


1.02 


0.89 


1.01 


1 


07 


1 


03 


c 


89 


c 


88 


1.05 


1.07 


ry^ 






















OM 


1 


01 


c 


iS 


1 


04 


c 


84 ^^m 1 m ^^^ 


1.01 


1 


05 


c 


91 


I 


■ 


f 


9Q 


^^H 1 OR H^^ 


1.02 


c 


92 


c 


81 


c 


99 


c 


88 




?tr 


7B^ 


^wr 




c 


46 


c 


3;i 




)t. 


1. 


93 


L 


93 


0.98 


1 06 


^^^1 1 


00 





86 


I 


■ 


c 


99 


c 


97 


1 02 


^^H 


0.91 


c 


84 





81 


c 


88 


c 


86 


c 


86 


0.90 


1 OB 


0.96 


c 


38 


c 


53 


c 


92 


c 


91 


c 


89 


0.94 


^^H 


1 00 


c 


91 


c 


55 


r 


96 


r 


95 


c 


92 


0.97 




0.89 


c 


56 


c 


78 


c 


90 


c 


90 


c 


86 


0.87 




0.93 


c 


59 


c 


50 


c 


94 


c 


94 


c 


89 


0.90 




0.97 


c 


92 


c 


52 


c 


98 


c 


98 


c 


92 


0.93 


^^H 


1.01 


c 


54 


c 


76 


c 


54 


c 


84 


c 


83 


0.85 


1 00 


1 05 


c 


36 


c 




1. 


■s^ 


I. 




I. 


Mh 




1 05 







69 





80 


r 


90 


r 


91 





88 


0.90 


^^H 


0.97 





85 





75 


c 


93 


c 


78 





87 


0.86 


0.97 


1 01 


c 


57 


c 


77 


c 


96 


c 


81 





89 


0.88 


1 00 




























1 07 


c 


79 


c 


73 


c 


36 


c 


79 


87 


0.80 


1 07 


^ ' 


51 


r 


75 


r 


39 


c 


82 


89 


0.82 ^m 


^K 


63 





77 


r 


92 


c 


84 


91 


^^WM 


^n 


66 





85 


r 


95 


c 


87 


t 


94 


-jj^^m 


^^2 




c 




r 




c. 




f 


97 




^m~^ 


9? 





90 


1 


03 


r 


94 


1 


00 


95 ^^H 


0.84 





67 





62 





71 


c 


60 





56 


0.55 


0.88 


0.85 


c 


71 


c 


55 


c 


73 


c 


63 





60 


0.60 


0.90 
































55 


c 




c 


95 


c 


77 


c 


85 


0.88 


■■ 


1 01 


c 


57 


c 


56 


c 


97 


c 


79 


c 


87 


0.90 


0.95 


c 


59 


c 


79 


c 


92 


c 


77 


c 


82 


0.85 


0.98 


r 


91 


r 


51 


r 


94 


c 


79 


c 


85 


0.87 


■■ 


1 00 





93 





82 





96 





81 





87 


0.88 


0.81 





61 





79 





91 





77 





82 


0.84 


0.88 





67 





a? 





96 





82 





88 


0.89 


0.95 


0.86 





85 





82 





96 





81 





86 


0.87 


0.92 


1 01 





93 





85 





98 





83 





88 


0.89 


^^H 


1 03 





93 





85 





98 





83 





89 


0.89 


^^1 


1.06 





97 





88 


1 


02 





87 





92 


0.92 


^^1 


1.00 





91 





83 





97 





82 





94 


0.88 


107 


1.03 





93 





84 





99 





84 





96 


0.89 


^^H 


1.05 





95 





85 


1 


01 





85 





99 


0.91 


^^1 


0.99 





90 





83 





97 





83 





93 


0.87 


1 06 


1 01 





92 





84 





99 





85 





95 


0.88 


^B 


1.03 





93 





86 


1 


01 





87 





97 


0.90 


1.05 





88 





84 


1 


01 





85 





93 


0.84 


1.08 





90 





85 


1 


02 


n 


86 


1) 


94 


0.85 


1 0(5 







91 





86 


1 


04 





88 


n 


96 


nsT ^^H 


1.04 





93 





84 


1 


01 





86 





91 


0.88 


^^H 


1.06 





94 


a 


85 


1 


02 





87 





93 


0.89 


^^1 







96 





87 


1 


04 





89 





95 


0.91 ^^H 


1.03 





S9 





85 





99 





86 





97 


0.83 


^^H 


1.05 





90 





86 







87 





98 


0.85 


^^1 


1.07 





91 





87 







89 


1 


00 


0.86 


^^1 


^^H 


S3 





83 





97 





86 





89 


0.53 ,0. 


^^H 


65 





85 





98 





87 





92 


(185 ^^H 


IHB ° 


87 





86 


1 


00 





89 





93 


087 ^^H 


0.85 





78 





77 





76 





83 





84 


0.79 


0.88 


0.87 





60 





79 





78 





85 





86 


0.81 


0.90 


0.75 





61 





60 





69 





63 





66 


0.62 


0.77 


0.77 





63 





63 





7? 





66 





68 


0.64 


0.79 


1 00 





85 





83 





94 





86 





90 


0.86 


1.00 


1 00 





85 





83 





94 





85 





90 


0.86 


1.00 


0.98 





83 





84 





94 





84 





89 


0.84 


1 01 


1.00 





84 





85 





96 





85 





90 


0.85 


1 02 


1.01 


c 


55 


c 


56 


c 


97 


c 


86 


c 


92 


0.86 


1.04 


1.02 


c 


34 


c 


55 


r 


95 


r 


34 


c 


90 


0.85 


1.01 


1.03 




35 


c 


56 


c 


96 


c 


85 


t 


91 




1 03 


1.05 


c 


56 


c 


57 


c 


97 


c 


87 


c 


92 


0.87 


1 04 


1.02 


c 


54 


c 


55 


c 


94 


c 


84 


c 


90 


0.85 


1 02 


1.04 


c 


55 


c 


57 


c 


96 


c 


85 


c 


91 


0.86 


1 03 


1.05 


t 


36 


t 


3b 


L 


9/ 


L 


bb 





92 


0.87 


1 04 


1.04 


c 


55 


c 


56 


c 


94 


c 


83 





91 


0.86 


1.02 


1.05 


c 


56 


c 


57 


c 


95 


c 


84 





92 


0.87 


1.03 


1.06 


c 


57 


c 


59 


c 


96 


c 


85 





93 


0.88 


1.04 




c 


36 


c 


57 


r 


97 


r 


34 


c 




0.86 


1.02 




c 


37 


c 


58 


c 


98 


c 


85 


t 


92 




1.03 


1.07 


c 


58 


c 


59 


c 


99 


c 


86 


c 


93 


0.88 


1.05 


1.05 


c 


56 


c 


58 


c 


97 


c 


85 


c 


91 


0.85 


1.03 


1.06 


r 


57 


r 


59 


r 


98 


c 


36 


r 


92 


0.86 


1 04 


1.07 


c 


58 


c 


90 


c 


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(a) 



(b) 








.00 





.50 





.75 


1 


.00 


l^^^^^^H 



Figure E-6. OCRs for WTC 1 core columns under original design loads, (a) 600 line, and 

(b) 900 line. 



liv 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Exterior Columns Behavior 

Analysis of the axial stress distribution in the columns under lateral wind loads indicated that the behavior 
of the lower portion of the towers at the basement floors was that of a braced frame, while the behavior of 
the super-structure was that of a framed tube system. Under a combination of the original WTC design 
dead and wind loads, tension forces were developed in the exterior walls of both towers. The forces were 
largest at the base of the building and at the corners. These tensile column loads were transferred from 
one panel to another through the column splices. The DCRs for the exterior wall splice connections under 
these tensile forces for both towers were shown to be less than 1.0. 

Resistance of the Towers to Shear Sliding and Overturning Moment 

The resistance of the towers to shear sliding and overturning due to wind was provided by the dead loads 
that acted on the exterior walls of the towers. Considering the resistance to shear sliding under wind load, 
the factor of safety was calculated to be between 10 and 1 1.5, while the factor of safety against 
overturning ranged from 1.9 to 2.7 for both towers. 

E.4.2 Baseline Performance of the Typical Floor Models 

The reference floor models were analyzed under gravity loads to establish their baseline performance. 
The following presents a summary of the results: 

• For the typical truss-framed floor (floor 96 of WTC 1), the DCRs for all floor trusses were less 
than 1.14 for the original WTC design loads and less than 0.86 for the ASCE 7-02 loading. 
Under the original WTC design loads, the DCR was less than 1.00 for 99.4 percent of the floor 
truss components. Inside the core, the DCRs for all floor beams were less than 1.08, and more 
than 99 percent of the floor beams had a DCR of less than 1 .0. The maximum mid-span 
deflections of the long span and short span zones under the original design loads were 
approximately 1.79 in. (~ L/400) and 0.57 in. (~ L/750), respectively, where L is the floor span. 

• For the typical beam-framed floor under the original WTC design loads, the DCRs for all floor 
beams were less than 1.0, except for two core beams where the DCRs in shear were 1.125 and 
1.09. The maximum mid-span deflections of the long span and short span zones under the 
original design loads were approximately 1.55 in. (~ L/450) and 0.70 in. (~ L/600), respectively. 

E.5 DEVELOPMENT OF TOWER AND AIRCRAFT IMPACT MODELS 

The WTC tower models for the impact analysis required considerably greater sophistication and detail 
than was required for the reference models. The reference models provided the basis for the more 
detailed models required for the impact simulations. The impact models of the towers, which utilized the 
structural databases described earlier, included the following refinements: 

• The material properties used in the impact models accounted for the highly nonlinear behavior of 
the tower and aircraft materials, including softening and failure of components, and strain rate 
sensitivity. 



NIST NCSTAR 1-2, WTC Investigation Iv 



Executive Summary 



• The impact simulations required a much higher level of detail than that in the reference global 
models. For instance, the impact analyses necessitated that the floors inside and outside the core 
in the impact region, as well as connections, be modeled in detail. In addition, structural 
components in the exterior walls and core of the towers were modeled using shell elements 
(instead of beam elements in the reference models) to properly capture the impact-induced 
damage to these components. 

• The size of the impact models required a very large mesh (more than ten million degrees of 
freedom). The SAP2000 program cannot accommodate this model size. 

• Contact and erosion algorithms were required for the impact analyses. That necessitated the use 
of appropriate software, specifically LS-DYNA, for the development of the impact models. 

As a result, three separate models were developed for the impact analyses. The first two were detailed 
models of the impact regions of the WTC 1 and WTC 2 towers. The third was a comprehensive model of 
the Boeing 767 aircraft. All models were developed using the LS-DYNA finite element code, which is a 
commercially available nonlinear explicit finite element code for the dynamic analysis of structures. The 
code has been used for a wide variety of crash, blast, and impact apphcations. 

One of the significant challenges in developing the tower and aircraft models for the global impact 
analyses was to minimize the model size while keeping sufficient fidelity in the impact zone to capture 
the deformations and damage distributions. The limitation was that for each analysis the combined 
aircraft and tower models should not exceed approximately 2.3 million nodes. These were distributed 
between the global WTC tower model and the aircraft so that the tower model would be about 1.5 million 
nodes and the aircraft about 0.8 million nodes. 

E.5.1 Development of Tower Impact Models 

The approach used to meet this model size limitation was to develop models for the various tower 
components at different levels of refinement. Components in the path of the impact and debris field were 
meshed with a higher resolution to capture the local impact damage and failure, while components outside 
the impact zone were meshed more coarsely to primarily capture their structural stiffness and inertial 
properties. A summary of the size of the global impact models of both towers is presented in Table E-6. 
As the table indicates, the towers were modeled primarily with shell elements, with the exception of the 
exterior wall bolted connections (beam and brick elements) and the floor truss diagonals (beam elements). 
The WTC 1 model extended between floors 92 and 100, while the WTC 2 model extended between floors 
77 and 85. 



Ivi NIST NCSTAR 1-2, WTC Investigation 









Executive Summary 




Table E-6. Summary of the global impact models \ 


or the WTC towers. 








WTC 1 Tower Model 


WTC 2 Tower Model 






Number of Nodes 


1,300,537 


1,312,092 






Hughes-Liu Beam Elements 


47,952 


53,488 






Belytschko-Tsay Shell Elements 


1,156,947 


1,155,815 






Constant Stress Solid Elements 


2,805 


2,498 





The global impact models of the WTC towers included the following components: 

• Exterior walls: The exterior columns and spandrels were modeled using shell elements with two 
mesh densities, a refined density in the immediate impact zone (typical element sizes were 4 in.) 
and a coarser far field density elsewhere (typical element sizes were 14 in.). For the bolted 
connections between exterior panels in the refined mesh areas, brick elements were used to model 
the butt plates, and beam elements were used for the bolts. The model of the impact face of 
WTC 1 is shown in Figure E-7. 

• Core columns and floors: Core columns were modeled using shell elements with two mesh 
densities, a refined density in the direct impact area and a coarser far field density elsewhere. 
Typical element sizes were 2 in. and 8 in. for the impact zone and far field, respectively. The 
spliced column connections were included in the model with proper failure criteria. The floors 
within the core were modeled using shell elements representing the floor slabs and beams. A 
generated model for the core of WTC 1 between floors 94 and 98 is shown in Figure E-8. 

• Truss floors: In the direct impact area, the floor model included shell elements for the combined 
floor slab and metal decking and for the upper and lower chords of the trusses. Beam elements 
were used for the truss diagonals. In the far field floor segments, simplified shell element 
representations were used for the floor slab and trusses, with typical element sizes of 30 in. A 
model assembled for the entire 96th floor of WTC 1 is shown in Figure E-9. 

• Interior building contents: The interior nonstructural contents of the towers were modeled 
explicitly. These included the partitions and workstations, which were modeled with shell 
elements in the path of the aircraft debris. The live load mass was distributed between the 
partitions and cubicle workstations. The resulting model of a floor with interior contents is 
shown in Figure E-10. 

Figure E-11 shows the assembled global impact model of WTC 1. 



NISTNCSTAR 1-2, WTC Investigation 



Ivii 



Executive Summary 



BC: fixed in Z 
translation only at 
free column ends 



r 



Truss floor and 
core structure 
floors 92-100 



Higher mesh_ 
density in 



< 



impact zone 



V 



II 1 1 1 II M II I M 1 1 1 1 Ml 1 1 1 1 M Mllllillil llililiiill 



■ lllll lill 1 1 1 1 1 111:1 1 1 1 1 1 Nil 1 1 1 1 1 1 11 IMIilill illli 1 1 i lliiil lili I 






Panel 
Numbers: 



133 127 



118 



Figure E-7. Impact face of the WTC 1 model, floors 91-101. 




Jl^\ 



Figure E-8. Model of the WTC 1 core, floors 94-98. 



Floor: 




93 

92 




Iviii 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Side 200 




Side 300 



Figure E-9. Model of the 96th floor of WTC 1. 

Workstations Modeled 
over Truss Floor Area 



Side 100 



Side 400 



Side 200 




Side 300 



Figure E-10. Model of the 96th floor of WTC 1 including interior contents. 



NISTNCSTAR 1-2, WTC Investigation 



lix 



Executive Summary 



Side 300 



Side 200 




Side 400 



Side 100 

Figure E-11. Multi-floor global impact model of the WTC 1 tower. 



Tower Material Constitutive Models 

The materials that were considered for the tower modeling included: (1) the several grades of steel used in 
the columns, spandrels, and floor trusses and beams of the WTC towers, (2) the concrete floor slabs, and 
(3) the nonstructural contents of the towers. These materials exhibit significant nonlinear rate-dependent 
deformation and failure behavior that need to be represented in the constitutive relationship. The 
following is a brief summary of the constitutive models used for these materials. 

WTC Tower Steel Constitutive Models — The primary constitutive model that was used for the tower 
steels was the Piecewise Linear Plasticity model in LS-DYNA. This model is sufficient to model the 
nonlinear rate-dependent deformation and failure of the steel structures. A tabular effective stress versus 
effective strain curve was used in this model with various definitions of strain rate dependency. The 
constitutive model parameters for each grade of steel were based on engineering stress-strain data 
developed by the mechanical and metallurgical analysis of structural steels part of the NIST Investigation. 
Finite element analyses of the test specimens were conducted with a fine and a medium mesh (similar to 
that used in the component level analysis) to capture the nonlinear material behavior up to failure 
(Figure E-12). The finite element analysis provided a validation that the constitutive model parameters 
were defined accurately and that the model could reproduce the measured response for the test conditions. 



Ix 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Grip 



Test Sample 




Fine Mesh 




Medium Mesli 




Figure E-12. Finite element models of the ASTM 370 rectangular tensile specimen. 

The first step in the constitutive model development process was to obtain a true stress-true strain curve. 
The typical approach was to select a representative test for each grade of steel and convert the engineering 
stress-strain curve to true stress-strain. The true stress-strain curve was extrapolated beyond the point of 
necking onset. This curve was the input used to specify the mechanical behavior in the simulation of the 
tensile test (Figure E-12). If necessary, the extrapolation of the true stress-strain behavior was adjusted 
until the simulation matched the measured engineering stress-strain response, including necking and 
failure. A summary of the true stress-strain curves used in the constitutive models for the various 
WTC tower steels are summarized in Figure E-13. 



NISTNCSTAR 1-2, WTC Investigation 



Ixi 



Executive Summary 



140 




40.^, 



sss^ 



-S-- 


36 ksi Model 


■■*■ 


42 ksi Model 


--»■ 


50 ksi Model 


• ■•■ 


55 ksi Model 


• *- 


60 ksi Model 



-■»- 


66 ks 


Model 


— s— 


70 ks 


Model 


-e-75ks 


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-ft- 


30 ks 


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-V- 


100ksiModel| 



20 
0.00 



I I I 



I I I 



I I I I I I 



I I I 



0.05 



0.25 



0.30 



0.10 0.15 0.20 

Plastic Strain 
Figure E-13. Tabular true stress-strain constitutive model curves for the tower steels. 



Strain-rate effects on the steel yield strength were included in the constitutive model for tower steels with 
the Cowper and Symonds rate effect model. The resulting rate effects used in the constitutive modeling 
of tower steels based on this model were compared to the measured high rate test data for the 50 ksi, 
75 ksi, and 100 ksi tower steels in Figure E-14. The comparison showed that the Cowper and Symonds 
model was capable of reproducing the rate effects for the range of data available. 

140 



120 - 



100 - 



<n 80 
w 



CO 



60 
40 [- 
20 - 







I I I I I I I I I I I I I I I I I M I I I M I I I I I I I I I I I I M I I I M I I I I I I I I I I I I 1 1 n I I [ I I I I n M I 



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• 1 00 ksi IVIaterial Tests 

♦ 75 ksi Material Tests 



I ' I I I I I I I I I I I n I I I I I I I ri I I I I I I t I I I I I I I I I I I r I r I I I I r I I I I r I r I I I I r I I I I r I ri I 



-3-2-10123 

-1. 



Log Strain Rate (s ) 
Figure E-14. Comparison of rate effects model and test data. 



Ixii 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Concrete Constitutive Models — The LS-DYNA material Type 16 (pseudo-tensor concrete model) was 
selected for modeling the concrete floor slabs due to its ability to accurately model the damage and 
softening of concrete, associated with low confinement. The model uses two pressure-dependent yield 
functions and a damage-dependent function to migrate between curves. This allows for implementation 
of tensile failure and damage scaling, which are more dominant material behaviors at low confinement. 
The pseudo-tensor model also accounts for the sensitivity of concrete to high strain rates. Material 
constitutive parameters for the pseudo-tensor model were developed. A simulation was performed of a 
standard unconfined concrete compression test to check the constitutive model behavior. The calculated 
compressive stress-strain response for the 3 ksi concrete was compared to measured compression data for 
2.3 ksi and 3.8 ksi strength concretes in Figure E-15. 



U7 
CO 
O 

CO 



Data [Wischers, 1978] fc = 3.8 ksi 

Data [Wischers, 1978] fc = 2.3 Itsi 

' Pseud o Tensor Model fc = 3.0 ksi 




Strain (%) 

Figure E-15. Comparison of the calculated unconfined compression behavior with 

concrete compression test data. 

Nonstructural Materials Constitutive Models — The primary infiuence of the nonstructural components 
on the impact behavior was their inertial contribution. The effects of their strength were small. As a 
result, relatively simple approximations of their constitutive behavior were used. Typically, a bilinear 
elastic-plastic constitutive model was applied for these materials to allow for efficient modehng of 
deformation and subsequent erosion from the calculations as their distortions became large. The abihty to 
include material failure and erosion of these soft materials was important for the stability of the impact 
analyses. 



E.5.2 



Development of Aircraft Impact Model 



The finite element model for the Boeing 767-200ER aircraft was constructed through a three-step process: 
(1) data collection, (2) data interpretation and engineering analysis, and (3) meshing of the structure. The 



NISTNCSTAR 1-2, WTC Investigation 



Ixiii 



Executive Summary 



focus of this effort was on gathering sufficient structural data and including adequate detail in the aircraft 
model so that the mass and strength distribution of the aircraft and contents were properly captured for 
implementation in the impact analyses. Structural data were collected for the Boeing 767-200ER aircraft 
from (1) documentary aircraft structural information, and (2) data from measurements on Boeing 767 
aircraft. 



A summary of the aircraft model size and parameters is presented in Table E-7. The complete model of 
the Boeing 767-200ER is shown in Figure E-16. The airframe model contained most of the significant 
structural components in the aircraft. The models of the fuselage, empennage, and wing structures were 
developed completely using shell elements. Models for the landing gear and engines were developed 
primarily using shell elements, but contained some brick elements as well. The typical element 
dimensions were between one and two in. for small components, such as spar or rib flanges, and three to 
four in. for large parts, such as the wing or fuselage skin. 

Table E-7. Boeing 767-200ER aircraft model parameters. 





American Airlines 11 


United Airlines 175 


No. Brick Elements 


70,000 


70,000 


No. Shell Elements 


562,000 


562,000 


No. SPH Fuel Particles 


60,672 


60,672 


Total Nodes 


740,000 


740,000 


Total Weight (Empty) 


183,5001b 


183,5001b 


ULD/Cargo Weight 


12,420 lb 


21,660 1b 


Cabin Contents Weight 


21,5801b 


10,420 1b 


Fuel Weight 


66,100 1b 


62,000 lb 


Total Weight (Loaded) 


283,600 lb 


277,580 lb 



Special emphasis was placed on modeling the aircraft engines due to their potential to produce significant 
damage to the tower components. The engine model was developed primarily with shell elements. The 
objective was to develop a mesh with typical element dimensions between one and 2 in. However, 
smaller element dimensions were required at many locations to capture details of the engine geometry. 
Brick elements were used for some of the thicker hubs and the roots of the compressor blades. The 
various components of the resulting engine model are shown in Figure E-17. Fuel was distributed in the 
wing, as shown in Figure E-18, based on a detailed analysis of the likely fuel distribution at the time of 
impact. 



Ixiv 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Time = 




^; 



Time = 




\c: 



Figure E-16. Finite element model of the Boeing 767-200ER. 



NISTNCSTAR 1-2, WTC Investigation 



Ixv 



Executive Summary 





Figure E-17. Pratt & Whitney PW4000 turbofan engine model. 




Figure E-18. Boeing 767-200ER with fuel load at time of impact. 

Aircraft Materials Constitutive Models 

The constitutive and failure properties for the aircraft materials were developed from data available in the 
open literature. Complete engineering stress-strain curves were obtained for various 2024 and 7075 
aluminum alloys that are commonly used in the construction of the Boeing 767 airframe structures. 
These curves were digitized for the various aluminum alloys. Representative stress-strain curves were 
then converted into true stress and true strain and used to develop tabular curves for constitutive models. 
The tabular constitutive model fits are shown in Figure E-19. No rate sensitivity of the aircraft materials 
was considered. 



Ixvi 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



120 




20 



OS 



Constitutive Model Fit: 
-*- 7075-T7351 Extrusion 
-♦- 2024-T3 Clad Sheet 
-■- 2024-T351X Extrusion 
-•- 7075-T651X Extrusion 



_i I I L. 



J_ 



A. 1 J L_ 



± 



_l I I L 



0.00 



0.05 



0.15 



0.20 



0.10 
Plastic Strain 
Figure E-19. True stress-strain curves developed for various aircraft aluminum alloys. 



E.5.3 



Component and Subassembly level analyses 



A large array of component and subassembly models were developed and used in the impact simulations. 
The primary objectives of the component modeling were to (1) develop understanding of the interactive 
failure phenomenon of the aircraft and tower components, and (2) develop the simulation techniques 
required for the global analysis of the aircraft impacts into the WTC towers. The approach taken for 
component modeling was to start with finely meshed, brick and shell element models of key components 
of the tower structure and progress to relatively coarsely meshed beam and shell element representations 
that were used for the global models. Other key technical areas were addressed in the component 
modeling, including material constitutive modeling, treatment of connections, and modeling of aircraft 
fuel. 

Examples of the component impact analyses conducted include: 

• Impact of a segment of an aircraft wing with an exterior column. 

• Detailed and simplified modeling of exterior panel bolted connection under impact loading. 

• Impact of a simplified plow type impactor with truss floor assembly. 

• Impact of fiael-filled wing segment with exterior wall panels (Figure E-20). 



NISTNCSTAR 1-2, WTC Investigation 



Ixvii 



Executive Summary 




a > 




t = 0.0 s 



t = 0.04 s 



Figure E-20. Calculated impact of a coarse mesh wing section laden with fuel modeled 

using SPH particles. 

The following results were obtained from the component impact analyses: 

• A 500 mph engine impact against an exterior wall panel resulted in a penetration of the 
exterior wall and failure of impacted exterior columns. If the engine did not impact a floor 
slab, the majority of the engine core would remain intact through the exterior wall 
penetration, with a reduction in speed between 10 percent and 20 percent. The residual 
velocity and mass of the engine after penetration of the exterior wall was sufficient to fail a 
core column in a direct impact condition. Interaction with additional interior building 
contents prior to impact or a misaligned impact against the core column could change this 
result. 

• A normal impact of the exterior wall by an empty wing segment from approximately mid- 
span of the wing produced significant damage to the exterior columns but not complete 
failure. Impact of the same wing section, but filled with fuel, resulted in extensive damage to 
the external panels of the tower, including complete failure of the exterior columns. The 
resulting debris propagating into the building maintained the majority of its initial momentum 
prior to impact. 

• Three different numerical techniques were investigated for modeling impact effects and 
dispersion of fuel: (1) standard Lagrangian finite element analysis with erosion, (2) Smoothed 
Particle Hydrodynamics (SPH) analysis, and (3) Arbitrary-Lagrangian-Eulerian analysis. Of 
these approaches, use of the SPH offered the best viable option due to its computational 
efficiency. 

The subassembly analyses were used as a transition between the component level analyses and the global 
impact analyses. With the subassembly analyses, more complex structural behavior not captured in the 
component analyses could be investigated with significantly shorter run times than required for the global 
analyses. The subassembly analyses were primarily used to investigate different modeling techniques and 
associated model size, run times, numerical stability, and impact response. The subassembly model used 
structural components from the impact zone on the north face of WTC 1 . The structural components in 



Ixviii 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



the subassembly model included the exterior panels, core framing, truss floor structures, and interior 
contents (workstations). Figure E-2 1 . 



Ext. Panel Numbers 
118 



Core Column Numbers 



Engine Impact Locatiorn — 
Center of panel 121 at 96"' floor 




Figure E-21. Final WTC tower subassembly model. 

The subassembly model was impacted by an aircraft engine and by a segment of a fuel- filled wing. The 
response of the structure to the engine impact is shown in Figure E-22. The following results were 
obtained from the subassembly impact analyses: 

• The deceleration profile of the impacting engine indicated that the response of the 
nonstructural building contents was dominated by the mass of the workstations, rather than 
by their strength. 

• Varying the strength of the floor concrete slab from 3 ksi to 4 ksi did not result in significant 
change in the impact response. It appears that the mass of the concrete slab had a greater 
effect on the engine deceleration and damage to the floor than did the concrete strength. 

• Varying the ductility of the weld zone in the exterior columns from 8 percent to 1 percent did 
not result in any noticeable difference in the damage pattern or the energy absorbed by the 
exterior panels, indicating that the weld ductility had a negligible effect on the impact 
response. 



NISTNCSTAR 1-2, WTC Investigation 



Ixix 



Executive Summary 




I ■■' -I '■' ii iz '■' 1 ■■■' ii 11 ''■' J- '■■^ ii il ^' -1 •"' A--' 



H 




(a) Time = 0.00 s 




' 



]\/i\/ \/'\/ \/l\/ \/'\/\/'i\/ \7\/ \/1aL-iU 



E.6 



(b) Time = 0.25 s 
Figure E-22. Engine impact and breakup behavior (side view). 

AIRCRAFT IMPACT INITIAL CONDITIONS 






1 • 
i I 



Three methods were used to determine the initial conditions for the two aircraft that impacted the towers. 
The first method used a comparison of videos from different positions to calculate the three-dimensional 
trajectory of the aircraft. The second method used the relative frame-by-frame motion in a single video 
scaled to the length of the aircraft in the video to calculate the impact speed. Finally, analysis of the 
impact damage on the face of each tower was used to refine the relative impact orientation and trajectory. 

The aircraft impact conditions matching the observed exterior wall damage are shown in Figure E-23 and 
Figure E-24 for WTC 1 and WTC 2, respectively. The aircraft and exterior wall models were used to 
visualize the impact scenario in the figures and the view shown was aligned with the aircraft trajectory. 
Matching the projected impact points of the wings, fuselage, engines, and vertical stabilizer onto the 
exterior wall of each tower to the observed damage pattern was an important constraint in the 
determination of impact conditions. The final set of impact conditions from the analyses are summarized 
in Table E-8. 



Ixx 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



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Figure E-23. WTCl impact conditions and the impact pattern. 



NISTNCSTAR 1-2, WTC Investigation 



Ixxi 



Executive Summary 



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Figure E-24. WTC 2 impact conditions and the impact pattern. 



Ixxii 



NIST NCSTAR 1-2, WTC Investigation 









Executive Summary 




Table E-8. Summary of refined aircraft im 


pact conditions. 








AA 11 (WTC 1) 


UAL 175 (WTC 2) 






Impact Speed (mph) 


443 ± 30 


542 + 24 






Vertical Approach Angle 
(Velocity vector) 


10.6° ± 3° below horizontal 
(heading downward) 


6° ± 2° below horizontal 
(heading downward) 






Lateral Approach Angle 
(Velocity vector) 


180.3° ± 4° clockwise from 
Structure North 


15° ± 2° clockwise from 
Structure North 






Vertical Fuselage Orientation 
Relative to Trajectory 


2° nose-up from the vertical 
approach angle 


1 ° nose-up from the vertical 
approach angle 






Lateral Fuselage Orientation 
Relative to Trajectory 


0° clockwise from lateral 
approach angle 


-3° clockwise from lateral 
approach angle 






Roll Angle (left wing downward) 


25° ± 2° 


38° ± 2° 





E.7 



GLOBAL IMPACT ANALYSES 



The objective of these analyses was to estimate the condition of the two WTC towers immediately 
following the aircraft impacts. This assessment included the estimation of the structural damage that 
degraded their strength and the condition and position of nonstructural contents such as partitions, 
workstations, aircraft fuel, and other debris that influenced the behavior of the subsequent fires in the 
towers. The global impact analyses were the primary method by which the damage to the towers was 
estimated. The global impact simulations provided, for each tower, a range of damage estimates. These 
included a base case based on reasonable initial estimates of all input parameters, along with a less severe 
and a more severe damage scenario. The less severe damage case did not meet two key observables: 
(1) no aircraft debris was calculated to exit the side opposite to impact and most of the debris was stopped 
prior to reaching that side, in contradiction to what was observed in photographs and videos of the impact 
event and (2) The subsequent structural response analyses of the damaged towers indicated that the 
towers would not have collapsed had the less severe damage results been used. As a resuh, this report 
provides detailed description of the results of the analyses pertaining to the base and the more severe 
cases, which were used as the initial conditions for the subsequent fire dynamics simulations, thermal 
analyses, and fire-structural response and collapse initiation analyses. Only a brief description is provided 
for the less severe damage results for comparison purposes. 



E.7.1 



WTC 1 Base Case Impact Analysis 



The combined aircraft and tower model for the base case WTC 1 global impact analysis is shown in 
Figure E-25. The base case impact analysis was performed for a 0.715 s duration following initial impact 
of the aircraft nose with the north exterior wall. The analysis was performed on a computer cluster using 
twelve 2.8 GHz Intel Xeon processors, each on a separate node of the cluster. The run time for this 
analysis was approximately two weeks. The calculations were terminated when the damage to the towers 
reached a steady state and the motion of the debris was reduced to a level that was not expected to 
produce any significant increase in the impact damage. The residual kinetic energy of the airframe 
components at the termination of a global impact simulation was typically less than one percent of the 
initial kinetic energy at impact. 



NISTNCSTAR 1-2, WTC Investigation 



Ixxiii 



Executive Summary 




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Figure E-25. WTC 1 global impact model. 

A side view of the base case WTC 1 global impact response is shown in Figure E-26. A corresponding 
top view of the impact response is shown in Figure E-27. The aircraft impact response was dominated by 
the impact, penetration, and fragmentation of the airframe structures. The entire aircraft fully penetrated 
the tower at approximately 0.25 s. The fiiselage structures were severely damaged both from the 
penetration through the exterior columns and the penetration of the 96th floor slab that sliced the fiaselage 



Ixxiv 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



structures in half. The downward trajectory of the aircraft structures caused the airframe to collapse 
against the floor, and the subsequent debris motion was redirected inward along a more horizontal 
trajectory parallel to the floor. The downward trajectory of the aircraft structures transferred sufficient 
vertical load such that the truss floor structures on the 95th and 96th floors collapsed in the impact zone. 




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Figure E-26. WTC 1 base case global impact analysis - side view. 



NISTNCSTAR 1-2, WTC Investigation 



Ixxv 



Executive Summary 




(a) Time=0.00 s 




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Figure E-27. WTC 1 base case global impact analysis - top view. 

The wing structures were completely fragmented by the exterior wall. The aircraft fuel cloud began to 
spread out after impact but remained relatively dense until the leading edge of the fuel reached the tower 
core. The aircraft fuel and debris cloud eventually penetrated most of the distance through the core before 
their motion was halted. 

The aircraft was severely broken into debris as a result of the impact with the tower. At the end of the 
impact analysis, the aircraft was broken into thousands of debris fragments of various sizes and masses. 
Larger fragments still existed for specific components, such as the engines. At the end of the simulation, 
the port engine was still inside the core, and the starboard engine was roughly one third of the distance 
from the core to the south exterior wall. Each had a speed of less than 50 mph. 

Exterior Wall Damage 

The exterior wall was the one structural system for which direct visual evidence of the impact damage 
was available. Therefore, the comparison of the calculated and observed exterior wall damage provided a 



Ixxvi 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



partial validation of the analysis methodologies used in the global impact analyses. A comparison of the 
north exterior wall observed and calculated damage from the base case WTC 1 global impact analysis is 
shown in Figure E-28. The comparison of the calculated and observed damage indicated that the 
geometry and location of the impact damage zone were in good agreement. This agreement in the 
position and shape of the impact damage served to validate the geometry of the aircraft model, including 
the aircraft orientation, trajectory, and flight distortions of the wings. 

The comparison also indicated a good agreement in the magnitude and mode of impact damage on the 
exterior wall. The exterior wall completely failed in the regions of the fuselage, engine, and fuel-filled 
wing section impacts. Damage to the exterior wall was observed all the way out to the wing tips, but the 
exterior columns were not completely failed in the outer wing and vertical stabilizer impact regions. 
Failure of the exterior columns occurred both at the bolted connections between column ends and at 
various locations in the column depending on the local severity of the impact load and the proximity of 
the bolted connection to the impact. The agreement of both the mode and magnitude of the impact 
damage served to partially validate the constitutive and damage modeling of the aircraft and exterior wall 
of the tower. 

Core Structural Damage 

The estimation of the damage to the core columns and core beams was important in determining the 
residual strength for the subsequent analyses of structural stability and collapse. The core had significant 
damage in the region close to the impact point. The columns in line with the aircraft fuselage failed on 
the impact side, and several of the core beams were also severely damaged or failed in the impact zone. 

The calculated damage to the core columns by row is shown in Figure E-29. A total of three columns 
were severed, and four columns were heavily damaged. The damage to the core floor framing for 
floors 95 and 96 is shown in Figure E-30. 



NIST NCSTAR 1-2, WTC Investigation Ixxvii 



Executive Summary 



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Figure E-28. Base case impact damage to the WTC 1 exterior wall. 



Ixxviii 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



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Figure E-29. Base case impact damage to the WTC 1 core columns. 




(a) Floor 95 core framing damage (b) Floor 96 core framing damage 

Figure E-30. Base case impact damage to the core beams of floors 95 and 96 of WTC 1. 



NISTNCSTAR 1-2, WTC Investigation 



Ixxix 



Executive Summary 



Floor Truss and Slab Damage 

An overall frontal view of the floor trusses in the impact zone, along with the calculated impact damage 
to the floor trusses, is shown in Figure E-31. The figure shows that the trusses experienced significant 
damage and sagging in the impact zone. A plan view of the calculated damage to the trusses on floors 95 
and 96 is shown in Figure E-32. The calculated impact response produced severe damage to the truss 
structures in the primary impact path of the fuselage from the exterior wall to the core. The truss floor 
system on floors 94 through 96 were damaged and sagged downward as a result of the impact loading. 
The calculated damage to the WTC 1 floor slab for floors 95 and 96 are shown in Figure E-33, where a 
similar pattern of response to that observed in the trusses can be seen for the floor slabs. 



Column 

135 



Column 
109 



Column 
151 



Column 
141 



Floor 96- 



Fioor 95 



Column 

107 



Column 
157 



Column 
115 



(a) Initial detailed truss structures 



Column 
151 




Floor 95 



(b) Calculated damage 
Figure E-31. Base case impact damage to the WTC 1 floor trusses (front view). 



Ixxx 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



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impact 



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113 




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107 




(a) Floor 95 slab damage 



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Figure E-33. Base Case impact damage to the slabs on floors 95 and 96 of WTC 1 

(plan view). 

Summary of Structural Damage 

The impact-induced structural damage described above was used as the initial conditions for the post- 
impact fire- structural analyses. Figure E-34 presents the cumulative damage on all affected floors and 
columns. The damage to the columns at the various levels is identified by the color of the circles, where 
red, blue, green, and yellow signify severed, heavily damaged, moderately damaged, and lightly damaged 
columns, respectively. The dotted boxes on the figures indicate areas where the impact created an 
opening in the floor. These were used to identify slab openings in the fire dynamics simulations. The 
solid boxes indicate areas in the floor system that had severe structural damage. These areas were 
removed from the subsequent structural analyses. Figure E-34 shows the damage to the exterior walls 
due to impact, based on the photographs of the north wall. Note the panel that was severed in the south 



NISTNCSTAR 1-2, WTC Investigation 



Ixxxi 



Executive Summary 



wall of the tower. While the analysis did not capture the failure of the connections at the ends of this 
panel due to the coarse mesh of the south wall, photographic evidence showed that this panel was 
knocked down by the impact. As a result, this panel was removed from the subsequent structural 
analyses. 



Severe Floor Damage 

Floor system 

structural damage 

Floor system 
removed 



Column Damage 
Severed Q 

Heavy Damage ^) 
Moderate Damage Q 
Light Damage 



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Fuel and Debris Distribution 

Another primary objective of the global impact analyses was to determine the initial conditions that 
influenced the initiation and propagation of the fires in the towers. These initial conditions included the 
distribution of the jet fuel in the towers, the distribution of tower contents and aircraft debris that provided 
flammable materials for the fires, and the condition of the partitions and walls that provided barriers to air 
flow and spreading of the fires. For the base case WTC 1 global impact analysis, the calculated 
distribution of the fuel in the tower and shape of the fuel cloud in a plan view and side view were shown 
previously in Figure E-26 and Figure E-27, respectively. Figure E-35 shows the distribution of the fuel 
and damage to the building contents due to impact. 

The bulk of the fuel and aircraft debris was deposited on floors 93 through 97, with the greatest 
concentration on floor 94. The calculated debris cloud included 17,400 lbs of debris and 6,700 lbs of 
aircraft fuel outside of the tower at the end of the impact analysis, either rebounding from the impact face 



Ixxxii 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



(north wall) or passing through the tower (south wall). This amount might have been larger in the 
calculation since the exterior walls were not modeled with windows that would contain the fuel cloud and 
other small debris inside the towers. In addition, the impact behavior of the aircraft fuel cloud did not 
include the ability to stick to, or wet, interior components. Rather, the aircraft fuel SPH particles tended 
to bounce off of internal structures. 



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Figure E-35. Calculated fuel distribution and debris damage in the base case WTC 1 

analysis. 

E.7.2 WTC 1 More Severe Impact Analysis 

The analysis of aircraft impacts into the WTC towers was subject to uncertainties in the input parameters 
such as: 

• Aircraft impact parameters: aircraft speed, horizontal and vertical angles of incidence, orientation, 
and location of impact. 

• Material properties: high strain rate material constitutive behavior and failure criteria for the 
towers and the aircraft. 

• Aircraft mass and stiffness properties, and the jet fuel distribution in the aircraft. 

• Tower parameters: structural strength and mass distribution, connection and joint positions 
relative to impact and joint failure behavior. 

• Nonstructural building contents that may share in absorbing energy imparted by the aircraft 
impact. 



NISTNCSTAR 1-2, WTC Investigation 



Ixxxiii 



Executive Summary 



Another important source of uncertainty is the inaccuracy associated with mathematical or numerical 
models. The inaccuracies of models, also known as modeling errors, are deterministic in nature, but are 
often treated as random variables to characterize the effects of the analysis methodologies on the 
calculated response. All of these variables did not necessarily have a significant effect on the estimated 
impact damage to the WTC towers. 

Because of the complexity of the problem and the limited number of parameters that could be varied in 
the global analyses, it was necessary to down-select a refined list of uncertainty parameters from all of the 
possible parameters. Therefore, variable screening was conducted using design of experiments 
methodology. Screening was first conducted at component and subassembly levels using orthogonal 
factorial design techniques in order to identify the most influential parameters and reduce the number of 
parameters to a more manageable number for the global impact analyses. The sensitivity analyses 
included engine impacts against core columns, wing section impacts against exterior panels, and engine- 
impact subassembly analyses. 

In addition to the base case impact analysis described in Section E.7.1, two more impact analyses were 
performed for each tower to provide a range of calculated impact-induced damage. These analyses 
included a more severe and a less severe case. Based on the three sensitivity analyses, the set of 
influential modeling parameters was reduced. The following parameters were selected for variation in the 
more severe and less severe global impact analyses: 

Impact speed. 

Vertical approach angle of the aircraft. 

Lateral approach angle of the aircraft. 

Total aircraft weight. 

Aircraft materials failure strain. 

Tower materials failure strain. 

Building contents weight. 

For the more severe case, the impact speed was increased to the upper bound obtained from the analysis 
of aircraft impact conditions, while the aircraft vertical trajectory angle was reduced to impart more 
impact energy inward toward the core. A 5 percent increase in the total aircraft weight was considered 
for the more severe case, while the failure strain was varied to be 125 percent of the baseline value to 
inflict more damage on the towers. For the tower model, the failure strains of the tower steels were 
reduced to 80 percent of the baseline value, and the mass of the building contents was reduced. These 
variations contributed to more severe damage to the tower structure, by making the tower structure 
weaker and the aircraft structure stronger. The opposite was done for the less severe case. This section 
provides some details of the WTC 1 more severe case, while Section E.7.3 provides a brief description of 
the WTC 1 less severe case. 

Exterior Wall Damage 

The calculated damage to the north wall from the more severe WTC 1 global impact analysis is shown in 
Figure E-36. A comparison of the north exterior wall observed (Figure E-28a) and calculated damage 

Ixxxiv NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



from the more severe WTC 1 global impact analysis (Figure E-36) indicated that the calculated and 
observed magnitude and mode of impact damage were still in good agreement. 

The overall agreement with the observed damage to the north wall was good for the base case and the 
more severe case, with the base case analysis providing the better match to the observed damage. The 
differences in apparent damage were largely due to panels that may have severed columns in one case and 
were removed at the connections in another. Toward the wing tips, where the columns and spandrels 
were not completely severed, the more severe impact damage analysis calculated greater damage to the 
exterior wall panels. As would be expected, the base case analysis calculated less damage to the exterior 
wall than the more severe case near the wing tips. 




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Figure E-36. Calculated more severe impact damage to the WTC 1 exterior wall. 

Core Structural Damage 

The core had extensive damage in the region close to the impact point. The columns in line with the 
aircraft fuselage failed on the impact side, and several of the core beams were also severely damaged or 
failed in the impact zone. In some cases, failure of the column splices located on floors 92, 95, and 98 
contributed significantly to the failure of the core columns. 

The calculated damage to the core columns by row is shown in Figure E-37, and the damage to the core 
framing for floors 95 and 96 is shown in Figure E-38. A total of six columns were severed, and three 
columns were heavily damaged in the more severe case, compared to three columns severed and four 
columns heavily damaged in the base case WTC 1 impact analysis. This shows a clear correlation 
between damage magnitude and impact severity. 



NISTNCSTAR 1-2, WTC Investigation 



Ixxxv 



Executive Summary 





(a) Columns 503-1003 



(b) Columns 504-1004 





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Figure E-37. More severe impact response of the WTC 1 core columns. 




(a) Floor 95 core framing damage 



(b) Floor 96 core framing damage 



Figure E-38. More severe impact damage to the core beams of floors 95 and 96 of 

WTCl. 



Ixxxvi 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Floor Truss and Slab Damage 

An overall frontal view of the calculated more severe impact damage to the floor trusses is shown in 
Figure E-39. The figure shows that the trusses experienced significant damage in the impact zone. A 
plan view of the calculated damage to the truss on floors 95 and 96 is shown in Figure E-40. The 
calculated impact response produced severe damage to the truss structures in the primary impact path of 
the fuselage from the exterior wall to the core. The truss floor system on floors 94 through 96 were 
damaged and sagged downward as a result of the impact loading. 

When the floor-by-floor damage was compared for the base case and more severe impact analyses, the 
damage appeared to be slightly less for the more severe impact analysis. The parameters used in the more 
severe global impact analysis would primarily contribute to an increased damage magnitude for the tower 
structures. However, the downward impact trajectory angle was reduced from the 10.6 degree angle in 
the base case analysis to a 7.6 degree angle in the more severe impact analysis. This would have the 
effect of directing more of the impact energy inward toward the tower core but reducing the normal 
downward force on the floor structures in the impact zone. As a result, the combined effects of the 
analysis parameter variations produced slightly less damage to the truss structure in the more severe 
impact analysis scenario. 



Column 
151 




Floor 95 



Figure E-39. Calculated more severe impact damage to the WTC 1 floor trusses 

(front view). 



NISTNCSTAR 1-2, WTC Investigation 



Ixxxvii 



Executive Summary 



Impact 



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(a) Floor 95 truss damage 



(b) Floor 96 truss damage 



Figure E-40. More severe impact damage to the trusses on floors 95 and 96 of WTC 1 

(plan view). 

The calculated more severe impact damage to the floor slabs for floors 95 and 96 of WTC 1 is shown in 
Figure E-41. The magnitude of floor slab damage was, in general, very similar for the base case and 
more severe global impact analyses. When the floor-by-floor damage was compared for the two analyses, 
the damage appeared to be slightly less for the more severe impact analysis. Similar to the truss damage, 
the reduced damage in the floor slabs is believed to be the result of the reduction in the downward impact 
trajectory angle from 10.6 to 7.6 degrees in the more severe impact analysis, reducing the normal 
downward force on the floor structures. 



Impact 



Impact 



CoTiimn 
113 




(a) Floor 95 slab damage 



(b) Floor 96 slab damage 



Figure E-41. More severe impact damage to the slabs on 1 floors 95 and 96 of WTC 1 

(plan view). 



Ixxxviii 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Summary of Structural Damage 

Figure E^2 presents the cumulative damage to WTC 1 on all affected floors and columns for the more 

severe case. 



1D1 103 10e 1CS 112 lis I1B 121 124 127 130 133 139 139 142 145 146 1S1 134 157 1S9 



Severe Floor Damage 

Floor system i — i 
structural damage I I 



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removed 




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Figure E-42. Cumulative structural damage to the floors and columns of WTC 1 (more 

severe case). 

Fuel and Debris Distribution 

The distribution of the fuel in the tower calculated from the more severe case is shown in Figure E-43. A 
comparison to the calculated damage for the base case WTC 1 impact analysis indicated that the content 
damage zone is very similar in width but extended further south through the tower in the more severe 
impact. The more severe impact produced significantly greater content damage on the far side of the core 
and extended more fully through the tower. 



NISTNCSTAR 1-2, WTC Investigation 



Ixxxix 



Executive Summary 



Time = d.685 



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Figure E-43. Calculated fuel distribution and debris damage in the more severe WTC 1 

analysis. 



E.7.3 



WTC 1 Less Severe Impact Analysis 



For the north exterior wall of WTC 1, the magnitude and mode of impact damage were in good agreement 
with the observed damage for the less severe impact scenario. The core had a limited damage confined to 
the region nearest to the impact point. Only one column was severed, and two columns were heavily 
damaged for the less severe case, compared to three severed columns and four heavily damaged columns 
in the base case WTC 1 impact analysis. 

The floor trusses experienced significant damage in the impact zone. The calculated impact response 
produced severe damage to the truss structures in the primary impact path of the fuselage. The truss 
structures were severely damaged from the exterior wall to the core. The truss floor system on floors 94 
through 96 were damaged and sagged downward as a result of the impact loading. When compared with 
the base case, the magnitude of damage to the floor trusses and floor slabs was slightly increased for the 
less severe impact analysis. The parameters used in the less severe global impact analysis would 
primarily contribute to a reduced damage magnitude for the tower structures. However, the downward 
impact trajectory angle was increased from the 10.6 degree angle in the base case analysis to a 
13.6 degree angle in the more severe impact analysis. This would have the effect of directing more of the 
impact energy downward, increasing the normal force on the floor structures in the impact zone. As a 
result, the combined effects of the analysis parameter variations produced a small increase in the damage 
to the truss structure in the less severe impact analysis scenario. 

A comparison to the base case and less severe case indicated that the building contents damage zone was 
very similar in width but did not extend as far through the tower in the less severe impact. The less severe 



xc 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



impact produced little content damage on the far side of the core and did not extend fully through the 
tower. No debris penetration of the south wall of the tower was observed for the less severe impact 
condition. 

E.7.4 WTC 2 Base Case Impact Analysis 

The WTC 2 base case impact analysis was performed for a 0.62 s duration following initial impact of the 
aircraft nose with the south exterior wall. The side view and top view of the base case WTC 2 global 
impact response is shown in Figure E-44 and Figure E-45, respectively. Full penetration of the aircraft 
into the tower was completed at 0.2 s after impact. The aircraft impact response was very similar to that 
of the WTC 1 impact and was dominated by the penetration and fragmentation of the airframe structures. 
The fuselage structures were severely damaged both from the penetration through the exterior columns 
and the penetration of the 81st floor slab that sliced the fuselage structures in half The downward 
trajectory of the aircraft structures caused the airframe to collapse against the floor, and the subsequent 
debris motion was redirected inward along a more horizontal trajectory parallel to the floor. The 
downward trajectory of the aircraft structures transferred sufficient vertical load that the truss floor 
structures on the 80th and 81st floors began to collapse in the impact zone by the end of the simulation. 

The aircraft wing structures and fuel tank were fragmented by the impact with the tower exterior. The 
aircraft fuel cloud started to spread out immediately after impact, but the leading edge of the fuel 
remained relatively dense until passing approximately one-third of the lateral distance through the tower 
core (approximately 0.2 s after impact). At 0.3 s after impact, the aircraft fuel cloud had penetrated 
approximately two-thirds the distance through the core and was spreading out. Beyond this time, the 
subsequent motion of the aircraft fragments and fuel debris cloud was noticeably slowed. The spread of 
the fuel and debris cloud was more rapid and extensive in the open truss floor regions than through the 
core as a result of the open volume above the workstations in the truss floor zone. 



NIST NCSTAR 1-2, WTC Investigation xci 



Executive Summary 



\ MH in irm 

^^BIHBBIIBBI IB I B B a Bl IB I BIB B Bl 




(a) Time=0.00 s 




(b) Time=0.50 s 
Figure E-44. WTC 2 base case global impact analysis - side view. 



xcn 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 




(a) Time=0.00 s 




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(b) Time=0.50 s 
Figure E-45. WTC 2 base case global impact analysis - plan view. 

Exterior Wall Damage 

A comparison of the south exterior wall observed and calculated damage from the base case WTC 2 
global impact analysis is shown in Figure E-46. The exterior wall completely failed in the regions of the 
fuselage, engine, and fuel-filled wing section impacts. Damage to the exterior wall extended to the wing 
tips, but the exterior columns were not completely failed in the outer wing and vertical stabilizer impact 
regions. Failure of the exterior columns occurred both at the bolted connections between column ends 
and at various locations in the column depending on the local severity of the impact load and the 
proximity of the bolted connection to the impact. 

The comparison of the calculated and observed damage indicated that the geometry and location of the 
impact damage zone were in good agreement. This agreement served to vahdate the geometry of the 
aircraft model, including the aircraft orientation, trajectory, and flight distortions of the wings. The 
agreement of both the mode and magnitude of the impact damage served to partially validate the 
constitutive and damage modeling of the aircraft and exterior wall of the tower. 



NISTNCSTAR 1-2, WTC Investigation 



xciii 



Executive Summary 



■ ■ 



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(b) Calculated damage 
Figure E-46. Base case impact damage to the WTC 2 exterior wall. 



XCIV 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Core Structural Damage 

The core had significant damage in the region close to the impact point, in particular the southeast corner 
of the core. The columns in line with the aircraft fuselage failed on the impact side, and several of the 
core beams were also severely damaged or failed in the impact zone. In some cases, failure of the column 
splices located on floors 77, 80, and 83 contributed significantly to the failure of the core columns. This 
was particularly true for the heavy column number 1001 at the southeast comer of the core that failed at 
the three sphce locations. 

The calculated damage to the core columns by row is shown in Figure E-47. A total of five columns 
were severed, and four columns were heavily damaged. The damage to the core beams for fioors 80 and 
81 is shown in Figure E^8. 




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(c) Columns 801-807 (d) Columns 701-708 

Figure E-47. Base case impact damage to the WTC 2 core columns. 



NISTNCSTAR 1-2, WTC Investigation 



xcv 



Executive Summary 




(a) Floor 80 core framing damage (b) Floor 81 core framing damage 

Figure E-48. Base case impact damage to the core beams of floors 80 and 81 of WTC 2. 

Floor Truss and Slab Damage 

An overall frontal view for the floor truss structure in the WTC 2 impact zone, along with the calculated 
base case impact damage to the trusses, is shown in Figure E-49. The figure shows that the trusses 
experienced significant damage in the impact zone, with the largest damage on floor 81. A plan view of 
the calculated damage to the trusses on floors 80 and 81 is shown in Figure E-50. The calculated impact 
response produced severe damage to the truss structures in the primary impact path of the fuselage. The 
truss structures were severely damaged from the exterior wall to the core. The truss floor system on 
floors 79 and 81 had sufficient damage from the impact that truss floor sections sagged downward. The 
calculated damage to the WTC 2 floor slabs for floors 80 and 81 is shown in Figure E-51, where a similar 
pattern of response to that observed in the trusses can be seen for the floor slabs. 



XCVl 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Floor 82 



Floor 80 
Floor 79 



Floor 78 



(a) Initial detailed truss structures 



Floor 82 




Floor 78 



(b) Calculated damage 
Figure E-49. Base case impact damage to the WTC 2 floor trusses (front view). 




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(b) Floor 81 truss damage 



Figure E-50. Base case impact damage to the trusses on floors 80 and 81 of WTC 2 

(plan view). 



NISTNCSTAR 1-2, WTC Investigation 



xcvn 



Executive Summary 




3S9 



(a) Floor 80 slab damage (b) Floor 81 slab damage 

Figure E-51. Base case impact damage to the slabs on floors 80 and 81 of WTC 2 

(plan view). 

Summary of Structural Damage 

The impact-induced structural damage described above provided the initial conditions for the post-impact 
fire-structural analyses. Figure E-52 presents the cumulative damage on all affected floors and columns. 
The damage to the columns at the various levels is identified by the color of the circles, where red, blue, 
green, and yellow signify severed, heavily damaged, moderately damaged, and lightly damaged columns, 
respectively. The dotted boxes on the figures indicate areas where the impact created an opening in the 
floor. These were used to identify slab openings in the fire dynamics simulations. The solid boxes 
indicate areas in the floor system that had severe structural damage. These areas were removed from the 
subsequent structural analyses. Figure E-52 also shows the damage to columns on the north perimeter 
wall, which the analysis did not capture due to the coarse mesh on the north wall. This damage was 
observed in photographs. As a result, this damage was accounted for in the subsequent structural 
analyses. 



xcvni 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



zn: 



212 235 2te 221 224 227 230 233 23B 239 242 



2« 251 254 257 259 



Severe Floor Damage 

Floor system i — i 
structural damage I I 



Floor system 
removed 



Column Damage 
Severed Q 

Heavy Damage Q 
Moderate Damage ^^ 
Light Damage 




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435 433 430 427 424 421 418 416 412 409 40S 403 401 



Figure E-52. Cumulative structural damage to the floors and columns of WTC 2 

(base case). 

Fuel and Debris Distributions 

The calculated distribution of the aircraft debris and fuel cloud from the base case WTC 2 global impact 
analysis was shown previously in Figure E-44 and Figure E-45. Figure E-53 shows the distribution of 
fuel and damage to the building contents due to impact. The bulk of the aircraft debris and fuel was 
arrested prior to exiting the tower structures. However, a significant amount of aircraft debris was 
calculated to exit the north and east sides of the tower (Sides 300 and 200 of WTC 2). 

The bulk of the fuel and aircraft debris was deposited in floors 78 through 80, with the greatest 
concentration of aircraft debris on floor 80, and the largest concentration of aircraft fuel on floors 79, 81, 
and 82. The calculated debris distribution included 55,800 lbs of debris and 10,600 lbs of aircraft fuel 
outside of the tower at the end of the impact analysis, either rebounding from the impact face or passing 
through the tower. The calculated mass outside the tower is believed to be larger than is realistic, since 
the exterior walls were not modeled with windows that could contain the fuel cloud and small debris 
inside the tower. In addition, treatment of the aircraft fuel cloud did not include the ability to stick to, or 
wet, interior components. Rather, the aircraft fuel SPH particles tended to bounce off of internal 
structures. 



NISTNCSTAR 1-2, WTC Investigation 



XCIX 



Executive Summary 



Time.-= - -D.BJ19. 




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Figure E-53. Calculated fuel distribution and debris damage in the base case 

WTC 2 analysis. 

E.7.5 WTC 2 More Severe Impact Analysis 

This section summarizes the resuhs of the more severe impact analysis for WTC 2. The parameters for 
the more severe and less severe damage cases for WTC 2 were similar to those for WTC 1. Section E.7.6 
provides a brief description of the WTC 2 less severe case. 

Exterior Wall Damage 

The calculated damage to the south wall from the more severe WTC 2 global impact analysis is shown in 
Figure E-54. A comparison of the south exterior wall observed (Figure E-46a) and calculated 
(Figure E-54) damage from the more severe WTC 2 global impact analysis indicated that the calculated 
and observed magnitude and mode of impact damage were still in good agreement. 

As was the case for WTC 1 , there were small differences in the damage estimates for the south wall of 
WTC 2 from the base case and the more severe case scenarios (compare Figure E-46b and Figure E-54). 
Overall, the agreement with the observed damage from photographs was very good for both cases. The 
most obvious differences were largely due to portions of panels that may have severed columns in one 
case or have been removed at the connections in another. 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 




Figure E-54. Calculated more severe impact damage to the WTC 2 exterior wall. 

Core Structural Damage 

The core had extensive damage in the region close to the impact point. The columns in line with the 
aircraft fuselage failed on the impact side, and several of the core beams were also severely damaged or 
failed in the impact zone. In some cases, failure of the column splices located on floors 77, 80, and 83 
contributed significantly to the failure of the core columns. 

The calculated damage to the core columns by row is shown in Figure E-55, and the damage to the core 
framing at floors 80 and 81 is shown in Figure E-56. A total often columns were severed, and one 
column was heavily damaged in the WTC 2 more severe case, compared to five columns severed and four 
columns heavily damaged in the base case WTC 2 impact analysis. This shows a clear correlation 
between damage magnitude and impact severity. 



NISTNCSTAR 1-2, WTC Investigation 



ci 



Executive Summary 




d:;tt 



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(c) Columns 801-807 (d) Columns 701-708 

Figure E-55. More severe impact damage to the WTC 2 core columns. 



en 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 





(a) Floor 80 core framing damage 



(b) Floor 81 core framing damage 



Figure E-56. More severe impact damage to the core beams of floors 80 and 81 of 

WTC2. 

Floor Truss and Slab Damage 

An overall frontal view for the calculated more severe impact damage to the trusses is shown in 
Figure E-57. The figure shows that the trusses experienced significant damage in the impact zone, with 
the heaviest damage on floor 81. A plan view of the calculated damage to the trusses on floors 80 and 81 
is shown in Figure E-58. The calculated impact response produced severe damage to the truss structures 
in the primary impact path of the fuselage. The truss structures were severely damaged from the exterior 
wall to the core. The truss floor system on floors 79 through 82 had sufficient damage from the impact 
that portions of the truss floor sections sagged downward as a result of the impact. 




Figure E-57. Calculated more severe impact damage to the WTC 2 floor trusses 

(front view). 

The magnitude of truss floor damage was very similar for the base case and more severe global impact 
analyses. The parameters used in the more severe global impact analysis would primarily contribute to an 
increased amount of damage for the tower structures. However, the downward impact trajectory angle 
was reduced from the 6 degree angle in the base case analysis to a 5 degree angle in the more severe 
impact analysis. This resulted in directing more of the impact energy inward toward the tower core, but 
reducing the normal downward force on the floor structures in the impact zone. As a result, the combined 
effects of the analysis parameter variations produced very similar damage to the truss structure. 



NISTNCSTAR 1-2, WTC Investigation 



cm 



Executive Summary 



The calculated damage to the WTC 2 floor slab for floors 80 and 81 for the more severe impact is shown 
in Figure E-59. The magnitude of floor slab damage was very similar for the base case and more severe 
global impact analyses due to the reasons explained above for the floor trusses. 



443- 



437 ■ 



Impact f^ 



417 





402 



359 



339 



319 



(a) Floor 80 truss damage 



(b) Floor 81 truss damage 



Figure E-58. More severe impact damage to the trusses on floors 80 and 81 of WTC 2 

(plan view). 




(a) Floor 80 slab damage 



(b) F oor 81 slab damage 



Figure E-59. More severe impact damage to the WTC 2 floor slab (plan view). 



CIV 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Summary of Structural Damage 

Figure E-60 presents the cumulative damage to WTC 2 on all affected floors and columns for the more 

severe case. 



^1 £03 £06 203 212 215 219 221 224 227 230 233 236 239 242 248 



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Figure E-60. Cumulative structural damage to the floors and columns of WTC 2 (more 

severe case). 

Fuel and Debris Distributions 

The distribution of the fuel in the tower calculated from the more severe case in a plan view and side view 
is shown in Figure E-61. A comparison to the calculated damage for the base case WTC 2 impact 
analysis indicated that the tower contents damage zone was similar, with a slight increase in damage for 
the more severe impact. 



NISTNCSTAR 1-2, WTC Investigation 



cv 



Executive Summary 



Time = q 58 



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Figure E-61. Calculated fuel distribution and debris damage in the more severe 

WTC 2 analysis. 

E.7.6 WTC 2 Less Severe Impact Analysis 

For the south exterior wall of WTC 2, the magnitude and mode of impact damage were in good 
agreement with the observed damage for the less severe impact scenario. The core had significant 
damage in the region close to the impact point. The columns in line with the aircraft fuselage failed on 
the impact side, and several of the core beams were also severely damaged or failed in the impact zone. 
In some cases, failure of the column sphces located on floors 77, 80, and 83 contributed significantly to 
the failure of the core columns. A total of three columns were severed, and two columns heavily 
damaged, compared to five severed columns and four heavily damaged columns in the base case WTC 2 
impact analysis. 

The truss floor system on floors 79 through 82 had sufficient damage from the impact that portions of the 
truss floor sections sagged downward as a result of the impact. The trusses experienced significant 
damage in the impact zone, with the heaviest damage on fioor 81. The calculated impact response 
produced severe damage to the truss structures in the primary path of the fiiselage. The truss structures 
were completely destroyed along the impact path on fioor 8 1 from the exterior wall to the core. 

When compared with the base case, the magnitude of damage to the fioor trusses and fioor slabs was 
slightly increased for the less severe impact analysis. The parameters used in the less severe global 
impact analysis would primarily contribute to a reduced damage magnitude for the tower structures. 
However, the downward impact trajectory angle was increased from the 6 degree angle in the base case 
analysis to an 8 degree angle in the less severe impact analysis. This would have the effect of directing 



cvi 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



more of the impact energy downward, increasing the normal force on the floor structures in the impact 
zone. As a result, the combined effects of the analysis parameter variations produced very similar 
damage to the truss structure. 

A comparison to the base case and less severe case indicated that the building contents damage zone was 
similar, with a slight reduction in damage for the less severe impact. 

E.7.7 Comparison with Observables 

The observables available to help validate the global impact analyses included the following: 

• Damage to the building exterior (exterior walls and floors in the immediate vicinity of the 
impact) documented by photographic evidence. 

• Aircraft debris external to the towers (landing gear for WTC 1 and a landing gear and an 
engine for WTC 2) as documented by photographic evidence. 

• Eyewitness accounts from survivors who were inside the towers (blocked or passable 
stairwells). 

An example of such comparisons was a detailed comparison between the observed and calculated damage 
(from the base case analysis) to the north wall of WTC 1 and the south wall of WTC 2. The comparison 
included the mode, magnitude, and location of failure around the hole created by the aircraft impact. The 
color code included the following: (1) green circles indicating a proper match of the failure mode and 
magnitude between the observed and calculated damage, (2) yellow circles indicating a proper match in 
the failure mode, but not the magnitude, (3) red circles indicating that the failure mode and magnitude 
predicted by the calculation did not match that was observed, and (4) black circles indicating that the 
observed damage was obscured by smoke, fire, or other factors. The comparisons shown in Figure E-62 
and Figure E-63 for WTC 1 and WTC 2, respectively, indicate that the overall agreement with the 
observed damage was very good. 



NIST NCSTAR 1-2, WTC Investigation evil 



Executive Summary 



BUULUuVumlfj 




1 1 1 1 1 1 1 1 1 1 1 1 

Correct mode and magnitude 

9 Corroct mode, magnitude Insufllciont 

incorrect mode 

^ Obacured on images 



Figure E-62. Comparison of observable and calculated base case impact damage to the 

north wall of WTC 1. 



|fttiiiiiiiiiip|pii^iiif^p^iiit 

:n:mr:n:Dn:n:iain:in:ia::!ii:Di:ni 








# Correct mode and magnitude 

^l mode, magnitude Insudicient 
t Incorrect mode 
I Obscured on images 

Figure E-63. Comparison of observable and calculated base case impact damage to the 

south wall of WTC 2. 



CVlll 



NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Not all of the observables were perfectly matched by the simulations due to the uncertainties in exact 
impact conditions, the imperfect knowledge of the interior tower contents, the chaotic behavior of the 
aircraft breakup and subsequent debris motion, and the limitations of the models. In general, however, 
the results of the simulations matched these observables reasonably well. 

E.8 FINDINGS 

Finding 1: The original design wind loads on the towers exceeded those established by NYCBC prior to 
1968, when the WTC towers were designed, and up to and including 2001. The original design load 
estimates were also higher than those required by other selected building codes of the time, including the 
relevant national model building code (BOCA). The prescriptive approach in these codes is 
oversimplified, and as a result, these codes are not appropriate for super-tall building design. 

Finding 2: In the majority of the cases, each of the two orthogonal shear components and of the two 
orthogonal overturning moment components at the base of the towers used in the original wind design 
were smaller than the CPP, RWDI, and refined NIST estimates. However, the most unfavorable 
combined peaks (resultant) from the original design were larger, or smaller, by at most 15 percent than 
estimates based on the CPP, RWDI, and NIST estimates. This is due to the conservative approach used to 
combine the loads in the original design. 

Finding 3: The estimated wind-induced loads on the towers varied by as much as 40 percent between the 
wind tunnel/climatological studies conducted in 2002 by CPP and RWDI. The primary reasons for these 
differences were due to the different approaches used in those studies to (1) estimate extreme wind 
speeds; (2) estimate wind profiles; (3) integrate aerodynamic, dynamic, and extreme wind climatological 
information; and (4) combine wind effects in two orthogonal directions and in torsion. Such disparity is 
indicative of the limitations and inconsistencies associated with the current state of practice in wind 
engineering for tall buildings. 

Finding 4: A comparison of wind speeds indicated significant differences among various specified 
design wind speeds. The basic wind speed specified in ASCE 7-02 for New York City is equivalent to an 
88 mph fastest-mile wind speed at 33 ft above ground for open terrain exposure. The wind speed 
specified in the NYCBC (2001) is 80 mph and is interpreted to be a fastest-mile wind speed at 33 ft above 
ground. For the original WTC design, the design wind speed was 98 mph averaged over 20 minutes at a 
height of 1,500 ft above ground, which is equivalent to a fastest-mile wind speed at 33 ft above ground of 
between 67 mph and 75 mph. The wind speed estimated by NIST for the three airports (La Guardia, 
Newark International Airport, and John F. Kennedy International Airport), regardless of direction, was 
equivalent to 96 mph fastest- mile wind speed. An evaluation of the wind speed specifications and the 
development of improved design wind speeds, as well as protocols for selection of site-specific wind 
speeds and directionality, are, therefore, in order. 

Finding 5: Under the original WTC design loads, the cumulative drifts at the top of the WTC towers 
ranged from H/263 to H/335. For the lower estimate state-of-the -practice case, those drifts ranged from 
H/253 to H/306. Under design loading conditions, the maximum inter-story drift was as high as h/230 
and h/200 for WTC 1 and WTC 2, respectively. Maximum inter-story drifts under the state-of-the 
practice case were about h/184 and h/200 for WTC 1 and WTC 2, respectively. For the refined NIST 
estimate case, the cumulative and inter-story drifts were about 25 percent larger than those from the state- 



NIST NCSTAR 1-2, WTC Investigation cix 



Executive Summary 



of-the-practice case. Total and inter-story drifts of the towers were larger than what is generally used in 
current practice. 

Finding 6: DCRs estimated from the original WTC design load case were in general close to those 
obtained for the lower-estimate state-of-the practice case. For both cases, a fraction of the structural 
components had DCRs larger than 1.0. These were mainly observed in both towers at (1) the exterior 
walls: (a) at the columns around the corners, (b) where the hat truss connected to the exterior walls, and 
(c) below floor 9; and (2) the core columns on the 600 line between floors 80 and 106 and at core 
perimeter columns 901 and 908 for much of their height. The DCRs obtained for the refined NIST 
estimate case were higher than those for the original WTC design and the lower-estimate state-of-the- 
practice load cases. 

Finding 7: The safety of the exterior walls, core columns, and hat truss members of the WTC towers on 
September 1 1 , 200 1 , was most likely not affected by the fraction of members for which the demand 
exceeded allowable capacity. 

Finding 8: The behavior of the lower portion of the towers at the basement floors was that of a braced 
frame, while the behavior of the super-structure was that of a framed tube system. Under a combination 
of the original WTC design dead and wind loads, tension forces developed in the exterior walls of both 
towers. The forces were largest at the base of the building and at the corners. The DCRs for the exterior 
wall splice connections under these tensile forces for both towers were shown to be less than 1.0. 

Finding 9: For the towers' resistance to shear sliding under wind loads, the factor of safety was between 
10 and 1 1.5, while the factor of safety against overturning ranged from 1.9 to 2.7 for both towers. 

Finding 10: For the typical truss-framed floor under the original WTC design gravity loads, the DCRs 
for all floor trusses were less than unity for 99.4 percent of the floor truss components, with a maximum 
of 1.14. Inside the core, the DCRs for all floor beams were less than 1.08, and more than 99 percent of 
floor beams had a DCR of less than 1.0. The maximum mid-span deflections of the long span and short 
span zones under the original WTC design loads were about L/400 and L/750, respectively, where L is 
the floor span. For the typical beam-framed floor under the original WTC design gravity loads, the DCRs 
for all floor beams were less than 1.0, except for two core beams where the DCRs in shear were 1.125 and 
1.09. The maximum mid-span deflections of the long span and short span zones under the original design 
loads were about L/450 and L/600, respectively. 

Finding 11: Documents from The Port Authority of New York and New Jersey indicated that the safety 
of the WTC towers and their occupants in an aircraft collision was a consideration in the original design. 
The documents indicate that a Boeing 707, the largest commercial aircraft at the time, flying at 600 mph 
was considered, and the analysis indicated that such collision would result in only local damage which 
could not cause collapse or substantial damage to the building and would not endanger the lives and 
safety of occupants not in the immediate area of impact. No documentary evidence of the aircraft impact 
analysis was available to review the criteria and methods used in the analysis of the aircraft impact into 
the WTC towers, or to provide details on the abihty of the WTC towers to withstand such impacts. 

Finding 12: The impact of a Boeing 767 engine at a speed of 500 mph on an exterior wall panel resulted 
in a complete penetration of the engine through the exterior wall and failure of impacted exterior 
columns. 



ex NIST NCSTAR 1-2, WTC Investigation 



Executive Summary 



Finding 13: Impact of an empty wing segment from approximately mid-span of the wing normal to the 
exterior wall produced significant damage to the exterior columns but not complete failure. Impact of the 
same wing section, but filled with fuel, resulted in extensive damage to the external panels of the tower, 
including complete failure of the exterior columns. 

Finding 14: The response of the nonstructural building contents and the floor concrete slab to an aircraft 
engine impact was dominated by the mass of the workstations and the concrete slab, rather than by their 
strength. 

Finding 15: The aircraft that impacted WTC 1 had a speed of 443±30 mph with a roll angle of 
25±2 degrees (port wing downward). The vertical approach downward angle was 10.6±3 degrees and the 
lateral approach angle was close to being normal to the north wall of the tower. For WTC 2, the 
impacting aircraft had a speed of 542±24 mph with a roll angle of 38±2 degrees (port wing downward). 
The vertical approach downward angle was 6±2 degrees, and the lateral approach angle was 15±2 degrees 
clockwise from the south wall of the tower. 

Finding 16: The aircraft impact on WTC 1 resulted in extensive damage to the north wall of the tower, 
which failed in the regions of the fuselage, engine, and fuel-filled wing section impacts. Damage to the 
exterior wall extended to the wing tips, but the exterior columns were not completely failed in the outer 
wing and vertical stabilizer impact regions. According to photographs, columns 1 12 to 144 along with 
column 151 were completely severed, while columns 145 to 148 were heavily damaged and columns 149 
to 150 were moderately damaged (for reference, columns 101 and 159 are located on the west and east 
corner, respectively, of the north wall). The results of the impact analyses matched well with this damage 
pattern to the north wall. Photographic evidence also indicated that an exterior panel with columns 329, 
330, and 331 on the south wall between floors 94 to 96 was dislodged. Failure of the exterior columns 
occurred both at the bolted connections between column ends and at various locations in the column 
depending on the local severity of the impact load and the proximity of the bolted connection to the 
impact. Subject to the uncertainties inherent in the models, the global impact simulations indicated that a 
total of three core columns were severed and four columns were heavily damaged in the base case, 
compared to six columns severed and three columns heavily damaged in the more severe case and one 
columns severed and two columns heavily damaged in the less severe case. In the analyses, the floor 
trusses, core beams, and floor slabs experienced significant impact-induced damage on floors 94 to 96, 
particularly in the path of the fuselage. The analyses indicated that the wing structures were completely 
fragmented due to the interaction with the exterior wall and as a result, aircraft fuel was dispersed on 
multiple floors. In addition, aircraft debris resulted in substantial damage to the nonstructural building 
contents (partitions and workstations) and also in dislodging of fireproofmg. The bulk of the fuel and 
aircraft debris was deposited in floors 93 through 97, with the largest concentration on floor 94. 

Finding 17: The aircraft impact on WTC 2 resulted in extensive damage to the south wall of the tower, 
which failed in the regions of the fuselage, engine, and fuel-filled wing section impacts. Damage to the 
exterior wall extended to the wing tips, but the exterior columns were not completely failed in the outer 
wing and vertical stabilizer impact regions. According to photographs, columns 410 to 436 and columns 
438 to 439 were completely severed, while column 437 was heavily damaged (for reference, columns 401 
and 459 are located on the east and west corner, respectively, of the south wall). The results of the impact 
analyses matched well with this damage pattern to the south wall. In addition, columns 407 to 409 were 
obscured by smoke, but the analysis results indicated that these columns were moderately damaged. 
Photographic evidence also indicated that columns 253, 254, 257, and 258 on the north wall were failed. 

