NIST NCSTAR1-6

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

(Chapters 1-6)

John L. Gross

Therese P. McAllister

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

NISTNCSTAR1-6

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

John L. Gross

Therese P. McAllister

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

NISTNCSTAR1-6

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

John L. Gross

Therese P. McAllister

Building and Fire Research Laboratory

National Institute of Standards and Technology

September 2005

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 1-6

Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-6, 425 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

One of the four main objectives of the National Institute of Standards and Technology (NIST)

investigation of the collapse of the World Trade Center (WTC) towers was to determine why and how the two towers collapsed. Events that played a significant role in the structural performance of the towers were the aircraft impact, the rapid ignition of fires on multiple floors, the growth and spread of fires and

the structural weakening resulting from effects of high temperatures. The passive fire protection applied

to the steel structural components in the WTC towers was investigated to provide information on the in- place condition of the fire protection before and after aircraft impact. Standard fire resistance tests were

conducted to establish the appropriate classification (fire resistance rating) of the original design of the

WTC floor system and to develop insight into the structural performance of the composite steel and

concrete floor system under exposure to a standard fire. Results of simulations of the aircraft impacts were used to predict damage to the structure, fire protection, and partition walls in the path of the debris

field. Characterization of the temperatures of the structural components, determined from simulated

WTC fires, allowed the calculation of the performance of major subsystems constituting the structural system of the towers including the core framing, the exterior wall (columns and spandrels), and full tenant

floors. Insights gained from these analyses were used, in turn, to formulate and execute nonlinear,

temperature-dependent finite element analyses of global structural systems to predict the collapse

sequence of each tower. The structural analyses were guided, and where possible validated, by

observ ations made from the review of thousands of photographs and video recordings. This report covers

the characterization of the conditions of the WTC towers before the attacks, their weakening due to the

aircraft impacts, the response of the structural systems to the subsequent growth and spread of fires, and

the progression of local failures that led ultimately to the total collapse of both towers.

Keywords: Buildings, collapse, fire, large deflections, stability, structural analysis, structural damage,

structural response to fire. World Trade Center.

NISTNCSTAR 1-6, WTC Investigation iii
Abstract

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iv NISTNCSTAR 1-6, WTC Investigation
Table of Contents

Abstract iii

List of Figures xi

List of Tables xxi

List of Acronyms and Abbreviations xxiii

Preface xxv Acknowledgments xxxv

Executive Summary xxxvii

Chapter 1 Introduction 1

1 . 1 Project Objectives 1

1 .2 Technical Approach and Tasks 2

1 .2. 1 Task A - Finite Element Reference Models 4

1 .2.2 Task B - Material Properties 5

1 .2.3 Task C - Passive Fire Protection 5

1 .2.4 Task D - Standard Fire Resistance Tests 5

1 .2.5 Task E - Aircraft Impact Damage 6

1.2.6 Task F - Observations and Timeline 6

1 .2.7 Task G - Temperatures of Structural Components 6

1 .2.8 Task H - Component and Subsystem Analyses 7

1 .2.9 Task I - Major Subsystem Analyses 7

1.2.10 Task J - Global Structural Analyses 7

1.2.11 Task K - Probable Collapse Sequence 8

1.3 Challenges 9

1.4 Report Organization 10

Chapter 2 Passive Fire Protection 13

2. 1 Fire Resistance of Structural Elements 13

2.2 Historical Review Related To Passive Fire Protection 14

2.2. 1 Building Code Requirements for the Design of the WTC Towers 14

2.2.2 New York City Building Code Requirements 14

2.2.3 Classification of WTC Towers 15

NISTNCSTAR 1-6, WTC Investigation v
Table of Contents

2.2.4 Response to Local Law 5/1973 16

2.2.5 Selection of Fire Resistive Materials 16

2.2.6 Specified Thickness of Fire Resistive Material 18

2.2.7 Upgrading SFRM on Floor Trusses 20

2.2.8 Need for Fire Resistance Tests 21

2.2.9 Maintenance of SFRM in Elevator Shafts 22

2.3 As-Applied Thickness of SFRM 22

2.3.1 1994 Measurements from WTC 1 Floors 23 and 24 22

2.3.2 Analysis of Photographs 23

2.3.3 Port Authority Data on Upgraded SFRM on Trusses 25

2.4 Effect of the Variability of SFRM Thickness on Thermal Response 25

2.4. 1 Effects of Thickness Variability and Gaps in SFRM 25

2.4.2 Thermally equivalent thickness of SFRM 27

2.4.3 Thickness of SFRM Used for Thennal Analyses 28

2.5 Summary of SFRM Thickness 29

2.6 Thermophysical Properties 30

2.6.1 Thermal Conductivity 31

2.6.2 Specific Heat Capacity 3

1

2.6.3 Density 32

2.6.4 Thermophysical Properties of Gypsum Panels 32

2.7 Estimation of SFRM Dislodged By to Aircraft Impact 33

2.7.1 In-place Density and Bond Strength 33

2.7.2 Specimen Preparation and Test Procedures 34

2.7.3 Test Results 35

2.8 References 36

Chapter 3 Fire Resistance Tests 37

3.1 Background 37

3.1.1 Motivation for Conducting Standard Fire Tests 37

3. 1 .2 Purpose of the Standard Fire Tests 37

3.2 Description of WTC Floor System 38

3.2.1 Structural System 38

3.2.2 Fireproofing Thickness 39

3.3 Fire Resistance Testing 39

vi N1ST NCSTAR 1-6, WTC Investigation
Table of Contents

3.3.1 General Description 39

3.3.2 Test Restraint Conditions and Ratings 39

3.3.3 Scale of Tests 40

3.3.4 Test Variables 40

3.4 Preparation of Test Assemblies 41

3.5 Description of Tests and Loading 43

3.5.1 Full-scale Tests (35 ft span) 43

3.5.2 Reduced-scale Tests (17 ft span) 44

3.6 Test Results 44

3.6.1 General Discussion of Tests 44

3.6.2 Fire Resistance Ratings 48

3.6.3 General Observations 49

3.6.4 Floor Deflections and Temperatures 5

1

3.6.5 Observations and Comparisons 54

3.7 Summary 55

Chapter 4 Structural Response of Components, Connections and Subsystems 57

4. 1 Mechanical Properties of Concrete and Steel 57

4.1.1 Concrete Properties and Failure Criteria 58

4. 1 .2 Steel Properties and Failure Criteria 60

4.2 Floor Subsystem Analysis 65

4.2.1 Description of Floor Subsystem 66

4.2.2 Truss Seats 69

4.2.3 Knuckles 79

4.2.4 Single Truss and Concrete Slab Section 84

4.2.5 Floor Subsystem Analysis 89

4.3 Exterior Wall Subsystem 96

4.3. 1 Exterior Wall Subsystem Description 97

4.3.2 One, Two, and Three-Story High Columns 100

4.3.3 Exterior Wall Section Analysis 103

4.3.4 Summary 119

4.4 References 119

NISTNCSTAR 1-6. WTC Investigation vii
Table of Contents

Chapters

Aircraft Impact Damage..... 121

5.1 Introduction 121

5.2 Methodology and Criteria for Developing Input Data From Aircraft Impact Analysis Results.. 122

5.2. 1 Core Column Damage 123

5.2.2 Structural Damage to Floor Slabs, Core Beams, and Floor Trusses 126

5.2.3 Fireproofmg Damage 129

5.2.4 Summary of Aircraft Impact Damage 133

5.3 Structural and Fireproofmg Damage to WTC 1 134

5.4 Structural and Fireproofmg Damage to WTC 2 141

5.5 Observations of Aircraft Impact Damage to the Pentagon 147

5.6 Summary 150

5.7 References 151

Chapters

Observations and Timeline of Structural Events 153

6.1 Introduction 153

6.2 Observations of Structural Events 154

6.2.1 WTC 1 Structural Response Observations 155

6.2.2 WTC 2 Structural Response Observations 167

Chapter 7 Structural Response of Major Tower Subsystems to Aircraft Impact Damage and Fire .185

7.1 Introduction 185

7.2 Core Subsystem 185

7.2. 1 Model and Method of Analysis 186

7.2.2 WTC 1 Core Analysis Results 188

7.2.3 WTC 2 Analysis Results 192

7 . 3 Full Floor Subsystem 1 94

7.3.1 Model and Method of Analysis 194

7.3.2 WTC 1 Analysis Results 200

7.3.3 WTC 2 Analysis Results 205

7.4 Exterior Wall Subsystem 212

7.4. 1 Finite Element Model and Methods of Analysis 213

7.4.2 WTC 1 Analysis Results 215

7.5 Summary of Subsystem Analyses 224

viii , NISTNCSTAR 1-6, WTC Investigation
Table of Contents

Chapter 8 Structural Response of the WTC Towers to Aircraft Impact Damage and Fire 229