NIST NCSTAR 1-2, WTC Investigation cxi 



Executive Summary 



Failure of the exterior columns occurred both at the bolted connections between column ends and at 
various locations in the column depending on the local severity of the impact load and the proximity of 
the bolted connection to the impact. Subject to the uncertainties inherent in the models, the global impact 
simulations indicated that a total of five core columns were severed and four columns were heavily 
damaged in the base case, compared to ten columns severed and one column heavily damaged in the more 
severe case and three columns severed and two columns heavily damaged in the less severe case. In some 
cases, failure of the column splices located on floors 77, 80, and 83 contributed significantly to the failure 
of the core columns. In the analyses, the floor trusses, core beams, and floor slabs experienced significant 
impact-induced damage on floors 79 to 81, particularly in the path of the fuselage. The analyses indicated 
that the wing structures were completely fragmented due to the interaction with the exterior wall, and as a 
result, aircraft fuel was dispersed on multiple floors. In addition, aircraft debris resulted in substantial 
damage to the nonstructural building contents (partitions and workstations) and also in dislodging of 
fireproofing. The bulk of the fuel was concentrated on floors 79, 81, and 82, while the bulk of the aircraft 
debris was deposited in floors 78 through 80, with the largest concentration on floor 80. 

Finding 18: Natural periods calculated from the reference global model of the WTC 1 tower matched 
well with those measured on the tower based on the analysis of data from accelerometers located atop 
WTC 1. The calculated period of oscillation in the N-S direction of the reference global model of WTC 2 
matched well with the period estimated immediately after aircraft impact based on a detailed analysis of 
the building motion, which was captured in a video footage of the WTC 2 impact. This indicated that the 
overall lateral stiffness of the tower was not affected appreciably by the impact damage. The maximum 
deflection at the top of the tower after impact was estimated from the footage to be more than 1/3 of the 
drift resulting from the original design wind loads. This indicated that the tower still had reserve capacity 
after losing a number of columns and floor segments due to aircraft impact. 

Finding 19: The towers sustained significant structural damage to the exterior walls, core columns, and 
floor systems due to aircraft impact. This structural damage contributed to the weakening of the tower 
structures, but did not, by itself, initiate building collapse. However, the aircraft impact damage 
contributed greatly to the subsequent fires and the thermal response of the tower structures that led 
ultimately to the collapse of the towers by: (1) dispersing jet fuel and igniting building contents over large 
areas, (2) creating large accumulations of combustible materials containing aircraft and building contents, 
and (3) increasing the air supply into the damaged buildings that permitted significantly higher energy 
release rates than would normally be seen in ventilation building fires, allowing the fires to spread rapidly 
on multiple floors. 



cxii NIST NCSTAR 1-2, WTC Investigation 



Chapter 1 

Introduction 



1.1 BACKGROUND 

As stated in the preface, the National Institute of Standards and Technology (NIST) investigation into the 
collapse of the World Trade Center (WTC) towers included eight interdependent projects (see Table P-1). 
The Baseline Structural Performance and Aircraft Impact Damage Analysis project had two primary 

tasks. These were: 

1. To develop reference structural models of the towers and use these models to establish the 
baseline performance of each of the two towers under gravity and wind loads. 

2. To estimate the damage to the towers due to aircraft impacts and establish the initial 
conditions for the fire dynamics modeling and the thermal-structural response and collapse 
initiation analyses. 

This report presents the details of the studies related to both tasks. For each task, the report provides the 
following: 

• Description of structural models: these include the reference structural models of the towers 
for the first task, and global impact models of the towers and a model of the aircraft for the 
second. 

• Description of apphed loads for analyses: these are gravity and wind load estimates for the 
first task, and aircraft impact initial conditions for the second. 

• Analysis results: these include the baseline performance analyses of the towers for the first 
task, and a description of the impact-induced damage to the towers for the second. 

The report is concluded by a set of findings (Chapter 8). The next sections provide the background, 
technical approach, and details for each task. 

1.2 REFERENCE MODELS AND BASELINE PERFORMANCE ANALYSIS 

The WTC towers used a structural system that, at the time of the design, incorporated a number of 
innovative features. Among these features were the use of a composite truss fioor system to provide 
lateral stability and diaphragm action to the towers, the use of wind tunnel testing to estimate static and 
dynamic wind effects, and the use of viscoelastic dampers to reduce wind-induced vibrations. Wind loads 
were a governing factor in the design of the structural components that made up the frame-tube steel 
framing system. Wind load capacity is also a key factor in determining the overall strength of the towers 
and is important in determining not only the ability of the towers to withstand winds but also the reserve 
capacity of the towers to withstand unanticipated events such as a major fire or impact damage. 



NISTNCSTAR 1-2, WTC Investigation 



Chapter 1 



Accurate estimation of the wind load on tall buildings is a challenging task, given that wind engineering 
is still an evolving technology. For example, as is shown later, estimates of the wind-induced response 
presented in two recent independent studies of the WTC towers differed from each other by about 40 
percent. The primary reasons for these differences appear to lie in the different approaches used in those 
studies to estimate extreme wind speeds, to estimate wind profiles, to integrate aerodynamic, dynamic, 
and extreme wind climatological information, and to combine wind effects in two orthogonal directions 
and in torsion. In this study, NIST developed refined estimates of wind effects using information 
provided in the two studies, a critical assessment of that information, and independent information 
concerning the wind climate. Furthermore, as shown in this study, the available prescriptive codes 
specify wind loads (pressures) on tall buildings that are significantly lower than wind tunnel-based loads. 
This case study provided an opportunity to assess effectively the current design practices and various 
code provisions on wind loads. 

The baseline performance of the WTC towers under gravity and wind loads were established in order to 
assess the towers' ability to withstand those loads safely and to evaluate the reserve capacity of the towers 
to withstand unanticipated events. The baseline performance study provides a measure of the behavior of 
the towers under design loading conditions, specifically: (1) total and inter-story drift (the sway of the 
building under design wind loads); (2) fioor defiections under gravity loads; (3) the stress demand-to- 
capacity ratio for primary structural components of the towers such as exterior walls, core columns, and 
fioor framing; (4) performance of exterior walls under wind loading, including distribution of axial 
stresses and presence of tensile forces; (5) performance of connections between exterior columns; and 
(6) resistance of the towers to shear sliding and overturning at the foundation level. 

For the purpose of establishing the baseline performance of the towers, various wind loads were 
considered in this study, including wind loads used in the original WTC design, wind loads based on two 
recent wind tunnel studies conducted in 2002 by Cermak Peterka Peterson, Inc. (CPP) and Rowan 
Williams Davis and Irwin, Inc. (RWDI) for a insurance litigation concerning the towers, and wind loads 
estimated by NIST by critically assessing information obtained from the CPP and RWDI reports and by 
bringing to bear state-of-the-art considerations. 

In order to develop the reference models and perform the baseline performance analyses, the following 
steps were undertaken: 

• Develop structural databases for the primary structural components of the WTC 1 and WTC 2 
towers from the original computer printouts of the structural design documents. 



• 



Develop reference structural analysis models that capture the intended behavior of each of the 
two towers using the generated databases. These reference models were used to establish the 
baseline performance of the towers and also served as a reference for more detailed models 
for aircraft impact damage analysis and thermal-structural response and collapse initiation 
analysis. The models included: (1) two global models (one for each tower) of the major 
structural components and systems of the towers, and (2) fioor models of a typical truss- 
framed floor and a typical beam-framed fioor. 



NIST NCSTAR 1-2, WTC Investigation 



Introduction 



• Develop estimates of design gravity (dead and live loads) and wind loads on each of the two 
towers for implementation into the reference structural models. The following three loading 
cases were considered: 

- Original WTC design loads case. Loads included dead and live loads as in original 
WTC design, in conjunction with original WTC design wind loads. 

- State-of-the-practice case. Loads included dead loads, current New York City Building 
Code (NYCBC 2001) live loads, and wind loads from the RWDI wind tunnel study, 
scaled in accordance with NYCBC 2001 wind speed. 

- Refined NIST estimate case. Loads included dead loads, live loads from the American 
Society of Civil Engineers (ASCE) 7-02 Standard (a national standard), and refined wind 
loads developed by NIST. 

• Perform structural analyses to establish the baseline performance of each of the two towers 
under design gravity and wind loads. 

The tasks outlined above were conducted by the firm of Leslie E. Robertson Associates, the firm 
responsible for the original structural engineering of the WTC towers, under contract to NIST for the 
development of the structural databases, reference structural models, and baseline performance analysis. 
NIST implemented a rigorous and comprehensive review procedure to ensure the integrity and objectivity 
of the output and results, including the structural databases, reference models, and baseline performance 
analysis. The review procedure included an in-house NIST review and a third-party review by the firm of 
Skidmore, Owings, and Merrill, under contract to NIST. 

Chapters 2 through 4 provide a description of the structural modeling and analysis of the baseline 
performance of the towers. For further details, the reader is referred to NIST NCSTAR 1-2A.' 

Chapter 2 presents the development of the reference structural models for WTC 1 and WTC 2, including 
the global tower models, typical floor models, and parametric studies conducted to support the 
development of the global models. The chapter provides a brief summary of the development of the 
structural databases. In addition, this chapter outlines the NIST and third-party review of the structural 
databases and reference models. 

Chapter 3 provides a discussion on the loading cases used in the baseline performance analyses, and 
outlines the development of the gravity and wind loads on the global tower models. In this chapter, 
special emphasis is placed on the estimates of the wind load cases used in this study. These include the 
original design wind loads, the state-of-the-practice wind loads (the CPP and RWDI wind studies), and 
the refined NIST estimates. The chapter concludes with a comparison of the various wind studies. 

Chapter 4 provides the results of the baseline performance analyses for the global tower models as well as 
the typical floor models. The results presented for the global models include total and inter-story drift, 
demand to capacity ratios for primary structural components of the towers, response of exterior walls 



This footnote is to one of the companion documents from this Investigation. A list of these documents appears in the Preface 
to this report. 

NIST NCSTAR 1-2, WTC Investigation : 



Chapter 1 



under wind loading, performance of connections between exterior columns, and resistance of the towers 
to shear sliding and overturning. For the floor models, these results include floor mid-span deflections 
and demand to capacity ratios for primary floor framing members. The chapter also outlines the review 
process of the basehne performance analyses. 

1.3 AIRCRAFT IMPACT DAMAGE ANALYSIS 

Buildings are not specifically designed to withstand the impact of fuel-laden commercial aircraft, and 
building codes in the United States do not require building designs to consider aircraft impact. However, 
after the crash of a B-25 bomber into the Empire State Building in 1945, designers of high-rise buildings 
became aware of the potential of aircraft collision with buildings. Documents obtained from The Port 
Authority of New York and New Jersey (PANYNJ) indicated that the safety of the WTC towers and their 
occupants in an aircraft collision was a consideration in the original design. A three-page white paper 
"Salient points with regard to the structural design of the World Trade Center towers", February 1964, 
from the PANYNJ (see Appendix A) indicated that the impact of a Boeing 707 or DC 8 aircraft flying at a 
speed of 600 mph was analyzed during the design stage of the WTC towers. The paper also addressed the 
hfe safety considerations following such impact. The paper stated that ". . .The Buildings have been 
investigated and found to be safe in an assumed collision with a large jet airhner (Boeing 707 - DC 8) 
traveling at 600 miles per hour. Analysis indicates that such colhsion would result in only local damage 
which could not cause collapse or substantial damage to the building and would not endanger the lives 
and safety of occupants not in the immediate area of impact." 

A three-page document "Period of Vibration due to plane crash at 80* floor," March 1964, from the 
PANYNJ included a calculation by the designer to estimate the period of vibration due to an aircraft 
impacting at the 80th floor of the towers. Although no conclusion was stated on the calculation sheet, it 
indicated that the design considered the possibility of aircraft impact on the towers. Aside from these two 
documents from the PANYNJ, no documentary evidence on the aircraft impact analysis was available to 
review the criteria and methods used in the analysis of the aircraft impact into the WTC towers or to 
provide details on the ability of the WTC towers to withstand such impacts. 

The Federal Emergency Management Agency (FEMA) 403 (2002) report indicated that it was assumed in 
the 1960s design of the WTC towers that a Boeing 707 aircraft, lost in fog and seeking to land at a nearby 
airport, might strike the towers while low on fuel and at a landing speed of 180 mph. 

A property risk assessment report, prepared for Silverstein Properties prior to leasing the WTC towers in 
2001, identified the scenario of an aircraft striking a tower as one of the maximum foreseeable losses. 
The assessment states "This scenario is within the realm of the possible, but highly unlikely. In the event 
[of] such an unlikely occurrence, what might result? The structural designers of the towers have publicly 
stated that in their opinion that either of the Towers could withstand such an impact from a large modern 
passenger aircraft. The ensuing fire would damage the skin in this scenario, as the spilled fuel would fall 
to the Plaza level where it would have to be extinguished by the NYC Fire Department." 

While the documents from the PANYNJ indicated that aircraft impact was considered in the design, there 
were two views expressed by the building designers during media interviews on whether the effects of the 



NIST NCSTAR 1-2, WTC Investigation 



Introduction 



subsequent fires and the implications on life safety were a consideration in the original design. One view^ 
suggested that an analysis was done indicating that the biggest problem would be the fact that all the fuel 
would dump into the building and there would be horrendous fire. For implications on life safety, this 
view suggested that a lot of people would be killed, but the building structure would still be there. The 
other view^ suggested that the fuel load and the subsequent fire damage may not have been considered in 
the design stage. 

For the events of September 11, 2001, the aircraft impact damage to the exterior of the WTC towers could 
be visibly identified from the video and photographic records collected. However, no visible information 
could be obtained for the extent of damage to the interior of the towers, including the structural system 
(floors and core columns), partition walls, and interior building contents. Such information was needed 
for the subsequent fire dynamics simulations and post-impact structural analyses. In addition, for the fire 
dynamics modeling, the dispersion of the jet fuel and the location of combustible aircraft debris were 
required. The estimate of the extent of damage to the fireproofmg on the structural steel in the towers due 
to impact was essential for the thermal and structural analyses. The aircraft impact damage analyses were 
the primary tool by which most of the information on the tower damage could be estimated. 

The focus of the analysis was to analyze the aircraft impacts into each of the WTC towers to provide the 
following: (1) estimates of probable damage to structural systems, including exterior walls, floor 
systems, and interior core columns; (2) estimates of the aircraft fuel dispersion during the impact; and 
(3) estimates of debris damage to the building nonstructural contents, including partitions and 
workstations. The analysis results were to be used to estimate the damage to fireproofmg based on the 
predicted path of the debris field inside the towers. This analysis thus estimated the condition of the two 
WTC towers immediately following the aircraft impacts and established the initial conditions for the fire 
dynamics modehng and the thermal-structural response and collapse initiation analysis. The impact 
analyses were conducted at various levels of complexity including: (1) the component level, (2) the 
subassembly level, and (3) the global level to estimate the probable damage to the towers due to aircraft 
impact. 

The WTC aircraft impact analysis was a challenging task for the following reasons: 

• The need to develop a comprehensive aircraft model that properly captured the stiffness and 
mass distributions of the aircraft, as well as the large scale fracture and fragmentation of the 
aircraft components. No such model was available at the beginning of the study. Associated 
with this task was the collection of information on the structure of the Boeing 767 aircraft 
from documentary aircraft structural information and data from measurements on a Boeing 
aircraft. 



• 



The towers and aircraft included a variety of materials that exhibited highly nonlinear, rate- 
dependent behavior with failure that need to be included in the models. Also, the various 
joints and connections (bolts and weldments) in the tower and aircraft structures presented 
complex behavior and failure. The constitutive behavior of these materials and connections 
was included in the models based on testing of tower steels or from data available in the open 
literature. 



^ J. Skilling in 1993 from James Glanz and Eric Lipton, "Qty in the Sky,'" Times Books, 2003. 

L.E. Robertson in 2001 from "The Tower Builder" by John Seabrook, The New Yorker, November 19, 2001. 



NISTNCSTAR 1-2, WTC Investigation 



Chapter 1 



• The WTC towers and Boeing 767 aircraft were large and complex structural systems. To 
include all of the primary structural components and details of both the aircraft and towers 
using refined finite element meshes in the impact models was prohibitive. As a result, 
coarser meshes were used in the impact simulations. That presented a challenge, since a very 
fine mesh was needed to properly capture the failure and fracture of components in these 
analyses. A large array of impact simulations at the component level were conducted to 
calibrate the failure and fragmentation of coarsely meshed aircraft and tower components 
against those models with fine meshes. 

• A significant portion of the weight of a Boeing 767 wing was from the fiiel in its integral fuel 
tanks. Upon impact, this fuel was responsible for large distributed loads on the exterior 
columns of the WTC towers and subsequently on interior structures, as it was dispersed 
inside the building. Modeling of the fluid- structure interaction is complex, but was deemed 
necessary to predict the extent of damage and the fuel dispersion within the building and to 
help establish the initial conditions for the fire dynamics modeling. A number of modeling 
options were investigated for possible application in the global impact simulations. 

• The impact analyses were subject to uncertainties in the input parameters such as initial 
impact conditions, material properties and failure criteria, aircraft mass and stiffness 
properties, connections response, the mass and strength of nonstructural contents, and 
modeling parameters. No information was available to determine a priori the sensitivity of 
the damage estimates to uncertainties in these parameters. Detailed sensitivity analyses using 
orthogonal factorial design were conducted at the component and subassembly levels to 
determine the most infiuential parameters that affect the damage estimates. The results of 
these analyses were used to provide a range of impact-induced damage estimates to the 
towers using the global models. 

The analyses of the aircraft impacts performed for this investigation are believed to be the highest-fidelity 
simulations ever performed for this impact behavior using state-of-the art analysis methodologies. 
Wherever possible, the models were validated against observables or supporting test data developed by 
the WTC investigation. 

In order to estimate the aircraft impact damage to the WTC towers, the following steps were undertaken: 

• Constitutive relationships were developed to describe the actual behavior and failure of the 
materials under the dynamic impact conditions of the aircraft. These materials included the 
various grades of steels used in the exterior walls, core columns, and fioor trusses of the 
towers, weldment metal, bolts, reinforced concrete, aircraft materials, and nonstructural 
contents. 

• Global impact models were developed for the towers and aircraft: The tower models 
included the primary structural components of the towers in the impact zone, including 
exterior walls, fioor systems, core columns, and connections, along with nonstructural 
building contents. A refmed finite element mesh was used for the areas in the path of the 
aircraft, and a coarser mesh was used elsewhere. The aircraft model included the aircraft 
engines, wings, fuselage, empennage, and landing gear, as well as nonstructural components 



NIST NCSTAR 1-2, WTC Investigation 



Introduction 



of the aircraft. The aircraft model also included a representation of the fuel using the smooth 
particle hydrodynamics approach. 

• Component and subassembly impact analyses were conducted to support the development of 
the global impact models: The primary objectives of these analyses were to (1) develop an 
understanding of the interactive failure phenomenon of the aircraft and tower components, 
and (2) develop the simulation techniques required for the global analysis of the aircraft 
impacts into the WTC towers, including variations in mesh density and numerical tools for 
modeling fluid-structure interaction for fuel impact and dispersion. The component and 
subassembly analyses were used to determine model simplifications for reducing the overall 
model size while maintaining fidelity in the global analyses. 

• Initial conditions were estimated for the impact of the aircraft into the WTC towers: These 
included the aircraft speed at impact, aircraft orientation and trajectory, and impact location 
of the aircraft nose. The estimates also included the uncertainties associated with these 
parameters. This step utilized the videos and photographs that captured the impact event and 
subsequent damage to the exterior of the towers. 

• Sensitivity analyses were conducted at the component and subassembly levels to assess the 
effect of uncertainties on the level of damage to the towers due to impact and to determine the 
most influential parameters that affect the damage estimates. The analyses were used to 
reduce the number of parameters that would be varied in the global impact simulations. 

• Analyses of aircraft impact into WTC 1 and WTC 2 were conducted using the global tower 
and aircraft models: The analysis results included the estimation of the structural damage 
that degraded the towers' strength and the condition and position of nonstructural contents 
such as partitions, workstations, aircraft fuel, and other debris that influenced the behavior of 
the subsequent fires in the towers. The global analyses included, for each tower, a "base 
case" based on reasonable initial estimates of all input parameters. They also provided a 
range of damage estimates based on variations of the most influential parameters. 

• Approximate analyses were conducted to provide guidance to the global finite element 
impact analyses: These included: (1) the analysis of the overall aircraft impact forces and 
assessment of the relative importance of the airframe strength and weight distribution, (2) the 
evaluation of the potential effects of the energy in the rotating engine components on the 
calculated engine impact response, (3) the influence of the static preloads in the towers on the 
calculated impact damage and residual strength predictions, and (4) the analysis of the load 
characteristics required to damage core columns compared to the potential loading from 
impact of aircraft components. 

The tasks outlined above were conducted in collaboration with experts from Applied Research 
Associates, Inc. under contract to NIST. Chapters 5 through 7 provide a summary of this study. For 
further details, the reader is referred to NIST NCSTAR 1-2B. 

Chapter 5 describes the global tower and aircraft impact models. The chapter provides the methodology 
used in the development for the models and the contents of the models, including geometry, element types 
and sizes, and boundary conditions. The chapter also includes a summary of the constitutive relationships 



NIST NCSTAR 1-2, WTC Investigation 



Chapter 1 



for the various materials used in the tower and aircraft models. Finally, the chapter provides a brief 
description of the components and subassembly models that were used to support and provide guidance to 
the development of the global models. 

Chapter 6 presents the methodology used to estimate the initial aircraft impact conditions. These 
included, for each aircraft, the impact speed, horizontal and vertical angles of incidence, roll angle, and 
impact location of the aircraft nose. Uncertainties in each of these parameters were also quantified. The 
estimates were based on videos that captured the approach of the impacting aircraft and photographs of 
the damage to the exterior walls of the towers. 

Chapter 7 presents the results of the global analyses of aircraft impact into WTC 1 and WTC 2 using the 
global tower and aircraft models. The global analyses included, for each tower, a "base case" based on 
reasonable initial estimates of all input parameters. They also provided a range of damage estimates of 
the towers due to aircraft impact. The chapter also provides a comparison between the simulation results 
and observables obtained from video and photographic evidence and eyewitness interviews, and a 
comparison of damage estimates from this study with those from prior studies. 



NIST NCSTAR 1-2, WTC Investigation 



Chapter 2 

Development of Reference Structural Models 



2.1 INTRODUCTION 

This chapter outlines the development of the reference structural models of the World Trade Center 
(WTC) towers. The models were used (1) to establish the baseline performance of the towers under 
design gravity and wind loads, and (2) as a reference for more detailed models used in other phases of the 
National Institute of Standards and Technology (NIST) Investigation, including the aircraft impact 
analysis and the thermal-structural response and collapse initiation analysis. The reference models were 
developed to capture the intended behavior of the WTC towers under design loading conditions and 
included the following: 

• A global model of the primary structural components and systems for each of the two towers. 

• A model of a typical truss-framed floor (tenant floor) and a model of a typical beam-framed 
floor (mechanical level) within the impact and fire regions. 

For the global models of the towers, the large amount of data required to construct the models dictated 
that a database of the primary structural components of the towers be developed from the original 
computer printouts of the structural design documents. The various databases, developed in Microsoft 
Excel format, were linked together using the relational database technique. The relational databases, 
developed using Microsoft Access, were generated in a format suitable for the development of the global 
finite element models of the towers. 

For the floor models, typical truss-framed floors existed on tenant floors such as floors 10 to 24, 26 to 40, 
50 to 58, 60 to 66, 68 to 74, 84 to 91, and 93 to 105 of WTC 1; and floors 14 to 24, 26 to 40, 50 to 58, 60 
to 74, 84 to 91, and 93 to 106 of WTC 2. Typical beam-framed floors existed on mechanical floors 
(floors 7, 41, 75, and 108) and near mechanical floors (floors 9, 43, 77, 107, 110, and roof) of both 
towers. 

Included in this chapter are descriptions of the reference structural models of the WTC towers, including 
the global and typical floor models. These models were linearly elastic and three-dimensional, and were 
developed using the Computers and Structures, Inc. SAP2000 Software (SAP2000 2002), Version 8. 
SAP2000 is a finite element software package that is customarily used for the analysis and design of 
building structures. 

This chapter describes the work conducted by Leslie E. Robertson Associates (LERA), the firm 
responsible for the original structural engineering of the WTC towers under contract to NIST, for the 
development of the structural databases and reference structural models. This chapter also summarizes 
the review process for the structural databases and reference models, including the third-party review by 
the firm of Skidmore, Owings, and Merrill (SOM) under contract to NIST and the in-house review by 
NIST. 



NISTNCSTAR 1-2, WTC Investigation 



Chapter 2 



Section 2.2 presents an overview of the structural database development and contents. Section 2.3 
describes the global models of the WTC 1 and WTC 2 towers. Sections 2.4 and 2.5 present the models 
for the typical truss-framed floor and beam-framed floor, respectively. Section 2.6 outlines the third- 
party review by SOM and the in-house review by NIST of the structural databases and reference models. 
Section 2.7 presents a summary of the chapter. 

2.2 DEVELOPMENT OF STRUCTURAL DATABASES 

The original structural drawings of the WTC towers were issued in two main formats: (1) large-size 
drawing sheets containing plan and elevation information, and (2) smaller book-sized drawings 
containing details and tabulated information of cross sectional dimensions and material properties. The 
large-size drawings referred to the structural drawing books in their notes, sections, and details. The 
structural databases, developed in Microsoft Excel file format, were generated from these drawing books 
and included modifications made after construction. The databases were generated for use in the 
development of the reference global models of the towers. 

The structural databases primarily contained the computer and hand-tabulated data for the major 
structural components of the towers from the following drawing books: 

• Drawing Book 1: exterior wall information, foundation to elevation 363 ft. 

• Drawing Book 2: exterior wall information, elevation 363 ft to fioor 9. 

• Drawing Book 3: core column information. 

• Drawing Book 4: exterior wall information, fioor 9 to fioor 110. 

• Drawing Book 5: beam schedule. 

Some additional information from Drawing Book 6 (core bracing schedule) and Drawing Book 9 (beams 
in the hat truss region) were included in the database files as it was utilized in the modehng of the towers. 
Modifications made after construction that were implemented in the structural databases included: 

• Strengthening of a number of core columns: This included core columns 501, 508, 703, 803, 
904, 1002, 1006, and 1007 from fioors 98 to 106 in both towers. These columns were 
reinforced using steel plates welded to the wide fiange core columns. 

• Reinforcing of two corner core columns (508 and 1008) at fioors 45 to 97 of WTC 2 due to 
the construction of a concrete vault at fioor 97. The reinforcement consisted of plates welded 
to the fianges of the built-up box columns (fioors 45 to 83) and the fianges of the rolled shape 
columns (floors 83 to 97). 

The tasks that were undertaken to develop the structural databases included: (1) scanning and digitization 
of the original drawing books, (2) a four-step quality control procedure, (3) cross section property 
calculations, and (4) the development of the relational databases, using Microsoft Access, to link the 
generated database files into a format suitable for the development of the structural global models. 



10 NIST NCSTAR 1 -2, WTC Investigation 



Development of Reference Structural Models 



For further details on the development of the structural databases, refer to Chapter 2 of NIST 
NCSTAR 1-2A. 

2.3 GLOBAL MODELS OF THE TOWERS 

Three-dimensional models of the 1 10-story above-grade structure and 6-story below-grade structure 
within the footprint of each of the two towers were developed. The global models for the towers 
consisted of all primary structural elements in the towers, including exterior walls (exterior columns, 
spandrel beams, and bracings in the basement floors), core columns, hat trusses, and rigid and flexible 
diaphragms representing the floor systems. 

For the development of the global models, each tower was divided into several sub-models that included: 

• Exterior walls, which in turn was divided into 

- Exterior wall, foundation to floor 4 

- Exterior wall trees (floors 4 to 9) 

- Exterior wall, floors 9 to 106 

- Exterior wall, floors 107 to 110 

• Core columns 

• Hat truss 

After these sub-models were assembled into a unified model, rigid and flexible diaphragms representing 
the floor systems, boundary conditions, gravity and wind loads, and masses were added to the unified 
model. 

The development of the WTC 1 and WTC 2 models were separate and consecutive endeavors. The 
lessons learned in the assembly of the WTC 1 model were applied to the development of the WTC 2 
model. While there were only minor differences in the basic structural systems of the two towers, there 
were significant differences in section and material properties, and additional column transfers at the 
basement levels in WTC 2 to create openings for the PATH subway line. 

Isometric views of the complete WTC 1 model, with exterior walls, core columns, bracings, hat trusses, 
and flexible floor diaphragms, are shown in Fig. 2-1. Elevations of the complete WTC 2 model showing 
similar systems are shown in Fig. 2-2. A summary of the size of the global models of WTC 1 and 
WTC 2 is presented in Table 2-1. The following presents the details of each of the sub-models used in 
the development of the unified global models for WTC 1 and WTC 2. 



NIST NCSTAR 1 -2, WTC Investigation 1 1 



Chapter 2 





Figure 2-1. Rendered isometric views of the WTC 1 global model. 



12 



NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 




Maiiiais!iii@iaBi; 



(a) 



4^ 



(b) 



Figure 2-2. Frame view of the WTC 2 model: (a) exterior wall elevation, and (b) interior 

section. 



NISTNCSTAR 1-2, WTC Investigation 



13 



Chapter 2 



Table 2-1. Approximate size of the reference structural models (rounded). 



Model 


Number of 
Joints 


Degrees of 
Freedom 


Number of 
Frame Elements 


Number of 
Shell Elements 


Total Number 
of Elements 


WTC 1 global moder 


53,700 


218,700 


73,900 


10,000 


83,900 


WTC 2 global model' 


51,200 


200,000 


73,700 


4,800 


78,500 


Typical truss-framed model 


28,100 


166,000 


27,700 


14,800 


42,500 


Typical beam-framed model 


6,500 


35,700 


7,500 


4,600 


12,100 



a. Model does not include floors except for flexible diaphragms at 17 floors as explained later. 



2.3.1 



Exterior Wall Modeling 



The exterior walls of the WTC towers were intended to resist approximately 50 percent of the gravity 
loads and all of the lateral loads (primarily wind loads) on the towers. While the exterior wall between 
floors 9 to 106 represented repetitive typical panels, significant variations existed at the lower floors and 
the upper portion of the walls. The exterior wall columns from the foundation level up to elevation 363 ft 
were spaced 10 ft in. on center. There were bracings in the plane of the exterior wall between the 
concourse level and the foundation. Between elevation 363 ft and floor 7, the single exterior wall 
columns spaced 10 ft in. on center transitioned to three columns spaced at 3 ft 4 in. on center. The 
exterior wall columns above floor 7, that were spaced 3 ft 4 in. on center, were connected to each other by 
spandrel plates, typically 52 in. deep. The exterior columns and spandrels were pre-assembled into 
exterior wall panels, typically three-columns wide by three-stories high. 

The exterior wall model for WTC 1 and WTC 2 consisted of prismatic and non-prismatic beam elements 
representing columns, spandrels, and bracings. The following describes the various parts of the exterior 
wall model. 

Foundation to Floor 4 

The sub-model of the exterior wall from the foundation level up to elevation 363 ft was developed using 
frame elements (also referred to as beam elements). Frame elements are typically used to model beams, 
columns, and truss members in planar and three-dimensional structures. They are modeled as straight 
lines connecting two nodes with six degrees of freedom (three translations and three rotations) at each 
node. The model was developed in a conventional manner, assigning joints and member connectivity as 
shown in the original WTC drawings. Below elevation 363 ft, columns were typically spaced at 10 ft and 
braced with spandrels and diagonals. Joints were defined at all locations where diagonals braced the 
columns. When coordinates were not given in the drawings, joint coordinates were determined based on 
the geometry of the diagonal. Structural details showed that the column-diagonal intersections were 
continuous. 

Spandrel centerline elevations were generally used to define joint coordinates. The SAP2000 program 
allows assignment of rigid zone factors to frame end offsets to account for the overlap of cross sections. 
At the intersection of columns and spandrels, 100 percent rigidity for the column and the spandrels were 
assigned due to the large size of both columns and spandrels. Figure 2-3 shows a frame and a rendered 
view of the exterior wall from foundation to floor 9 of the WTC 1 model. The figure also shows the core 
columns and core bracings. 



14 



NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 




i 



ra 



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klillJIIil kJUkllilJ 
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Figure 2-3. Frame view and rendered view of the WTC 1 model (foundation to floor 9). 

Exterior Wall Trees (Floor 4 to 9) 

The panels of the exterior walls between elevation 363 ft and elevation 418 ft 1 1 1/2 in. were called 
exterior wall trees. At the exterior wall trees, the typical exterior wall columns transitioned from a 
spacing of 10 ft to a spacing of 3 ft 4 in. A typical exterior wall tree panel was divided into five levels; 
level B, C, D, E, and F as shown in Fig. 2-4. For each panel in the model, the three exterior columns 
from above elevation 418 ft 1 1 1/2 in. continued down to level D. At that level, the three columns were 
connected by a horizontal rigid element to become one member, which extended down to elevation 363 ft. 

Both prismatic and non-prismatic frame elements were used to model the exterior wall trees. Non- 
prismatic elements were used to accurately model the tapering columns as well as the complex geometry 
of the tress at the transition from three columns to a single column. For further details on the modehng 
details of the exterior wall trees, refer to Chapter 3 of NIST NCSTAR 1-2A. The final model of a typical 
tree is illustrated in Fig. 2-5. 



NIST NCSTAR 1-2, WTC Investigation 



15 



Chapter 2 



WORTHINGTON, SKiLLING. HILLE & JACKSON 



Civil & Structural Enginaen 



r^e WORLD TRADE CENTER 



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byNIST. 

Figure 2-4. Exterior wall tree panel (taken from Drawing Book 2, page 2-AB2-2). 



16 



NISI NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 



Figure 2-5. Frame and rendered view of an exterior wall tree. 

Floor 9 to 106 

The typical exterior panels were modeled using frame elements representing columns and spandrels. In 
plan, the columns and spandrels were joined at nodes located at the outside face of the spandrel, 6 1/2 in. 
from the exterior column reference line (Fig. 2-6). Thus the columns were offset horizontally, or 
'inserted' at this node, using an insertion point located at the centerline of the interior plate 3 as shown in 
the figure. Insertion points were not adjusted for spandrel thickness. In elevation, the columns and 
spandrels were joined at the spandrel centerline, located typically 12 1/2 in. below the reference floor 
elevation (Fig. 2-6). The spandrels were then located correctly without the need for offsets to be defined. 

As Fig. 2-6 indicates, nodes at five elevations were defined for a typical exterior wall panel. These 
included nodes at the three representative floor levels (defined at the spandrel centerlines), as well as the 
upper and lower column splices. 



NISTNCSTAR 1-2, WTC Investigation 



17 



Chapter 2 



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Source: Reproduced with permission of Tlie Port Authority of New York and New Jersey. Enhanced by 
NIST. 

Figure 2-6. Typical WTC tower exterior wall panel. 

To develop a frame model of the exterior panel, a parametric study of typical three-column, three- 
spandrel exterior wall panels was performed using two modeling techniques (see Fig. 2-7). The first 
model was a detailed shell element model of the panel, and the second was a simplified frame element 
model similar to that used throughout the global models. Shell elements are typically used to model the 
plate and membrane behaviors in planar and three-dimensional structures. They can be used in a three- or 
four-node formulation with six degrees of fi"eedom (three translations and three rotations) at each node. 
For the detailed shell element model, each plate of each column and spandrel was explicitly modeled, 
including internal column stiffeners. 

The parametric study assumed that the detailed shell model best represented the as-built performance of 
the panel, and therefore, was used to tune the performance of the simplified frame model. The purposes 
of the parametric study were to (1) match the axial stiffness of the frame model with the detailed shell 



A//ST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 



model under gravity loads and (2) match the inter-story drift of the two models under lateral loads in the 
plane of the panel by modifying the rigidity of the column/spandrel intersections in the simplified frame 
model. 
















































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































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ZL 




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k 



















■■.[2 








■._u 


iU 






^ 



A 



(b) 



(c) 



Figure 2-7. (a), (b) Shell element, and (c) frame element models of a typical exterior 

wall panel. 

For comparing the axial stiffness of the simplified frame model of the panel with the detailed shell model, 
both models were loaded vertically by applying identical gravity loads to the three columns. The two 
models were simply supported at the bottom of the columns. The results indicated that the shell model 
was stiffer than the equivalent frame element model due to the contribution of the spandrel beams to the 
axial stiffness of the panel. This is due to the rigidity of the spandrel beams and the proximity between 
the columns. The parametric study on a wide range of panels over the height of the towers showed that 
the axial stiffness of the columns in the bottom third of the towers should be increased by a factor in the 
range of 25 percent to 28 percent, and the columns in the middle and upper thirds of towers should be 
increased by a factor in the range of 20 percent to 28 percent. Based on these figures, a 25 percent 
increase in the axial stiffness of exterior columns was selected as a reasonable representation for the panel 
vertical stiffness between floors 9 and 106. This was achieved using a frame property multiplier of 1.25 
for the cross-sectional area of the exterior wall columns. 



NISTNCSTAR 1-2, WTC Investigation 



19 



Chapter 2 



For studying the lateral deformation of the exterior panels, panel properties were taken from three 
different areas of the building at floors 79 to 82, 53 to 56, and 23 to 26. The deformations at points A, B, 
I, and II (Fig. 2-8) were studied for the three different panels. The top most columns were connected via 
a rigid link and loaded in the plane of the panel and perpendicular to the columns with a lateral load of 
100 kip. The boundary conditions included roller supports at the spandrel ends and pin supports at the 
bottom of the columns as seen in Fig. 2-7. 























A 


• 


_, 


- 




• \ 




i • 






1 : 

B 


• 


ill 
























Figure 2-8. Selection of column and spandrel rigidity of typical exterior wall panel. 

The lateral displacements calculated for the detailed shell and simplified frame models of typical exterior 
wall panels with varied column and spandrel intersection rigidities are shown in Table 2-2. The table 
indicates that using a column rigidity of 50 percent and a spandrel rigidity of 100 percent in the frame 
model produced deflection results consistent with the shell model. This was achieved in the global 
models by assigning 50 percent rigidity for the columns and 100 percent rigidity for the spandrels at the 
column-spandrel intersection. 

A similar study was conducted for the corner exterior panels to develop a simplified frame model that 
matched the behavior of a detailed shell element model of the corner panels. These models are shown in 
Fig. 2-9. For details of the study, refer to NIST NCSTAR 1-2A. Based on the results of this parametric 
study, 25 percent rigidity for the columns and 50 percent rigidity for the spandrels were assigned to the 
exterior wall corner panels. Also, an area modifier was used to provide a 25 percent increase in the axial 
stiffness of the two continuous columns of the corner panels. No modifier was used for the intermittent 
columns at the corners. 



20 



NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 



Table 2-2. Lateral displacement (in.) for the shell and frame models of typical 
exterior wall panel with varied column and spandrel rigidities. 





Lateral displacement (in) 


Floor 79-82 


Shell model 


Frame model (Rigidity) | 


No rigidity 


C:50%, S:100% 


C:100%, S:100% 


A 


0.60 


1.04 


0.59 


0.35 


B 


0.28 


0.52 


0.29 


0.18 


1 


0.45 


0.78 


0.44 


0.26 


II 


0.45 


0.78 


0.44 


0.26 




Floor 53-56 | 


Shell model 


Frame model (Rigidity) | 


No rigidity 


C:50%, S:100% 


C:100%, S:100% 


A 


0.26 


0.43 


0.27 


0.18 


B 


0.12 


0.22 


0.14 


0.11 


1 


0.19 


0.32 


0.2 


0.15 


II 


0.19 


0.32 


0.2 


0.15 




Floor 23-26 | 


Shell model 


Frame model (Rigidity) | 


No rigidity 


C:50%, S:100% 


C:100%, S:100% 


A 


0.21 


0.37 


0.21 


0.12 


B 


0.1 


0.18 


0.1 


0.06 


1 


0.16 


0.28 


0.16 


0.09 


II 


0.16 


0.28 


0.16 


0.09 





Figure 2-9. Shell element and frame models of typical exterior wall corner panel. 

Floor 107 to 110 

The exterior wall members from floors 107 to 110 were typically rolled shapes with a yield strength of 
42 ksi or 50 ksi (where not shown in the drawings as 50 ksi, a yield strength of 42 ksi was used). Frame 
elements were used to model the columns and spandrels of the exterior walls at these floors. Spandrel 
depths varied at floors 108 and 110. A weighted average of spandrel depth was calculated in order to 



NISTNCSTAR 1-2, WTC Investigation 



21 



Chapter 2 



define the average centerline elevation of the spandrels and, therefore, the node elevation for the entire 
floor. 

2.3.2 Core Columns Modeling 

The core columns were typically built-up box members at the lower floors and transitioned into rolled 
structural steel shapes at the upper floors for both towers. Core columns were modeled as frame elements 
spanning from node to node, defined at the representative fioor elevations (centerline of spandrels). 
Splices in core columns occurred typically 3 ft above the floor level. In the models, however, the splice 
was considered to occur at the floor level, and nodes were only defined at these levels. Most three-story 
column pieces were unique. In the model, a section for each three-story piece was defined and assigned 
to each of the three frame members that represented that column. 

Core column coordinates were tabulated based on the structural drawings. Column locations were 
typically referenced at their centerlines. However, columns on lines 500 and 1000 were located in plan 
drawings along most of their height according to the face of the column into which the fioor trusses 
connected (i.e., WTC 1 north face for 500 series columns and south face for 1000 series columns). The 
centerline of these columns was based on their dimensions given in the drawing books. Where these 
column centerlines varied along the height of the towers (typically 1 1/2 in. between three-story pieces), a 
representative location was chosen to define the column node. Thus, the column coordinate at floor 106 
was used as a constant along the tower height because at this level, these columns aligned with the hat 
truss above. 

2.3.3 Hat Truss Modeling 

In both WTC 1 and WTC 2, a truss system referred to as a 'hat truss' was constructed between fioor 107 
and the roof The hat truss system was intended to support the load of the antenna on top of the tower and 
to interconnect the exterior walls to the core. Four trusses spanning perpendicularly to the long direction 
of the core and four trusses spanning perpendicularly to the short direction of the core were constructed 
atop the towers (refer to Figs. 2-10 and 2-1 1). The wide fiange core columns represented the vertical 
members of the hat trusses. The diagonals were primarily wide fiange rolled sections, with the exception 
of the end diagonals between the core and the exterior walls, which were built-up box sections. The 
majority of the horizontal members in the hat truss system were wide fiange and built-up box section 
fioor beams. 

Members of the hat truss were modeled using frame elements. These frame elements between floors 107 
and 110 were assigned to the model according to plan and elevation drawings of the hat truss. Node 
locations were set to coincide with the centerline of spandrels at the exterior wall. All columns and 
diagonals shown in the drawings were included in the model. Floor beams that did not participate in the 
hat truss system were not included in the model, unless they were used to transfer truss chords to the core 
columns. Flexible floor diaphragms (see Section 2.3.4) were used to represent the floors within the hat 
truss area. 

In general, diagonals and columns of the hat truss were assumed to be non-composite, and floor beams 
were assumed to be composite. Hat truss diagonals, main chords, and main columns were modeled with 
continuous joints. However, hat truss beams had pinned ends. 



22 NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 




Figure 2-10. As-modeled plan of the WTC 1 hat truss. 




Figure 2-11. Rendered 3-D model of the WTC 1 hat truss. 



NISTNCSTAR 1-2, WTC Investigation 



23 



Chapter 2 



2.3.4 Flexible and Rigid Floor Diaphragm Modeling 

For floors with high in-plane stiffness, a rigid diaphragm constraint causes all of its constrained joints to 
move together as a planar diaphragm that is rigid against in-plane deformation. This is customarily done 
in practice for lateral force analyses to reduce the size of the building models. For most floors of the 
WTC towers, this constraint provided for a sufficiently accurate representation of the flow of forces and 
deformations for global structural response. Where the flow of forces and deformations would be 
significantly affected by the use of rigid diaphragms in the global models, the floors were modeled as 
flexible diaphragms. 

Flexible diaphragms were used at the floors of the towers in the core of the atrium area, in the mechanical 
floors, and in the floors of the hat trusses. The floors modeled using flexible diaphragms included: 

• Atrium levels: floors 3, 4, 5, 6, 7, and 9. 

• Mechanical levels: floors 41, 42, 43, 75, 76, and 77. 

• Hat truss levels: floors 107, 108, 109, 110, and roof 

The flexible floor diaphragms consisted of equivalent shell element floor models attached to all exterior 
wall columns and core columns. 

The floor models developed as described in Sections 2.4 and 2.5 (see Fig. 2-12) were used to develop the 
flexible diaphragm stiffness used within the global models. For that purpose, parametric studies were 
conducted to compare the diaphragm stiffness of the two different floor models for both the typical truss- 
framed floor and the beam-framed floor. The simplified floor models duplicated the representation of the 
exterior wall columns and spandrels, core columns, and their boundary conditions. The difference 
between the detailed and simplified models was that the floor framing for the simphfied models, both 
inside and outside the core, was replaced by a course mesh of shell elements (see Fig. 2-13). The 
material properties of the simplified shell model matched the properties of the concrete floor outside the 
core in the respective floor model. 

The detailed and simplified floor models were loaded in the plane of the floors with a lateral load of 180 
lb/ft on both the windward and leeward faces. The column base supports were released for the exterior 
wall columns along the loaded faces and for all core columns to allow lateral translation only in the 
direction of loading. The horizontal deflections of both floor models were calculated on both the 
windward and leeward sides of the model. Both the total horizontal deflection of the slab and the relative 
displacement between the windward and leeward sides were compared between the models. The shell 
thickness of the simplified model was modified to match the in-plane stiffness determined by the detailed 
floor models. 

The deformations from the lateral load case using the 75th floor model of WTC 2 are illustrated in 
Fig. 2-12, while Fig. 2-13 shows the deformations of the simplified floor model. Figure 2-14 shows the 
lateral deflection of the north and south sides of the floor model under lateral load applied in the north 
direction using the detailed and equivalent floor models. 



24 NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 




Figure 2-12. Deflection of typical beam-framed floor model due to lateral loading 

(exaggerated scale). 



kiM.la t t f .lu li j .l i J nl ■) n l II I I II iiJ i .i I I I i.i ij III il l iJ I J ii l. ' il I J ii t»* i il . Uui« l 4yMj*» ) ri i * .l i«. l liM i 




Figure 2-13. Deflection of equivalent floor model due to lateral loading 

(exaggerated scale). 



NISTNCSTAR 1-2, WTC Investigation 



25 



Chapter 2 




0.003 



East Corner 



West Corner 



Figure 2-14. Deflections of the north and south faces of the floor for the detailed and 

equivalent floor models. 

2.3.5 Boundary Conditions 

The global models of the WTC 1 and WTC 2 towers were pin-supported at the bottom of the models, 
i.e., all translations were restrained and all rotations were permitted at the foundation level. No restraints 
were provided to account for the effect of floors at the basement levels outside the footprint of the towers. 



2.3.6 



Results of Modal Analysis 



A modal analysis was conducted to estimate the natural frequencies of the WTC 1 and WTC 2 towers. 
The mass of the towers was estimated from the construction and superimposed dead loads only (see NIST 
NCSTAR 1-2A for further details). No live loads were used in estimating the floor masses for the modal 
analysis. The calculated first six periods and frequencies for WTC 1 and WTC 2 are presented in 
Table 2-3 without P-A effects and in Table 2-4 with P-A effects. P-A effects refer to secondary effects of 
column axial loads (designated P) and lateral deflection (designated A) on the moments in members, and 
hence the term P-A. Results of modal analysis without P-A effects are relevant for small-amplitude 
vibrations, while those with P-A effects are relevant for large-amplitude vibrations. As expected, the 
natural periods estimated with the P-A effects were longer than those without the P-A effects. The mode 
shapes can be found in Chapter 3 of NIST NCSTAR 1-2A. 



26 



NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 



Table 2-3. Calculated first six periods and frequencies without P-A effects. 



Direction 

of 
Motion 


WTC 1 


WTC 2 


Mode 


Frequency 
(Hz) 


Period 

(s) 


Mode 


Frequency 
(Hz) 


Period 

(s) 


N-S 


1 


0.088 


11.4 


2 


0.093 


10.7 


E-W 


2 


0.093 


10.7 


1 


0.088 


11.4 


Torsion 


3 


0.192 


5.2 


3 


0.192 


5.2 


N-S 


4 


0.233 


4.3 


5 


0.263 


3.8 


E-W 


5 


0.263 


3.8 


4 


0.238 


4.2 


Torsion 


6 


0.417 


2.4 


6 


0.417 


2.4 



Table 2-4 


. Calculated first six periods and frequencies with P-A effects. 


Direction 

of 
Motion 


WTC 1 


WTC 2 


Mode 


Frequency 
(Hz) 


Period 

(s) 


Mode 


Frequency 
(Hz) 


Period 

(s) 


N-S 


1 


0.083 


12.1 


2 


0.089 


11.2 


E-W 


2 


0.088 


11.3 


1 


0.083 


12.1 


Torsion 


3 


0.189 


5.3 


3 


0.192 


5.2 


N-S 


4 


0.227 


4.4 


5 


0.250 


4 


E-W 


5 


0.250 


4 


4 


0.227 


4.4 


Torsion 


8 


0.455 


2.2 


8 


0.455 


2.2 



Table 2-5 presents a comparison of the calculated first three natural frequencies and periods (N-S 
direction, E-W direction, and torsion) against measured frequencies and periods for WTC 1 . These 
measurements were based on analyzing acceleration records obtained from accelerometers installed atop 
WTC 1. The measurements were taken during the period from 1978 through 1994 for wind speeds 
ranging from 11.5 mph to 41 mph. As the table indicates, the trend is for longer periods measured at 
larger wind speeds. The table also includes the values of the natural periods and frequencies predicted in 
the original design. The table shows good agreement between the calculated and measured periods, 
especially for the periods calculated without P-A effects (small amplitude vibrations). Thus, Table 2-5 
indicates that the reference global model provided a reasonable representation of the actual structure. 



NISTNCSTAR 1-2, WTC Investigation 



27 



Chapter 2 



Table 2-5. Comparison of measured and calculated natural frequencies and periods for 

WTCl. 



Data Source/ 
Event Date 


Wind Speed & 
Direction 


Frequency (HZ) 


Period (s) 


Direction of Motion 


Direction of Motion 


N-S 


E-W 


Torsion 


N-S 


E-W Torsion 


Historical Data 


October 11, 1978 


11.5mph, E/SE 


0.098 


0.105 


0.211 


10.2 


9.5 


4.7 


January 24, 1979 


33 mph, E/SE 


0.089 


0.093 


0.203 


11.2 


10.8 


4.9 


March 21, 1980 


41 mph, E/SE 


0.085 


0.092 


0.201 


11.8 


10.9 


5.0 


December 11, 1992 


- 


0.087 


0.092 


- 


11.5 


10.9 


- 




February 2, 1993^ 


20 mph, NW 


0.085 


0.093 


0.204 


11.8 


10.8 


4.9 


March 13, 1993^ 


32 mph, NW 


0.085 


0.094 


0.199 


11.8 


10.6 


5.0 


March 10, 1994^ 


14 mph, W 


0.094 


0.094 


0.196 


10.6 


10.6 


5.1 


December 25, 1994^ 


N 


0.081 


0.091 


- 


12.3 


11.0 


- 


Average of Measured Data 


Average | - | 0.088 | 0.094 | 0.202 | 11.4 | 10.6 | 4.9 


Orginal Design - Predicted Values 


Theoretical Value | - | 0.084 | 0.096 | - | 11.9 | 10.4 | 


Reference Global Model 


LERA/NIST-WTC 1 
without P-Delta 




0.088 


0.093 


0.192 


11.4 


10.7 


5.2 


LERA/NIST-WTC 1 
with P-Delta 




0.083 


0.088 


0.189 


12.1 


11.3 


5.3 



Notes: 

^Reported frequency value is the average of the SW corner, NE corner, and center core frequency measurements. 

^Reported frequency is based on center core data only. 

The period of oscillation in the N-S direction of WTC 2 was estimated immediately after aircraft impact 
based on a detailed analysis of the building motion, which was captured in video footage (Figure E-6 of 
Appendix E) of the WTC 2 impact (see NIST NCSTAR 1-5 A). A frequency analysis of the displacement 
of the tower at the 70th floor, shown in Fig. 2-15, resulted in a fundamental mode in the N-S direction 
with a period of approximately 1 1 .4 s, a torsional mode with a period of 5.3 s, and two higher 
translational modes with periods of 3.9 and 2.2 s. Periods were accurate to within ±0. 1 s. The measured 
fundamental period of 1 1.4 s ±0.1 s was nearly identical to the calculated period from the model (1 1.2 s 
with P-A effects for large-amplitude vibrations). Also, the measured torsional period and the higher 
translational period were almost identical to the calculated periods from the model with P-A effects (5.2 s 
and 4.0 s for the torsional and higher translational modes, respectively). 

The maximum displacement of the WTC 2 tower at floor 70 was measured to be about 12 in., while the 
maximum sway of the tower at the top was approximately 27 in. (NIST NCSTAR 1-5 A). 



28 



NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 



20 



■^ 15 - 



o 
o 



10 - 



:£ 5 



o 
1^ 



H » «■:■?* 



E 

o 

tn 

b 



■10 



■15 



-20 




Figure 2-15. Displacement of floor 70 of WTC 2 after impact based on video analysis 

(NIST NCSTAR 1-5A). 

The impact of the aircraft into WTC 2 caused the tower to sway back and forth for almost four minutes. 
The estimated period of oscillation was found to be nearly equal to the calculated first mode period of the 
undamaged structure, indicating that the overall lateral stiffness of the tower was not affected appreciably 
by the impact damage. The maximum deflection at the top of the tower was estimated to be more than 
1/3 of the drift resulting from the original design wind loads (about 65 in. in the N-S direction) as 
calculated from the baseline analysis (see Chapter 4). Since the lateral stiffness of the building before and 
after impact was essentially the same, it can be concluded that the additional stresses in the columns due 
to this oscillation were roughly 1/3 of the column stresses resulting from the original design wind loads, 
assuming linear behavior and assuming that the oscillation mode shape and the static deflected shape 
under design wind loads were identical. The building demonstrated an ability to carry this additional load 
and therefore, still had reserve capacity. This was confirmed by the structural analysis of the damaged 
towers reported in NIST NCSTAR 1-6. 



2.4 



TYPICAL TRUSS-FRAMED FLOOR MODEL— FLOOR 96 OF WTC 1 



The majority of the floors of the WTC towers were tenant floors, where the areas outside of the core were 
constructed of steel trusses acting in a composite fashion with concrete slabs cast over metal deck. The 
trusses consisted of double angle top and bottom chords with round bar webs that extended to the 
concrete slab to provide the composite action (shear knuckles). Two trusses were placed at every other 
exterior column line, resulting in a 6 ft 8 in. spacing between truss pairs. The typical floor consisted of 
three truss zones: a long span zone, a short span zone, and a two-way zone as shown in Fig. 2-16. The 
span of the trusses was about 36 ft in the short direction and 60 ft in the long direction. The two-way 
zone included trusses in the long span direction (primary trusses) as well as bridging trusses (secondary 



NIST NCSTAR 1-2, WTC Investigation 



29 



Chapter 2 



trusses) normally found elsewhere. The secondary trusses had additional strength and connectivity to 
enable them to act in tandem with the long spanning trusses to form a two-way spanning truss grid. The 
floor trusses were pre-assembled into floor panels as defined in the contract drawings. The floor panels 
included primary trusses, bridging trusses, deck support angles, metal deck, and strap anchors. A typical 
composite beam and slab construction was used for the floors inside the core. 




Figure 2-16. Typical floor truss framing zones. 

In order to select the typical truss-framed floor within the expanded impact and fire zones of both towers, 
the drawings for floors 80 to 100 were reviewed to identify structural similarities. Appendix G of NIST 
NCSTAR 1-2A provides the details of this study. It was found that floor 96 of WTC 1 (96A) represented 
the typical truss-framed floor in the expanded impact and fire region for WTC 1 and WTC 2. An 
isometric view of the typical truss-framed floor model is shown in Fig. 2-17. Table 2-1 includes a 
summary of the size of the 96A floor model. The floor model consisted primarily of frame elements with 
the exception of the floor slabs, which were modeled using shell elements. The following summarizes the 
major components of the typical truss-framed floor model. 



30 



NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 




Figure 2-17. Typical truss-framed floor model (floor 96 of WTC 1), slab not shown. 



2.4.1 



Primary Trusses 



The primary trusses consisted of double angle top and bottom chords, which were 29 in. out-to-out of the 
chords. The distance between the centroid of the two chords was 28.05 in. For a typical long-span truss, 
C32T1, the top chord consisted of two angles 2 in. by 1.5 in. by 0.25 in. and the bottom chord consisted 
of two angles 3 in. by 2 in. by 0.37 in.; both chords were short legs back-to-back. The top chords acted 
compositely with a 4 in. concrete slab on 1 1/2 in. metal deck. The distance from the centroid of the top 
chord to the neutral axis of the transformed composite slab with top chord was calculated to be 1.93 in. In 
the model, therefore, 30.0 in. (28.05 in. + 1.93 in. =29.98 in.) was assumed as the typical distance 
between the top and bottom chords for both short- and long-span primary trusses, see Fig. 2-18. The 
shell element representing the floor slab was located at the same level as the beam elements representing 
the top chord. 

In the long-span truss zone, the two individual primary trusses, which were part of the same floor panel 
and attached to the same column, were separated, typically by a distance of 7 1/8 in. At the joint between 
panels, the distance between the abutting long-span trusses was 7 1/2 in. Therefore, in the model, 
7 1/2 in. was used as the spacing between all long-span primary trusses. In the short-span truss zone, two 
individual trusses which attached to the same column were separated by a distance that varied between 
4 7/8 in., 5 in., and 5 1/4 in. In the model, the typical spacing between all short-span double trusses was 



NISTNCSTAR 1-2, WTC Investigation 



31 



Chapter 2 



5 in. The long span trusses in the two-way zone had an as-modeled length of 58 ft 10 in., while the long- 
span trusses in the one-way zone had an as-modeled length of 59 ft 8 in. 



C32T1 (Primary Truss Section) 

2" 



Web member extension 
into slab 



2.48" 



L2x 1.5x0.25 
13x2x0.37 



^ 4" Slab 



0.414" 



^/r 



28.05" 



29.98" 

= 30" 



V 




N.A. Combined Slab + 
Primary Truss Double L 



0.537" 



ACTUAL 



(Note: 2"" Truss of Pair Not Shown) 
MODEL 



Figure 2-18. Typical primary truss cross-section, as-designed and as-modeled. 

The diagonal web bars for the primary trusses were most often 1.09 in. diameter bars. Therefore, for 
double angle shapes in the primary trusses, 1.09 in. was taken as the distance between the two angles. 
This holds true for primary trusses where bar diameters varied between 0.92 in. and 1.14 in. The as- 
designed truss diagonals had end fixity, but were considered pinned in the model. Pinning the diagonals 
provided an upper bound of the gravity load stresses. To mitigate the effect of the pinned member 
approach, end length offsets were used for the truss diagonals to account for the difference between the 
as-built and the as-modeled unbraced length of the diagonal. A similar approach was used for the 
diagonals of the bridging trusses. 

In 30 percent of the floor area, truss members were supplemented with cover plates. The members with 
additional plates included top chords, web members, and most typically bottom chords. The primary 
truss top chords were reinforced with an additional set of double angles at truss end connections. At these 
locations, the work points for the section were located at the centroid of the composite double angle and 
concrete slab. Plates 3/8 in. by 3 in. connected the bottom chord of the primary truss pairs together at 
each end and at the intersection with a bridging truss. These plates were included in the model. 



2.4.2 



Bridging Trusses 



The bridging trusses were 24 in. deep, edge-to-edge, with double angle chords. For a typical bridging 
truss, 24T1 1, the top and bottom chords consisted of two angles 1.5 in. by 1.25 in. by 0.23 in. The 
distance between the centroid of the two chords was 23.26 in. The distance used as the offset between the 
top and bottom chords for all bridging trusses was taken as 23.25 in. (Fig. 2-19). The distance between 
the top chord of the bridging truss and the top chord of the primary trusses and equivalent slab plate for 
truss 24T1 1 was calculated to be 3.39 in. and was selected to be 3.375 in. for all bridging trusses. As in 
the as-designed structure, the bridging truss was not connected along its length to the slab shell elements 



32 



NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 



in the model. The intersection of the top chords of the primary and bridging trusses was modeled using 
vertical rigid links, connected in turn to the slab shell elements representing the concrete slab. 

For bridging trusses in the model, a 0.75 in. angle gap was used for trusses with web bar diameters that 
varied between 0.75 in. and 0.98 in. 



24T11 (Bridging Truss Section) 



4" Slab 



L1.5X 1.25x0.23 




/ 



T.O.S. 



1.5" 



5.868" 



0.368" 



23.26" 




N.A. Combined Slab + 
Primary Truss Double L 



= 23.25" 



Rigid Link to Slab at 
Primary Truss and 
Bridging Truss 
Intersection 



0.368" 



ACTUAL 



MODEL 



Figure 2-19. Typical bridging truss cross-section, as-designed and as-modeled. 



2.4.3 



Concrete Slab and Metal Deck 



Outside the core, the primary trusses acted compositely with the 4 in. concrete slab on 1 1/2 in. metal 
deck. In the model, the average depth of the slab plus deck was modeled as 4.35 in. The concrete slab 
consisted of lightweight concrete with a self-weight of 100 pcf and a design compressive strength, 
f'c= 3,000 psi. The concrete modulus of elasticity, Ec, was 1,810 ksi. These values were consistent with 
those included in the WTC Structural Design Criteria Book. In the as-designed structure, composite 
action was achieved by the shear connection provided by the web bar extending above the top chord and 
into the slab (shear knuckle). This composite action was modeled by assuming a rigid connection 
between the concrete slab and the top chord at the intersection with the diagonal (knuckle location). 

Typically, inside the core, the beams acted compositely with a 4 1/2 in. formed concrete slab. The 
concrete slab consisted of normal weight concrete with a self-weight of 150 pcf and a design compressive 
strength, f'c= 3,000 psi. The concrete modulus of elasticity, Ec, was 3,320 ksi. In the as-designed 
structure, composite action was achieved using shear stud connectors between the beam tops and the slab. 
This composite action was modeled by assuming a rigid connection between the concrete slab and the 
floor beams. 

The floors of the WTC towers had an in- floor electrical distribution system of electrified metal deck and 
trench headers. The effects of the in-slab trench headers were included in the model by reducing the slab 
shell element thickness. A 1 ft 8 in. wide shell panel (the typical truss-floor shell mesh size) was reduced 
in thickness from 4.35 in. to 2.35 in. or 1.35 in. at the trench header locations. 



NISTNCSTAR 1-2, WTC Investigation 



33 



Chapter 2 



2.4.4 



Viscoelastic Dampers 



Viscoelastic dampers were used to reduce the wind-induced vibrations and were located where the bottom 
chords of the long span, short span, and bridging trusses intersected the exterior columns. The dampers 
were defined in Drawing Book D. The dampers resisted static and quasi-static loads (such as gravity 
loads) at the time of load application. Immediately following load application, the dampers shed load 
until the stress in the dampers was dissipated. A placeholder element was located in the model at the 
damper location. 



2.4.5 



Strap Anchors 



Exterior columns not supporting a truss or truss pair were anchored to the floor diaphragm by strap 
anchors. These strap anchors were connected to the columns by complete penetration welds. The strap 
anchors were then connected to the slab with shear stud connectors and to the top chords of the trusses by 
fillet welds. The straps were included in the model and located in the plane of the centroid of the 
composite top chord. Also, in the model the work points intersected with the centerline of the column and 
used a rigid link to attach back to the spandrel (see Fig. 2-20). 




Attachment 
PL to Slab 



Attachment 
PL to Slab 



PL = Plate 



Spandrel PL 

Figure 2-20. Strap anchors modeling, slab not shown. 



2.5 



TYPICAL BEAM-FRAMED FLOOR MODEL— FLOOR 75 OF WTC 2 



Beam-framed floors were used for the mechanical floors within the towers. These floors were 
constructed using rolled structural steel shapes. The beam framing for the typical floor system consisted 
of W27 and W16 beams in the long- and short-span regions, respectively. Typical beam spacing was 6 ft 
8 in. The steel beams acted in composite fashion with the normal weight concrete slab on metal deck. 
The deck spanned in the direction of the primary beams and was supported typically at 6 ft 8 in. intervals 
by a 4C5.4 deck support channel. A 2 in. concrete topping slab was placed on top of the structural slab. 



34 



NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 



The core area was framed similarly to the core of the truss-framed floors, but the steel beams were 
typically larger, and the concrete slab was 6 in. deep. 

As described in Section 2.4 for truss-framed floors, the structural drawings were reviewed to identify 
structural similarities between the beam-framed floors within the expanded impact and fire zones of both 
towers (see Appendix G of NIST NCSTAR 1-2A). It was found that floor 75 of WTC 2 (75B) 
represented the typical beam-framed floor in the expanded impact zone for WTC 2 (floors 74B to 88B). 
There were no beam- framed floors within the expanded impact zone of WTC 1. An isometric view of the 
typical beam-framed floor model is presented in Fig. 2-21. Table 2-1 includes a summary of the size of 
the 75B floor model. The following presents the major structural systems and components of the beam- 
framed floor model. 




Figure 2-21. Typical beam-framed floor model (floor 75 of WTC 2). 



2.5.1 



Composite Beams 



The beams in the model were located at the elevation of the centerline of the concrete slab. The insertion 
point for the beams was set at the beam top flange, and then the beam was offset down by one-half the 
thickness of the slab. The beam was rigidly linked with the slab to simulate the composite action. This 
option provided for accurate estimation of the composite stiffness of the floor. 



2.5.2 



Horizontal Trusses 



Exterior columns that did not support a beam were connected to the floor for bracing purposes by 
horizontal trusses. These exterior horizontal trusses were anchored to the columns with complete joint 
penetration welds. The horizontal trusses were then connected with shear stud connectors to the slab. 



NIST NCSTAR 1-2, WTC Investigation 



35 



Chapter 2 



The truss angles (typically 4 in. by 4 in. by 5/16 in.) were then connected to the top flange of the beams. 
In the model, the work points intersected with the centerline of the column and used a rigid link to attach 
back to the spandrel. The truss members were located in the plane of the centroid of the composite top 
chord (see Fig. 2-22). 




Spandrel Plate 

Figure 2-22. Horizontal truss modeling, slab not shown. 



2.5.3 



Concrete Slab and Metal Deck 



Outside the core on the mechanical floors, the beams acted compositely with a 5 3/4 in. concrete slab on 
1 1/2 in. metal deck. The average depth of the slab in the model was taken as 6.1 in. The concrete slab 
consisted of normal weight concrete with a self-weight of 150 pcf and a design compressive strength of 
typically f'c= 3,000 psi. The concrete modulus of elasticity, Ec, was 3,320 ksi. Typically, inside the core, 
the beams acted compositely with a 6 in. formed concrete slab. The concrete slab consisted of normal 
weight concrete with the same properties as concrete outside the core. 

The mechanical floors had a 2 in. maximum depth topping slab, both inside and outside the core. The 
topping slab stiffness was not included in the models, but this dead weight was accounted for in the 
baseline performance analyses. 



2.5.4 



Viscoelastic Dampers 



Viscoelastic dampers were located below the bottom flange of the beams where the beams intersected the 
exterior columns. Similar to the typical truss-framed floor model, a placeholder element was located in 
the model at the damper location. 



36 



NIST NCSTAR 1-2, WTC Investigation 



Development of Reference Structural Models 



2.6 REVIEW OF THE STRUCTURAL DATABASES AND REFERENCE 

MODELS OF THE TOWERS 

The following summarizes the results of the third-party review by the firm of SOM and the in-house 
NIST review for the developed structural databases and reference models. 

2.6.1 Structural Databases 

The third-party review by SOM included random checks of the digitized structural databases and cross 
section property calculations. The review indicated no discrepancies between the developed databases 
and the original drawing books. Also, for cross section property calculations, the review indicated good 
agreement (within 1 percent) between the properties in the developed databases and those estimated by 
SOM. 

The in-house NIST review included the following steps: (1) line-by-line review of all database files, 
(2) random checks on the developed databases by the NIST investigator, and (3) calculation of all cross 
section properties and comparison with those in the developed databases. The review indicated minor 
discrepancies between the developed databases and the original drawing books. For cross section 
property calculations, good agreement was obtained between the properties in the developed databases 
and those estimated by NIST. The discrepancies between the developed databases and the original 
drawing books were reported to LERA, and they implemented the changes and modified the databases 
accordingly. Consequently, the structural databases were approved by NIST and were made available for 
other phases of the NIST investigation. 

2.6.2 Reference Structural Models 

The third-party review by SOM included: (1) random checks of the consistency of the developed 
reference models with the original structural drawings and drawing books, (2) verification and validation 
of the models (including reviewing assumptions and level of detail), and (3) performing analyses using 
various loading conditions to test the accuracy of the models. The review concluded that the developed 
models were consistent with the original design documents, and that, in general, the modeling 
assumptions and level of detail in the models were accurate and suitable for the purpose of the 
Investigation. The SOM review identified two areas where the models needed to be modified. The first 
was the effect of additional vertical stiffness of the exterior wall panels due to the presence of the spandrel 
beams (see Section 2.3.1). The second area was the modeling of the connections of the fioor slab to the 
exterior columns of the 75B floor model (Section 2.5), where this connection appeared to be fixed while it 
would be appropriate to model it as pinned. 

The in-house NIST review included: (1) checks on the consistency of the developed reference models 
with the original structural drawings and drawing books, (2) verification and validation of the models 
(including reviewing assumptions and level of detail), and (3) and performing analyses using various 
loading conditions to test the accuracy of the models. The review indicated minor discrepancies between 
the developed reference models and the original design documents. Similar to the third-party review, the 
in-house NIST review identified the proper modeling of the vertical stiffness of the exterior wall panels 
and the accurate modeling of the floor slab connections to the exterior columns in the 75B floor model as 
areas that needed to be modified in the models. 



NIST NCSTAR 1 -2, WTC Investigation 3 7 



Chapter 2 



In addition, NIST conducted a workshop for NIST investigators and contractors to review the reference 
structural models developed by LERA. The workshop attendees included experts from LERA (two 
experts); SOM (two experts); Teng and Associates (one expert, contractors on probable structural 
collapse); Professor Kaspar Willam (contractor on thermal-structural analysis); Dr. David M. Parks 
(contractor on computational mechanics for aircraft impact analysis); Apphed Research Associates (two 
experts, contractor on analysis of aircraft impact into the WTC towers), as well as all key investigators 
from NIST (17 experts). The purpose of the workshop was to discuss the methodology, assumptions, and 
details of the developed reference models. The feedback from the workshop was included in the final 
review of the models. The minutes of the workshop were made public. 

The discrepancies between the developed models and the original design documents, as well as the areas 
identified by both the third-party and the NIST in-house review as needing modification, were reported to 
LERA, which implemented the changes and modified the models accordingly. Subsequently, the 
reference structural models were approved by NIST and were made available for use in other phases of 
the NIST investigation. 

2.7 SUMMARY 

This chapter described the development of the reference structural models for the WTC towers. These 
reference models were used to establish the baseline performance of the towers and also serve as a 
reference for more detailed models for the aircraft impact damage analysis and the thermal-structural 
response and collapse initiation analysis. The main types of the models developed were: 



• 



• 



Two global models of the towers, one each for WTC 1 and WTC 2. The models included all 
primary structural components in the towers, including exterior walls (columns and spandrel 
beams), core columns, exterior wall bracing in the basement floors, hat trusses, and rigid and 
flexible diaphragms representing the floor systems. To validate the global models, the calculated 
natural frequencies of WTC 1 were compared with those measured on the tower, and good 
agreement between the calculated and measured values was observed. 

One model each of a typical truss-framed floor (floor 96 of WTC 1) and a typical beam-framed 
floor (floor 75 of WTC 2) in the impact and fire zones in the two towers. The models included all 
primary structural components in the fioor system, including primary and bridging trusses, 
beams, strap anchors and horizontal trusses, concrete slabs, and viscoelastic dampers. Both 
models were developed using frame elements, except for the concrete slabs which were modeled 
using shell elements with typical element sizes of 20 in. and 40 in. for the truss-framed floor and 
the beam framed floor, respectively. 

Prior to the development of the reference models, databases of the primary structural components of the 
towers were developed from the original computer printouts of the structural design documents and 
modifications made after construction. These databases facilitated the development of the global models 
of the towers. 

The structural databases and reference structural models were developed by LERA and were reviewed by 
SOM and NIST. 



38 NIST NCSTAR 1 -2, WTC Investigation 



Development of Reference Structural Models 



2.8 REFERENCES 

SAP2000 (2002), Linear and Nonlinear Static and Dynamic Analysis and Design of Three-Dimensional 
Structures Basic Analysis Reference, Computers & Structures Inc., Berkeley, CA. 



NIST NCSTAR 1 -2, WTC Investigation 3 9 



Chapter 2 



This page intentionally left blank. 



40 NIST NCSTAR 1-2, WTC Investigation 



Chapters 

Wind Loads on the WTC Towers^ 



3.1 INTRODUCTION 

Wind loads were a governing factor in the design of the World Trade Center (WTC) towers' perimeter 
frame-tube system. The study of the wind loads on the WTC towers was required for evaluating: (1) the 
baseline performance of the towers under design loading conditions, (2) the towers' reserve capacity to 
withstand unanticipated events such as a major fire or impact damage, and (3) design practices and 
procedures. 

The accurate estimation of the wind loads on tall buildings is challenging, since wind engineering is still 
an evolving technology. As is shown in this chapter, estimates of the wind-induced response presented in 
two recent independent studies of the WTC towers differed from each other by about 40 percent. This 
discrepancy is indicative of limitations of the current state of practice in wind engineering for tall 
buildings. Also, as will be shown later in this chapter, wind loads (pressures) specified in current 
prescriptive codes differ significantly from the loads estimated from wind tunnel-based studies. The 
study of the wind loads on the WTC towers provided an opportunity to assess current design practices and 
various code provisions on wind loads. 

This chapter outlines the loading cases applied to the reference global models of the WTC towers 
(Section 2.3) to establish the towers' baseline performance. The following sources were used to develop 
the loads for the various loading cases: 

• Design Criteria document of the WTC towers, prepared by Worthington, Skilling, Helle & 
Jackson (WSHJ) (henceforth referred to as Design Criteria). 

• WTC architectural and structural drawings (henceforth WTC Dwgs). 

• Wind reports prepared by WSHJ in the 1960s, describing the development of design wind 
loads for the WTC towers (henceforth WSHJ Wind Reports). 

• Reports from two independent wind tunnel studies concerning the WTC towers, conducted in 
2002 by Cermak Peterka Peterson, Inc. (henceforth CPP) and Rowan Williams Davis and 
Irwin, Inc. (henceforth RWDI) for insurance litigation. 

• Current New York City Building Code (henceforth NYCBC 2001). 

• Current American Society of Civil Engineers (ASCE 7) Standard (henceforth ASCE 7-02). 



This chapter was co-authored by Emil Simiu and Fahim Sadek of National Institute of Standards and Technology (NIST). 
NIST NCSTAR 1 -2, WTC Investigation 4 1 



Chapter 3 



Three loading cases were considered for the baseline performance analysis. They included: 

• Original WTC design loads case: Dead and live loads as in original WTC design in 
accordance with the Design Criteria, and original WTC design wind loads from WSHJ Wind 
Reports. 

• State-of-the-practice case: Dead loads as in original design; NYCBC 2001 live loads; and 
wind loads from RWDI wind tunnel study, scaled in accordance with NYCBC 2001 wind 
speed. This wind load was considered to be a lower estimate state-of-the-practice case. As 
will be explained later, the CPP wind tunnel study produced larger wind loads and was, 
therefore, considered to be an upper estimate state-of-the-practice case. 

• Refined NIST estimate case: Dead loads as in original design; live loads from American 
Society of Civil Engineers (ASCE) 7-02 (a national standard); and wind loads developed by 
NIST from a critical assessment of information obtained from the RWDI and CPP reports, 
and state-of-the-art considerations. 

The purpose of considering the original WTC design loads case was to evaluate structural performance 
under original design loading conditions and ascertain whether those loads and the corresponding design 
were adequate given the knowledge available at the time of the design. In addition, this loading case was 
useful in evaluating the towers' reserve capacity to withstand unanticipated events such as those of 
September 11, 2001. The purpose of considering the state-of-the-practice case and the refined NIST 
estimate case was to better understand and assess the effects of successive changes in standards, codes, 
and practices on wind design for tall buildings, with a view to helping improve standard provisions for 
wind loads in the future. The study provided a unique opportunity to achieve this objective. 

The gravity loads applied to the global WTC models consisted of dead loads and live loads (LLs), 
appropriately combined as stipulated in the Design Criteria. Dead loads were applied to the reference 
global models in two parts: construction dead loads (CDLs) and superimposed dead loads (SDLs), based 
on the WTC Dwgs and the Design Criteria. 

• CDL is defined as the self-weight of the structural system, including fioor slabs, beams, truss 
members, columns, spandrel beams, and so forth. 

• SDL is defined as the added dead load associated with architectural, mechanical, electrical, 
and plumbing systems; such as curtain walls, ceilings, partitions, fioor finishes, mechanical 
equipment and ducts, transformers, and so forth. 

Three independent sets of live loads were combined with the dead loads: 

• The first set was taken from the Design Criteria and was used with the original WTC design 
loads case. 

• The second set was taken from NYCBC 2001 and was used for the state-of-the-practice case. 

• The third set was taken from ASCE 7-02 and was used for the refined NIST estimate case. 
The live loads given in ASCE 7-02 are essentially identical to the NYCBC 2001 live loads. 



42 NIST NCSTAR 1-2, WTC Investigation 



Wind Loads on the WTC Towers 



For each live load set, live load reductions for column design were taken from their respective source. 
Refer to Chapter 4 of NIST NCSTAR 1-2A for further details on the estimation of gravity loads in the 
reference global models of WTC 1 and WTC 2. 

Sections 3.2, 3.3, and 3.4 present, respectively, the original WTC design wind loads, the state-of-the- 
practice wind loads, and the refined NIST wind load estimates. Section 3.5 provides a comparison of the 
various wind loading cases. 

3.2 ORIGINAL WTC DESIGN WIND LOADS 

Wind loads were determined for the original design of the WTC towers through the development and 
implementation of a boundary-layer wind-tunnel study, which simulated the mean and fluctuating 
(turbulence) properties of the wind from ground to gradient height by using the knowledge and techniques 
available in the 1960s. Aeroelastic wind tunnel tests were conducted at a 1:500 scale at Colorado State 
University (CSU), and at a 1 :400 scale at the National Physical Laboratory (NPL), Teddington, United 
Kingdom. Results from the tests conducted at NPL and CSU were in good qualitative and quantitative 
agreement. The original WTC wind loads were taken from summaries given in Part IV of the WSHJ 
Wind Reports. For further details, refer to NIST NCSTAR 1-1 A. 

Wind tunnel data were collected for each tower for wind approaching from 24 wind directions in 
15 degree increments. Part IV of the WSHJ Wind Reports provided equations for the wind-induced 
shears and overturning moments in the towers at 2 1 elevations, z, along the building height, H, at 
increments of 0.05H. For each wind direction, sets of coefficients were provided for use in these 
equations to obtain the static and the dynamic components of shear and overturning moment in the N-S 
and E-W directions. Coefficients were also provided for calculating torsional moments. Based on these 
equations, shears in the two orthogonal directions x andy, and torsions, were calculated for each wind 
direction. The equivalent effective static shear forces and overturning moments at each level consisted of 
sums of the respective static and dynamic components. For details see NIST NCSTAR 1-1. The wind 
speeds at 1,500 ft above ground averaged over 20 min, used in the original design, were assumed to be 
independent of direction and were estimated to be 98 mph. 

Considering the 24 different wind directions and the four combinations of the static and dynamic parts of 
the N-S and E-W components of the building forces listed below, there were 96 different wind load cases 
for each tower. 

N-S (Static + Dynamic) and E-W (Static + Dynamic) 

N-S (Static + Dynamic) and E-W (Static - Dynamic) 

N-S (Static - Dynamic) and E-W (Static + Dynamic) 

N-S (Static - Dynamic) and E-W (Static - Dynamic) 

The static and dynamic shears and overturning moments in the N-S and E-W directions were calculated 
for all 96 loading cases. In order to determine the most severe of the 96 loading cases for each tower, the 
wind-induced shears and overturning moments were compared, for each direction, at heights z/H = 0.75, 



NIST NCSTAR 1-2, WTC Investigation 43 



Chapter 3 



0.50, 0.25 and 0. The wind loading cases producing the maximum shears in either of the two orthogonal 
directions were identified for application to the global models. 

To compare overturning moments for each loading case, the moments in the two orthogonal directions 
were combined vectorially (i.e., the magnitude of the resultant is equal to the square root of the sum of the 
squares of the components, and the direction p of the resultant is the arc whose tangent is equal to the 
ratio of they- and x-components). The load cases were grouped by the angle P using increments of 
45 degrees, resulting in eight groups of load cases. For each p group, at z/H = 0.75, 0.50, 0.25, and 0, the 
wind load cases that generated the maximum resultant moment were identified for application to the 
reference global models. Eight groups of maximum moment plus four directions of maximum shear at 
four heights in the towers would result in 48 different loading cases. Some individual wind load cases, 
however, produced a maximum resultant moment and/or a maximum shear at more than one elevation in 
the towers. As a result, 16 loading cases were identified for WTC 1, and 17 loading cases were identified 
for WTC 2. 

For the floors modeled in the global models by rigid diaphragms, the wind forces were apphed as 
concentrated loads at the geometric center of the building. The torsional moments were also taken into 
account. For the floors with flexible diaphragms (see Chapter 2), the forces based on tributary areas were 
resolved into point loads at the perimeter columns. At these floors, the torsional moment was represented 
by four identical concentrated forces applied parallel to the four faces of the tower at the center column of 
each face. For each loading case, the orthogonal wind forces were subdivided into windward and leeward 
forces based on the direction of the wind. The distribution of forces between the windward and leeward 
sides was based on Figure 6-6 of the ASCE 7-02 Standard (see Chapter 4 of NIST NCSTAR 1-2A for 
more details). 

3.3 STATE-OF-THE-PRACTICE WIND LOADS 

For the WTC towers, two wind tunnel tests and wind engineering studies based thereon were conducted 
in 2002 by independent laboratories as part of insurance litigation unrelated to the NIST investigation. 
The tests and studies were conducted by CPP and by RWDI. The results of both studies were made 
available to NIST. Since the CPP and RWDI studies are representative of current practices, their wind 
load estimates are considered "state-of-the-practice wind loads." 

CPP study. The CPP wind tunnel tests modeled the terrain surrounding the WTC towers over an area 
with a radius of about 2,300 ft. Measurements were made only on the south tower. In one test the south 
tower was modeled by using a high-frequency force-balance (HFFB) device. In a second test the south 
tower was modeled aeroelastically. The test scale was 1 :400, and testing was conducted for 36 wind 
directions at 10 degrees intervals. The wind-induced loads and responses were determined by combining 
the wind tunnel test data with (a) directional non-hurricane wind speed data recorded at three major 
airports in the New York area for periods of about 25 years, and (b) hurricane wind speed data (the source 
of the hurricane data was not indicated in the study). The directional wind tunnel and wind speed data 
were combined by using the sector-by-sector approach, described and assessed in Section 3.4 of this 
chapter. Wind effects corresponding to a damping ratio of 2.5 percent were provided for the south tower 
only, for nominal 50 year and 720 year mean recurrence intervals and consisted of peak shear force and 
bending moment components for two orthogonal directions and peak torsional moments. The peak 
components were combined in accordance with the "companion point-in-time" method, for example, by 



44 NIST NCSTAR 1-2, WTC Investigation 



Wind Loads on the WTC Towers 



using the full peak load in one direction and the loads in the other direction and in torsion at the time of 
occurrence of that peak. The CPP report considered 10 such combinations. 

RWDI study. The RWDI wind tunnel tests modeled the terrain surrounding the WTC towers over an area 
with a radius of about 4,000 ft. The tests used an HFFB model for each of the towers and an aeroelastic 
model for the north tower only. The test scale was 1 :500, and testing was conducted for 36 wind 
directions at 10 degree intervals. Corrections were made to account for the effects on the flow of the 
presence of building models (i.e., of wind tunnel blockage). Estimates of the full-scale wind effects and 
responses were obtained by combining the wind tunnel test data with a statistical model of winds for New 
York City, including surface wind measurements taken at three airports between 1948 and 1995 and 
proprietary simulated hurricane winds provided by Applied Research Associates (Raleigh, NC). The 
directional wind tunnel and wind speed data were combined by using an out-crossing approach developed 
by RWDI. Two sets of wind effects on the towers were developed by scaling the wind loads to the design 
wind speeds provided in the NYCBC 2001 and to the basic wind speeds specified by the ASCE 7-98 
Standard. The wind effects were obtained, for a damping ratio of 2.5 percent, as peak shear forces and 
bending moments for two orthogonal directions, and peak torsional moments. The peak components were 
combined using the "principle of companion loads" entailing weighting combination factors based on 
engineering judgment. The RWDI report considered 24 such combinations. 

Note. For both the CPP and RWDI studies, tests were conducted for the two-tower configuration and for 
a single tower configuration. For the purposes of this investigation, only the two-tower configuration was 
considered. As was mentioned earlier, the CPP study provided results for the south tower only, while the 
RWDI study provided wind load estimates for both towers. In the absence of CPP estimates for the north 
tower, the state-of-the-practice wind loads considered in the baseline study for the north and south towers 
were selected to be the RWDI wind loads scaled in accordance with a wind speed equivalent to the 
NYCBC 2001 wind speed. The latter was interpreted to be the 80 mph fastest-mile wind speed at 30 ft 
elevation over open terrain. In the baseline performance study, these wind loads were applied to the 
reference global models using the directional and torsional load combination factors presented in the 
RWDI reports. The application of the wind loads at each floor of the global models was similar for the 
lower-estimate state-of-the-practice case and for the original WTC design case. 

The wind loads from RWDI were smaller than those obtained from CPP for WTC 2 (see Section 3.4). 
Therefore, RWDI loads may be viewed in this study as a "lower-estimate state-of-the-practice" case. 

3.4 REFINED NIST ESTIMATE OF WIND EFFECTS 

NIST completed an independent analysis to estimate the wind loads that would be appropriate for use in 
designing the towers. The analysis was based on results provided by CPP and RWDI, with refinements 
that drew on the state of the art in wind engineering. The objective of this analysis was to better 
understand and assess the effects of successive changes in standards, codes, and practices, not to assess 
the adequacy of the original design wind loads. The analysis yielded refined estimates of wind effects for 
the north and south WTC towers. These estimates made use of independent extreme wind climatological 
estimates developed by NIST (Appendix B), based on airport wind speed data obtained from the National 
Climatic Data Center, National Oceanic and Atmospheric Administration, and on the NIST hurricane 
wind speed database - the only such database publicly available at present (see Appendix B for details). 



NIST NCSTAR 1-2, WTC Investigation 45 



Chapter 3 



The estimates of wind-induced forces and moments provided in this report relied primarily on RWDI 
results, since no results for WTC 1 were available from CPP. However, the estimates took into account a 
comparative assessment of the RWDI and CPP results for WTC 2. 

3.4.1 Summary Comparison by Weidlinger Associates, Inc., of CPP and RWDI 

Estimates 

A useful summary comparison between CPP and RWDI estimates of maximum base moments and shear 
forces on WTC 2 induced by ASCE 7-98 wind loads is contained in a memorandum by Weidlinger and 
Associates.^ As indicated in that memorandum, the values presented in Table 3-1 are based on nominal 
basic wind speeds (i.e., 500 yr speeds divided by square root of 1.5 for RWDI, and 720 yr loads divided 
by 1.6 for CPP). 

Table 3-1. Approximate maximum base moments for WTC 2 induced by ASCE 7-98 

standard wind loads. 



Wind Tunnel Study 


\My\ (Ib-ft) 


m (ib-ft) 


RWDI (Table 2a) 
CPP (Upper Table, p. 21) 


lO.le+9 
14.0e+9 


ll.le+9 
15.5e+9 



For the CPP results, the wind directions associated with the largest \My\ and \Mx\ moments were 
205 degrees and 215 degrees, respectively (CPP report. Upper Table, p. 21; degrees was defined as 
True North). Both RWDI and CPP results indicated that the critical base moments occurred for a wind 
direction of about 210 degrees. This agreement suggested that a comparison between those results was 
warranted in some detail for the 202.5 to 225 degree range. (The reason for the choice of this range was 
that hurricane data in the NIST database are provided for the 16 half-octants of the compass.) Such a 
comparison is presented in this report. 



3.4.2 



Review of CPP Estimates 



Independent estimates by NIST of the 720 yr, 3 s peak gust speeds for the 202.5 degree and 225 degree 
angles were 104.1 mph and 91.1 mph, respectively (Appendix B, Fig. 1). Linear interpolation between 
these estimates yielded a 720 yr, 3 s peak gust speed of 99.8 mph for 210 degrees. CPP estimated the 
720 yr peak 3 s peak gust speed at 210 degrees to be about 117.5 mph.^ Therefore, the CPP results were 
modified through multiplication by the factor (99.8/1 17.5)^=1/1.386. Owing to the dynamic character of 
the response, multiphcation by the square of the ratio of the speeds is not rigorously correct, but in the 
absence of sufficiently detailed information it can serve as a useful approximation. A similar conclusion 
was reached in a letter by RWDI to NIST."* 



Memorandum on Comparison of RWDI and CPP Design Wind Loads, from N.N. Abboud and A. Jain, Weidlinger Associates, 
Inc., November 11,2003. 

This is obtained through multiplication of the 93 mph speed (basis of design speeds for the 210 degree angle, CPP report, 
p. 10, upper curve) by the square root of 1.6. 

Letter on World Trade Center wind tunnel investigations, by P.A. Irwin, RWDI, November 7, 2003. 



46 



NIST NCSTAR 1-2, WTC Investigation 



Wind Loads on the WTC Towers 



In addition to their multiplication by the factor 1/1.386, the CPP results were modified to account for the 
use by CPP of the sector-by-sector approach to integrating aerodynamic data and extreme -wind 
climatological data. The sector-by-sector approach is not valid from a physical point of view. This was 
also noted by RWDI.^ In attempting to explain the differences between the CPP and RWDI estimates, 
RWDI assumed that the use of the sector-by-sector approach contributed to the overestimation of the 
response by CPP. This assumption was due to the difficulty of analyzing the CPP report. Such analysis 
required a special study by NIST, reported in Appendix C,'' which concluded that, in fact, the sector-by- 
sector approach as applied by CPP underestimated the wind effects corresponding to a specified mean 
recurrence interval. According to preliminary estimates that would need to be confirmed by research 
using, e.g., Bonferroni bounds (see Appendix C), it was assumed that the underestimation was about 15 
percent. Therefore, the CPP results, modified via multiplication by the factor 1/1.386, were further 
modified via multiplication by the factor 1.15. The reduction factor applied to the estimated CPP effects 
was, therefore, about 1.15/1.386*1/1.205. 

Conclusion. The CPP moments presented in Table 3-1 were reduced via application of the factor 1/1.205 
from approximately 14.0e+9 Ib-ft and 15.5e+9 Ib-ft to approximately 1 1.62e+9 Ib-ft and 12.86e+9 Ib-ft, 
respectively. To within the limitations inherent in the information available for this investigation, and to 
within the approximations noted, these reduced values are reasonable estimates of the actual responses of 
interest. 

3.4.3 Review of RWDI Estimates 

According to the conclusion of Section 3.4.2 concerning the modified CPP results, the RWDI results 
underestimated the moments for the directions being considered. This conclusion is consistent with the 
fact that RWDI assumed wind profiles in hurricanes to be flatter (to increase more slowly with height) 
than wind profiles in non-hurricane winds. This assumption, and its effect on the RWDI estimates, were 
confirmed in the RWDI Response to NIST's Questions, September 2003.^ The RWDI assumption 
regarding the relative flatness of hurricane wind profiles was based on a calculation of the ratio between 
wind speeds at 500 m and at 10 m over open terrain, based on the formula y(500 m)/y(10 m) = 
(500 m/10 m)*'''*=1.73. In this calculation it was assumed that, in the power law model of the atmospheric 
boundary layer over open terrain, wind speeds increase monotonically up to an elevation of at least 
1,640 ft (500 m). This assumption is not consistent with accepted practice, according to which in the 
power law model the mean speed increases with elevation only up to a gradient height which, for open 
terrain, is about 900 ft (275 m or so), rather than 1,640 ft (500 m) ~ see, for example, the ASCE 7 
Standard. An unconventional model such as the relatively flat hurricane profile model invoked by RWDI 
is not supported by measurements in the atmosphere. A recent article in Nature (Powell, Vickery, and 
Reinhold, 2003) indicated that the increase of hurricane wind speeds with height is consistent with the 
logarithmic law (see Figure 2 of the article). This is also true of extratropical storm winds. It is also 
noted that the ASCE 7 Standard does not differentiate between wind speed profiles in hurricane and non- 



Letter on Review of Wind Tunnel tests, RWDI Reference #02-13 10 by P.A. Irwin to M. Levy of Weidlinger Associates, dated 

October 2, 2002. 

See also E. Simiu and J. J. Filliben, "Wind Tunnel Testing and the Sector-by-Sector Approach to Wind Directionality Effects," 

J. Struct. Eng., ASCE, July 2005, pp. 1 143-1 145. 

Responses to NIST's Questions on "Wind-Induced Structural Responses, World Trade Center, Project Number 02. 13 lOA and 

02.13 lOB, October 2002, by RWDI, Prepared for Hart- Weidlinger", Hart-Weidlinger, September 12, 2003. 

NIST NCSTAR 1-2, WTC Investigation 47 



Chapter 3 

hurricane winds, even though wind profiles affecting velocity pressures are defined therein up to 500 ft 
above ground, where the effect of wind profile differences would be significant. 

In response to a NIST query,* the use of a ratio of approximately 1.1 between tower responses to 88 mph 
and 80 mph wind speeds (note 3 at the bottom of Tables 3b and 3c in the RWDI report) was ascribed to 
the assumption that hurricane wind profiles are relatively fiat. This justification is not viewed as 
satisfactory for the reasons indicated in the preceding paragraph. In view of the current state of the art, 
according to which hurricane and non-hurricane wind profiles are substantially similar, a ratio of 
approximately (88/80)^=1.21 is more appropriate than the ratio of approximately 1.1 used by RWDI. 

Also, it is not clear that the weighting of hurricane wind speeds in proportion to their squares, as used by 
RWDI in the out-crossing method, is warranted. No justification was provided in the RWDI report for 
the weighting procedure based on squares of speeds, nor did RWDI list any reference pertaining to this 
approach. In the standard Peaks-Over-Threshold approach applied to extreme wind speeds by, among 
others, Simiu and Heckert (1996), all data above a threshold are affected by the weighting factor 1, while 
all the data below the threshold are weighted by the factor zero; the analysis is carried out for a large 
number of thresholds to ascertain the range of thresholds for which the estimates being sought are stable. 
The use of data lower than the lowest acceptable threshold results in the underestimation of the extreme 
wind effects being sought. Therefore, it can be expected that the use of such data, albeit weighted in 
accordance with the RWDI procedure, will have a similar effect. More generally, "concerns that the 
crossing-rate" (i.e., the out-crossing) "method may underestimate extreme wind-induced effects which 
depend upon both on wind speed and wind direction" were noted by Isyumov et al. (2003). 

The University of Western Ontario (UWO) conducted an independent estimate of wind effects which by 
and large were reasonably close to the RWDI results.'^ It appears, however, that the assumptions used by 
UWO and RWDI with respect to hurricane wind profiles were the same or similar. According to UWO, 
the CPP directionahty approach would appear to overestimate the 50 year response. This view is not 
consistent with the conclusions of Appendix C to this report. 

A direct, full quantitative assessment and verification of the RWDI results was judged not to be possible 
given the information available. Nevertheless, as was shown earlier in connection with the wind profiles, 
a partial quantitative assessment was made, which indicated that the actual response would be higher than 
the RWDI estimated response. Given this assessment, and an estimate of the actual response based on the 
modified CPP estimates, the conclusion that the response was underestimated by RWDI by a factor of 
about 10 percent to 20 percent was judged to be warranted. The difference between the NIST estimate of 
the response and the RWDI estimate is smaller than the difference between the CPP estimate and the 
NIST estimate of the response. 

Conclusion. Based on the discussion presented above, loads associated with the refined NIST estimates 
case and consistent with the design wind speed in the ASCE 7-98 and ASCE 7-02 Standards can be 
estimated approximately by using the RWDI results multiplied by a factor equal to the ratio of the 
modified CPP estimates (see Section 3.4.2) to the corresponding RWDI estimates. This factor varies 



Letter on Response to NIST Questions of March 30, 2004, by N.N. Abboud, Weidlinger Associates, Inc., April 6, 2004. 
9 

Report Regarding the Review of the World Trade Center Twin Towers (NY) Wind Studies Carried Out by RWDI and CPP 

UWO File W020, cover letter to N. Abboud, Weidlinger Associates, dated November 3, 2003. 
48 NIST NCSTAR 1-2, WTC Investigation 



Wind Loads on the WTC Towers 



from (14.0e+9/1.205)/(10.1e+9) =1.15 to (15.5e+9/1.205)/(ll.le+9)=1.159. Therefore, the factor 1.15 
was recommended for baseline analysis. However, the actual factor could be anywhere between, say, 
1.10 and 1.20. 

3.4.4 Comments by Third Party Reviewer (Skidmore, Owings & Merrill LLP - SOM) - 

Appendix D 

SOM served as a third party reviewer for the wind load estimation by NIST. According to SOM, it would 
have been desirable for the measured fundamental period of vibration of the north tower to be used in lieu 
of the calculated periods for either tower. According to the RWDI report, for the south tower the 
fundamental periods for the x-direction (a) not accounting for P-A effects, and (b) accounting for P-A 
effects were 12.341 s and 13.292 s, respectively (Appendix A of RWDI report), the difference between 
them being about 7 percent. The respective estimated x-direction base shears in the RWDI report were 
9.45e+06 lb and 9.71e+06 lb, respectively, the difference in this case being about 2.7 percent. In the 
y-direction, the differences between the respective shears were less than 1 percent. In view of the 
uncertainties in the measurement and calculation of the natural periods it is concluded that the differences 
between shears inherent in the differences between natural periods noted by SOM are not significant. 

With respect to the NIST assessment of the CPP and RWDI results, SOM stated that the approach taken 
by NIST was reasonable, but that SOM was not able to confirm the precise values put forth in the NIST 
report. SOM noted that quantitative assessments and corrections were made by NIST to the CPP report, 
and that NIST made only qualitative assessments of the RWDI report. As was indicated in Section 3.4.3, 
this is indeed the case, except for the quantitative assessment related to wind profiles. No other 
quantitative assessments were possible, either by NIST or SOM. SOM's inability to confirm precise 
values is understandable in view of the lack of sufficient clarity in portions of the CPP and RWDI reports. 
NIST's intent was to recommend reasonable estimates, not precise values. The estimates may be 
somewhat larger or smaller than the non-attainable precise values. In NIST's judgment approximate 
bounds to these estimates are defined by the interval of about 1.1 to 1.2 (see Conclusion to Section 3.4.3). 

SOM emphasized the urgent need to put order in the field of wind tunnel testing and the estimation of 
wind effects through standards developed by consensus. NIST fully agrees with this view. It also agrees 
with SOM's suggestion that the ASCE 7 Standard specify the use of an importance factor larger than 
unity for buildings representing a substantial hazard to human life in the event of failure. Currently, the 
ASCE 7 Standard specifies an importance factor larger than unity for buildings designed in accordance 
with the Standard's "analytical procedure." It does not require the use of the importance factor for 
buildings whose wind loads are estimated by the "wind tunnel procedure." In fact, neither the CPP nor 
the RWDI wind loads were augmented by the use of a 1.15 importance factor. It is also noted that even if 
an importance factor of 1 . 1 5 were required to augment wind effects estimated by the wind tunnel method, 
there could be some confusion over the definition of" buildings ... where more than 300 people 
congregate in one area" (Table 1-1 of the ASCE 7 Standard), for which an importance factor of 1.15 is 
specified in the "analytical procedure." The question arises whether buildings like the WTC towers are 
included in that definition. This is not indicated clearly in the ASCE 7 Standard, in which the term "area" 
may be interpreted by some engineers as being restricted to, e.g., auditoria, rather than apartment 
buildings or other structures with an occupancy of more than 300 people. NIST also believes that the 
importance factor should be risk-based, rather than prescribed arbitrarily. 



NIST NCSTAR 1-2, WTC Investigation 49 



Chapter 3 

An increase of the wind pressures by an importance factor of 1 . 1 5 to account for the large population of 
some tall buildings (over 5,000 individuals) is specified in Table 1604.5 of the 2003 International 
Building Code (IBC 2003), which is otherwise mostly based on ASCE 7-02. A consensus should be 
reached on whether 5,000 is the appropriate threshold. 

In addition, it does not appear appropriate for a tall building with significant dynamic effects to have the 
same load factor as an ordinary, rigid building: the tall building response depends on dynamic response 
parameters with uncertainties, including, in particular, uncertainty with respect to damping, that should 
affect the wind load factor applicable to the tall building. This is especially true of buildings designed in 
accordance with the "wind tunnel procedure." Therefore, research into differences between wind load 
factors for rigid and flexible buildings is warranted. 

3.4.5 Summary 

The lateral wind loads on the towers, consistent with the ASCE 7-98 and ASCE 7-02 design wind speed 
requirements, were estimated by using the effective static floor-by-floor wind loads presented in Table 5a 
(without P-A effects) or Table 5b (with P-A effects) of the RWDI report (north tower) for WTC 1 and 
Table 3a (without P-A effects) or Table 3b (with P-A effects) of the RWDI report (south tower) for 
WTC 2.'° These effective static floor-by-floor wind loads were multiplied by the factor 1.15 (see 
Section 3.4.3) and by the factors indicated in footnote (3) to Tables 3 and 5 in RWDI to account for the 
ratio between the ASCE 7 and NYCBC wind speeds. The loads so obtained were applied to the reference 
global model of each tower using the load combinations presented in Table 6a of RWDI (north tower) and 
Table 4a of RWDI (south tower). The loads put forth in this section were used along with the load factors 
given in Section 2 of ASCE 7-02. 

It would have been desirable to perform more elaborate calculations providing more comprehensive and 
precise results than those presented in this document. However, given the information available, this was 
not practicable. 

3.5 COMPARISONS OF WIND LOADS, WIND SPEEDS, AND PRACTICES 

The purpose of this section is to provide comparisons among wind loads and wind speeds applicable to 
the WTC towers in accordance with various codes, standards, and estimation procedures. Sections 3.5.1 
and 3.5.2 present comparisons among wind loads and among wind speeds, respectively. Section 3.5.3 
compares wind engineering features used to perform the response estimates for the original design and the 
CPP, RWDI, and NIST estimates. 

3.5.1 Wind Loads 

Tables 3-2 and 3-3 provide a summary of the wind-induced base shears and base moments" on WTC 1 
and WTC 2, respectively, based on the 1938 and 1968 versions of the NYCBC, the RWDI study, the CPP 
study, the refined NIST estimates, and the original design. The wind loads are expressed in terms of two 



For the WTC 2 tower Tables 3b and 3c in the RWDI report (South Tower) were inadvertently switched. The loads accounting 

for P-A effects are in fact given in Table 3c of the report. 

All base moments presented in this chapter are calculated at the foundation level. 

50 NIST NCSTAR 1-2, WTC Investigation 



Wind Loads on the WTC Towers 



orthogonal components (N-S and E-W for base shears, and about N-S and about E-W for base 
moments), and of vectorial measures of the most unfavorable combined peaks. The vectorial measures 
are an indication of the relative conservatism of various methods for combining wind effects in the x and 
y directions, and were defined as the largest of a set of vectorial sums ofx andy components, as follows: 

• For the RWDI estimates, the set consisted of 24 vectorial sums, each corresponding to one of 
24 X andy load combinations considered in the RWDI report. The combined x and y values 
were weighted as indicated in Section 3.3. 



• 



For the CPP estimates, the set consisted of 10 vectorial sums, each sum corresponding to one 
of 10 X andy load combinations considered in the CPP report. The combined x andy values 
conformed to the "companion-load-in-time" approach described in Section 3.3. 

• For the original WTC design estimates, the set consisted of 24 vectorial sums of peak x andy 
values, each corresponding to one of the 24 wind directions considered in the original design, 
as described in Section 3.2. 

The NIST estimates were in all cases equal to 1.15 times the RWDI estimates based on the ASCE 7-98 
Standard. 

Table 3-4 is a summary of design base shears and base moments based on prescriptive provisions at the 
time of the design in the 1938 and 1968 New York City Building Codes, the 1964 New York State Code, 
the 1965 Building Officials and Code Administrators Basic Building Code (BOCA/BBC), and the 1967 
Chicago Municipal Code. 

Tables 3-2 and 3-3 indicate that the two orthogonal components of the original design wind load 
estimates exceeded in all cases their counterparts based on the New York City Building Code (a 
prescriptive code) prior to 1968, when the WTC towers were designed, and from 1968 to date. Table 3-4 
shows that the design values were also higher than those required by other prescriptive building codes of 
the time, including the relevant national model building code. It is noted, however, that the prescriptive 
approach in these codes is oversimplified, and that these codes are therefore not appropriate for super-tall 
building design. In fact, wind effects obtained from three separate wind-tunnel-based studies (for the 
original WTC design, the CPP, and the RWDI studies) were in all cases higher than wind effects based on 
the prescriptive codes. 

The two orthogonal base shear and base moment components used in the original design were in the 
majority of cases smaller than the CPP, RWDI, and NIST estimates. However, the vectorial measures of 
the most unfavorable combined peaks for the original design were larger, or smaller, by at most 
10 percent or so, than those based on the CPP, RWDI, and NIST estimates. This is due to the 
conservative procedure used to combine the loads in the original design. For example, NIST estimates 
were higher by about 1 percent than the most unfavorable original design wind loads for WTC 1 , and 
lower by about 5 percent than the most unfavorable original design loads for WTC 2. 



NIST NCSTAR 1 -2, WTC Investigation 5 1 



Chapter 3 



Table 3-2. Comparison of wind load estimates for WTC 1 based 


on various sources. 


Source 


Year 


Base Shear 10^ kip 


Base Moment 10''kip-ft 


N-S 


E-W 


Most 

unfavorable 

combined 

peak 


About 

N-S 


About 
E-W 


Most 

unfavorable 

combined 

peak 


NYC Building Code 


1938 


5.3 


5.3 




4.2 


4.2 




NYC Building Code 


1968 to 
date 


9.3 


9.3 




7.7 


7.7 




RWDI / NYC Building 
Code 


2002 


11.4 


10.5 


13.0 


10.1 


10.5 


12.2 


RWDI / ASCE 7-98 


2002 


12.3 


11.3 


14.0 


10.8 


11.4 


13.1 


CPP / NYC Building 
Code 


2002 


NA 


NA 


NA 


NA 


NA 


NA 


CPP / ASCE 7-98 


2002 


NA 


NA 


NA 


NA 


NA 


NA 


NIST / third-party SOM 
review 


2004 


14.1 


13.0 


16.1 


12.4 


13.1 


15.1 


Original WTC Design 


1960s 


9.8 


10.6 


14.0 


10.3 


9.1 


13.7 



52 



NIST NCSTAR 1-2, WTC Investigation 



Wind Loads on the WTC Towers 



Table 3-3. Comparison of wind load estimates for WTC 2 based 


on various sources. 


Source 


Year 


Base Shear 10^ kip 


Base Moment 10* kip- ft 


N-S 


E-W 


Most 

unfavorable 

combined 

peak 


About 

N-S 


About 
E-W 


Most 

unfavorable 

combined 

peak 


NYC Building Code 


1938 


5.3 


5.3 




4.2 


4.2 




NYC Building Code 


1968 to 
date 


9.3 


9.3 




7.6 


7.6 




RWDI / NYC Building 
Code 


2002 


9.7 


11.1 


12.3 


10.1 


9.2 


11.3 


RWDI / ASCE 7-98 


2002 


10.6 


12.2 


13.5 


11.1 


10.1 


12.4 


CPP / NYC Building 
Code 


2002 


NA 


NA 


NA 


NA 


NA 


NA 


CPP /ASCE 7-98' 


2002 


15.1 


15.3 


17.1 


15.5 


14.0 


17.0 


NIST / third-party SOM 
review 


2004 


12.2 


14.0 


15.5 


12.8 


11.6 


14.3 


Original WTC Design 


1960s 


13.1 


10.1 


16.5 


8.8 


12.6 


15.2 



a. Using ASCE 7-98 Sections 6.5.4.1 and 6.6. 



Table 3-4. Base shears and base moments due to wind loads based on various 

building codes. 



Building Code 


1938 
NYC Code 


1968 to data 
NYC Code 


1964 
NY State Code 


1965 
BOCA/BBC 


1967 

Chicago 

Municipal Code 


Base Shear 
(103 kip) 


5.3 


9.3 


9.5 


9.8 


8.7 


Base Moment 
(lO^kip-ft) 


4.2 


7.7 


7.6 


8.5 


7.5 



3.5.2 



Wind Speeds 



A comparison of wind speeds is presented in Table 3-5. The ASCE 7-02 Standard specifies a basic 
design wind speed for New York City of 104 mph at 33 ft above ground for open terrain exposure. This 
speed is equivalent to an 88 mph fastest- mile wind speed at 33 ft above ground. The wind speed specified 
by the NYCBC 2001 is 80 mph and is interpreted to be a fastest-mile wind speed at 33 ft above ground. 
For the original WTC design, a design wind speed of 98 mph averaged over 20 minutes at a height of 
1,500 ft above ground was used. This speed is equivalent to a fastest-mile wind speed at 33 ft above 



NISTNCSTAR 1-2, WTC Investigation 



53 



Chapter 3 

ground in open terrain of about 67 mph, based on wind tunnel measurements by CPP'^, and of about 75 
mph, based on the National Building Code of Canada (NBC) provision for centers of large cities'^. (A 
similar provision was deleted from ASCE 7 due to its uncertainty.) The 50 yr 3 s peak gust speed 
estimated by NIST for the three airports (La Guardia, Newark International Airport, and John F. Kennedy 
International airport), including hurricanes, was about 1 12 mph (see Figure 2 of Appendix B), regardless 
of direction. This speed is equivalent to a 96 mph fastest-mile wind speed. Note that the ASCE 7 basic 
wind speed does not correspond to a 50 yr event. Basic wind speeds in the ASCE 7 Standard are defined 
as wind speed estimates corresponding to a 500 year mean recurrence interval, divided by the square root 
of the load factor 1.5. For hurricane-prone regions, the ratio of 500 year speeds to 50 year speeds is 
typically larger than vl-5 . Therefore, the mean recurrence intervals of basic speeds in hurricane-prone 
regions typically exceed 50 years. Table 3-5 shows that significant differences exist among various 
specified design wind speeds, just as significant differences were noted between, say, base shears and 
moments estimated by different laboratories for various wind directions. An evaluation of the wind speed 
specifications and the estimation of improved design wind speeds, as well as protocols for selection of 
site-specific wind speeds as fimctions of direction, are, therefore, in order. 



A 98 mph wind speed averaged over 20 minutes at a height of 1,500 ft above ground is equivalent to a wind speed averaged 
over 1 hr at 1,500 ft above ground at the building site, of 98/1.03=95 mph (see Fig. C6-2, ASCE 7-02 Commentary). By using 
the power law applied to centers of large cities, this speed is approximately equivalent to an hourly mean wind speed at 
1,000 ft above ground at the building site of 95 (1000/1500) =81 mph. According to wind tunnel measurements in the CPP 
report, this is equivalent to a 3 s peak gust at 33 ft above ground in open terrain of about 81 mph, or to a fastest-mile wind 
speed at 33 ft above ground over open terrain of 81/1.26=64 mph. As a check, the averaging time for a 64 mph fastest-mile 
wind speed is 3,600/64=56 s. The ratio of wind speed averaged over 56 s to the hourly mean speed is 1.26 (ASCE 7-02 
Commentary). The ratio of the 3 s speed to the hourly speed is about 1.525. The ratio of the 3 s speed to the fastest-mile speed 
averaged over 56 s is, therefore, about 1.21. Therefore, a 81/1.21= 67 mph fastest-mile wind speed at 33 ft above ground in 
open terrain corresponds approximately to a 98 mph 20-min speed at 1,500 ft elevation at the building site. 
The hourly wind speed at 1,700 ft above ground at the building site (gradient height for centers of large cities according to 
NBC Canada) is (1,700/1,500) x 95=100 mph (gradient mean hourly speed). The nominal hourly mean speed at 33 ft above 
ground in open terrain is (33/900) x 100=59 mph. The fastest-mile wind speed at 33 ft above ground in open terrain is, to a 
first approximation, 59 x 1.22=72 mph. As a check, the averaging time for a 72 mph fastest-mile wind speed is 3,600/72=50 s. 
The ratio of the fastest-mile speed to the mean hourly speed is 1.265. Therefore, 59x1.265=75 mph fastest-mile wind speed at 
33 ft above ground in open terrain corresponds to a 98 mph 20-min speed at 1,500 ft elevation above the building site. 

54 NIST NCSTAR 1-2, WTC Investigation 



Wind Loads on the WTC Towers 



Table 3-5. Comparison between various design wind speeds. 



Source 


Wind Speed (fastest-mile at 33 ft above ground over 
open terrain) 


ASCE 7-02 


88mph 


NYCBC 


80 mph" 


Original WTC design 


67 - 75 mph 


NIST estimate 


96mph 



a. This wind speed is assumed to be defined as a fastest-mile speed, even though no such definition is explicitly included in the 
NYCBC. 



3.5.3 



Wind Engineering Practices Pertaining to Tall Buildings 



Table 3-3 shows that, for reasons explained in Section 3.4, the wind-induced loads on the towers 
estimated by CPP and RWDI differ by about 40 percent. Table 3-6 shows differences among wind 
engineering features of the original design, the CPP study, the RWDI study, and the refined NIST 
estimates. 





Table 3-6. Comparison 


between the various wind studies. 


Wind Study 


Type of Wind Tunnel 
Testing 


Wind Profile 


Integration of 

Aerodynamics with 

Climatology 


Original Design 


Aeroelastic 


Conventional hurricane wind 
profile 


Extreme wind rosette 
assumed circular 


CPP 


HFFB and aeroelastic 


Conventional hurricane 
wind profile 


Sector-by-sector approach 


RWDI 


HFFB and aeroelastic 


Hurricane profile flatter than 
conventional profile 


Out-crossing based on 

sample including weighted 

low wind speeds 


Refined NIST 
Estimates 


Estimates based on RWDI 
and CPP tests 


Conventional hurricane wind 
profile 


Correction to sector-by- 
sector approach 



Such differences highlight the limitations of the current state of practice in wind engineering for tall 
buildings and the need to put order in the field of wind tunnel testing and wind effects estimation. 

The state of the practice with respect to wind loading and response is defined by the relevant assumptions, 
procedures, and methodologies accepted by professionals engaged in the design of super-tall buildings. 
Such professionals are structural engineers with unique experience in structural matters, but no special 
expertise in wind engineering. Therefore, they must rely for definitions of wind loading and response on 
specialized wind engineering practitioners. The state of practice is therefore de facto defined by the 
advice accepted by practicing structural engineers from wind engineering specialists. 

North American structural engineers rely primarily on design wind loads estimated, from wind tunnel 
tests and extreme wind speed data, by three commercial wind engineering organizations: The Boundary 



NISTNCSTAR 1-2, WTC Investigation 



55 



Chapter 3 



Layer Wind Tunnel of UWO (London, Ontario), RWDI (Guelph, Ontario), and CPP (Fort Collins, 
Colorado). Wind load estimates by these three organizations are not necessarily mutually compatible. 
Therefore, what the state of the practice depends largely upon the preferences structural engineers have 
for the practices implicit in the advice offered by these organizations, as well as upon prior experience in 
working with any of them, and wind-study cost considerations. Although some criteria for wind tunnel 
testing are available in Sect. 6. 6 of ASCE 7-02 and in the ASCE Manual Wind Tunnel Studies of 
Buildings and Structures (1999), they are not sufficient to guide these preferences. 

No consensus exists on the wind loading estimates provided by the wind engineering experts. In addition, 
because, in general, the estimates are proprietary and confidential, no scrutiny of the technical basis of the 
estimates being provided is generally possible, nor are building inspectors equipped to offer such scrutiny. 
Finally, and most importantly, the basis for the estimates provided to the structural engineer is commonly 
presented in a manner that, according to some users, lacks clarity, transparency, and sufficient detail, so 
that not only the structural engineering user but even specialized wind engineering experts can have 
difficulty in following and checking key aspects of the calculations on which the estimates are based. 

The state of the art in wind engineering for tall buildings is more advanced than the state of the practice. 
It offers the potential for developing a consensus of acceptable practices based on information and 
procedures representing the advanced knowledge currently available. Such consensus requires the use of 
publicly accessible data and methodologies. The realization that transparency and public scrutiny of wind 
engineering models is in the public interest is illustrated by the recent decision of the Florida Department 
of Insurance to forgo the use of mutually inconsistent "black box" models for which justifications are not 
available in any detail, in favor of the development of an open, public model of hurricane-induced losses. 

For the reasons discussed in this section it is necessary that the following issues be considered: 

• Methods for estimating wind effects with specified mean recurrence intervals that account for the 
directionality of extreme wind speeds, the aerodynamic response, and the dynamic response. 

• Protocols for conducting wind tunnel tests. 

• Criteria for flow structure modeling, including mean wind profiles and turbulence features, for 
various types of wind storms, including hurricanes. 

• Protocols for site-specific estimation of extreme wind speeds from National Oceanic Atmospheric 
Administration (NOAA) and other sources of data for non-hurricane winds. 

• Estimates of hurricane wind speeds for all U.S. hurricane-prone regions, similar to estimates 
currently performed for Florida by NOAA's Hurricane Research Division. 

• Load combinations, and material-specific (e.g., steel, concrete, and composites) responses to peak 
loads. 

Consensus standards need to be developed that would ensure that the current state of practice will be 
brought to a level consistent with the state of the art. 



56 NIST NCSTAR 1-2, WTC Investigation 



Wind Loads on the WTC Towers 



3.6 REFERENCES 

ASCE 7-02: American Society of Civil Engineers, ASCE 7 Standard Minimum Design Loads for 
Buildings and Other Structures, Reston, VA, 2002. 

BOCA/BBC 1965: BOCA Basic Building Code, Fourth Edition, Building Officials and Code 
Administrators, Chicago, IL. 

CPP report: Data Report, Wind-Tunnel Tests - World Trade Center, Cermak Peterka Petersen, Inc. 
August 2002. 

Design Criteria: Design Criteria document for the WTC towers developed by Worthington, Skilling, Helle 
& Jackson. 

IBC 2003: International Building Code, International Code Council, Falls Church, VA, 2003. 

Isyumov, N., Mikitiuk, M.J., Case, P.C, Lythe, G.R., and Welburn, A., (2003), "Predictions of Wind 
Loads and Responses from Simulated Tropical Storm Passages," Proceedings of the Eleventh 
International Conference on Wind Engineering, June 2-5 2003, Lubbock, TX, D.A. Smith and C.W. 
Letchford, eds, Texas Tech University, Lubbock, TX, USA. 

Municipal Code of Chicago Relating to Buildings (as amended to and including January 1, 1967), 1967 
Index Publishing Corp., Chicago, IL 

New York State Building Code, 1964, State Building Construction Code Applicable to General Building 
Construction, Building Code Bureau, State of New York, New York, NY. 

NYCBC 2001: Building Code of the City of New York, 2001 Edition, Gould Publications, Binghamton, 

NY. 

Powell, M.D., Vickery, P. J., and Reinhold, T.A., (2003), "Reduced drag coefficients for high wind speeds 
in tropical cyclones. Nature, Vol. 422, pp. 279-283. 

RWDI report: (South Tower) Final Report, Wind-Induced Structural Responses, World Trade Center - 
Tower 2, Rowan Williams Davis and Irwin, Inc., October 4, 2002. 

RWDI report: (North Tower) Final Report, Wind-Induced Structural Responses, World Trade Center - 
Tower 1, Rowan Wilhams Davis and Irwin, Inc., October 4, 2002. 

Simiu, E. and Heckert, N.A., (1996), "Extreme Wind Distribution Tails: A Peaks Over Threshold 
Approach," J. Struct. Eng. 122 539-547. 

Wind Tunnel Studies of Buildings and Structures (1999), ASCE Manuals and Reports on Engineering 
Practice No. 67, N. Isyumov (ed.), American Society of Civil Engineers, Reston, VA. 

WSHJ Wind Reports: A series of wind reports developed by Worthington, Skilling, Helle & Jackson, 
outlining the development of design wind loads for the WTC towers (see NIST NCSTAR 1-1). 



NIST NCSTAR 1-2, WTC Investigation 57 



Chapter 3 

WTC Dwgs: WTC architectural and structural drawings. 



58 NIST NCSTAR 1 -2, WTC Investigation 



Chapter 4 

Baseline Performance of the WTC Towers 



4.1 INTRODUCTION 

This chapter presents the results of the baseline performance analysis for the World Trade Center (WTC) 
towers. Results are presented for the global models under the three gravity and wind loading cases 
described in Chapter 3. These cases included the original WTC design load case (henceforth referred to 
as original design case), the lower-estimate state-of-the-practice case (henceforth SOP case), and the 
refined National Institute of Standards and Technology (NIST) estimate case (henceforth refined NIST 
case). Baseline performance results are also presented for the typical truss-framed and beam-framed floor 
models under gravity loads only. Baseline performance results include basic information about the 
towers' behavior under design loading conditions, pertaining to total and inter-story drift (the maximum 
sway of the building under design wind loads), floor deflections, demand/capacity ratios of primary 
structural components, exterior columns response (shear lag effects and presence of tensile forces), 
performance of connections, and resistance of the towers to shear sliding and overturning. The baseline 
performance analyses in this chapter were conducted under design loading conditions. Analyses under in- 
service loads of the towers before and after aircraft impact were conducted and reported in NIST 
NCSTAR 1-6. 

This chapter reports on the work conducted by the firm of Leslie E. Robertson Associates (LERA) on the 
baseline performance analyses. The results were reviewed by Skidmore, Owings, and Merrill (SOM) and 
NIST. The reviews included checking the various load vectors, analysis procedure and results, and design 
parameters. 

Section 4.2 presents the results of the baseline performance analysis for the global WTC 1 and WTC 2 
models under the three loading cases. Similarly, Section 4.3 presents the baseline performance results for 
the two typical floor models. Section 4.4 outlines the third-party review by SOM and the in-house review 
by NIST of the baseline performance analyses. Section 4.5 presents a summary of the chapter. 

4.2 BASELINE PERFORMANCE OF THE GLOBAL MODELS 

4.2.1 Analysis Methodology 

This section presents the details of the analysis procedure, including staged construction analysis, the load 
combinations, and method of estimation of the demand/capacity ratios (DCRs) for the structural 
components. 

The global models were analyzed under the three loading cases identifled in Chapter 3. For applying the 
gravity loads to the global models, the nonlinear staged construction analysis function in SAP2000 was 
used. The purpose of using the staged construction methodology in the analysis was to provide, at the top 
of the towers, a reasonably accurate distribution of dead loads between the core columns and the exterior 
walls. The hat truss system that was installed atop the towers distributed gravity and wind loads between 
the core and the exterior walls. The construction dead loads (CDLs) and superimposed dead loads 

NIST NCSTAR 1-2, WTC Investigation 59 



Chapter 4 



(SDLs), put in place prior to the completion of the hat truss system, were not distributed through the hat 
truss. In order reasonably to differentiate between those loads distributed through the hat truss system 
and those that were not, the construction sequence was considered in the analysis. 

The global model of each tower was subdivided into two portions: floor 106 and below, and the area 
above floor 106 that included the hat truss. In the first stage, the lower portion of the global model was 
loaded with all of the CDL and SDL associated with floor 106 and below. In the second stage, the 
portion of the full model above floor 106 was activated, and the CDL and SDL associated with the upper 
floors were placed on the Ml computer model. Live loads on the whole model were applied to the full 
building with the hat truss engaged in the second stage. This methodology approximated well the way in 
which the towers were constructed. 

For all analysis cases, the DCRs for structural components were estimated using the Allowable Stress 
Design (ASD) procedure as specified in the American Institute of Steel Construction (AISC) 
Specification for Structural Steel Buildings - Allowable Stress Design and Plastic Design - 9* Edition, 
1989. The DCRs were calculated by dividing component demands by component capacities, taken at 
unfactored (working) loads and at working stresses, not at ultimate loads or yield stresses. These DCRs 
for the structural components were determined as follows: 

1 . The component demands were obtained from the results of the baseline performance analysis 
using the reference global models, and working loads based on the following load combinations: 

• For the original WTC design loading case and for the lower estimate, state-of-the- 
practice case, the load combinations were those specified by the AISC Specification 
(1989) and the New York City Building Code (NYCBC) 2001: 

Dead Load 

Dead Load + Live Load 

Dead Load + Live Load + Wind Load 

Dead Load + Wind Load 

• For the refined NIST estimate case, the load combinations were those specified by the 
American Society of Civil Engineers (ASCE 7-02) Standard: 

Dead Load 

Dead Load + Live Load 

Dead Load + Wind Load 

Dead Load + 0.75 x (Live Load + Wind Load) 

0.6 X Dead Load + Wind Load 

2. The component capacities were based on the nominal steel strength as specified in the original 
design documents and using the AISC Specification (1989): 

• For the original design loading case and for the lower estimate, state-of-the-practice case 
(consistent with NYCBC 2001), a one-third increase in the allowable stress was 



60 NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



considered for load cases that included wind, as specified at the time of the design and as 
is currently specified in NYCBC 2001 and AISC Specification (1989). 

• For the refined NIST estimate case, where loads were based on the ASCE 7-02 Standard, 
load combinations were taken from the ASCE 7-02 Standard, which does not allow the 
one-third increase in allowable stresses. 

The interaction equation in AISC Specifications (1989) estimates the DCR as the larger of the following 
two equations for members subjected to both axial compression and bending stresses: 



la I ^mx ibx , my I by 



DCR = -^ + '^^V^ — + 



^" (^-jfr)Fbx i^-jfr)Fby 

ex ey 

O.6OF3, F,, F,^ 

For the case when f^/ F^ < 0.15 , the following equation is permitted in lieu of the previous two 
equations: 

^a ^bx ^by 

where the subscripts x and y indicate the axis of bending about which a particular stress or design property 
apphes, and 

F^ and F^ are the axial compressive stress and compressive bending stress, respectively, that 
would be permitted if axial force alone or if bending moment alone existed. 

f^ and ffj are the computed axial stress and compressive bending stress at a given point, 
respectively. 

Fj is the Euler buckling stress divided by a factor of safety. 

C^ is a coefficient that depends on column curvature caused by applied moment. 

A review of the basic design equations and allowable stresses for combined axial and bending stresses for 
the 6th Edition of the AISC Specifications (1963), which was in effect at the time of the design, indicated 
that they are essentially identical to those of the 9th Edition (1989) design equations and allowable 
stresses. There are, however, some variations between the 6th and 9th Editions of the specification. The 
1 963 Specification did not specifically address biaxial bending in the combined stress equations. In 
addition, the allowable stress formulations for bending with lateral torsional buckling are somewhat 
different between the two design specifications. 

For the original design loading case, the SAP2000 program was used directly to estimate the DCRs using 
the equations presented above. For the lower-estimate, state-of-the-practice case and the refined NIST 
estimate case, a second order analysis that accounted for P-A effects was used to estimate member 



NIST NCSTAR 1 -2, WTC Investigation 6 1 



Chapter 4 



demands under the applied gravity and wind loads. The P-A analysis resulted in a moment magnification 
in the components of the global models; and as a result, the terms C^ and (1 — f^ / F^) were assigned a 
unit value in the above equations to estimate component DCRs. For these cases, DCRs were calculated in 
Excel spreadsheets using results obtained by the SAP2000 computer program. 

For further details, see Chapter 5 of NIST NCSTAR 1-2A. 



4.2.2 



Total and Inter-Story Drift 



The calculated total drift of both WTC 1 and WTC 2 induced by the three loading cases is presented in 
Table 4-1. The table lists calculated total drift values at the top of the tower, in absolute terms and as a 
fraction of the height, H, from the foundation level to the roof (referred to in the table as the drift ratio). 
According to LERA, limiting total building drift under wind loads was not part of the original 
WTC design criteria (see NIST NCSTAR 1-2A). Instead, inter-story drifts were determined at the design 
stage and were compared with the ability of the architectural building systems such as the partitions and 
the exterior cladding, to accommodate these inter-story drifts. Accordingly, there is no project-specific 
data available to which the total drifts may be compared. Figure 4-1 presents the deflected shape of 
WTC 1 under the three loading cases. Similarly, Fig. 4-2 shows the inter-story drift distribution along 
the height of the tower, normalized to the story height. The plots are presented for the E-W and N-S 
directions for the wind load combination that produced the maximum cumulative drift for each case. 
Similar plots for WTC 2 can be found in Chapter 5 of NIST NCSTAR 1-2A. 



Table 4-1. Total drift for WTC 1 and WTC 2 


under the three loading cases. 


Loading 

Case 


WTC 1 


WTC 2 


E-W 


N-S 


E-W 


N-S 


Total 
Drift (in.) 


Drift 
Ratio 


Total 
Drift (in.) 


Drift 
Ratio 


Total 
Drift (in.) 


Drift 
Ratio 


Total 
Drift (in.) 


Drift 
Ratio 


Original 
design case 


56.6 


H/304 


55.7 


H/309 


51.2 


H/335 


65.3 


H/263 


SOP case 


56.8 


H/303 


68.1 


H/253 


59.7 


H/287 


56.1 


H/306 


Refined 

NIST case 


70.6 


H/244 


83.9 


H/205 


75.6 


H/227 


71.0 


H/242 



Under the original WTC design loads, the cumulative drifts at the top of the WTC 1 tower were about 
56.6 in. (H/304) and 55.7 in. (H/309) in the E-W and N-S direction, respectively. For WTC 2 the drifts 
were about 51.2 in. (H/335) in the E-W direction and 65.3 in. (H/263) in the N-S direction. For the state- 
of-the-practice case, the drifts for WTC 1 were larger than those from the original design case by about 
0.5 percent and 22 percent for the E-W and N-S directions, respectively; for WTC 2 the drift was larger 
than that from the original design case by about 16 percent and 15 percent for the E-W and N-S drift, 
respectively. These differences are commensurate with those between the base shears for the two cases. 



62 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



The drifts obtained from the refined NIST estimate case were about 25 percent larger than those from the 
state-of-the practice case for both towers. 

As Fig. 4-2 indicates, the inter-story drift varied over the height of the tower. Under the original design 
loading case, the maximum inter-story drift was as high as h/225 and h/195 for WTC 1 and WTC 2, 
respectively, where h is the story height. Maximum inter-story drifts under the state-of-the practice case 
were about h/185 and h/200 for WTC 1 and WTC 2, respectively. For the refined NIST estimate case, 
these inter-story drifts were about 25 percent larger than those from the state-of-the practice case for both 
towers. 

Currently no building codes specify a drift limit for wind design. The ASCE 7-02 Standard states in 
Section B.1.2 that the drift of structures due to wind effects shall not impair the serviceabihty of the 
structure. The commentary to this section of the standard indicates that drift limits in common usage for 
building design are on the order of 1/400 to 1/600 of the building (for total drift) or story height (for inter- 
story drift) to minimize damage to cladding and nonstructural walls and partitions. Structural engineers 
often use in their practice the criterion that total drift ratios should not exceed H/400 to H/500 for 
serviceability considerations and to enhance overall safety and stability (including P-A effects). Typical 
drift limits used in practice (H/400 to H/500) are superimposed on the drift plots shown as the shaded 
areas in Fig. 4-1. Reducing the drift of the WTC towers to the range of H/400 to H/500 (about 43 in. to 
34 in.) would entail enhancing the stiffness and/or the damping capacity of the towers. 

For inter-story drifts, structural engineers often use in their practice an inter-story drift limit in the range 
of h/300 to h/400. This is primarily done for serviceability considerations. Typical inter-story drift limits 
used in practice (h/300 to h/400) are superimposed on the inter-story drift plots shown as the shaded areas 
in Fig. 4-2. Similar to total drift, inter-story drifts of the towers were larger than what is generally used in 
practice today. 

4.2.3 Demand/Capacity Ratios 

The DCR statistics for WTC 1 obtained from the reference global model under the original WTC design 
loading case, the lower-estimate state-of-the-practice case, and the refined NIST estimate case are 
summarized in Tables 4-2, 4-3, and 4-4, respectively. The statistics include, for each member category, 
the total number of members, the mean value of the DCRs, their coefficient of variation (C.O.V.), 
the percentage of components with DCR greater than 1.0 and greater than 1.05, the number of 
components with a DCR greater than 1.05, and the maximum calculated DCR. The DCR statistics for 
WTC 2 under the three loading cases were comparable to those presented herein for WTC 1. See 
Chapter 5 of NIST NCSTAR 1-2A for details. 

Fig. 4-3 shows the distribution of DCRs for the four exterior walls of WTC 1 under the original design 
load case. Close-up views are shown for the exterior walls below fioor 9 in Fig. 4-4. DCRs for the 
WTC 1 core columns are provided in Fig. 4-5. Similar plots for WTC 2 DCRs can be found in Chapter 5 
of NIST NCSTAR 1-2A. 



NIST NCSTAR 1-2, WTC Investigation 63 



Chapter 4 

1600 
1400 

I— —I 
■•-* 

r 1200 
% 

^ 1000 

! 800 

> 

I 600 

o) 400 

^ 200 





/ ~ 


"' / 






/ 


7 








/ 






/ 


7 








t J 


|/ 






















E-W direction 


fl 




Drift jflAx=56.6 



1600 1 
1400 



r 1200 

% 



1000 

800 

■g 600 

§) 400 
a> 

^ 200 





to 

CQ 
0) 

> 
o 





/ - 


"' / 








/ 

/ ' 
1 / 


7 








1 1 


/ 








/ 








/ 


/ 








/ 










/ 




N-S direction 


/ 


Drift „^,=55.7 

1 1 1 



20 40 60 80 100 
Cumulative Drift [in.] 



20 40 60 80 100 
Cumulative Drift [in.] 



(a) Original WTC design wind loads 



1600 
1400 

I— n 

^ 1200 

> 

^ 1000 

! 800 

> 

■9 600 



O) 400 
^ 200 





1 - 


"' 1 






/ 
1 

1 ' 


7 






1 > 
1 > 


/ 






::/ 








1 
1 

1 A 


/ 








if 
if 

i' 










I; 


E-W direction 




Drift ^,=56.8 

1 1 





1600 




1400 


1— 1 














1200 


0) 




> 




0) 

-I 


1000 


to 




m 

0) 


800 


> 




o 






600 


+.» 






400 


0) 




I 


200 








/ ~ 


- 1 


/ 




1 ' 




/ 




1 ' 
, 1 


/ 






1 ' 

■ ■■'/ 


/ 






/7 










f 








if 




N-S direction 






Drift MAx=68-l 

1 1 



20 40 60 80 100 
Cumulative Drift [in.] 



20 40 60 80 100 
Cumulative Drift [in.] 



1600 
1400 
1200 
1000 



to 

^ 800 

> 

I 600 



O) 400 

^ 200 





/ ~ 


-/ 


/ 




/ 




/ 




1 ' 
1 ' 

, / 


/ 








''// 


/ 






/ 
/ 


/ 








;jf 








jf 


E-W direction 


( 


Drift ^,=70.6 

1 1 



(b) Lower estimate, state-of-the-practice wind loads 

1600 

1400 



5- 1200 

0) 

^ 1000 

to 
m 



800 
> 

S 600 

Co 

*^ 

O) 400 

^ 200 







1 - 


"/ 




/ 




1 


/ 


y 


/ 




1 1 
1 1 

, 1 


> 


/ 






1 ' 


/ 






/ 

1 


./ 










/ 












N-S direction 






Drift ^,=83.9 



20 40 60 80 100 
Cumulative Drift [in.] 



20 40 60 80 100 
Cumulative Drift [in.] 



(c) Refined NIST wind loads 
Figure 4-1. Cumulative drift diagrams for WTC 1 under the three wind loading cases. 



64 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



1600 
1400 

I— —I 

c 
r 1200 

> 

^ 1000 

! 800 

> 

I 600 
+j 

■§) 400 
'53 

^ 200 











■^ 










► 






/- 






f 




1/ 


^ 




/ 






1 E- 


W direction 



1600 
1400 

I— —I 

if 

r 1200 

a> 

> 

0) 



1000 
800 
600 
S 400 
^ 200 




0.00 0.20 0.40 0.60 

Interstory Drift/Story Height [%] 





>-.^' — ' 






"^ 


X 








} 








h 








c 






/ 


< 




J 


/ 




^^^ 


z* 


N- 


S direction 




0.00 0.20 

Interstory Drift/Story Height [%] 



1600 

1400 

£l200 

<"1000 



CO 

m 

% 
o 



800 



w 400 
X 
200 



•* 


-*-r ^1 








^ 


v 








\ 






i 


^=- 






^ 






1/ 


^ 






/ 








! ^ 


-w 


direction 



(a) Original WTC design wind loads 

1600 

1400 



£,1200 
■" 1000 



CO 

% 

o 



800 





0.00 0.20 0.40 0.60 

Interstory Drift/Story Height [%] 



.ff 400 
X 

200 







•« 


^ — ' 










^-v 


^, 










) 










A 








L 










/ 


y^ 








y 


( 






^^ 


y 


/ 




N-S direction 



0.00 0.20 0.40 0.60 0.80 
Interstory Drift/Story Height [%] 



1600 
1400 

I— —I 

r 1200 

> 

^ 1000 

\ 800 

> 

I 600 



O) 400 
'5 

X 



200 



















L 






i h 


- 




\f 






\f 






/ 






r \ 

1 E-W direction 



(b) Lower estimate, state-of-the-practice wind loads 

1600 





1400 


1—1 










1200 


0) 




> 




0) 


1000 


CO 




m 




11) 


800 


> 




o 




<8 


600 


+-< 




r 




D) 


400 


lU 




X 






200 




0.00 0.20 0.40 0.60 0.80 

Interstory Drift/Story Height [%] 















"""-^^ 












) 






/ 


^ 






L 








r 








/ 




- ~ 1 — 


r^ 


N-S direction 

' — 1 1 




0.00 0.20 0.40 0.60 0.80 

Interstory Drift/Story Height [%] 



(c) Refined NIST wind loads 
Figure 4-2. Inter-story drift diagrams for WTC 1 under the three wind loading cases. 



NISTNCSTAR 1-2, WTC Investigation 



65 



Chapter 4 



Table 4-2 and Figures 4-3 through 4-5 indicate that under the original WTC design loading case, most 
structural members had a DCR of less than 1.0. A fraction of the structural members had DCRs in excess 
of 1.0. These were mainly observed in the exterior walls and core columns. 

The types of members in the exterior walls that had DCRs larger than 1.0 were calculated for a 
combination of axial load and bending under the combination of gravity and wind loads. These included: 
(1) columns at the corners, (2) where the hat truss connected to the exterior wall, and (3) below floor 9. 
The members in these locations would be expected to experience a large degree of stress. The corner 
columns had some of the highest calculated forces under wind loading. The hat truss-to-exterior wall 
connections interconnected two major structural systems with large concentrated load transfers. The 
exterior wall below floor 9 was a highly variable and articulated structural system that had large 
calculated forces. 

The core columns that had DCRs larger than 1.0 were calculated for axial stresses due to gravity loads 
and were generally located: (1) on the 600 column line between floors 80 and 106, and (2) at core 
perimeter columns 901 and 908 for much of their height. The gravity loads on these columns were 
affected significantly by assumptions about tributary areas, construction dead loads and superimposed 
dead loads, and the sequence of construction of the hat truss. According to LERA, the high degree of 
stress calculated at these core columns was likely associated with differences in these assumptions 
between the original and current computations. 

The results indicated a number of members throughout the structures with DCRs larger than unity, which 
is inconsistent with the design requirements of the AISC, ASD Specification. One possible explanation 
may lie in the computer-based structural analysis and software techniques employed for this baseline 
performance study in comparison with those utilized in the original design nearly forty years ago. An 
example is the contribution of secondary moments in the various elements, which may have gone 
undetected in the original analysis and design. The exterior walls of the towers might have the potential 
for significant redistribution of the loads of members with large DCRs to adjacent members. Demand- 
capacity ratios greater than 1.0 detected in core columns and hat truss members are less easily resolved as 
the ability for redistribution may be limited. 

While it is a normal design practice to achieve a DCR less than unity, the safety of the WTC towers on 
September 1 1 , 200 1 , was most likely not affected by the fraction of members for which the demand 
exceeded capacity due to the following reasons: 

• The allowable stress design method has an inherent factor of safety for structural components. 
The safety factor is about 1.67 and 1.92 for yielding and buckling, respectively, for components 
subjected primarily to gravity loads, such as core columns. The factor of safety is reduced by 1/3 
for components subjected to wind loads, such as the exterior walls, due to the 1/3 increase in the 
allowable stresses. 

• After reaching the yield strength, structural steel components continue to possess significant 
reserve capacity, thus allowing for load redistribution to other components that may still be in the 
elastic range. 



66 NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



The DCRs presented herein were estimated using the design live loads. On September 11, 2001, 
the towers were subjected to in-service live loads, which are considered to be approximately 
25 percent of the design live loads. 



• 



On September 11, 2001, the wind loads were minimal, thus providing significantly more reserve 
capacity for the exterior walls. 

A comparison between Tables 4-2 and 4-3 indicates that the DCRs estimated from the original 
WTC design load case were, in general, close to those obtained for the lower-estimate state-of-the 
practice case for WTC 1. Comparing Tables 4-2 through 4-4, it was found that the DCRs obtained for 
the refined NIST estimate loading case were higher than those from the original WTC design and the 
lower-estimate state-of-the-practice load cases, owing to the following reasons: 

• The refined NIST estimated wind loads were higher than those used in the lower-estimate state- 
of-the-practice case by about 25 percent (about 10 percent difference between the RWDI loads 
scaled to the NYCBC 2001 wind speed and RWDI loads scaled to the ASCE 7-02 wind speed, in 
addition to the 15 percent increase estimated by NIST, Section 3.4). It is noted that the NIST 
estimated wind loads were about 20 percent smaller than those estimated by CPP (an upper- 
estimate state-of-the practice case, see Chapter 3). 

• The original WTC design and the state-of-the-practice cases used NYCBC load combinations, 
which result in lower DCRs than the ASCE 7-02 load combinations used for the refined NIST 
case. 

Similar observations and conclusions could be made for the DCRs estimated for WTC 2 for the three 
loading cases, see NIST NCSTAR 1-2A. 

As part of the in-house NIST review into the baseline performance analyses, the DCRs were estimated 
using the Strength Design procedure as specified in the AISC Load & Resistance Factor Design (LRFD) 
Specification for Structural Steel Buildings (1993). The analysis was conducted for the exterior wall 
columns from fioor 9 to fioor 106, and for core columns of WTC 1 for the refined NIST estimate case. 
For this analysis, the load combinations were those specified by the ASCE 7-02 Standard: 

1 .4 Dead Load 

1.2 Dead Load +1.6 Live Load 

1.2 Dead Load + 0.8 Wind Load 

1.2 Dead Load + Live Load +1.6 Wind Load 

0.9 Dead Load + 1.6 Wind Load 

The DCRs estimated using LRFD for the refined NIST case are presented in Table 4-5, along with the 
DCRs obtained from the ASD method. The results indicate that the mean DCRs estimated using the 
LRFD procedure were smaller than those using the ASD procedure by about 15 percent. 



NIST NCSTAR 1-2, WTC Investigation 67 



Chapter 4 



Table 4-2 


Statistics of OCRs for WTC 1 under ori 


ginal design load case. 












Percentage 


Percentage 














of 


of 


Number of 








Mean 


c.o.v. 


components 


components 


components 


Maximum 




Number of 


Calculated 


of 


with DCR > 


with DCR > 


with DCR > 


Calculated 


Member Type 


Members 


DCR 


DCR 


1.0 


1.05 


1.05 


DCR 


Exterior Wall 
















Columns 
















Below floor 1 


628 


0.77 


0.19 


4.3 


2.7 


17 


1.36 


Floor 1 to 9 


1,122 


0.74 


0.25 


3.3 


0.5 


6 


1.27 


Floor 9 to 106 


31,086 


0.76 


0.12 


1.1 


0.4 


121 


1.31 


Above floor 106 


578 


0.73 


0.31 


12.3 


10.0 


58 


1.46 


Exterior Wall 
















Spandrels 
















Below floor 1 


420 


0.44 


0.46 


0.7 


0.7 


3 


1.28 


Floor 1 to 9 


610 


0.34 


0.45 


1.1 


1.0 


6 


1.30 


Floor 9 to 106 


31,160 


0.31 


0.30 











0.83 


Above floor 106 


836 


0.35 


0.69 


1.9 


1.7 


14 


1.55 


Core Columns 


5,219 


0.86 


0.14 


10 


5.3 


278 


1.36 


Hat Truss System 
















Columns 


239 


0.47 


0.45 


0.4 


0.4 


1 


1.26 


Beams 


499 


0.24 


0.87 


0.4 


0.2 


1 


1.07 


Braces 


279 


0.47 


0.53 


2.5 


0.7 


2 


1.06 


Exterior Wall 
















Bracing 
















Below floor 1 


200 


0.72 


0.16 


2 


1 


2 


1.16 


Above floor 106 


12 


0.40 


0.52 











0.75 



68 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



Table 4-3. Statistics of OCRs for WTC 1 


under the lower estimate, state-of-the 








practice case 
















Percentage 


Percentage 














of 


of 


Number of 






Number 


Mean 


c.o.v. 


components 


components 


components 


Maximum 




of 


Calculated 


of 


with DCR > 


with DCR > 


with DCR > 


Calculated 


Member Type 


Members 


DCR 


DCR 


1.0 


1.05 


1.05 


DCR 


Exterior Wall 
















Columns 
















Below floor 1 


628 


0.77 


0.19 


6.1 


4.0 


25 


1.30 


Floor 1 to 9 


1,122 


0.78 


0.26 


13.1 


5.2 


58 


1.15 


Floor 9 to 106 


31,086 


0.78 


0.13 


2 


0.9 


281 


1.44 


Above floor 1 06 


578 


0.71 


0.31 


10.7 


7.6 


44 


1.36 


Exterior Wall 
















Spandrels 
















Below floor 1 


420 


0.49 


0.46 


4 


2.4 


10 


1.26 


Floor 1 to 9 


610 


0.37 


0.45 


1.3 


1.1 


7 


1.22 


Floor 9 to 106 


31,160 


0.32 


0.29 











0.80 


Above floor 106 


836 


0.35 


0.70 


1.9 


1.7 


14 


1.57 


Core Columns 


5,219 


0.86 


0.14 


9.9 


5.3 


278 


1.36 


Hat Truss System 
















Columns 


239 


0.45 


0.50 


0.4 


0.4 


1 


1.26 


Beams 


499 


0.23 


0.93 


0.2 


0.2 


1 


1.07 


Braces 


279 


0.41 


0.60 


1.1 








1.03 


Exterior Wall 
















Bracing 
















Below floor 1 


200 


0.76 


0.16 


2.5 


2 


4 


1.18 


Above floor 106 


12 


0.35 


0.47 











0.64 



NISTNCSTAR 1-2, WTC Investigation 



69 



Chapter 4 



Table 4-4. Statistics of OCRs for WTC 1 


under the refined NIST estimate case. 










Percentage 


Percentage 














of 


of 


Number of 






Number 


Mean 


c.o.v. 


components 


components 


components 


Maximum 




of 


Calculated 


of 


with DCR > 


with DCR > 


with DCR > 


Calculated 


Member Type 


Members 


DCR 


DCR 


1.0 


1.05 


1.05 


DCR 


Exterior Wall 
















Columns 
















Below floor 1 


628 


1.04 


0.24 


52.5 


47.3 


297 


1.95 


Floor 1 to 9 


1,122 


1.11 


0.27 


69.0 


63.6 


714 


1.69 


Floor 9 to 106 


31,086 


1.10 


0.14 


72.1 


59.7 


18572 


2.05 


Above floor 106 


578 


0.81 


0.28 


19.7 


14.2 


82 


1.57 


Exterior Wall 
















Spandrels 
















Below floor 1 


420 


0.81 


0.46 


22.1 


21.4 


90 


2.05 


Floor 1 to 9 


610 


0.61 


0.45 


8.0 


4.3 


26 


2.03 


Floor 9 to 106 


31,160 


0.52 


0.29 


0.5 


0.3 


109 


1.32 


Above floor 106 


836 


0.41 


0.68 


2.4 


1.9 


16 


1.82 


Core Columns 


5219 


0.84 


0.15 


8.9 


5.2 


270 


1.40 


Hat Truss System 
















Columns 


239 


0.53 


0.49 


3.8 


0.8 


2 


1.26 


Beams 


499 


0.26 


0.93 


1.8 


1.4 


7 


1.30 


Braces 


279 


0.49 


0.55 


6.1 


2.5 


7 


1.10 


Exterior Wall 
















Bracing 
















Below floor 1 


200 


1.11 


0.18 


73.0 


62.0 


124 


1.76 


Above floor 106 


12 


0.52 


0.42 











0.90 



70 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 








Mit%:'-'-:'^'M 





I . fi 



t:::::::::4: 





M i n i. 



SJ'^>^W:i 










:*j: 



^, 




.;:-|;::::ji:; 



t;:rT),:: ;:::::: :::::::::::: 
E^^3!Er^:r::::::-:;: 

'^ ■ ■ ■ V ,^.„. .. ff ..fff .„ 



;::^:::: 



i:::r?::(: 



E^^" 





^ 



^"U"Jbi 



(a) 



(b) 



(c) 



(d) 



0.00 



0.50 



0.75 



1.00 



Figure 4-3. OCRs for the exterior walls of WTC 1 under original design case, (a) north 
elevation, (b) east elevation, (c) south elevation, and (d) west elevation. 



NISTNCSTAR 1-2, WTC Investigation 



71 



Chapter 4 



! I I I . I I I I I 



T"T 



LJLMUJUJlJUJlMUJUJLML^ 




_LLL 



(a) 



JTV 



/K/K/k A A\ A /i\ /h /[\/[\A\/kAA /k a a a /i\ /i\ 

(b) 




00 



,50 



75 



00 



rg 



iror 



Figure 4-4. RCRs for WTC 1 under original design loads below floor 9, (a) north 

elevation, and (b) east elevation. 



72 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 






7K7K7K7K7K7K7K7K7K7K7K7K7K7K7K'^7K 



(c) 



JTTT 



TTT 



1 1 1 1 1 1 1 1 1 1 1 'I I 



T^ 



/ivfy1\ A /T\ A /K /K /N /N /N /N /K /K /N /N /N A /K /N 

(d) 








.00 







,50 







.75 



00 



Figure 4-4. (c) south elevation, and (d) west elevation (continued). 