8.1 Introduction 229

8.2 Global Model of Towers 230

8.2. 1 Model Description 230

8.2.2 Model Modifications 233

8.3 Analysis Methodology 235

8.4 Results of WTC 1 Analysis 237

8.4. 1 WTC 1 Structural Response to Aircraft Impact Damage 237

8.4.2 WTC 1 Structural Response to Elevated Temperatures 243

8.4.3 WTC 1 Hat Truss Members and Connections 251

8.5 Results of WTC 2 Analysis 253

8.5.1 WTC 2 Structural Response to Aircraft Impact Damage 254

8.5.2 WTC 2 Structural Response to Elevated Temperatures 260

8.5.3 WTC 2 Hat Truss Members and Connections 273

8.6 Structural Response of the WTC Towers Without Impact Damage and Subjected to the Same

Fires 279

8.7 Summary of Structural Response of the WTC Towers 28

1

Chapter 9 Probable Collapse Sequences 285

9.1 Introduction 285

9.2 Methodology 285

9.2. 1 Key Observed Events and Conditions 286

9.2.2 Collapse Hypotheses 286

9.2.3 Mathematical Modeling - Analysis Interdependencies 288

9.2.4 Sensitivity Studies to Identify Influential Variables 290

9.2.5 Evaluation of Collapse Hypotheses 294

9.3 Probable Collapse Sequences 296

9.3.1 Probable Collapse Sequence of WTC 1 297

9.3.2 Probable Collapse Sequence of WTC 2 306

9.4 Discussion and Summary 3 1

9

9.4. 1 Structural Response to Impact Damage and Fire 320

9.4.2 Structural Response to Fire Without Impact Damage 32

1

9.4.3 Time to Collapse 322

9.4.4 Factors that Affected Performance 322

NIST NCSTAR 1-6, WTC Investigation ix
Table of Contents

Chapter 10

Findings 329

10.1 Passive Fire Protection 329

10.2 Building Code Requirements for Structural Fire Resistance 329

10.2.1 Selection of Fire Resistive Materials 330

10.2.2 Equivalent Thickness of SFRM 330

10.3 Fire Resistance Tests 330

10.3.1 Structural Performance 331

10.3.2 Fire Resistance Ratings .331

10.4 Structural Response of Components 332

10.4.1 Floor System 332

10.4.2 Exterior Wall System 332

10.5 Fireproofmg and Partition Damage Due to Aircraft Impact 332

10.6 Observations and Timeline 333

10.6.1 WTC 1 333

10.6.2 WTC 2 334

10.7 Structural Response of Major Tower Subsystems 335

10.7.1 Isolated Core Subsystem 335

10.7.2 Full Floor Subsystem 335

10.7.3 Isolated Exterior Wall Subsystem 335

10.8 Structural Response to Aircraft Impact Damage and Fire 336

10.8.1 General Findings 336

10.8.2 Performance with Intact fireproofmg 337

10.9 Probable Collapse Sequences 337

10.9.1 Role of the Building Core 337

10.9.2 Role of the Building Floors 337

10.9.3 Role of Exterior Frame-Tube 338

10.9.4 Probable Collapse Sequences •. 338

X NISTNCSTAR 1-6, WTC Investigation
List of Figures

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

Figure 1-1. Structural Analysis Sequence 4

Figure 2-1 . Mock up of floor truss system 1

7

Figure 2-2. Demonstration of application of Monokote sprayed fire-resistive material to floor

trusses 18

Figure 2-3. Example of measurement procedure used to estimate SFRM thickness from

photographs 24

Figure 2—4. Example of "gap" in fire-resistive material on diagonal member of a bridging floor

truss 26

Figure 2-5. Thermo-mechanical response of 1 in. bar compared with uniform thickness SFRM 28

Figure 3-1 . Floor system of the WTC towers 38

Figure 3-2. Schematic of floor system viewed along the main steel trusses, (not to scale) 39

Figure 3-3. Drawing of 35 ft truss and end detail 42

Figure 3^. Trusses being fit-up in test frame of 35 ft test assembly 43

Figure 3-5. 35 ft span test assembly loaded with concrete blocks and water-filled containers 43

Figure 3-6. 1 7 ft span test assembly loaded with concrete blocks, water-filled containers and

hydraulic actuators 44

Figure 3-7. Unexposed surface of Assembly No. 1 after loading equipment was removed 45

Figure 3-8. Unexposed surface of Assembly No. 2 after loading equipment was removed 46

Figure 3-9. Unexposed surface of Assembly No. 3 after loading equipment was removed 47

Figure 3-10. Unexposed surface of Assembly No. 4 after loading equipment was removed 48

Figure 3-1 1 . Fire exposure side of the 35 ft restrained test assembly after almost 2 hours of fire exposure 49

Figure 3-12. Sections cut through concrete slab to confirm extent and depth of spalling 50

Figure 3-13. Measurement of remaining slab thickness after spalhng 50

Figure 3-14. Detail of spalling concrete at east end of Assembly No. 1 51

Figure 3-15. Deflection measured at the center of each assembly 52

Figure 3-16. Deflection measured at the center of each assembly divided by the span 52

Figure 3-17. Average temperature of the unexposed surface for all four tests 53

NIST NCSTAR 1-6. WTC Investigation xi
List of Figures

Figure 3-18. Average temperatures of the bottom chord for Test Nos. 1, 2 and 3 (3/4 in. thick

SFRM) 54

Figure 4-1 . Temperature-dependent concrete properties 59

Figure 4-2. Concrete stress-strain curves 60

Figure 4-3. Temperature-dependent properties for all steels 63

Figure 4^. Stress-strain relationships for Material ID 1 steel 64

Figure 4-5. Strain behaviors at elevated temperatures for Material ID 1 steel 65

Figure 4-6. Maximum plastic strain from the finite element analysis and limiting plastic strain 65

Figure 4-7. Floor structural subsystem (WTC 1, Floor 96) 67

Figure 4-8. Primary truss components 68

Figure 4-9. Plan view of truss seat connection, straps, and horizontal studs 69

Figure 4-10. Truss seat detail location on northeast quadrant of Floor 96 of WTC 1. Reproduced

with permission of The Port Authority of New York and New Jersey 70

Figure 4-11. Finite element model of exterior seat 71

Figure 4-12. Failure sequence of the exterior seats against tensile force 72

Figure 4-13. Capacity of exterior seat against tensile force (Detail 141 1 in Fig. 4-10) 73

Figure 4-14. Capacity of interior seat against vertical and horizontal force (Detail 22 in Fig. 4-10). ..... 73

Figure 4-15. Truss seat capacity against vertical force. Reproduced with permission of The Port Authority of New York and New Jersey. Enhancements by NIST 77

Figure 4-16. Truss seat capacity against horizontal force. Reproduced with permission of The

Port Authority of New York and New Jersey. Enhancements by NIST 78

Figure 4-17. Results of interior truss seat model at 500 °C 80

Figure 4-1 8. Laclede Steel Company shear tests of a knuckle 8

1

Figure 4-19. Compressive stresses in longitudinal shear finite element model (4,100 psi concrete) 82

Figure 4-20. Shear force versus displacement from finite element model for longitudinal shear of

two knuckles (4,100 psi concrete) 82

Figure 4-21. Compressive stresses in transverse shear finite element model (2,500 psi concrete) 83

Figure 4-22. Shear force versus displacement from finite element model for transverse shear of

two knuckles (2,500 psi concrete) 83

Figure 4-23. Composite truss and concrete slab model 85

Figure 4-24. Vertical displacement at 700 °C. 87

Figure 4-25. Displacement versus temperature 87

Figure 4-26. Comparison of detailed and reduced truss models 88

Figure 4-27. Converted ANSYS model for Floor 96 of WTC 1 90

Figure 4-28. Long span trusses of converted ANSYS model for Floor 96 of WTC 1 91

xii NIST NCSTAR 1-6, WTC Investigation
List of Figures

Figure 4-29. Core floor beams and columns of converted ANSYS model for Floor 96 of WTC 1 91

Figure 4-30. Full floor model before impact damage is included (without concrete floor slab) 94

Figure 4-3 1 . Break element locations in the floor model (Floor 96, WTCl) 96

Figure 4-32. Exterior wall section model 98

Figure 4-33. Schematic of exterior column cross-section 98

Figure 4-34. One-story exterior column model 101

Figure 4-35. Load-deflection of column at room temperature (RT) and 700 °C 101

Figure 4-36. Local buckling of column at room temperature 102

Figure 4-37. Plastic hinge in column at room temperature 102

Figure 4-38. Deformed shape of column at maximum axial load at 700 °C 103

Figure 4-39. SAP2000 shell model of prefabricated panel 105

Figure 4—40. ANSYS model of prefabricated panel showing finite element mesh 106

Figure 4-41 . ANSYS model of prefabricated panel showing boundary conditions and loading 106

Figure 4-42. Deflection of prefabricated panel under 100 kip lateral load 107

Figure 4—43. Typical spandrel splice layout for exterior wall section model 108

Figure 4-44. Modehng of a typical interior spandrel splice in the exterior wall section model 108

Figure 4—45. Column Splice Model used in Exterior Wall Model 109

Figure 4—46. Exterior wall subsystem model with boundary conditions 110

Figure 4-47. Typical meshing of exterior wall model components 1 10

Figure 4-48. Exterior wall model temperature time-histories 112

Figure 4-49. Spandrel plate deformations 1 14

Figure 4-50. Lateral deflections for Case 6 and Case 7 1 14

Figure 4-5 1 . Structural response (out-of-plane deformations) for temperature time history DBARE

and pulled-in at three disconnected floors for Case 8. (lOX displacement

magnification) 115

Figure 4-52. Column splice contact element status for temperature time history and DBARE and

pulled at three disconnected floors for Case 8 1 16

Figure 4-53. Column splice bolt stresses for temperature time history and DBARE and pulled at three disconnected floors for Case 8 116

Figure 4-54. Structural response (out-of-plane deformations) for temperature time history DBARE

and pushdown with three disconnected floors for Case 9 (lOX displacement

magnification) 117

Figure 4-55. Out-of-plane deformation as a function of pushdown displacement after appHcation

of temperamre DBARE with three disconnected floors for Case 9 117

Figure 4-56. Total reaction at column base resulting from pushdown with temperature DBARE

and three disconnected floors for Case 9 118

NIST NCSTAR 1-6. WTC Investigation xiii
List of Figures

Figure 4-57. Individual column reaction during pushdown with temperature DBARE and three

disconnected floors for Case 9 118

Figure 5-1 . Validation of Aircraft Impact Analysis Prediction With Observations for WTC 1 North Exterior Wall Damage 122

Figure 5-2. Validation of Aircraft Impact Analysis Prediction With Observations for WTC 2

South Exterior Wall Damage 123

Figure 5-3. Core column damage levels 124

Figure 5-^. Impact damage to WTC 1 Floor 95 for Case A (plan view) 127

Figure 5-5. Damage to WTC 1 Floor 95 framing and slab for Case A 128

Figure 5-6. Debris and fuel field in WTC 1 Case B analysis of aircraft impact 131

Figure 5-7. WTC 1 Case B aircraft impact damage to framing, partitions, and furnishings on

Floor 95 131

Figure 5-8. WTC 1 Case B Floor 95 partitions and furnishings layout prior to impact (layout

provided in model only where interaction with aircraft was expected) 132

Figure 5-9. WTC 1 Case B Floor 95 partitions and furnishings layout after impact with overlay

showing extent of dislodged insulation from direct debris impact 132

Figure 5-10. Definition of structural floor and occupancy floor 133

Figure 5-1 1. Plan view of WTC 1 Case B insulation and column damage for Occupancy Floor 95 133

Figure 5-12. Plan view of WTC 1 Case B damage to Structural Floors 95 134

Figure 5-13. Plan view of WTC 1 Case A cumulative damage for Floors 93 to 98 135

Figure 5-14. Plan view of WTC 1 Case A insulation and column damage to Occupancy Floors 93

to 98 136

Figure 5-15. Plan view of WTC 1 Case A damage to Structural Floors 93 to 98 137

Figure 5-16. Plan view of WTC 1 Case B cumulative damage from Floors 93 to 98 138

Figure 5-17. Plan view of WTC 1 Case B insulation and column damage to Occupancy Floors 93

to 98 139

Figure 5-18. Plan view of WTC 1 Case B damage to Structural Floors 93 to 98 140

Figure 5-19. Plan view of WTC 2 Case C cumulative damage from Floors 78 to 83 141

Figure 5-20. Plan view of WTC 2 Case C insulation and column damage to Occupancy Floors 78

to 83 '. 142

Figure 5-21. Plan view of WTC 2 Case C damage to Structural Floors 78 to 83 143

Figure 5-22. Plan view of WTC 2 Case D cumulative damage from Floors 78 to 83 144

Figure 5-23. Plan view of WTC 2 Case D insulation and column damage to Occupancy Floors 78

to 83 145

Figure 5-24. Plan view of WTC 2 Case D damage to Structural Floors 78 to 83 146

Figure 5-25. Overlay of WTC Footprint on Pentagon damage area from aircraft impact (original

figure from ASCE, 2003) 148

XIV NISTNCSTAR 1-6, WTC Investigation
List of Figures

Figure 5-26. TA,^ical damage to spirally reinforced columns in the Pentagon impacted and bent by

large debris (ASCE. 2003) 149

Figure 5-27. Typical damage to spirally reinforced colunms in the Pentagon not impacted or bent

by large debris^(ASCE, 2003) 1 50

Figure 6-1 . Initial aircraft impact damage on WTC 1 north face 157

Figure 6-2. Initial aircraft impact damage on WTC 1 west and south faces minutes after impact

(exact time of image is unknown) 158

Figure 6-3. SFRM knocked off north exterior columns. Arrows show where fireproofmg was damaged or missing 159

Figure 6-4. South face of WTC 1 with fire visible only on the west side at 9:25. Arrow shows

region where debris pile under missing panel was obser\'ed 159

Figure 6-5. Fires on WTC 1 south face at 9:40 a.m. Note lack of inward bowing 160

Figure 6-6. WTC 1 exterior columns bowing inward across most of the south face between

Floors 95 to 97 (or 98) at 10:23 a.m. Note buckled panel at SW comer 161

Figure 6-7. Expulsion of smoke and debris at WTC 1 Floor 98 on the east, north, and west faces 162

Figure 6-8. Smoke expulsion at Floor 98 from north and west faces as collapse initiates 163

Figure 6-9. Smoke expulsion at Floor 98 from north and east faces at collapse initiation 164

Figure 6-10. Rotation of WTC 1 building section above the aircraft impact zone toward the south

as viewed fi^om due north. Note that there is no tilt in the east or west directions 165

Figure 6-11. WTC 1 tilt to the south of approximately 8 degrees was measured before smoke and

debris obscured view. Note view is from west and tilt is directly south 166

Figure 6-12. Aircraft impact into WTC 2 and fireball, view from the east 171

Figure 6-13. Hanging object (noted by arrows) in east windows of Floor 82 appears to be edge of

Floor 83 172

Figure 6-14. Debris piles at windows where fire are burning at the northeast comer 172

Figure 6-15. Hanging object (noted by arrows) in north windows of Floor 79 appears to be edge of

Flool-80 173

Figure 6-16. Image showing damage to fireproofmg on east face of WTC 2 due to intemal impact.

Red arrows highlight areas where fireproofmg has been damaged. The blowup to the

right shows a column where red Tnemec primer paint is visible 174

Figure 6-17. WTC 2 exterior columns bowing inward across north side of the east face between

Floors 77 to 83 at 9:2 1 a.m 1 75

Figure 6-18. Inward Bowing of east Face of WTC 2 at 9:21 a.m 175

Figure 6-19. Inward Bowing of east Face of WTC 2 at 9:53 a.m 176

Figure 6-20. Inward Bowing of east Face of WTC 2 at 9:53 a.m 177

Figure 6-2 1 . WTC 2 exterior columns bowing inward across the east face between Floors 77 to 83

at 9:59 a.m 178

NIST NCSTAR 1-6. WTC Investigation xv
List of Figures

Figure 6-22. View of WTC 2 buckling of east wall near northeast comer as collapse initiates from

northeast 179

Figure 6-23. View of east wall buckling and WTC 2 collapse from southeast 180

Figure 6-24. View of upper building section of WTC 2 tilting to the east 181

Figure 6-25. View of upper building section of WTC 2 tihing to the east from the northeast 182

Figure 6-26. Kink on southeast comer near Floor 106 formed after collapse initiation 183

Figure 7-1. Isolated core models 188

Figure 7-2. Vertical displacement of WTC 1 isolated core model with impact damage and gravity

loads 189

Figure 7-3. North side vertical displacement of the WTC 1 core model at 100 min for Case A

temperatures 190

Figure 7^. South side vertical displacement of the WTC 1 core model at 100 min for Case B

temperatures 191

Figure 7-5. Vertical displacement of the WTC 2 isolated core model with impact damage and

gravity loads (south and east faces) 192

Figure 7-6. Vertical displacement of the WTC 2 core model at 60 min for Case C temperatures

(south and east faces) 193

Figure 7-7. South and east side vertical displacement of the WTC 2 core model at 60 min for Case D temperatures 194

Figure 7-8. Full floor model. 195

Figure 7-9. Fireproofmg damage to WTC 1 Floor 97 for Case A i and Case A 198

Figure 7-10. WTC 1 Floor 97 comparison of tmss temperatures for Case A i and Case A 199

Figure 7-11. Vertical deflection of WTC 1 Floor 95 for Case Bi at 10 min 202

Figure 7-12. Vertical deflection of WTC 1 Floor 96 for Case Bi at 10 min 202

Figure 7-13. Vertical deflection of WTC 1 Floor 97 for Case Bi at 100 min 203

Figure 7-14. Vertical deflection of WTC 1 Floor 98 for Case Bi at 100 min 203

Figure 7-15. Vertical deflection of WTC 1 Floor 99 for Case Bi at 100 min 204

Figure 7-16. Loss of vertical supports in WTC 1 Floor 97 and Floor 98 for Case Bi 204