NISTNCSTAR 1-2, WTC Investigation 



73 



Chapter 4 



106 FL 


105 FL 


104 FL 


103 FL 


102 FL 


101 FL 


100 FL 


99 FL 


98 FL 


97 FL 


96 FL 


95 FL 


94 FL 


93 FL 


92 FL 


91 FL 


90 FL 


89 FL 


88 FL 


87 FL 


86 FL 


85 FL 


84 FL 


83 FL 


82 FL 


81 FL 


80 FL 


79 FL 


7eFL 


77 FL 


76 FL 


75 FL 


74 FL 




72 FL 


71 FL 


70 FL 


69 FL 


66 FL 


67 FL 


66 FL 


65 FL 


64 FL 


63 FL 


62 FL 


61 FL 


60 FL 


59 FL 


58 FL 


57 FL 


56 FL 


55 FL 


54 FL 


53 FL 


52 FL 


51 FL 


50 FL 


49 FL 


48 FL 


47 FL 


46 FL 


45 FL 


44 FL 


43 FL 


42 FL 


41 FL 


40 FL 


39 FL 


38 FL 


37 FL 


36 FL 


35 FL 


34 FL 


33 FL 


32 FL 


31 FL 


30 FL 


29 FL 


28 FL 


27 FL 


26 FL 


25 FL 


24 FL 


23 FL 


22 FL 


21 FL 


20 FL 


19 FL 


18 FL 


1/FL 


16 FL 


15 FL 


14 FL 


13 FL 


12 FL 


11 FL 


10 FL 


09 FL 


08 FL 


07 FL 


06 FL 


05 FL 


04 FL 


03 FL 


02 FL 


01 FL 


B1 FL 


B2FL 


B3FL 


B4FL 


B5FL 



TOWER A, DCR of CORE COLUMN 
500's COLUMN NUMBER 


501 


502 


503 


504 


505 


506 


507 


508 




1.05 1 1.00 1 0.92 1 1.01 1 0.88 1 1.01 


0.98 


101 




1 07 





R3 




0.94 





95 


1.02 


1.02 HUlH 1 m ^^^ 1 ni 


1 01 


1 


07 






c 


JS 




1 


01 


1 07 


1 oa 





83 


C 


80 


C 


32 


c 


85 


c 


90 


c 


93 





88 





90 





90 


C 


as 


C 


97 


c 


90 


f 


95 


f 




c 


93 





96 


r 


96 


1 


01 


1 


53 


c 


95 


1 


01 


1 


04 


c 


97 


1 


J1 


1 


01 




74 






c 


36 





74 


c 


92 


c 


34 


c 


35 


c 


36 


t 


78 


c 


91 


c 


91 


c 


78 


c 


96 


c 


38 


c 


90 


c 


90 


c 


82 


c 


95 


c 


95 


c 


82 


1 


01 


c 


92 


c 


94 


c 


95 


c 


73 


c 


34 


c 


34 





73 


c 


88 


c 


31 


c 


33 


c 


73 


c 


77 


c 


37 


c 


37 


c 


77 


c 


91 


c 


34 


c 


36 


c 


77 


(- 


dU 


L 


91 


L 


91 


I 


8U 


I 


95 


c 


3/ 


c 


90 


c 


3U 


c 


76 


C 


30 


c 


30 


c 


76 


c 


91 


c 


78 


c 


30 


c 


59 


c 


79 


C 


33 


c 


33 


c 


79 


c 


95 


c 


31 


c 


33 


c 


71 


c 


82 


C 


37 


c 


36 


c 


82 


c 


98 


c 


34 


c 


36 


c 


74 


c 


70 


C 


78 


c 


77 





70 


c 


86 


c 


75 


c 


33 


c 


70 


c 


73 


C 


31 


c 


30 





73 


c 


89 


c 


78 


c 


36 


c 


73 


c 


75 


c 


33 


c 


33 


f 


75 


f 


9? 


r 


31 


r 







75 


c 


71 


c 


93 


c 


30 





71 


c 


92 


c 


38 


c 


99 


c 


52 


c 


73 


c 


96 


c 


32 





73 


c 


95 


c 


90 




c 


55 


c 


76 


c 


99 


c 


35 


c 


76 


c 


98 


c 


93 


1 on 


c 


57 


c 


83 


c 


91 


c 


90 


c 


83 


c 


89 


c 


37 


c 


94 


c 


53 


c 


86 


c 


94 


c 


93 


c 


86 


c 


92 


c 


90 


c 


96 


c 


56 


c 


8S 


c 


97 


c 


95 


c 


88 


c 


94 


c 


93 


c 


99 


c 


58 


f 


88 


r 


91 


r 


93 


c 


88 


c 


9? 


c 


38 


r 


94 


r 


38 


c 


91 


c 


94 


c 


96 


c 


91 


c 


95 


c 


90 


c 


97 


c 


91 


c 


95 


c 


98 


1 


JO 


c 


95 


c 


99 


c 


93 


1 00 


c 


95 


c 


66 


c 


57 


c 


68 





66 


c 


67 


c 


55 


69 


c 


56 


c 


67 


c 


59 


c 


70 





67 





68 


c 


56 


70 


c 


58 


c 


83 


[ 


34 


c 


35 


c 


83 


c 


89 


c 


34 


0.86 


c 


54 


c 


84 


[ 


36 


c 


36 


c 


84 


c 


90 


c 


35 


87 


c 


55 


































c 


82 


c 


34 


c 


35 


c 


82 


c 


85 


c 


34 


c 


36 


c 


53 


c 


84 


c 


35 


c 


37 


c 


84 


c 


87 


c 


35 


c 


38 


c 


55 





86 





87 





89 





86 





88 





87 





90 





87 





81 





80 





83 





81 





85 





82 





83 





82 





84 





83 





86 





84 





89 





85 





86 





85 





85 





84 





87 





85 





89 





85 





87 





86 





87 





86 





89 





87 





92 





88 





89 





87 





88 





87 





91 





88 





93 





89 





91 





89 





90 





89 





93 





90 





95 





91 





93 





91 





86 





87 





89 





86 





92 





88 





88 





86 





87 





88 





91 





87 





94 





89 





90 





88 





89 





90 





92 





89 





96 





91 





91 





89 





86 





87 





88 





86 





93 





88 





88 





87 





88 





88 





89 





88 





95 





89 


89 





88 





89 





90 





91 





89 





96 





91 


0.91 





90 





86 





85 





88 





86 





91 





87 


86 





87 





88 





87 





90 





88 





92 





88 


88 





88 





89 


t) 


88 


t) 


91 





89 





94 





89 





89 





89 





84 





86 





89 





84 





91 





87 





87 





85 





85 





88 





90 





85 





93 





89 





89 





86 





87 





89 





92 





87 





94 





90 





90 





87 





85 





86 





88 





85 





92 





87 





87 





86 





87 





88 





90 





87 





94 





89 





88 





87 





88 


n 


89 


n 


91 





88 





95 





90 





90 





88 





86 





86 





88 





86 





93 





87 





86 





86 





87 





87 





89 





87 





94 





88 





87 





87 





88 





88 





90 





88 





96 





89 





89 





89 





80 





80 





83 





80 





89 





83 





80 





81 





83 





82 





85 





83 





91 





85 





82 





83 





63 


t) 


R? 


t) 


64 





63 





69 





83 





63 





64 





65 





64 





66 





65 





70 





64 





54 





65 





84 





85 





87 





84 





95 





86 





86 





85 





85 





86 





87 





85 





96 





87 





86 





85 





84 





84 





86 





84 





94 





85 





85 





84 





85 





85 





87 





85 





95 





86 





86 





85 


c 


86 


c 


36 


c 


38 


c 


86 


c 


96 


c 


37 


c 


37 


c 


56 


c 


83 


c 


33 


c 


37 


c 


83 


c 


95 


c 


35 


c 


35 


c 


53 


c 


84 


c 


35 


c 


38 


c 


84 


c 


96 


c 


36 


c 


36 


c 


55 


c 


85 


c 


36 


c 


39 


c 


85 


c 


97 


c 


37 


c 


37 


c 


56 


f 


84 


r 


34 


r 


37 


c 


84 


c 


95 


r 


35 


c 


35 


r 


35 


c 


85 


c 


35 


c 


38 


c 


85 





96 


c 


36 


86 


c 


56 


c 


86 


c 


36 


c 


39 


c 


86 





97 


c 


37 


0.87 


c 


57 


c 


84 


c 


34 


c 


37 


c 


84 





94 


c 


35 


83 


c 


54 


c 


85 


c 


35 


[ 


38 





85 





95 


c 


36 


84 


c 


55 


c 


86 


c 


36 


[ 


39 


c 


86 





96 


c 


37 


85 


c 


56 


c 


85 


c 


34 


[ 


35 


c 


85 


c 


95 


c 


35 


83 


c 


55 


c 




c 


35 


c 




r 


85 


r 


96 


r 






r 


36 


c 


86 


c 


36 


c 


37 


c 


86 


c 


97 


c 


37 


C 


35 


c 




c 


83 


c 


32 


c 


37 


c 


83 


c 


96 


c 


35 


C 


33 


c 




c 


84 


c 


33 


c 


38 


c 


84 


c 


97 


c 


36 


C 


34 


c 




c 


85 


c 


34 


c 


39 


c 


85 


c 


98 


c 


37 


C 


35 


c 




c 


84 


c 


32 


c 


37 


c 


84 


c 


94 


c 


35 


C 


33 


c 




c 


85 


c 


33 


c 


38 


c 


85 


c 


95 


c 


36 


C 


34 


c 




t 


86 


L 


34 


L 


39 


I 


86 


I 


96 


c 


3/ 


c 


3b 


6 


36 


c 


84 


c 


33 


c 


37 


c 


84 


c 


95 


c 


35 


c 


33 


C 


54 


c 


85 


c 


33 


c 


38 


c 


85 


c 


96 


c 


36 


c 


34 


C 


55 


c 


86 


c 


34 


c 


38 


c 


86 


c 


97 


c 


37 


c 


35 


C 


56 


c 


85 


c 


33 


c 


37 


c 


85 


c 


96 


c 


36 


c 


33 


c 


55 


c 


86 


c 


34 


c 


37 


c 


86 


c 


97 


c 


36 


c 


54 


c 


56 


r 


87 


c 


35 


c 




r 


87 


r 




r 


37 


r 


35 


r 


37 


c 


86 


c 


33 


c 


36 


c 


86 


c 


95 


c 


36 


c 


33 


c 


36 


c 


86 


c 


34 


c 


37 


c 


86 


c 


96 


c 


37 


c 


34 


c 


57 


c 


88 


c 


35 


c 


39 


c 


38 


c 


97 


c 


38 


c 


35 


c 


58 


c 


92 


c 


33 


c 


92 


c 


92 


1 04 


c 


90 


c 


39 


c 


54 


c 


92 


c 


36 


c 


93 


c 


92 


1 04 


c 


91 


c 


90 


c 


58 


c 


78 


c 


76 


c 


78 


c 


78 


0.81 


c 


77 


c 


75 


c 


78 


t 


/9 


L 


// 


L 


/9 


u 


/9 


82 


c 


/8 


c 


/6 


c 


ya 


c 


79 


c 


77 


c 


79 





79 


82 


c 


78 


c 


77 


c 


79 


c 


77 


c 


75 


c 


78 





77 





80 


c 


77 


c 


75 


c 


77 


c 


84 


c 


31 


c 


33 


c 


84 


c 


87 


c 


32 


81 


c 


54 


c 


86 


c 


32 


c 


35 


c 


86 


c 


89 


c 


33 


82 


c 


57 


c 


83 


c 


33 


c 


36 


c 


83 


c 


90 


c 


34 


83 


c 


53 


: 


8? 


r 


3? 


r 


■15 


r 


8? 


r 


90 


r 




82 


r 


3? 


[ 


83 


c 


33 


c 


86 


c 


83 


c 


91 


c 


35 


0.83 


c 


53 


[ 


83 


c 


31 


c 


83 


c 


83 


c 


89 


c 


30 


0.80 


c 


54 


[ 


89 


c 


36 


c 


88 


c 


89 


c 


95 


c 


35 


0.85 


c 


91 





90 





86 





89 





90 





96 





86 


0.86 





91 



(a) 



TOWER A, DCR of CORE COLUMN 
600's COLUMN NUMBER 


601 


602 


603 


604 


605 


606 


607 


608 




94 1 It:; ^^H 1 


01 


1.01 


1.05 ^^H 1) 


80 


1.03 ^^^^^^B 






^^^^^H u 


h;; 


^^m TuiT^^Bi 


94 


0.96 


(iTrr^^B 1 


34 


1 "^ ^^* 1 


01 


1 03 


1 ni ^^H r 


93 


^^^H^^^^l 1 


08 


^ 


1 07 ^^H 1 


02 


^^■iTie 1 1.05 1 c 






1 07 1 1 OS^^^H 


^^^H^^HH 


93 


1.05 ^^^^^^^^^M 


^^^^^^^HH 


99 


^^^^l^^^l^^^^^l 


^^■iTii 1 nnn 1 n 




1.02 1 i.orTTTin^^B 


^^■~Tri6 ^^Hi 


01 


'Tri7~^^m~u)r'^^U 


^^^HHiIm 


1 


Ub 


^^^^^^^^■^^^^^1 




I 




1 n=; 1 1 n^^H^^^H 


^^^^1 


01 


c 


96 




Tp^^^^^^M 1 on ^^H 


^^H 1 


34 


c 


99 


1 


1/ ^^^^^H 1 it4 ^^H 


^^M (■ 


98 


f 


93 


r 


33 


f 


98 


1.03 


0.98 1.07 


^^H 1 


02 


c 


95 


c 


86 


1 


02 


1.07 


3K^™ 


^^H 1 


}5 


c 


98 


c 


59 


1 


05 


^^1 


j^r^H 


^^w 




c 


89 


r 


93 


f 


92 


1 02 


0.94 


1.03 




f 


9? 


r 


96 


c 




1.06 


0.97 


1.08 


1.08 n 


01 





94 


1 


00 





99 ^^H 1 00 


^^ 


^^M i; 


46 


L 


43 


I. 


1/ 


( 




1.00 


^^M r 


99 





95 





90 


n 


90 ^^H 1 


1.04 


^^H 1 


11 


r 


98 


r 


93 


f 


93 ^^M 1 05 


1.0R 


^H~c 


93 


t 


90 


r 


96 





96 


0.99 


f 


9f; ^^H 


1 


01 


■E 


37 


c 


93 


c 


99 





99 


1.02 


1 


00 


^^H 


1 


04 




17 


f 


99 


1 


14 


1 


13 


1.07 


1 


05 


^^1 


1 


OR 


-c 


38 


f 


93 


r 


77 





82 


1.07 


f 


8fi 


f 


97 


1 


04 


C 


90 


f 


96 


r 




f 


^4 ^^^ I 






■)1 


1 


OR 


r 


9? 


f 


98 


i: 


76 





87 ^^H r 


90 


1 


05 


[ 


76 





63 





66 





62 





68 


C 


65 





66 


c 


85 


c 


7R 


r 


57 


r 


70 


r 


55 


r 


79 


r 


5fl 


r 


70 


f 


36 


[ 


97 


c 


34 




89 


c 


52 


c 


91 


c 


56 


c 


86 


1 


00 


[ 


97 


c 


35 


c 


89 


c 


52 


c 


92 


c 


56 


c 


86 


1 


01 


1 


DO 


c 


36 


c 


91 


c 


54 


c 


93 


c 


58 


c 


88 


1 


04 


- 


no 


r 


33 


c 


87 


r 


57 


f 


85 


r 


53 


f 


85 


f 


99 


- 


03 


c 


34 


c 


88 


c 


53 


c 


86 


c 


54 


c 


86 


1 


02 


1 


05 





85 





90 





85 





88 





fl6 





88 


1 


04 





80 





78 





82 





80 





84 





83 





83 





86 


n 


84 





81 





85 





R4 


n 


88 





R7 





87 





91 


n 


83 





80 





85 





R4 


n 


87 





RR 





86 





90 


n 


89 





8? 





89 





R7 


n 


89 





RR 





89 


1 


07 


n 


90 





8? 





89 





R7 





89 





RR 





89 


^^H 





92 





84 





92 





90 


92 





90 





91 


^^1 





90 





81 





89 





86 


88 





87 





88 





99 





91 





83 





90 





88 


89 





88 





89 


1 


01 





93 





84 





92 





89 





91 





89 





91 


1 


03 





90 





81 





86 





90 





87 





87 





87 


1 


05 





91 





82 





87 





91 





89 





88 





88 


I 


37 
OS 





93 





83 





88 





93 





90 





89 





89 





90 





81 





86 





90 





87 





fl6 





86 





92 





82 





87 





92 





88 





67 





87 


I 


37 
04 





93 





83 





88 





93 





89 





R8 





88 





87 





81 





86 





90 





91 





86 





83 





8R 





87 





87 





91 





92 





R7 





84 


1 


OR 





89 





83 





88 





93 





93 





R8 





85 


1 


OR 





87 





81 





86 





90 





90 





RR 





83 


1 


04 





89 





87 





87 





91 





91 





fl7 





83 


1 


05 





90 





83 





88 





92 





93 





88 





84 


1 


07 





88 





81 





86 





93 





90 





84 





82 


1 


03 





89 





82 





87 





94 





91 





85 





83 


1 


05 





91 





83 





88 





96 





92 





86 





84 


1 


07 





83 





77 





81 





86 





84 





80 





75 





92 





85 





78 





83 





69 





86 





81 





77 





95 





64 





56 





60 





65 





65 





61 





58 





70 





66 





56 





63 





68 





68 





64 





61 





72 





85 





77 





83 





R8 





90 





84 





81 





92 





85 





76 





82 





88 





90 





84 





81 





92 


n 


84 





77 





a? 





88 


n 


88 





R4 





80 





90 


n 


85 





78 





83 





90 





89 





R5 





81 





91 


[ 


87 


c 


79 


t 


84 


c 


91 


c 


90 


c 


56 


c 


82 


c 


93 


[ 


85 


c 


78 


t 


81 


c 


58 


c 


87 


c 


54 


c 


81 


c 


91 


[ 


86 


c 


79 


t 


82 


c 


59 


c 


88 


c 


55 


c 


82 


t 


92 


t 


RR 


r 


30 


r 


8? 


r 


91 


f 


90 


r 


56 


r 


87 


c 


93 


[ 


84 


c 


77 


c 


81 


c 


59 


c 


87 


c 


53 


c 


80 


c 


92 


[ 


85 


c 


77 


c 


82 


c 


90 


c 


88 


c 


54 


c 


81 


c 


93 


[ 


86 


c 


78 


c 


83 


c 


91 


c 


89 


c 


55 


c 


82 


c 


94 


c 


87 


r 


77 


r 


82 


r 




c 


■17 


c 


34 


r 




f 


97 


c 


83 





78 


r 


a? 





90 





88 


c 


R5 





87 


f 


94 


[ 


84 


c 


78 


c 


83 


c 


91 


c 


89 


c 


36 


c 


83 


c 


95 


[ 


83 


c 


77 


c 


82 


c 


59 


c 


88 


c 


53 


c 


80 


c 


93 


t 


84 


r 


78 


c 


8? 


r 


90 


f 


88 


r 


54 


r 


81 


c 


94 


c 


85 


c. 


79 


r 


83 


r 


91 





89 


r 


R5 


f 


87 


c 


95 


[ 


82 


c 


78 


c 


81 


c 


59 


c 


88 


c 


52 


c 


80 


c 


94 


[ 


83 


c 


78 





81 


c 


90 


c 


89 


c 


52 


c 


81 


c 


95 


[ 


84 


c 


79 





82 


c 


91 


c 


89 


c 


53 


c 


82 


c 


96 


c 


87 


r 


78 


r 




r 


97 


r 




r 


57 


r 




r 


94 


[ 


83 


c 


79 


t 


82 


c 


93 


c 


89 


c 


53 


c 


82 


c 


96 


[ 


84 


c 


79 


c 


83 


c 


94 


c 


90 


c 


53 


c 


83 


c 


97 


[ 


82 


c 


78 


c 


81 


c 


92 


c 


88 


c 


52 


c 


80 


c 


93 


c 


83 


c 


79 


c 


8? 


r 


93 


c 


89 


r 


5? 


f 


80 


c 


94 


[ 


84 


c 


79 


c 


83 


c 


94 


90 


c 


53 


c 


81 


c 


95 


[ 


81 


c 


77 


c 


80 


c 


93 


90 


c 


51 


c 


80 


c 


93 


[ 


82 


c 


78 


c 


81 


c 


93 


91 


c 


52 


c 


80 


c 


94 


c 


83 


r 


79 


r 


82 


r 


94 


92 


r 


57 


r 




r 


95 


c 


87 





78 


r 


81 





93 


90 


r 


R7 





79 


r 


94 


[ 


83 


c 


78 


c 


82 


c 


94 


0.91 


c 


52 


c 


79 


c 


95 


[ 


84 


c 


30 


c 


83 


c 


95 


0.92 


c 


54 


c 


80 


c 


96 


[ 


92 


c 


76 


c 


90 


1 


04 


1 01 


c 


97 


c 


87 


1 


03 


t 


9? 


r 


31 


c 


90 


1 


14 


1 02 


r 


9R 


f 


87 


1 


14 


[ 


57 


c 


53 





56 


c 


52 


f 


60 


c 


50 


c 


55 


c 


61 


[ 


59 


c 


55 


c 


57 


c 


53 


C 


62 


c 


52 


c 


56 


c 


62 


[ 


59 


c 


55 


c 


58 


c 


54 


C 


63 


c 


53 


c 


57 


c 


62 


c 


57 


r 


54 


r 


56 


r 


57 


r 


fil 


r 


51 


r 


55 


r 


50 




66 


c 


51 


t 


64 


c 


71 





70 


c 


59 


[ 


63 


c 


69 


c 


69 


c 


52 


t 


66 


c 


74 





72 


c 


71 


c 


54 


c 


72 


[ 


80 


c 


79 




82 


c 


92 


c 


90 


c 


58 


c 


79 


c 


58 


t 


79 


r 


78 




81 


r 


91 


f 


89 


r 


57 


f 


79 


f 


3R 


t 


81 


r 


79 


r 


8? 


r 


93 





90 


r 


5R 


r 


SO 


t 


89 


[ 


77 


c 


75 


c 


81 


c 


59 


c 


87 


c 


57 


c 


78 


c 


54 


[ 


79 


c 


33 


c 


89 


c 


99 


97 


c 


95 


c 


86 


c 


95 





80 





83 





90 


1 


00 


0.98 





96 





87 





96 



(b) 








.00 





.50 





.75 


1 


.00 


l^^^^^^H 



Figure 4-5. DCRs for WTC 1 core columns under original design loads, (a) 500 line, and 

(b) 600 line. 



74 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



106 FL 


105 FL 


104 FL 


103 FL 


102 FL 


101 FL 


100 FL 


99 FL 


98 FL 


97 FL 


96 FL 


95 FL 


94 FL 


93 FL 


92 FL 


91 FL 


90 FL 


89 FL 


88 FL 


87 FL 


86 FL 


85 FL 


84 FL 


83 FL 


82 FL 


81 FL 


80 FL 


79 FL 


78 FL 


77 FL 


76 FL 


75 FL 


74 FL 


73 FL 


72 FL 


71 FL 


70 FL 


69 FL 


68 FL 


67 FL 


66 FL 


65 FL 


64 FL 


63 FL 


62 FL 


61 FL 


60 FL 


59 FL 


58 FL 


57 FL 


56 FL 


55 FL 


54 FL 


53 FL 


52 FL 


51 FL 


50 FL 


49 FL 


48 FL 


47 FL 


46 FL 


45 FL 


44 FL 


43 FL 


42 FL 


41 FL 


40 FL 


39 FL 


38 FL 


37 FL 


36 FL 


35 FL 


34 FL 


33 FL 


32 FL 


31 FL 


30 FL 


29 FL 


28 FL 


27 FL 


26 FL 


25 FL 


24 FL 


23 FL 


22 FL 


21 FL 


20 FL 


19 FL 


18 FL 


17 FL 


16 FL 


15 FL 


14 FL 


13 FL 


12 FL 


11 FL 


10 FL 


09 FL 


08 FL 


07 FL 


06 FL 


05 FL 


04 FL 


03 FL 


02 FL 


01 FL 


Bl FL 


B2FL 


B3FL 


B4FL 


B5FL 



TOWER A, DCR of CORE COLUMN 
700's COLUMN NUMBER 


701 702 703 704 705 706 707 708 




0.78 


t 


99 


t 


J6 


L 


35 


L 


32 


t 


bU 


L 


/8 


t 


32 


0.85 


1 


07 


r 


il 


C 


11 


C 


40 


C 


57 


I 


84 


r 


39 


1.00 


c 


99 


c 


78 


C 


16 


C 


40 


C 


58 


C 


79 


c 


79 


1.07 


1 


Ob 





8b 





21 


u 


A! 


u 


64 





84 





8b 


0.93 


c 


37 


"o 


79~ 


"o 


3i" 


"o 


60~ 


"o 


64" 


"o 


80~ 


~o 


72~ 


0.99 


t 


91 


L 


ib 


L 


36 


(. 


66 


u 


6H 


L 


84 


t 


lb 


1.05 


c 


96 


c 


59 


C 


42 


73 





75 


C 


89 


c 


30 


0.82 


t: 


85 


i 


/b 


L 


35 


0.79 





69 


L 


84 


i. 


n 


0.86 


i 


89 


i 


iU 


L 


39 


86 





/■i 


L 


88 


t 


/b 


0.90 


i 


93 


i 


M 


L 


44 


93 


t 


/H 


L 


92 


t 


/9 


0.86 


i 


80 


i 


71 


L 


3/ 


82 





12 


L 


83 


t 


32 


0.90 


c 


84 


c 


75 





41 


C 


87 


c 


77 


C 


87 


c 


36 


0.94 


L 


8/ 


t 


79 


L 


44 


(. 


93 


t 


HI 


L 


91 


L 


90 


0.80 


L 


80 


L 


74 


L 


42 


(. 


89 


t 


lb 


L 


83 


L 


lb 


0.83 


L 


83 


t 


7/ 


L 


4b 


L 


93 


t 


/9 


L 


86 


L 


('9 


0.87 


L 


86 


t 


iU 


L 


48 


(. 


98 


t 


83 


L 


90 


L 


32 


0.78 


l 


79 


c 


76 


r 


6? 


f 


81 


c 


7R 


C 


S3 


C 


75 


0.81 


C 


82 


c 


79 





65 


C 


85 


c 


81 


C 


86 


C 


78 


0.84 


L 


84 


i 


i2 





69 


L 


89 


L 


8b 


L 


SB 


C. 


31 


0.77 


L 


80 


i. 


/H 


I. 


=6 




-.)/ 


L 


33 


I 


Sb 


t 


^6 


0.79 


C 


82 


c 


79 


r 


f^u ^^^H r 


37 


I 


SB 


f 


78 


0.82 





85 


n 


a? 





71 ^^H n 


9(1 





91 





81 


0.79 





79 





77 





70 ^^H 


84 





78 





75 


0.81 


I 


HV 


c 


W 


I. 


/A ^^H ( 


3/ 


I 


81 


c 


(-H 


0.84 


c 


84 


c 


32 


c 


76 


1.06 


c 


90 


C 


83 


c 


30 


0.78 


I 


87 


c 


lb 


I. 


b5 


c 


44 


I 


94 


c 


34 


0.81 


<:. 


91 


r 


19 


c 


KU ^^^ (' 


98 


I 


9H 


r 


37 


0.84 


a 


94 





92 





73 ^^H 1 


n? 


1 


01 





91 


0.67 





60 


c 


46 


c 


46 


C 


66 


c 


50 


C 


59 


c 


58 


0.69 





6b 


i 


53 





5b 


1- 


61 


t 


bl 





64 


L 


(-O 


0.84 


L 


8b 


i 


i2 


L 


/6 


1- 


69 


t 


94 


L 


89 


t 


90 


0.85 


L 


84 


i 


33 





/b 


1- 


/O 


t 


94 


L 


90 


t 


91 


0.87 


C 


86 


c 


34 


c 


76 


[ 


72 


c 


97 


C 


92 


c 


93 


0.85 


L 


81 


L 


/9 





rz 


L 


69 


t 


92 


L 


8/ 


t 


91 


0.87 


L 


83 


L 


30 





/4 


L 


12 


t 


94 


L 


89 


L 


93 


0.89 





84 


U 


81 





/b 


U 


/4 


u 


9/ 





91 





95 


0.69 





80 





77 





72 





71 





83 





87 





76 


0.72 


C 


85 


c 


30 


[ 


77 


C 


78 


c 


89 


C 


92 


c 


79 


0.71 





82 





80 





74 





76 





87 





90 





79 


0.87 





84 





81 





75 





78 





89 





92 





93 


0.88 





84 





81 





76 





78 





89 





92 





94 


0.90 





86 





83 





/6 





83 





93 





9b 





9/ 


0.87 





79 





83 





79 


80 





87 





90 





95 


0.89 





81 





84 





81 


82 





88 





92 





96 


0.91 





82 





85 





82 


84 





89 





93 





98 


0.86 





S3 





81 





78 


81 





84 





Sfi 





95 


0.88 





84 





82 





79 


0.83 





85 





87 





96 


0.89 





86 





83 





80 


0.85 





86 





88 





98 


0.87 





79 





81 





77 


0.81 





87 





90 





92 


0.89 





80 





8? 





78 


0.83 





88 





91 





94 


0.90 





81 





83 





79 


85 





89 





92 





95 


0.88 





82 





80 





76 


83 





84 





84 





91 


0.90 





83 





81 





77 


84 





85 





85 





92 


0.91 





84 





82 





/8 


86 





SI 





86 





93 


0.89 


c 


78 


c 


79 





75 


C 


84 


c 


82 


c 


87 


c 


92 


0.91 





79 





80 





76 





85 





83 





88 





93 


0.92 





80 





81 





77 





87 





84 





89 





94 


0.90 





81 





78 





74 





84 





85 





82 





92 


0.92 


c 


82 


c 


79 


i: 


75 


c 


86 


c 


86 


c 


S3 


c 


93 


0.93 





83 





80 





77 





88 





88 





85 





94 


0.92 





77 





75 





72 





79 





81 





79 





86 


0.95 





80 





76 





75 





82 





83 





81 





89 


0.66 


c 


72 


c 


56 





56 


[ 


57 


c 


64 


c 


73 


c 


57 


0.68 





76 





59 





60 





60 





69 





77 





69 


0.90 





78 





79 





78 





76 





90 





87 





91 


0.90 





// 





79 





// 


u 


/6 


u 


89 





8/ 





90 


0.87 


c 


78 


c 


79 





78 


c 


77 


c 


89 


c 


SO 


c 


39 


0.88 





79 





79 





79 





79 





90 





81 





91 


0.90 


L 


80 


i 


30 


L 


80 


1- 


80 


c 


91 


L 


82 


t 


92 


0.87 


c 


73 


c 


78 


c 


77 


t 


78 


c 


89 


c 


83 


c 


90 


0.88 


I 


/4 


t 


/9 


L 


/8 


L 


/9 


t 


9U 


L 


84 


L 


91 


0.89 


I 


/b 


t 


79 


L 


/8 


L 


80 


t 


91 


L 


8b 


L 


92 


0.86 


I 


/6 


t 


78 





/6 





/8 


t 


89 


L 


// 


L 


90 


0.87 


I 


// 


L 


79 





// 





80 


t 


90 


L 


/8 


L 


91 


0.88 


l 


77 


c 


30 


r 


78 


81 


c 


91 


C 


79 


C 


92 


0.85 


n 


77 


c 


78 





76 


0.79 


c 


88 


C 


80 


C 


91 


0.86 


i 


/8 


(. 


78 





/6 


80 


t 


39 


L 


81 


L 


92 


0.87 


L 


/8 


(. 


/9 





// 


82 


t 


9U 


L 


82 


t 


93 


0.86 


L 


// 


i 


// 





/b 


80 


t 


84 


L 


83 


t 


90 


0.87 


L 


// 


i 


// 


L 


/6 


C 


81 


t 


3b 


L 


84 


t 


92 


0.88 


C 


78 


c 


78 


c 


76 


c 


82 


c 


36 


C 


85 


C 


93 


0.86 


L 


// 


t 


/6 





/4 


L 


80 


t 


31 


L 


83 


t 


91 


0.87 


L 


// 


t 


// 





/b 


L 


81 


t 


82 


L 


84 


t 


92 


0.88 


L 


/8 


L 


78 





/6 


(. 


83 


t 


83 


L 


Bb 


t 


93 


0.85 


L 


// 


L 


76 





/6 


(. 


81 


t 


80 


L 


83 


t 


91 


0.86 


C 


78 


c 


77 





77 


82 


c 


81 


C 


83 


C 


92 


0.87 


L 


/9 


i 


/8 





/8 


83 


t 


82 


L 


84 


C 


93 


0.86 


L 


// 


i 


/6 





/6 


78 


c 


/9 


L 


81 


C. 


39 


0.87 


L 


/8 


i 


// 





1! 


79 


c 


HU 


L 


82 


c: 


90 


0.88 


L 


/8 


i 


// 





1! 


80 


c 


HI 


L 


83 


t 


91 


0.86 


C 


77 


c 


76 





73 


C 


82 


c 


80 


C 


82 


c 


39 


0.87 


L 


/8 


t 


// 





n 


L 


83 


t 


HO 


L 


82 


t 


39 


0.88 


L 


/8 


t 


// 





/4 


L 


84 


t 


HI 


L 


83 


t 


90 


0.86 


L 


// 


L 


74 





/b 


L 


/8 


t 


HO 


L 


81 


L 


38 


0.87 


L 


/8 


t 


7b 


L 


lb 


L 


/9 


t 


HI 


L 


82 


L 


39 


0.88 


C 


79 


c 


76 


r 


77 


f 


81 


c 


32 


C 


84 


c 


90 


0.96 


C 


89 


c 


33 


i: 


88 


C 


98 


c 


88 





69 


c 


36 


0.97 


L 


90 


i 


34 


L 


89 


1- 


99 


t 


89 


L 


/b 


c 


3/ 


0.69 





53 


i 


51 





b'i 


l 


4b 





bl 





b4 


(. 


59 


0.71 


L 


54 


i 


53 





bb 


I 


46 





b3 





bb 


(. 


/I 


0.71 





5b 


i 


53 





b! 


I 


4/ 





b3 





b6 


L 


/I 


0.68 


C 


54 


c 


52 





56 


C 


46 


c 


52 


C 


55 


c 


59 


0.78 


I 


58 


(. 


59 





6/ 


L 


bl 


t 


b9 


L 


59 


t 


/9 


0.81 


I 


59 


t 


50 





69 


L 


52 


L 


60 


L 


61 


t 


31 


0.85 


I 


/6 


L 


75 





63 


L 


90 


t 


H2 


L 


80 


t 


3/ 


0.84 


{. 


/6 


L 


ft 


L 


bl 


( 


90 


t 


31 


L 


80 


t 


36 


0.85 


C 


76 


C 


75 


c 


63 


C 


92 


c 


82 


C 


81 


c 


37 


0.81 


t- 


73 


i 


/4 


(- 


58 


I 


Bb 


t 


/H 


L 


/6 


c. 


34 


0.83 


(. 


// 


i 


31 


(- 


6/ 


I 


92 


t 


Hb 


L 


Bl 


c. 


94 


0.84 


u 


/8 





82 


u 


6/ 





93 





H6 





81 





94 



(c) 



TOWER A, DCR of CORE COLUMN 
800's COLUMN NUMBER 


801 802 803 804 805 806 807 




c 


36 


L 


95 


(. 


b4 


0.37 


L 


59 


0.74 


D.90 


L 


93 


1 


03 


I 


63 


0.47 


{. 


64 


0.79 


D.98 


I 


32 


I 


89 


[ 


59 


0.48 


C 


58 


0.73 


0.90 


C 


37 


C 


95 


[ 


66 


0.56 


C 


52 


0.77 


0.97 


L 


93 


1 


01 


L 


4/ 


0.64 


(. 


56 


0.81 


1.04 


L 


n 


L 


/9 


L 


bl 


0.49 


{. 


50 


0.76 


0.90 


L 


II 


L 


82 


L 


bb 


0.55 


(. 


54 


0.80 


0.95 


L 


31 


L 


85 


L 


bB 


0.60 


(. 


il 


0.84 


1.01 


L 


3b 


L 


80 


L 


bb 


0.62 


(. 


b3 


0.75 


0.74 


C 


89 


C 


83 


C 


58 


0.68 


C 


55 


0.78 


0.78 


L 


94 


L 


86 


I 


61 


0.73 


L 


bH 


0.81 


0.82 





n 





69 


I 


53 


0.67 


L 


bl 


0.77 


0.74 





n 





/I 


I 


b6 


0.72 


L 


b9 


0.80 


0.78 


L 


lb 





/3 


I 


bB 


0.77 


L 


62 


0.83 


0.81 


L 


n 


L 


65 


I 


b4 


0.70 


L 


bl 


0.75 


0.77 


C 


73 


C 


67 


[ 


57 


0.75 


C 


59 


0.78 


0.80 


L 


lb 


L 


69 


L 


59 


0.79 


t 


51 


0.80 


0.83 


L 


n 


L 


66 


L 


56 


0.73 


t 


50 


0.74 


0.73 


L 


n 


L 


6/ 


L 


58 


0.77 


t 


52 


0.76 


0.76 


L 


lb 


L 


69 


L 


60 


0.80 


(. 


5b 


0.79 


0.78 


L 


lA 


L 


/I 


L 


/U 


0.76 


L 


fi 


0.72 


0.73 


C 


76 





73 


C 


72 


0.79 


C 


76 


0.74 


0.75 


L 


/H 





lb 


1- 


lb 


0.82 


L 


I'i 


0.76 


0.78 





lA 





/O 


1- 


n 


0.70 


L 


lb 


0.72 


0.74 


L 


lb 





12 


1- 


/3 


0.73 


L 


/9 


0.74 


0.76 


L 


/H 





/4 


(. 


lb 


0.76 


L 


31 


0.76 


0.78 


L 


H4 





64 


(. 


80 


0.79 


L 


34 


0.78 


0.80 


C 


37 


C 


67 


t 


83 


0.83 


C 


37 


0.81 


0.82 


L 


9U 


L 


69 


L 


86 


0.87 


t 


90 


0.83 


0.85 


L 


69 


L 


45 


L 


bb 


0.62 


L 


54 


0.50 


0.68 


L 


/O 


L 


49 


L 


bB 


0.65 


L 


61 


0.57 


0.71 


L 


H9 


L 


/4 


L 


/3 


0.73 


t 


91 


0.80 


0.90 


L 


90 


L 


/4 


L 


/4 


0.73 


L 


91 


0.80 


0.91 


C 


92 


C 


76 


C 


76 


0.75 


C 


94 


0.82 


0.93 


L 


39 





12 


1- 


/3 


0.73 


L 


3b 


0.78 


0.87 


I. 


91 





/4 


1- 


/b 


0.75 


L 


3H 


0.80 


0.89 








lb 



















73 





72 





74 


0.72 





B5 


0.78 


0.76 


C 


76 


C 


77 


C 


78 


0.79 


c 


92 


0.83 


0.79 





76 





75 





78 


0.76 





90 


0.82 


0.79 





90 





77 





80 


0.78 





H6 


0.83 


0.93 





91 





77 





SO 


0.79 





H6 


0.84 


0.94 





93 





80 





84 


0.82 





90 


0.87 


0.97 





87 





74 





80 


0.77 





85 


0.82 


0.92 





89 





76 





82 


0.79 





86 


0.84 


0.94 





91 





77 





84 


0.80 





88 


0.85 


0.95 


c 


88 


C 


79 


c 


81 


0.79 


c 


32 


0.79 


0.93 





90 





80 





83 


0.81 





83 


0.81 


0.95 





92 





82 





Bb 


0.83 





Hb 


0.82 


0.97 


c 


88 


C 


75 


[ 


82 


0.78 


c 


36 


0.83 


0.94 





89 





77 





84 


0.80 





B7 


0.85 


0.95 





91 





78 





85 


0.81 





89 


0.86 


0.97 





87 





80 





83 


0.80 





83 


0.79 


0.92 


c 


88 


C 


81 


c 


84 


0.81 


c 


35 


0.81 


0.94 





90 





82 





86 


0.83 





86 


0.82 


0.95 





86 





76 





86 


0.79 





88 


0.83 


0.91 





87 





78 





SB 


0.80 





89 


0.85 


0.92 





89 





79 





89 


0.81 





91 


0.86 


0.94 





87 





80 





87 


0.76 





85 


0.80 


0.91 





89 





81 





88 


0.78 





87 


0.81 


0.93 





91 





83 





90 


0.79 





HH 


0.82 


0.94 





70 





78 





82 


0.75 





79 


0.77 


0.73 





73 





80 





85 


0.78 





H2 


0.80 


0.76 





63 





71 





60 


0.55 





70 


0.62 


0.66 


c 


65 


c 


75 


[ 


62 


0.58 


c 


74 


0.64 


0.68 





87 





76 





81 


0.80 





B7 


0.83 


0.91 





8/ 





lb 





81 


0.79 





H6 


0.83 


0.91 





H6 





11 





83 


0.73 





B7 


0.84 


0.90 


c 


87 


c 


78 


c 


84 


0.75 


c 


39 


0.85 


0.91 


c 


89 


c 


79 


[ 


85 


0.76 


c 


90 


0.86 


0.92 


L 


8/ 


L 


80 


t 


82 


0.77 


L 


31 


0.79 


0.89 


L 


HH 


L 


81 


t 


83 


0.78 


L 


32 


0.80 


0.90 


L 


H9 


L 


82 


L 


Bb 


0.79 


L 


i'i 


0.81 


0.91 


L 


H6 


L 


lb 


L 


83 


0.74 


L 


34 


0.82 


0.91 


L 


HH 


L 


lb 


L 


84 


0.75 


L 


36 


0.83 


0.92 


C 


89 


I 


11 


[ 


85 


0.76 


C 


37 


0.84 


0.93 


C 


86 


C 


78 


C 


84 


0.78 


C 


37 


0.85 


0.90 


I. 


8/ 


L 


/8 


(. 


8b 


0.79 


L 


3H 


0.86 


0.91 


L 


HH 


L 


/9 


(. 


86 


0.80 


L 


39 


0.87 


0.92 


L 


HH 


L 


80 


(. 


8b 


0.81 


L 


3b 


0.80 


0.91 


L 


H9 


L 


81 


L 


86 


0.82 


(- 


36 


0.80 


0.92 


C 


90 


C 


82 


[ 


87 


0.83 


C 


37 


0.82 


0.93 


L 


3H 


L 


81 


L 


84 


0.78 


L 


36 


0.83 


0.91 


L 


39 


L 


82 


L 


Bb 


0.79 


L 


3/ 


0.84 


0.92 


L 


9U 


L 


83 


L 


86 


0.80 


L 


3H 


0.85 


0.93 


L 


3H 


L 


82 


L 


84 


0.81 


t 


3/ 


0.85 


0.91 


L 


39 


L 


83 


L 


Bb 


0.82 


L 


3H 


0.86 


0.92 


C 


90 


C 


84 


C 


87 


0.83 


C 


39 


0.87 


0.93 


C 


HH 


C 


82 


C 


85 


0.79 


C 


36 


0.84 


0.89 


L 


H9 


L 


83 


(. 


86 


0.80 


(. 


3/ 


0.85 


0.90 


L 


9U 


L 


84 


(. 


8/ 


0.81 


(. 


3H 


0.86 


0.91 


L 


HH 


L 


82 


(. 


86 


0.82 


(. 


36 


0.85 


0.89 


L 


H9 


L 


83 


L 


8/ 


0.83 


(. 


il 


0.86 


0.90 


C 


90 


C 


84 


[ 


88 


0.83 


C 


33 


0.87 


0.90 


L 


HH 


L 


83 


t 


8/ 


0.79 


L 


il 


0.85 


0.91 


L 


H9 


L 


83 


L 


BB 


0.80 


t 


38 


0.86 


0.92 


L 


91 


L 


8b 


(. 


90 


0.82 


L 


39 


0.88 


0.93 





lb 


L 


84 


(. 


9/ 


0.87 


L 


90 


0.72 


0.88 


L 


30 


L 


8b 


(. 


9/ 


0.88 


L 


91 


0.78 


0.88 
























(. 


12 


(. 


56 


I 


56 


0.54 


(. 


bl 


0.56 


0.71 


C 


72 


C 


b7 


[ 


57 


0.55 


C 


58 


0.57 


0.72 


L 


/O 


L 


b6 


L 


5b 


0.54 


L 


56 


0.57 


0.69 


L 


30 


L 


60 


L 


63 


0.60 


L 


a 


0.61 


0.80 


L 


i'6 


L 


62 


L 


6b 


0.62 


L 


5b 


0.62 


0.83 


L 


H9 


L 


82 


L 


BB 


0.94 


t 


3b 


0.84 


0.92 


C 


88 


C 


82 


C 


88 


0.94 


r 


84 


84 


0.91 


C 


89 


C 


S3 


C 


89 


0.96 





85 


85 


0.92 


L 


8b 


(. 


IV, 


t 


86 


0.91 





80 


80 


0.87 


L 


8/ 


(. 


82 


t 


9b 


0.99 





8/ 


85 


0.97 





88 





83 





96 


1.00 





88 


86 


0.98 



(d) 



0.00 



0.50 



0.75 



1.00 



Figure 4-5. (c) 700 line, and (d) 800 line (continued). 



NISTNCSTAR 1-2, WTC Investigation 



75 



Chapter 4 



TOWER A, DCR of CORE COLUMN 
900's COLUMN NUMBER 


901 


902 


903 


904 


905 


906 


907 


908 




10.76 


0.98 


(■ 


97 0.80 C 


H», ^^^ 1 


T) 1 n7fi 


0.B2 


1.05 


1 


02 1 0.71 1 C 


d^Kj 


wwm 


0.B3 


0.95 


n 


oa ^1^ r 


I^^H 


^^M 


0.92 


1.01 


n 


as ^^H r 


^ ^'°'^ 


1 


^^M 


1.01 


1.07 


1 


03 





89 


f 




1.05 


1 


07 


^SP 


0.B7 


0.96 


r 


^R 


<:. 


98 


L 


78 


0.89 


f 


98 


0.84 


0.94 


1.01 


r 


^8 




04 


r 


84 


0.93 




f)3 


0.91 


1.01 


1.05 





91 


■ 







90 


0.97 


1 


08 


0.98 


1.02 


0.92 


c. 


m 


i 


99 


<:. 


88 


0.90 


f 


94 


1.00 




0.96 


r 


^3 




05 


r 


93 


0.93 


r 


98 


1.06 


^^B inn 





86 


■ 







99 


0.97 


1 


0? 


^^ 


0.91 


0.84 


r 


^1 


i 


88 


<:. 


86 


0.86 


f 


90 


1 08 


0.96 


0.88 


c 


^3 


<:. 


92 


<:. 


91 


0.89 


r 


94 


1.14 


1 00 


0.91 


c 


^5 


i 


96 


i 


95 


0.92 


f 


97 


1.20 


0.89 


0.86 


c 


78 


<:. 


90 


<:. 


90 


0.86 


f 


87 


1 04 


0.93 


0.89 


c 


^0 


i 


94 


<:. 


94 


0.89 


f 


90 


m 

1.00 


0.97 


0.92 


c 


^2 


r 


98 


r 


98 


0.92 


( 


93 


1.01 


0.84 


c 


76 


r 


84 


r 


84 


0.83 


( 


85 


1.05 


0.86 


r 


78 


r 


87 


r 


88 


0.8b 


f 




1.05 




0.89 


n 


80 





90 


n 


91 


0.88 





90 


^^H 


0.97 


0.85 





75 





93 





78 


0.87 





86 


0.97 


1.01 


0.87 


c 


77 


r 


96 


<:. 


81 


0.89 


( 


88 


1.00 


1.04 


0.90 


c 


79 


<:. 


99 


<:. 


83 


0.92 


( 


91 


1 04 


1.07 


0.79 


r 


73 


r 


86 


<:. 


79 


0.87 


r 


80 


1 07 


^^H 


0.81 


c 


75 


c 


89 


c 


82 


0.89 


c 


82 


1.11 


^^H 


0.83 


t 


// 


L 


92 


L 


84 


0.91 


0B4 


1.14 


^^H 


0.86 


t 


85 


L 


95 


L 


8/ 


0.94 


0B6 


1.19 


^^H 


0.89 


t 


88 


L 


99 


L 


91 


0.97 


0B9 


1.24 


^H 


0.92 


c 


9(1 


1 


.13 


I. 


94 


1.00 


92 


1.30 


0.84 


0.67 


c 


62 


c 


71 


c 


60 


0.56 


55 


0.38 


0.85 


0.71 


(. 


-ih 


(. 


/3 


(. 


^3 


0.60 


t 


■M 


0.90 


0.98 


0.85 


c 


84 


c 


94 


c 


76 


0.8b 


[ 


88 


1.05 


0.98 


0.85 


t 


84 


L 


95 


L 


// 


0.8b 


t 


88 


1.06 
1.03 


1.01 


0.87 


t 


86 


L 


9/ 


L 


/9 


0.87 


t 


90 


0.95 


0.89 


t 


/9 


L 


92 


L 


// 


0.82 


t 


85 


0.98 


0.91 


t 


81 


L 


94 


L 


/9 


0.8b 


t 


8/ 


1.06 


1.00 


0.93 





82 





96 





81 


0.87 





88 


0.81 


0.81 





79 





91 





77 


0.82 





84 


0.88 


0.87 





82 





96 





82 


0.88 





89 


0.95 


0.86 


0.85 





82 





96 





81 


0.86 





87 


0.92 


1.01 


0.93 





85 





98 





S3 


0.88 





89 


1.03 


0.93 





85 





98 





83 


0.89 


89 


1.06 


0.97 





88 


1 


02 





87 


0.92 


92 


1.,4 


1.00 


0.91 





83 





97 





82 


0.94 


0B8 


1 07 


1.03 


0.93 





84 





99 





84 


0.96 





39 


ii 

1.06 


1.05 


0.95 





85 


1 


01 





85 


0.99 





91 


0.99 


0.90 





83 





97 





83 


0.93 





87 


1.01 


0.92 





84 





99 





85 


0.9b 





88 


1.04 


1.03 


0.93 





86 


1 


01 





87 


0.97 





90 


1.06 


0.88 





84 


1 


01 





85 


0.93 





84 


1.08 


0.90 





85 


1 


02 





86 


0.94 





85 


1.06 


1.04 


0.91 





86 


1 


04 





88 


0.96 





37 


^m 


0.93 





84 


1 


01 





86 


0.91 


88 


i.11 


^6 

1.03 


0.94 





85 


1 


02 





87 


0.93 


89 


1.13 


0.96 





87 


1 


04 





89 


0.9b 


91 


1.15 


0.89 





85 





99 





86 


0.97 


83 


1.09 


1.05 


0.90 





86 


1 


01 





87 


0.98 


85 


1.11 


1.07 


0.91 





87 


1 


03 





89 


1.00 





36 


1.13 


^^M 


0.83 





r:^ 


n 


97 


n 


86 


89 





83 


1 08 


^^H (IHS 





85 





98 





87 


0.92 





85 ^m 


^^H 0.87 





86 


1 


00 





89 


0.93 





B7 ■ 





85 


0.78 





77 





76 





83 


0.84 





79 


0.88 





87 


0.80 





79 





78 





85 


0.86 





81 


0.90 





75 


0.61 





60 





69 





63 


0.66 





fi? 


0.77 





77 


0.63 


n 


63 





72 





66 


0.68 





54 


0.79 




00 


0.85 





83 





94 





86 


0.90 





86 


1.00 




00 


0.85 





83 





94 





85 


0.90 





86 


1.00 




98 


0.83 





84 





94 





84 


0.89 





84 


1.01 




00 


0.84 





85 





96 





85 


0.90 





85 


1.02 




01 


0.85 


t 


^6 


L 


9/ 


L 


86 


0.92 


t 


86 


1.04 




02 


0.84 


t 


^5 


L 


95 


L 


84 


0.90 


t 


85 


1.01 




03 


0.85 


t 


^6 


L 


96 


L 


8b 


0.91 


t 


86 


1.03 




05 


0.86 


t 


8/ 


L 


9/ 


L 


8/ 


0.92 


t 


8/ 


1.04 




02 


0.84 


t 


85 


L 


94 


L 


84 


0.90 


t 


85 


1.02 




04 


0.85 


t 


8/ 


L 


96 


L 


8b 


0.91 


t 


86 


1.03 




05 


0.86 


t 


88 


L 


9/ 


L 


86 


0.92 


t 


8/ 


1.04 




04 


0.85 


t 


86 


L 


94 


L 


83 


0.91 


t 


86 


1.02 




05 


0.86 


t 


8/ 


L 


95 


L 


84 


0.92 


t 


8/ 


1.03 




06 


0.87 


t 


89 


L 


96 


L 


8b 


0.93 


t 


88 


1.04 




0.86 


t 


8/ 


L 


9/ 


L 


84 


0.91 


t 


86 


1.02 




0.87 


t 


88 


L 


98 


L 


8b 


0.92 


t 


8/ 


1.03 




0^ 


0.88 


t 


89 


L 


99 


L 


86 


0.93 


t 


88 


1.05 




05 


0.86 


t 


88 


L 


9/ 


L 


8b 


0.91 


t 


85 


1.03 




06 


0.87 


t 


^9 


L 


98 


L 


86 


0.92 


t 


86 


1.04 




0^ 


0.88 


t 


9U 


L 


99 


L 


8/ 


0.93 


t 


8/ 


1.05 




05 


0.87 


t 


^9 


L 


9/ 


L 


86 


0.92 


t 


86 


1.06 




0^ 


0.88 


t 


9U 


L 


98 


L 


8/ 


0.93 


t 


8/ 


1.07 




18 


0.89 


r 


91 


r 


99 


r 


87 


0.94 


r 


88 


^^H 


0.87 





88 





96 





87 


0.92 





87 


1.06 


^^H 


0.88 


c 


^9 


C 


97 


C 


88 


0.93 


c 


88 


^^8 


^^H 


0.89 


c 


90 


C 


98 


C 


89 


0.94 


89 


^^H 


0.86 


c 


iS 


C 


99 


C 


86 


0.93 


37 


1 ng 


^^H 


0.87 


c 


90 


1 


10 


C 


87 


0.94 


88 


1.09 


^^H 


0.88 


c 


91 


1 


01 


C 


88 


0.95 


89 


1.10 


^^H 


0.87 


c 


90 


c 


98 


C 


86 


0.94 


87 


1.08 


^^H 


0.88 


c 


91 


c 


99 


C 


87 


0.9b 


88 


1.09 


^^1 


0.89 


c 


93 


1 


01 


C 


89 


0.96 


89 


1.11 


c 


91 


0.86 


c 


^9 


c 


97 


C 


99 


1.03 


97 


1 06 


c 


98 


0.91 


c 


95 


1 


03 


1 


00 


1 04 


( 


97 


1 06 


c 


=i3 


0.60 


c 


=il 


c 


62 


C 


67 


0.59 


( 


59 


0.62 


c 


=>5 


0.62 


c 


=>3 


c 


R4 


c 


69 


0.61 


C 


60 


0.64 


c 


=>5 


0.63 


c 


=>4 


c 


R5 


c 


70 


0.62 


( 


61 


0.64 


c 


=i3 


0.61 


c 


1? 


c 


63 


c 


67 


0.60 


( 


59 


0.62 


c 


7? 


69 


c 


71 


c 


72 


c 


80 


0.68 


( 


67 


0.71 


c 


75 


0.71 


c 


73 


r 


74 


c 


83 


0.70 


( 


69 


0.74 




:i7 


0.89 


c 


92 


1 


ni 


1 


10 


0.96 


C 


90 


1.07 




:ifi 


0.88 


c 


91 


1 


00 


c 


98 


0.94 


C 


88 


^^6 

1.01 




:i8 


0.89 


c: 


9? 


1 


D1 


c 


99 


0.96 


C 


90 




:i? 


0.85 


c 


89 


c 


97 


c 


93 


0.92 


C 


86 




:i5 


0.87 


c 


98 


1 


07 


1 


05 


1.00 


C 


94 


^^H 




06 


0.88 





99 


1 


08 


1 


06 


1.01 





95 


^^1 



(e) 



0.00 



0.50 



0.75 



TOWER A, DCR of CORE COLUMN 
lOOO's COLUMN NUMBER 


1001 


1002 


1003 


1004 


1005 


1006 


1007 


1008 



'C 


93 


^^1 


1.07 


0.82 


H^i ^^^ 1 


0.98 


c 


99 


C 


84 


^^1 


0.91 


1 ni 1 oiu^^H 


1 03 


c 


87 


1 


07 


1 


01 


0.84 


C 


mi ^^^ 1 


0.90 


c 


93 


I 


■ 


1 


07 


0.91 


C 


H'l ^^^^^^ 


0.95 


1 


10 






I 


■ 


98 


1 


1(1 ■■[14/ 


1 01 


c 


83 


1 


10 


C 


93 


0.85 


C 


J9 1 1.03 1 0.97 


0.83 


c 


88 


1 


06 


c 


98 


0.91 


c 


H■^ ^1^ 1 


0.89 


c 


93 


I 


■ 


1 


0? 


0.96 


c 


H/ ^^^^^^ 


0.94 


c 


74 


c 


84 


c 


89 


0.90 


c 


38 


C 


78 


0.91 


0.74 


I. 


/8 


(. 


89 


(. 


93 


0.95 


t 


92 


i 


32 


0.95 


0.78 


L 


82 


(. 


93 


c 


9/ 


1.00 


t 


96 


i 


36 


1.00 


0.82 


U 


66 


(. 


82 


1- 


/5 


0.87 


t 


35 


i 


/6 


0.83 


0.66 


L 


69 


(. 


86 


(. 


// 


0.91 


c: 


m 


i 


/9 


0.87 


0.70 


L 


rz 


(. 


89 


(. 


80 


0.95 


c. 


91 


i 


32 


0.90 


0.73 


L 


69 


(. 


/9 


1- 


/2 


0.91 


c. 


/8 


i 


/4 


0.80 


0.69 


r 


71 


c 


82 


[ 


75 


0.95 


r 


10 


r 


77 


0.83 


0.72 


r 


74 


c 


8b 


c 


77 


0.98 


r 


S3 


r 


30 


0.86 


0.74 


r 


81 


c 


77 


[ 


66 


0.86 


r 


78 


c 


70 


0.77 


0.81 


c 


84 


[ 


79 


[ 


68 


0.89 


c 


30 


c 


73 


0.80 


0.84 


L 


8/ 


L 


82 


I 


/O 


0.92 


t 


33 


t 


fb 


0.83 


0.87 


L 


82 


t 


92 


t 


/6 


0.91 


t 


78 


t 


31 


0.93 


0.82 


L 


Bb 


t 


9b 


t 


/8 


0.94 


t 


31 


t 


34 


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6b 





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L 


91 


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93 





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(f) 



1.00 



Figure 4-5. (e) 900 line, and (f) 1000 line (continued). 



76 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



Table 4-5. Statistics of OCRs for WTC 1 under the refined NIST estimate case using 







LRFD and 


ASD. 
















Percentage 


Percentage 














of 


of 


Number of 






Number 


Mean 


C.O.V. 


components 


components 


components 


Maximum 




of 


Calculated 


of 


with DCR > 


with DCR > 


with DCR > 


Calculated 


Member Type 


Members 


DCR 


DCR 


1.0 


1.05 


1.05 


DCR 


Exterior Wall 
















Columns, floor 9-106 
















LRFD 


31,086 


0.96 


0.15 


35.6 


24.0 


7,461 


1.72 


ASD 


31,086 


1.10 


0.14 


72.1 


59.7 


18,572 


2.05 


Core Columns 
















LRFD 


5,219 


0.73 


0.16 


2.9 


1.8 


92 


1.26 


ASD 


5,219 


0.84 


0.15 


8.9 


5.2 


270 


1.40 



4.2.4 



Exterior Columns Axial Loads and Stresses 



The distribution of the normal stresses due to axial loads (axial column load divided by column cross 
sectional area) in the four exterior wall columns of WTC 1 due to wind loads only is presented in 
Fig. 4-6. The stresses are presented for the original design wind loads blowing from west to east and do 
not include the influence of gravity loads. The axial stresses are presented for the exterior wall columns 
at level B6 and floor 39. Fig. 4-7 shows a 3-dimensional plot of the same stresses at floors B6 and 39. 
The plots show both the tensile and compressive stresses on the columns induced by wind loading, where 
the shear lag effects can be observed. Similar plots were obtained for WTC 2 (NIST NCSTAR 1-2A). 

At the B6 level, the plots indicate that there were significant differences in stresses between the two 
columns at a given corner. For example, at the southwest corner at level B6, the stresses at columns 359 
(south wall) and 401 (east wall) were about 25 ksi and 15 ksi, respectively. This indicates significant 
deformations in the corner panels at the basement floors. Much smaller differences were observed in the 
stresses at the floor 39. This indicates that the behavior of the lower portion of the tower at the basement 
floors resembled that of a braced frame, while the behavior of the super-structure resembled that of a 
framed tube system. 

A framed tube structure consists of closely spaced exterior columns tied together at each floor with deep 
spandrel beams, thereby creating a rigid wall-like structure around the building exterior (i.e., a hollow 
tube with perforated openings for windows) (Khan and Amin 1973; Taranath 1988). The behavior of the 
framed tube structure is hybrid, showing characteristics of both pure tube and pure frame behaviors. The 
overturning moments of the lateral loads are primarily resisted by the tube action, i.e., axial shortening 
(compression) and elongation (tension) of the columns on all sides of the tube. The shear from the lateral 
loads is primarily resisted by the frame action (in-plane bending of columns and spandrels) of the two 
sides of the building parallel to the direction of the lateral load (webs). Since the perimeter walls have a 
tendency to behave as a thin-walled tube structure, shear stresses and strains are large, and as a result the 
distribution of bending stresses is affected. Therefore, the bending stresses in the side walls (webs) are no 
longer proportional to the distance from the neutral axis and are larger near the flanges. The same large 
stresses occur in the flanges near the webs, and the stresses at the center of the flanges (normal to the 
lateral load) are reduced or 'lag' behind the stresses near the webs (parallel to load). Bending stresses in 



NIST NCSTAR 1-2, WTC Investigation 



77 



Chapter 4 



the webs are also affected in a similar manner. This phenomenon is known as shear lag and can be 
clearly shown in Fig. 4-6(b). In the framed tube system, the floor diaphragms play a key role since they 
carry lateral forces to the side walls of the building, thereby allowing for the tube action to take place. In 
addition, floor diaphragms provide lateral support for the stability of the columns, and under torsion, they 
assure that the cross-sectional shape of the structure is maintained at each level by their in-plane shearing 
resistance. 

An investigation into the behavior of the exterior wall columns indicated that under the original 
WTC design dead and wind loads (no live loads were considered), tension forces were developed in the 
exterior walls of the towers. The tension forces from the combination of dead and wind loads for the four 
exterior walls of WTC 1 are illustrated in Fig. 4-8. Similar plots for WTC 2 can be seen at Chapter 5 of 
NIST NCSTAR 1-2A. As the figure indicates, the tensile forces were largest at the base of the building 
and at the corners. 



Tower A: West to East Wind (AON-E-) 

(Gravity Loads not included) 




100 Face (Nortii Coulumns) - FL B6 



Tower A: West to East Wind (AON-E-) 

(Gravity Loads not included) 



30 



20 



10 




259 



-20 



201 



200 Face (East Columns) - FL B6 



Tower A: West to East Wind (AON-E-) 

(Gravity Loads not included) 




300 Face (South Columns) - FL B6 



30 - 
20 - 

1 1° - 

S - 

« -10 - 

-20 - 

-30 - 


Tower A: West to East Wind (AON- 
(Gravity Loads not included) 


E-) 


£ 

.2 
en 

£ 

£ 
O 
"in 
in 

s; 

a. 
E 
o 
O 


401 


459 ♦ 


*''-^-^K^N,^>^»^.^.,..,^,^K^^4^^ 








400 Face (West Columns) - FL B6 





(a) Floor B6 

Figure 4-6. Distribution of normal stresses in the exterior walls of WTC 1 due to original 
WTC wind loads only at (a) floor B6, and (b) floor 39. 



78 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



Tower A: West to East Wind (AON-E-) 

(Gravity Loads not included) 




100 Face (North Columns) - FL 39 



6 

4 

2 

; 
-2 
-4 
-6 



Tower A: West to East Wind (AON-E-) 
(Gravity Loads not included) 




200 Face (East Columns) - FL 39 



Tower A: West to East Wind (AON-E-) 
(Gravity Loads not included) 




300 Face (South Columns) - FL 39 



-10 



Tower A: West to East Wind (AON-E-) 

(Gravity Loads not included) 




400 Face (West Columns) - FL 39 



(b) Floor 39 

Figure 4-6. Distribution of normal stresses in the exterior walls of WTC 1 due to original 
WTC wind loads only at (a) floor B6, and (b) floor 39 (continued). 



NISTNCSTAR 1-2, WTC Investigation 



79 



Chapter 4 



30 



Wind 
Direction 



20 




Wind Loads at Level B6 



Figure 4-7. Three-dimensional distribution of normal stresses in the exterior walls of 
WTC 1 due to original WTC wind loads only at floors B6 and 39. 



80 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



Wind 
Direction 



30 T 



20 



10 



30 



20 



10 




Wind Loads at Floor 39 



-10 



-20 



-30 



Figure 4-7. Three-dimensional distribution of normal stresses in the exterior walls of 
WTC 1 due to original WTC wind loads only at floors B6 and 39 (continued). 



NISTNCSTAR 1-2, WTC Investigation 



Chapter 4 



m-m 110-119 lZO-129 130-139 140-149 150-159 



200-209 210-219 220-229 230-239 240-249 250-259 




(a) 












(b) 









100 




500 




1000^^^^^^^ 



Figure 4-8. Tension force distribution (l<ip) in the exterior wall columns of WTC 1 under 
original design dead and wind loads (no live loads included), (a) 100 face (north), and 

(b) 200 face (east). 



82 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



300-309 310-319 320-329 330-339 340-349 350-359 



400-419 410-419 420-429 430-439 440-449 450-459 





(c) 
















(d) 









100 




500 




lOOO^^^H 



Figure 4-8. (c) 300 face (south), and (d) 400 face (west) (continued). 



NISTNCSTAR 1-2, WTC Investigation 



83 



Chapter 4 



4.2.5 



Exterior Columns Splice Connection 



The axial tensile column loads estimated in Section 4.2.4 under dead and wind loads were transferred 
from one panel to another through the column splices. The exterior wall column splice capacities were 
calculated from the original details and compared to the tension forces for all four faces of WTC 1. The 
DCRs for the exterior wall splice connections for WTC 1 are summarized in Table 4-6. As can be 
observed from Table 4-6 and from a similar table for WTC 2 (NIST NCSTAR 1-2A), the DCRs were less 
than unity for all walls of both towers. 

Table 4-6. Maximum calculated DCRs for exterior wall column splices for WTC 1 under 

original design dead and wind load case. 



Exterior Wall 


Exterior Wall 


Maximum 


Face 


Column Splices 


Calculated DCR 




Below floor 1 


0.64 


100 Face 


Floor 1 to 9 


0.31 


(North) 


Floor 10 to 41 


0.96 




Above floor 42 


0.26 




Below floor 1 


0.53 


200 Face 


Floor 1 to 9 


0.32 


(East) 


Floor 10 to 41 


0.63 




Above floor 42 


0.14 




Below floor 1 


0.54 


300 Face 


Floor 1 to 9 


0.26 


(South) 


Floor 10 to 41 


0.77 




Above floor 42 


0.15 




Below floor 1 


0.59 


400 Face 


Floor 1 to 9 


0.36 


(West) 


Floor 10 to 41 


0.84 




Above floor 42 


0.26 



4.2.6 



Resistance of the Towers to Shear Sliding and Overturning Moment 



The dead loads that acted on the exterior walls of the towers provided resistance to shear sliding and 
overturning induced by wind loads. Considering the resistance to shear sliding under wind loads, the 
factor of safety was estimated to be approximately 1 1.5 and 10 for WTC 1 and WTC 2, respectively. This 
was calculated by dividing the resisting force due to dead load on the exterior walls (a coefficient of 
friction of 0.7 was used) by the wind shear (maximum base shear) at the foundation level. 

For the resistance of the towers to overturning due to wind loads, the factors of safety for WTC 1 were 
estimated to be approximately 2.3 and 2.6 for overturning about a north-south axis and an east-west axis, 
respectively. For WTC 2, these factors of safety were about 1.9 and 2.7 for overturning about a north- 
south axis and an east-west axis, respectively. These factors of safety were calculated by dividing the 



84 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



resisting moment due to dead load on the exterior walls by the overturning moment due to wind loads 
taken at the foundation level (maximum base moments). 

4.3 BASELINE PERFORMANCE OF THE TYPICAL FLOOR MODELS 

This section presents the results of the baseline performance analysis for the typical floor models under 
gravity (dead and live) loads. These models included the typical truss-framed floor (floor 96 of WTC 1, 
see Section 2.4) and the typical beam- framed floor (floor 75 of WTC 2, see Section 2.5). 

For application to the floor models, gravity loads were separated into three categories: CDLs, SDLs, and 
live loads (LLs). CDL is defined as the self-weight of the structural system, including floor trusses, floor 
beams, and concrete slabs. SDL is defined as the added dead load associated with architectural and 
mechanical/electrical/plumbing systems (curtain wall, floor finishes, mechanical equipment and ducts, 
transformers, etc.) The CDL and SDL were based on the WTC architectural and structural drawings and 
on the original WTC Design Criteria. For the estimation of the dead loads on the floor models, see 
Chapter 6 of NIST NCSTAR 1-2 A. 

Two independent sets of live loads were applied in combination with the dead loads. The first was taken 
from the original WTC Design Criteria and the second from the ASCE 7-02 Standard. The live loads in 
the NYCBC 2001 are essentially identical to the ASCE 7-02 live loads. Live load reductions were taken 
from the original WTC Design Criteria and from the ASCE 7-02 Standard, each for use with its respective 
live loads. For the typical beam-framed floor, it was found that the original WTC design criteria live 
loads, NYCBC 2001 loads, and the ASCE7-02 Standard loads were nearly identical. The only difference 
was that the live load for the corridors within the core was 100 psf in the original WTC design criteria, 75 
psf in NYCBC 2001, and 80psf in ASCE 7-02. As a result, only the original WTC design criteria loads 
were applied to the beam- framed floor model. 

For the baseline performance analysis for the floor systems, DCRs for structural components were 
estimated using the ASD procedure as specified in the AISC Specification (1989), see Section 4.2.1. 

4.3.1 Typical Truss-Framed Floor 

For the CDL, SDL, and LL applied to this floor and for the selection of the design parameters for 
estimating the DCRs, see Chapter 6 of NIST NCSTAR 1-2A. 

The maximum mid-span deflections for each of the long-span, short-span, and two-way zones for the 
original WTC Design Criteria and ASCE-7-02 total loads are provided in Table 4-7. 



NIST NCSTAR 1-2, WTC Investigation 85 



Chapter 4 



Table 4-7. Summary of maximum deflections for typical truss-framed floor under dead 

and live loads for areas outside of core. 



Criteria 


Two- Way Zone 


Long Span 


Short Span 


WTC Design 
Criteria 


1.44 in. 


1.79in. ~L/400 


0.57 in. = L/750 


ASCE 7-02 


1.14 in. 


1.43in. =L/500 


0.44 in. ~ L/980 



The Design Criteria for the towers specified that the floor trusses were to be cambered for construction 
dead loads and proportioned such that the deflection under SDL and LL did not exceed L/360. Table 4-7 
clearly shows that this criterion was met. 

For the components of the truss-framed floors, DCRs were calculated using the SAP2000 program. 
Calculations were made for the bottom chords, the diagonals and the verticals of the trusses, and for the 
beams and girders of the core. 

DCR statistics for the truss-framed floor model are summarized in Table 4-8 for the original design 
loading case and in Table 4-9 for the ASCE 7-02 loading case. For the area outside the core, the DCRs 
for all floor trusses were less than 1.14 for the original WTC design loads and less than 0.86 for the 
ASCE 7-02 loading and (by comparison) for the NYCBC 2001 loading. Under the original WTC design 
loading, the DCR was less than 1.00 for 99.4 percent of the floor truss components. Inside the core, the 
DCRs for all floor beams were less than 1.08, and more than 99 percent of the members had a DCR of 
less than 1.0. 

For the area outside the core, the average ratio of the DCRs estimated from the ASCE 7-02 loading to the 
DCRs from the original WTC design loading for all floor trusses was about 0.80. 



86 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



Table 4-8. DCR statistics for the typical truss-framed floor under the original design 

load case. 



Member Type 


Number 


Mean 


C.O.V. 


Percentage 


Percentage 


Number of 


Maximum 




of 


Calculated 


ofDCR 


of 


of 


components 


Calculated 




Members 


DCR 




components 

with DCR > 

1.0 


components 

with DCR > 

1.05 


with DCR > 
1.05 


DCR 


One-Way Long 
















Span Zone 
















Web members 


1,792 


0.44 


0.61 


3.7 


1.28 


23 


1.14 


Bottom chord 


1,038 


0.74 


0.26 











0.99 


members 
















One-Way Short 
















Span Zone 
















Web members 


640 


0.33 


0.61 











0.92 


Bottom chord 


288 


0.37 


0.32 











0.55 


members 
















Two- Way Zone 
















Web members 


3,086 


0.30 


0.80 


0.3 


0.26 


8 


1.06 


Bottom chord 


2,035 


0.48 


0.54 











0.94 


members 
















Bridging Trusses 
















within One-Way 
















Span Zones 
















Web members 


692 


0.16 


1.25 


1 








1.02 


Bottom chord 


327 


0.12 


1.33 











0.95 


members 
















Core Beams 
















Beams within core 


1,361 


0.33 


0.67 


0.9 


0.3 


4 


1.07 


Core perimeter 


686 


0.36 


0.58 


1.0 


0.6 


4 


1.08 


channels 

















NISTNCSTAR 1-2, WTC Investigation 



87 



Chapter 4 



Table 4-9. DCR statistics for floor the typical truss-framed floor under the ASCE 7-02 

loading case. 



Member Type 


Number 


Mean 


C.O.V. of 


Percentage 


Percentage 


Maximum 




of 


Calculated 


DCR 


of 


of 


Calculated 




Members 


DCR 




components 

with DCR > 

1.0 


components 

with DCR > 

1.05 


DCR 


One-Way Long Span 














Zone 














Web members 


1,792 


0.35 


0.60 








0.86 


Bottom chord members 


1,038 


0.59 


0.25 








0.80 


One-Way Short Span 














Zone 














Web members 


640 


0.26 


0.65 








0.69 


Bottom chord members 


288 


0.30 


0.33 








0.43 


Two- Way Zone 














Web members 


3,086 


0.24 


0.79 








0.78 


Bottom chord members 


2,035 


0.38 


0.55 








0.74 


Bridging Trusses within 














One-Way Span Zones 














Web members 














Bottom chord members 


692 


0.11 


1.55 








0.95 




327 


0.09 


1.44 








0.81 


Core Beams 














Beams within core 


1,361 


0.28 


0.64 


0.1 


0.1 


1.05 


Core perimeter channels 


686 


0.28 


0.61 








0.86 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



4.3.2 Typical Beam-Framed Floor 

For the CDL, SDL, and LL applied to this floor, see Chapter 6 of NIST NCSTAR 1-2 A. 

The maximum mid-span deflections of the long-span and short-span zones under the original WTC design 
loads were approximately 1.55 in. (~ L/450) and 0.70 in. (~ L/600), respectively. The Design Criteria for 
the towers specifled that the floor beams be proportioned such that the deflection would not exceed L/360 
under total load. If the beams were cambered for construction dead loads, the flnal deflection could not 
exceed L/360 under SDL + LL. The calculated deflections clearly showed that this criterion was met. 

Using the SAP2000 computer program, DCRs were calculated for the components of the floor framing. 
Only two beams running in the east-west direction and cantilevering off of comer core columns 501 and 
508 had DCRs larger than 1.0 under the original WTC design loading. For these two beams, the DCRs 
from the axial load and moment interaction equation were less than 1.0, while the DCRs in shear were 
1.125 and 1.09. 

Fig. 4-9 shows the distribution of DCRs for the floor framing. The figure shows the location of the two 
beams with DCR greater than 1.0. DCR statistics for the beam- framed floor model are summarized in 
Table 4-10 for the original design loading case. The statistics are provided for member groups that are 
shown in Fig. 4-10. 



!l 1 
















^>^ 




" ' 




















=> 




<- 














• 


> 






































































1 


1 1 1 1 1 




4-" 






-- 


r^ 1 






1 1 


\ 


i«^' 1 


h 
























^^ 




1 1 ' 


1 


































' 
















N 


- 




1 — 










► 








- 




































^- 






















— ' — 1 — ■ — 1 


1 












! 


" 
































































>< 




















« 


1 




































> 








1 


1 1 




1 








L-. 



0.00 



0.50 



0.75 



1.00 



IP 



Figure 4-9. DCRs for the typical beam-framed floor under original WTC design criteria 

loading. 



NIST NCSTAR 1-2, WTC Investigation 



89 



Chapter 4 



Core Beams 



Comer 
Beams 




Long Span 
Beams 



Short Span 
Beams 



>x 



Figure 4-10. Beam-framed floor member groups. 



Table 4-10. DCR statistics for the typical beam-framed floor under the original design 

loading case. 



Member Type 


Number of 
Members 


Mean Calculated 
DCR 


C.O.V. of 
DCR 


Maximum 
Calculated DCR 


Long Span Beams 


156 


0.64 


0.16 


0.83 


Short Span Beams 


84 


0.65 


0.12 


0.89 


Core Beams 


156 


0.31 


0.77 


1.13 


Corner Beams 


32 


0.49 


0.35 


0.90 



4.4 



REVIEW OF BASELINE PERFORMANCE ANALYSES 



As was the case for the structural databases and models, the baseline performance analyses outlined in 
this chapter for the global WTC models and the floor models were reviewed by SOM and NIST. The 
reviews included the following: (1) checks on the accuracy of load vectors (gravity and wind) as 
developed in Chapter 3; (2) reviews of the adequacy of the analysis procedures, including staged 
construction analysis, P-A effects, modal analysis, etc.; and (3) checks on the proper use of load 
combinations and component capacity estimates. The reviews indicated that the baseline performance 
analyses were appropriate. The reviews also included a thorough review of the report on baseline 
performance analysis, that resulted in substantial modifications to the report. 



4.5 



SUMMARY 



This chapter presented the results of the baseline performance analysis for the WTC 1 and WTC 2 towers. 
For the global models of the towers, three gravity and wind loading cases were considered: (1) the 



90 



NIST NCSTAR 1-2, WTC Investigation 



Baseline Performance of the WTC Towers 



original WTC design load case, (2) the lower-estimate state-of-the-practices case, and (3) the refined 
NIST estimate case. 

Under the original WTC design loads, the cumulative drifts at the top of the WTC towers ranged from 
H/263 to H/335. For the lower- estimate state-of-the-practice case, those drifts ranged from H/253 to 
H/306. The drifts obtained from the refined NIST estimate case were about 25 percent larger than those 
from the state-of-the practice case. While currently no building codes specify a drift limit for wind 
design, structural engineers often use in their practice the criterion that drift ratios should not exceed 
H/400 to H/500 for serviceability considerations and to enhance overall safety and stability (including 
P-A effects). Reducing the drift of the WTC towers to the range of H/400 to H/500 would entail 
enhancing the stiffness and/or damping characteristics of the buildings. 

Structural engineers often use in their practice an inter-story drift limit in the range of h/300 to h/400 for 
serviceability considerations. Under design loading conditions, the maximum inter-story drift was as high 
as h/230 and h/200 for WTC 1 and WTC 2, respectively. Maximum inter-story drifts under the state-of- 
the practice case were about h/184 and h/200 for WTC 1 and WTC 2, respectively. For the refined NIST 
estimate case, these inter-story drifts were about 25 percent larger than those from the state-of-the practice 
case. Similar to total drift, inter-story drifts of the towers were larger than what is generally used in 
practice. 

The DCRs were based on the allowable stress design (ASD) procedure and were estimated using the 
AISC Specifications (1989). The results indicated that DCRs estimated from the original WTC design 
load case were, in general, close to those obtained for the lower estimate state-of-the practice case. For 
both cases, a fraction of structural components had DCRs larger than 1.0. These were mainly observed in 
both towers at (1) the exterior walls at the columns around the corners, where the hat truss connected to 
the exterior walls, and below floor 9; and (2) the core columns on the 600 hne between floors 80 and 106 
and at core perimeter columns 901 and 908 for much of their height. The DCRs obtained for the refined 
NIST estimate case were higher than those from the original WTC design and the lower-estimate state-of- 
the-practice load cases, owing to the following reasons: (1) the NIST estimated wind loads were higher 
than those used in the state-of-the-practice case by about 25 percent, and (2) the original WTC design and 
the state-of-the-practice cases used NYCBC load combinations, which result in lower DCRs than the 
ASCE 7-02 load combinations used for the refined NIST case. The DCRs estimated using the load and 
resistance factor design (LRFD) procedure for exterior and core columns were, on average, smaller than 
those using the ASD procedure by about 1 5 percent. 