Figure 7-17. Vertical deflection of WTC 2 Floor 79 for Case Ci at 60 min •. 206

Figure 7-18. Vertical deflection of WTC 2 Floor 80 for Case Ci at 60 min 206

Figure 7-19. Vertical deflection of WTC 2 Floor 81 for Case Ci at 60 min 207

Figure 7-20. Vertical deflection of WTC 2 Floor 82 for Case Ci at 60 min 207

Figure 7-21 . Vertical deflection of WTC 2 Floor 83 for Case Ci at 60 min 208

Figure 7-22. Loss of vertical supports in WTC 2 Floor 82 and Floor 83 for Case Ci 208

Figure 7-23. Vertical deflection of WTC 2 Floor 79 for Case Di 210

Figure 7-24. Vertical deflection of WTC 2 Floor 80 for Case Di 210

xvi NISTNCSTAR 1-6, WTC Investigation
List of Figures

Figure 7-25. Vertical deflection of WTC 2 Floor 81 for Case Di 211

Figure 7-26. Vertical deflection of WTC 2 Floor 82 for Case Di 211

Figure 7-27. Vertical deflection of WTC 2 Floor 83 for Case Di 212

Figure 7-28. Isolated exterior wall segments from WTC 1 and WTC 2 214

Figure 7-29. Boundary conditions applied on the isolated exterior wall segment 214

Figure 7-30. Locations of WTC 1 disconnections and pull-in forces over five floors for Case B 216

Figure 7-3 1 . Inward displacement of the WTC 1 south wall at 80 min of the Case B temperatures

with floor disconnections and 6 kip pull-in forces over five floors 217

Figure 7-32. Inward displacement of the WTC 1 south wall at 100 min of the Case B temperatures

with floor disconnections and 6 kip pull-in forces over five floors 218

Figure 7-33. Out-of-plane displacements of the WTC 2 east wall calculated with 0.5 kip pull-in

force with uniform magnitude distribution at 32 min 220

Figure 7-34. Out-of-plane displacements of the WTC 2 east wall calculated with 5.0 kip pull-in

force with uniform magnitude distribution at 1 8 min 22

1

Figure 7-35. Out-of-plane displacements of east wall calculated with pull-in force of 1 .0 kip on the

south half and 4.0 kip on the north half of the WTC 2 east wall 222

Figure 7-36. Out-of-plane displacements of east wall of WTC 2 calculated with pull-in forces of

1 .5 kip on the south half and 5.0 kip on the north half 223

Figure 8-1. Displaced shape of WTC 1 and WTC 2 at the end of gravity load analysis 232

Figure 8-2. Office area and core floors and core beams 233

Figure 8-3. Hat truss with labeled outriggers 233

Figure 8-4. Vertical displacement at Floor 99 of WTC 1. Total displacements are shown before

aircraft impact and incremental displacements, with total displacements in parentheses, are shown after impact 238

Figure 8-5. Vertical displacement of WTC 1 east and north exterior walls before aircraft impact 238

Figure 8-6. Vertical displacement of east and north exterior walls of WTC 1 after aircraft impact

for Case B 239

Figure 8-7. Vertical displacement of the east and north side of the WTC 1 core before aircraft impact 239

Figure 8-8. Vertical displacement of the east and north side of the WTC 1 core after aircraft impact for Case B 240

Figure 8-9. Maximum demand-to-capacity ratio for axial force in core columns between Floor 93

and Floor 99 of WTC 1 before aircraft impact 241

Figure 8-1 0. Maximum demand-to-capacity ratio for axial force in core columns between Floor 93

and Floor 99 of WTC 1 after aircraft impact for Case B 241

Figure 8-1 1 . Out-of-plane displacement of south wall of WTC 1 at 80 min for Case B 243

Figure 8-12. Out-of-plane displacement of south wall of WTC 1 at 100 min for Case B conditions

with 5 kip pull-in forces 244

NISTNCSTAR 1-6. WTC Investigation xvii
List of Figures

Figure 8-13. Time history of maximum out-of-plane displacement of WTC 1 south wall for Case B with 5 kip pull-in forces 244

Figure 8-14. Distribution of axial force in exterior columns at Floor 98 of WTC 1 south wall for Case B with 5 kip pull-in forces 245

Figure 8-15. Distribution of axial force in exterior columns at Floor 98 of WTC 1 east wall for

Case B with 5 kip pull-in forces 246

Figure 8-16. Vertical displacement of west and south exterior walls of WTC 1 at 80 min for Case B 246

Figure 8-17. Vertical displacement of west and south exterior walls of WTC 1 at 100 min for

Case B 247

Figure 8-18. Change in vertical displacement at Floor 99 of WTC 1 from the state before impact

to 100 min for Case B 247

Figure 8-19. Maximum elastic-plus-plastic-plus-creep strain for columns between Floor 93 and

Floor 99 of WTC 1 at 10 min for Case B (strain values are in percent) 248

Figure 8-20. Maximum elastic-plus-plastic-plus-creep strain for columns between Floor 93 and

Floor 99 of WTC 1 at 40 min for Case B (strain values are in percent) 249

Figure 8-2 1 . Maximum elastic-plus-plastic-plus-creep strain for columns between Floor 93 and

Floor 99 of WTC 1 at 100 min for Case B with 5 kip pull-in forces (strain value are

in percent) 249

Figure 8-22. Maximum demand-to-capacity ratio for axial force in core columns between Floor 93

and Floor 99 of WTC 1 at 80 min for Case B conditions 250

Figure 8-23. Maximum demand-to-capacity ratio for axial force in core columns between Floor 93

and Floor 99 of WTC 1 at 100 min for Case B with 5 kip pull-in forces 25

1

Figure 8-24. Tension demand-to-capacity ratio for core column splices at WTC 1 Floor 106 at 100 min for Case B with 5 kip pull-in forces 252

Figure 8-25. Location and label of outriggers and supporting columns for WTC 1 253

Figure 8-26. Vertical displacement before impact of WTC 2 exterior wall for Case D 255

Figure 8-27. Vertical displacement after impact of WTC 2 exterior wall for Case D 256

Figure 8-28. Vertical displacement before impact of WTC 2 core for Case D 256

Figure 8-29. Vertical displacement after impact of WTC 2 core for Case D 257

Figure 8-30. Lateral displacements after impact above WTC 2 Floor 86 in the x-direction (northsouth) for Case D 257

Figure 8-3 1 . Lateral displacements after impact above WTC 2 Floor 86 in the y-direction (eastwest) for Case D 258

Figure 8-32. Core column loads (kip) before impact at WTC 2 Floor 83 for Case D (compression

is positive) 259

Figure 8-33. Core column loads (kip) after impact at WTC 2 Floor 83 for Case D (compression is positive) 260

Figure 8-34. Vertical displacement of exterior wall of WTC 2 at 20 min for Case D 262

xvni NISTNCSTAR 1-6, WTC Investigation
! Figures

Figure 8-35. Vertical displacement at Floor 83 of WTC 2 at 20 min for Case D (note the tilt toward east and south) 262

Figure 8-36. Vertical displacement of exterior wall of WTC 2 at 43 min for Case D 263

Figure 8-37. Vertical displacement at Floor 83 of WTC 2 at 43 min for Case D (note the tilt toward east and south) 263

Figure 8-38. Out-of-plane displacement of the east wall of WTC 2 at 20 min for Case D 264

Figure 8-39. Out-of-plane displacement of the east wall of WTC 2 at 43 min for Case D 266

Figure 8—40. Variation of maximum out-of-plane displacement on the east wall of WTC 2 over the

time for Case D 266

Figure 8—41 . Vertical displacement of core of WTC 2 at 20 min for Case D 267

Figure 8—42. Vertical displacement of core of WTC 2 at 43 min for Case D 267

Figure 8^3. Core column loads (kip) at Floor 83 of WTC 2 at 20 min for Case D (compression is positive) 268

Figure 8^4. Core column loads (kip) at Floor 83 of WTC 2 at 43 min for Case D (compression is positive) 268

Figure 8^5. Total displacements of WTC 2 above Floor 86 at 43 min of Case D (deformed shape

magnified 20 times). Note the tilt toward east and south 269

Figure 8^6. Axial force in the east wall columns at Floor 83 of WTC 2 for Case D (compression

is positive) 269

Figure 8-47. Axial force in the west wall columns at Floor 83 of WTC 2 for Case D (compression

is positive) 270

Figure 8^8. Axial force in the south wall columns at Floor 83 of WTC 2 for Case D (compression

is positive) 270

Figure 8—49. Axial force in the north wall columns at Floor 83 of WTC 2 for Case D (compression

is positive) 271

Figure 8-50. Axial force in Floor 83 columns of WTC 2 before impact for Case D (compression is positive) 271

Figure 8-5 1 . Axial force in Floor 83 columns of WTC 2 at 43 min for Case D (compression is positive) 272

Figure 8-52. Maximum elastic-plus-plastic-plus-creep strains at 20 min for columns between

Floor 78 and Floor 83 of WTC 2 for Case D (strain values are in percent) 272

Figure 8-53. Maximum elastic-plus-plastic-plus-creep strains at 43 min for columns between

Floor 78 and Floor 83 of WTC 2 for Case D (strain values are in percent) 273

Figure 8-54. Axial force in core columns (kip) at WTC 2 Floor 105 (at hat-truss level) before

impact for Case D (compression is positive) 274

Figure 8-55. Axial force in core columns (kip) at WTC 2 Floor 105 (at hat-truss level) after impact

for Case D (compression is positive) 275

Figure 8-56. Axial force in core columns (kip) at Floor 105 (at hat-truss level) of WTC 2 for Case D conditions (compression is positive) 276

Figure 8-57. State of core column splices at Floor 105 of WTC 2 277

NISTNCSTAR 1-6. WTC Investigation xix
List of Figures

Figure 8-58. Location and IDs of outriggers and supporting columns 278

Figure 8-59. Temperatures of two adjacent trusses (left) and two adjacent perimeter columns

(right) exposed to simulated fires in WTC 1 . Data plotted in blue are for structural

steel components with fireproofing; data in red are for steel components without

fireproofmg (from NISTNCSTAR 1-5) 281

Figure 9-1. Critical analysis inter-dependencies 289

Figure 9-2. Full analysis tree for influential parameter effects 291

Figure 9-3. Pruned analysis tree for influential parameter effects 292

Figure 9-4. Data used for validating probable collapse sequences 295

Figure 9-5. Variability in global reserve capacity using model predictions and observables for sequential analyses with imperfect information 296

Figure 9-6. Vertical displacement contour of the detailed truss model under thermal loading 297

Figure 9-7. Combined flexural and catenary action in the floor system 297

Figure 9-8. WTC 1 probable collapse sequence 299

Figure 9-9. Vertical displacement of Floors of WTC 1 for Case B' at 100 min 302

Figure 9-10. Loss of vertical supports observed in Floor 97 and Floor 98 of WTC 1 for Case B' (Ix displacement magnification) 302

Figure 9-11. Inward bowing of the WTC 1 south wall of WTC 1 at 10:23 a.m 303

Figure 9-12. Inward bowing of south wall of WTC 1 global model with creep at 100 min for Case

B with 5 kip pull-in forces (5x displacement magnification) 304

Figure 9-13. Expulsion of smoke and debris at WTC 1 Floor 98 on the east, north, and west faces 305

Figure 9-14. WTC 2 probable collapse sequence 307

Figure 9—15. Vertical displacements of Floors 79 through Floor 88 of WTC 2 at 40 min (Case D) 311

Figure 9-16. Floor sagging observed on the east wall of WTC 2 at different stages 312

Figure 9-17. Out-of-plane displacements on the east wall of WTC 2 (Case D) 313

Figure 9-1 8. Out-of-plane displacement estimates of the east wall of WTC 2 from photographs 3 14

Figure 9-19. Variation of maximum out-of-plane displacement on the east wall of WTC 2

(Case D) 315

Figure 9-20. Total column loads at Floor 83 of the east wall of WTC 2 at different stages

(Case D) 315

Figure 9-21. Maximum elastic + plastic + creep strains for columns between Floor 78 and Floor

83 of WTC 2 at different stages (Case D) (strain values are in percent) 316

Figure 9-22. Inward bowing of the east wall of WTC 2 when buckled at 43 min for Case D

(4x displacement magnification) 317

Figure 9-23. Inward bending of exterior columns of the west wall of WTC 2 just before collapse 318

Figure 9-24. Total displacements of WTC 2 above Floor 86 at 43 min for Case D. Note tilt toward

east and south (20x displacement magnification) 319

XX NISTNCSTAR 1-6, WTC Investigation
List of Tables

Table P-1 . Federal building and fire safety investigation of the WTC disaster.Error! Bookmark not defined.

Table P-2. Public meetings and briefings of the WTC Investigation Error! Bookmark not defined.

Table 2-1. Average SFRM thickness from six measurements taken in 1994 on each of 16

random floor trusses on floors 23 and 24 of WTC 1 23

Table 2-2. Summary of specified, in-place and thermally equivalent thickness of SFRM 29

Table 2-3. Measured thermal conductivity as a function of temperature 31

Table 2—4. Summary of physical characteristics of BLAZE-SHIELD DC/F specimens tested at NIST 35

Table 3-1 . Results of ASTM E 1 1 9 Standard Fire Tests 49

Table 4-1 . Steel types used in WTC 1 and WTC 2 61

Table 4-2. Interior seat capacity against vertical force 74

Table 4-3. Exterior seat capacity against vertical force 75

Table 4-4. Interior seat capacity against horizontal tensile force 75

Table 4-5. Exterior seat capacity against horizontal tensile force 76

Table 4-6. Knuckle shear capacity reduction for elevated temperatures 84

Table 4-7. Comparison of SAP2000 and ANSYS results for gravity load case 92

Table 4-8. Comparison of SAP2000 and ANSYS Modal Analysis Results 92

Table 4-9. Types of break elements 95

Table 4-10. Column section properties 99

Table 4-11. Spandrel Splice Details 99

Table 4-12. Column Splice Details 99

Table 4-13. Prefabricated panel validation results 107

Table 4-14. Thermal loading conditions used in the exterior wall model 1 1

1

Table 4-15. Analysis cases for exterior wall section model 113

Table 5-1. WTC 1 Case A core column damage 124

Table 5-2. WTC 1 Case B core column damage 125

Table 5-3. WTC 2 Case C core column damage 125

Table 5^. WTC 2 Case D core column damage 126

NISTNCSTAR 1-6, WTC Investigation xxi
List of Tables

Table 6-1. WTCl Timeline of Observed Structural and Fire Events 154

Table 6-2. WTC2 Timeline of Observed Structural and Fire Events 167

Table 6-3. Possible floor damage observed in photos of WTC 2 windows 168

Table 7-1. Maximum vertical displacement of WTC 1 floors for Case Ai 199

Table 7-2. Maximum vertical displacement of WTC 1 floors for Case Bi 199

Table 7-3. Maximum vertical displacement of WTC 2 floors for Case Ci 203

Table 7^. Maximum vertical displacement of WTC 2 floors for Case Di 207

Table 8-1. Total column loads at Floor 98 of WTC 1 for Case B conditions 240

Table 8-2. Total column loads at Floor 105 of WTC 1 for Case B conditions 240

Table 8-3. Demand-to-capacity ratio for axial force in outriggers of WTC 1 for Case B 25

1

Table 8-4. Total column loads at WTC 2 Floor 83 for Case D (Compression is positive) 256

Table 8-5. Total column loads at WTC 2 Floor 105 for Case D (Compression is positive) 257

Table 8-6. Change in total column loads before and after aircraft impact. (Loads After Impact) -

(Loads Before Impact) (Compression is positive) 262

Table 8-7. Change in total column loads between 40 min and 43 min. (Loads at 43 min) -

(Loads at 40 min) (Compression is positive) 262

Table 8-8. Demand-to-capacity ratios for outriggers of WTC 2 for Case D conditions (outrigger

IDs are shown in Fig. 8-3) 276

Table 9-1 . Aircraft impact analysis parameters 290

Table 9-2. Fire dynamics analysis parameters 291

Table 9-3. Thermal analysis parameters 291

Table 9-4. Structural response analysis parameters 292

Table 9-5 . Observations for WTC 1 296

Table 9-6. Total column loads at Floor 98 and Floor 105 of WTC 1 for Case B 299

Table 9-7. Key observations on WTC 2 304

Table 9-8. Total column loads at Floor 83 of WTC 2 for Case D. Compression is positive 308

Table 9-9. Total column loads at Floor 105 of WTC 2 for Case D. Compression is positive 308

Table 9-10. Change in total column loads when the east wall of WTC 2 buckles (Case D,

compression is positive) 317

XXll NISTNCSTAR 1-6, WTC Investigation
List of Acronyms and Abbreviations

Acronyms

ASTM ASTM International

BPS Building Performance Study

FEMA Federal Emergency Management Agency

MIG Metal Inert Gas (welding)

NIST National Institute of Standards and Technology

PANYNJ Port Authority of New York and New Jersey

SEAoNY Structural Engineers Association of New York

T TO use United States Lode

ULI Underwriters Laboratories Inc. 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

°C degrees Celsius

°F degrees Fahrenheit

ft feet

in. inch

L liter m meter

|im micrometer

min minute

s second

NISTNCSTAR 1-6, WTC Investigation xxin
List of Acronyms and Abbreviations

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XXIV NISTNCSTAR 1-6, WTC Investigation
Preface

Genesis of This Investigation

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 107-231), 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.