While it is a normal design practice to achieve a DCR less than unity, the safety of the WTC towers on 
September 1 1 , 200 1 , was most likely not affected by the fraction of members for which the demand 
exceeded capacity due to the following: (1) The inherent factor of safety in the allowable stress design 
method, (2) the load redistribution capability of ductile steel structures, and (3) on the day of the attack, 
the towers were subjected to in-service live loads (a fraction of the design live loads) and minimal wind 
loads. 

Analysis of the axial stress distribution in the columns under lateral wind loads indicated that the behavior 
of the lower portion of the towers at the basement floors was that of a braced frame, while the behavior of 
the super-structure was that of a framed tube system. Under a combination of the original WTC design 
dead and wind loads, tension forces were developed in the exterior walls of both towers. The forces were 
largest at the base of the building and at the corners. These tensile column loads were transferred from 

NIST NCSTAR 1 -2, WTC Investigation 9 1 



Chapter 4 



one panel to another through the column splices. The DCRs for the exterior wall splice connections under 
these tensile forces for both towers were shown to be less than 1.0. 

The resistance of the towers to shear sliding and overturning due to wind was provided by the dead loads 
that acted on the exterior walls of the towers. Considering the resistance to shear sliding under wind load, 
the factor of safety was calculated to be between 10 and 1 1.5, while the factor of safety against 
overturning ranged from 1.9 to 2.7 for both towers. 

Two typical floor models were each analyzed under gravity loads. The following is a summary of the 
results: 



• 



For the typical truss-framed floor, the DCRs for all floor trusses were less than 1.14 for the 
original WTC design loads and less than 0.86 for the ASCE 7-02 loading. Under the original 
WTC design loads, the DCR was less than 1.00 for 99.4 percent of the floor truss 
components. Inside the core, the DCRs for all floor beams were less than 1.08, and more 
than 99 percent of the floor beams had a DCR of less than 1.0. The maximum mid-span 
deflections of the long-span and short-span zones under the original WTC design loads were 
approximately 1.79 in. (~ L/400) and 0.57 in. (~ L/750), respectively. 

• For the typical beam-framed floor under the original WTC design loads, the DCRs for all 
floor beams were less than 1.0 except for two core beams where the DCRs in shear were 
1.125 and 1.09. The maximum mid-span deflections of the long-span and short-span zones 
under the original design loads were approximately 1.55 in. (~ L/450) and 0.70 in. (~ L/600), 
respectively. 

4.6 REFERENCES 

AISC Specification (1989): American Institute of Steel Construction, Specification for Structural Steel 
Buildings - Allowable Stress Design and Plastic Design - 9* Edition, Chicago, IL, 1989. 

AISC Specification (1993): American Institute of Steel Construction, Specification for Structural Steel 
Buildings - Load & Resistance Factor - 2nd Edition, Chicago, IL, 1993. 

ASCE 7-02: American Society of Civil Engineers, ASCE 7 Standard Minimum Design Loads for 
Buildings and Other Structures, Reston, VA, 2002. 

Khan, F.R., and Amin, N.R., (1973), "Analysis and Design of Framed Tube Structures for Tall Concrete 
Buildings," The Structural Engineer, 51(3), pp. 85-92. 

NYCBC 2001: Building Code of the City of New York, 2001 Edition, Gould Publications, 
Binghamton, NY. 

Taranath, B.S., (1988), Structural Analysis & Design of Tall Buildings, McGraw-Hill, Inc., New York, 
USA. 



92 NISI NCSTAR 1-2, WTC Investigation 



Chapters 

Development of Tower and Aircraft Impact Models 



5.1 introduction 

This chapter describes the structural models used in the analysis of aircraft impact into the World Trade 
Center (WTC) towers. The WTC tower models for the impact analysis required considerably greater 
sophistication and detail than was required for the reference models described in Chapter 2. The 
reference models provided the basis for the more detailed models required for the impact simulations. 
The impact models of the towers, which utilized the structural databases described in Chapter 2 (see also 
NIST NCSTAR 1-2A), included the following refinements: 

• The material properties used in the impact models accounted for the highly nonlinear 
behavior of the tower and aircraft materials, including softening and failure of components, 
and strain rate sensitivity. 

• The impact simulations required a much higher level of detail than that in the reference global 
models. For instance, the impact analyses necessitated that the floors inside and outside the 
core in the impact region, as well as connections, be modeled in detail. In addition, structural 
components in the exterior walls and core of the towers were modeled using shell elements 
(instead of beam elements in the reference models) to properly capture the impact-induced 
damage to these components. 

• The size of the impact models required a very large mesh (more than ten million degrees of 
freedom). The SAP2000 program cannot accommodate this model size. 

• Contact and erosion algorithms were required for the impact analyses. That necessitated the 
use of appropriate software, specifically LS-DYNA (LS-DYNA 2003), for the development 
of the impact models. 

Three separate models were developed for conducting the impact analyses. The first two were detailed 
models of the WTC 1 and WTC 2 towers in the impact region. The third model was a comprehensive 
model of the Boeing 767 aircraft. All models were developed for the LS-DYNA fmite element code, 
which is a commercially available nonlinear explicit fmite element code for the dynamic analysis of 
structures. The code has been used for a wide variety of crash, blast, and impact applications. The 
models were developed using the TrueGrid model generation program (TrueGrid Manual 2001). The 
input data for TrueGrid included a set of commands that defmed the model geometry, material properties, 
boundary conditions, and mesh sizes. The output from TrueGrid was a complete LS-DYNA input file for 
the desired analysis. 

One of the significant challenges in developing the tower and aircraft models for the global impact 
analyses was to minimize the model size while keeping sufficient fidelity in the impact zone to properly 
capture the characteristics of the impact response. The hmitation was a model size that could be run on a 
32-bit computer, since additional memory was needed to decompose a model with greater than ~ 

NIST NCSTAR 1-2, WTC Investigation 93 



Chapter 5 



2.3 million nodes. Based on this limitation, each combined aircraft and tower model could not exceed 
2.3 million nodes. These were distributed between the global WTC tower model and the aircraft so that 
the tower model would be about 1.5 million nodes and the aircraft about 0.8 million nodes. The approach 
used to meet this objective was to develop models for the various tower components at different levels of 
refinement. Components in the path of the impact and debris field were meshed with a higher resolution 
to capture the local impact damage and failure, while components outside the impact zone were meshed 
more coarsely to primarily capture their structural stiffness and inertial properties. As a result, an array of 
component and subassembly analyses were performed to optimize the finite element mesh densities and 
study the infiuence of a number of modeling options on the calculated response. 

Section 5.2 and 5.3 provide the details and methodology used to develop the global tower and aircraft 
models, respectively, including constitutive relationships used for the various materials in the towers and 
aircraft. Section 5.4 provides a summary of the component level and subassembly analyses used to 
support the development of the global tower and aircraft models. Section 5.5 is a summary of the chapter. 

5.2 DEVELOPMENT OF TOWER IMPACT MODELS 

Given the complexity of the towers' structure, a key aspect of developing the global models was 
automating the mesh generation process. The component model generation files were developed in a 
parameterized format to support automated mesh generation. For that purpose, the electronic structural 
databases developed by the firm of Leshe E. Robertson Associates, R.L.L.P. under contract to NIST 
within the framework of Project 2, and reviewed and approved by NIST (see Chapter 2), were utilized. 
Visual Basic programs were developed to interface with the structural databases and to automatically 
write master level TrueGrid input files for mesh generation. These programs were used to generate the 
models for the core columns and exterior walls. 

An example of such programs is presented in Figure 5-1, which shows the user interface for the program 
that generated the models of the exterior wall panels. In this program, the user identified the tower, upper 
and lower fioor boundaries, and left and right (as viewed from outside the building) panel numbers. 
Additionally, the user could specify a fine mesh region, typically in the area of the aircraft impact. This 
program extracted information from the database and wrote a master TrueGrid file. Information not 
available in the database but included in the drawing books, such as the weld specifications, were 
included in the program. The automatically generated TrueGrid files included the geometry and material 
specification for the columns, butt plates, spandrels, welds, bolts, and spandrel splice plates. Node 
tolerance specifications (nodal merging commands) were also automatically generated to define the 
connectivity of adjacent parts in the model. 

A summary of the model size and element types for the global tower models is presented in Table 5-1. 
The following sections provide the details of the various components used in the tower models. 



94 NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



^ File Edit View Inseri: Formal: Tools Dai:a Window Help 
^^^i,^^^||3 *fe^| rVReply withChanges... EndPevie'.'j.,, . 



-fwi 



ToWer I A tI 



■© 





ff Mukiple Panels 






Left Panel | 103 






Right Panel | ,57 






Top Floor 1 iQil 






Bottom Floor | 91 










Left Panel | 109 


Right Panel | 14^ 


Top Floor 1 gg 


Bottom Floor | 94 









^ Single Panel 

Panel Number \~ 

Top Floor I' 

Bottom Floor Fi 



Create True Grid Input 






I 
CQlumr 2 Lower Splice 



jSpliceDetlBPI Fy5D |Fillet W11 1 No bolts | Dia Bolts |3rade Bolt^age Bolt=|o12 Field \i 13 Field fill 
' 421 1 .625 3^ 4 7^ A325 3.5 ^ 

Figure 5-1. User interface for exterior panel generator. 
Table 5-1. Summary of the size of the global impact tower models. 





WTC 1 Tower Model 


WTC 2 Tower Model 


Number of Nodes 


1,300,537 


1,312,092 


Hughes-Liu Beam Elements 


47,952 


53,488 


Belytschko-Tsay Shell Elements 


1,156,947 


1,155,815 


Constant Stress Solid Elements 


2,805 


2,498 



5.2.1 



Exterior Wall Model Development 



The exterior walls were constructed as an assembly of panels. The most common panel types on the 
exterior of the towers consisted of three columns and spans over three floors. The columns in each panel 
were attached together by spandrel plates, typically at each floor level. The construction of the exterior 
wall model required the generation of a parameterized model for each panel type that was located in the 
tower regions near the impact zones. 

The complete exterior wall model in the impact zone for each tower was generated by placing the various 
panels in the actual locations with their dimensions and material specifications. The impact face for the 
global WTC 1 (north wall) and WTC 2 tower (south face) models are shown in Figure 5-2 and 
Figure 5-3, respectively. A refined mesh was used in the immediate impact zone for improved accuracy 
of the impact response, and a coarse mesh was used outside the impact zone for improved computational 
efficiency. All panels were primarily constructed from Belytschko-Tsay shell elements. The reader is 



NISTNCSTAR 1-2, WTC Investigation 



95 



Chapter 5 



referred to the LS-DYNA Theoretical Manual for a complete description of this element type. For 
modeling the bolted column connection between columns, constant stress brick elements were used to 
model the butt plates in the refined panels, and Hughes-Liu beam elements were used for the bolts 
connecting the butt plates in the refined impact zone. Section 5.4.2 describes the details of the model for 
the exterior column connections. The column ends for the coarse far field exterior wall panels were 
merged together to create a perfect bond between column ends. 



BC: fixed in Z 
translation only at 
free column ends 



r 



Floor: 



Truss floor and 
core structure 
floors 92-100 



Higher mesh 

density in 
impact zone 




iiiiiiiiiiniiiiiiimiiiiiiiiiiiiiiiiiin 




Panel 
Numbers 



Figure 5-2. Impact face of the WTC 1 global model, floors 91-101. 



96 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



Column 457 

Numbers 



451 445 439 433 427 421 415 409 403 



Floor 
Number 



Truss floor and 

core structure 

floors 77-85 



Higher mesh 
density in 



impact zone 



BC: fixed in Z 
translation only at 
free column ends 




Figure 5-3. Impact face of the WTC 2 global model, floors 75-86. 

The model of the spandrel splice plate connection is shown in Figure 5-4. Twelve nodes on the splice 
plate were attached to the spandrels using the spot weld tied node algorithm (LS-DYNA Type 7 tied 
interface). The spot weld approximated the connection of the individual bolts connecting the spandrel 
splice plates. Failure of these connections occurred through deformation of the splice plates and/or 
spandrel and ductile failure of the materials. The placement of the spandrel splice plates was limited to 
the higher resolution impact zone for the exterior wall. The far-field coarse panel models were merged 
together as shown in Figure 5-5. The influence of the spandrel sphce connection on the impact response 
and exterior wall damage was investigated using engine component impact analyses (see Chapter 5 of 
NISTNCSTAR1-2B). 

Each three-column, three story panel in the impact zone contained 5,304 nodes, 5,202 shell elements, 
78 brick elements, and 12 beam elements. The corresponding element sizes in the impact zone were a 
1 in. element for the weld zone and 4 in. elements for the exterior column. A typical element dimension 
for the far field exterior panels was 14 in. 

The boundary conditions at the top and bottom of the exterior wall were constrained vertical 
displacements. The lateral degrees of freedom and rotation about the vertical axis were not constrained. 
The free lateral displacements at the model boundary allowed for the tower model to have a rigid body 
velocity following the impact. Since the natural period of the tower was in the range of 10 to 11 s (see 
Chapter 2), the tower provided little structural resistance to the translation at the model boundary during 
the less than one second impact event. 



NISTNCSTAR 1-2, WTC Investigation 



97 



Chapter 5 



Medium 
panels 




Spot welded nodes 

Figure 5-4. Model of the spandrel splice plate connection. 



med-to-med 

connection 

(spliced] 



coarse-to-med 

connection 
(spliced] 



Spliced 
connection 




coarse-to- 
coarse conn. 

(merged nodes] 

i 




Coarse 

panels 



Figure 5-5. Placement of spandrel splice plates in the exterior wall model. 



98 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



5.2.2 Core Columns and Floors Model Development 

Core column models were generated as a group in single floor sections. Dimensions and material 
specifications were assigned automatically, as specified in the WTC structural databases. The boundary 
conditions at the top and bottom of the core model and the column sphces were automatically generated. 
An example of the model of the WTC 1 core columns for floors 95 to 97 is shown in Figure 5-6. 
Different colors correspond to different material assignments for the various column sections. 



Core Columns: Floors 95-97 



fl 



I 




« 



I 



so ksi columns: shown as red 
36 ksi columns: shown as purple 

42 ksi columns: shown as orange 
Figure 5-6. Model of the WTC 1 core columns and connections, floors 95-97. 

Both wide flange and box core columns were modeled with Belytschko-Tsay shell elements. Two mesh 
densities were used in the model, a refined density in the direct impact area and a coarse far field density 
elsewhere. Typical element dimensions were 2 in. and 8 in. for the impact zone and far field, 
respectively. A single wide flange column in the impact zone had 552 shell elements and 600 nodes per 
floor, while a box column in the impact zone had 864 shell elements and 900 nodes per floor. 

The wide flange-to-wide flange core column connections were modeled by splice plates placed on the 
outer side of each flange, as shown in Figure 5-7. The connection between the splice plate and column 
flange was modeled with a surface-to-surface tied interface without failure, which resulted in a perfect 
bond between the nodes of the splice plate and the flange of the adjacent column. If the columns were 
pulled apart, the elements at the splice plate spanning the gap between column ends would be stretched. 
Failure of the sphce plate in the model resulted from ductile failure of the splice plate in the elements 
spanning the connection. 



NISTNCSTAR 1-2, WTC Investigation 



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Chapter 5 




Splice Plates 
Figure 5-7. Detail of wide flange core columns splices 

A typical box column-to-wide flange column connection is shown in Figure 5-8. The thick box column 
cap was modeled with shell elements and was perfectly merged into the lower box column. The 
connection between the wide flange column and the box column cap was an edge-to-surface tied interface 
without failure, which resulted in a perfect bond between the nodes of the wide flange column and the 
element segments of the box column cap plate. Failure of this connection would occur only when 
deformations and strains of this connection were sufficiently high to fail the elements in the columns 
adjacent to the joint. 




-Column Cap/Splice Plate 



Figure 5-8. Detail of box column-to-wide flange core columns connection. 



100 



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Development of Tower and Aircraft Impact Models 



The approach for assembling the core floors in the global model was to generate models of typical floors 
in the impact zone and repeat them in the surrounding floors. For WTC 1, a model of floor 96 inside the 
core was developed and used for modeling floors 92 through 100. This approach was also used for floors 
77 through 85 as the impact zone in WTC 2. Figure 5-9 shows the WTC 1 core prototype of the 96th 
floor with and without the concrete floor slab. The entire model was developed with Belytschko-Tsay 
shell elements. Mesh density was set independently from floor to floor to obtain higher accuracy in the 
impact zone and computational economy in the surrounding floors. A typical core floor with the higher 
impact zone mesh density had approximately 66,000 shell elements and 76,000 nodes. This included core 
floor slab, floor beams, connections, and core columns over a height of one floor. 




Figure 5-9. Model of the core of floor 96 of WTC 1 (with and without floor slab). 

The various connection details between core beams are illustrated in Figure 5-10. Core perimeter beams 
were joined with splice plates in the same manner as the wide flange column end connections described 
above. Interior beams were connected with node-to-surface tied connections. This contact algorithm 
constrained the nodes to move with the same relative motions as the adjacent surface elements and was 



NISTNCSTAR 1-2, WTC Investigation 



101 



Chapter 5 

appropriate for modeling a strong welded connection. An automatically generated model for the 
assembly of WTC 1 core floors 94 through 98 is shown in Figure 5-11. 



Node-to-Surface 
Tied Interface at 
Floor Beam 
Connections 



Splice Plates 
at Column 
Connections 



Perimeter Beams 
Connected with 
Splice Plates 



Figure 5-10. Model detail of core column and beam connections. 





Figure 5-11. Model of the WTC 1 core, floors 94-98. 



5.2.3 



Truss Floor Model Development 



The approach to the development of the truss floor model was very similar to other portions of the tower 
structure. Initially, parameterized component models were developed for segments of long-span trusses, 
short-span trusses, and corner two-way trusses. These parameterized models were then called repeatedly 
for generation and placement of the floor truss segments within the complete tower models. The 



102 



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Development of Tower and Aircraft Impact Models 



individual truss floor segments spanned the distance from the exterior wall to the core. An example of a 
truss floor segment used in the global model is shown in Figure 5-12. In the double truss sections, the 
two trusses were modeled explicitly with the proper dimensions. 




Figure 5-12. Model of a truss floor segment. 

The floor truss model was developed using a uniform layer of Belytschko-Tsay shell elements for the 
combined floor slab and metal decking, Belytschko-Tsay shell elements for the truss upper and lower 
chord components, and Hughes-Liu beam elements for the round bar truss diagonals. The upper chord 
was attached to the floor slab using a tied interface. This approach, using shell elements as opposed to 
solid brick elements for the floor slab, was adapted to reduce the model size requirements. Development 
of a model with matching mesh density in the slab and truss structures (nodal alignment for a merged 
connection) resulted in a much larger model size. Bridging trusses were modeled in a similar fashion to 
the primary trusses. 

A series of dampers were installed in the WTC towers between the floor truss lower chord and the 
spandrel on the exterior wall. The primary function of these dampers was to reduce the vibration of the 
building under wind loading. These dampers, however, were of low mass and the arrangement of the 
damper and saddle (member attaching the damper to the bottom chord of the truss), along with their 
connections, had virtually no strength in the transverse direction. Under impact conditions, the aircraft 
apphed transverse forces to the damper assembly due to the downward motion of the aircraft (see 
Chapter 6). Also, due to the short duration of the impact event (less then one second), damping was not 
included in the analyses. As a result, the dampers were considered to have sufficiently low mass and 
strength and were therefore not included in the impact analyses. 

The mesh refinement used in this model for the truss floor would result in a very large global tower model 
size if used throughout the structure. The model for the long-span truss floor segment (Figure 5-12) 
contained 2,737 nodes, 362 beam elements, and 1,878 shell elements. Constructing a global impact tower 
model with these detailed floor segments was not practical due to model size limitations. A complete 
floor would result in approximately 200,000 nodes for a single truss floor structure. As a result, detailed 
floor segments were used only in the impact zone, and a simplifled floor truss model was used elsewhere. 
The far-field floor truss was modeled with a significantly reduced mesh resolution, as shown in 
Figure 5-13, and provided the appropriate inertial properties and structural stiffness of the floor. The 
trusses were modeled with an effective shell element in place of the vertical truss structure and a beam 
element along the lower chord. These element dimensions were on the order of 30 in. and would not be 
able to accurately model a local collapse behavior of the trusses. The floor slab model was similar to the 



NIST NCSTAR 1 -2, WTC Investigation 1 03 



Chapter 5 

floor slab in the impact zone, but with a typical element dimension of 30 in. compared to an element 
dimension of approximately 10 in. in the impact zone. 

Figure 5-14 and Figure 5-15 show the truss floor connection details at the exterior and core, respectively. 
The models for the truss seat connections were developed using shell elements and attached using the tied 
interface algorithm. The failure of these seats occurred only as a result of exceeding the ductility of the 
seat or truss structures. A detailed model of floor 96 of WTC 1 is shown in Figure 5-16. 



From Top 



3-D View from Bottom 




Beain elements to match 
weight distribution 



Simplified steel truss shells 
Combined concrete/corrugated decking 



Figure 5-13. Simplified far field truss floor model. 



Truss Beam 



Spandrel 



Ext. Columns 




Support/contact 

between truss 

beams 



Seat brackets 
tied to column 



Tied connection 
between bracket 
and truss beam 



Figure 5-14. Truss floor connection detail at exterior wall. 



104 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



Truss Beam 



Core Perimeter 
Beam 




Figure 5-15. Truss floor connection detail at core perimeter. 



Side 200 




Side 300 



Figure 5-16. Detailed model of floor 96 of WTC 1. 

5.2.4 Interior Contents Model Development 

The interior nonstructural contents of the towers were modeled explicitly in the tower models used for the 
global impact analyses. The live load weight was distributed between gypsum walls and cubicle 
workstations that covered the truss floor area. The distribution of the gypsum walls was obtained from 



NISTNCSTAR 1-2, WTC Investigation 



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Chapter 5 



architectural drawings and other information gathered as part of Project 5 of the NIST investigation 
(NIST NCSTAR 1-5). Similarly, data gathered by NIST for the floor layout plans in the impact zone 
were used to develop the approximate placement of workstations over the truss floor area. The resulting 
model of a floor with interior contents is shown in Figure 5-17. 

Workstations Modeled 
over Truss Floor Area 



Side 100 



Side 200 




Side 400 



Side 300 



Figure 5-17. Model of floor 96 of WTC 1, including interior contents. 

The densities of specific materials were scaled to obtain the desired magnitudes for the service live loads 
and superimposed dead loads. The densities of the tower contents (workstations and gypsum walls) were 
scaled by the appropriate ratios to obtain the desired distribution of live loads in the core and truss floor 
areas. The densities of all the remaining tower structural components were scaled proportionately to 
obtain the desired superimposed dead loads. These additional loads were important for obtaining an 
accurate mass distribution in the towers and inertial effects in the impact response. The in-service live 
load used was assumed to be 25 percent of the design live load on the floors inside and outside the core. 
The in-service live load was selected based on a survey of live loads in office buildings (Culver 1976) and 
on engineering judgment. The uncertainty in the amount of in-service live load was accounted for in the 
sensitivity analyses (Chapter 8 of NIST NCSTAR 1-2B) and in the global impact simulations (Chapter 7 
of this report). 

The partitions and workstations were modeled using shell elements. The model of the building contents 
(partitions and workstations) over a single floor, as shown in Figure 5-17, had 101,733 nodes and 
97,284 shell elements. To include the complete distribution of the building contents over five floors in 
the global impact model would require approximately 500,000 nodes. As a result, the global models 
included the partitions and workstations only in the region of each floor directly in the path of the aircraft 
impact and debris. Using this approach significantly reduced the computational requirements needed to 



106 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



include the building contents' inertial contributions. For example, the WTC 1 global impact model 
included only 160,410 nodes and 148,858 shell elements for the partitions and workstations in the impact 
path over five floors. These building content distributions for both tower models are shown in the 
following section. 



5.2.5 



Global Impact Models Assembly 



The multiple floor global model of the impact zone in WTC 1 is shown in Figure 5-18. The model 
included the complete floors inside and outside the core, the exterior walls, and core structures for floors 
92 through 100. The boundary conditions at the top and bottom of the exterior and core columns were 
constrained vertical displacements. This allowed for free translations of the tower structure in the 
longitudinal and lateral directions and rotation about the vertical axis. The higher resolution exterior wall 
panels in the impact zone can be seen on the impact face of the tower model (side 100). 



Side 300 



Side 200 



£, 

^ 




Side 400 



Side 100 
Figure 5-18. Global impact model of the WTC 1 tower. 

The WTC 1 global impact model with the exterior wall removed is shown in Figure 5-19. The figure 
shows how the model was optimized to reduce mesh size and eliminate computational requirements 
outside of the immediate impact and damage zone. The nonstructural building contents (partitions and 
workstations) were modeled only in the path of the aircraft impact and debris cloud. These components 
are shown separately in Figure 5-20. 



NISTNCSTAR 1-2, WTC Investigation 



107 



Chapter 5 



In the assembled global model, the core columns for floors 93 through 98 of WTC 1 were modeled with 
higher resolution than that in the floors above and below the direct impact zone. This higher mesh 
resolution was needed to capture the local damage that occurred from direct impact of aircraft structures 
and debris. 



Floor: 




Far-field truss floor for 
remainder of the tower 



Detailed truss floor 
sections in impact zone. 



Figure 5-19. Interior structures and contents of the WTC 1 global impact model. 



Workstations and 

walls distributed in 

Impact path only 



View lool<ing in impact direction 



Impact 
direction 




Figure 5-20. Nonstructural building contents in the WTC 1 global impact model. 



108 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



The WTC 2 global impact model is shown in Figure 5-21. The model included the complete floor inside 
and outside the core for floors 77 through 85. The exterior wall panels at the bottom end of the model 
extended downward below floor 75. The boundary conditions at the top and bottom of the exterior and 
core columns were the same as those for the WTC 1 model. The higher resolution exterior wall panels in 
the impact zone can be seen on the impact face of the WTC 2 tower model (Side 400). 



Side 200 



Side 100 



Y^X 









m 



m\m^ 








Side 300 



Side 400 

Figure 5-21. Global impact model of the WTC 2 tower. 



The WTC 2 global impact model with the exterior wall removed is shown in Figure 5-22. The 
nonstructural building contents were again modeled only in the path of the aircraft impact and debris 
cloud. These components are shown separately in Figure 5-23. Similarly, the truss floor structures near 
the impact zone were modeled in greater detail as seen in Figure 5-22. These detailed sections of the 
truss floor were positioned adjacent to Side 400 (south face) for floors 78 through 81 and side 300 (east 
face) for floors 81 and 82. The surrounding truss floor structures were modeled with the far- field truss 
model. 



NISTNCSTAR 1-2, WTC Investigation 



109 



Chapter 5 




Floors 81 & 82, Side 300 
Floors 78-81, Side 400 

Figure 5-22. Interior structures and contents of the WTC 2 global impact model. 




View looking in approximate 
impact direction 



Workstations and 

walls distributed in 

impact path only 



Impact 
direction 




Figure 5-23. Nonstructural building contents in the WTC 2 global impact model. 



5.2.6 



Tower Material Constitutive Models 



The development of constitutive models that properly captured the actual behavior of the WTC towers 
under the dynamic aircraft impact conditions was an important requirement for high fidelity simulation of 



110 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



the aircraft impact damage. The primary materials that were considered included: (1) the several grades 
of steel used in the columns, spandrels, and floor trusses and beams of the WTC towers; (2) the concrete 
floor slabs; and (3) the nonstructural contents of the towers. These materials exhibit significant nonlinear 
rate-dependent deformation and failure behavior over the range of strain rates expected in the impact 
scenario. The following is a brief summary of the constitutive models used for these materials. 
Additional details can be found in Chapter 2 of NIST NCSTAR 1-2B, where constitutive models were 
described for bolt material and weldments. It also includes a discussion on the effect of mesh size on 
failure criteria. 

WTC Tower Steel Constitutive Models 

The primary constitutive model used for the several grades of the tower steels was the Piecewise Linear 
Plasticity model. This model is sufficient to model the nonlinear dynamic deformation and failure of steel 
structures. A tabular effective stress versus effective strain curve can be used in this model with various 
definitions of strain rate dependency. The constitutive model parameters for each grade of steel were 
based on engineering stress-strain data provided by the mechanical and metallurgical analysis of 
structural steels part of the NIST Investigation (see NIST NCSTAR 1-3D). Finite element analyses of the 
test specimens (ASTM Designation A 370 - 03 a) were conducted with a fine and a medium mesh (similar 
to that used in the component level analysis) to capture the nonlinear material behavior up to failure, see 
Figure 5-24. The finite element analysis also provided a validation that the constitutive model parameters 
were defined accurately and that the model could reproduce the measured response for the test conditions. 



Grip Test Sample 




Fine Mesh 




Medium Mesli 




Figure 5-24. Finite element models of the ASTM 370 rectangular tensile specimen. 

The first step in the constitutive model development process was to obtain a true stress-true strain curve. 
The typical approach was to select a representative test for each grade of steel and convert the engineering 
stress-strain curve to true stress-strain. The true stress-strain curve was extrapolated beyond the point of 
necking onset. This true stress-strain curve was then approximated by a piecewise linear curve in tabular 



NIST NCSTAR 1-2, WTC Investigation 



HI 



Chapter 5 



form, which was used to specify the mechanical behavior in the constitutive model. The final step was to 
simulate the tensile test (Figure 5-24). If necessary, the extrapolation of the true stress-strain behavior 
was adjusted until the simulation matched the measured engineering stress-strain response including 
necking and failure (the portion of the stress-strain curve beyond the maximum engineering stress). The 
true stress-strain curves used in the constitutive models for the various WTC tower steels are summarized 
in Figure 5-25. 



140 




Plastic Strain 
Figure 5-25. Tabular true stress-strain constitutive model curves for the tower steels. 

Elevated strain rates can influence the strength and ductility of structural materials. For the materials and 
strain rates of the WTC tower impact analyses, these strain rate effects are expected to be somewhat small 
compared to the effects of the baseline (static) strength and failure modeling. Strain-rate effects on the 
steel yield strength were included in the constitutive model for tower steels with the Cowper and 
Symonds rate effect model. The functional form for the rate effects on strength is governed by the 
equation: 



^.=^ji+ern 



Jo 



c 



where a and a are the yield strengths at strain rates of £ and zero, respectively. C and p are the 



yo 



Cowper and Symonds parameters. 

A series of high-rate characterization tests was performed on tower steels by the mechanical and 
metallurgical analysis of structural steels part of the NIST Investigation (NIST NCSTAR 1-3D) at strain 
rates between 100 and 1000 s"\ The Cowper and Symonds model parameters C 



112 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



and p were then fit to the test data and were provided in the following functional form for a strain rate in 



s" and a yield strength in ksi: 



• Log(C) = -7.55 + 0.324ayo-0.00153( Oy.Y 

• p = 6.7824 



The resulting rate effects used in the constitutive modehng of tower steels based on the Cowper and 
Symonds model are compared to the measured high rate test data for the 50 ksi, 75 ksi, and 100 ksi tower 
steels in Figure 5-26. The comparison shows that the Cowper and Symonds model was capable of 
reproducing the rate effects for the range of data available. 



140 



120 - 



100 



lA 




J^ 








in 


80 


M 




(U 








^-l 




V) 

•a 


60 


CD 




>- 





40 - 



20 - 







1 1 1 1 1 1 1 1 1 1 1 1 1 M 1 1 1 1 1 1 1 M 1 1 1 1 1 1 1 1 1 1 1 1 n 1 1 1 1 1 1 1 1 1 1 1 1 1 n 1 1 1 n 1 1 1 1 1 1 1 1 1 1 u I 



G &■• ■© £>•■ 



■ e Q -o ^ 



o ^..♦....^ o- o- o ■«'■ 



_ A fi"*- 



-A- &■ '^■■' 



-A ii- 



•■-O c>- 



-» %• 



-O — 



...<>..... -o- 



6- 



...«.♦■■ 



> A 



A 50 ksi Material Tests 

• 1 00 l<si Material Tests 

♦ 75 ksi Material Tests 



I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1 I I I I I I 1 I I I I I I I I I I I 1 I I I I r I 1 n I I I I I I I I I r I I 



-3-2-10123 

-1. 



Log Strain Rate (s ) 
Figure 5-26. Comparison of rate effects model and test data. 



Concrete Constitutive IVIodels 

The LS-DYNA material Type 16 (pseudo-tensor concrete model) was selected for modeling the concrete 
floor slabs due to its ability to accurately model the damage and softening of concrete, associated with 
low confinement. The model used two pressure-dependent yield functions and a damage-dependent 
function to migrate between curves. This allowed for implementation of tensile failure and damage 
scaling, which are more dominant material behaviors at low confinement. The pseudo-tensor model also 
accounted for the sensitivity of concrete to high strain rates. The reader is the reader is referred to the 
LS-DYNA user's manual (2003) for a detailed description of the model, and to NIST NCSTAR 1-2B for 
the model parameters used in the analysis. 

Material constitutive parameters for the pseudo-tensor model were developed for both 3 ksi and 4 ksi 
compressive strength lightweight concrete. A simulation was performed of a standard unconfmed 
concrete compression test to check the constitutive model behavior. The simulated behavior of the 



NIST NCSTAR 1-2, WTC Investigation 



113 



Chapter 5 

concrete specimen is shown in Figure 5-27. The calculated compressive stress-strain response for the 
3 ksi concrete was compared to measured compression data for 2.3 ksi and 3.8 ksi strength concretes in 
Figure 5-28 (Wischers 1978). 

For subsequent global analyses, a 4 ksi concrete was used, instead of the 3 ksi concrete strength specified 
in the original design, to account for factors such as aging and the difference between specified nominal 
and actual concrete strength. The same material parameters were used for the concrete in both the core 
(normal weight concrete) and truss floor (lightweight concrete) areas. 



HI 1 1 1 


1 1 


























































































































i^^Y 1 1 


1 1 1 1 ii.y 



III 1 1 


1 1 1 1 


1 






^ 












'^^ i 






1 






1 






-I 






1 













































































































































m 1 


1 1 1 1 


1 III 



Initial configuration 2% compression 

Figure 5-27. Finite element analysis of the unconfined compression test. 



114 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



4'- 



-I 1 1 r 



~i 1 1 r 



"T 1 1 r 



T 1 1 r 



•■♦- Data [Wise hers, 197B] fc = 3.8 ksi 
•■•■■ Data [Wischers, 1978] fc = 2.3 ksi 
— Pseudo Tensor Model fc = 3.0 ksi 




Strain (%) 

Figure 5-28. Comparison of the calculated unconfined compression behavior with 

concrete compression test data. 

Experimental characterization of the strain rate effects on concrete is difficult, and there is a wide scatter 
in data that is infiuenced by concrete type, strength, and the testing methods applied. In general, elevated 
strain rate loading has a greater influence on the tensile strength than on the compressive strength. 
However, in the aircraft impact response of the WTC towers, the majority of the high-rate damage occurs 
with impact and penetration of the floor slab by hard components such as the aircraft engine. As a result, 
the strain rate effects for compressive loading were used for the constitutive model. The strain rate effects 
were added to the model in tabular form. The rate effects curve used in the model is shown in 
Figure 5-29, based on the work of Bischoff and Perry (1991) and Ross et al. (1992). The curve was 
selected to provide a relatively smooth fit to the available compressive rate effects data on compressive 
strength. 



NISTNCSTAR 1-2, WTC Investigation 



115 



Chapter 5 



N I I I I I I I I I I I I I I I I I J I I I I I I I I I I I I I I I M I I J I I I I I I I I I 1 1 I I I I M I I I I I I I I I I I I I I I I I I I I 1 I 



.2 
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-4-3-2-101234 

Log Strain Rate {s ) 
Figure 5-29. Tabular concrete strain rate effects curve. 

Nonstructural Materials Constitutive Models 

In general, the primary influence of the nonstructural components on the impact behavior was their 
inertial (mass) contribution. The effects of their strength were small. As a result, relatively simple 
approximations of their constitutive behavior were used. Typically, a simple elastic-plastic model was 
apphed for these materials to allow for efficient modeling of deformation and subsequent erosion from 
the calculations as their distortions became large. The ability to include material failure and erosion of 
these soft materials was important for the stability of the impact analyses. 

Based on a survey on the strength of various nonstructural building components (see Chapter 2 of 
NIST NCSTAR 1-2B), a bihnear elastic-plastic constitutive model with a yield strength of 500 psi and a 
failure strain of 60 percent was used. The large failure strain for these materials was used to prevent large 
scale erosion of the contents before the momentum transfer from the aircraft debris had occurred. 



5.3 



DEVELOPMENT OF AIRCRAFT MODEL 



The finite element model of the Boeing 767-200ER aircraft was constructed through a three-step process: 
(1) data collection, (2) data interpretation and engineering analysis, and (3) meshing of the structure. 
A major focus of this effort was gathering sufficient structural data and including adequate detail in the 
aircraft model so that the mass and strength distribution of the aircraft and its contents were properly 
captured for implementation in the impact analyses. Structural data were collected for the 
Boeing 767-200ER aircraft from: (1) documentary aircraft structural information, and (2) data from 
measurements on Boeing 767 aircraft. 



116 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



The objective of the aircraft model development was to properly simulate the impact damage and aircraft 
breakup, and their effects on the WTC towers. Key requirements were to simulate the mass distribution, 
dynamic impact response, fragmentation, and progress of the aircraft components and debris into and 
through the towers. The modeling approach was to model the airframe completely using shell elements 
as opposed to a shell element skin and beam elements for the airframe. Shell elements in the airframe 
provided higher fidelity simulation of the impact response and fragmentation behavior. As a result of the 
model size constraints, some of the details and smaller structural elements were not modeled explicitly. 
Where modeling simplifications were required, component analyses were applied to ensure that the 
impact strength and breakup behavior were maintained. 

An example of this approach was the development of the wing model. A section of the aircraft wing 
structure was first modeled with a very fine mesh of the detailed wing structure to establish a baseline 
behavior for aircraft structural failure and fragmentation upon impacting the exterior wall of the 
WTC towers. A coarser and simplified version of the same wing section was subsequently developed, 
and the failure criteria were modified to obtain similar impact and fragmentation behavior to the fine, 
detailed version. Section 5.3.2 describes how this model was constructed and the methodology used for 
developing the coarsely meshed wing section. A similar mesh resolution and failure criteria were used 
throughout the rest of the aircraft model. 

Similar to the global towers structural model, the LS-DYNA model of the aircraft was generated and 
meshed using the TrueGrid software (TrueGrid Manual 2001). The complete model for the Boeing 767- 
200ER is shown in Figure 5-30. A summary of the model size and weight parameters for the aircraft that 
impacted WTC 1 (American Airlines flight 1 1 [AA 11]) and the aircraft that impacted WTC 2 (United 
Airlines flight 175 [UAL 175]) is presented in Table 5-2. The weight cited for the unit load device 
(ULD) and seats included the empty weights plus the passenger or cargo weight. Carry on luggage and 
catering weight was distributed to the seats, and freight and cargo luggage weight was distributed to the 
ULD. 

Fuel was distributed in the wings as shown in Figure 5-31, based on a detailed analysis of the fuel 
distribution in the aircraft wings at the time of impact (see Chapter 4 of NIST NCSTAR 1-2B for analysis 
details). The wings of the aircraft were also deflected from the surface model geometry to represent their 
in-flight condition, as shown in Figure 5-32. A cubic function of the wing span was used with a tip 
deflection of approximately 52 in., which was estimated from the impact pattern seen in photographs of 
the WTC towers and from the damage documented on the exterior panels. 

The following sections outline the overall aircraft model developed for the impact analysis. Details in 
modeling each major component including the wings, engines, fuselage, empennage, and landing gear are 
provided. 



NIST NCSTAR 1 -2, WTC Investigation 1 1 7 



Chapter 5 



Table 5-2. Boeing 767-200ER aircraft model parameters. 




AAll 


UAL 175 


No. Brick Elements 


70,000 


70,000 


No. Shell Elements 


562,000 


562,000 


No. SPH Fuel Particles 


60,672 


60,672 


Total Nodes 


740,000 


740,000 


Total Weight (Empty) 


183,500 1b 


183,5001b 


ULD/Cargo Weight 


12,420 lb 


21,660 1b 


Cabin Contents Weight 


21,5801b 


10,420 1b 


Fuel Weight 


66,1001b 


62,000 lb 


Total Weight (Loaded) 


283,600 lb 


277,580 lb 



118 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



Tlme= 




Time = 




Ck 



Figure 5-30. Finite element model of the Boeing 767-200ER. 



NISTNCSTAR 1-2, WTC Investigation 



119 



Chapter 5 




Figure 5-31. Boeing 767-200ER with fuel load at time of impact. 



Without wing deflection 




Figure 5-32. Boeing 767-200ER model wing deflections. 



120 



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Development of Tower and Aircraft Impact Models 



5.3.1 



Airframe Model Development 



The airframe model developed for the Boeing 767-200ER contained most of the significant structural 
components in the aircraft. The models of the fuselage, empennage, and wing structures were developed 
completely using Belytschko-Tsay shell elements. Models for the landing gear and engines were 
primarily developed using shell elements, but contained some brick elements as well. The model was 
developed in a parameterized form, where the mesh resolution was determined by a single element 
characteristic size parameter. This approach was selected early in the development to allow flexibility in 
the model size and resolution as the model development and impact analyses progressed. The objective 
was to develop a mesh with typical element dimensions between one and two in. for small components, 
such as spar or rib flanges, and element dimensions of three to four in. for large parts such as the wing or 
fuselage skin. 

Detailed models of the empennage and landing gear are shown in Figure 5-33 and Figure 5-34, 
respectively. Ribs, spars, rudder, and elevator were all modeled in detail in the empennage. Tires and 
hubs, the main strut and truck, and support bracing were all included in the landing gear model. The 
underside of the airframe in the model is shown in Figure 5-35, illustrating the position of the retracted 
main landing gear in the wheel well. Also shown in the figure are the Unit Load Devices (ULDs shown 
in red with blue edges). The density of these containers was scaled to include the weight of the cargo. 




(a) Top view 





(b) Side view (c) Oblique view 

Figure 5-33. Empennage model of the 767-200ER aircraft. 



NISTNCSTAR 1-2, WTC Investigation 



121 



Chapter 5 







Nose Gear 



Collapsed Drag 
and Side Brace 



vf ^ Miln Gear 

Figure 5-34. Retracted landing gear components for the 767-200ER aircraft model. 




Figure 5-35. Underside of the 767 airframe model (skin removed) showing retracted 

landing gear, engine, and ULDs. 



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Development of Tower and Aircraft Impact Models 



Figure 5-36 shows the model of the wing structure, including the center wing, which attaches the port and 
starboard wings. The wing stringers were not explicitly modeled to help reduce the size of the model. 
The stringers have a z-section geometry with typical dimensions of approximately one in. flanges and a 
two in. web with a thickness of approximately 1/8 in. These stringers run along the wing span over the 
top and bottom of the wing ribs. To account for the weight and strength of the riveted skin/stringer 
construction, an 'effective' wing skin was used, as discussed in Section 5.3.2. 




(a) Complete wing model 




(b) Center wing structures 
Figure 5-36. Complete wing structures for the 767 aircraft model. 

A model of the fuselage was assembled with a stringer and frame construction supporting the external 
skin, as shown in Figure 5-37. A tied interface was used to connect the stringers to the frames and skin 
using the tied surface-to-surface algorithm in LS-DYNA, where nodes on a slave surface were 
constrained to nodes on a master surface, provided they were within a certain distance of the master 
surface node. This distance was a function of the element thickness or diagonal length. The wing was 
integrated into the fuselage structure through attachment of the center wing to the keel and front and rear 
spar bulkheads, as shown in Figure 5-38. These components were also attached using a tied interface. 
Due to model size constraints, the forward and aft portions of the fuselage were modeled without the 
detailed stringer/frame construction. Instead, the weight of these components was smeared into the skin 
by increasing the skin thickness and scaling down the strength by a factor of 40 percent, as described in 
the component analyses (see Chapter 4 of NIST NCSTAR 1-2B). 



NISTNCSTAR 1-2, WTC Investigation 



123 



Chapter 5 



stringers 



Frames 



Bulkheads 




Figure 5-37. Model of fuselage interior frame and stringer construction. 





Rear Spar 
Bulkhead 


Wheel Well 
Pressure Deck 

Wheel Well 


Center 


>^ 


*«_. _ Bulkhead 


Wing 

1 


/?) 


OTS^// 




Main Landing Gear 
Wheel Well 

Figure 5-38. Integration of the fuselage and wing structures. 

The density of various parts of the aircraft was increased to account for the mass of structural and 
nonstructural components not specifically modeled. Density scale factors and total weights for each 
major component are shown in Table 5-3. The difference in scale factors for flights AA 1 1 and UAL 175 
were due to differences in passenger and cargo weight. In both cases, the weight of the cargo, passengers, 
and crew were incorporated in the ULD (cargo weight) and the seats (passenger, crew and carry on 
luggage weight). The weight of the modeled wings and empennage were doubled to account for the 
weight of small structural details, such as stiffeners, not specifically modeled, as well as hydraulic lines 
and fluid pumps, actuators, inboard flaps and outboard ailerons, flap and rudder connections, and other 



124 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



nonstructural components. The weight of the landing gear was increased by a smaller amount (1.5) to 
account for hydraulic fluid and smaller structural components not included in the model. The weight of 
the fuselage was adjusted to match the published empty weight for the aircraft. That the scale factor for 
the fuselage was larger than for other components was reasonable as many heavy items in the fuselage 
were not specifically modeled (e.g. electronics, air conditioning, power units, ductwork, electronic wiring, 
cargo floor, actuator motors, insulation, hydraulics, galley and lavatories). These structural and non- 
structural components could not be modeled in detail due to the constraints on model size. 



Table 5-3. Density scale factors and weights 


for aircraft components 


Major Aircraft 
Component 


Density Scale 
Factor (AA 11) 


Total Weight 
(AA 11) 


Density Scale 
Factor (UAL 175) 


Total Weight 
(UAL 175) 


Wings 


2.0 


37,000 lb 


2.0 


37,000 lb 


Empennage 


2.0 


8,350 lb 


2.0 


8,350 lb 


Fuselage 


6.68 


103,050 lb 


6.68 


103,050 lb 


Landing Gear 


1.5 


8,400 lb 


1.5 


8,400 lb 


Engines (with cowlings) 


1.2 


20,100 1b 


1.2 


20,1001b 


ULD 


1.43 


12,400 lb 


2.50 


21,6501b 


Seats 


1.29 


28,200 lb 


0.78 


17,0501b 


Fuel 


1.0 


66,100 1b 


1.0 


62,000 lb 


Total Weight 




283,600 lb 




277,600 lb 



5.3.2 



Wing Section Component Model Development 



A wing section model was developed to perform the component and subassembly level analyses (See 
Chapters 5 and 6, respectively, of NIST NCSTAR 1-2B). The full wing contained 35 ribs, with rib 1 
closest to the fuselage and rib 35 near the wing tip. The wing section model described herein included the 
section of the wing from rib 14 to rib 18 and is shown in Figure 5-39. 

The wing structure of the Boeing 767 contains a riveted stringer-skin construction between the front and 
rear spars. This part of the structure was not included in the wing model as it added significant 
complexity and size to the model. In order to reduce the size of the model for the global impact analysis, 
an 'effective' wing skin was developed to account for the weight and strength of the riveted skin/stringer 
construction. A simplified wing section model, containing a uniform stringer-skin construction and a 
simple rectangular cross-section, was also developed to determine the strength and weight of the effective 
skin. Both wing section component models utilized Belytschko-Tsay shell elements. The parameters of 
the effective wing skin model (39) were developed by calibrating this model against the simplified wing 
section model that included the main spars, wing ribs, leading edge ribs, nose beams, leading edge slats, 
and outboard flaps. Refer to Chapter 4 of NIST NCSTAR 1-2B for further details. 



NIST NCSTAR 1-2, WTC Investigation 



125 



Chapter 5 





(a) Small wing section model (b) Internal structure (skin removed) 

Figure 5-39. Wing section model for component level and subassembly analyses. 



5.3.3 



Engine Model Development 



Initial sources indicated that the Pratt & Whitney PW4000 engine and the General Electric CF6-80 engine 
were on the aircraft that impacted the WTC towers (FEMA 2002). For this reason, the Engine Reference 
Manuals were obtained from Pratt & Whitney for the PW4000 turbofan engine. A detailed finite element 
model of the PW4000 engine was developed from these manuals. 

After the engine model was developed, the engine types on each aircraft were clarified by the Aviation 
Safety Network (http://aviation-safety.net/). AA 1 1 was powered by two General Electric CF6-80A2 
engines. UAL 175 was powered by two Pratt & Whitney JT9D-7R4D engines. However, careful review 
of these engines indicated that the PW4000 turbofan engine was very similar to the General Electric 
CF6-80A2 and the PW JT9D-7R4D engines. Comparisons of specific physical characteristics of the 
engines are given in Table 5-4. The JT9D-7R4D and PW4000-94 are almost identical as they are in the 
same family of Pratt & Whitney aircraft engines. The PW4000 was labeled the "new technology JT9D" 
when it began replacing the latter engine on 767s built after 1987. The PW4000-94 is 5.8 percent heavier 
than the JT9D-7R4 but produces up to 10 percent more thrust. Aside from an additional set of long stator 
blades and elongated exit nozzle, the CF6-80C2 is also of similar weight and dimensions to the PW4000. 
Due to these similarities, the PW4000 engine model was used for all impact simulations. Differences in 
the weights of aircraft components were accounted for in the uncertainty analyses. 

Table 5-4. Boeing 767 Engine Comparison. 



Engine 


Pratt & Whitney 
PW4000-94 


Pratt & Whitney 
JT9D-7R4'''' 


General Electric 
CFG-BOCZ'" 


Fan Blade Diameter 


94 (in.) 


94 (in.) 


93 (in.)^ 


Length 


153 (in.) 


153 (in.) 


161-168 (in.)' 


Dry Weight 


9,400 (lb) 


8,885 (lb) 


9135-9860 (lb) 



a. The JT9D-7R4 and PW4000-94 are almost identical: (1) They are in the same family of Pratt & Whitney aircraft engines, and 
(2) the PW4000 was labeled the "new technology JT9D" when it began replacing the latter engine on 767s built after 1987. 

b. The PW4000-94 is 5.8 percent heavier than the JT9D-7R4 but produces up to 10 percent more thrust. 

c. The CF6-80C2 has an additional set of long stator blades for the excess fan air that is not present in the PW4000. 

d. The second stage compressor blades in the CF6-80C2 are closer to the central shaft than the PW4000 and do not appear to 
have counter weights. 

e. Reference value of 106 in. also found - may include cowling. 

f. The "tail" of the CF6-80C2 is much longer than the PW4000. This potentially accounts for the additional 15 in. in length. 



126 



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Development of Tower and Aircraft Impact Models 



Figure 5-40. The engine is an important component of the aircraft with the potential to produce 
significant damage to the WTC tower structures. As a result, special emphasis was given to the 
development of the engine model to include all the details of the engine construction. 




Used with Permission. 

Figure 5-40. Pratt & Whitney PW4000 turbofan engine. 

The approach used to capture the geometry of the engine was to start with a cross-sectional drawing 
provided by Pratt & Whitney that clearly showed many of the engine geometric details. In addition, the 
drawing had sufficient detail that the component thicknesses could be estimated. The primary structural 
components were identified and approximated with simplified geometry as illustrated in Figure 5-41. 
Known engine dimensions were used to determine the scale factor for the drawing. The simplified 
geometry of the engine structures could then be captured using a common digitization procedure. 



NISTNCSTAR 1-2, WTC Investigation 



m 



Chapter 5 



47! n 




Used with Permission. Enhanced by NIST. 
Figure 5-41. PW4000 engine cross-sectional geometry and simplification. 

Once the engine internal geometry was captured, the digitized geometry was imported into TrueGrid and 
used to generate surface definitions and part geometries for the engine model. The engine model was 
developed using primarily shell elements with typical element dimensions between 1 in. and 2 in. 
Smaller element dimensions were required at many locations to capture details of the engine geometry. 
Brick elements were used for some of the thicker hubs and the roots of the compressor blades. The 
various components of the resulting engine model are shown in Figure 5-42. A summary of the elements 
used in the engine model is given in Table 5-5. 



128 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 




Figure 5-42. Pratt & Whitney PW4000 turbofan engine model. 
Table 5-5. Engine model parameters. 





PW4000 Engine Model 


No. Brick Elements 


9,560 


No. Shell Elements 


54,788 


Total Nodes 


101,822 


Preliminary Engine Model Weight 


7,873 lb 


Adjusted Engine Model Weight 


9,447 lb 



After the known primary structural components of the engine were included in the engine model, the 
weight of the model was calculated to be 7,873 lb. The dry weight of the PW4000 engine was listed at 
9,400 lb and the JT9D-7R4 and CF6-80C2 engines weigh between 8,885 and 9,860 lb. These engine 
weights were approximately 20 percent larger than the initial model weight. The difference in weight 
potentially resulted from the nonstructural components (tubing, pumps, seals, bearings, etc.) that were not 
included in the model. To account for the difference, the density of all of the material models used for 
engine components was increased by 20 percent. This effectively smeared the missing mass in proportion 
to the original mass distribution in the model. The resulting adjusted engine model weight was 9,447 lb. 



NISTNCSTAR 1-2, WTC Investigation 



129 



Chapter 5 



5.3.4 



Aircraft Material Constitutive Models 



The constitutive and failure properties for the aircraft materials were developed from data available in the 
open literature. The principal sources of data for the airframe materials were the Military Handbook 
(MIL-HDBK-5F), 1987 and the Aerospace Structural Metals Handbook [Brown, et al. 1991]. Additional 
sources of data were used to verify and supplement the information obtained from these primary data 
sources. 

Complete engineering stress-strain curves were provided in the MIL-HDBK-5F for various 2024 and 
7075 aluminum alloys that are commonly used in the construction of the Boeing 767 airframe. These 
curves were digitized for the various 2024 and 7075 alloys. Representative stress-strain curves were then 
converted into true stress and true strain and used to develop tabular curves for constitutive models. The 
calculated true stress-strain curves and tabular constitutive model fits are shown in Figure 5-43 and 
Figure 5^4, respectively. Appropriate failure criteria for the aircraft materials were developed using the 
fine and coarse wing component models, see NIST NCSTAR 1-2B. No rate sensitivity of the aircraft 
materials was considered. 



120 



: — \ — I — I — I — I — I — I — I — I — I — I — I — I — I — ] — I — I — I — I — I — 1 — I — I — r 



100 E- 



..^--*- 



v^-^*- 



£ 60 H ^^ pv0-^' 




.-& 



..+■■■" „ & 



.<»-e' 



■•+■■■ 7075-T7351 Extrusion 
■o- 2024-T3 Clad Sheet 
■■&■" 2024-T351X Extrusion 
■O" 7075-T651X Extrusion 



L 




0.00 



_L 



J I I I I I I I I I I I ! I I I I I I I I L 



0.05 



0.20 



0.25 



0.10 0.15 

True Strain 
Figure 5-43. True stress-strain curves developed for various aircraft aluminum alloys. 



130 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



120 




20 



Constitutive Model Fit: 
-*- 7075-T7351 Extmsion 
-♦- 2024-T3 Clad Sheet 
-m- 2024-T351X Extrusion 
-•- 7075-T651X Extrusion 



0<i— ' 1- 



_L 



X 



_L 



0.00 



0.05 



0.15 



0.20 



0.10 
Plastic Strain 

Figure 5-44. Tabular stress-strain curves developed for various aircraft aluminum alloys. 



5.4 



COMPONENT AND SUBASSEMBLY LEVEL ANALYSES 



The primary objectives of the component modeling were to (1) develop understanding of the interactive 
failure phenomenon of the aircraft and tower components, and (2) develop the simulation techniques 
required for the global analysis of the aircraft impacts into the WTC towers. The approach taken for 
component modeling was to start with finely meshed, brick and shell element models of key components 
of the tower structure and progress to relatively coarsely meshed beam and shell element representations 
that were used for the global models. This was done to support the development of the reduced finite 
element global models appropriate for high fidelity global impact analyses, as modehng each component 
with fine details in the global models would be too demanding from a computational standpoint as was 
explained in Section 5.1. In addition to determining the optimal element size and type for global 
modeling, other key technical areas were addressed in the component modeling, including material 
constitutive modeling, treatment of connections, and modeling of aircraft fuel. The component analyses 
were also used in the uncertainty analyses to assess the effects of uncertainties associated with the aircraft 
and WTC towers on the level of damage to the towers after impact and to determine the most influential 
modeling parameters that affect the damage estimates (see Chapter 8 of NIST NCSTAR 1-2B). 

The subassembly analyses were considered as a transition between the component analyses and the global 
impact analyses. The subassembly analyses were primarily used to investigate different modeling 
techniques and associated model size, run times, numerical stability, and impact response. The 
subassembly model was also used to investigate the sensitivity of the impact response to model 
parameters as well as for the uncertainty analyses (Chapter 8 of NIST NCSTAR 1-2B). 

A large array of component and subassembly models were developed and used in the impact simulations. 
Examples of such analyses are included in this section. The reader is referred to Chapters 5 and 6 of 
NIST NCSTAR 1-2B for fiarther details of the component and subassembly analyses, respectively. 



NIST NCSTAR 1-2, WTC Investigation 



131 



Chapter 5 



5.4.1 



Exterior Column Impacted with an Empty Wing 



The objective of this analysis was to develop a model with a coarse mesh that could be applied to the 
global impact analyses and still capture the impact damage properly. The analysis used an empty wing 
section impacting an exterior wall column. The empty wing section model was selected to produce 
significant column damage at an impact speed of 470 mph without completely failing the column. 

These calculations used a preliminary failure criterion. The exterior column modeled was constructed 
entirely with 55 ksi steel and the spandrel plates with 42 ksi steel. Both a model with a fine mesh of brick 
elements and a model with a coarser mesh of shell elements were developed. These models included a 
specific description of the weld geometry, with different properties. In the fine brick element model, the 
failure strain for the base metal, weld metal, and heat affected zone (HAZ) were all set at a uniform 
plastic strain of 64 percent, corresponding to the base metal ductility. Failure strains in the coarse shell 
element models were then adjusted until a similar impact damage and failure mechanism were obtained. 
A comparison of parameters for the two models is given in Table 5-6. 

The calculated impact response is shown in Figure 5-45. The column model on the left has the fine mesh 
of brick elements, and the column model on the right has the coarse mesh of shell elements. Contours of 
resultant displacements are shown on the column components. The figure indicates that the overall 
response was similar in both magnitude and damage mode. The reduction in model refinement resulted in 
a significant reduction in run time from over 600 min to 9 min. This comparison demonstrated the 
significance of the mesh refinement on capturing local stress and strain concentrations and the resulting 
effect on the impact response. 



y 



I 



Fringe Levels 

2.000e+001 

1.800e+001 

1.E00e+001 . 

l.ilOOe+001 

1.200e+001 . 

1.000e+001 

8.000e+00a. 

G.OOOe+OOO, 

I.OOOe+OOO. 

2.000e+00a. 

O.OOOe+000 

(a) Fine brick element column (b) Coarse shell element column 

Figure 5-45. Exterior column response comparison, showing contours of the 

displacement magnitude (in.). 



132 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



Table 5-6. Exterior column component analyses comparison. 



5.4.2 



Column Model Type 


Fine Brick Model 


Coarse Shell 
Model 1 


Number of Brick Elements 


473,208 


352 


Number of Shell Elements 





9,192 


Number of Beam Elements 





8 


Minimum Element Dimension 


0.0625 in. 


1.0 in. 


Bulk Material Failure Strain 


64% 


12% 


Weld Zone Failure Strain 


64% 


2% 


Calculation Time (CPU)" 


444 min 


3 min 


Elapsed Time 


624 min 


9 min 



a. Simulation of 0.035 second duration impact response performed on 11 CPUs. 



Bolted Connection Modeling 



The objective of this analysis was to develop connection models for the global impact analyses that 
accurately captured the capacity and failure modes of the bolted connection between exterior columns. 
Component models of the exterior column butt plate connections are shown in Figure 5-46. The detailed 
model (a) included individual bolts and butt plates modeled with solid brick elements. The simplified 
model (b) used coarse brick element butt plates joined by beam elements representing the bolts. A 
dynamic analysis was carried out to calibrate the beam element bolt model. The loading condition was a 
dynamic separation of the two butt plates. The velocity profile used to separate the butt plates was a 
linearly increasing separation velocity between the butt plates with an initial velocity of zero and a 
velocity of 43 fps at a time of 5.0 ms, obtained from a preliminary engine impact analysis against the 
exterior wall. 





(a) Brick element bolts 
Figure 5-46. Modeling of exterior column bolted connection. 



(b) Beam element bolts 
(butt plates shown as transparent) 



Failure strain in the beam models was calibrated such that the beam bolts failed at the same time as the 
brick element bolts. Failure of the bolts occurred at a time of approximately 3.0 ms. These connection 
models were used in the corresponding brick and shell models of the exterior column component impact 



NISTNCSTAR 1-2, WTC Investigation 



133 



Chapter 5 



analyses shown previously in Figure 5-45. Connection failure at the column ends was quite similar in 
both cases as shown in Figure 5-47. Failure of the connection is illustrated for both connection models at 
the same time, 35 ms, after impact with the empty wing segment. The primary failure mode for both 
models was a tensile failure of the bolts and subsequent separation of the column end butt plates. 





(a) Brick element bolts (b) Beam element bolts 

Figure 5-47. Failure comparison of exterior column bolted connection treatments. 



5.4.3 



Floor Assembly Component Analysis 



Floor truss impact analyses were carried out to develop a coarse representation of the truss floor, for use 
in the global impact simulations, that properly captured the impact response characteristics of the fine 
model of the floor system. For that purpose, detailed floor component models used a combination of 
brick elements for the concrete slab, beam elements for the truss round bar diagonals, and shell elements 
for the remainder of the structures, including the truss upper and lower chords and metal decking. This 
model is shown in Figure 5-48. A less-refined model, similar to that used in the global impact models, 
was then developed with coarser shell and beam elements as shown in Figure 5-49. This model reduced 
the size of the floor model by an order of magnitude and the run times by more than 80 percent (see the 
comparison in Table 5-7). 

Table 5-7. Truss floor assembly component analyses comparison. 



Model Type 


Fine Brick Model 


Coarse Shell Model 


No. Beam Elements 


6,928 


3,440 


No. Brick Elements 


230,778 





No. Shell Elements 


148,256 


39,000 


Total Nodes 


372,084 


48,971 


CPU Time 


16,796 s (4.7 h) 


2,482 s (0.7 h) 


Elapsed Time 


26,553 s (7.3 h) 


4,454 s (1.2 h) 



The concrete constitutive model used in the brick elements of the detailed floor model was the pseudo- 
tensor model described in Section 5.2.6. The coarse floor model used an effective material model for the 



134 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



concrete and metal decking so that these parts would not need to be meshed separately. As the pseudo- 
tensor model is developed for brick elements, and does not work for shell elements, a piecewise plasticity 
model was used for the effective slab-decking behavior. A tabular stress-strain curve was developed 
based on the rule of mixtures of the elastic-plastic metal decking with the unconfined compressive 
behavior for the concrete. The combined slab and decking stress-strain curve was compared to the 
concrete unconfined compressive behavior in Figure 5-50. The strength of the combined floor slab was 
dominated by the concrete strength at low strain levels (below 1 percent strain). However, as the concrete 
was fragmented and removed as debris, the residual strength was equivalent to that of the metal deck 
alone and remained ductile until a strain of 30 percent was reached. The strain rate effects used for the 
combined concrete slab and metal decking were those used for concrete as shown in Figure 5-29. 

The impactor used in the component modeling was a simplified plow type impactor, which produced 
repeatable damage, not complicated by all the debris and randomness associated with an engine-floor 
impact. The weight of the plow impactor was comparable to an engine, and the impact speed was 
500 mph, applied horizontally. An example analysis with a plow impactor and the fine mesh floor model 
is shown in Figure 5-51. The calculated impact damage with the coarser shell element floor system 
model is shown in Figure 5-52. This component impact configuration was useful for comparing the 
differences in response with changes in the modeling methods or refinement. 



Brick Element 
Concrete Slab 




Shell Elements 
for Metal Decking 



Beam & Shells Elements 
for Truss Structure 




View from Below 



Figure 5-48. Detailed model of the truss floor system. 



NISTNCSTAR 1-2, WTC Investigation 



135 



Chapter 5 



Shell Elements for 

Combined Concrete 

and Metal Decking 




Beam & Shell Elements 
for Truss Structures 



View from Below 
Figure 5-49. Simplified model of the truss floor system. 



5 - 



T 



1 I I I I r I I I I I I I I f 1 E I I I I F 1 r 



T 



T 



Concrete Constitutive Behavior 
Composite Constitutive Model 




q<|l_i I I I I I I I I I I I I I I I I L 



-e- 



j I I I I I I I i_ 



0.000 



0.005 



0,010 



0.015 

Strain 



0:020 



0.025 



0.030 



Figure 5-50. Constitutive behavior for the combined concrete and metal decking. 



136 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 




Impact response at 0.10 s 
Figure 5-51. Floor assembly impact response with brick element concrete slab. 




Impact response at 0.10 s 
Figure 5-52. Floor assembly impact response with shell element concrete slab. 



NISTNCSTAR 1-2, WTC Investigation 



137 



Chapter 5 



5.4.4 



Modeling of Aircraft Wing Section Impact with Fuel 



A significant portion of the weight of a Boeing 767 wing is from the fuel in its integral fuel tanks. At the 
time of impact, it is estimated that each aircraft had approximately 1 0,000 gal of fuel onboard. Upon 
impact, this fiael was responsible for large distributed loads on the exterior columns of the WTC towers 
and subsequently on interior structures, as it flowed into the building, potentially having a significant 
effect on the damage inflicted on the building structure. Modeling of the fluid-structure interaction was 
necessary to predict the extent of this damage and the fuel dispersion within the building to help establish 
the initial conditions for the fire dynamics modehng. 

A number of approaches to solving fluid-structure interaction problems are available in LS-DYNA. One 
approach is the standard Lagrangian finite element analysis with erosion, where the fuel is modeled using 
a deformable mesh. This approach accounts for the inertial effects of the fuel, but does not simulate well 
the fuel flow during impact due to limitations on mesh distortion. The Arbitrary-Lagrangian-Eulerian 
(ALE) method was developed as a good approach to solve fluid and solid material interaction. With this 
methodology, fluids are modeled with an Eulerian mesh, which allows for materials to flow between 
mesh elements. Solid materials are modeled with a moving Lagrangian mesh. With ALE, both mesh 
types can interact. An alternative approach is to use mesh-free methods such as Smoothed Particle 
Hydrodynamics (SPH). SPH modeling for fuel effects has the advantage of a smaller mesh size and 
potentially much faster run times than ALE analyses. Both ALE and SPH methods were applied to the 
analysis of fuel impact and dispersion and are compared in this study. 

A small wing segment was used for performing component level analyses of the wing with fuel. The 
segment was considered to be completely filled with fuel (approximately 850 gal). Figure 5-53 shows 
the wing section model with an SPH and ALE mesh for the fuel, shown in blue. The fuel was modeled 
with 6,720 SPH fuel particles and 1 10,825 ALE elements for the fuel and surrounding air region, shown 
in Figure 5-54. The impacted structures were two exterior wall panels as shown in Figure 5-54. 

An ALE mesh, surrounding the wing segment and the panels, was needed for the fuel to flow into. In 
ALE analyses, material is advected from one element to the next so that a mesh is needed for initially 
"empty" regions. In this case, this mesh was filled with stationary air to interact with the fuel. 




(a) SPH mesh (b) ALE mesh 

Figure 5-53. SPH and ALE fuel models in the small wing segment. 



138 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



The wing segment trajectory was that of a normal impact at 500 mph at mid-height between spandrels. 
The wing was oriented with no pitch, yaw, or roll. Therefore, the leading edge impacted the panels with 
the sweep angle of the wing relative to the fuselage. The two exterior panels were constrained rigidly at 
the butt plates and at the floor slab locations. Refer to Chapter 5 of NIST NCSTAR 1-2B for the fuel 
modeling parameters used in the ALE and SPH analyses. 



Rigid constraint at butt plates 



Exterior panel 
of WTC tower 



ALE air region 




Wing Segment with 
~ 850 gallons fuel 



Rigid constraint at 
floor slab locations 



Figure 5-54. Wing segment, fuel, and exterior panel configuration. 

Results of the impact analysis of the wing section using the ALE and SPH approaches are shown in 
Figure 5-55 and Figure 5-56, respectively. In both cases, the columns of the exterior panels were 
completely destroyed due to impact. Close-ups of the damage to the exterior panels are shown in 
Figure 5-57. Figure 5-58 and Figure 5-59 show close-up comparisons of the fuel dispersion and wing 
break up predicted by the two fuel modeling approaches. While both modeling approaches gave 
comparable results for the damage to the exterior wall panels, the SPH modeling method predicted greater 
fuel dispersion and wing break up than when using ALE, as can be shown clearly in the side views 
(Figure 5-59). Without experimental data, it is difficult to evaluate which method provides a more 
accurate solution. 

Run-times from these component analyses clearly indicated that the SPH method was more practical for 
the global impact analyses. The SPH model ran about 10 times faster than the ALE method, as it required 
a smaller mesh and did not need to rezone after each time step, as was done in the ALE method. In 



NIST NCSTAR 1-2, WTC Investigation 



139 



Chapter 5 

addition, the ALE method required a mesh for both the fuel region and the air zone into which the fuel 
could flow. Therefore, the SPH method was selected as the modeling technique for the global analyses. 




t = 0.0 s 



t = 0.01s 



'Hhk > D.DItt 




xi. 




t = 0.02 s 



t = 0.03 s 




Y^.a 




t = 0.04 s 



Figure 5-55. Impact response of a wing section laden with fuel modeled using 

ALE approach. 



140 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 





t = 0.0 s 



t = 0.01s 



itocT Minn 




t = 0.02 s 




t^Ji 



a > 




t = 0.04 s 



Figure 5-56. Impact response of a wing section laden with fuel modeled using 

SPH approach. 



NISTNCSTAR 1-2, WTC Investigation 



141 



Chapter 5 





(a) With SPH approach (b) With ALE appraoch 

Figure 5-57. Exterior panels after impact with a wing segment with fuel. 



142 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



Time = 0.039999 










= 



40 • 







(b) ALE analysis 
Figure 5-58. Top view of structural damage and fuel dispersion at 0.04 s. 



NISTNCSTAR 1-2, WTC Investigation 



143 



Chapter 5 



rime = 0.039999 



a 



Time = 0.039999 




(a) SPH analysis 



^ 




(b) ALE analysis 
Figure 5-59. Side view of structural damage and fuel dispersion at 0.04 s. 



144 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



5.4.5 



Engine Impacts Subassembly Analyses 



This subassembly model was developed using structural components from the impact zone on the north 
face of WTC 1. The model, shown in Figure 5-60, was used to evaluate the response of structural 
connections, material and failure models, and other issues affecting the global impact analyses. The 
model was three floors tall, spanning floors 95-97, three exterior panels wide, and extended from the 
exterior wall through to the first two rows of core columns. The exterior wall in the subassembly model 
included the exterior panels that extend into floors 95-97, as well as two panels above and below the 
panel, spanning all three floors. The structural components in the final subassembly model included the 
exterior panels, core framing, truss floor structures, and interior contents (workstations). 

The vertical displacements were constrained at the top and bottom of the free ends of the core columns. 
For the exterior columns, a bolted connection was added to an adjacent butt plate for which the vertical 
motions were constrained. The lateral displacements were constrained at the free spandrel edges and at 
the sides of the truss floor structures. 

Ext. Panel Numbers 



r 



118 



Core Column Numbers 

>. 



Engine Impact Locatiorn — 
Center of panel 121 at 96'" floor 




Figure 5-60. Tower subassembly model. 

In this impact simulation, the engine had an initial speed of 413 mph and a trajectory with a lateral 
approach angle of 4 degrees from the exterior panel normal and vertical approach angle of 7.6 degrees 



NISTNCSTAR 1-2, WTC Investigation 



145 



Chapter 5 



below the horizontal. The impact point was centered approximately 6 ft below the 97th floor so that the 
initial impact did engage a significant portion of the truss floor structures. The calculated impact 
response of the subassembly is shown in Figure 5-61. The engine penetrated the exterior wall and 
continued into the interior of the building along the initial downward trajectory. As the engine continued 
into the subassembly model, it plowed through the interior building contents (workstations) and 
eventually skipped off of the truss floor slab at floor 96. 




Figure 5-61. Response of the subassembly model to engine impact. 

A side view of the impact behavior at different time instants during the response is shown in Figure 5-62. 
The engine penetrated the exterior wall following the initial downward trajectory. As the engine 
continued downward, it impacted the workstations and the truss floor structures of floor 96. The engine 
motion was redirected by the impact with the truss floor and continued its motion toward the core 
penetrating additional workstations. At a time of 0.25 s, the engine entered the core as shown in 
Figure 5-62(c). The impact conditions of this analysis resulted in a collision of the engine with core 
column 503. The speed-time history of the engine core in this impact analysis is shown in Figure 5-63. 
The deceleration that occurred in the first 5 ms was primarily from the penetration of the exterior wall and 
the floor slab and truss of floor 97. 



146 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 




I ■■' -I '■' iZ iZ '■' 1 ■■■' ii 11 ''■' J- '■■^ ii ll ^' -1 •"' A--' 



H 




(a) Time = 0.00 s 



:m 






■7r^~''V"7VTT"7rrTTT'V^''VlTr 




(b) Time = 0.05 s 




(c) Time = 0.25 s 
Figure 5-62. Subassembly-engine impact and breakup response (side view). 



NISTNCSTAR 1-2, WTC Investigation 



147 



Chapter 5 



500 



400 - 



Q. 

E 

— ' 300 

£r 
u 
o 

I 

* 200 
lU 



1 1 


1 1 1 1 1 1 1 


1 1 1 1 1 1 1 1 1 1 1 1 1 1 


1 2 


Z. Exterior Wall 


Impact 




_ 


'Y' 






-j 


\ V 




Core Column Impact 


-_ 


— 




\ 





-_ 




V 


\ 


" 1 t r 1 1 t 


1 1 1 1 r 1 1 


1 r 1 1 1 1 1 1 1 r 1 1 r r 


' 



100 - 





0.00 0.05 0.10 0.15 0.20 0.25 0.30 

Time (s) 
Figure 5-63. Speed history for the engine subassembly impact analysis. 

This subassembly model was used to investigate the effect of a number of modeling parameters on the 
response and damage estimates. These parameters included the strength of the building nonstructural 
contents and the concrete slab strength. The reader is referred to Chapter 6 of NIST NCSTAR 1-2B for 
further details. The same configuration was also used to study the sensitivity of the response to 
1 1 parameters as part of the uncertainty analysis, see Chapter 8 of NIST NCSTAR 1-2B. 



5.5 



SUMMARY 



The towers were modeled primarily with shell elements with the exception of the exterior wall bolted 
connections (beam and brick elements) and the floor truss diagonals (beam elements). The WTC 1 model 
extended between floors 92 and 100, while the WTC 2 model extended between floors 77 and 85. The 
global impact models of the WTC towers included the following components: 

• Exterior walls: The exterior columns and spandrels were modeled using shell elements with 
two mesh densities, a refined density in the immediate impact zone and a coarser far field 
density elsewhere. For the bolted connections between exterior panels in the refined mesh 
areas, brick elements were used to model the butt plates, and beam elements were used for 
the bolts. 

• Core columns and floors: Core columns were modeled using shell elements with two mesh 
densities, a refined density in the direct impact area and a coarser far field density elsewhere. 
The spliced column connections were included in the model with proper failure criteria. The 



148 



NIST NCSTAR 1-2, WTC Investigation 



Development of Tower and Aircraft Impact Models 



floors within the core were modeled using shell elements representing the floor slabs and 
beams. 

• Truss floor: In the direct impact area, the floor model included shell elements for the 
combined floor slab and metal decking, and for the upper and lower chords of the trusses. 
Beam elements were used for the truss diagonals. In the far field floor segments, simphfied 
shell element representations were used for the floor slab and trusses. 

• Interior building contents: The interior nonstructural contents of the towers were modeled 
explicitly. These included the partitions and workstations, which were modeled with shell 
elements in the path of the aircraft debris. The live load mass was distributed between the 
partitions and cubicle workstations. 

The Boeing 767-200ER aircraft model was developed based on (1) documentary aircraft structural 
information, and (2) data from measurements on Boeing 767 aircraft. The airframe model contained most 
of the significant structural components in the aircraft. The models of the fuselage, empennage, and wing 
structures were developed completely using shell elements. Models for the landing gear and engines were 
primarily developed using shell elements, but contained some brick elements as well. The typical 
element dimensions were between 1 in. and 2 in. for small components, such as spar or rib flanges, and 
3 in. to 4 in. for large parts such as the wing or fuselage skin. 

Special emphasis was placed on modeling the aircraft engines due to their potential to produce significant 
damage to the tower components. The engine model was developed primarily with shell elements. The 
objective was to develop a mesh with typical element dimensions between 1 in. and 2 in. However, 
smaller element dimensions were required at many locations to capture details of the engine geometry. 
Brick elements were used for some of the thicker hubs and the roots of the compressor blades. 