NISTNCSTAR 1-6. WTC Investigation XXV
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 life 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 building1 and fire safety investiqation of the WTC disaster. e S I ^ 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, constaiction, 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 abnonnal 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

Detennine 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.

xxvi NISTNCSTAR 1-6, WTC Investigation
Preface

NIST WTC Investigation Projects

Baseline Performance & Impact

Damage Thermal and

Tenabillty Environment

Video/

Photographic

Records

Oral History Data

Emergency

Response

Records

Analysis of Codes and

Practices Active Fire Protection

Nisr

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 Br\ an. Professor Emeritus, University of Maryland

• Da\ id Collins, President, The Preview Group, Inc.

• Glenn Corbett, Professor, John Jay College of Criminal Justice

• Philip DiNenno, President, Hughes Associates, Inc.

NISTNCSTAR 1-6, WTC Investigation xxvu I
Preface

• Robert Hanson, Professor Emeritus, University of Michigan

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

Inc.

• Kathleen Tiemey, 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 preliminary 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.

XXVlll NISTNCSTAR 1-6, WTC Investigation
Preface

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

Date Location Principal Agenda

June 24. 2002 New York Cit>-. NY Public meeting: Public comments on the Draft Plan for the pending WTC Investigation.

August 21. 2002 Gaithersburg. MD Media briefing announcing the fonnal 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 Mav 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 fonmilating final recommendations.

June 18. 2004 New York City. NY Media/public 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 Public 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 public comment session.

VJai lllCl oUUi iVlJ_/ 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 public

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 resuhing from the WTC Investigation

and the R&D program, and (b) provide practical guidance and tools to better prepare facihty 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 1-6, WTC Investigation XXIX
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 pubhcations are:

NIST (National Institute of Standards and Technology). 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Final Report on the Collapse ofthe World Trade

Center Towers. NIST NCSTAR 1. Gaithersburg, MD, September.

NIST (National Institute of Standards and Technology). 2006. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Final Report on the Collapse of World Trade Center 7. NIST NCSTAR 1 A. 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 ofStructural 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 ofthe World Trade Center Disaster: Design and Construction ofStructural 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 ofthe World

Trade Center Disaster: Comparison ofBuilding 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 ofthe 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 ofthe 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 ofthe World

Trade Center Disaster: Comparison of Codes, Standards, and Practices in Use at the Time ofthe

Design and Construction of World Trade Center I, 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 ofthe World Trade Center Disaster: Comparison ofthe 1968 and Current (2003) New

XXX NIST NCSTAR 1-6, 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 ofthe 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 ofthe 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

ofthe 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 ofthe World

Trade Center Disaster: Design, Installation, and Operation ofFuel System for Emergency Power in World Trade Center 7. NIST NCSTAR 1-1 J. National Institute of Standards and Teclinology.

Gaithersburg, MD, September.

Sadek, F. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster:

Baseline Structural Performance and Aircraft Impact Damage Analysis ofthe 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 ofthe World Trade Center Disaster: Reference Structural Models and Baseline Performance Analysis of

the World Trade Center Towers. NIST NCSTAR 1-2 A. 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 ofthe World Trade

Center Disaster: Analysis ofAircraft 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 ofthe World Trade Center

Disaster: Mechanical and Metallurgical Analysis ofStructural 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 ofthe 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 1-6, WTC Investigation XXXI
Preface

Banovic, S. W. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center j Disaster: Steel Inventojy and Identification. NIST NCSTAR 1-3B. National Institute of Standards

| and Technology. Gaithersburg, MD, September.

j

J Banovic, S. W., and T. Foecke. 2005. Federal Building and Fire Safety Investigation ofthe World \ Trade Center Disaster: Damage and Failure Modes ofStructural 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 ofthe World

Trade Center Disaster: Mechanical Properties ofStructural 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 ofthe World Trade Center Disaster: Physical Properties ofStructural Steels. NIST

NCSTAR 1-3E. National Institute of Standards and Technology. Gaithersburg, MD, September.

Evans, D. D., R. D. Peacock, E. D. Kuhgowski, W. S. Dols, and W. L. Grosshandler. 2005. Federal

Building and Fire Safety Investigation ofthe 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 ofthe 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

ofthe 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 ofthe World

Trade Center Disaster: Fire Alarm Systems. NIST NCSTAR 1-4C. National Institute of Standards i and Technology. Gaithersburg, MD, September. \

Ferreira, M. J., and S. M. Strege. 2005. Federal Building and Fire Safety Investigation ofthe

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 ofthe World Trade , Center Disaster: Reconstruction ofthe 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 1-5A. National Institute of Standards and Technology. Gaithersburg, MD, '

: September.

xxxn NIST NCSTAR 1-6, WTC Investigation
Preface

Hamins, A., A. Maranghides, K. B. McGrattan, E. Johnsson, T. J. Ohlemiller, M. Donnelly,

J. Yang. G. Mulholland. K. R. Prasad, S. Kukuck, R. Anleitner and T. McAllister. 2005. Federal

Building and Fire Safety' Investigation ofthe World Trade Center Disaster: Experiments and

Modeling ofStructural Steel Elements Exposed to Fire. NIST NCSTAR 1-5B. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Ohlemiller, T. J.. G. W. Mulholland, A. Maranghides, J. J. Filliben, and R. G. Gann. 2005. Federal

Building and Fire Safety^ Investigation ofthe World Trade Center Disaster: Fire Tests ofSingle

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. Reinhom, and P. A. Hough. 2005.

Federal Building and Fire Safety Investigation ofthe 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 ofthe World Trade Center Disaster: Experiments and

Modeling ofMultiple 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 ofthe World Trade Center Disaster: Computer Simulation ofthe 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 ofthe World

Trade Center Disaster: Fire Structure Interface and Thermal Response ofthe 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 ofthe World Trade

Center Disaster: Structural Fire Response and Probable Collapse Sequence ofthe World Trade Center

Towers. NIST NCSTAR 1-6. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Carino, N. J., M. A. Stames, J. L. Gross, J. C. Yang, S. Kukuck, K. R. Prasad, and R. W. Bukowski.

2005. Federal Building and Fire Safety Investigation ofthe 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 ofthe World Trade Center Disaster: Fire Resistance Tests ofFloor 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 1-6. WTC Investigation XXXIll
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 ofthe World Trade Center Disaster: Global Structural

Analysis ofthe Response ofthe 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 ofthe World

Trade Center Disaster: Structural Fire Response and Probable Collapse Sequence of World Trade

Center?. (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 ofthe World Trade

Center Disaster: Structural Analysis ofthe 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 ofthe World Trade Center

Disaster: Analysis ofSeptember 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 ofthe 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 ofthe 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 ofPublished Accounts ofthe 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 ofthe World Trade Center

Disaster: Technical Documentation for Sun>ey 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 ofthe World

Trade Center Disaster: The Emergency Response Operations. NIST NCSTAR 1-8. National Institute of

Standards and Technology. Gaithersburg, MD, September.

xxxiv NIST NCSTAR 1-6, WTC Investigation
Acknowledgments

The work reported herein w as conducted with the assistance of several contractors and, in addition, many

companies and individuals contributed in a substantial way.

The structural analyses presented in this report were conducted in collaboration with:

• Simpson Gumpertz & Heger Inc. (SGH). of Waltham. Massachusetts; Dr. Mehdi Zarghamee,

Project Leader, whose work included the development of structural models and the conduct of the temperamre-dependent analyses for the prediction of the structural performance of components

and subsystems of the WTC towers and for the determination of collapse of each tower .

• Computer Aided Engineering Associates Inc. (CAEA), Woodbury, Connecticut; Dr. Peter

Barrett. Project Leader, who provided technical assistance to SGH and to NIST on the complex

computer analyses.

• Dr. R. Shankar Nair of Teng Associates. Prof Daniele Veneziano of MIT, and Prof Kaspar

Willam of the University of Colorado, who provided expertise in the areas of computational

mechanics, structural behavior of tall buildings and reliability.

The work in determining the fireproofmg of the WTC towers during their construction and the upgrading

of fireproofmg on the steel trusses that took place in the 1990's involved many companies and

individuals. Their cooperation and assistance in providing the needed information is gratefully

acknowledged. The companies and key individuals included:

• Isolatek International. Paulette Kaminski

• Laclede Steel Co. David McGee

• Morse Zehnter Associates, Roger Morse

• Leslie E. Robertson Assoc. (LERA), William Faschen and Richard Garlock

In addition to providing information on the fireproofmg, the Port Authority of New York and New Jersey

(PANYNJ), and in particular Saroj Bohl, Joe Englot, and Frank Lombardi, researched and provided answers to specific questions at the request of NIST.

Investigative work into the fireproofmg of the WTC was led by Dr. Monica Stames. Data analysis and

determination of fireproofmg for the finite element calculations was made by Dr. Nicholas Carino.

Concrete properties at elevated temperatures were established by Dr. Long Phan for component and

subsystem finite element analyses.

NISTNCSTAR 1-6. WTC Investigation XXXV
Acknowledgments

Large-scale furnace tests of floor assemblies were conducted by Underwriters Laboratories Inc. (UL),

Northbrook, Illinois, and the contribution of the many individuals who contributed to this effort, led by

Joe Treadway and Frederick Hervey, is gratefully acknowledged. The cooperation and dedication of the

project team are greatly appreciated.

In addition, the following WTC Investigation projects made significant contributions:

• Project 2: Baseline Structural Performance and Aircraft Impact Damage Prediction, led by Dr. Fahim Sadek, provided reference models and the aircraft impact simulations allowing the

estimation of structural and fireproofmg damage.

• Project 3: Mechanical and Metallurgical Analysis of Structural Steel, led by Dr. Frank W. Gayle,

provided the mechanical characteristics of the tower steels that were used in the constitutive

models for the finite element analyses. In addition, Dr. Tim Foecke conducted the photographic

interpretation and estimation of bowing of the exterior walls of both towers.

• Project 5: Reconstruction of Thermal and Tenability Environment, led by Dr. Richard Gann,

provided temperature histories for the structural components for use in the finite element analyses

of components, subsystems and global systems. In addition. Dr. William M. Pitts provided

assistance in the collection, cataloging and interpretation of the many videos and photographs

relevant to this project.

NIST acknowledges the parties to the insurance litigation for voluntarily making their findings available

toNIST.

XXXVl NISTNCSTAR 1-6, WTC Investigation
Executive Summary

E.1 PURPOSE AND SCOPE

One of the four objectives of the National Institute of Standards and Technology (NIST) investigation of

the collapse of the World Trade Center (WTC) towers was to detemiine why and how the two towers

(WTC 1 and WTC 2) collapsed following the initial impacts of the aircraft. Both the north and south

towers of the World Trade Center were severely damaged by the impact of Boeing 767 aircraft, yet they

remained standing for some time. The ensuing fires were observed to move through both buildings and

eventually, both buildings collapsed. The probable collapse sequence for each of the WTC towers as well

as the extent and relative importance of the damage caused by the aircraft impact and subsequent

weakening by fires were investigated under this project. Structural Response and Collapse Analysis of

WTC Towers to Aircraft Impact Damage and Fire Conditions.

Events that played a significant role in the structural performance of the towers were the aircraft impact,

rapid ignition of fire on multiple floors, and the growth and spread of fire in each tower. Detailed

information was required on the condition of the structural system and its passive fire protection system

(also referred to as fireproofing or thermal insulation), both before and after the aircraft impact, and

during the ensuing fires that elevated temperatures in the structural members. The purpose of this project,

then, was to analyze the response of the WTC towers to fires—both with and without aircraft damage

and to determine the probable sequence of structural collapse for each tower. Specifically, the Structural

Response and Collapse Analysis project intended to:

• Determine the pre- and post-aircraft impact condition of the passive fire protection used to thermally insulate the structural members and provide resistance to fire damage,

• Conduct tests of structural components and systems under fire conditions to quantify their behavior,

• Evaluate the response of floor and column systems under impact and fire conditions to understand their response,

• Evaluate the response of the WTC towers under impact and fire conditions, with and without

aircraft impact damage, and

• Develop and evaluate failure hypotheses, resulting in the probable sequence of structural events leading to collapse for each WTC tower.

The unprecedented complexity and sophistication of these analyses required the use of various strategies

for managing the computational demands while adequately capturing the essential physics. The overall

approach—from impact analysis to collapse initiation—combined mathematical modeling, statistical and

probability-based analysis, laboratory testing, and analysis of photographic and videographic records.

Data were collected from a number of sources and included structural plans and specifications, thermal

and mechanical (adhesion/cohesion) properties of sprayed fire-resistive material (SFRM), SFRM

NIST NCSTAR 1-6. WTC Investigation xxxvn
Executive Summary

thickness and condition in the towers, and recorded observations of structural events subsequent to

aircraft impact and prior to collapse. Information about tower construction was obtained from original

drawings, design and construction specifications, project documents including correspondence and

reports, and records provided by The Port Authority of New York and New Jersey (PANYNJ or Port

Authority), Leslie E. Robertson Associates (LERA), Silverstein Properties, and a number of contractors

that had worked on the design, construction, or modifications of the towers. Information about the events

that occurred in each tower on September 1 1

, 200 1, was obtained from analysis of available photographic

and videographic records, eyewitness accounts, and mechanical and metallurgical analysis of recovered

structural steel.

Computer simulations were used to model the complete sequence of events leading to the initiation of

collapse of the WTC towers. The analyses simulated the damage to the towers resulting from aircraft impact, the spread of multi-floor fires, the heating and thermal weakening of structural components, and

the progression of local structural failures that led to the collapse of the buildings. The structural response

analyses relied upon the following information:

• Reference global structural models of the WTC 1 and WTC 2 towers, and typical floor and

exterior wall subsystem models (NIST NCSTAR 1-2)

• Extent of damage to the structural systems and interior contents of the WTC 1 and WTC 2

towers resulting from aircraft impact (NIST NCSTAR 1-2)

• Temperature-dependent mechanical properties of the steels, welds, and bolts used in the

construction of the towers, including elastic, plastic, and creep properties from 20 °C to 700 X (NIST NCSTAR 1-3)

• Time-temperature histories for structural components and connections for standard fires (e.g.,

ASTM E 1 19) and actual fires based on fire dynamics simulations (NIST NCSTAR 1-5).

• Photographic and videographic records with time stamps that documented the observed

sequence of events (NIST NCSTAR 1-5).

E.2 METHODOLOGY AND ANALYSIS RESULTS

E.2.1 Overview and Approach

The interdependence of the analyses of significant events is illustrated in Fig. E-1. Reference structural models were first developed and used to determine the baseline perfomiance of each tower prior to September 11, 2001. The reference models were then used as a basis for the aircraft impact damage

models and the structural response models to ensure consistency between structural models. The aircraft impact analysis determined damage to the interior of the building, including the structural system, passive

fire protection, partition walls, and furnishings for each tower. The analysis also provided an estimate of

the fuel dispersion in the towers. These results provided initial conditions to the fire dynamics analysis,

thermal analysis, and structural analysis. The fire dynamics analysis simulated the grov^h and spread of

fires and produced gas temperature histories for each floor involved in fire. The fire dynamics model

accounted for window breakage and damage to interior partition walls and floors (both affecting

ventilation conditions), and the distribution of debris and ftjel. The thermal analysis used the heat transfer

XXXVlll NIST NCSTAR 1-6, WTC Investigation
Executive Summary

model to determine temperature histories for the various structural components. The thermal analysis

required input from the structural analysis model, fire dynamics analysis results, damage to fire protection, and tem.perature-dependent thennal material properties. The structural temperature histories,

also referred to as thermal loads, were input to the structural analysis, along with the structural impact

damage and temperature-dependent material properties, to determine the structural response of each

tower.

Reference Structural Models

SAP

Baseline

Performance

Analysis

1 SAP

Reference

Model

Conversion

Fire Dynamics

Analysis

I

Aircraft Impact

Damage Analysis

LS-DYNA

Resolution

1-4 in. 10-^ s / Structural / Model \ Conversion

Time scale range between analyses:

10 orders of magnitude

Length scale range between

analyses: 5 orders of magnitude

'ReferenceV

Model \

Conversion y/

Structural Response and -I Failure Analysis

ANSYS

Resolution

1 to 60 in. 600 s

Collapse Sequence

Figure E-1. Critical analysis inter-dependencies.

The WTC towers were large, complex structural systems. To include all of the structural components and

connections and their associated behavior and failure mechanisms using refined finite element meshes

would ha\ e been prohibitive. The analysis approach used was a variant of the well-established substructuring approach, adapted for the analysis of structures with highly nonlinear behavior that progressed

from individual components to major subsystems to global systems, as shown in Fig. E-2. The

component analyses were conducted to identify critical behavior and failure mechanisms that contributed

to the global structural response of each tower. The subsystem analyses incorporated the behavior and

failure mechanisms identified in the component studies, with modifications to reduce the model size and

complexity, thereby enhancing computational performance, without adversely affecting the quality of the

results. Whenever modeling modifications were used, they were validated against the detailed component

model results. The global analyses incorporated critical behavior and failure mechanisms, determined

from subsystem analyses, while making necessary modifications in the level of modeling detail.

NISTNCSTAR 1-6. WTC Investigation XXXIX
Executive Summary

Component Subsystems Global

Shear knuckle

Truss seat connections

Single truss and concrete

WTC 1 • Isolated core

® South exterior face

* Full floors 93 to 99

WTC 1 WTC 2

slab Full floor

Column splice connection

WTC 2

Isolated core

East exterior face

Full floors 79 to 83

Single story column

Nine story column

Nine story-nine column

exterior wall panel

Detailed nonlinear analyses to detemiine components

behavior and failure mechanism.

Nonlinear analyses with

models of reduced

complexity for component

behavior and failure

mechanism to determine

major subsystem behavior

and sequential failure

Nonlinear analyses

to determine global

behavior and

probable collapse

sequence.

Figure E-2. Structural Analysis Sequence.