In support of the development of the global models of the towers and aircraft, a large array of component 
and subassembly models were developed and used in the impact simulations. Examples of such analyses 
included: 

Impact of a segment of an empty aircraft wing with an exterior column. 

Detailed and simplified modeling of exterior panel bolted connection under impact loading. 

Impact of a simplified plow type impactor with truss floor assembly. 

Impact of fuel-filled wing segment with exterior wall panels. 

Impact of an aircraft engine with a subassembly from the exterior wall though the core of the 
towers. 

These component and subassembly analyses provided guidance on the optimal element size and type for 
global modeling, material constitutive modeling, treatment of connections, and modeling of aircraft fuel. 
They were also used for the sensitivity analyses conducted to assess the effects of uncertainties associated 
with various parameters on the level of damage to the towers and to determine the most influential 
modeling parameters that affect the damage estimates. 



NIST NCSTAR 1 -2, WTC Investigation 1 49 



Chapter 5 

5.6 REFERENCES 

"TrueGrid Manual, Version 2.1.0," XYZ Scientific Applications, Inc., September 2001. 

"LS-DYNA Keyword User's Manual," Livermore Software Technology Corporation, Version 970, 
April 2003. 

"LS-DYNA Theoretical Manual," Livermore Software Technology Corporation, May 1998. 

ASTM Designation A 370 - 03a, "Standard Test Methods and Definitions for Mechanical Testing of Steel 
Products," Approved June 10, 2003. 

Bischoff, P.H., Perry, S.H., "Compressive Behavior of Concrete at High Strain Rates," Materials and 
Structures, vol. 24, 1991, pp. 425-450. 

Boeing Company, Aircraft Description of Boeing 767, Section 2, Document number D6-58328, 
www.boeing.com, 1989. 

Brown, W. F. Jr., Mindlin, H., and Ho, C.Y., (Eds.), 1991, Aerospace Structural Metals Handbook, 
CINDAS/Purdue University Publishers, Volumes 3 & 4. 

Culver, C.G., 1976, Survey Results for Fire Loads and Live Loads in Office Buildings, NBS Building 
Science Series, No. 85, National Bureau of Standards, Washington, DC. 

FEMA 304, May 2002, World Trade Center Building Performance Study: Data Collection, Preliminary 
Observations, and Recommendations, Federal Emergency Management Agency. 

Military Handbook, 1987, Metallic Materials and Elements for Aerospace Vehicle Structures, U.S. Dept. 
of Defense, MIL-HDBK-5F. 

Ross, C.A., Kuennen, S.T., Tedesco, J.W., "Effects of Strain Rate on Concrete Strength," Session on 
Concrete Research in the Federal Government, ACI Spring Convention, Washington, D.C., 
March 1992. 

Wischers, G., "Application of Effects of Compressive Loads on Concrete," Betontech, Berlin, Nos. 2 and 
3, 1978. 



1 50 NIST NCSTAR 1 -2, WTC Investigation 



Chapter 6 

Aircraft Impact Initial Conditions 



6.1 introduction 

In order to determine the impact loading on the World Trade Center (WTC) towers, the aircraft impact 
initial conditions needed to be estimated. These initial conditions included the aircraft speed, aircraft 
orientation and trajectory, and location of aircraft nose at impact. The estimates also included the 
uncertainties associated with these parameters. This chapter describes the estimation of the initial impact 
conditions of the aircraft which impacted the WTC towers from available records. These records 
included the videos that captured the two impact events and photographs of the damage to the exterior of 
both towers. 

Two videos captured the approach and impact of the American Airlines Flight 1 1 (AA 1 1) aircraft that 
impacted the WTC 1 tower, and several videos captured the United Airlines Flight 175 (UAL 175) 
aircraft that impacted the WTC 2 tower. In addition, a large body of photographic evidence was available 
that could be used to determine the impact location and orientation relative to the towers. These videos 
and photographs were analyzed to estimate, with the best accuracy possible, the impact speed, horizontal 
and vertical angles of incidence, and roll angle of each aircraft during impact with each tower, as well as 
the location of impact. 

The analysis of the initial aircraft impact conditions was performed in two steps. The first step was to 
perform an analysis of the video footage of the two impact events. This analysis compared the various 
videos and used visual references and known dimensions and positions of towers to determine the flight 
conditions prior to impact (Section 6.2). The second step was to use photographs of the impact damage to 
refine the details of the impact position, orientation, and trajectory (Section 6.3). 

The impact orientation and trajectory parameters are defined in Figure 6-1. In this figure, two vectors 
were defined, one for the velocity vector of the aircraft (the trajectory) and one for the orientation of the 
aircraft. These two vectors may not be coincident. Both vectors were described in terms of a vertical 
angle around structure east, as shown in the figure, and a lateral angle, which was measured clockwise 
around the tower axis from structure north. The orientation was also described in terms of a wing-tip roll 
angle, as shown in the figure. 

The resolution of the video footage was not sufficient to measure the wing defiections or impact points 
more accurately than within ± 6 ft. In the two videos that captured the WTC 1 impact, there also was not 
enough resolution to obtain an accurate orientation of the aircraft. Consequently, the impact point and 
roll angle of AA 1 1 were determined using only the still-frame photographs of the impact damage to the 
north wall of WTC 1. Since the UAL 175 impact was captured by several videos, the trajectory and 
orientation measurements could be made from the available video footage. Similar to WTC 1 , the impact 
location was primarily determined from the still frame photography of the damaged WTC 2 south wall. 
The following sections describe the analysis methodologies used to determine the motion parameters and 
impact conditions. 



NIST NCSTAR 1 -2, WTC Investigation 1 5 1 



Chapter 6 




Vertical 

Orientation 

Angle 



Structure 
East 



Lateral Approach 
Angle 



Orientation 
Vector 



Velocity 
Vector 




Figure 6-1. Definition of the aircraft impact parameters. 

Section 6.2 of this chapter provides the details of the motion analysis methodology based on video 
footage of the two impact events. This included a complex and a simplified motion analysis. Section 6.3 
presents the procedure used to refine the initial impact conditions obtained from the motion analysis 
based on the damage to exterior walls documented in photographs. Section 6.4 is a comparison between 
the impact conditions estimated from this study with those reported or estimated previously. Section 6.5 
is a summary of the chapter. 



6.2 



MOTION ANALYSIS METHODOLOGY 



6.2.1 



Videos Used in the Analysis 



The first task in the analysis of the aircraft impact conditions was to review and select appropriate videos 
and photographs that could be used for the estimation of the impact initial conditions. An extensive 
library of video and photographic evidence of the WTC tower impacts was collected by National Institute 
of Standards and Technology (NIST) (see NIST NCSTAR 1-5A). The available videos were reviewed to 
select the best video footage of the aircraft's approach and impact with each tower. The videos were 
digitized and stored in AVI files. The WTC 1 aircraft impact was captured in two videos, and both were 
used in the analysis. Several videos captured the aircraft impact into WTC 2 tower, and seven of them 
were selected for the analysis. The image coordinates of the aircraft nose, tail, wing tips, aileron, and 
several locations on the towers were measured in each frame of the videos. Adobe Photoshop was used to 
determine the image coordinates. Table 6-1 provides a summary of the videos used to analyze the impact 
initial conditions. Still images from each of these video records are provided in Appendix E. 



152 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Initial Conditions 



Table 6-1. Videos used for the analysis of aircraft impact initial conditions. 



Digitized Video File 


Original 

Video 

Format 


Tower 
Impact 


Description 


VI 


NTSC 


WTC 1 


Footage taken at ground level at the comer of Church and 
Lispenard streets. Taken north and east of the towers. 


V2 


PAL 


WTC 1 


Footage taken from the entrance of the Brooklyn Battery 
Tunnel, heading west. Taken south and east of the towers. 


V3 


NTSC 


WTC 2 


Footage taken from a helicopter north and west of the towers. 


V4 


NTSC 


WTC 2 


Footage taken at ground level near the Castle Clinton National 
Monument. Footage taken south and east of the towers. 


V5 


NTSC 


WTC 2 


Footage taken from Brooklyn, south and east of the towers. 


V6 


NTSC 


WTC 2 


Footage taken from the 13th floor of a building in John Street, 
east of the towers. 


V7 


NTSC 


WTC 2 


Footage taken at ground level from the comer of Church and 
Liberty. Taken south and east of the towers. 


V8 


NTSC 


WTC 2 


Footage taken from a helicopter north of the towers. 


V9 


NTSC 


WTC 2 


Footage taken from a moving vehicle on FDR drive, heading 
west just before the Brooklyn Bridge. Footage taken north 
and east of the towers. 



The second column in Table 6-1 lists the original format of the various videos that were analyzed. All 
the videos with the exception of one used the National Television System Committee (NTSC) video 
format, which is the standard television format in the United States. The V2 video used the Phase 
Alternating Line (PAL) video format, which is common in Europe and parts of Asia. Any image data 
from the interlaced field of the videos were neglected. It was also assumed that the digitized NTSC 
videos had a rate of 29.97 images per second, while the PAL videos had a rate of 25 images per second. 
The digitized images had sizes of 720x480 pixels (NTSC) and 720x576 pixels (PAL). The original video 
footage was assumed to have an aspect ratio of 1.33/1, so the X-values of the measured image coordinates 
were adjusted to account for the actual aspect ratio. The image coordinates were also shifted relative to 
the locations of fixed points in the field of view (corners of a tower) to eliminate the effects of movement 
and shaking of the camera. 



6.2.2 



Complex Motion Analysis 



A complex motion analysis was the method originally used in this study to calculate the speed and the 
orientation and trajectory vectors of the aircraft. However, subsequent analysis methodologies, as 
discussed in the following sections, provided more accurate estimates of speed and orientation. The 
quahty and limited video footage available produced larger uncertainty using the complex motion 
analysis methodology. Therefore, this analysis methodology was only used to define the aircraft 
trajectory. Following is a discussion on the complex motion analysis and an assessment of its accuracy. 

The methodology used in this analysis to determine the aircraft impact conditions was previously 
developed for other applications (Cilke 1995). Figure 6-2 describes the analysis procedure. The image 
coordinates of the moving object (the aircraft) and two stationary positions on the structures within the 
field-of-view were triangulated with the known real -world positions of the structures and camera. The 



NISTNCSTAR 1-2, WTC Investigation 



153 



Chapter 6 



camera was assumed to be a pin-hole type camera, i.e., all the light rays pass through a single focal point 
and project onto a flat surface that records the image. The result was the definition of the vector 
extending from the camera to the aircraft. However, the position of the aircraft along the vector was still 
unknown. The vector was then intersected with a surface defined by a set of vectors extending from a 
second camera to the measured object in multiple frames. The result was the real-world position of the 
aircraft at one instance in time. The global positions of other points on the aircraft and positions of the 
aircraft in multiple frames were then used to define the orientation and trajectory of the aircraft. Note that 
in ideal test conditions, where the video cameras and reference positions are precisely surveyed and the 
camera field-of-views are designed, the uncertainties in the measured object velocities range from 
1 percent to 1.5 percent. 



Step 1 : 




Location of the object 

along the vector not 

known. 



Step 2: 



(Xn , Yn , Zii) 




Intersect vector 

Camera from one camera 

No. 1 with data from a 

second camera. 



Camera 
No. 2 



Figure 6-2. Complex motion analysis to measure object motions using multiple 

cameras. 



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NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Initial Conditions 



For the WTC aircraft motion analysis, various locations on the two towers were used as fixed reference 
locations. The four corners of the towers at three floor levels were used, as they could be easily identified 
in the video footage. These three levels included the top floor and two mechanical floors. Additional 
points on the WTC 1 antenna were also used as reference locations. The tower reference positions were 
at the center of each beveled corner. The coordinates of the reference locations were determined by using 
the original construction drawings of the towers. While the locations on the structure could be 
determined with high fidelity, the coordinates of the cameras had to be estimated through an iterative 
process. 

With the camera locations estimated, motion analyses were performed using the complex motion analysis 
technique. For the WTC 1 aircraft impact, image data from the VI video were correlated with the data 
from the V2 video. The analysis produced a speed of 435 mph ± 30 mph for the WTC 1 aircraft at time 
of impact. 

For the WTC 2 aircraft impact, data from each of the V4, V5, and V9 videos were correlated with the 
other two cameras to determine the motion of the aircraft prior to impact. The other cameras were less 
effective with this analysis technique (see Chapter 7 of NIST NCSTAR 1-2B for further details). Image 
data from the three videos converged to a tight set of trajectory angles and aircraft orientations (see 
Table 6-3). The uncertainties in the measured angles were derived from three components. First, there 
was a significant amount of scatter in the measured image coordinates. The perceived motion and 
orientation of the aircraft varied between frames, due to the relatively low resolution of the images and 
the motion of the camera fields-of-view. The scatter in the image data contributed to approximately 
±2 degrees to ±4 degrees of the image uncertainty. The tips of the aircraft wings were more difficult to 
define accurately; the scatter in the wing measurements led to an estimated uncertainty of ±4 degrees in 
the roll angle. Second, the uncertainties in the camera locations contributed to the uncertainties in 
measured angles. Since there was more uncertainty in the cameras' horizontal positions than the vertical 
positions, the measured horizontal azimuths had larger uncertainties (±2 degrees). Third, the potential 
distortion in the field of view would distort the measured angles. The uncertainty of the measured angles 
due to image distortion was estimated to be ±1 degree. 

The initial analyses using the complex motion methodology indicated the UAL 175 aircraft impact speed 
to be about 497 mph, which was heavily based on the V4 footage. However, subsequent analyses showed 
that the cameras did not provide an accurate aircraft impact speed due to three possible causes. First, the 
range of the camera could only be estimated. If the camera was close to the object motion, the range of 
the camera would have a significant effect on the perceived scale of the object in motion. Second, the 
scale of the image was determined from the dimensions of the towers in the field-of-view, which took up 
a relatively small portion of the field of view. As a result, the uncertainties in the measured image 
distances increased. Third, and most important, there were measurable distortions in the camera fields of 
view. For example, in the V5 video, the camera pans from left to right, tracking the aircraft as it 
approaches the south tower. The tower initially appears from the right edge of the image and moves to 
the center. The length of the aircraft (which remained in the center of the field-of view) appeared to 
decrease by 1.5 percent. However, the width of the south tower's east edge appeared to decrease by 
7 percent, indicating a significant distortion in the field of view. As a result, a simplified motion analysis 
procedure was used to determine the speed of the WTC 2 aircraft as described in the next section. The 
complex motion analysis was used only to define the aircraft orientation and trajectory vectors. 



NIST NCSTAR 1 -2, WTC Investigation 1 55 



Chapter 6 



6.2.3 



Simplified Motion Analysis 



This procedure determined the impact speed by scaling the displacement of the aircraft within the field of 
view with the apparent fuselage length of the aircraft. Figure 6-3 depicts the simplified procedure to 
determine the aircraft speed. For several videos of the WTC tower impacts, linear regressions were 
performed for the image coordinates as functions of time. The displacements of the nose, tail, and wing 
tips were measured. The apparent length of the fuselage within each image was determined from the nose 
and tail regression lines, and the apparent displacement of the aircraft between images was normalized to 
the apparent length of the fuselage. Multiplying the result by the length of the aircraft determined the 
aircraft speed (there are constant time steps between frames). Finally, a geometric correction was made if 
the fuselage orientation and trajectory were not aligned. 

The length of the fuselage of the Boeing 767-200ER aircraft is 155 ft (see Chapter 7 of NIST 
NCSTAR 1-2B). However, for the simplified motion analysis, the fuselage was assumed to have an 
apparent length of 153 ± 2 ft. The adjustment in apparent fuselage length was a result of the relatively 
low resolution of the video footage. As a sharp object entered a region captured by a single pixel, the 
background dominated the pixel color value until the object entered by a significant fraction. The low 
resolution could not accurately capture the shape of the aircraft nose and tail, and the aircraft nose in the 
videos appeared to be blunter than the actual nose of the Boeing 767-200ER. The average length of the 
fuselage in the videos analyzed was approximately 75 pixels (but varied depending on the footage). It 
was assumed that the resolution effect resulted in an apparent loss of approximately a half pixel at each 
end of the fuselage (one ft at each end of the fiaselage). As a result, the apparent length of the fuselage in 
the video footage was approximately 2 ft less than the actual length. 






Image 1 



Image 2 





Speed = 



(d34) 



(L,+L,)I2 



Image 4 
(Actual plane length)(lmage Rate) 



Figure 6-3. Simplified motion analysis procedure to determine aircraft speed. 

The simplified motion analysis technique was used for the analysis of aircraft speed for both tower 
impacts. For the WTC 1 impact, only the VI video could be used to determine the aircraft speed with this 
technique. The second video, V2, could not be used to obtain an accurate measure of speed as the aircraft 



156 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Initial Conditions 



was traveling away from the camera. The simplified analysis produced a speed of 451 + 30 mph, which 
was 16 mph higher than the value obtained from the complex motion analysis technique. Both of these 
values for the WTC 1 impact speed were within the uncertainties in the corresponding analyses. As a 
result, the WTC 1 aircraft impact speed provided in Table 6-3 was selected as the average of the two 
speeds obtained using the complex and simplified motion analysis methodologies. 

Five videos with a viewing angle approximately perpendicular to the UAL 175 flight direction were used 
to estimate the aircraft speed at the time of impact. The results of the simplified motion analyses from 
each camera for UAL 175 are provided in Table 6-2. The uncertainties in the table were based on the 
scatter in the measured displacements, the aircraft length within the image, and uncertainty in the actual 
aircraft length as seen in the images due to unknown orientation. A systematic error in calculating the 
aircraft speed was introduced due to the lateral fuselage orientation relative to trajectory. The uncertainty 
in this value was due to the aircraft maneuvers during its approach. In calculating the uncertainty in the 
speed, an uncertainty of ±3 degrees in orientation was assumed. 



A speed estimate was then calculated from the individual videos. A mean value was calculated using the 
weighted average of the mean values. The measurement precision (the reciprocal of the variance) was 
used as a weight factor on the mean values. If measurements were independent, the uncertainty in the 
mean could be calculated by summing the individual measurement precisions, giving 443 ±21 mph for 
AA 1 1 and 542 ±14 mph for UAL 175. However, some uncertainties were systematic and the actual 
bound on the uncertainty was larger as a result. Therefore, the uncertainty range was increased to ± 30 
mph and ± 24 mph for AA 1 1 and UAL 175, respectively. A summary of the impact conditions derived 
from video analysis (both complex and simplified motion analyses) is presented in Table 6-3. 

Table 6-2. Measured UAL 175 impact speeds using the 
simplified analysis methodology. 



Video Reference 


Calculated Aircraft Speed 


V4 


573 ± 55 mph 


V5 


556 ± 27 mph 


V6 


535 + 23 mph 


V7 


523 ±31 mph 


V9 


557 ± 53 mph 


Best Estimate Speed 


542 ± 24 mph 



NISTNCSTAR 1-2, WTC Investigation 



157 



Chapter 6 



rable 6-3. Summary of measured aircraft impact conditions from video analysis. 




AA 11 (WTC 1) 


UAL 175 (WTC 2) 


Impact Speed (mph) 


443 ± 30 


542 + 24 


Vertical Approach Angle 
(Velocity vector) 


10.6° ± 3° below horizontal 
(heading downward) 


8° ± 4° below horizontal 
(heading downward) 


Lateral Approach Angle 
(Velocity vector) 


180.3° ± 4° clockwise from 
Structure North 


19° ±6° clockwise from 
Structure North 


Vertical Fuselage Orientation 
from horizontal 


— 


3° ± 4° below horizontal 
(heading downward) 


Lateral Fuselage Orientation 
from Structure North 


— 


8° ± 6° clockwise from 
Structure North" 


Roll Angle (left wing downward) 


25° ±4° 


38° ±4° 



a. Structure North is approximately 29 degrees clockwise from True North. 

Initial results from the simplified motion analysis produced a mean speed for UAL 175 of 546 mph. This 
speed was therefore used in the global impact analysis, discussed in Chapter 7. Subsequent refinement of 
the analysis and associated uncertainties produced the slightly lower mean value of 542 mph as discussed 
above. Since this difference in speed was less than 1 percent and well within the uncertainty range, the 
speed used for the impact analysis was not modified. 



6.3 



REFINEMENT OF AIRCRAFT IMPACT CONDITIONS 



Estimates of the aircraft impact locations, orientations, and trajectories were further refined based on the 
damage patterns documented on the exterior walls of the WTC towers. The general approach was to 
visualize the aircraft within the range of flight conditions estimated from the video analysis (Section 6.2) 
and project the impact points of the wings, fuselage, engines, and vertical stabilizer onto the exterior wall 
of each tower. A damage pattern was then estimated and compared to that obtained previously from 
analysis of the film and photographic evidence. 

The estimated damage to the north face of WTC 1 is shown in Figure 6-4 along with approximate impact 
locations for various aircraft components of AA 1 1 , including the wind tips, vertical stabilizer tip, and 
engines. For AA 11 , it was found that the fuselage orientation needed to be 2 degrees above the vertical 
approach angle (2 degrees nose-up). The difference in the lateral approach angle and the fuselage 
orientation from structure north was 0°. 

An example impact condition is shown in Figure 6-5, where the vertical approach angle was 10.6 degrees 
(fuselage orientation from horizontal = 8.6 degrees) and the lateral approach angle was 180° (fuselage 
orientation from structure north =180 degrees). The position of the vertical stabihzer tip was the most 
critical factor in determining this relationship. The impact points of the wing tips were known to within 
approximately ±2 ft. This corresponded to an uncertainty in the roll angle of approximately ±2 degrees. 
Since no accurate orientation information could be derived from the video analysis, analysis of the 
damage pattern was critical in estimating the aircraft orientation at the time of impact. 



158 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Initial Conditions 



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(vertical approach angle = 10.6°, lateral approach angle = 0°). 



NISTNCSTAR 1-2, WTC Investigation 



159 



Chapter 6 



The estimated damage to the face of WTC 2 is shown in Figure 6-6, along with approximate impact 
locations for various aircraft components of UAL 175. From these impact locations, the combinations of 
flight conditions that were consistent with the observed impact damage could be estimated. 

Figure 6-7 shows the south face of WTC 2 with the aircraft model positioned in the impact orientation 
and location estimated from the video analysis (Table 6-3). The viewpoint of the figure was along the 
trajectory axis so that the projection of each aircraft component onto the tower face represented its 
approximate impact location, assuming no significant structural deformation prior to impact with the 
building exterior. During the impact simulation, little structural deformation was observed in parts of the 
aircraft that had not yet impacted the towers. The vertical stabilizer, the last part of the aircraft to enter 
the building and the part that had the longest time to experience structural deformation, impacted close to 
this projected impact location. 

The impact conditions shown in Figure 6-7, which were based on video analysis alone, would cause the 
starboard wing tip to miss the building and were, therefore, not physically possible. Also shown in the 
figure are the estimated impact locations for the wing tips, vertical stabilizer, and engines. These also did 
not align well with the observed impact damage. Translation of the aircraft alone did not account for the 
discrepancy in the impact point shown in the figure. Both a translation of 3.3 ft higher and 9.8 ft further 
west were needed, along with a specific relationship between the trajectory and orientation in order for 
the impact pattern to match. The final impact points, defined as the location where the nose of each 
aircraft initially contacted the towers, are provided in Table 6-5. 

It was found that a strict relationship between the aircraft trajectory and orientation needed to be 
established in order to achieve an impact pattern consistent with the damage observed on the south wall of 
WTC 2. The fuselage orientation needed to be Idegree above the vertical approach angle (i.e., 1 degrees 
nose-up). The difference in the lateral approach angle and the fuselage orientation from structure north 
was 3 degrees as listed in Table 6-3. An example impact condition for UAL 175 is shown in Figure 6-8, 
where the vertical approach angle was 6 degrees (fuselage orientation from horizontal= 5 degrees) and the 
lateral approach angle was 13 degrees (fuselage orientation from structure north= 10 degrees). Larger or 
smaller angles resulted in projected impact points with the engines spaced too far horizontally or 
vertically or with the tip of the vertical stabilizer in the wrong location. Also, note that the impact point 
of the nose had been moved from original estimates, as previously discussed, and that the roll angle was 
maintained. A second example of an acceptable impact condition, this time with a lateral approach angle 
of 17 degrees, is shown in Figure 6-9. 

The relationship between aircraft trajectory and orientation was then used to reduce the uncertainty of 
these parameters. The uncertainty in the vertical approach angle from the video analysis varied from 
4 degrees to 12 degrees, as shown in Table 6-3, and the fuselage orientation from horizontal varied from 
-1 degrees to 7 degrees. As a 1 degree difference needed to be maintained in order for the impact pattern 
to match the observed damage, uncertainty in the vertical approach angle was reduced to 6 degrees + 
2 degrees and the fuselage orientation from horizontal to 4 degrees ± 1 degrees. Uncertainty in the lateral 
approach angle and the fuselage orientation from structure north was similarly reduced, as shown in 
Table 6-4. The impact points of the wing tips were known to within approximately ± 2 ft. This 
corresponded to an uncertainty in the roll angle of approximately ±2 degrees. 



1 60 NIST NCSTAR 1 -2, WTC Investigation 



Aircraft Impact Initial Conditions 



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damaged face of WTC 2. 



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NISTNCSTAR 1-2, WTC Investigation 



161 



Chapter 6 



Table 6-4. Aircraft impact locations on the WTC towers . 





Horizontal 
Location 


Vertical 
Location 


AAll (WTC 1) 


2.0 ± 3 ft west of 
tower centerline 


1.6. ±4 ft above 
floor 96 


UAL 175 (WTC 2) 


23.1 ±3 ft east of 
tower centerline 


0.6. ± 4 ft above 
floor 81 



Table 6-5. Summary of refined aircraft impact conditions. 





AA 11 (WTC 1) 


UAL 175 (WTC 2) 


Impact Speed (mph) 


443 + 30 


542 + 24 


Vertical Approach Angle 
(Velocity vector) 


10.6° ± 3° below horizontal 
(heading downward) 


6° ± 2° below horizontal 
(heading downward) 


Lateral Approach Angle 
(Velocity vector) 


180.3° + 4° clockwise from 
Structure North 


15° ±2° clockwise from 
Structure North 


Vertical Fuselage Orientation 
Relative to Trajectory 


2° nose-up from the vertical 
approach angle 


1° nose-up from the vertical 
approach angle 


Lateral Fuselage Orientation 
Relative to Trajectory 


0° clockwise from lateral 
approach angle 


-3° clockwise from lateral 
approach angle 


Roll Angle (left wing downward) 


25°+ 2° 


38° ± 2° 



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(vertical approach angle = 6°, lateral approach angle = 13°). 



162 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Initial Conditions 



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(vertical approach angle = 6°, lateral approach angle = 17°). 

Although the lateral approach angle of UAL 175 had a nominal value of 15 degrees, additional observable 
information was used to define the most probable fiight condition. Figure 6-10 shows the top view of 
WTC 2 with the engines and landing gears in their pre-impact location. Also shown is the projected 
trajectory of the starboard engine of UAL 175, with an initial lateral approach trajectory of 13 degrees 
instead of 15 degrees, assuming the engine was not significantly deflected as it passed through the 
building. With this lateral trajectory, the starboard engine would exit the tower at the north east corner, 
consistent with the observables from video and photographic evidence. As a result, a lateral approach 
trajectory of 13 degrees was used for all WTC 2 impact simulations. 

It is possible that the tower structure and/or contents could have deflected the engine from its initial 
lateral trajectory. The global simulations used a standard configuration for building contents similar to 
WTC 1. This conflguration did not cause substantial deviation in the trajectory of the starboard engine. 
This lateral trajectory was, therefore, the most likely and was adopted for the global analyses. 



NISTNCSTAR 1-2, WTC Investigation 



163 



Chapter 6 



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approach angle of 13"". 



6.4 COMPARISON WITH PREVIOUS ESTIMATES OF AIRCRAFT IMPACT 

INITIAL CONDITIONS 

Alternate analyses and values of the aircraft impact initial conditions were performed and reported by 
other studies. The objective of this chapter was to provide an independent assessment using the full 
database of video and photographic evidence collected and maintained by NIST. Many of these data 
sources may not have been available in the previous analyses. In this section, a comparison is presented 
between the aircraft impact conditions estimated in this study and those reported earlier. This comparison 
provides an opportunity to review the methodologies applied, as well as assists in the determination of the 
uncertainties in the impact conditions. The comparison includes estimates or analyses performed by the 
Federal Emergency Management Agency (FEMA), Federal Bureau of Investigation (FBI) (reported in the 
New York Times), Hart-Weidlinger, and MIT, and the analyses presented in this chapter (NIST). The 
analysis methodologies and data sources used for the FEMA and FBI estimates of the impact speeds were 
not available. As a result, an evaluation of those estimates of impact conditions and determination of 
their uncertainties could not be made. In addition, preliminary estimates of the speed based on a 
simphfied analysis of a single video footage for each tower (VI video for WTC 1 and V6 video for 
WTC 2) were conducted by Project 5 (NIST NCSTAR 1-5A) and are included in this comparison. 

Table 6-6 compares the results of the motion analyses for the AA 1 1 impact. Both the Hart-Weidlinger 
and Massachusetts Institute of Technology (MIT) analyses utilized the Doppler shift of the engine noise 
to determine the aircraft speed. The Hart-Weidlinger velocity analysis was based on AA 1 1 approaching 
the north tower at an angle 4 degrees shallower than the analyses presented here (NIST analysis in 
Table 6-6). If the Hart-Weidlinger analysis had the aircraft approaching at a steeper angle, it would have 
reported a speed much closer to the MIT and NIST analyses. The difference between the speed estimated 
from this study and that from the simplified analysis (NIST NCSTAR 1-5A) was about 5 percent. 
However, both speeds were well within the uncertainty range. One significant difference in the analyses 



164 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Initial Conditions 



of the AA 1 1 impact conditions was that none of the previous analyses had the opportunity to utilize the 
V2 video. This second video from a different location was very helpful to determine the motion 
parameters of the AA 1 1 . 

Table 6-6. AA 11 (WTC 1) aircraft impact analysis comparison. 





FEMA" 


FBI'' 


Hart- 
Weidlinger" 


MIT"" 


NIST 
Simplified 
Analysis" 


NIST 


Best Estimate Speed (mph) 


470 


494 


500 


429 


466 


443 


Speed Error Estimate (mph) 






+ 30/ -50 


±51 


±34 


±30 


Lateral Approach Angle 
(clockwise) 






4.3° 






0.3° ±4° 


Vertical Approach Angle 
(downward) 






6.2° 






10.6° ±3° 


Aircraft Roll (left wing 
down) 






20.7° 






25°± 2° 



a. FEMA World Trade Center Building Performance Study, May 2002. Analysis methodology or data source not available. 

b. Lipton, E. and J. Glanz, 2002, "First Tower to Fall Was Hit at Higher Speed, Study Finds," The New York Times, 
February 23 

c. Levy, M. and Abboud N., 2002, "World Trade Center - Structural Engineering Investigation," Hart-Weidlinger. 

d. The Towers Lost and Beyond, Massachusetts Institute of Technology, Eduardo Kausel. 

e. NIST NCSTAR 1-5A. 

Table 6-7 compares the results of the various motion analyses for the UAL 175 impact. The Hart- 
Weidhnger and the analyses presented here were consistent with the exception of the lateral approach 
angle. The MIT estimates of impact speed were low compared to the other analyses. However, assuming 
a lateral approach angle of 20 degrees would have increased the MIT estimate of the UAL 175 impact 
speed to about 524 mph. The simplified analysis (NIST NCSTAR 1-5A) yielded a speed that was very 
close to that obtained in this study. 



Table 6-7. UAL 175 (WTC 2) aircraft impact analysis comparison. 






FEMA' 


FBI*' 


Hart- 
Weidlinger" 


MIT** 


NIST 
Simplified 
Analysis" 


NIST 


Best Estimate Speed (mph) 


590 


586 


550 


503 


545 


542 


Speed Error Estimate (mph) 








±38 


±18 


±24 


Lateral Approach Angle 
(clockwise) 






11.7° 


15° 




15° ±2° 


Vertical Approach Angle 
(downward) 






2.7° 


0° 




6° ±2° 


Aircraft Roll (left wing 
down) 






30.1° 






38°±2° 



a. FEMA World Trade Center Building Performance Study, May 2002. Analysis methodology or data source not available. 

b. Lipton, E. and J. Glanz, 2002, "First Tower to Fall Was Hit at Higher Speed, Study Finds," The New York Times, 
February 23 

c. Levy, M. and Abboud N., 2002, "World Trade Center - Structural Engineering Investigation," Hart-Weidlinger. 

d. The Towers Lost and Beyond, Massachusetts Institute of Technology, Eduardo Kausel. 

e. NIST NCSTAR 1-5A. 



NIST NCSTAR 1-2, WTC Investigation 



165 



Chapter 6 



6.5 SUMMARY 

Three methods were used to estimate the impact conditions for the two aircraft that impacted the 
WTC towers. The initial impact conditions included aircraft speed, horizontal and vertical angles of 
incidence, roll angle of each aircraft, and the location of nose impact with each tower. The estimates also 
included the uncertainties associated with these parameters. The first method used a comparison of 
videos from different positions to calculate the three-dimensional trajectory of the aircraft. The second 
method used the relative frame-by- frame motion in a single video, scaled to the length of the aircraft in 
the video to calculate the impact speed. Finally, analysis of the impact damage on the face of each tower 
was used to refine the relative impact orientation and trajectory. This was done by matching the projected 
impact points of the wings, fuselage, engines, and vertical stabilizer onto the exterior wall of each tower 
to the observed damage pattern. 

6.6 REFERENCES 

Cilke, R.W., (1995), "Motion Picture Analysis Procedures for Impact Parameters of Air-Delivered 
Weapons," FCT-LR-95-6, prepared by Applied Research Associates, Inc., Letter Report to the 
Defense Nuclear Agency, June. 

FEMA, May 2002, "World Trade Center Building Performance Study: Data Collection, Preliminary 
Observations and Recommendations," FEMA 403. 

Kausel, E., "The Towers Lost and Beyond," Massachusetts Institute of Technology, May, 2002, 
http://web.mit.edu/civenv/wtc/index.html. 

Levy, M. and Abboud N., August 1, 2002, "World Trade Center - Structural Engineering Investigation," 
Hart-Weidlinger Technical Report, Prepared for Wachtell, Lipton, Rosen, and Katz. 

Lipton, E. and J. Glanz, 2002, "First Tower to Fall Was Hit at Higher Speed, Study Finds," The New 
York Times, February 23. 



1 66 NIST NCSTAR 1 -2, WTC Investigation 



Chapter? 

Aircraft Impact Damage Results 



7.1 introduction 

This chapter presents the results of the analyses of the aircraft impacts into the two World Trade Center 
(WTC) towers. The analysis results include the estimation of the structural damage and the condition and 
position of nonstructural contents such as partitions, workstations, aircraft fuel, and other debris that 
influenced the behavior of the subsequent fires in the towers. These results were used to provide the 
initial conditions for the subsequent structural analyses (level of damage to columns and floor systems) 
and damage to fireproofmg due to debris impact. The global impact simulations provided, for each tower, 
a range of damage estimates. These included the base case, based on reasonable initial estimates of all 
input parameters, along with a less severe and a more severe damage scenario. The less severe damage 
case did not meet two key observables: (1) no aircraft debris was calculated to exit the side opposite to 
impact and most of the debris was stopped prior to reaching that side, in contradiction to what was 
observed in photographs and videos of the impact event (see Section 7.10), and (2) the fire-structural and 
collapse initiation analyses of the damaged towers (NIST NCSTAR 1-6) indicated that the towers would 
not have collapsed had the less severe damage results been used. As a result, this chapter provides 
detailed description of the results of the analyses pertaining to the base case and the more severe case, 
which were used as the initial conditions for the fire dynamics simulations (NIST NCSTAR 1-5F), 
thermal analyses (NIST NCSTAR 1-5G), and fire-structural response and collapse initiation analyses 
(NIST NCSTAR 1-6). Only a brief description is provided for the less severe damage results for 
comparison purposes. The details of the less severe damage estimates can be found in National Institute 
of Standards and Technology (NIST) NCSTAR 1-2B. 

Section 7.2 provides a description of the analysis methodology, including assumptions and limitations. 
Sections 7.3 and 7.4 provide detailed description of the impact analysis results for the base case and the 
more severe case, respectively for WTC 1. Sections 7.6 and 7.7 provide similar results for WTC 2. 
Sections 7.5 and 7.8 provide a brief description of the less severe case resuhs for WTC 1 and WTC 2, 
respectively. The last three sections present different comparisons. Section 7.9 presents a comparison of 
the impact response between WTC 1 and WTC 2. Section 7.10 compares the simulation results with 
observables based on video and photographic evidence as well as eyewitness interviews. Section 7.1 1 
presents a comparison between the damage estimates from this study with those from previous studies. A 
summary is provided in Section 7.12. 

7.2 ANALYSIS METHODOLOGY, ASSUMPTIONS, AND LIMITATIONS 

The impact analyses were performed using the LS-DYNA finite element code (LS-DYNA Version 971). 
LS-DYNA is a commercially available nonlinear explicit finite element code for the dynamic analysis of 
structures (LSTC 2003) and has been used for a wide variety of crash, blast, and impact applications. The 
impact analyses used a variety of capabilities and algorithms in LS-DYNA. A brief description of these 
capabilities is described in this section. A significantly detailed description of the analysis methods is 
provided in the LS-DYNA Theoretical Manual (1998). 



NIST NCSTAR 1 -2, WTC Investigation 1 67 



Chapter 7 



The impact simulations used a nonlinear transient analysis with an explicit dynamics solver. This solver 
allows for simulating softening and failure of components in the analysis. The analysis solved the 
dynamic system of equations with a very small time increment (AT a 0.8 |as), and with external loading 
defined as the initial conditions of the aircraft (velocity vector and location of the aircraft, see Chapter 6). 
Such analysis utilizes a number of capabilities that might not be customarily used in structural 
engineering applications. These include the following: 

• Element erosion: Damage and failure of components were included in the models through 
the constitutive algorithms. Damage criteria (such as maximum plastic strain) were tracked 
for each element within the constitutive model evaluation, and elements were eroded when 
the failure criteria were exceeded. This allowed for a direct evaluation of damage and failure 
within the impact simulations. The eroded elements allow for the initiation and extension of 
fracture in the model. Eroded elements no longer supported any stress, and the strains in the 
eroded elements were no longer calculated. The associated mass of the elements remained 
with the nodes in the calculation. If adjacent elements did not reach the failure surface, the 
nodes remained attached to the structure. If all of the elements connected to a specific node 
failed, the node became a free particle. Free nodes can either be eliminated from the 
calculation or remain in the calculation with associated inertial properties and potential for 
impacts against other structural components (free nodes remain in contact algorithms). 

• Contact behavior: A contact algorithm was used to detect contact between two bodies and to 
estimate the forces generated by this contact. Overall contact in the impact analyses was 
modeled using the automatic single surface contact algorithms in LS-DYNA. Interacting 
components were defined by a material list, and contact segments were automatically 
generated by LS-DYNA. This greatly simplified the specification of contact between various 
components in the aircraft and tower structures. The type 1 soft constraint option was used in 
the contact algorithm that determined the contact stiffness based on stability considerations, 
time step size, and nodal mass. This soft constraint option was found to be more robust than 
the default penalty formulation for modeling the complex contact behaviors in large impact 
and crash simulations. 

• Complex failure modes: In specific applications, unique algorithms were required to 
introduce failure modes in the analysis. These were primarily used in modeling the response 
and failure of connections (see Chapter 5 for description of some of these connections). An 
example is the splice between core columns, where the connection between the splice plate 
and column fiange was modeled with a surface-to-surface tied interface without failure. This 
resulted in a perfect bond between the nodes of the splice plate and the fiange of the adjacent 
column. When columns were pulled apart, the elements at the sphce plate spanning the gap 
between column ends would be stretched. Failure of the splice plate in the model resulted 
from ductile failure of the splice plate in the elements spanning the connection. 



• 



Fluid-structure interaction: This was needed to model the fuel impact on the exterior wall 
and the subsequent dispersion inside the towers. The Smooth Particle Hydrodynamics (SPH), 
which utilizes a mesh free approach, was used to model the fuel in the impacting aircraft. In 
this approach, fuel was modeled as particles that were allowed to interact with the structure of 
the aircraft and tower. 



1 68 NIST NCSTAR 1 -2, WTC Investigation 



Aircraft Impact Damage Results 



The finite element meshes used in the impact analyses typically used elements with single point 
integration. The biggest disadvantage of the single point integration is the potential for hourglassing or 
zero energy modes. There are several methodologies for controlling hourglass modes in LS-DYNA. The 
typical approach used in the impact analyses was to apply a viscous hourglass control where a viscous 
damping was introduced that suppressed the formation of hourglass modes, but did not significantly 
influence the global modes. 

As mentioned in Chapter 5, the global impact simulations were limited by the maximum finite element 
model size that could be executed on the available 32-bit computer clusters. The primary assumptions 
and limitations of the global impact analyses were the result of reducing the model size to meet this 
limitation, as well as to achieve a run time that allowed the global impact analyses to be completed within 
the duration of the investigation. 

Although the analyses were performed on a 32 bit computer cluster, the precision used in the analyses can 
be controlled by the analysis software. Both single precision and double precision versions of LS-DYNA 
were available for the impact analyses. In general, single precision analyses are more efficient and the 
precision is sufficient for the type of impact simulation being performed. However, when the dimensions 
of the structure being analyzed are sufficiently large, the single precision analyses can introduce rounding 
errors in the analyses. The rounding errors occur since the analysis is resolving deformations or 
analyzing element penetrations on a local scale that is several orders of magnitude smaller than the 
controlling dimension. 

In preliminary simulations, the coordinate system for the models of the tower structures was located near 
the base of the tower. As a result, the largest dimensions were the vertical position of the structures in the 
impact zone. This large vertical dimension controlled the size scale in the impact analyses and introduced 
rounding errors that were manifested as unstable element behaviors. To eliminate this precision problem, 
the tower model coordinate system was moved to be centered on the impact zone of the tower. The 
largest controlling dimension was therefore the distance across tower (significantly smaller than the 
height of the tower). After adjusting the coordinate location, the unstable element behaviors were no 
longer observed. 

To confirm the adequacy of the single precision analysis, subassembly impact analyses (Section 5.4.5) 
were performed on the same model in both single and double precision. The comparison of the two 
analyses showed no substantial difference in the impact response and damage. 

Specific assumptions and limitations introduced in the analyses to meet the computational and time 
constraints included: 

• Tower structures away from the impact zone had a coarse mesh resolution, and as a result, 
damage in these regions may not have been accurately captured. An example is the potential 
damage to the exterior wall on the far side of the tower (opposite to impact). As debris 
passed through the building and struck a panel on the far side, the coarse mesh and merged 
boundary conditions at column ends (as opposed to bolted connections in the impact zone) 
underestimated the secondary impact damage. 

• Tower contents (workstations and nonstructural walls) were only included in the expected 
path of the aircraft impact and subsequent debris cloud. Therefore, debris and fuel that 



NIST NCSTAR 1 -2, WTC Investigation 1 69 



Chapter 7 



• 



passed beyond this region could move more freely through the structure, only interacting with 
primary structural components. Also, the workstation layout from WTC 1 was used for 
WTC 2. That added an additional uncertainty to the nonstructural building contents for 
WTC 2. 

The analysis of the impact response of the aircraft fuel cloud had several limitations. Smooth 
Particle Hydrodynamics (SPH) was used to model the fuel in the impacting aircraft. The air 
in and around the towers was not modeled, so the deceleration of the fuel particles in the 
cloud by aerodynamic resistance was not included. The contact algorithm for the fuel 
particles and tower did not include a sticking or "wetting" behavior so the fuel particles 
would bounce off of components in the tower. The results of these limitations would spread 
the fuel cloud over a larger region in the simulation. Finally, the deflagration of the fuel was 
not modeled, and the resulting dynamic over-pressures in the tower from the combustion 
process were not included in the analysis. 

Windows were not modeled on the exterior of the tower. The open space between the 
exterior columns allowed ftiel particles and small debris fragments from the aircraft and 
tower to escape that may have been contained if the windows were included. Note, however, 
that the weight of the windows was added to the columns as part of the superimposed dead 
loads. 

The rotational velocity of the spinning aircraft engine components was not modeled. The 
effects of the rotational kinetic energy, spin stabilization of the engine trajectory, or potential 
for engine thrust during impact were, therefore, not included in the analysis. An analysis was 
performed to estimate the magnitude of the effects of this assumption (see Chapter 1 of 
NIST NCSTAR 1-2B). The analysis indicated that the rotational kinetic energy of each 
engine was approximately one percent of the aircraft initial kinetic energy. In addition, much 
of this rotational energy was probably dissipated by internal deformations of the engine 
components following impact with the tower exterior. Therefore, this approximation should 
have had a small influence on the global impact damage. 

Aeroelastic forces were not applied to the aircraft wings since the resulting stresses were not 
expected to affect the impact response. A wing tip deflection of 52 in. was applied to the 
aircraft model based on photographic evidence. 

Gravitational acceleration was modeled during the impact analyses to include the gravity 
effects on debris movement and potential contributions to the collapse of the damaged truss 
floor regions. However, initial service loads (stresses) in the tower and aircraft were not 
included. The material internal energy associated with the elastic service loads were small 
compared to the material internal energy capacity. Therefore, their execution was not 
expected to have a significant influence on the dynamic impact response and deformation. 
Simplified analyses were performed to evaluate the magnitude of the effects of this 
assumption for the impact response of a core column (see Chapter 10 of NIST 
NCSTAR 1-2B). These analyses indicated that ignoring the static preload in the column had 
little influence on either the dynamic column deformations or the reserve capacity of the 
column. 



1 70 NIST NCSTAR 1 -2, WTC Investigation 



• 



Aircraft Impact Damage Results 



• The impact analyses were subject to uncertainties in the input parameters, such as initial 
impact conditions, material properties and failure criteria, aircraft mass and stiffness 
properties, mass distribution inside the towers, the jet fuel distribution and dispersion, 
connections behavior, the presence of nonstructural building contents, etc. Sensitivity 
analyses were conducted as described in Chapter 8 of NIST NCSTAR 1-2B to assess the 
effects of these parameters on the damage estimates. The global analyses not only provided a 
"base case" based on reasonable initial estimates of all input parameters, but also provided a 
range of damage estimates based on variations of the most influential parameters, identified 
in the sensitivity analyses. 

7.3 WTC 1 BASE CASE IMPACT ANALYSIS - CASE A 

This case is referred to as Case A in the remainder of the WTC Investigation reports. This analysis 
provided an estimate of the impact damage based on reasonable initial estimates of all the variables 
obtained from photographic evidence, material testing, and data in the open literature. The combined 
aircraft and tower model used for the base case global impact conditions of WTC 1 is shown in 
Figure 7-1. 



NIST NCSTAR 1 -2, WTC Investigation 1 7 1 



Chapter 7 




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Figure 7-1. WTC 1 global impact model. 

The WTC 1 base case analysis was performed for a 0.715 s duration following initial impact of the 
aircraft nose with the north exterior wall. The analysis was performed on a computer cluster using twelve 
2.8 GHz Intel Xeon processors, each on a separate node of the cluster. The run time for this analysis was 
approximately two weeks. The progress of the global impact simulations was monitored on average 
every two days. The calculations were terminated when the damage to the towers reached a steady state 



172 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



and the motion of the debris was reduced to a level that was not expected to produce significant impact 
damage. The residual kinetic energy in the airframe components at the termination of the global impact 
simulation was less than one percent of the initial kinetic energy at impact. 

7.3.1 Impact Response 

The impact response of WTC 1 is shown in side and plan views in Figure 7-2 and Figure 7-3, 
respectively. The response is shown at intervals of 0.1 s from impact through the initial 0.5 s of the 
response. The initial 0.1 s of the response, shown in Figure 7-2(b) and Figure 7-3(b), was dominated by 
the impact, penetration, and fragmentation of the forward fuselage structures. The engines and wing 
sections were just starting to impact the exterior wall. The forward fuselage structures were severely 
damaged both from the penetration through the exterior columns and the interaction with the 96* floor 
slab that sliced the fuselage structures in half The downward trajectory of the aircraft structures caused 
the airframe to collapse against the floor, and the subsequent debris motion was redirected inward along a 
more horizontal trajectory parallel to the floor. 

By 0.2 s after impact, the wings completely penetrated the exterior wall, and only the tail structures were 
still outside the tower, as shown in Figure 7-2(c) and Figure 7-3(c). The wing structures were 
completely fragmented by the penetration through the exterior wall. The aircraft fuel cloud was starting 
to spread out but was still relatively dense, and the leading edge of the fuel was just reaching the tower 
core. The downward trajectory of the aircraft structures transferred sufficient vertical load that the truss 
floor structures on floors 95 and 96 were starting to collapse in the impact zone. 

At 0.3 s after impact, the aircraft was completely inside of the tower (full penetration completed at 
approximately 0.25 s), as shown in Figure 7-2(d) and Figure 7-3(d). The airframe was mostly broken up, 
but some large sections of the aft fuselage and tail were still intact, having penetrated through the opening 
in the north wall created by the forward fuselage structures. The aircraft fuel cloud penetrated 
approximately half the distance through the core and was spreading out. However, the subsequent motion 
of the aircraft fragments and fuel debris cloud began to be noticeably slowed beyond this time. The fuel 
and debris did continue to spread through the tower, but at a much slower rate, as seen in the remaining 
images in Figure 7-2 and Figure 7-3. 



NIST NCSTAR 1 -2, WTC Investigation 1 73 



Chapter 7 



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Figure 7-2. WTC 1 base case global impact analysis (side view). 



174 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



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Figure 7-2. WTC 1 base case global impact analysis (side view) (continued). 



NISTNCSTAR 1-2, WTC Investigation 



175 



Chapter 7 





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Figure 7-3. WTC 1 base case global impact analysis (plan view). 



176 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




(d) Time=0.30 s 




(e) Time=0.40 s 




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(f) Time=0.50 s 
Figure 7-3. WTC 1 base case global impact analysis (plan view) (continued). 



NISTNCSTAR 1-2, WTC Investigation 



111 



Chapter 7 



The load transfer of the aircraft impact can be described by the time-history of the aircraft momentum as 
shown in Figure 7-4. The momentum plotted was for all of the aircraft structures and contents (including 
fuel), normalized by the initial momentum magnitude. The curve illustrates an initial rate of load transfer 
during the first 0.1 s of impact as the forward fuselage penetrated the exterior wall and impacted the 
interior structures. Between 0.1 s and 0.25 s, a more rapid load transfer rate was observed as the area of 
the impact became larger (extending outward in the wing impact regions) and a higher percentage of the 
aircraft mass was impacting the interior structures. At 0.25 s, the aircraft completely penetrated the 
building and retained approximately 30 percent of its initial momentum. Beyond this time, the rate of 
load transfer was steadily decreasing with very little load transfer after approximately 0.5 s. 



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Figure 7-4. Normalized aircraft momentum for the WTC 1 base case impact. 

The aircraft was severely broken into thousands of debris fragments of various sizes and mass as a result 
of the impact with the tower. Larger fragments occurred for specific components such as the engines. At 
the end of the simulation, the port engine was still inside the core, and the starboard engine was roughly 
one third of the distance from the core to the south exterior wall. Each engine had a speed of less than 
50 mph. 



7.3.2 



Tower Structural Damage 



The structural damage to the WTC 1 tower by the base case impact conditions is described in this section. 
The primary structural components of interest were the exterior wall, core columns and core framing 
components, and the floor structures and concrete floor slab. Only limited results are presented in this 
chapter. Refer to Chapter 9 of NIST NCSTAR 1-2B for further details. 



178 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Exterior Wall Damage 

The exterior wall was the one structural system for which direct visual evidence of the impact damage 
was available. Therefore, the comparison of the calculated and observed exterior wall damage can 
provide a partial vahdation of the analysis methodologies used in the global impact analyses. A 
comparison of the north exterior wall observed and calculated damage from the base case WTC 1 global 
impact analysis is shown in Figure 7-5. The calculated impact damage to the exterior wall is shown with 
color fringes representing plastic strain magnitude, with undamaged sections in blue and strains at or 
above 5 percent shown in red. The schematic of observed damage was developed from inspections of the 
film and photographic data collected on the tower after impact. Both the observed and calculated damage 
wall regions illustrate a region of the exterior wall from column 108 to column 152, extending from floor 
91 to floor 100 (spandrels at floors 92 through 100). 

The comparison of the calculated and observed damage indicated that the geometry and location of the 
impact damage zone were in good agreement. This agreement in the position and shape of the impact 
damage served to validate the geometry of the aircraft model, including the aircraft orientation, trajectory, 
and flight distortions of the wings. 

The comparison also indicated good agreement in the magnitude and mode of impact damage on the 
exterior wall. The exterior wall completely failed in the regions of the fuselage, engine, and fuel- filled 
wing section impacts. Damage to the exterior wall was observed all the way out to the wing tips, but the 
exterior columns were not completely failed in the outer wing and vertical stabilizer impact regions. 
Failure of the exterior columns occurred both at the bolted connections between column ends and at 
various locations in the column depending on the local severity of the impact load and the proximity of 
the bolted connection to the impact. The agreement of both the mode and magnitude of the impact 
damage served to partially validate the constitutive and damage modeling of the aircraft and exterior wall 
of the tower. Section 7. 10. 1 provides a detailed comparison of the calculated and observed damage mode 
and magnitude. 

Core Structural Damage 

The estimation of the damage to the core columns and core beams was important in determining the 
residual strength for the subsequent analyses of structural stability and collapse. The core had significant 
damage in the region close to the impact point. The columns in line with the aircraft fuselage failed on 
the impact side, and several of the core beams were also severely damaged or failed in the impact zone. 

The calculated damage to the core columns by row is shown in Figure 7-6. The columns are shown with 
color fringes representing plastic strain magnitude, with undamaged sections in blue and strains at or 
above 5 percent shown in red. A summary of the column damage is listed in Table 7-1. The qualitative 
classification of the column damage levels is shown in Figure 7-7. This classification levels were light 
damage, moderate damage, heavy damage, and failed (severed). The light damage level was defined as 
having evidence of impact (low level plastic strains), but without significant structural deformations. The 
moderate damage level had visible local distortions of the column cross section (e.g. bending in a flange) 
but no lateral displacements of the column centerline. The heavy damage classification was for impacts 
that produced significant global deformation, resulting in a permanent deflection of the column centerline. 



NIST NCSTAR 1 -2, WTC Investigation 1 79 



Chapter 7 

The severed columns were completely failed and could carry no residual load. The damage to the core 
floor framing for floors 95 and 96 is shown in Figure 7-8. 

P-A effects generated due to the sway of the towers after impact, as observed in video evidence, were not 
expected to affect or impose additional damage to the core columns. The core columns were designed as 
axially loaded members without continuity of framing, and thus would not develop significant P-A 
moments (see Chapter 5 of NIST NCSTAR 1-2A). 



1 80 NIST NCSTAR 1 -2, WTC Investigation 



Aircraft Impact Damage Results 



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Figure 7-5. Base case impact damage to the WTC 1 exterior wall. 



NISTNCSTAR 1-2, WTC Investigation 



181 



Chapter 7 





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Figure 7-6. Base case impact damage to the WTC 1 core columns. 



182 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Table 7-1. Summary of core column damage for the base case WTC 1 impact. 


Column 


Location 


Damage Level 


Lateral Deflection of 
Column Centerline (in.) 


Column 503 


Floor 96 


Heavy 


18 


Column 504 


Floors 92-96 


Severed 




Column 505 


Floors 93-96 


Heavy 


20 


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10 


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Severed 




Column 605 


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Floor 96 


Moderate 




Column 703 


Floor 96 


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Column 704 


Floor 94 


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18 


Column 705 


Floor 95 


Moderate 




Column 706 


Floors 93-95 


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Column 802 


Floor 96 


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Figure 7-7. Classification of damage levels in core columns. 



NISTNCSTAR 1-2, WTC Investigation 



183 



Chapter 7 



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Figure 7-8. Base case impact damage to the core beams of floors 95 and 96 of WTC 1. 

Floor Truss and Slab Damage 

An overall frontal view of the floor trusses in the impact zone along with the calculated impact damage to 
the floor trusses is shown in Figure 7-9. The figure shows that the trusses experienced significant 
damage and sagging in the impact zone. A plan view of the calculated damage to the trusses on floors 95 
and 96 is shown in Figure 7-10. The calculated impact response produced severe damage to the truss 
structures in the primary impact path of the fuselage from the exterior wall to the core. The truss floor 
system on floors 94 through 96 were damaged and sagged downward as a result of the impact loading. 

The calculated damage to the WTC 1 floor slab for floors 95 and 96 are shown in Figure 7-11. The 
fringes of damage were set such that the concrete slab failed in the regions colored red (2 percent plastic 
strain was used, corresponding to the zero strength strain limit for the concrete in unconfmed 
compression). At these strain levels, the concrete slab was severely damaged and probably removed, 
exposing the supporting metal decking. Beyond 2 percent plastic strain, the strength of the floor slab was 
severely reduced in the analyses to model the residual strength of the metal deck after the concrete failure, 
breakup, and removal. At a plastic strain of 30 percent, corresponding to failure levels for the metal 
decking material, the elements were eroded (seen as holes ruptured in the floor slabs shown). 



184 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Column 

135 



Column 
151 



Column 
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Floor 96 



Floor 95 



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115 



Column 
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Column 

107 




(b) Calculated damage 
Figure 7-9. Base case impact damage to the WTC 1 floor trusses (front view). 



NISTNCSTAR 1-2, WTC Investigation 



185 



Chapter 7 



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(plan view). 



186 



A//ST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Impact 



Impact 



Column 
113 




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(b) Floor 96 slab damage 



Figure 7-11. Base Case impact damage to the slabs on floors 95 and 96 of WTC 1 

(plan view). 

Summary of Structural Damage 

The impact-induced structural damage described above was used as the initial conditions for the post- 
impact fire-structural analyses. Figure 7-12 shows a summary of the structural damage to the core 
columns and floor systems at floors 93 through 97 of WTC 1 for the base case (Case A). The damage to 
the columns at the various levels is identified by the color of the circles, where red, blue, green, and 
yellow signify severed, heavily damaged, moderately damaged, and lightly damaged columns, 
respectively. The dotted boxes on the figures indicate areas where the impact created an opening in the 
floor. These were used to identify slab openings in the fire dynamics simulations (NIST NCSTAR 1-5F). 
The solid boxes indicate areas in the fioor system that had severe structural damage. These areas were 
removed from the subsequent structural analyses (NIST NCSTAR 1-6). 

Figure 7-13 presents the cumulative damage to WTC 1 on all affected floors and columns. The figure 
shows the damage to the exterior walls due to impact based on the photographs of the north wall. Note 
the panel that was severed in the south wall of the tower. While the analysis did not capture the failure of 
the connections at the ends of this panel due to the coarse mesh of the south wall, photographic evidence 
showed that this panel was knocked down by the impact (see Section 7.10.1). As a result, this panel was 
removed from the subsequent structural analyses (NIST NCSTAR 1-6). 



NIST NCSTAR 1-2, WTC Investigation 



187 



Chapter 7 



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of WTC 1 (base case). 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



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of WTC 1 (base case) (continued). 



101 103 106 109 112 115 llfl 121 124 127 130 133 138 130 li? 145 148 151 154 157 159 



Severe Floor Damage *« — ; 

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359 3S7 364 3S1 343 34S 342 



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Figure 7-13. Cumulative structural damage to the floors and columns of WTC 1 

(base case). 



NISTNCSTAR 1-2, WTC Investigation 



Chapter 7 



7.3.3 Fuel and Debris Distributions 

The global impact results presented in this section include the distribution of the jet fuel inside the tower, 
the damage to the building contents (partition walls and workstations), and the aircraft debris distribution 
in the towers. For the base case WTC 1 global impact analysis, the calculated distribution of the fuel in 
the tower in a plan view and side view is shown in Figure 7-14. At the termination of the global impact 
analysis, the residual momentum of the jet fuel in the impact direction was less than one percent of the 
initial momentum, indicating that the fuel cloud was nearly at rest at about 0.715 s. 

To more clearly present the calculated response of the structures that influenced the fire propagation, the 
structural components were removed from the visualization, with the exception that the core columns 
were maintained in the visualizations for reference positions. A plan view of the response of the 
remaining building contents and aircraft debris is shown in Figure 7-15. Similar plan views of floors 95 
and 96 response of the building contents and debris field are provided in Figure 7-16 and Figure 7-17, 
respectively. The bulk of the aircraft debris and fuel were arrested prior to exiting the far side of the 
tower core. A small amount of aircraft debris was calculated to exit the south wall of the tower. 

Plots of debris distribution and damage to tower contents at the end of the impact simulation similar to 
those in Figure 7-1 6(c) and Figure 7-1 7(c) were used to estimate the damage to fireproofing. The extent 
of dislodged fireproofing was estimated by considering fireproofing damage only to structural 
components in the direct path of debris. For details of the methodology and the extent of fireproofing 
damage, see NIST NCSTAR 1-6. 

A quantitative characterization of the fuel and aircraft debris distribution was obtained by slicing the 
model at vertical floor locations and calculating the mass at each floor level. A summary of the floor-by- 
floor fuel and debris distributions is given in Table 7-2. The bulk of the fuel and aircraft debris was 
deposited in floors 93 through 97, with the greatest concentration on floor 94. Approximately 18,000 lb, 
or 7 percent, of aircraft mass was eliminated from the debris cloud at the final state as a result of the 
erosion in the aircraft structures due to impact and breakup. This mass was not accounted for in the fuel 
and debris distributions provided in Table 7-2. A first approximation would be to increase the airframe 
debris distribution proportionately to account for the eroded mass. This eroded mass was maintained in 
the calculation but was no longer included in the contact algorithm. As a result, any residual momentum 
at the time of erosion could not be subsequently transferred to the tower. 

The calculated debris cloud included 17,400 lbs of debris and 6,700 lbs of aircraft fuel outside of the 
tower at the end of the impact analysis, either rebounding from the impact face (north wall) or passing 
through the tower (south wall). This amount might have been larger in the calculation, since the exterior 
walls were not modeled with windows that could contain the fuel cloud and small debris inside the 
towers. In addition, the impact behavior of the aircraft fuel cloud did not include the ability to stick to, or 
wet, interior components. Rather, the aircraft fuel SPH particles tended to bounce off of internal 
structures. 

The physics of fuel impact and dispersion in this type of impact event is complex and no appropriate 
validation data could be found. The fuel starts as a continuous fluid within the tanks and ends up 
distributed both on the tower structures and as small droplets that interact with the atmosphere 
surrounding the impact zone. No single analysis technique is currently available that can analyze this full 
range of fuel dispersion without significant uncertainties. 

1 90 NIST NCSTAR 1 -2, WTC Investigation 



Aircraft Impact Damage Results 



Both the SPH and ALE analysis techniques (see Section 5.4.4) available for the analysis of the fuel 
impact and dispersion had limitations. Details of the fuel behavior such as the wetting of the fuel against 
tower structures and interior contents or the physics of the fuel breakup into droplets are not accurately 
reproduced in either analysis technique. However, the momentum transfer from the fuel to the tower 
structures and subsequent impact damage produced by the fuel can be modeled by both analysis 
techniques. 

The detailed predictions of the fuel dispersion and distribution using SPH in the global impact analyses 
had significant uncertainties in the absence of improved validation testing. However, some aspects of the 
distribution had a higher confidence. The floors confined the vertical motion of the fuel, and the floor-by- 
floor distribution of fuel was controlled more by the geometry of the tower and impact conditions. As a 
result, this distribution by floor has a higher level of confidence. Similarly, the interior contents and 
partition walls, and the damage to these structures, controlled the spread of fuel. 



NIST NCSTAR 1 -2, WTC Investigation 1 9 1 



Chapter 7 



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Figure 7-14. Calculated fuel distribution in the base case WTC 1 analysis. 



192 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




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for the base case. 



NISTNCSTAR 1-2, WTC Investigation 



193 



Chapter 7 




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Figure 7-16. Calculated floor 95 contents and fuel distribution (base case). 



194 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 







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Figure 7-17. Calculated floor 96 contents and fuel distribution (base case). 



NISTNCSTAR 1-2, WTC Investigation 



195 



Chapter 7 



Table 7-2. Fuel and aircraft debris distribution for the base case WTC 1 impact. 


Tower Location 


Aircraft Fuel 


Aircraft Debris 


Total Outside Tower 


6,700 lb 


17,4001b 


WTC 1 Floor 92 


8101b 


260 1b 


WTC 1 Floor 93 


6,1001b 


22,600 lb 


WTC 1 Floor 94 


16,100 1b 


96,000 lb 


WTC 1 Floor 95 


12,200 lb 


28,000 lb 


WTC 1 Floor 96 


11,700 1b 


19,4001b 


WTC 1 Floor 97 


9,500 lb 


6,000 lb 


WTC 1 Floor 98 


2,200 lb 


6,000 lb 


WTC 1 Floor 99 


7701b 


90 1b 


Total Weight 


66,1001b 


196,000 1b 



7.4 



WTC 1 MORE SEVERE IMPACT ANALYSIS - CASE B 



This case is referred to as Case B for the remainder of the WTC Investigation reports. 

In addition to the base case impact analysis described in Section 7.3, two more impact analyses were 
performed for each tower to provide a range of calculated impact-induced damage. The variations in 
impact analysis parameters were developed based on the results of the sensitivity analyses and additional 
evaluations of the parameter uncertainties (see Chapter 8 of NIST NCSTAR 1-2B). These analyses 
included a more severe and a less severe case. Presented in this section is the more severe case. 

The parameters for the more severe WTC 1 impact scenario are compared to the corresponding 
parameters in the base case analysis in Table 7-3. For the flight parameters, the impact speed was 
472 mph in the more severe impact scenarios, which was the upper bound obtained from the analysis of 
aircraft impact conditions described in Chapter 6. The aircraft vertical trajectory angle was varied from 
10.6 degrees in the base case to 7.6 degrees for the more severe impact case, which resulted in more 
impact energy directed inward toward the core. The lateral trajectory was not varied since the impact was 
close to being centered on the tower and normal to the north face of WTC 1. A small variation in the 
lateral approach angle would have had little effect on the energy of the aircraft debris entering the tower 
and core. 

The parameters varied for the aircraft model were the weight of the aircraft and the ductility of the aircraft 
materials. A 5 percent increase in the total aircraft weight was considered for the more severe case. The 
failure strain was varied to be 125 percent of the baseline value. This relatively large variation in aircraft 
material ductility was used for multiple reasons. First, no material characterization testing of specimens 
cut from a 767 were performed as part of this Investigation. All of the material properties used for the 
aircraft was obtained from sources available in the open literature. Secondly, the variation in ductility 
was used as the single parameter in this analysis to evaluate the uncertainties in the energy absorption 
capacity of aircraft materials. An increase in aircraft material strength would have had a similar effect to 
an increase in material ductility for producing increased impact damage to the towers. Finally, the 



196 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



material failure parameters were influenced by the resolution of the models in the impact analysis. The 
mesh refinement effects introduced an increased uncertainty on the failure strains in these analyses. 

Table 7-3. Input parameters for the more and less severe WTC 1 impact analysis. 



Analysis Parameters 


Base Case 


More Severe 


Less Severe 


Flight 
Parameters 


Impact speed 


443 mph 


472 mph 


414 mph 


Trajectory - pitch 


10.6° 


7.6° 


13.6° 


Trajectory - yaw 


0.0° 


0.0° 


0.0° 


Orientation - pitch 


8.6° 


5.6° 


11.6° 


Orientation - yaw 


0.0° 


0.0° 


0.0° 


Aircraft 
Parameters 


Weight 


100 percent 


1 05 percent 


95 percent 


Failure Strain 


100 percent 


125 percent 


75 percent 


Tower 
Parameters 


Failure Strain 


100 percent 


80 percent 


120 percent 


Live Load Weight 


25 percent 


20 percent 


25 percent 



Finally, the parameters varied for the tower model were the ductility of the steel used in the tower 
construction and the weight of the contents inside the tower. A variation of 20 percent was used to 
account for the uncertainty in failure strain for the tower materials. The combination of increasing the 
aircraft material ductilities by 25 percent and reducing the tower material ductilites by 20 percent covered 
a wide range in relative aircraft and tower strength assumptions. The variations in internal tower contents 
(live load weight in Table 7-3) are specified as a percentage of the design live load. 

Table 7-3 provides also the parameters used in the less severe damage case. As can be seen from the 
table, the parameters are selected to provide for a stronger tower and a weaker aircraft to yield less 
damage to the tower structure. 



7.4.1 



Impact Response 



The impact response of WTC 1 for the more severe case is shown in side and plan views in Figure 7-18 
and Figure 7-19, respectively. The response is shown at intervals of 0.1 s from impact through the initial 
0.5 s of the response. Comparing the more severe impact response in Figure 7-18 and Figure 7-19 with 
the base case response in Figure 7-2 and Figure 7-3, it can be seen that the two responses were very 
similar with two exceptions. These were the slightly compressed time scale and the larger amount of 
debris exiting the south wall in the more severe case. These differences were due to the larger impact 
speed, the increased weight and material toughness of the aircraft, and the reduced contents mass and 
material toughness of the towers for the more severe case. 



NISTNCSTAR 1-2, WTC Investigation 



197 



Chapter 7 




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Figure 7-18. WTC 1 more severe global impact analysis (side view). 



198 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



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Figure 7-18. WTC 1 more severe global impact analysis (side view) (continued). 



NISTNCSTAR 1-2, WTC Investigation 



199 



Chapter 7 




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Figure 7-19. WTC 1 more severe global impact analysis (plan view). 



200 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 







(d) Time=0.30 s 

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Figure 7-19. WTC 1 more severe global impact analysis (plan view) (continued). 



NISTNCSTAR 1-2, WTC Investigation 



201 



Chapter 7 

7.4.2 Tower Structural Damage 

Exterior Wall Damage 

A comparison of the north exterior wall observed and calculated damage from the more severe WTC 1 
global impact analysis is shown in Figure 7-20. The calculated and observed magnitude and mode of 
impact damage on the exterior wall were still in good agreement for the more severe impact analysis. 

Comparing Figure 7-5 and Figure 7-20, it can be concluded that the overall agreement with the observed 
damage to the north wall was good for the base case and the more severe case, with the base case analysis 
providing the better match to the observed damage. The differences in apparent damage were largely due 
to panels that may have severed columns in one case and were removed at the connections in another. 
Toward the wing tips, where the columns and spandrels were not completely severed, the more severe 
impact damage analysis calculated higher damage to the exterior wall panels. These columns had the 
largest amount of material with plastic strains above 5 percent (shown in red in the figure). As would be 
expected, the base case analysis calculated less damage to the exterior wall than the more severe case near 
the wing tips. 



202 NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



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Figure 7-20. More severe impact damage to the WTC 1 exterior wall. 



NISTNCSTAR 1-2, WTC Investigation 



203 



Chapter 7 



Core Structural Damage 

The core had extensive damage in the region close to the impact point. The columns in line with the 
aircraft fijselage failed on the impact side, and several of the core beams were also severely damaged or 
failed in the impact zone. In some cases, failure of the column splices located on floors 92, 95, and 98 
contributed significantly to the failure of the core columns. 

The calculated damage to the core columns by row is shown in Figure 7-21, and the damage to the core 
framing for floors 95 and 96 is shown in Figure 7-22. A summary of the core column damage is 
provided in Table 7-4, with the qualitative classification of the column damage levels provided 
previously in Figure 7-7. A total of six columns were severed, and three columns were heavily damaged 
in the more severe case, compared to three columns severed and four columns heavily damaged in the 
base case WTC 1 impact analysis. This shows a clear correlation between damage magnitude and impact 
severity. 





(a) Columns 503-1003 



(b) Columns 504-1004 





Fringe Levels 
5.000e-02 
I.BOOe-OZ 
1.000e-02 
3.500e-0Z 
3.000e-0Z 
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O.OOOe+00 



I 



(c) Columns 505-1005 (d) Columns 506-1006 (e) Reference scale 

Figure 7-21. More severe impact response of the WTC 1 core columns. 



204 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




(a) Floor 95 core framing damage 



(b) Floor 96 core framing damage 



Figure 7-22. More severe impact damage to the core beams of floors 95 and 96 of 

WTCl. 



Table 7-4. Summary of core column damage for the more severe WTC 1 impact. 


Column 


Location 


Damage Level 


Lateral Deflection of 
Column Centerline (in.) 


Column 503 


Floor 95-96 


Severed 




Column 504 


Floors 92-96 


Severed 




Column 505 


Floors 93-96 


Severed 




Column 506 


Floors 93-95 


Heavy 


24 


Column 603 


Floors 96-97 


Moderate 




Column 604 


Floors 92-96 


Severed 




Column 605 


Floors 94-95 


Moderate 




Column 606 


Floors 94 


Light 




Column 702 


Floor 97 


Light 




Column 703 


Floor 96 


Moderate 




Column 704 


Floors 92-96 


Severed 




Column 705 


Floor 95 


Moderate 




Column 706 


Floors 93-95 


Severed 




Column 802 


Floor 96 


Light 




Column 803 


Floors 96-97 


Moderate 




Column 804 


Floor 94-96 


Moderate 




Column 805 


Floors 93-95 


Heavy 


20 


Column 903 


Floor 96 


Light 




Column 904 


Floors 95-96 


Heavy 


19 


Column 905 


Floor 95 


Light 





NISTNCSTAR 1-2, WTC Investigation 



205 



Chapter 7 



The strong correlation between the core damage and impact severity was expected. All of the parameter 
variations were expected to produce an increase in core damage. The flight parameters had an increasing 
impact speed and a shallower impact angle, directing more energy toward the core. The aircraft had an 
increasing weight and higher material toughness. The tower had reduced mass in the contents and a 
reduced material toughness. All of these variations contributed toward the increased core damage with 
impact severity. 

Floor Truss and Slab Damage 

An overall frontal view of the floor truss structure in the impact zone along with the calculated more 
severe impact damage to the floor trusses is shown in Figure 7-23. The figure shows that the trusses 
experienced significant damage in the impact zone. A plan view of the calculated damage to the truss on 
floors 95 and 96 is shown in Figure 7-24. The calculated impact response produced severe damage to the 
truss structures in the primary impact path of the fuselage from the exterior wall to the core. The truss 
floor system on floors 94 through 96 was damaged and sagged downward as a result of the impact 
loading. 

When the floor-by-floor damage was compared for the base case and more severe impact analyses, the 
damage appeared to be slightly less for the more severe impact analysis. The parameters used in the more 
severe global impact analysis would primarily contribute to an increased damage magnitude for the tower 
structures. However, the downward impact trajectory angle was reduced from the 10.6 degree angle in 
the base case analysis to a 7.6 degree angle in the more severe impact analysis. This would have the 
effect of directing more of the impact energy inward toward the tower core but reducing the normal 
downward force on the floor structures in the impact zone. As a result, the combined effects of the 
analysis parameter variations produced slightly less damage to the truss structure in the more severe 
impact analysis scenario. 



206 NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Column 
135 



Column 
109 



Floor 95 



Column 
151 



Floor 96- 



Column 
141 



Column 
107 



Column 
157 



Floor 95 



Column 
151 



(a) Initial detailed truss structures 



Column 
115 




(b) Calculated damage 
Figure 7-23. More severe impact damage to the WTC 1 floor trusses (front view). 



NISTNCSTAR 1-2, WTC Investigation 



207 



Chapter 7 



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Figure 7-24. More severe impact damage to the trusses on floors 95 and 96 of WTC 1 

(plan view). 

The calculated more severe impact damage to the WTC 1 floor slab for floors 95 and 96 is shown in 
Figure 7-25. The magnitude of floor slab damage was, in general, very similar for the base case and 
more severe global impact analyses. When the floor-by-floor damage was compared for the two analyses, 
the damage appeared to be slightly less for the more severe impact analysis. Similar to the truss damage, 
the reduced damage in the floor slab is believed to be the result of the reduction in the downward impact 
trajectory angle from 10.6 to 7.6 degrees in the more severe impact analysis, reducing the normal 
downward force on the floor structures. 



208 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Impact 



Impact 



Column 
113 




(a) Floor 95 slab damage (b) Floor 96 slab damage 

Figure 7-25. More severe impact damage to the slabs on floors 95 and 96 of WTC 1 

(plan view). 

Summary of Structural Damage 

Figure 7-26 shows a summary of the structural damage to the core columns and floor systems at floors 93 
through 97 of WTC 1 for the more severe case (Case B). Figure 7-27 presents the cumulative damage to 
WTC 1 on all affected floors and columns. 



NISTNCSTAR 1-2, WTC Investigation 



209 



Chapter 7 



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(d) Floor 96 



Figure 7-26. Summary of the floor-by-floor structural damage to the floors and columns 

of WTC 1 (more severe case). 



210 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



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Figure 7-26. WTC 1 more severe global impact analysis (plan view) (continued). 



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Figure 7-27. Cumulative structural damage to the floors and columns of WTC 1 (more 

severe case). 



NISTNCSTAR 1-2, WTC Investigation 



211 



Chapter 7 



7.4.3 



Fuel and Debris Distribution 



The distribution of the fuel in the tower, calculated from the more severe case, in a plan view and side 
view is shown in Figure 7-28. At the termination of the global impact analysis, the residual momentum 
of the jet fuel was less than one percent of the initial momentum, indicating that the fuel cloud was nearly 
at rest. To more clearly present the calculated response of the structures that influenced the fire 
propagation, the structural components were removed from the visualization, with the exception that the 
core columns were maintained in the visualizations for reference positions. A plan view of the response 
of the remaining building contents and aircraft debris is shown in Figure 7-29. 

The calculated damage to the WTC 1 contents for the more severe impact case is shown in plan views for 
floors 95 and 96 in Figure 7-30 and Figure 7-31, respectively. A comparison to the calculated damage 
for the base case WTC 1 impact analysis indicated that the content damage zone was very similar in 
width, but extended further south through the tower in the more severe impact. The more severe impact 
produced significantly greater content damage on the far side of the core and extended more fully through 
the tower. 



A summary of the floor-by-floor fuel and debris distributions is given in Table 7-5. The bulk of the fuel 
and aircraft debris was deposited in floors 93 through 97, with the greatest concentration on floor 94. The 
calculated debris cloud included 46,800 lbs of debris and 7,500 lbs of aircraft fuel outside of the tower at 
the end of the impact analysis, either rebounding from the impact face (north wall) or passing through the 
tower (south wall). This amount might have been larger in the calculation due to the reasons mentioned 
previously for the base case impact (see Section 7.3.3). Comparing Figure 7-29 and Table 7-5 with 
Figure 7-15 and Table 7-2, it can be seen that the amount of debris exiting the south wall of the tower in 
the more severe case was much larger than that from the base case. 

Table 7-5. Fuel and aircraft debris distribution for the more severe WTC 1 impact. 



Tower Location 


Aircraft Fuel 


Aircraft Debris 


Total Outside Tower 


7,500 lb 


46,800 lb 


WTC 1 Floor 92 


1.2001b 


15 1b 


WTC 1 Floor 93 


5,800 lb 


39,1001b 


WTC 1 Floor 94 


14,100 lb 


59,900 lb 


WTC 1 Floor 95 


13,600 1b 


22,500 lb 


WTC 1 Floor 96 


13,300 lb 


21,5001b 


WTC 1 Floor 97 


9,600 lb 


5,200 lb 


WTC 1 Floor 98 


3,1001b 


7,300 lb 


WTC 1 Floor 99 


1,1001b 


400 1b 


Total Weight 


69,300 lb 


202,700 lb 



212 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



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Figure 7-28. Calculated fuel distribution in the more severe WTC 1 analysis. 



NISTNCSTAR 1-2, WTC Investigation 



213 



Chapter 7 




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Figure 7-29. Plan view of calculated WTC 1 building, fuel, and aircraft debris distribution 

for the more severe case. 



214 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




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Figure 7-30. Calculated more severe WTC 1 impact response of floor 95 contents. 



NISTNCSTAR 1-2, WTC Investigation 



215 



Chapter 7 




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Figure 7-31. Calculated more severe WTC 1 impact response of floor 96 contents. 



216 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



7.5 WTC 1 LESS SEVERE IMPACT ANALYSIS - BRIEF DESCRIPTION 

This section presents a brief description of the resuhs from the less severe damage case. The reader is 
referred to NIST NCSTAR 1-2B for further details. 

For the north exterior wall of WTC 1 , the magnitude and mode of impact damage were still in good 
agreement with the observed damage for the less severe impact scenario. 

The core had a limited damage confined to the region nearest to the impact point. Only one column was 
severed, and two columns were heavily damaged for the less severe case, compared to three severed 
columns and four heavily damaged columns in the base case WTC 1 impact analysis. The failure of the 
column splices located on floors 92 and 95 contributed to the failure of the core column. 

The floor trusses experienced significant damage in the impact zone. The calculated impact response 
produced severe damage to the truss structures in the primary impact path of the fuselage. The truss 
structures were severely damaged from the exterior wall to the core. The truss floor system on floors 94 
through 96 were damaged and sagged downward as a result of the impact loading. 

When compared with the base case, the magnitude of damage to the floor trusses and floor slabs was 
slightly increased for the less severe impact analysis. The parameters used in the less severe global 
impact analysis would primarily contribute to a reduced damage magnitude for the tower structures. 
However, the downward impact trajectory angle was increased from the 10.6 degree angle in the base 
case analysis to a 13.6 degree angle in the more severe impact analysis. This would have the effect of 
directing more of the impact energy downward, increasing the normal force on the floor structures in the 
impact zone. As a result, the combined effects of the analysis parameter variations produced a small 
increase in the damage to the truss structure in the less severe impact analysis scenario. 

A comparison to the base case and less severe case indicated that the building contents damage zone was 
very similar in width but did not extend as far through the tower in the less severe impact. The less severe 
impact produced little content damage on the far side of the core and did not extend fully through the 
tower. Little or no debris penetration of the south wall of the tower was expected for the less severe 
impact condition. 

7.6 WTC 2 BASE CASE IMPACT ANALYSIS - CASE C 

This case is referred to as Case C for the remainder of the WTC Investigation reports. The combined 
aircraft and tower model used for the base case global impact conditions of WTC 2 is shown in 
Figure 7-32. The WTC 2 base case impact analysis was performed for a 0.62 s duration following initial 
impact of the aircraft nose with the south exterior wall. 



NIST NCSTAR 1 -2, WTC Investigation 2 1 7 



Chapter 7 




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Figure 7-32. WTC 2 global impact model. 



7.6.1 



Impact Response 



The base case global aircraft impact response of WTC 2 is shown in side views and plan views in 
Figure 7-33 and Figure 7-34, respectively. The response is shown at intervals of 0.1 s from impact 
through the initial 0.5 s of the response. The initial 0.1 s of the base case global aircraft impact response, 
shown in Figure 7-33(b) and Figure 7-34(b), was dominated by the impact, penetration, and 



218 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



fragmentation of the forward fuselage structures. The engines and leading portions of the wings 
penetrated the exterior wall. The forward fuselage structures were severely damaged both from the 
penetration through the exterior columns and the interaction with the 81st floor slab that sliced the 
fuselage structures in half The downward trajectory of the aircraft structures caused the airframe to 
collapse against the floor, and the subsequent debris motion was redirected inward along a more 
horizontal trajectory parallel to the floor. The higher impact speed and short truss floor span in this 
impact orientation had the forward fuselage structures well into the tower core by this time. 

By 0.2 s after impact, the full penetration of the aircraft into the tower was just completed, as shown in 
Figure 7-33(c) and Figure 7-34(c). The airframe was mostly broken up, but some large sections of the 
aft fuselage and tail were still intact, having penetrated through the opening in the south wall produced by 
the forward fuselage structures. The aircraft fuel cloud was starting to spread out, but was still relatively 
dense, and the leading edge of the fuel was approximately one-third through the tower core. By 0.2 s, the 
downward trajectory of the aircraft structures transferred sufficient vertical load that the truss floor 
structures on floors 80 and 81 were starting to collapse in the impact zone. 

At 0.3 s after impact, the aircraft fuel cloud had penetrated approximately two-thirds the distance through 
the core and was spreading out, as shown in Figure 7-33(d) and Figure 7-34(d). However, the 
subsequent motion of the aircraft fragments and fuel debris cloud began to be noticeably slowed beyond 
this time. The fuel and debris continued to spread through the tower, but at a much slower rate, as seen in 
the remaining images in Figure 7-33 and Figure 7-34. The spread of the fuel and debris cloud was more 
rapid and extensive in the open truss floor regions than through the core as a result of the open volume 
above the workstations in the truss floor zone. 

The load transfer of the base case WTC 2 aircraft impact can be described by the time-history of the 
aircraft momentum as shown in Figure 7-35. The curve illustrates an initial rate of load transfer during 
the first 0. 1 s of impact as the forward fuselage penetrated the exterior wall and impacted the interior 
structures. Between 0.1 s and 0.2 s, a more rapid load transfer rate was observed as the area of the impact 
became larger (extending outward in the wing impact regions) and a higher percentage of the aircraft 
mass was impacting the interior structures. At 0.2 s, the aircraft completely penetrated the building and 
retained approximately 30 percent of its initial momentum. Beyond this time, the rate of load transfer 
was steadily decreasing, with very little load transfer after approximately 0.4 s. The behavior was very 
similar to that of the base case WTC 1 impact, shown in Figure 7-4, but with a slightly compressed time 
scale resulting from the higher impact speed on WTC 2. 



NIST NCSTAR 1 -2, WTC Investigation 2 1 9 



Chapter 7 



\ MH in irm 

^^BIHBBIIBBI IB I B B a Bl IB I BIB B Bl 




(a) Time=0.00 s 



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(c) Time=0.20 s 
Figure 7-33. WTC 2 base case global impact analysis (side view). 



220 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



MM in in m m m mi 1 

IMMAl HMMl in HAMMI IMMl IHHIH-IBH 




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(f) Time=0.50 s 
Figure 7-33. WTC 2 base case global impact analysis (side view) (continued). 



NISTNCSTAR 1-2, WTC Investigation 



111 



Chapter 7 




(a) Time=0.00 s 

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Figure 7-34. WTC 2 base case global impact analysis (plan view). 



222 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




(d) Time=0.30 s 



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(e) Time=0.40 s 




(f) Time=0.50 s 
Figure 7-34. WTC 2 base case global impact analysis (plan view) (continued). 



NISTNCSTAR 1-2, WTC Investigation 



223 



Chapter 7 



E 

3 
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I I I I I I I I I I I I M I I I I I I I I I I I I I r I I I I I I I I I r I I I I I I I I I I I I I i M I I I I I I r 



WTC 2 Baseline Impact Analysis 



Engines & Wings 
Impacting Exterior Wall 



Entire Aircraft 
Inside Tower 



Q Q I 1 I I I I 1 I I I I 1 I I r I r I I I I r I I I I r I I r I r I I I I 1 I I I I r I I 1 I 1 rl I I I I r I I I I I I I i-H 

0.0 0.1 0.2 0.3 0.4 0.5 0.6 

Time (s) 
Figure 7-35. Normalized aircraft momentum for the WTC 2 base case impact. 

The aircraft was severely broken into thousands of debris fragments of various sizes and mass as a result 
of the impact with WTC 2. Larger fragments occurred for specific components, such as the engines and 
landing gear components. This behavior was very similar to the WTC 1 aircraft breakup. A discussion of 
the location of the engines at the end of the simulation is presented in Section 7.10.2. 



7.6.2 



Tower Structural Damage 



The structural damage to the WTC 2 tower by the base case impact conditions is described in this section. 
The primary structural components of interest were the exterior wall, core columns and core framing 
components, and the floor structures and concrete floor slab. Only limited results are presented herein. 
Refer to Chapter 9 of NIST NCSTAR 1-2B for further details. 

Exterior Wall Damage 

A comparison of the south exterior wall observed and calculated damage from the base case WTC 2 
global impact analysis is shown in Figure 7-36. The calculated impact damage to the exterior wall is 
shown with color fringes representing plastic strain magnitude, with undamaged sections in blue and 
strains at or above 5 percent shown in red. The schematic of observed damage was developed from 
inspections of the film and photographic data collected on the tower after impact. Both the observed and 
calculated damage regions shown in Figure 7-36 illustrate a region of the exterior wall from column 402 
to column 446, extending from floor 76 to floor 86 (spandrels at floors 77 through 86). 



224 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



The exterior wall completely failed in the regions of the fuselage, engine, and fuel-filled wing section 
impacts. Damage to the exterior wall extended to the wing tips, but the exterior columns were not 
completely failed in the outer wing and vertical stabilizer impact regions. Failure of the exterior columns 
occurred both at the bolted connections between column ends and at various locations in the column, 
depending on the local severity of the impact load and the proximity of the bolted connection to the 
impact. 

The initial observation from the comparison of the calculated and observed damage was that the geometry 
and location of the impact damage zone were in good agreement. This agreement in the position and 
shape of the impact damage served to validate the geometry of the aircraft model, including the aircraft 
orientation, trajectory, and flight distortions of the wings. The agreement of both the mode and 
magnitude of the impact damage served to partially validate the constitutive and damage modeling of the 
aircraft and exterior wall of the tower. Section 7.10.2 provides a detailed comparison of the calculated 
and observed damage mode and magnitude. 

Core Structural Damage 

The core had significant damage in the region close to the impact point, in particular the southeast corner 
of the core. The columns in line with the aircraft fuselage failed on the impact side, and several of the 
core beams were also severely damaged or failed in the impact zone. In some cases, failure of the column 
splices located on floors 77, 80, and 83 contributed significantly to the failure of the core columns. This 
was particularly true for the heavy column number 1001 at the southeast corner of the core that failed at 
the three splice locations. 

The calculated damage to the core columns by row is shown in Figure 7-37. The columns are shown 
with color fringes representing plastic strain magnitude, with undamaged sections in blue and strains at or 
above 5 percent shown in red. A summary of the column damage is listed in Table 7-6. The qualitative 
classification of the column damage levels were provided previously in Figure 7-7. The damage to the 
core beams for floors 80 and 81 is shown in Figure 7-38. 



NIST NCSTAR 1-2, WTC Investigation 225 



Chapter 7 



u y y 



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Figure 7-36. Base case impact damage to the WTC 2 exterior wall. 



226 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




'^.._i ■ 'i |i 1 i| ■■-' ■ |- 




(a) Columns 1001-1008 



(b) Columns 901-908 



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Figure 7-37. Base case impact damage to the WTC 2 core columns. 

Floor Truss and Slab Damage 

An overall frontal view for the floor truss structure in the WTC 2 impact zone, along with the calculated 
base case impact damage to the trusses, is shown in Figure 7-39. The figure shows that the trusses 
experienced significant damage in the impact zone, with the largest amount of damage on floor 81. A 
plan view of the calculated damage to the trusses on floors 80 and 81 is shown in Figure 7^0. The 
calculated impact response produced severe damage to the truss structures in the primary impact path of 
the fuselage. The truss structures were severely damaged from the exterior wall to the core. The truss 
floor system on floors 79 and 81 had sufficient damage from the impact that truss floor sections sagged 
downward as a result of the impact. 



NISTNCSTAR 1-2, WTC Investigation 



227 



Chapter 7 



Table 7-6. Summary of core column damage for the base case WTC 2 impact. 



Column 


Location 


Damage Level 


Lateral Deflection of 

Column Centerline 

(in.) 


Column 801 


Floor 79 


Heavy 


10 


Column 901 


Floors 79-82 


Severed 




Column 902 


Floor 79 


Heavy 


32 


Column 903 


Floors 77-83 


Severed 




Column 904 


Floor 79 


Moderate 




Column 905 


Floor 79 


Heavy 


18 


Column 1001 


Floors 77-83 


Severed 




Column 1002 


Floors 79-81 


Severed 




Column 1003 


Floor 80 


Severed 




Column 1004 


Floor 80 


Heavy 


18 




(a) Floor 80 core framing damage (b) Floor 81 core framing damage 

Figure 7-38. Base case impact damage to the core beams of floors 80 and 81 of WTC 2. 



228 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Floor 82 



Floor 80 
Floor 79 

Floor 78 



(a) Initial detailed truss structures 



Floor 82 




Floor 78 



(b) Calculated damage 
Figure 7-39. Base case impact damage to the WTC 2 floor trusses (front view). 

The calculated damage to the WTC 2 floor slabs for floors 80 and 81 is shown in Figure 7-41. The 
fringes of damage were set such that the concrete failed in the regions colored red (2 percent plastic 
strain). In these regions, it is expected that the concrete had been severely damaged and potentially 
removed, exposing the supporting metal decking. The strength of the floor slab was severely reduced in 
the analysis beyond this strain to model the residual strength of the metal deck after the concrete failure, 
breakup, and removal. At a plastic strain of 30 percent, corresponding to failure levels for the metal 
decking material, the elements were eroded (seen as holes ruptured in the floor slab shown). 



NIST NCSTAR 1-2, WTC Investigation 229 



Chapter 7 




417— I I f ' V' ■ ! ■ 

(a) Floor 80 truss damage 



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(b) Floor 81 truss damage 

Figure 7-40. Base case impact damage to the trusses on floors 80 and 81 of WTC 2 

(plan view). 



230 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




(a) Floor 80 slab damage 



443 



Impact 




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Figure 7-41. Base case impact damage to the slabs on floors 80 and 81 of WTC 2 

(plan view). 

Summary of Structural Damage 

The impact-induced structural damage described above was used as the initial conditions for the post- 
impact fire-structural analyses. Figure 7-42 shows a summary of the structural damage to the core 



NISTNCSTAR 1-2, WTC Investigation 



231 



Chapter 7 



columns and floor systems at floors 77 through 83 of WTC 2 for the base case (Case C). The damage to 
the columns at the various levels is identified by the color of the circles, where red, blue, green, and 
yellow signify severed, heavily damaged, moderately damaged, and lightly damaged columns, 
respectively. The dotted boxes on the figures indicate areas where the impact created an opening in the 
floor. These were used to identify openings in the floor slab in the fire dynamics simulations (NIST 
NCSTAR 1-5F). The solid boxes indicate areas in the floor system that had severe structural damage. 
These areas were removed from the subsequent structural analyses (NIST NCSTAR 1-6). 

Figure 7^3 presents the cumulative damage to WTC 2 on all affected floors and columns. The figure 
shows the damage to the south exterior wall due to impact, based on photographs of the south walls. Note 
that damage to columns 407 through 409 was based on the analysis results, since this area was obscured 
by smoke in the photographs. Figure 7-43 also shows the damage to columns on the north perimeter 
wall, which the analysis did not capture due to the coarse mesh on the north wall. This damage was 
observed in photographs (see Section 7.10.2). As a result, this damage was accounted for in the 
subsequent structural analyses (NIST NCSTAR 1-6). 



232 NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 





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Figure 7-42. Summary of the floor-by-floor structural damage to the floors and columns 

of WTC 2 (base case). 



NISTNCSTAR 1-2, WTC Investigation 



233 



Chapter 7 



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Figure 7-42. Summary of the floor-by-floor structural damage to the floors and columns 

of WTC 2 (base case) (continued). 



234 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



zn: 



212 235 2te 221 224 227 230 233 23B 239 242 



2« 251 254 257 259 



Severe Floor Damage 

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Column Damage 
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Moderate Damage ^^ 
Light Damage 




J69 457 454 451 448 445 442 



435 433 430 427 424 421 418 416 412 409 40S 403 401 



Figure 7-43. Cumulative structural damage to the floors and columns of WTC 2 

(base case). 

7.6.3 Fuel and Debris Distributions 

The global impact results presented in this section include the distribution of the jet fuel and aircraft 
debris in the WTC 2 tower, and the damage to the building contents (partition walls and workstations). 
For the base case WTC 2 global impact analysis, the calculated distribution of the fuel in the tower and 
shape of the fuel cloud in a plan view and side view are shown in Figure 7^4. At the end of the analysis, 
the residual momentum of the jet fuel in the impact direction was less than one percent of the initial 
momentum, indicating that the fuel cloud was nearly at rest at about 0.62 s. 

To more clearly present the calculated response of the structures that influenced the fire propagation, the 
structural components were removed from the visualization, with the exception that the core columns 
were maintained in the visuahzations for reference positions. A plan view of the response of the 
remaining building contents and aircraft debris are shown in a plan view in Figure 7-45. Similar plan 
views of floor 80 and 81 slices through the building contents and debris field are provided in Figure 7^6 
and Figure 7-47, respectively. The bulk of the aircraft debris and fuel was arrested prior to exiting the 
tower structures. However, a significant amount of aircraft debris was calculated to exit the north and 
east sides of the tower (Sides 300 and 200 of WTC 2). 



NISTNCSTAR 1-2, WTC Investigation 



235 



Chapter 7 



Plots of debris distribution and damage to tower contents at the end of the impact simulation similar to 
those in Figure 7-46(c) and Figure 7-47(c), were used to estimate the damage to fireproofmg. The extent 
of dislodged fireproofing was estimated by considering fireproofmg damage only to structural 
components in the direct path of debris. For details of the methodology and the extent of fireproofmg 
damage, see NIST NCSTAR 1-6. 

A quantitative characterization of the fuel and aircraft debris distribution was obtained by slicing the 
model at vertical floor locations and calculating the mass at each floor level. A summary of the floor-by- 
floor fuel and debris distributions is given in Table 7-7. The bulk of the fuel and aircraft debris was 
deposited in floors 78 through 80, with the greatest concentration of aircraft debris on floor 80, and the 
largest concentration of aircraft fuel on floors 79, 81, and 82. Approximately 14,000 lb, or 5 percent, of 
the total aircraft mass was ehminated from the debris cloud in the final state as a result of the erosion in 
the aircraft structures due to impact and breakup. This eroded mass was maintained in the calculation but 
eliminated from consideration in the contact algorithm. As a result, any residual momentum at the time 
of erosion could not be subsequently transferred to the tower. 

The calculated debris distribution included 55,800 lbs of debris and 10,600 lbs of aircraft fuel outside of 
the tower at the end of the impact analysis, either rebounding from the impact face or passing through the 
tower. These estimates of mass outside the tower were expected to be overestimated in the calculation 
since the exterior walls were not modeled with windows that could contain the fuel cloud and small debris 
inside the towers. In addition, the impact behavior of the aircraft fuel cloud did not include the ability to 
stick to, or wet, interior components. Rather the aircraft fuel SPH particles tended to bounce off of 
internal structures (see Section 7.3.3). 



236 NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Time.-= - -D.BJ19. 



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(b) Side view 
Figure 7-44. Calculated fuel distribution in the base case WTC 2 analysis. 



NISTNCSTAR 1-2, WTC Investigation 



237 



Chapter 7 




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Figure 7-45. Plan view of calculated WTC 2 building, fuel, and aircraft debris distribution 

for the base case. 



238 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




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Figure 7-46. Calculated floor 80 contents, and fuel distribution (base case). 



NISTNCSTAR 1-2, WTC Investigation 



239 



Chapter 7 




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Figure 7-47. Calculated floor 81 contents and fuel distribution (base case). 



240 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Table 7-7. Fuel and aircraft debris distribution for the base case WTC 2 impact. 


Tower Location 


Aircraft Fuel 


Aircraft Debris 


Total Outside Tower 


10,600 1b 


55,8001b 


WTC 2 Floor 77 


1,3001b 


400 1b 


WTC 2 Floor 78 


6,200 lb 


4,800 lb 


WTC 2 Floor 79 


11,400 1b 


16,2001b 


WTC 2 Floor 80 


6,000 lb 


83,8001b 


WTC 2 Floor 81 


14,400 lb 


27,300 lb 


WTC 2 Floor 82 


10,600 1b 


3,600 lb 


WTC 2 Floor 83 


1,5001b 


4,300 lb 


WTC 2 Floor 84 


2001b 


5001b 


Total Weight 


62,000 lb 


197,600 1b 



7.7 



WTC 2 MORE SEVERE IMPACT ANALYSIS - CASE D 



This case is referred to as Case D for the remainder of the WTC Investigation reports. 

In addition to the base case impact analysis described in Section 7.6, two more impact analyses were 
performed for WTC 2 to provide a range of calculated impact-induced damage. The variations in impact 
analysis parameters were developed based on the results of the sensitivity analyses and additional 
evaluations of the parameter uncertainties (see Chapter 8 of NIST NCSTAR 1-2B). These analyses 
included a more severe and a less severe case. Presented herein is the more severe case only. 

The parameters for the more severe impact scenario are compared to the corresponding parameters in the 
base case analysis in Table 7-8. For the flight parameters, the impact speed was 570 mph in the more 
severe impact scenario, which was the upper bound obtained from the analysis of aircraft impact 
conditions described in Chapter 6. The aircraft vertical trajectory angle was also varied from 6 degrees in 
the base case to 5 degrees for the more severe impact scenario, which resulted in more impact energy 
directed inward toward the core. The lateral trajectory was not varied in this analysis so that the starboard 
engine trajectory was aligned with exiting the northeast corner of the tower, as was observed from 
photographic evidence (see Section 7.10.2). 



NIST NCSTAR 1-2, WTC Investigation 



241 



Chapter 7 



Table 7-8. Input parameters for the more severe WTC 2 impact analysis. 



Analysis Parameters 


Base Case 


More Severe 


Less Severe 


Flight 
Parameters 


Impact Velocity 


546 mph 


570 mph 


521 mph 


Trajectory - pitch 


6.0° 


5.0° 


8.0° 


Trajectory - yaw 


13.0° 


13.0° 


13.0° 


Orientation - pitch 


5.0° 


4.0° 


7.0° 


Orientation - yaw 


10.0° 


10.0° 


10.0° 


Aircraft 
Parameters 


Weight 


1 00 percent 


1 05 percent 


95 percent 


Failure Strain 


1 00 percent 


1 1 5 percent 


75 percent 


Tower 
Parameters 


Contents Strength 


1 00 percent 


80 percent 


1 00 percent 


Failure Strain 


1 00 percent 


90 percent 


120 percent 


Live Load Weight 


25 percent 


20 percent 


25 percent 



For WTC 2, the variations in the parameters from the base case were similar to those for WTC 1 (see 
Table 7-3), with two exceptions. The first exception was the introduction of the strength of the building 
contents as a parameter. There was less information available about the layout of building contents in the 
WTC 2 impact zone and therefore a larger uncertainty associated with the contents was assumed (the 
workstation layout from WTC 1 was used for WTC 2). Thus, in the more severe case, the contents 
strength was reduced to 80 percent of the baseline value. 

The second exception was the failure strains for the aircraft and tower materials. For the more severe 
WTC 1 analysis, 125 percent and 80 percent of the baseline values were used for the aircraft and tower 
failure strains, respectively. For the more severe WTC 2 analysis, 115 percent and 90 percent of the 
baseline values were used. The more severe WTC 2 analysis was the final global impact analysis 
performed. Based on the previous analyses, the variation in damage levels indicated that the WTC 2 
more severe impact analysis would produce impact damage state that was not viable (e.g., the amount of 
debris exiting the north wall). To ensure that a viable damage state was obtained, the aircraft and tower 
materials were adjusted to the values presented in Table 7-8. 

Table 7-8 provides also the parameters used in the less severe damage case. As can be seen from the 
table, the parameters are selected to provide for a stronger tower and a weaker aircraft to yield less 
damage to the tower structure. 



7.7.1 



Impact Response 



The impact response of WTC 2 for the more severe case is shown in side and plan views in Figure 7-48 
and Figure 7-49, respectively. The response is shown at intervals of 0.1 s from impact through the initial 
0.5 s of the response. Comparing the more severe impact response in Figure 7-48 and Figure 7-49 with 
the base case response in Figure 7-33 and Figure 7-34, it can be seen that the two responses were very 
similar with two exceptions. These were the slightly compressed time scale and the larger amount of 
debris exiting the north wall in the more severe case. These differences were due primarily to the larger 
impact speed, the increased weight and material toughness of the aircraft, and the reduced contents mass 
and material toughness of the towers for the more severe case. 



242 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




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Figure 7-48. WTC 2 more severe global impact analysis (side view). 



NISTNCSTAR 1-2, WTC Investigation 



243 



Chapter 7 



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Figure 7-48. WTC 2 more severe global impact analysis (side view) (continued). 



244 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




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Figure 7-49. WTC 2 more severe global impact analysis (plan view). 



NISTNCSTAR 1-2, WTC Investigation 



245 



Chapter 7 



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Figure 7-49. WTC 2 more severe global impact analysis (plan view) (continued). 



246 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



7.7.2 Tower Structural Damage 

Exterior Wall Damage 

A comparison of the south exterior wall observed and calculated damage from the more severe WTC 2 
global impact analysis is shown in Figure 7-50. The calculated impact damage to the exterior wall is 
shown with color fringes representing plastic strain magnitude, with undamaged sections in blue and 
strains at or above 5 percent shown in red. The mode and magnitude of the calculated and observed 
impact damage on the exterior wall were still in good agreement in this more severe impact analysis. 

As was the case for WTC 1 , there were small differences in the damage estimates for the south wall of 
WTC 2 from the base case and the more severe case scenarios (compare Figure 7-36 and Figure 7-50). 
Overall, the agreement with the observed damage from photographs was very good for both cases. The 
most obvious differences were largely due to portions of panels that may have severed columns in one 
case or have been removed at the connections in another. 



NIST NCSTAR 1-2, WTC Investigation 247 



Chapter 7 




(a) Schematic of observed damage 



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Figure 7-50. More severe impact damage to the WTC 2 exterior wall. 

Core Structural Damage 

The core had extensive damage in the region close to the impact point. The columns in line with the 
aircraft fuselage failed on the impact side, and several of the core beams were also severely damaged or 
failed in the impact zone. In some cases, failure of the column splices located on floors 77, 80, and 83 
contributed significantly to the failure of the core columns. 



248 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



The calculated damage to the core columns by row is shown in Figure 7-51, and the damage to the core 
framing at floors 80 and 81 is shown in Figure 7-52. A summary of the column damage is provided in 
Table 7-9, with the qualitative classification of the column damage levels provided previously in 
Figure 7-7. A total often columns were severed, and one column was heavily damaged in this WTC 2 
more severe case, compared to five columns severed and four columns heavily damaged in the base case 
WTC 2 impact analysis. 








































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Figure 7-51. More severe impact damage to the WTC 2 core columns. 

The strong correlation between the core damage and impact severity was expected. All of the parameter 
variations would be expected to produce an increase in core damage. The flight parameters had an 
increasing impact speed and a shallower impact angle, directing more energy toward the core. The 
aircraft had an increasing weight and higher material toughness. The tower had reduced mass in the 
contents and a reduced material toughness. All of these variations contributed toward the increased core 
damage with impact severity. 



NISTNCSTAR 1-2, WTC Investigation 



249 



Chapter 7 




(a) Floor 80 core framing damage 




(b) Floor 81 core framing damage 



Figure 7-52. More severe impact damage to the core beams of floors 80 and 81 of 

WTC2. 



250 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Table 7-9. Summary of core column damage for the more severe WTC 2 impact. 


Column 


Location 


Damage Level 


Lateral Deflection of 
Column Centerline (in.) 


Column 602 


Floor 79 


Moderate 




Column 605 


Floor 79 


Moderate 




Column 701 


Floors 79-80 


Severed 




Column 702 


Floor 79 


Heavy 


16 


Column 703 


Floor 79 


Moderate 




Column 704 


Floor 79 


Light 




Column 705 


Floors 78-79 


Light 




Column 705 


Floor 78 


Light 




Column 801 


Floors 79-80 


Severed 




Column 802 


Floors 77-80 


Severed 




Column 803 


Floors 77-80 


Severed 




Column 804 


Floor 79 


Light 




Column 901 


Floors 80-81 


Severed 




Column 902 


Floor 79 


Moderate 




Column 903 


Floors 77-83 


Severed 




Column 904 


Floors 79-81 


Moderate 




Column 905 


Floors 79 & 81 


Light 




Column 907 


Floor 81 


Light 




Column 1001 


Floors 77-83 


Severed 




Column 1002 


Floors 79-83 


Severed 




Column 1003 


Floors 79-83 


Severed 




Column 1004 


Floors 79-83 


Severed 




Column 1005 


Floors 79-81 


Moderate 





Floor Truss and Slab Damage 

An overall frontal view for the floor truss structure in the WTC 2 impact zone, along with the calculated 
more severe impact damage to the trusses, is shown in Figure 7-53. The figure shows that the trusses 
experienced significant damage in the impact zone, with the heaviest damage on floor 81. A plan view of 
the calculated damage to the trusses on floors 80 and 81 is shown in Figure 7-54. The calculated impact 
response produced severe damage to the truss structures in the primary impact path of the fuselage. The 
truss structures were severely damaged from the exterior wall to the core. The truss floor system on 
floors 79 through 82 had sufficient damage from the impact that portions of the truss floor sections 
sagged downward as a result of the impact. 



NISTNCSTAR 1-2, WTC Investigation 



251 



Chapter 7 



Floor 82 



Floor 80 
Floor 79 

Floor 78 



(a) Initial detailed truss structures 




(b) Calculated damage 
Figure 7-53. More severe impact damage to the WTC 2 floor trusses (front view). 

The magnitude of truss floor damage was very similar for the base case and more severe global impact 
analyses. The parameters used in the more severe global impact analysis would primarily contribute to an 
increased damage for the tower structures. However, the downward impact trajectory angle was reduced 
from the 6 degree angle in the base case analysis to a 5 degree angle in the more severe impact analysis. 
This resulted in directing more of the impact energy inward toward the tower core, but reducing the 
normal downward force on the floor structures in the impact zone. As a result, the combined effects of 
the analysis parameter variations produced very similar damage to the truss structure. 

The calculated damage to the WTC 2 floor slabs for floors 80 and 81 for the more severe impact is shown 
in Figure 7-55. The magnitude of floor slab damage was very similar for the base case and more severe 
global impact analyses due to the reasons explained above for the floor trusses. 



252 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



443- 



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Figure 7-54. More severe impact damage to the trusses on floors 80 and 81 of WTC 2 

(plan view). 



NISTNCSTAR 1-2, WTC Investigation 



253 



Chapter 7 




(a) Floor 80 slab damage (b) Floor 81 slab damage 

Figure 7-55. More severe impact damage to the WTC 2 floor slab (plan view). 

Summary of Structural Damage 

Figure 7-56 shows a summary of the structural damage to the core columns and floor systems at floors 77 
through 83 ofWTC 2 for the more severe case (Case D). Figure 7-57 presents the cumulative damage to 
WTC 2 on all affected floors and columns. 



254 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



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of WTC 2 (more severe case). 



NISTNCSTAR 1-2, WTC Investigation 



255 



Chapter 7 



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Figure 7-56. Summary of the floor-by-floor structural damage to the floors and columns 

of WTC 2 (more severe case) (continued). 



256 



A//ST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Severe Floor Damage 

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severe case). 



7.7.3 



Fuel and Debris Distributions 



The distribution of the fuel in the tower calculated from the more severe case in a plan view and side view 
is shown in Figure 7-58. At the termination of the global impact analysis, the residual momentum of the 
jet fuel was less than one percent of the initial momentum, indicating that the fuel cloud was nearly at 
rest. To more clearly present the calculated response of the structures that influenced the fire propagation, 
the structural components were removed from the visualization, with the exception that the core columns 
were maintained in the visuahzations for reference positions. A plan view of the response of the 
remaining building contents and aircraft debris is shown in Figure 7-59. 

The calculated damage to the WTC 2 contents for the more severe impact case is shown in plan views for 
floors 80 and 81 in Figure 7-60 and Figure 7-61, respectively. A comparison to the calculated damage 
for the base case WTC 2 impact analysis indicated that the tower contents damage zone was similar, with 
a slight increase in damage for the more severe impact. 



A summary of the floor-by-floor fuel and debris distributions is given in Table 7-10. The bulk of the fuel 
and aircraft debris was deposited in floors 78 through 80, with the greatest concentration on floor 80. The 
calculated debris cloud included 121,000 lbs of debris and 14,800 lbs of aircraft fuel outside of the tower 
at the end of the impact analysis, either rebounding from the impact face (north wall) or passing through 



NISTNCSTAR 1-2, WTC Investigation 



257 



Chapter 7 

the tower (south wall). This amount might have been larger in the calculation due to the reasons 
mentioned previously for the base case impact (see Section 7.3.3). Comparing Figure 7-59 and 
Table 7-10 with Figure 7-45 and Table 7-7, it can be seen that the amount of debris exiting the north 
wall of the tower in the more severe case was much larger than that from the base case. 



Fable 7-10. Fuel and aircraft debris distribution for the more severe WTC 2 impact. 


Tower Location 


Aircraft Fuel 


Aircraft Debris 


Total Outside Tower 


14,800 lb 


121,000 1b 


WTC 2 Floor 77 


1,3001b 


300 1b 


WTC 2 Floor 78 


7,400 lb 


2,500 lb 


WTC 2 Floor 79 


12,500 lb 


16,4001b 


WTC 2 Floor 80 


7,200 lb 


40,700 lb 


WTC 2 Floor 81 


10,000 1b 


21,4001b 


WTC 2 Floor 82 


10,200 1b 


1,4001b 


WTC 2 Floor 83 


1,4001b 


1,1001b 


WTC 2 Floor 84 


3001b 


400 1b 


Total Weight 


65,100 1b 


205,200 lb 



258 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Time = q 58 



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Figure 7-58. Calculated fuel distribution in the more severe WTC 2 analysis. 



NISTNCSTAR 1-2, WTC Investigation 



259 



Chapter 7 




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Figure 7-59. Plan view of calculated more WTC 2 building, fuel, and aircraft debris 

distribution for the more severe case. 



260 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




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Figure 7-60. Calculated floor 80 contents and fuel distribution (more severe case). 



NISTNCSTAR 1-2, WTC Investigation 



261 



Chapter 7 




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Figure 7-61. Calculated floor 81 contents and fuel distribution (more severe case). 



262 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



7.8 WTC 2 LESS SEVERE IMPACT ANALYSIS - BRIEF DESCRIPTION 

This section presents a brief description of the resuhs from the less severe damage case. The reader is 
referred to NIST NCSTAR 1-2B for further details. 

For the south exterior wall of WTC 2, the magnitude and mode of impact damage were still in good 
agreement with the observed damage for the less severe impact scenario. 

The core had significant damage in the region close to the impact point. The columns in line with the 
aircraft fuselage failed on the impact side, and several of the core beams were also severely damaged or 
failed in the impact zone. In some cases, failure of the column splices located on floors 77, 80, and 83 
contributed significantly to the failure of the core columns. A total of three columns were severed, and 
two columns heavily damaged, compared to five severed columns and four heavily damaged columns in 
the base case WTC 2 impact analysis. 

The truss floor system on floors 79 through 82 had sufficient damage from the impact that portions of the 
truss floor sections sagged downward as a result of the impact. The trusses experienced significant 
damage in the impact zone, with the heaviest damage on floor 81. The calculated impact response 
produced severe damage to the truss structures in the primary path of the fuselage. The truss structures 
were completely destroyed along the impact path on floor 81 from the exterior wall to the core. 

When compared with the base case, the magnitude of damage to the floor trusses and floor slabs was 
slightly increased for the less severe impact analysis. The parameters used in the less severe global 
impact analysis would primarily contribute to a reduced damage magnitude for the tower structures. 
However, the downward impact trajectory angle was increased from the 6 degree angle in the base case 
analysis to an 8 degree angle in the less severe impact analysis. This would have the effect of directing 
more of the impact energy downward, increasing the normal force on the floor structures in the impact 
zone. As a result, the combined effects of the analysis parameter variations produced very similar 
damage to the truss structure. 

A comparison to the base case and less severe case indicated that the building contents damage zone was 
similar, with a slight reduction in damage for the less severe impact. 

7.9 COMPARISON BETWEEN WTC 1 AND WTC 2 

The comparison of the aircraft impact response and resulting tower damage for WTC 1 and WTC 2 was 
complicated by the differences in the two impact scenarios. The base case WTC 1 impact was close to 
centered and perpendicular on the face of the tower, with the long-span trusses between the impact point 
and the core. The WTC 1 impact scenario resulted in a debris trajectory where almost all of the aircraft 
debris would pass through the core. The baseline impact conditions for WTC 1 were a 443 mph collision 
with a downward impact trajectory angle of 10.6 degrees. In contrast, the baseline WTC 2 impact was off 
center and angled away from the core, resulting in a significant fraction of the aircraft debris cloud 
outside (east) of the core. The WTC 2 impact had short-span trusses between the impact point and the 
core. Finally, the baseline impact conditions for WTC 2 were a 542 mph collision with a downward 
impact trajectory angle of 6 degrees. 



NIST NCSTAR 1-2, WTC Investigation 263 



Chapter 7 



7.9.1 Exterior Wall Damage 

The calculated exterior wall damage for the base case WTC 1 and WTC 2 impacts are compared in 
Figure 7-62. Despite the differences in impact conditions, the mode and magnitude of damage to the 
exterior walls were quite similar in both towers. This was because the impact loads distributed over the 
majority of the aircraft structures were much larger than the exterior column rupture strength. The details 
of the failure mode (column deformation and rupture or failure and separation of bolted column end 
connections) were determined by the proximity of the floor slab and column joints to the impact point. 
For both impacts, the wing tip structures imparted damage, but did not completely fail the columns. 

7.9.2 Core Column Damage 

The calculated core column damage for the base case WTC 1 and WTC 2 impacts are compared in 
Figure 7-63. In the WTC 1 impact, there were three columns severed and four columns heavily damaged. 
The calculated region of significant core column damage appeared to extend three column rows deep into 
the core. In contrast, the calculated damage for the WTC 2 impact included five columns severed and 
four columns heavily damaged, and the region of significant core column damage appeared to extend four 
column rows deep. This increase in core damage was even more significant since the impact zone was 
15 floors lower in WTC 2 (and therefore designed to carry more gravity loads), and as a result the core 
columns were heavier and more resistant to impact damage in the WTC 2 impact zone. 

The differences in the core column damage between WTC 1 and WTC 2 can be explained by two primary 
factors. The first was that the WTC 2 impact speed was 23 percent higher (approximately 50 percent 
larger impact energy), and the shallower impact angle directed more impact energy inward toward the 
core. The second factor was that the orientation of the core relative to the impact was different in the two 
towers, as the core was closer to the impact point in WTC 2. As a result, WTC 2 had reduced energy 
absorbing capacity due to the shorter floor structures and less building contents between the impact point 
and the core. 



264 NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



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Figure 7-62. Comparison of base case impact damage to the exterior walls of WTC 1 and 

WTC 2. 



NISTNCSTAR 1-2, WTC Investigation 



265 



Chapter 7 




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Figure 7-63. Comparison of base case impact damage to the core columns of WTC 1 and 

WTC 2. 



266 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



7.9.3 



Floor Truss Damage 



The calculated floor truss damage for the base case WTC 1 and WTC 2 impacts are compared in 
Figure 7-64. The comparison shows that the WTC 1 floor truss had greater damage and collapse of the 
truss floor despite the lower aircraft impact energy. The greater truss floor damage and deflection in 
WTC 1 can be explained by two factors. The primary factor was that the WTC 1 downward impact 
trajectory was nearly twice as steep as that of the WTC 2 impact. As a result, the steeper impact angle 
directed more impact energy normal to the floor slab. The vertical component of the impact load in 
WTC 1 was approximately 40 percent higher than in WTC 2. The secondary factor was that the damage 
to the long-span truss floors in the WTC 1 impact zone produced larger displacements than the 
corresponding damage level to the short-span truss region in WTC 2. 



Column 
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Figure 7-64. Comparison of base case impact damage to floor trusses of WTC 1 and 

WTC 2. 



7.10 



COMPARISON WITH OBSERVABLES 



The observable evidence available to help validate the global impact analyses included the following: 

• Damage to the building exterior documented by photographic evidence. 

• Floor damage visible from the building exterior documented by photographic evidence. 



NISTNCSTAR 1-2, WTC Investigation 



267 



Chapter 7 



• Aircraft debris external to the towers as documented by photographic evidence. 

• Eyewitness accounts from survivors who were inside portions of the buildings. 

Another observable was that each tower remained standing after sustaining the impact-induced structural 
damage. Analyses of the structural response of the damaged towers immediately after impact, presented 
in NIST NCSTAR 1-6, showed that this observable was met for both towers. Sections 7.10.1 and 7.10.2 
compare, for WTC 1 and WTC 2, respectively, these observables with the results of the simulations. 

7.10.1 Comparison with Observables on WTC 1 

Damage Comparison on the North Exterior Wall of WTC 1 

The most valuable observable from a modeling standpoint was the damage to the impacted exterior wall 
of each tower. The impact damage to the exterior walls was well documented, and the impact response 
did not depend much on unknown parameters, such as the detailed office layout on each floor. Good 
agreement of the calculated and observed damage profile indicated that the geometric modeling of the 
aircraft and the initial trajectory and orientation of the aircraft were accurate. The agreement of both the 
mode and magnitude of the structural damage on the impact wall served to partially validate the 
constitutive and damage modeling of the aircraft and exterior wall structures of the tower. The agreement 
in exterior wall damage, based on the modeling methodologies described in this report, contributed to the 
confidence that the damage predictions for the interior of the towers were reasonably estimated. 

Figure 7-65 provides the results of a detailed comparison between the observed and calculated damage 
(from the base case analysis) on the north wall of WTC 1. The comparison includes the mode, 
magnitude, and location of failure around the hole created by the aircraft impact. The color code included 
the following: (1) green circles indicating a proper match of the failure mode and magnitude between the 
observed and calculated damage; (2) yellow circles indicating a proper match in the failure mode, but not 
the magnitude; (3) red circles indicating that the failure mode and magnitude predicted by the calculation 
did not match that was observed; and (4) black circles indicating that the observed damage was obscured 
by smoke, fire, or other factors. The comparison shown in Figure 7-65 indicates that the overall 
agreement with the observed damage was very good. 



268 NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




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Figure 7-65. Comparison of observable and calculated base case impact damage to the 

north wall of WTC 1. 

Damage Comparison on the South Exterior Wall of WTC 1 

The exterior panel from column 329 to 331 between floors 94 through 96 on the south face of WTC 1 was 
knocked free by landing gear and possibly other debris (see NIST NCSTAR 1-5A). These columns were 
located in the center of the south wall of the WTC 1 . In both the base case and more severe damage 
global analyses, aircraft debris impacted the south face of the tower, as shown in Figure 7-66 and 
Figure 7-67, and exited the building. The figures also show the calculated landing gear debris for both 
simulations. None of the debris impacting the south wall happened to contain landing gear fragments. In 
the base case analysis, the debris impacted columns 328 to 330 at floor 96. In the more severe impact 
analysis, debris impacted columns 328 to 333 on both floors 95 and 96. In the base case analysis, very 
httle damage was done to the exterior panels on the south wall. However, damage was heavy in the more 
severe damage analysis, as shown in Figure 7-68. 



NIST NCSTAR 1-2, WTC Investigation 



269 



Chapter 7 



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Figure 7-66. Base case aircraft debris distribution in WTC 1. 



270 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



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Figure 7-67. More severe damage aircraft debris distribution in WTC 1. 



NISTNCSTAR 1-2, WTC Investigation 



271 



Chapter 7 



Column: 333 332 331 330 329 328 327 



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analysis. 

Because of model size constraints, the panels on the south face of WTC 1 were modeled with a very 
coarse resolution. Neither the spandrel splice joints nor exterior column butt joints were modeled. 
Column ends and spandrel edges were merged together. The model therefore underestimated the damage 
to the tower on this face. The calculated damage produced by the more severe impact, shown in 
Figure 7-68, indicated that columns 329-331 on floors 94 through 96 sustained substantial damage. Had 
a fine mesh been used on these columns, it is likely that they would have failed on floor 95, and possibly 
on 94 and 96. Based on the failure modes observed on the north face and on the speed and mass of the 
debris, the panel would potentially be knocked free by failing at the connections. 

Landing Gear Trajectory Comparison 

A portion of the main landing gear of AA 1 1 exited WTC 1 at the 94th or 95th floor and landed at the 
corner of Rector St. and West St. The debris consisted of a tire, wheel, brake assembly and hub of a main 
landing gear, as shown in Figure 7-69. Based on the final position of the landing gear and assuming the 
landing gear to be a projectile with a horizontal initial velocity, the exit speed of the landing gear from the 
south wall of WTC 1 can be estimated to be about 105 mph. Note that there is a significant uncertainty in 
this estimate associated with the exit trajectory, aerodynamic effects, landing position rather than final 
resting position of debris, etc. Another piece of landing gear debris, shown in Figure 7-70, was found 
embedded in what is postulated to be the panel containing columns 329, 330, 331, running from the 93rd 
to the 96th floors. This panel was dislodged from the building and found at Cedar Street near its 
intersection with West Street. As little other damage had been documented on the south face of WTC 1, 



272 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



it is postulated that the landing gear debris that landed at the corner of Rector St. and West St. also exited 
through this panel location. 

The amount of aircraft debris found to exit WTC 1 in the global impact analyses varied, as shown in 
Figure 7-67 and Figure 7-68. However, no portion of the landing gear was observed to exit the tower in 
the simulations, but rather was stopped inside, or just outside, of the core. In order to simulate the 
trajectory of specific pieces of aircraft debris, a fairly precise knowledge of the internal configuration of 
the building was needed. This is especially true with components passing through the core of the 
building, where some of the most massive building contents and partition walls were present. 
Uncertainties regarding the internal layout of each floor, such as the location of hallways or walls, could 
make the difference between debris from a specific component passing through or being stopped inside 
the tower. In addition, modeling uncertainties and assumptions might play a role in not matching the 
observable. 



^©2001 Charfes Marsh 




Figure 7-69. Landing gear found at the corner of West and Rector Streets. 



NISTNCSTAR 1-2, WTC Investigation 



273 



Chapter 7 




Figure 7-70. Landing gear found embedded in exterior panel 
knocked free from WTC 1. 

Stairwell Disruption Comparison 

According to eyewitness interviews, stairwells 1 (referred to also as stairwell A), 2 (stairwell C), and 3 
(stairwell B) inside the core were impassable at floor 92 and possibly above after the impact of AA 1 1 
(see NIST NCSTAR 1-7). The calculated base case stairwell disruption is shown in Figure 7-71 for 
floors 93 through 97. Stairwell positions are outlined with red boxes in the figure. No debris or 
disruption was observed to the core on floor 92 in the calculation, therefore, it is not shown in the figure. 
Recall that the global model for WTC 1 only contained partition walls in the core on floors 94 through 97. 
Therefore, the abihty to ascertain damage and/or debris in the stairwell on floors 92 and 93 was limited. 
The floor slab was removed from the view on floors 94 through 97 so that debris is more visible. 

Based on the calculated damage to, or debris in, the stairwells on floors 94 to 96, all three stairwells 
appear impassable. Given that falling debris in these areas would cause further subsequent damage to the 
floors below, as well as block passage on these floors, this result was reasonably consistent with the 
eyewitness accounts. 



274 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



N 



f 




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t 



(a) Floor 93 





(b) Floor 94 



(c) Floor 95 





(d) Floor 96 (e) Floor 97 

Figure 7-71. Base case stairwell disruption in WTC 1. 

Floor Damage Visible on the North Face of WTC 1 

One location where the damage to the WTC 1 truss floors could be observed was through the opening in 
the tower exterior produced by the aircraft impact. A photograph of the impact damage on the north face 
of WTC 1 is shown in Figure 7-72(a). The magnitude of damage was difficult to quantify as a result of 



NISTNCSTAR 1-2, WTC Investigation 



275 



Chapter 7 



the strong contrast in lighting between the tower interior and exterior and the smoke inside the building. 
However, the photograph shows that the truss floor was heavily damaged and/or removed in the primary 
impact zone. The depth of the floor damage extending into the tower could not be determined. 




© 2001 Roberto Rabanne / Corbis 
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 JJJ 



(a) Observed Damage 




(b) Calculated damage 
Figure 7-72. Observed and calculated WTC 1 damage (front view). 



276 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



A corresponding image of the calculated damage to the tower structures is shown in Figure 7-65(b). The 
structures beyond the start of the core were removed and replaced with a black background for 
comparison with the photograph. Although a quantitative comparison of the calculated and observed 
damage could not be made from the available damage photographs, the truss floor damage appeared to be 
consistent. 

7.10.2 Comparison with Observables on WTC 2 

Damage Comparison on the South Wall of WTC 2 

Figure 7-73 provides the results of a careful comparison between the observed and calculated damage 
(from the base case analysis) on the south wall of WTC 2. The comparison includes the mode, 
magnitude, and location of failure around the hole created by the aircraft impact. The comparison 
indicates that the overall agreement with the observed damage was very good. 

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Figure 7-73. Comparison of observable and calculated base case impact damage to the 

south wall of WTC 2. 



NISTNCSTAR 1-2, WTC Investigation 



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Chapter 7 



Damage Comparison on the North Wall of WTC 2 

From photographic evidence, such as that shown in Figure 7-74, damage on the north wall at the 
northeast corner of WTC 2 was documented and is shown in Figure 7-75. As mentioned earlier, there 
was significant uncertainty as to the actual layout of the workstations and other building contents on the 
impacted floors of the towers. Recall that generic workstation configurations were used to model these 
building contents, as shown in the northeast comer of WTC 2 in Figure 7-76(a). Uncertainties regarding 
this layout, such as missing partition walls and workstations, could make the difference between debris 
from a specific component passing through or being stopped inside the structure. The base case impact 
response of the northeast corner of WTC 2 on the 81st floor is shown in Figure 7-76(b). 




Figure 7-74. Impact damage to the northeast corner of the exterior wall of WTC 2. 



278 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Column 259 - unbroken and straight 
Column 258 - broken bolt connection 

- unloaded 
Column 257 - broken bolt connection 

-unloaded 
Column 256 - bows out slightly (inconclusive) 
Column 255 - unbroken and straight 
Column 254 - column severed over a 6 ft section. 

- outer web intact but not load bearing 
Column 253 - column severed over a 6 ft section 
Column 252 - appears intact and straight 
Column 251 

to 201 - intact and straight 



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Figure 7-75. Documented damage to the northeast corner of 

floor 81 of WTC 2. 

Aircraft debris on floor 81 of WTC 2 is shown in Figure 7-76(c), with the coloring depicting the residual 
speed of the debris field. Notice that some of the debris in this figure, weighing approximately 3,800 lb, 
was traveling at 1 10-150 mph and was projected to impact between columns 252 and 256. The leading 
debris was portions of the starboard main landing gear main strut and main landing gear beam. That 
significant debris was projected to impact in the region of significant damage shows positive agreement 
with damage evidence available for the north wall of WTC 2. 



NISTNCSTAR 1-2, WTC Investigation 



279 



Chapter 7 





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Figure 7-76. Base case response on the northeast corner of floor 81 of WTC 2. 



280 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



Stairwell Disruption Comparison 

According to eyewitness interviews, stairwells 2 and 3 on floor 78 of WTC 2 were impassable (see NIST 
NCSTAR 1-7). Stairwell 1 (referred to also as stairwell A), which was located in the northwest corner of 
the core, was passable. The calculated base case stairwell disruption is shown in Figure 7-77. 
Stairwells 1 and 2 (stairwell C) on floor 78 of WTC 2 were outside of the core column region as is shown 
in the figure. These stairwells were not included in the WTC 2 model. Therefore, a good assessment 
could not be made for stairwell 2. However, disruption to stairwell 3 (stairwell B) is shown in 
Figure 7- 77. By the damage shown in the figure, the stairwell appears to be impassable. As no damage 
or debris was seen in the northwest corner of the tower, the top right in the figure, stairwell 1 in this area 
of the core was likely unaffected. Both of these assessments were consistent with the eyewitness 
accounts. 




Figure 7-77. Base case stairwell disruption on floor 78 in WTC 2. 

Landing Gear Trajectory Comparison 

A portion of the landing gear of UAL 175 exited WTC 2 and landed on the roof of 45 Park Place (see 
FEMA 2002). No photographic evidence was available to document the size of the fragment and whether 
this was a nose or main landing gear. From the damage to the building, the landing gear fragment might 
have exited somewhere along the north wall between column 251 and the northeast comer on floor 81. 
Based on the final position of the landing gear and assuming the landing gear to be a projectile with a 
horizontal initial velocity, the exit speed of the landing gear from the north wall of WTC 2 can be 
estimated to be about 1 02 mph. Note that there is a significant uncertainty in this estimate associated with 
the exit trajectory, aerodynamic effects, landing position rather than final resting position of debris, etc. 

The calculated aircraft debris distribution and landing gear and engine debris distributions for UAL 175 
are shown in Figure 7-78 and Figure 7-79 for the base case and more severe case, respectively. A 
portion of the port main landing gear was seen to exit the building at approximately 230 mph in the more 
severe impact analysis, as shown in Figure 7-79(b). No landing gear debris exited the building in the 
base case. At the conclusion of the simulation, the base case analysis had a substantial piece of the 
starboard main landing gear still moving at approximately 130 mph that was expected to impact the 
northeast corner. 



NIST NCSTAR 1-2, WTC Investigation 



281 



Chapter 7 






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Figure 7-78. Base case damage aircraft debris distribution in WTC 2. 



282 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




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(b) Calculated engine and landing gear debris (t = 0.58 s) 
Figure 7-79. Aircraft debris distribution in the more severe WTC 2 impact. 



NISTNCSTAR 1-2, WTC Investigation 



283 



Chapter 7 



Engine Trajectory Comparison 

A portion of an engine also exited the tower at the northeast corner of the building and was found at the 
intersection of Murray and Church Streets. From the damage to the building, it was believed that the 
engine exited the building in this corner of WTC 2. Based on this trajectory, it was estimated that the 
engine exited the building at approximately 120 mph. The engine trajectories predicted from the base 
case global analysis are shown in Figure 7-80, which indicates that the engine that exited from the 
northeast corner of the tower is likely the starboard engine. The dotted line indicates the extrapolated 
engine flight path based in the initial trajectory of the starboard engine. Notice that this trajectory would 
result in engine fragments exiting at the northeast corner. In the simulations, the engines were projected 
to stop short of this position, although they follow the extrapolated trajectory reasonably well. 

Speed time-histories for the aft portion of the starboard engine are shown in Figure 7-81. The engine 
would typically breakup into smaller fragments from the forward section of the engine and a larger 
section from the aft end, as shown in Figure 7-82. In all simulations, the speed was seen to drop by 
approximately 200 mph due to impact with the exterior panel, floor slab, and floor truss. Interaction with 
these portions of the structure ended by approximately 0.12 s. This initial impact from the base case is 
shown in Figure 7-83. The engine debris then continued through the tenant space of the 81st floor, 
plowing through the workstations and contents. Whether or not the fragment passed over these contents, 
or if other debris and fuel removed the contents from the engine's path, affected the deceleration of the 
fragment. At the end of the simulation, the speed of the aft portion of the engine was below 80 mph, and 
it was more than 60 ft from the northeast corner of the building. For these calculations, it was estimated 
that the building contents would likely stop the engine fragment prior to impacting the northeast corner of 
the exterior wall. 

None of the three WTC 2 global impact simulations resulted in a large engine fragment exiting the tower. 
However, the impact behavior suggests that only minor modifications would be required to achieve this 
response. For example, if the starboard engine impact location was lowered by 1 to 2 ft, which is within 
the aircraft impact geometry uncertainty range, the engine would likely have had a greater residual speed 
inside the tower, over 1 00 mph. In the global analyses performed, the engine impacted the underside of 
the 82nd floor, as shown in Figure 7-76. This resulted in a large reduction in speed of approximately 
200 mph. In the component analyses, the engine speed decreased by roughly 60 mph when impacting an 
exterior panel alone. This additional speed would likely result in a large engine fragment exiting the 
northeast corner of the tower. 

Other minor modifications to the model could also result in a large engine fragment exiting the building. 
As mentioned previously, there was significant uncertainty in the distribution of building contents on the 
floors of the impact area. If any portion of the east side of WTC 2 was relatively free of office materials, 
the engine fragment would have been free to move relatively unrestricted and would have experienced 
little loss of speed. After the engine entered the structure, and without office materials, the engine 
fragment would only slow due to friction with the floor slab and occasional interaction with floor trusses 
above. After initially entering the building, the engine did not further penetrate the floor slab. Removing 
much of the building contents from the east side would result in the starboard engine fragment impacting 
the northeast corner of the tower with sufficient speed to exit the building. Little or no difference in core 
damage would result, as debris in this area had no chance of impacting the core. 



284 NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




t = 




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Figure 7-80. Starboard engine fragment trajectory in the base case global analysis of 

WTC2. 



NISTNCSTAR 1-2, WTC Investigation 



285 



Chapter 7 



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0,7 



286 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 





(a) Undamaged engine 



(b) Large engine fragment 



2001 George Marengo 




(c) Engine fragment found at Murray and Church St. 
Figure 7-82. Calculated and observed engine damage. 



NISTNCSTAR 1-2, WTC Investigation 



287 



Chapter 7 





t = 0.07 s 



t = 0.08 s 





t = 0.09 s 



t = 0.11 s 



Figure 7-83. Starboard engine impact with the south face of WTC 2 in the base case 

global analysis. 

Floor Damage Visible on the South Face of WTC 2 

One location where the damage to the WTC 2 truss floor could be observed was through the opening in 
the tower exterior produced by the aircraft impact. A similar comparison for the WTC 1 truss floor 
damage was shown in Figure 7-72. The magnitude of damage was difficult to quantify as a result of the 
strong contrast in lighting between the tower interior and exterior and the smoke inside the building. This 
was worse for WTC 2, where the prevailing wind and fire conditions resulted in larger quantities of 
smoke exiting through the opening on the impact face. The partial photographic evidence did suggest that 
a similar level of truss floor damage in the impact zone occurred for WTC 2. The severity and the depth 
of the floor damage extending into the tower could not be determined. Although a quantitative 
comparison of the calculated and observed damage could not be made from the available damage 
photographs, the truss floor damage appeared to be consistent. 



288 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



The 'Cold Spot' on the North Face of WTC 2 

A 'cold spot' was observed on the north face of the tower between columns 238 and 250 on floors 80, 81, 
and 82. The cold spot was a region of the tower where no debris could be seen from the exterior of the 
tower and no significant fires were observed prior to tower collapse. 

Much of the explanation for the cold spot was obtained from an analysis of the debris trajectory aligned 
with the cold spot. The debris path, obtained by projecting the width of the cold spot along the initial 
lateral impact trajectory of the aircraft, is shown in Figure 7-84 (13 degrees relative to the tower face 
normal). This region was aligned laterally with the left side of the fuselage and the port wing structures. 
Considering the baseline impact orientation and trajectory, shown in Figure 7-85, it can be seen that 
much of the wing debris impacted on floors lower than the observed cold spot. Only debris from very 
close to the fuselage would be expected on floor 80 or above. The debris from the port wing, including 
the majority of the aircraft fuel in the left side tanks, entered at floors 78 and 79. 

The base case WTC 2 global analysis calculated a small amount of aircraft debris passing through the 
cold zone on floors 80 and 81. However, the building contents were not completely modeled over the 
entire path in this section. After clearing the core region, the debris in the calculation had primarily an 
open path to the cold spot on the north wall of WTC 2. If all of the internal contents had been included, it 
is likely that all of this debris would have been stopped before reaching the cold spot. 

The comparison of the calculated and observed impact response cold spot is inconclusive. Much of the 
absence of damage and aircraft debris in this region is explained by the impact orientation and trajectory. 
Much of this region was not directly in the path of significant aircraft fuel and debris. In addition, the 
debris aligned with the cold spot would be required to pass through a significant portion of the core. A 
more accurate analysis of the impact mechanics leading to the formation of a cold spot would require a 
specific survey of the tenant layout, including both contents that acted as a barrier to the debris and walls 
that provided a barrier to subsequent fire propagation. 



NIST NCSTAR 1-2, WTC Investigation 289 



Chapter 7 



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Figure 7-84. Projected debris path for the WTC 2 north face cold spot. 

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290 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



7.10.3 Summary 

In general, not all of these observables were perfectly matched by the impact simulations due to the 
uncertainties in exact impact conditions, the imperfect knowledge of the interior tower contents, the 
chaotic behavior of the aircraft breakup and subsequent debris motion, and the limitations of the models. 
In general, however, the results of the simulations matched these observables reasonably well. Examples 
where the simulations matched the observables included: (1) the damage to the exterior walls of both 
towers, (2) the disruption to the stairwells in both towers, (3) the landing gear trajectory and the cold spot 
on WTC 2. 

7.11 COMPARISON WITH PREVIOUS STUDIES 

Two previous studies were conducted to estimate the impact damage to the WTC towers. These studies 
were performed by Massachusetts Institute of Technology (MIT) (Wierzbicki, Xue, and Hendry- 
Brogan, 2002) and Weidlinger Associates, Inc. (WAI) (Levy and Abboud 2002). The MIT study used an 
energy balance approach to estimate damage to the core columns. Estimates were made for the initial 
kinetic energy of the impacting aircraft, and the internal energy absorbed in fragmentation of the aircraft 
and damage to the tower exterior columns, floor slab, and core columns. The energy absorbed by the core 
was used to estimate the number of failed core columns. 

The WAI study used the FLEX finite element code to calculate the aircraft impact damage to both towers. 
The FLEX family of finite element modeling software (Vaughan 1997) was developed and maintained by 
WAI. FLEX is an explicit, nonhnear, large deformation transient analysis finite element code for the 
analysis of structures subjected to blast, impact, and shock loadings. The overall code architecture is 
similar to that of LS-DYNA, used to calculate the aircraft impact damage in this investigation. 

In the WAI calculations, the aircraft and WTC towers models were composed of beam and shell elements. 
The aircraft model consisted of 27,000 shell elements and 23,000 beam elements. The aircraft fuel was 
included in the model by increasing the mass of the structures in the wing box. The tower models 
included the exterior wall on the impact face and the floor structures and the core frame for floors 91-101 
and floors 76-86 for WTC 1 and WTC 2, respectively. The tower models had flxed boundary conditions 
at the top and bottom floors. 

7.11.1 Comparison of Exterior Wall Damage 

The calculated base case impact damage to the exterior north wall of WTC 1 from this study is compared 
to the impact damage calculated by WAI in Figure 7-86. The flgure also shows a schematic of the 
damage observed in photographic evidence. Figure 7-87 shows a similar comparison for the south wall 
of WTC 2. In both towers, the base case impact damage estimated in this study closely matched the 
observed damage. The damage proflles in the WAI impact simulations had some noticeable differences. 
The first was that the damage profile included complete failure of the exterior columns over the entire 
length of the wings and to the top of the vertical stabilizer. The second difference was that the failure 
mode of the exterior walls was dominated more by local rupture of the columns adjacent to the impact 
point with less infiuence of the bolted connections on panel failure and removal. 



NIST NCSTAR 1 -2, WTC Investigation 29 1 



Chapter 7 



The differences in the damage profiles in the two calculations most likely resulted from a variety of 
differences in the models. One major difference between the two studies was in the fidelity of the aircraft 
models. The WAI Boeing 767 model was based on their model of a Lockheed C-141B military transport. 
In the WAI model, the external geometry of the C-141B was modified to fit the dimensions of the 767, 
but the internal components, such as stiffener configuration, as well as material thicknesses and properties 
remained the same. The differences in the internal structure and materials could affect the way the 
aircraft responded to the impact. The aircraft model used in this study also contained an order of 
magnitude more elements (70,000 bricks, 562,000 shells, and 61,000 SPH particles) than the WAI model 
(27,000 shell elements and 23,000 beam elements). The higher resolution of the NIST model could also 
account for significant differences in the determination of the impact load distribution and resulting 
exterior damage. Additionally, the NIST model explicitly modeled the fuel. If the fuel mass in the WAI 
model was spread out further toward the wing tips as part of the wing structure, it would be expected that 
the calculated column damage would extend over a wider portion of the wings. 

Secondary differences in the WAI and NIST impact analyses included, but were not limited to, variations 
in impact conditions (impact speed, orientation and trajectory, location, etc.), aircraft model differences 
(airframe geometry, component thicknesses, mass distribution, material properties, etc.) and tower model 
differences (material properties, geometry, joint modeling, number of elements, etc.). 



292 NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 




(a) Schematic of observed damage 



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Figure 7-86. Comparison of impact damage to the north wall of WTC 1. 



NISTNCSTAR 1-2, WTC Investigation 



293 



Chapter 7 




(a) Schematic of observed damage 



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Figure 7-87. Comparison of impact damage to the south wall of WTC 2. 



294 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



7.11.2 Comparison of Core Column Damage 

Table 7-1 1 compares the estimated core column damage from the various studies. For WTC 1, MIT 
(Wierzbicki, Xue, and Hendry-Brogan 2002) estimated that 4 to 12 core columns were failed. This MIT 
estimate of core columns was based on energy balance calculations and corresponded to a damage 
distribution ranging from four columns failed over a three-story length to 12 columns failed over a single 
floor length. The expected distribution of damage would fall between these bounds, with some columns 
damaged on a single floor and others with damage distributed on multiple floors. WAI gave two 
estimates for core column failure. The first estimate of 23 core columns failed and five damaged was 
obtained from the FLEX impact analysis. The second estimate of 20 failed columns was the number used 
in their collapse analysis. The NIST base case impact damage of three severed and four heavily damaged 
and less severe estimate of one severed and two heavily damaged fall below both the MIT and WAI 
estimates. The more severe estimate of six severed and three heavily damaged falls in the middle of the 
MIT range, but still well below the WAI estimates. 

A similar trend in the predicted damage to the core columns was found in the WTC 2 analysis. MIT 
estimated seven to 20 columns failed (from seven columns failed over a three-story length to 20 columns 
failed over a single fioor length). WAI calculated 14 core columns failed and another 10 damaged in their 
FLEX analysis, but reduced the number of failed columns to five for their collapse analysis. The NIST 
base case impact damage of five severed and four heavily damaged, as well as the more severe estimate 
of 10 severed and one heavily damaged fall in the middle of the range predicted by MIT. The less severe 
impact scenario predicted fewer columns severed and heavily damaged than the MIT and WAI studies. 

The MIT prediction of the number of failed core columns agreed remarkably well with the NIST 
estimates using their simphfied analysis. Differences may be a result of the estimates of material 
properties and structural geometry used (MIT did not have access to the detailed structural drawing of the 
WTC towers for their study), approximations in the estimates of damage mode and resulting energy 
absorption, as well as the fact that the MIT study did not include the energy absorbed by internal tower 
contents. 

The WAI impact analysis predicted much greater core column failure and damage than the NIST 
estimates. One reason for the greater damage prediction may be the lack of internal tower contents in the 
WAI model, such as workstations and other live loads. This study found that the internal tower material 
absorbed a significant amount of the impact energy and, therefore, reduced the loads applied to the core 
columns. Another reason for the greater damage prediction in the WAI study could result from the 
aircraft model. As noted above, the WAI aircraft impact simulation overpredicted the extent of column 
damage and failure on the exterior wall. It is possible to assume that the aircraft model would also 
overpredict the damage to the core columns, especially that this damage configuration resulted in an 
unstable tower (Levy and Abboud, 2002). 

In conducting a collapse analysis, WAI used engineering estimates to reduce the number of failed 
columns from that predicted by their FLEX model to stabilize the tower immediately after impact. 
Despite this adjustment, the WAI study still estimated significantly greater damage for WTC 1 than the 
MIT and NIST studies. For WTC 2 their adjusted estimate falls in hne with the MIT and NIST studies. 

In general, the MIT and WAI studies appear to over-predict the damage to the core columns compared to 
the NIST estimates. 



NIST NCSTAR 1-2, WTC Investigation 295 



Chapter 7 



Table 7-11. Comparison of damage to core columns from various studies 



WTC Impact Study 


WTC 1 Core Column Damage 


WTC 2 Core Column Damage 


MIT 
Impact Analysis 


4-12 Severed 


7-20 Severed 


WAI 

Impact Analysis 


23 failed & significantly damaged 
Plus 5 damaged 


14 failed and significantly damaged 
Plus 10 damaged 


WAI 

Collapse Analysis 


20 Failed 


5 Failed 


NIST Base Case 
Impact Analysis 


3 Severed 
Plus 4 Heavily Damaged 


5 Severed 
Plus 4 Heavily Damaged 


NIST More Severe 
Impact Analysis 


6 Severed 
Plus 3 Heavily Damaged 


10 Severed 
Plus 1 Heavily Damaged 


NIST Less Severe 
Impact Analysis 


1 Severed 
Plus 2 Heavily Damaged 


3 Severed 
Plus 2 Heavily Damaged 



7.12 



SUMMARY 



Presented in this chapter were estimates of damage to the WTC towers due to aircraft impact, calculated 
from the global impact simulations. The results indicated significant structural damage to the exterior 
walls, core columns, and floor systems in the affected floors. This structural damage contributed to the 
weakening of the tower structures, but did not, by itself, initiate building collapse. The aircraft impact 
damage, however, contributed greatly to the subsequent fires and the thermal response of the tower 
structures that led ultimately to the collapse of the towers by: 

• Dispersing jet fuel and igniting building contents over large areas 

• Creating large accumulations of combustible materials containing aircraft and building 

contents 

• Increasing the air supply into the damaged buildings that permitted significantly higher 
energy release rates than would normally be seen in ventilation building fires, allowing the 
fires to spread rapidly on multiple floors (see NIST NCSTAR 1-5F) 

Other effects of the impact on the towers were investigated in other projects of the Investigation based on 
the results reported herein. These included: (1) damage and dislodging of fireproofmg from structural 
components in the direct path of the debris (see NIST NCSTAR 1-6), (2) damage to the sprinkler and 
water supply systems in the path of the aircraft debris (see NIST NCSTAR 1-4), and (3) damage to 
ceilings that enabled unabated heat transport over the floor-to-ceiling partition walls and to structural 
components (see NIST NCSTAR 1-5D). 



296 



NIST NCSTAR 1-2, WTC Investigation 



Aircraft Impact Damage Results 



7.13 REFERENCES 

"LS-DYNA Keyword User's Manual," Livermore Software Technology Corporation, Version 970, 
April 2003. 

FEMA, May 2002, "World Trade Center Building Performance Study: Data Collection, Preliminary 
Observations and Recommendations," FEMA 403. 

Kausel, E., "The Towers Lost and Beyond," Massachusetts Institute of Technology, May, 2002, 
http://web.mit.edu/civenv/wtc/index.html. 

Levy, M. and Abboud N., August 1, 2002, "World Trade Center - Structural Engineering Investigation," 
Hart-Weidlinger Technical Report, Prepared for Wachtel, Lipton, Rosen, and Katz. 

Vaughan, D. K., FLEX User's Guide, Report UG8298, Weidlinger Associates, Los Altos, CA, May 1983 
plus updates through 1997. 

Wierzbicki, T., Xue, L., and Hendry-Brogan, M. (2002). "Aircraft impact damage of the World Trade 
Center towers." Tech. Rep. No. 74, Impact and Crashworthiness Lab, Massachusetts Institute of 
Technology, Cambridge, MA. 



NIST NCSTAR 1-2, WTC Investigation 297 



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298 NIST NCSTAR 1-2, WTC Investigation 



Chapters 

Findings 



8.1 BASELINE PERFORMANCE ANALYSIS 

8.1.1 Wind Loads on the World Trade Center Towers 

Various wind load estimates for the World Trade Center (WTC) towers were considered in this study. 
These included: (1) wind loads used in the original WTC design, (2) wind loads based on two recent wind 
tunnel studies conducted in 2002 by Cermak Peterka Peterson, Inc. (CPP) and Rowan Williams Davis and 
Irwin, Inc. (RWDI) for insurance litigation concerning the towers, and (3) refined wind loads estimated 
by National Institute of Standards and Technology (NIST) by critically assessing information obtained 
from the CPP and RWDI reports and by bringing to bear state-of-the-art considerations. The following 
summarizes the study findings. 

Finding 1: The original design wind loads on the towers exceeded those established in the prescriptive 
provisions of the New York City Building Code (NYCBC) prior to 1968, when the WTC towers were 
designed, and up to and including 2001. The original design load estimates were also higher than those 
required by other selected building codes of the time (Chicago and New York State), including the 
relevant national model building code. Building Officials and Code Administrators (BOCA). The 
prescriptive approach in these codes is oversimplified, and as a result, these codes are not necessarily 
appropriate for super-tall building design. This fmding is supported by the fact that wind effects obtained 
from three separate wind-tunnel-based studies (the original WTC design, the CPP, and the RWDI studies) 
were in all cases higher than wind effects based on the prescriptive codes. 

Finding 2: In the majority of the cases, each of the two orthogonal shear components and of the two 
orthogonal overturning moment components at the base of the towers used in the original wind design 
were smaller, than the CPP, RWDI, and refmed NIST estimates. However, the most unfavorable 
combined peaks (resultant) from the original design were larger, or smaller by at most 1 5 percent, than 
estimates based on the CPP, RWDI, and NIST estimates. This is due to the conservative approach used to 
combine the loads in the original design. For example, the refined NIST estimates were higher by as 
much as 15 percent than the most unfavorable original design wind loads for WTC 1, and lower by about 
5 percent than the most unfavorable original design loads for WTC 2. 

Finding 3: The estimated wind-induced loads on the towers varied by as much as 40 percent between the 
wind tunnel/climatological studies conducted in 2002 by CPP and RWDI. The primary reason for these 
differences was the different approaches used in those studies to (1) estimate extreme wind speeds; 
(2) estimate wind profiles; (3) integrate aerodynamic, dynamic, and extreme wind climatological 
information; and (4) combine wind effects in two orthogonal directions and in torsion. Such disparity is 
indicative of the limitations and inconsistencies associated with the current state of practice in wind 
engineering for tall buildings. Among the issues that need to be considered are: 

• Estimation methods for combining directional wind loads, integrating climatological (wind) 
and aerodynamic (wind tunnel) data. 

NIST NCSTAR 1-2, WTC Investigation 299 



Chapter 8 



• Evaluation of the wind speed specifications and the development of improved design wind 
speeds, as well as protocols for selection of site-specific wind speeds and directionality. 

• Protocols for conducting the wind tunnel tests. 

• Profiles of hurricane and non-hurricane winds. 

• Load combinations, and material-specific responses to peak loads. 

Finding 4: A comparison of wind speeds indicated significant differences among various specified 
design wind speeds. The basic wind speed specified in American Society of Civil Engineers 
(ASCE 7-02) for New York City is equivalent to an 88 mph fastest-mile wind speed at 33 ft above ground 
for open terrain exposure. The wind speed specified in the New York City Building Code (2001) is 
80 mph and is interpreted to be a fastest-mile wind speed at 33 ft above ground. For the original WTC 
design, a design wind speed of 98 mph averaged over 20 minutes at a height of 1,500 ft above ground was 
used. This speed is equivalent to a fastest- mile wind speed at 33 ft above ground in open terrain of 
between 67 mph and 75 mph. The wind speed estimated by NIST for three airports (La Guardia, Newark 
International Airport, and John F. Kennedy International Airport), regardless of direction, was equivalent 
to 96 mph fastest-mile wind speed. An evaluation of the wind speed specifications and the development 
of improved design wind speeds, as well as protocols for selection of site-specific wind speeds and 
directionality, are, therefore, in order. 

8.1.2 Baseline Performance of the Global Tower Models 

The global models of the towers were analyzed under the following gravity and wind loading cases: 
(1) the original WTC design load case, (2) the lower-estimate state-of-the-practice case (NYCBC 2001 
gravity loads plus wind loads from the RWDI study, scaled in accordance with NYCBC 2001 wind 
speed), and (3) the refined NIST estimate case (gravity loads from ASCE 7-02 plus refined wind loads 
developed by NIST). The following summarizes the findings from the analyses. 

Finding 5: Under the original WTC design loads, the cumulative drifts at the top of the WTC towers 
ranged from H/263 to H/335, where H is the building height. For the lower-estimate state-of-the-practice 
case, those drifts ranged from H/253 to H/306. Under design loading conditions, the maximum inter- 
story drift was as high as h/230 and h/200 for WTC 1 and WTC 2, respectively, where h is the story 
height. Maximum inter-story drifts under the state-of-the practice case were about h/184 and h/200 for 
WTC 1 and WTC 2, respectively. For the refined NIST estimate case, the cumulative and inter-story 
drifts were about 25 percent larger than those from the state-of-the practice case. Currently no building 
codes specify a drift limit for wind design. The commentary to Section B.1.2 of the ASCE 7 Standard 
indicates that drift limits in common usage for building design are on the order of 1/400 to 1/600 of the 
building (for total drift) or story height (for inter-story drift) to minimize damage to cladding and 
nonstructural walls and partitions. Structural engineers often use in their practice the criterion that total 
drift ratios should not exceed H/400 to H/500 for serviceability considerations and to enhance overall 
safety and stability (including second order P-A effects). For inter-story drifts, structural engineers often 
use in their practice an inter-story drift limit in the range of h/300 to h/400. This is primarily done for 
serviceability considerations. Similar to total drift, inter-story drifts of the towers were larger than what is 
generally used in current practice. 



3 00 NIST NCSTAR 1 -2, WTC Investigation 



Findings 



Finding 6: The demand/capacity ratios (DCRs), based on the allowable stress design procedure, 
estimated from the original WTC design load case were in general close to those obtained for the lower- 
estimate state-of-the practice case. For both cases, a fraction of the structural components had DCRs 
larger than 1.0. These were mainly observed in both towers at (1) the exterior walls: (a) at the columns 
around the comers, (b) where the hat truss connected to the exterior walls, and (c) below floor 9; and (2) 
the core columns on the 600 line between floors 80 and 106 and at core perimeter columns 901 and 908 
for much of their height. The DCRs obtained for the refined NIST estimate case were higher than those 
for the original WTC design and the lower-estimate state-of-the-practice load cases, owing to the 
following reasons: (1) the NIST estimated wind loads were larger than those used in the state-of-the- 
practice case by about 25 percent, and (2) the original WTC design and the state-of-the-practice cases 
used NYCBC load combinations, which result in lower DCRs than the ASCE 7-02 load combinations 
used for the refined NIST case. 

Finding 7: The safety of the WTC towers on September 1 1, 2001 was most likely not affected by the 
fraction of members for which the demand exceeded allowable capacity due to: (1) the inherent factor of 
safety in the allowable stress design method, (2) the load redistribution capability of ductile steel 
structures, and (3) on the day of the attack, the towers were subjected to in-service live loads (a fraction of 
the design live loads) and minimal wind loads. 

Finding 8: The behavior of the lower portion of the towers at the basement floors resembled that of a 
braced frame, while the behavior of the super-structure resembled that of a framed tube system based on 
the analysis of the axial stress distribution in the columns under wind loads. Under a combination of the 
original WTC design dead and wind loads, tension forces developed in the exterior walls of both towers. 
The forces were largest at the base of the building and at the corners. These tensile column loads were 
transferred from one panel to another through the column splices. The DCRs for the exterior wall splice 
connections under these tensile forces for both towers were shown to be less than 1.0. 

Finding 9: For the towers' resistance to shear sliding under wind loads, the factor of safety was between 
10 and 1 1.5, while the factor of safety against overturning ranged from 1.9 to 2.7 for both towers. 

8.1.3 Baseline Performance of the Typical Floor Models 

Finding 10: For the typical truss-framed floor under the original WTC design gravity loads, the DCRs 
for all floor trusses were less than unity for 99.4 percent of the floor truss components with a maximum of 
1.14. Inside the core, the DCRs for all floor beams were less than 1.08, and more than 99 percent of floor 
beams had a DCR of less than 1.0. The maximum mid-span deflections of the long-span and short-span 
zones under the original design loads were approximately 1.79 in. (~ L/400) and 0.57 in. (~ L/750), 
respectively, where L is the floor span. For the typical beam-framed floor under the original design 
gravity loads, the DCRs for all floor beams were less than 1.0 except for two core beams, where the 
DCRs in shear were 1.125 and 1.09. The maximum mid-span deflections of the long-span and short-span 
zones under the original design loads were approximately 1.55 in. (~ L/450) and 0.70 in. (~ L/600), 
respectively. 



NIST NCSTAR 1-2, WTC Investigation 301 



Chapter 8 

8.2 AIRCRAFT IMPACT DAMAGE ANALYSIS 

8.2.1 Safety of the WTC Towers in Aircraft Collision 

Finding 11: Buildings are not specifically designed to withstand the impact of fuel-laden commercial 
aircraft, and building codes in the United States do not require building designs to consider aircraft 
impact. Documents obtained from The Port Authority of New York and New Jersey indicated that the 
safety of the WTC towers and their occupants in an aircraft collision was a consideration in the original 
design. The documents indicate that a Boeing 707, the largest commercial aircraft at the time, flying at 
600 mph was considered and that the analysis indicated that such collision would result in only local 
damage which could not cause collapse or substantial damage to the building and would not endanger the 
lives and safety of occupants not in the immediate area of impact. No documentary evidence of the 
aircraft impact analysis was available to review the criteria and methods used in the analysis of the 
aircraft impact into the WTC towers, or to provide details on the ability of the WTC towers to withstand 
such impacts. 

8.2.2 Preliminary Impact Analyses (Component and Subassembly Levels) 

Component and subassembly impact analyses were conducted with the objectives of (1) developing 
understanding of the interactive failure phenomenon of the aircraft and tower components, (2) developing 
simulation techniques required for the global analysis of the aircraft impacts into the WTC towers, and 
(3) investigating different modeling techniques and associated model size, run times, numerical stability, 
and impact response. The following summarizes the analyses' findings: 

Finding 12: Impact of a Boeing 767 engine at a speed of 500 mph on an exterior wall panel resulted in a 
complete penetration of the engine through the exterior wall and failure of impacted exterior columns. If 
the engine did not impact the floor slab, the majority of the engine core would remain intact through the 
exterior wall penetration, with a reduction in speed between 10 and 20 percent. The residual velocity and 
mass of the engine after penetration of the exterior wall was sufficient to fail a core column in a direct 
impact condition. Interaction with interior building contents prior to impact, or a misaligned impact 
against the core column, could alter this response of the core column. 

Finding 13: An impact of an empty wing segment from approximately mid-span of the wing normal to 
the exterior wall produced significant damage to the exterior columns but not complete failure. Impact of 
the same wing section, but filled with fuel, resulted in extensive damage to the external panels of the 
tower, including complete failure of the exterior columns. The resulting debris propagating into the 
building maintained the majority of its initial momentum prior to impact. 

Finding 14: The response of the nonstructural building contents and the fioor concrete slab to an aircraft 
engine impact was dominated by the mass of the workstations and the concrete slab, rather than by their 
strength. 

8.2.3 Aircraft Impact Damage Results 

The global analyses of the aircraft impacts into the WTC towers provided the following: (1) estimates of 
probable damage to structural systems, (2) estimates of the aircraft fuel dispersion during the impact, and 
(3) estimates of debris damage to the building nonstructural contents, including partitions and 

3 02 NIST NCSTAR 1 -2, WTC Investigation 



Findings 



workstations. The global analyses included, for each tower, a "base case" based on reasonable initial 
estimates of all input parameters. They also provided a range of damage estimates of the towers due to 
aircraft impact. These included a more severe and a less severe damage estimates. The initial impact 
conditions were estimated based on a detailed analysis of video records that captured the approach and 
impact of the aircraft with the towers and the photographs of the exterior tower damage. The following 
summarizes the analyses findings: 

Finding 15: The aircraft that impacted WTC 1 had a speed of 443±30 mph with a roll angle of 
25±2 degrees (port wing downward). The vertical approach downward angle was 10.6±3 degrees, and the 
lateral approach angle was close to being normal to the north wall of the tower. For WTC 2, the 
impacting aircraft had a speed of 542±24 mph, with a roll angle of 38±2 degrees (port wing downward). 
The vertical approach downward angle was 6±2 degrees, and the lateral approach angle was 13±2 degrees 
clockwise from the south wall of the tower. 

Finding 16: The aircraft impact on WTC 1 resulted in extensive damage to the north wall of the tower, 
which failed in the regions of the fuselage, engine, and fuel-filled wing section impacts. Damage to the 
exterior wall extended to the wing tips, but the exterior columns were not completely failed in the outer 
wing and vertical stabilizer impact regions. According to photographs, columns 1 12 to 144 along with 
column 151 were completely severed, while columns 145 to 148 were heavily damaged, and columns 149 
to 150 were moderately damaged (for reference, columns 101 and 159 are located on the west and east 
corner, respectively, of the north wall). The results of the impact analyses matched well with this damage 
pattern to the north wall. Photographic evidence also indicated that an exterior panel with columns 329, 
330, and 331 on the south wall between floors 94 to 96 was dislodged. Failure of the exterior columns 
occurred both at the bolted connections between column ends and at various locations in the column, 
depending on the local severity of the impact load and the proximity of the bolted connection to the 
impact. Subject to the uncertainties inherent in the models, the global impact simulations indicated that a 
total of three core columns were severed, and four columns were heavily damaged in the base case, 
compared to six columns severed and three columns heavily damaged in the more severe case and one 
columns severed and two columns heavily damaged in the less severe case. In the analyses, the floor 
trusses, core beams, and floor slabs experienced significant impact-induced damage on floors 94 to 96, 
particularly in the path of the fuselage. The analyses indicated that the wing structures were completely 
fragmented due to the interaction with the exterior wall and as a result, aircraft fuel was dispersed on 
multiple floors. In addition, aircraft debris resulted in substantial damage to the nonstructural buildings 
contents (partitions and workstations) and also in dislodging of fireproofmg. The bulk of the fuel and 
aircraft debris was deposited in floors 93 through 97, with the largest concentration on floor 94. 

Finding 17: The aircraft impact on WTC 2 resulted in extensive damage to the south wall of the tower, 
which failed in the regions of the fuselage, engine, and fuel-filled wing section impacts. Damage to the 
exterior wall extended to the wing tips, but the exterior columns were not completely failed in the outer 
wing and vertical stabilizer impact regions. According to photographs, columns 410 to 436 and columns 
438 to 439 were completely severed, while column 437 was heavily damaged (for reference, columns 401 
and 459 are located on the east and west corner, respectively, of the south wall). The results of the impact 
analyses matched this damage pattern to the south wall well. In addition, columns 407 to 409 were 
obscured by smoke, but the analysis results indicated that these columns were moderately damaged. 
Photographic evidence also indicated that columns 253, 254, 257, and 258 on the north wall were failed. 
Failure of the exterior columns occurred both at the bolted connections between column ends and at 



NIST NCSTAR 1-2, WTC Investigation 303 



Chapter 8 



various locations in the column, depending on the local severity of the impact load and the proximity of 
the bolted connection to the impact. Subject to the uncertainties inherent in the models, the global impact 
simulations indicated that a total of five core columns were severed, and four columns were heavily 
damaged in the base case, compared to ten columns severed and one column heavily damaged in the more 
severe case and three columns severed and two columns heavily damaged in the less severe case. In some 
cases, failure of the column splices located on floors 77, 80, and 83 contributed significantly to the failure 
of the core columns. In the analyses, the floor trusses, core beams, and floor slabs experienced significant 
impact-induced damage on floors 79 to 81, particularly in the path of the fuselage. The analyses indicated 
that the wing structures were completely fragmented due to the interaction with the exterior wall and as a 
result, aircraft fuel was dispersed on multiple floors. In addition, aircraft debris resulted in substantial 
damage to the nonstructural buildings contents (partitions and workstations) and also in dislodging of 
fireproofing. The bulk of the fuel was concentrated on floors 79, 81, and 82, while the bulk of the aircraft 
debris was deposited in floors 78 through 80, with the largest concentration on floor 80. 

Finding 18: Natural periods calculated from the reference global model of the WTC 1 tower matched 
well with those measured on the tower based on the analysis of data from accelerometers located atop 
WTC 1. The calculated period of oscillation in the N-S direction of the reference global model of WTC 2 
matched well with the period estimated immediately after aircraft impact based on a detailed analysis of 
the building motion which was captured in a video footage of the WTC 2 impact. This indicated that the 
overall lateral stiffness of the tower was not affected appreciably by the impact damage. The maximum 
deflection at the top of the tower after impact was estimated from the footage to be more than 1/3 of the 
drift resulting from the original design wind loads. This indicated that the tower still had reserve capacity 
after losing a number of columns and floor segments due to aircraft impact. 

Finding 19: The towers sustained significant structural damage to the exterior walls, core columns, and 
floor systems due to aircraft impact. This structural damage contributed to the weakening of the tower 
structures, but did not, by itself, initiate building collapse. However, the aircraft impact damage 
contributed greatly to the subsequent fires and the thermal response of the tower structures that led 
ultimately to the collapse of the towers by: (1) dispersing jet fuel and igniting building contents over large 
areas, (2) creating large accumulations of combustible materials containing aircraft and building contents, 
and (3) increasing the air supply into the damaged buildings that permitted significantly higher energy 
release rates than would normally be seen in ventilation building fires, allowing the fires to spread rapidly 
on multiple floors. 



3 04 NIST NCSTAR 1 -2, WTC Investigation 



AppencfxA 

Salient Points with Regard to the Structural Design of the 

World Trade Center Towers 



Reproduced with permission of The Port Authority of New York and New Jersey. 



NIST NCSTAR 1-2, WTC Investigation 305 



Appendix A 



^Ut^y..c)^^ 



Saliert points with regard to the structural design of Ihe 
World Trade Center "jowers: 

1. Tna structural analysis carried out by the firm of Worthington, • 
Skilling, Halle & Jackson is the most complete and detailed of any 
ever made for any building structure. The preliminary calculations 
.•■.lone cover 1,200 pages' and involve over 100 detailed drawings. 

2. The buildings have been designed for wind loads of 45 lbs, per square 
foot which is 1\, times the New York City Building Code requirements 
of 20 lbs. per square foot, the design load for the Empire State, 
Pan American and Chrysler Buildings. In addition to static wind 
-o.-,ci, z. co:'.-.plete c'.ynamic analysis hit bean iiade to take into account 
extremely high velocity gusts. 

3. The buildings have been investigated and found to be safe in an assumed 
collision with a large jet airliner (Boeing 707 - DC 8) travelling 

at 600 miles per hour. Analysis indicates that such collision would 
result in only local damage which could not cause collapse or 
substantial damage to the building and would not endanger the lives 
and safety of occupants not in the immediate area of impact. 

4- Because of its configuration, which is essentially that of a beam 209' 
deep, the towers are actually far less daring structurally than a 
conventional building such as the Empire State where the spine or 
braced area of the building -is far smaller in relation to the height. 

5. The building as designed is sixteen times stiffer than a conventional 
structure. The design concept Is so sound that the Structural Engineer 
has been able to be ultra-conservative Iri his design without adversely 
affecting the economics of the structure. This is not the case' With 
conventional buildings where a more radical approach must be used if 
the building is to be constructed at reasonable cost. 



3 06 A//ST NCSTAR 1 -2, WTC Investigation 



Salient Points witli Regard to tlie Structural Design of the WTC Towers 



-2- 

6. The structural concept is new but the design principles, the stress 
analysis and the theories of cr.echar.ics upon which the design is based 
are well kno\vn and are in accordance viUi ^--o^ c.i^l'acox'ir^; practice. 

7. The design has been reviewed by some of the most knowledgeable people 

in the construction industry. In a letter to John Skllling, the Structural 
Engineer for The World Trade Center, the Chief Engineer of the American 
Bridge Division of U. S. Steel Corporation said: 

"In revievjing this design ^■.'ith ol;; ,3:.cvc;ir.s cr.'. Cc.-.str-.iction Dcpr.rtments, 
we are very optimistic that you have turaid e, r.cw page in u;-.c C.^::±^i\ of 
structural steel. It is high time that some new thinking be applied in 
our industry. In the viords of our General Manager of Operating, Lester i 
Larison, he said - 'It was the best damn thing that he has seen come % 
down the pike in his 46 years of experience. Imagine designing a 100- 
story building for under 30 pounds per scuare foot, '" 

8. The Engineering News-Xecord of January 30th carries a aeries of-qjaotations 
from people in the building industry with regard to The World Trade Center 
design. 

A. James Ruderman, one of the outstanding New York Structural Engineers 

says that "The structural design of the tower buildings shows a 

commendable job of rethinking, where ideas were given a lot of 
thought and not just treated routinely." 

B. Harold Bernhard, partner, Shreve, Lamb and Harmon Associates, 
Architects, says "It's a magnificent project." 

9. In an editorial in the sa.T.e issue of the Record is the comment: 

"Thus, the PKYA will not build as high as permitted all over its property, 
despite the high land costs in dovmtovTn Manhattan. Instead, the twin 
towers will occupy only 12% of the site. This plan should please the 
nu.Tierous vociferous critics of other recent New York projects not 



surrounded by large open scaces. It al sn no-rm^^c n,. 



■L _ T... J < , 



NIST NCSTAR 1 -2, WTC Investigation 3 07 



Appendix A 



-3- ■ 

wi,th no setbacks witho-at violating zoning regulations, • Over-all, the 
design not only appears to be esthetically preferable to a set-back ' • 
silhouette, but also lends itself to more economical construction and 
use of space. The PNVA, in addition, has engaged noted architects and 
consulting engineers to design the project. From the preliminary data 
released, it appears that the design of the twin towers will laark an 
important advar.ce in skyscraper construction. Tall buildings are 
handicapped economically because the cost of structural framing and the 
space consumed by vertical transportation rise rapidly with increasing 
height. The Trade Center designers have departed from usually con- • 
ventional practices to cut these costs." 
10. We have been informed that the structural engineering firm of Ammann & 
l-Jhitney has been approached by a leading New York architect with a 
request that this structural system be reviewed for possible incorporation 
in a large office building which the architect is presently designing. 
LI. The skyscraper is one of America's contributions to World Architecture. 
New York is the capital of skyscraper construction in the United States. 
The design of the towers of The World Trade Center is based on the lessons 
l&&m&d in constructing all the tens of millions of square feet of high 
rise buildings in this great city. The towers may be said to be the 
first buildings of the 21st Century and the design concepts which they 
embody will be incorporated in some measure in every future high rise 
building ever built. 



MPLrfg 
2-3-64 



308 NIST NCSTAR 1 -2, WTC Investigation 



AppencixB 

Estimation of Sectorial Extreme Wind Speeds^ 



Abstract 

We present a procedure for estimating extreme wind speeds corresponding to a sector-by- 
sector approach to the estimation of extreme wind effects. We provide details of the data 
sets and their treatment, as well as details of the estimates themselves, in a manner 
intended to be thorough, clear, and transparent. Efforts in the direction of clarity and 
transparency are in our view necessary if estimates of extreme winds and their effects are 
to meet the need for effective scrutiny by users and building authorities, and if a solid 
technical basis for a consensus among practitioners, standards organizations, and 
professional organizations is to be created in the near future. 

Introduction 

The estimation of extreme wind speeds at a given site is, in principle, straightforward. 
However, in practice, for any given location, differences between approaches used by 
various wind engineers or other professionals can lead to widely divergent estimates. To 
assess any particular extreme wind speed estimates it is necessary to scrutinize with care 
the procedure on which that estimate is based. This requires, in turn, that the procedure, 
each of its steps, and the attendant calculations, be explained clearly, transparently, in 
sufficient detail, and in a manner that should be independently verifiable by users or 
building inspection authorities. For an example of detailed assessment of an extreme 
wind speed estimation methodology and attendant calculations, see (Coles and Simiu, 
2003). 

At this time no sufficient guidance is available in standards for (a) the estimation of 
extreme wind speeds on buildings subjected to wind tunnel testing and (b) the integration 
of those wind speeds with aerodynamic data. Several procedures are used by various 
practitioners, but no professional consensus appears to exist on how discrepancies 
between the respective estimates can be reconciled or how the various methods should be 
amended to avoid situations - which do occur in actual practice - wherein various 
estimates of wind effects corresponding to the same nominal mean recurrence interval 
can differ by as much as 50 percent. 

Some wind engineering professionals perform estimates of structural responses 
corresponding to winds blowing from each of a number of sectors. The sectors we 
consider here are the half-octants bisected by the NNE, NE, ENE,....,N compass 
directions. Those winds are referred to as sectorial wind speeds. In this paper we describe 
the estimation of sectorial wind speeds. 



^ This appendix was co-authored by William P. Fritz and Emil Simiu of NIST. 

NISTNCSTAR 1-2, WTC Investigation 309 



Appendix B 



This paper is intended to serve as a contribution to the professional debate that, in our 
opinion, is needed to create a robust basis for a consensus on extreme wind estimation. 
We present here a procedure for estimating sectorial extreme wind speeds in a region 
with both hurricane and non-hurricane winds, and show in some detail a numerical 
example illustrating the procedure. To fix the ideas we will consider a site close to New 
York City (NYC). 

Extreme wind speed data 

Hurricane wind speed data. We make use in this note of the NIST simulated hurricane 
wind speed database which, to our knowledge, is the only non-proprietary hurricane 
database currently in existence. The database is available online at the following link on 
the worldwide web: ftp://ftp.nist.gov/pub/bfrl/emil/hurricane/datasets/ . This subdirectory 
contains the relevant data sets of simulated hurricane wind speeds in nautical miles per 
hour (nmi/hr) at 10 meters above ground in open terrain, averaged over 1-min. There are 
55 files with data for locations ranging from milepost 150 (file2.dat; near Port Isabel, TX) 
to milepost 2850 (file56.dat; near Portland, ME), spaced at 50 mile intervals. The 
structure of each data file is as follows: 

Line 1: Milepost identifier, plus other information not needed for the analysis 

program. 
Line 2: Blank, usually. In some files, the milepost number is repeated here. 
Line 3: URATE and NSTRMS. URATE is the estimated annual rate of 
occurrence of hurricanes at and near this milepost, and NSTRMS is the 
number of simulated storms used to create the data. For all data sets 
included in this subdirectory, NSTRMS=999. 
Lines 4-1003: The wind speed data for each of the NSTRMS simulated storms. 
There are a total of 18 numbers on each line. The first 16 are the 
maximum wind speeds in 16 specified directions, beginning with NNE 
and moving clockwise to N. The 17* number is the maximum wind 
speed for ANY direction (i.e., the largest of the previous speeds). The 
final number (18th) number in each line is the storm identifier. 

The NIST data sets are based on the "highly regarded work of Batts et al. (1980)," 
(unpublished report prepared for Insurance Services Office, Inc., New York City, 1994 
by Robert H. Simpson, former director of the National Hurricane Center and creator with 
Herbert Saffir of the well-known Saffir-Simpson hurricane intensity scale). A variety of 
other hurricane models are currently available, although the data based thereon are, to our 
knowledge, proprietary. Agreement between wind speeds near the coastline based on the 
NIST data sets and on data sets based on other models is very good. At milestone 2500 
(one of the milestones tabulated in Simiu and Scanlan (1996, p. 117) that is closest to 
New York City), the estimated hurricane mean hourly speeds at 10 m above ground in 
open terrain according to Batts et al. (1980), Simiu, Heckert and Whalen (1996) (both 
based on the NIST database), Georgiou et al. (1983), and Vickery and Twisdale (1995) 
are, respectively, about 30 m/s, 30 m/s, 30 m/s, and 29 m/s for the 50-year speeds, and 
45 m/s, 43 m/s, 47 m/s, and 45 m/s for the 2000-year speeds. In evaluating these 
differences it should be kept in mind that sampling errors in the estimation of hurricane 
wind speeds in the New York City area have estimated coefficients of variation of 



310 NIST NCSTAR 1-2, WTC Investigation 



Estimation of Sectorial Extreme Wind Speeds 



roughly 10% for 50-year speeds and 20% for 500-year speeds (Coles and Simiu, 2003). 
Note that the sampling errors depend less on the number of simulated hurricanes in the 
database (999 in our case) than on the number of historical hurricanes (about 100) used to 
obtain statistics of the climatological parameters on which the simulations are based (i.e., 
radii of maximum wind speeds, atmospheric pressure defect, hurricane translation speed 
and direction, and so forth). Those statistics differ relatively little among the various 
simulation packages. It is the authors' understanding that hurricane wind speeds for the 
State of Florida, corresponding to various probabilities of exceedance, are currently being 
estimated by the NOAA Hurricane Research Division. In our opinion it would be 
desirable that this effort be expanded to cover all U.S. hurricane -prone regions. 

Treatment of hurricane wind speed data. The data listed in the NIST database need to be 
rank-ordered for reasons explained subsequently in this note. The rank-ordered data for 
the location of interest (file 50, milestone 2550 - nearest to NYC - in the NIST database) 
and for the 202.5°and 225° sectors of interest are listed in Table 1. Note that for these 
sectors hurricane translation speeds and the relevant vortex speeds within the hurricanes 
at and near NYC are in many instances of opposite signs, resulting in relatively small and 
therefore negligible, or even vanishing, total hurricane wind speeds. It is therefore 
sufficient to show in the table only the largest 55 of the total of 999 data, while keeping 
in mind that all the 999 data should be accounted for in the calculations. 

Table 1, Rank-ordered wind speeds (nmi/hr at 10m above ground in open terrain, 
averaged over 1-min) from NIST database for 202,5°and 225° sectors at 

milepost 2550. 





ssw 


SW 




SSW 


SW 




SSW 


SW 


Rank.m 


202.5° 


225° 


Rank.m 


202.5° 


225° 


Rank.m 


202.5° 


225° 


1 


88.81 


86.73 


21 





29.56 


41 





22.64 


2 


74.49 


61.79 


22 





28.96 


42 





21.59 


3 


73.75 


52.37 


23 





28.95 


43 





21.56 


4 


46.59 


47.91 


24 





27.89 


44 





21.25 


5 


39.68 


42.82 


25 





27.79 


45 





20.62 


6 


17.46 


41.97 


26 





27.74 


46 





20.09 


7 


14.35 


41.59 


27 





27.59 


47 





20.04 


8 


13.81 


37.13 


28 





27.35 


48 





19.07 


9 


13.51 


36.4 


29 





27.13 


49 





18.82 


10 


6.8 


35.85 


30 





27.01 


50 





18.55 


11 


4.88 


34.77 


31 





26.63 


51 





16.97 


12 


3.49 


33.64 


32 





26.59 


52 





16.67 


13 





32.41 


33 





26.45 


53 





15.49 


14 





31.79 


34 





25.82 


54 





15.14 


15 





31.75 


35 





25.58 


55 








16 





31.13 


36 





25.28 








17 





30.64 


37 





24.16 








18 





30.59 


38 





23.58 








19 





30.01 


39 





23.04 








20 





29.86 


40 





22.98 









Non-hurricane extreme wind speed data. In this paper we make use of wind speeds 
recorded using ASOS (Automated Surface Observing System) during the period 1983- 
2002, made available to NIST by the NOAA's National Climatic Center for three airports 
near NYC: La Guardia (LGA), Newark International Airport (EWR), and 
John F. Kennedy International Airport (JFK). The wind speed data sets include the peak 
5-s gust speed multiplied by a factor of 10, for every hour within the period of record, in 



NISTNCSTAR 1-2, WTC Investigation 



311 



Appendix B 



m/s. The data were recorded at 20 ft (6.1m) above ground until May 1, 1996 at LGA and 
JFK and until July 1, 1996 at EWR. They were recorded at 10 m above ground thereafter. 

Treatment of non-hurricane wind speed data. The results being sought are expressed in 
terms of 3-s peak gust speeds at 10 m above ground in open (airport) terrain. Therefore, 
all data need to be transformed from 5-s peak gust speeds to 3-s peak gust speeds. This 
can be done to within a sufficient approximation through multiplication of the 5-s speeds 
by a factor of 1.02 (see ASCE 7-02 Standard, Figure C6.2). The data not recorded at 10m 
must also be adjusted to correspond to a 10 m elevation above ground. This involves the 
use of the power law 

y(zi)/y(z2)=(zi/z2)" (1) 

where, for 3-s peak gust speeds, the exponent a = 1/9.5 for Exposure C (see ASCE 7-02 
Standard). 

Note that in the data sets each wind speed is associated with the direction from which the 
wind is blowing. The directions from which the wind is blowing are measured in a 
clockwise direction from true north, and are recorded for 36 angles in 10 degree 
increments. 

Data should be excluded from the analysis if (1) the record provides no direction for a 
recorded wind speed (this is the case for a relatively small number of speeds), and (2) if 
the data have a quality code other than 'good', as provided explicitly in the NOAA data 
set. Only one measurement at JFK (the maximum speed in the 50° sector in 1987) and 
two measurements at LGA (the maximum speeds in the 210° sector in 1983 and in the 
200° sector in 1984) had a quality code other than 'good'. 

Maximum wind speeds are extracted from an airport data set for each of the 36 wind 
directions for each year of record. For example, 20 years of maximum hourly wind 
speeds produce 36 x 20 values. The dates of major hurricanes of record for NYC during 
these 20 years should be checked against the dates of each tabulated maximum wind 
speed. Data recorded on September 27 and 28, 1985 (hurricane Gloria) and August 19 
and 20, 1991 (hurricane Bob) (Neumann et al., 1993) should not be considered and the 
largest non-hurricane wind speeds in the records should be used instead. 

The 36 directions are reduced through an appropriate scheme to 16 directions that match 
the NIST hurricane data. This can be accomplished by defining the wind speed data set 
associated with, say, the 22.5° sector as the set of maximum yearly wind speeds from the 
NOAA data sets for the 10°, 20°, 30° and 40° sectors. This definition is somewhat 
conservative, since the 22.5° sector is associated with the narrower sector 11.25° to 
33.75°, rather than the sector 5°-45°. However, in our opinion this conservatism is 
warranted by the fact that the data samples at our disposal are limited to 20 years. A 
longer than 20-year data set for the 11.25° to 33.75° sector may contain wind speeds that, 
during a 20-year interval, have actually blown within the small sectors 5° to 11.25° and 
33.75° to 45°. This minor conservatism affecting wind speeds is an empirical and 



312 NIST NCSTAR 1-2, WTC Investigation 



Estimation of Sectorial Extreme Wind Speeds 



reasonable way of accounting for possible sampling errors with respect to the direction of 
extreme speeds, for which to our knowledge no applicable theory is available to date. 

Estimates of extreme wind speeds 

Estimation of extreme wind speeds regardless of whether they are associated with 
hurricanes or non-hurricane winds. Estimates of extreme wind speeds at 10 m above 
ground in open terrain at or near the site must take into account both hurricane and non- 
hurricane winds. We are interested in estimates of sectorial wind speeds, that is, wind 
speeds that occur in a specified sector defined by the azimuth of its bisector and the total 
angle swept by the sector. For specificity, in this note we illustrate our estimates of 
sectorial wind speeds for the 22.5° sectors defined by the bisectors with a 202.5° and a 
225° azimuth (i.e., for the SSE and SE directions). 

Let the probability of non-exceedance of the wind speed v be denoted by P(V<v). This 
probability represents the probability that hurricane wind speeds do not exceed v and that 
non-hurricane wind speeds do not exceed v. Denoting the probability that hurricane wind 
speeds do not exceed v by Ph(V<v) and the probability that non-hurricane speeds do not 
exceed v by PNH(y<v), and noting that the occurrences of hurricane and non-hurricane 
speeds are independent events, we have 

PiV<v)= PHiV<v) PNHiV<v). (2) 

The corresponding mean recurrence interval of the wind speed Vis, by definition, 

iV=l/[l-P(y<v)]. (3) 

Estimation of probabilities Ph(V<v). For wind speeds blowing from any one of the 16 
compass directions (corresponding to the 16 half-octants) the following procedure is 
used: 

• Extract from the NIST database the hurricane mean rate of arrival (// = 
0.305/year) and, for the wind direction of interest, the 999 hurricane wind speed 
data for New York City (milestone 2550). 

• Rank-order the 999 data. (This was done in Table 1.) If the hurricane mean arrival 
rate URATE (henceforth denoted in this paper by //) was 1/year, the highest speed 
would have a 999-year (or approximately 1,000-year) mean recurrence interval. 
However, if // <1, then the mean recurrence interval of the highest speed in the set 
is 999/ ju. (For example, if the mean arrival rate were one hurricane every two 
years (// =0.5), then the mean recurrence interval of the highest speed in the set 
would be 999/0.5=1998, or about 2000 years.) 

• The m-th largest speed in the set of 999 speeds corresponds to a mean recurrence 
interval iV=999/(// m). For example, if - as is the case for New York City area - 
the estimated mean rate of arrival is 0.305, the mean recurrence intervals of the 
first highest, second highest, and 65 highest speed are about 
999/0.305=3275 years, 999/(0.305 x 2)=1640 years, and 999/(0.305 x 65)=50 
years, respectively. Conversely, the hurricane wind speed with an JV/j-year mean 



NIST NCSTAR 1-2, WTC Investigation 313 



Appendix B 



recurrence interval corresponds to the m-th largest wind speed in the set, where 
m=999/( ju Nh). 
• The probability that this wind speed does not exceed v is defined as follows: 

PHiV<V) = l-l/NH. (4) 

Other estimation procedures are available, however to date there is no definitive 
consensus on which procedure is to be preferred. Some analysts believe that extreme 
value distributions are inadequate owing to their validity, strictly speaking, under 
asymptotic assumptions only; others believe that WeibuU distributions are not appropriate 
since they are distributions of the smallest values, rather than distributions of the largest 
values. In spite of its theoretical non-optimality in terms of the precision of some 
estimates, the non-parametric approach used in this paper appears to be relatively non- 
controversial and appears to have been adopted by other analysts of hurricane wind 
speeds. 

Estimation of probabilities PjvH(y<v). The iVjvH-year mean recurrence interval may be 
estimated by using techniques discussed in Simiu and Scanlan (1996, Appendix A1.7). 
Although other distributional models may be adopted, the least controversial model for 
extreme wind speeds of non-hurricane origin appears to date to be the Type I extreme 
value distribution. The mean recurrence interval associated with the non-hurricane wind 
speed V is then 



Nhh = exp 



V-v 

+ 0.577 

0.78s 



(5) 



The mean, v , and standard deviation, s, are calculated from the yearly maximum wind 3- 
s peak gust speeds at 10 m above ground in open terrain for the sector of interest. The 
probability that the wind speed, V, does not exceed v is 

PjvH(V<v) = l-l/iVjvH. (6) 

The requisite probability P(y<v) can be obtained from Eqs. 2, 4, and 6. 

Numerical example 

We seek the 50-, 500- and 720-year winds blowing from the sectors nominally associated 
with the 202.5° and 225° sectors for the area around New York City. We use 20 years of 
non-hurricane wind speed data measured at LGA and the NIST hurricane wind speed 
data for those sectors. The choice of the LGA data set is commented upon subsequently. 

Let us first consider the 3-s peak gust speed y=100 mph at 10 m above ground in open 
terrain, and calculate its mean recurrence interval (Eq. 3). Recall that the estimated 
hurricane arrival rate at milepost 2550 is // = 0.305/year. The 100 mph, 3-sec gust wind 
speed is divided by 1.525 (for conversion to mean hourly speeds), then divided by 
1.15 (for conversion to nmi/hr) and finally multiplied by 1.25 (for conversion to 1-min 

314 NIST NCSTAR 1-2, WTC Investigation 



Estimation of Sectorial Extreme Wind Speeds 



averaging time) (see ASCE 7-02 Standard, Figure C6.2). The 1-min speed at 10 m above 
ground in open terrain corresponding to the 100 mph peak 3-s speed is therefore 71.3 
nmi/hr. This value ranks in Table 1 m = 3.1 and m=1.6 for the 202.5° and 225° sectors, 
respectively. The mean recurrence intervals of a 100 mph, 3-sec gust hurricane speed are 
therefore: 

999 

iV„__. = = 1057 years 



N 



0.305(3.1) 
999 



H,225° 



= 2047 years 



0.305(1.6) 

and the probability that the 100 mph, 3-sec wind does not exceed v is 

1 



H, 202.5^ 



(100mph,3-s<v)=l 



(100mph,3-s<v)=l 



1057 
1 

2047 



0.99905 



= 0.99951. 



Note that if a Poisson-based approach to the estimation of the mean recurrence intervals 
was adopted, instead of the approach used in this paper, the results would be identical for 
practical purposes. The mean recurrence interval obtained by the Poisson-based approach 
is iV=l/{l-exp{-)[i[m/(999+l)]}}. This yields 1058 years for 202.5° sector and 2049 years 
for the 225° sector. 



For non-hurricane winds, maximum hourly wind speeds at LGA airport are shown in 
Table 2 for the two directions considered and for each of 20 consecutive years (1983 to 
2002). The original speeds in m/s, averaged over 5-sec, and affected by a scale factor of 
10 from the NOAA data set are provided in Table 2 along with their converted values in 
3-s peak gusts in mph at 10 meters. Also shown are the four directions of the NOAA 
data from which the maximum value is drawn for the 202.5° and 225° sectors. The mean 
(v ) and standard deviation (s) of each set of 20 values are also provided. 

Table 2, Maximum non-hurricane wind speeds (mph, 3-s), LaGuardia (LGA), 







202.5° 


225" 






190',200° 


210°,220' 


210°,220',230°,240° 


Year 


0.1m/s,5- 


sec 


Mph,3-sec 


0.1in/s,5-sec 


mph,3-sec 


1983 


319 




77 


267 


64 


1984 


268 




65 


268 


65 


1985 


118 




28 


108 


26 


1986 


113 




27 


103 


25 


1987 


170 




41 


118 


28 


1988 


154 




37 


134 


32 


1989 


149 




36 


154 


37 


1990 


154 




37 


113 


27 


1991 


113 




27 


149 


36 


1992 


138 




33 


118 


28 


1993 


128 




31 


128 


31 


1994 


118 




28 


128 


31 


1995 


118 




28 


113 


27 


1996 


154 




37 


103 


24 


1997 


113 




26 


149 


34 


1998 


118 




27 


118 


27 


1999 


144 




33 


118 


27 


2000 


134 




31 


113 


26 


2001 


123 




28 


123 


28 


2002 


123 




28 


123 


28 


mean 






35.3 




32.6 


std 






13.0 




11.5 



NISTNCSTAR 1-2, WTC Investigation 



315 



Appendix B 



The mean recurrence interval of the 100 mph, 3-sec gust as a non-hurricane wind is 
therefore: 



iV„ 



N 



JVH,225° 



exp 



exp 



100-35.3 
0.78(13.0) 

100-32.6 



- + 0.577 



+ 0.577 



1051 years 



3265 years 



0.78(11.5) 
and the probabiUty that a 100 mph, 3-sec wind does not exceed v is 



JVH,202.5" 



(100mph,3-s<v)=l 



1 



(100mph,3-s<v)=l 



1051 
3265 



0.99905 



= 0.99969. 



In our opinion it would be desirable that a concerted effort be made that would engage 
NOAA on the one hand and wind and structural engineering professionals on the other, 
aimed at making wind speed observations archived by NOAA available in a suitable, user 
friendly format to the structural engineering community. The mean recurrence interval 
for the peak 3-s gust 100 mph speed, regardless of whether it is associated with hurricane 
or non-hurricane winds, is calculated using Eqs. 2, 4, and 6: 

= 527 years 



N 



202.5° 



225 -1 



1-P(100<v) 
1 



l-(0.99905)(0.99905) 
1 



P(100<v) 1-(0.99951)(0.99969) 



1250 years. 



The procedure just described was followed for wind speeds between 60 and 105 mph. 
The mean recurrence interval of the wind speeds - regardless of whether they are 
associated with hurricane or non-hurricane winds - is plotted in Figure 1 for the two 
sectors. The mean recurrence intervals for the V=100 mph above are marked with a circle 
in the respective plots. 



1B0O 
1B00 
HOO 
1200 
■ 1D00 
BOO 
BOO 
400 
200 






75 BO 85 90 
Wind speed (mph) 



(a) 




Figure 1, Combined mean recurrence intervals as a function of peak 3-s gust wind 
speed for the (a) 202,5° and (b) 225° sectors. 



316 



NIST NCSTAR 1-2, WTC Investigation 



Estimation of Sectorial Extreme Wind Speeds 



Estimates of the 50-, 500- and 720-year, 3-s peak gust winds are obtained from Figure 1 
and are shown in Table 3. 

Table 3, Estimates of the NYC 50-, 500- and 720-year speeds, regardless of whether 

they are associated with hurricane or non-hurricane winds, at 10m above ground in 

open terrain for the 202,5° and 225° sectors, 

JV-year wind (mph,3-s) 
Sector 50-yr 500-yr 720-yr 
202.5° 69.8 99.1 104.1 
225° 63.0 86.3 91.1 

Choice of LGA sectorial data versus EWR and/or JFK sectorial data 

The estimated sectorial wind speeds associated with the 202.5° and 225° directions were 
found to differ significantly for the LGA and EWR records, on the one hand, and the JFK 
record on the other. This may be due to relatively large sampling errors associated with 
wind directionality. In view of the uncertainties associated with sectorial wind speeds it 
appeared prudent to consider the LGA data above, whose variability for the sectors of 
interest is largest. Had the EWR data been considered instead, the final results would 
have been marginally lower. However, had the JFK results been used, the results would 
have been significantly smaller. This is due to the absence in the JFK record of some of 
the relatively high wind speeds that are present in the sectors of interest for LGA and 
EWR. This is an example of the occurrence of significant sampling errors in a sectorial 
wind speed record. 

Rather than making use of the LGA data set alone, the analyst may be tempted to use a 
"super-station" comprising the data from the LGA, EWR, and JFK stations. However, in 
our opinion this consolidation of the three data sets into one larger data set would provide 
an inadequate basis for performing more precise estimates. The reason for this statement 
is that the three stations are relatively close to each other. The respective wind speed 
records are not necessarily independent, and gust speeds contain variabilities associated 
with turbulent fluctuations that may mask the actual correlations between the three 
records. In our opinion the issue of superstations constructed for stations that are 
geographically close needs to be researched in the future. 

Comparison of extreme wind speed estimates at the three NYC airports 

It was noted in the previous section that sectorial speeds can vary fairly significantly from 
station to station. It is of interest to compare extreme wind speed estimates at EWR, JFK 
and LGA without regard to wind direction. To do this, maximum wind speeds, 
regardless of their direction, are used in the procedure described earlier in lieu of sectorial 
wind speeds. That is, we consider hurricane winds from column 17 in file 50 of the NIST 
database and maximum yearly non-hurricane winds from the NOAA data set. Thus, non- 
hurricane data consist of 20 observations for each of the three NYC airports. Mean 



NIST NCSTAR 1-2, WTC Investigation 317 



Appendix B 



recurrence intervals of wind speeds at each airport, regardless of whether they are 
associated with hurricane or non-hurricane winds, and regardless of their direction, are 
plotted in Figure 2. The 50-year 3-s peak gust speed at each airport, regardless of 
direction, is 112.2 mph. 



100 
90 
80 
70 
60 
50 
40 
30 



1 


1 1 1 1 


---- EWR 

- JFK 

— LGA 








1 1 1 1 1 7 


1 1 1 1 \ J 


i i i i i/ 


: : j 1 "Jf 


1 1 1 Jy^'' 


\ \ L^^^ 1 


i ^^^Jf^^^l^ \ \ 


i i i i i 



60 



70 



BO 90 100 
Wind speed (mph) 



110 



120 



Figure 2, Mean recurrence intervals of wind speeds - regardless of whether they are 
associated with hurricanes or non-hurricane winds, and regardless of direction - for 

LGA, EWR, and JFK airports. 

For any specified wind speed, the mean recurrence interval is generally shorter for winds 
regardless of their direction than for winds blowing from one sector only. The remarkable 
agreement between the estimates of extreme wind speeds at the three airports contrasts 
with the far less satisfactory agreement observed for the sectorial wind speeds. In other 
words, sectorial wind speeds appear to exhibit significant sampling errors for which, as 
mentioned earlier, no applicable theory or research appear to be available to date. This 
justifies, in our opinion, the use of the data set among the three available airport data sets 
that yields the most conservative results. In light of these remarks, we believe that caution 
is also warranted on the use of overly refined schemes for estimating extreme wind 
speeds for any one angular sector in approaches to wind directionality effects other than 
the sector-by-sector approach, e.g., the up-crossing approach. 

Summary and conclusions 

We presented a procedure for estimating extreme wind speeds corresponding to a sector- 
by-sector approach to the estimation of extreme wind effects. We provided details of the 
data sets and their treatment, as well as details of the estimates themselves, in a manner 
intended to be both clear and transparent. Efforts in the direction of clarity and 
transparency are in our view indispensable if estimates of extreme winds and their effects 
are to meet the need for effective scrutiny by users and building authorities, and if a solid 
technical basis for a consensus practitioners, standards organizations, and professional 
organizations is to be created in the near future. 



318 



NIST NCSTAR 1-2, WTC Investigation 



Estimation of Sectorial Extreme Wind Speeds 



In the authors' opinion it would be desirable (1) that the NOAA's Hurricane Research 
Division expand in the future its current efforts aimed at estimating hurricane wind 
speeds, with a view to covering all U.S. hurricane-prone regions, and (2) that NOAA's 
wind speed archives for non-hurricane wind speeds be made available to the wind and 
structural engineering communities in a suitable, user-friendly format to be agreed upon 
by NOAA and qualified representatives of those communities. 



Acknowledgement 

We wish to thank William Brown of the National Climatic Center (National Weather 
Service) for providing valuable help on the LaGuardia, Newark International Airport, and 
John F. Kennedy International Airport data sets, and information on the anemometer 
height history for those sets. 

References 

American Society of Civil Engineers. ASCE Standard ASCE 7-02, Minimum Design 

Loads for Buildings and Other Structures, American Society of Civil Engineers, 

Reston, Virginia, 2002. 
Batts, M.E., Russell, L.R., and Simiu, E. (1980), "Hurricane Wind Speeds in the United 

States," Journal of the Structural Division, ASCE 100 2001-2015. 
Coles, S., and Simiu, E. (2003), "Estimating Uncertainty in the Extreme Value Analysis 

of Data Generated by a Hurricane Simulation Model," Journal of Engineering 

Mechanics 129 1288-1294. 
Georgiou, P.N., Davenport, A.G., and Vickery, B.J. (1983), "Design Wind Loads in 

Regions Dominated by Tropical Cyclones," Journal of Wind Engineering and 

Industrial Aerodynamics 13 139-152. 
Neumann, C.J., Jarvinen, B.R., McAdie, C.J., and Elms, J.D. (1993), "Tropical Cyclones 

of the North Atlantic Ocean, 1871-1992," Historical Climatology Series 6-2, 

National Climatic Data Center, Ashville, NC in cooperation with the National 

Hurricane Center, Coral Gables, PL. 
Simiu, E., Heckert, N.A., and Whalen, T.M. (1996), "Estimates of Hurricane Wind 

Speeds by the 'Peaks over Threshold' Method," NIST Technical Note 1416, 

National Institute of Standards and Technology, Gaithersburg, MD. 
Simiu, E. and Scanlan, R.H. (1996), Wind Effects on Structures, New York: Wiley. 
Vickery, P.J. and Twisdale, L.A. (1995), "Prediction of Hurricane Windspeeds in the 

U.S.," Journal of Structural Engineering 121 1691-1699. 



NIST NCSTAR 1-2, WTC Investigation 319 



Appendix B 



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320 NIST NCSTAR 1-2, WTC Investigation 



AppendxC 

Wind Tunnel Testing and the Sector-By-Sector 
Approach to Wind Directionality Effects^ 



ABSTRACT 

We examine the sector-by-sector approach used by some wind tunnel operators to specify 
extreme wind effects. According to this criterion the design of a structural member 
subjected to wind loads is adequate if the stresses induced by the largest sectorial wind 
speed with a 50-yr mean recurrence interval does not exceed the maximum allowable 
wind-induced stress for that member, sectorial wind speeds with a 50-yr mean recurrence 
interval being estimated separately for each of the eight 45° (or the sixteen 22.5°) 
azimuthal sectors. We show that this approach leads to estimates of wind effects that are 
unconservative (i.e., on the unsafe side), owing to their failure to consider the overall 
effects of winds blowing from all sectors. 
INTRODUCTION 

The sector-by-sector approach to the estimation of wind directionality effects consists of 
estimating, separately, the wind speeds with a 50-yr mean recurrence interval (MRI) for 
winds blowing from each of the eight 45° sectors of the horizontal plane. Those wind 
speeds are referred to as the 50-yr sectorial speeds. For defmiteness we consider the case 
of eight 45° sectors and of a 50-yr MRI, but the same definition can be extended for 
sixteen 22.5° sectors and any desired MRI. 



To appear in the Journal of Structural Engineering, ASCE, July, 2005. This appendix was coauthored by 
Emil Simiu, ASCE, NIST Fellow, Structures Group, National Institute of Standards and Technology, Gaithersburg, 
MD 20899-861 1, and James J. Filliben, Leader, Statistical Engineering Group, National Institute of Standards and 
Technology, Gaithersburg, MD 20899-8980. 



NISTNCSTAR 1-2, WTC Investigation 321 



Appendix C 

Some wind tunnel operators specify wind effects based on the following criterion, 
henceforth referred to as the sectorial design criterion: for any given member, the 
maximum allowable wind-induced effect, R, (e.g., the maximum allowable wind-induced 
stress) must not be exceeded by the largest of the wind effects Qj,5o (/=1,2,..,8) induced by 
the eight 50-yr sectorial speeds v,;5o. We denote by k the sector where this largest wind 
effect, denoted by 2/t,5o, occurs. The purpose of this work is to show that the sectorial 
design criterion is unconservative (i.e., on the unsafe side) relative to the physically- 
based criterion, henceforth referred to as the regular design criterion, which states that 
the maximum allowable wind-induced effect R should not be exceeded by the 50-year 
effect induced by wind blowing from any direction (rather than just from the sector k). 

It would be desirable to address this question by making use of the joint extreme 
value probability distributions (including correlations) of the wind speeds at the location 
of interest. Unfortunately, to our knowledge, expressions for such distributions do not 
exist. Bounds for the joint probabilities of interest may be estimated (Simiu et al., 1985; 
Simiu, Leigh, and Nolan, 1986), but such an approach can be unwieldy owing to 
combinatorial explosion problems. For the purposes of this work, which is addressed to 
structural engineers, it also has the drawback of not being sufficiently intuitive. 

ASSESSMENT OF THE SECTORIAL DESIGN CRITERION 

Intuitive Approach. Let Vj^nj denote the sectorial wind speeds that blow from the sector 7" 
(1<7<8) and cause the allowable wind effect R (the subscript A^ denotes the mean 
recurrence interval of the wind speed v^^a^). For7=A: we have Nk = 50 years. For j^k the 
mean recurrence intervals A^ exceed 50 years. (If A^ were 50 years or less for any 77^ k, 

322 NIST NCSTAR 1-2, WTC Investigation 



Wind Tunnel Testing and tlie Sector-By-Sector Approacli to Wind Dir Effts 

then R would be attained under sectorial wind speeds Vj^so, rather than under the sectorial 
wind speed v,t,5o, which would be contrary to the sectorial design criterion.) 

Let Fq(Q<R) denote the probability that the largest yearly wind effect Q, regardless of 
the direction from which the wind blows, does not exceed R. If the number of sectors 
were limited to one, then we would have, with notations similar to those used earlier, 

Fe(e<i?)=Prob (vi<V5o) = 1 - 1/50=0.98, 
where vi denotes the wind speed inducing the effect Q. In this particular case the sectorial 
design criterion would be adequate. 

For multi-directionally defined wind speeds and responses the following relation 
is consistent with the use of the sectorial design criterion: 

Fq(Q<R) = Prob(vi<vi,M, V2<V2,N2, ■•-, n<Vi,m) (1) 

in which one of the indexes 7=1, 2, .., 8 has the value k, to which there corresponds the 
sectorial speed Vk,Nk with Nk=50 years, all other N/s being larger than 50 years. Let us 
consider the following three cases: positively correlated speeds, independent speeds, and 
negatively correlated speeds. For each of these cases we will examine the probability 
F(Q<R). If it were true that F(Q<R)=0.9S, the sectorial design criterion design would be 
adequate. If F(Q<R)<0.9S, the design performed in accordance with the sectorial design 
criterion would be unconservative. If F(g</?)>0.98 the opposite would be the case. 

Case 1. The speeds v\, V2,..., vg avQ perfectly, positively correlated. This means that 
for dXXji^k, we have Vj= Uj Vk, where ay are constants. Therefore, 

Fq{Q<R) = Prob(vi<Vi,5o) (2) 

=0.98. 



NIST NCSTAR 1-2, WTC Investigation 323 



Appendix C 

Equation 2 is valid because, by the definition of the sectorial design criterion, the 
occurrence of the event v/i<Vk,5o implies the occurrence of the events Vj<Vj^nj for all j. It 
follows that in Case 1 the sectorial design criterion is adequate. 

Case 2. The speeds vi, V2,..., vg are mutually independent. The mutual correlations 
of pairs of sectorial speeds then vanish. This implies 

Fq{Q<R) = Prob(vi<vi,M, V2<V2,A^, . . . , V8<V8,w8) (3a) 

= Prob(vi<vi,M) Prob(v2<V2,iV2)- • ■ Prob(v8<V8,w8) (3b) 

< 0.98, (3c) 

i.e., the mean recurrence interval of the event Q<R is equal to or less than 50 years. The 
inequality (3c) holds because in Eqs. 3, as in Eq. 1, one of the indexes7=l, 2, .., 8 has the 
value k, to which there corresponds the sectorial speed Vk,Nk with Nk=5Q years, and all 
other A^'s are equal to or larger than 50 years. Consider, for example, the case in which 
the effects from one of the sectors were dominant, that is, the mean recurrence interval of 
the event that winds from that sector would cause R to be exceeded would be 50 years, 
while for the other sectors the corresponding mean recurrence intervals would be much 
longer, say 250 years. Then, Fq{Q<R)={\ - 1/50) x (1 - 1/250)^ = 0.98 x 0.996^ « 0.95, 
corresponding to a mean recurrence interval of the event Q>R equal to 1/(1 - 0.95)=20 
years. In other words, the sectorial design criterion would lead to an underestimation of 
the wind effect. It is reasonable to expect that this statement remains true even if the 
correlations do not vanish but are relatively small. 

Case 3. The speeds vi, V2,..., V8 have negative correlations. To illustrate the 
significance of this case from the point of view of the problem considered in this note, we 
consider the model consisting of one die with two sets of numbers, one in blue and one in 

324 NIST NCSTAR 1-2, WTC Investigation 



Wind Tunnel Testing and tlie Sector-By-Sector Approacli to Wind Dir Effts 

red, as follows. For faces 1, 2, 3, 4, 5, 6, the blue numbers are 1, 2, 3, 4, 5, 6, and the red 
numbers are 6, 5, 4, 3, 2, 1, respectively. The correlation coefficient between the red and 
blue outcomes is -1. The probability of the event of throwing a 4 or larger number, 
regardless of color, is 1 ~ to which there corresponds a mean recurrence interval of one 
throw. (Blue and red numbers would correspond in our analogy to north and south winds, 
say.) 

Instead the model just described, we now consider a model consisting of one die with 
two sets of numbers, one in blue and one in red, but with the following sets of numbers 
for faces 1, 2, 3, 4, 5, 6. Blue: 1, 2, 3, 4, 5, 6, and red: 1, 2, 3, 4, 5, 6, respectively. In this 
case the correlation coefficient between the red and blue outcomes is 1 {perfect positive 
correlation). The probability of throwing a 4 or larger number, regardless of color, is 1/2, 
to which there corresponds a mean recurrence interval of two throws, rather than one 
throw, as in the case of the die with negative correlation. If exceeding the critical value 4 
is undesirable, it is seen that the case of negative correlation is more unfavorable than the 
case of positive correlation (the undesirable outcome occurs more frequently in the 
former than in the latter case). 

It is of interest to also consider the case of throwing two ordinary dice, one with the 
blue numbers 1, 2, 3, 4, 5, and 6, and the other with red numbers 1, 2, 3, 4, 5, and 6. In 
this case the correlation vanishes, and the probability of getting in a throw of the two 
dice an outcome of 4 or larger is 27/36=0.75, i.e., the mean recurrence interval of this 
outcome is 1.33 throws. Again, this outcome occurs more frequently than in the case of 
positive perfect correlation, which is consistent with our earlier comparison between Case 
1 and Case 2. 



NIST NCSTAR 1-2, WTC Investigation 325 



Appendix C 

The preceding arguments suggest that considering the case of strongly positive 
correlation when the correlation is in fact low or negative would overestimate the mean 
recurrence interval of the critical event. This statement is valid not only for the cases of 
perfect positive correlation and negative or zero correlation. This can be checked by 
considering, for example: (a) Instead of a die with perfectly negatively correlated red and 
blue outcomes, one in which the blue and the red numbers are 1, 2, 3, 4, 5, 6, and 4, 3, 2, 
2, 1, 1, respectively; for this die the correlation coefficient is -0.75, and the mean 
recurrence interval of an outcome of 4 or larger, regardless of color, is 1 .5 throws, (b) 
Instead of the two dice considered earlier, two dice with blue and red numbers 1, 2, 3, 4, 
5, 6, and 1, 1, 2, 2, 3, 4; in this case the correlation coefficient is again zero, and the mean 
recurrence interval of a blue or red outcome of at least four is 1.7 throws, (c) Instead of 
the die with perfectly positive correlation, one in which the blue and red numbers are 1, 
2, 3, 4, 5, 6, and 1, 1, 2, 2, 3, 4, respectively; in this case the correlation coefficient is 0.86 
and the mean recurrence interval of an outcome of 4 or larger, regardless of color, is 2 
throws. Thus, the mean recurrence interval of this outcome is, again, shorter for both the 
uncorrelated case (1.7 throws) and the negatively correlated (1.5 throws) case that it is for 
the positively correlated case (2 throws). 

Our choice of an intuitive argument is deliberate - it is intended to render our finding 
as clear as possible to practicing structural engineers, who may or may not have a 
theoretical probabilistic background. More basic probabilistic arguments are now 
adduced that strengthen and generalize our finding, without injecting unduly elaborate 
probabilistic manipulations. 



326 NIST NCSTAR 1-2, WTC Investigation 



Wind Tunnel Testing and tlie Sector-By-Sector Approacli to Wind Dir Effts 

Probabilistic approach. The advantage of a probabilistic argument is that is it more 
general. We invoke the definition of conditional probability: 

P{E,\E,) = ^^^^^^ (4a) 



P{EAE,) = ^^^^^^ . (4b) 

from which it follows: 

P{E„E,) = P{E,\E,)P{E,) 

(5a,b) 
= P{E,\E,)P{E,). 

In Eqs. 4 and 5 P(Ei,E2) is the probability of occurrence of both events Ei and E2, 
P{E\\E2) is the conditional probability of occurrence of event E\ given that event E2 has 
occurred, P{E2) is the probability of event E2, and similar definitions hold for the second 
the above equalities. It follows from Eqs. 5 that 

P{E,,E2)<m:m{PiE,),PiE2)} (6a) 

For three events E\, E2, and £3, it can be shown that 

P{Ei,E2M <mm{P{E,),P{E2\V{E,)}, (7) 

By induction, Eq. 7 may be extended for any number of events Em {m=\,2,...). 

Let the event v,<v,;5o be denoted by Ej. The application of the extension of Eq. 7 for 8 
events Ej (i.e., to Eq. 1) shows that Fq{Q<R) < 0.98. 

Another, more intuitive way of conveying this result is the following. If the structure 
was strengthened so that it could fail only in direction k, the return period of the 



NIST NCSTAR 1-2, WTC Investigation 327 



Appendix C 

exceedances of R would be 50 years. Hence for the unstrengthened structure the return 
period must be shorter. 
CONCLUSION 

We conclude that, except for the case of strong positive correlations between sectorial 
wind speed - a case that is rarely if ever encountered in nature, - designs based on the 
sectorial design criterion underestimate the 50-year wind-induced effects, and are 
therefore unconservative (on the unsafe side). Results of calculations based on Bonferroni 
bounds (Simiu et al, 1985, and Simiu, Leigh, and Nolan, 1986) are consistent with this 
conclusion. However, owing to combinatorial explosion issues those calculations could 
not be conducted to the degree of usefulness rendered possible by current computational 
capabilities. We believe similar calculations should be performed in the future by using 
such capabilities. Pending such calculations, the assumption of independence among 
sectorial wind speeds provides a lower bound of the actual mean return period of interest. 
A rigorous estimation of probabilities Fq{Q<R) by reducing the multidirectional 
problem to a one-dimensional problem was described by Rigato, Chang, and Simiu 
(2001) for structures with no dynamic amplification effects. A similar solution applicable 
for structures exhibiting dynamic effects is in progress. 
References 

Rigato, A., Chang, P., and Simiu, E., "Database-assisted Design, Standardization, and Wind 
Direction Effects," J. Struct. Eng., 127 855-860 (2001). 

Simiu, E., Hendrickson, E., Nolan, W., Olkin, I., and Spiegelman, C, "Multivariate Distributions 
of Directional Wind Speeds," J. Struct. Eng., Ill 939-943 (1985). 

Simiu, E., Leigh, S., and Nolan, W., "Environmental Load Direction and Reliability Bounds," J. 
Struct. Eng., 112, 1199-1203 (1986). 

328 NIST NCSTAR 1-2, WTC Investigation 



AppencixD 

SOM Project 2, Progress Report No. 3, WTC Wind 

Load Estimates 



NIST NCSTAR 1-2, WTC Investigation 329 



SOM 



NIST - World Trade Center Investigation 
PROJECT 2: Baseline Structural Performance and Aircraft Impact Damage Analysis 



Progress Report No. 3 
WTC Wind Load Estimates 

Outside Experts for Baseline Structural Performance 



13 April 2004 



Skidmore, Owings & Merrill LLP 

Suite 1000, 224 South Michigan Avenue, Chicago, Illinois 60604 
312 554-9090, Fax 312 360-4545, www.som.com 



SOM 



1.0 Table of Contents 



1.0 Table of Contents 1 

2.0 Overview 2 

2.1 Project Overview 2 

2.2 Report Overview 2 
3.0 NIST-Supplied Documents 2 

3.1 RWDI Wind Tunnel Reports 2 

3.2 Cermak Peterka Petersen, Inc. Wind Tunnel Reports 2 

3.3 Correspondence 3 

3.4 NIST Report 3 
4.0 Discussion and Comments 3 

4.1 General 3 

4.2 Wind Tunnel Reports and Wind Engineering 3 

4.2.1 CPP Wind Tunnel Report 4 

4.2.2 RWDI Wind Tunnel Report 4 

4.2.3 Building Period used in Wind Tunnel Reports 5 

4.2.4 NYCBC Wind Speed 5 

4.2.5 Incorporating Wind Tunnel Results in Structural Evaluations 5 

4.2.6 Summary 6 

4.3 NIST Recommended Wind Loads 6 
5.0 References 6 



Progress Report No. 3 
WTC Wind Load Estimates 



2.0 Overview 

2.1 Project Overview 

The objectives for Project 2 of the WTC Investigation include the development of reference 
structural models and design loads for the WTC Towers. These will be used to establish the 
baseline performance of each of the towers under design gravity and wind loading conditions. 
The work includes expert review of databases and baseline structural analysis models developed 
by others as well as the review and critique of the wind loading criteria developed by NIST. 

2.2 Report Overview 

This report covers work on the development of wind loadings associated with Project 2. This 
task involves the review of wind loading recommendations developed by NIST for use in 
structural analysis computer models. The NIST recommendations are derived from wind tunnel 
testing/wind engineering reports developed by independent wind engineering consultants in 
support of insurance litigation concerning the WTC towers. The reports were provided 
voluntarily to NIST by the parties to the insurance litigation. 

As the third party outside experts assigned to this Project, SOM's role during this task was to 
review and critique the NIST developed wind loading criteria for use in computer analysis 
models. This critique was based on a review of documents provided by NIST, specifically the 
wind tunnel/wind engineering reports and associated correspondence from independent wind 
engineering consultants and the resulting interpretation and recommendations developed by 
NIST. 



3.0 NIST-Supplied Documents 

3. 1 Rowan Williams Davies Irwin (RWDI) Wind Tunnel Reports 
Final Report 

Wind-Induced Structural Responses 
World Trade Center - Tower 1 
New York, New York 
Project Number: 02-1310A 
October 4, 2002 

Final Report 

Wind-Induced Structural Responses 

World Trade Center - Tower 2 

New York, New York 

Project Number:02-1310B 

October 4, 2002 

3.2 Cermak Peterka Petersen, Inc. (CPP) Wind Tunnel Report 

Data Report 

Wind-Tunnel Tests - World Trade Center 

New York, NY 

CPP Project 02-2420 

August 2002 



SOM 



3.3 Correspondence 



Letter dated October 2, 2002 

From: Peter Irwin/RWDI 

To: Matthys Levy/Weidlinger Associates 

Re: Peer Review of Wind Tunnel Tests 

World Trade Center 

RWDI Reference #02-1310 

Weidlinger Associates Memorandum dated March 19, 2003 

From: Andrew Cheung 

To: Najib Abboud 

Re: ERRATA to WAI Rebuttal Report 

Letter dated September 12, 2003 

From: Najib N. Abboud/Hart- Weidlinger 

To: S. Shyam Sunder and Fahim Sadek (sic)/NIST 

Re: Responses to NIST's Questions on: 

"Wind-Induced Structural Responses, World Trade 

Center, Project Number 02-1310A and 02-1310B 

October 2002, By RWDI, Prepared for Hart- 

Weidlinger" 

Letter dated April 6, 2004 

From: Najib N. Abboud /Weidlinger Associates 
To: Fahim Sadek and Emil Simiu 

Re: Response to NIST's question dated March 30, 2004 regarding "Final Report, Wind- 
Induced Structural Responses, World Trade Center - Tower 2, RWDI, Oct 4, 2002" 



3.4 NIST Report 



Estimates of Wind Loads on the WTC Towers 
Emil Simiu and Fahim Sadek 
April 7, 2004 



4.0 Discussion and Comments 



4.1 General 

This report covers a review and critique of the NIST recommended wind loads derived from wind 
load estimates provided by two independent private sector wind engineering groups, RWDI and 
CPP. These wind engineering groups performed wind tunnel testing and wind engineering 
calculations for various private sector parties involved in insurance litigation concerning the 
destroyed WTC Towers in New York. There are substantial disparities (greater than 40%) in the 
predictions of base shears and base overturning moments between the RWDI and CPP wind 
reports. NIST has attempted to reconcile these differences and provide wind loads to be used for 
the baseline structural analysis. 



Progress Report No. 3 
WTC Wind Load Estimates 



4.2 Wind Tunnel Reports and Wind Engineering 

The CPP estimated wind base moments far exceed the RWDI estimates. These differences far 
exceed SOM's experience in wind force estimates for a particular building by independent wind 
tunnel groups. 

In an attempt to understand the basis of the discrepancies, NIST performed a critique of the 
reports. Because the wind tunnel reports only summarize the wind tunnel test data and wind 
engineering calculations, precise evaluations are not possible with the provided information. For 
this reason, NIST was only able to approximately evaluate the differences. NIST was able to 
numerically estimate some corrections to the CPP report but was only able to make some 
qualitative assessments of the RWDI report. It is important to note that wind engineering is 
an emerging technology and there is not consensus on certain aspects of current practice. 
Such aspects include the correlation of wind tunnel tests to full-scale (building) behavior, 
methods and computational details of treating local statistical (historical) wind data in overall 
predictions of structural response, and types of suitable aeroelastic models for extremely tall and 
slender structures. It is unlikely that the two wind engineering groups involved with the WTC 
assessment would agree with NIST in all aspects of its critique. This presumptive disagreement 
should not be seen as a negative, but reflects the state of wind tunnel practice. It is to be expected 
that well-qualified experts will respectfully disagree with each other in a field as complex as wind 
engineering. 

SOM's review of the NIST report and the referenced wind tunnel reports and correspondence has 
only involved discussions with NIST; it did not involve direct communication with either CPP or 
RWDI. SOM has called upon its experience with wind tunnel testing on numerous tall building 
projects in developing the following comments. 

4.2.1 CPP Wind Tunnel Report 

The NIST critique of the CPP report is focused on two issues: a potential overestimation 
of the wind speed and an underestimation of load resulting from the method used for 
integrating the wind tunnel data with climatic data. NIST made an independent estimate 
of the wind speeds for a 720-year return period. These more rare wind events are 
dominated by hurricanes that are reported by rather broad directional sectors (22.5 
degree). The critical direction for the towers is from the azimuth direction of 205 to 210 
degrees. This wind direction is directly against the nominal "south" face of the towers 
(the plan north of the site is rotated approximately 30 degrees from the true north) and 
generates dominant cross-wind excitation from vortex shedding. The nearest sector data 
are centered on azimuth 202.5 (SSW) and 225 (SW). There is a substantial drop (12%) 
in the NIST wind velocity from the SSW sector to the SW sector. The change in velocity 
with direction is less dramatic in the CCP 720-year velocities or in the ARA hurricane 
wind roses included in the RWDI report. This sensitivity to directionality is a cause for 
concern in trying to estimate a wind speed for a particular direction. However, it should 
be noted that the magnitude of the NIST interpolated estimated velocity for the 210 
azimuth direction is similar to the ARA wind rose. The reduction of forces has been 
estimated by NIST based on a square of the velocity, however, a power of 2.3 may be 
appropriate based on a comparison of the CPP 50-year (nominal) and 720-year base 
moments and velocities. 

The NIST critique of the CPP use of sector by sector approach of integrating wind tunnel 
and climatic data is fairly compelling. The likelihood of some degree of underestimation 



SOM 



is high but SOM is not able to verify the magnitude of error (15%) which is estimated by 
NIST. This estimate would need to be verified by future research, as noted by NIST. 

4.2.2 RWDI Wind Tunnel Report 

The NIST critique of RWDI has raised some issues but has not directly estimated the 
effects. These concerns are related to the wind velocity profiles with height used for 
hurricanes and the method used for up-crossing. 

NIST questioned the profile used for hurricanes and had an exchange of correspondence 
with RWDI. While RWDI's written response is not sufficiently quantified to permit a 
precise evaluation of NIST's concerns, significant numerical corroboration on this issue 
may be found in the April 6 letter (Question 2) from N. Abboud (Weidlinger Associates) 
to F. Sadek and E. Simiu (NIST). 

NIST is also concerned about RWDI's up-crossing method used for integrating wind 
tunnel test data and climatic data. This method is computationally complex and 
verification is not possible because sufficient details of the method used to estimate the 
return period of extreme events are not provided. 

4.2.3 Building Period used in Wind Tunnel Reports 

SOM noted that both wind tunnel reports use fundamental periods of vibrations that 
exceed those measured in the actual (north tower) buildings. The calculation of building 
periods are at best approximate and generally underestimate the stiffness of a building 
thus overestimating the building period. The wind load estimates for the WTC towers are 
sensitive to the periods of vibration and often increase with increased period as 
demonstrated by a comparison of the RWDI base moments with and without P-Delta 
effects. Although SOM generally recommends tall building design and analysis be based 
on P-Delta effects, in this case even the first order period analysis (without P-Delta) 
exceeds the actual measurements. It would have been desirable for both RWDI and CPP 
to have used the measured building periods. 

4.2.4 NYCBC Wind Speed 

SOM recommends that the wind velocity based on a climatic study or ASCE 7-02 wind 
velocity be used in lieu of the New York City Building Code (NYCBC) wind velocity. 
The NYCBC wind velocity testing approach does not permit hurricanes to be 
accommodated by wind tunnel testing as intended by earlier ASCE 7 fastest mile 
versions because it is based on a method that used an importance factor to correct 50-year 
wind speeds for hurricanes. Because the estimated wind forces are not multiplied by an 
importance factor, this hurricane correction is incorporated in analytical methods of 
determining wind forces but is lost in the wind tunnel testing approach of determining 
wind forces. 

4.2.5 Incorporating Wind Tunnel Results in Structural Evaluations 

It is expected that ASCE 7 load factors will also be used for member forces for evaluating 
the WTC towers. Unfortunately, the use of ASCE 7 with wind tunnel-produced loadings 
is not straight forward. Neither wind tunnel report gives guidance on how to use the 
provided forces with ASCE 7 load factors. 



Progress Report No. 3 
WTC Wind Load Estimates 



The ASCE 7 load factors are applied to the nominal wind forces and, according to the 
ASCE 7 commentary, are intended to scale these lower forces up to wind forces 
associated with long return period wind speeds. The approach of taking 500-year return 
period wind speeds and dividing the speeds by the square root of 1.5 to create a nominal 
design wind speed; determining the building forces from these reduced nominal design 
wind speeds; and then magnifying these forces by a load factor (often 1.6) is, at best, 
convoluted. For a building that is as aerodynamically active as the WTC, an approach of 
directly determining the forces at the higher long return period wind speeds would be 
preferred. The CPP data did provide the building forces for their estimates of both 720- 
years (a load factor of 1.6) and the reduced nominal design wind speeds. A comparison 
of the wind forces demonstrates the potential error in using nominal wind speeds in lieu 
of directly using the underlying long period wind speeds. 

It should also be noted that the analytical method of calculating wind forces in ASCE 7 
provides an importance factor of 1.15 for buildings such as the WTC in order to provide 
more conservative designs for buildings with high occupancies. Unfortunately, no 
similar clear guidance is provided for high occupancy buildings where the wind loads are 
determined by wind tunnel testing. Utilizing methods provided in the ASCE 7 
Commentary would suggest that a return period of 1800 years with wind tunnel-derived 
loads would be comparable to the ASCE 7 analytical approach to determining wind loads 
for a high occupancy building. 

It would be appropriate for the wind tunnel private sector laboratories or NIST, as future 
research beyond the scope of this project, to address how to incorporate wind tunnel 
loadings into an ASCE 7-based design. 

4.2.6 Summary 

The NIST review is critical of both the CPP and RWDI wind tunnel reports. It finds 
substantive errors in the CPP approach and questions some of the methodology used by 
RWDI. It should be noted that boundary layer wind tunnel testing and wind engineering 
is still a developing branch of engineering and there is not industry-wide consensus on all 
aspects of the practice. For this reason, some level of disagreement is to be expected. 

Determining the design wind loads is only a portion of the difficulty. As a topic of future 
research beyond the scope of this project, NIST or wind tunnel private sector laboratories 
should investigate how to incorporate these wind tunnel-derived results with the ASCE 7 
Load Factors. 

4.3 NIST Recommended Wind Loads 

NIST recommends a wind load that is between the RWDI and CPP estimates. The NIST 
recommended values are approximately 83% of the CPP estimates and 115% of the RWDI 
estimates. SOM appreciates the need for NIST to reconcile the disparate wind tunnel results. It 
is often that engineering estimates must be done with less than the desired level of information. 
In the absence of a wind tunnel testing and wind engineering done to NIST specifications, NIST 
has taken a reasonable approach to estimate appropriate values to be used in the WTC study. 
However, SOM is not able to independently confirm the precise values developed by NIST. 

The wind loads are to be used in the evaluation of the WTC structure. It is therefore 
recommended that NIST provide clear guidelines on what standards are used in the evaluations 
and how they are to incorporate the provided wind loads. 



SOM 



5.0 References 



[1] American Society of Civil Engineers, Minimum Design Loads for Buildings and Other 
Structures, ANSI/ASCE 7-02, 2002. 

[2] American Society of Civil Engineers, Minimum Design Loads for Buildings and Other 
Structures, ANSI/ASCE 7-93, 1993. 



Progress Report No. 3 
WTC Wind Load Estimates 



Appendix E 
Still Images of the Video Records Used in Chapter 6 



This appendix provides still images of the video records (Figures E-1 through E-9) used to estimate the 
initial impact conditions of the aircraft that impacted World Trade Center (WTC) 1 and WTC 2 
(see Chapter 6). A short description of each of these videos is provided in Table 6-1. 




Figure E-1. Still image from Video VI (WTC 1 impact). 



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339 



Appendix E 




Figure E-2. Still image from Video V2 (WTC 1 impact). 



340 



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still Images of the Video Records Used in Chapter 7 




©2001 WABC-TV 






Figure E-3. Still image from Video V3 (WTC 2 impact). 



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Appendix E 




Figure E-4. Still image from Video V4 (WTC 2 impact). 



342 



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still Images of the Video Records Used in Chapter 7 




Figure E-5. Still image from Video V5 (WTC 2 impact). 



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Appendix E 




Figure E-6. Still image from Video V6 (WTC 2 impact). 



344 



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still Images of the Video Records Used in Chapter 7 




Figure E-7. Still image from Video V7 (WTC 2 impact). 



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Appendix E 




Figure E-8. Still image from Video V8 (WTC 2 impact). 



346 



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still Images of the Video Records Used in Chapter 7 




Figure E-9. Still image from Video V9 (WTC 2 impact). 



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Appendix E 



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348 NIST NCSTAR 1-2, WTC Investigation