Analysis of the global behavior and determination of probable collapse sequences for both WTC 1 and

WTC 2, which included work performed by other projects, was divided into the following tasks:

A. Develop finite element models based on reference models. Reference models faithfully represented

the actual structures. These reference models became the basis for all subsequent finite element analyses.

B. Develop the constitutive relationships for the materials used in the construction of the towers. Mechanical and chemical properties were determined for steel specimens recovered from the WTC site to assure that the materials used were in conformance with properties specified in the original design. The

mechanical properties at high loading rates for the aircraft impact analyses and at elevated temperatures

(from room temperature to 800 °C) for the thermal and structural analyses were also determined from the

steel specimens.

C. Characterize the passive passive fire protection applied to the structural steel. Neither the type of

materials nor the required thicknesses of SFRM were identified in the contract documents or

specifications. Estimates of the characteristics and condition of SFRM were needed for the thermal and

structural modeling of the towers.

D. Conduct standard fire resistance tests of composite truss floor system. Tests were conducted to: (1)

establish the baseline fire resistance rating of the composite truss floor system used in the WTC towers,

(2) understand the influence of thermal restraint by testing the floor system under both thermally

unrestrained and restrained conditions, and (3) provide experimental data to validate and provide

guidance to the development of the floor models and to interpret the analyses results.

xl NISTNCSTAR 1-6, WTC Investigation
Executive Summary

E. Establish the damage to the structure, passive fire protection, and partition walls as a result of aircraft impact. The aircraft impact resulted in significant damage to the exterior, floor, and core structures of the

buildings. The jet fuel dispersed inside the towers ignited the building contents and furnishings as well as influenced the amount of oxygen reaching the fires. The fire protection of steel components was

dislodged in areas of direct debris impact.

F. Document observ^ations and data related to structural events. NIST validated analysis results with key

observations obtained from its extensive collection of over 7,000 photographs and over 150 hours of

videotape documenting the events at the World Trade Center on September 1 1, 2001. Key observations were used in the analyses in three ways: ( 1) to determine input parameters, (2) to impose time-related constraints upon an analysis, or (3) to validate analysis results.

G. Compute temperature histories for structural components subjected to fires. To determine how the

towers were affected by the fires, estimates of the growth and spread of fires over time were developed

using fire dynamics simulations. Temperature histories of the steel structural components and concrete

floor slabs were predicted in thennal analyses.

H. Conduct component and subsystem analyses. These analyses provided understanding of the nonlinear

behavior of structural components and subsystems under gravity and thermal loading and were used to develop reduced models for the global analyses. The components and subsystems considered included:

(1) typical floor subsystem with (a) the shear knuckles, (b) truss seats, and (c) a single truss and concrete

slab section; and (2) a nine-story by nine-column exterior wall subsystem with (a) bolted connection

between exterior columns, (b) bolted connection between spandrels, (c) single exterior coluirms with

spandrel sections, and (d) single exterior wall panel with three columns and three spandrels.

I. Conduct analyses of major subsystems. Analyses of three major subsystems - the isolated core framing

subsystem, an exterior wall subsystem, and the composite floor subsystems - were analyzed to determine

their ability to resist and redistribute loads after impact damage and response to elevated temperatures.

The subsystem models used reduced models from the component analyses, which kept the analysis

tractable while including nonlinear features and failure modes. These analyses were crucial for determining critical structural behaviors, including floor sagging under thermal loading, the resulting pullin forces, and the inward bowing of the exterior walls.

J. Conduct a separate global analysis for each tower. These analyses determined the relative roles of

impact damage and fires with respect to structural stability, sequential failures of components and

subsystems, and probable collapse initiation sequences. Each global model was first evaluated for

stability under gravity loads with structural impact damage. Temperature histories were applied in 10 min

intervals and linearly ramped to the next temperature state. Pull-in forces from sagging floors were also

applied during the appropriate 10 min intervals. The question of how the WTC towers would have

responded to the same fires without the aircraft impact damage was considered to determine the

vulnerability of the towers to collapse initiated by conventional large fires.

K. Determine the probable collapse sequence for each tower. A probable collapse sequence for each

tower was determined. The collapse sequences were evaluated against key observables.

NISTNCSTAR 1-6. WTC Investigation xli
Executive Summary

E.2.2 Structural Response

To conduct the global analysis of each tower, input data were collected from numerous sources, including

fire dynamics, thermal, and impact analyses, as already described.

Thermal analyses to simulate the elevated temperatures of the structural components and consequent

weakening required an assessment of the condition of the passive fire protection, including thermal

properties and SFRM thicknesses. Additionally, tests of the WTC floor system under standard fire conditions provided insights into the dominant behavior of the floors at elevated temperatures and

allowed validation of analytical results. Interpretation of the aircraft impact study results led to a determination of likely damage to load bearing structural elements and an estimation of damage to, and

consequent loss of, passive fire protection of the floor trusses, core columns and beams, and exterior

columns and spandrels. Properties of the materials of construction, including mechanical properties at room and elevated temperatures as well as thermal characteristics, were needed. The structural analyses

of components, subsystems and, ultimately, the global systems could be accomplished with this information.

Passive Fire Protection for Structural Components

Passive fire protection delays the transfer of heat to structural components by providing an insulation

barrier. Increasing thickness of passive fire protection materials, commonly referred to as fireproofmg,

correspondingly increases the time delay before the structural component temperature begins to rise. The

amount of time delay for a given fire protection method, such as SFRM, is not predicted for design

purposes because the actual fire conditions vary; instead, the relative performance is defined by

comparative testing with the ASTM Standard Fire Test.

The structural steel in the WTC towers was sprayed with SFRM or protected with rigid fire-rated gypsum

panels. SFRM is supplied as dry ingredients, and water is added at the time of application. The water

mixes with the cementitious materials and allows the SFRM to adhere weakly to the steel. With time, the

cementitious materials harden, and excess water evaporates resulting in a covering of insulation with some cohesive strength.

Three SFRM products that were used in the towers include:

• BLAZE-SHIELD DC/F for floor trusses, core columns, and the exterior surfaces of the

exterior columns and spandrels

• BLAZE-SHIELD II for upgrades to floor trusses, which started in the 1990's

• W.R. Grace and Co., Monokote (sprayed cementitious vermiculite) for the interior surfaces of

the exterior columns and spandrels

The gypsum panels were used to form fire resistant enclosures around steel core columns, stairwells,

mechanical shafts, and the core area in the towers. The core column thermal insulation varied according

to the column location and exposure to occupied spaces. Column surfaces in public access areas were

protected with gypsum enclosures, while the remaining surfaces were protected with SFRM.

xlii NISTNCSTAR 1-6, WTC Investigation
Executive Summary

The following information was required to determine the in-place condition of the passive fire protection

before and after aircraft impact and to conduct thermal analysis of structural components:

• Thermophysical properties of the passive fire protection materials,

• Effect of gaps in thermal insulation and variability of insulation thickness,

• Effective thickness of thermal insulation for use in thermal-structural analyses that accounts

for thickness variability effects,

• Adhesive and cohesive strengths of BLAZE-SHIELD SFRM products (vermiculite product is no longer available).

Thermophysical properties were determined with ASTM standard tests for BLAZE-SHIELD DC/F,

BLAZE-SHIELD n, and Monokote MK-5 SFRM products and for gypsum board. The specific heat,

thermal conductivity, and density of each material were determined for temperatures ranging from 25 °C

to 1200 °C. The standard tests used for SFRM products were ASTM C 1 1 13 (1999), ASTM E 1269

(2001), ASTM E 1131 (1998), and ASTM E 228 (1995). The standard tests used for the gypsum board

products were ASTM D 5334 (2000b) and ASTM E 1269 (2001). Densities were calculated from the

thermogravimetric analysis and linear thermal expansion measurements.

Analyses showed that when the SFRM thickness is variable, the isotherms in the steel depend upon the

shape of the SFRM surface contour. Thus, the temperature history at any point in the steel depends on the

local thickness of the insulation. It was shown that an increase in thickness variability reduced the time to reach a certain temperature. In addition to the effect of variation in thickness, the effect of gaps in the

SFRM coating was smdied. As expected, thermal analysis resuhs indicated that the exposed steel heated

quickly and transmitted heat to the adjacent interior steel. However, the temperature rise quickly

dissipated as the distance from the gap increased. Review of available photographs showed that gaps were a relatively infrequent occurrence in most floor truss areas. Because there was insufficient

information to determine the frequency of occurrence of these gaps or their typical locations, insulation

gaps were not considered in the thermal modeling.

SFRM thickness measurements were determined from analysis and interpretation of photographs showing

the condition of the originally applied material. Finite element simulations were used to determine a thermally equivalent uniform thickness of SFRM for the original variable-thickness applied to the floor

trusses. These values were used in the thermal analyses for determining temperature histories of

structural components.

No information was available about the condition of thermal insulation for the exterior columns and

spandrel beams, and little information was available for the core beams and columns. For thermal

analyses of the towers, the SFRM on these elements was taken to have uniform thicknesses equal to the

specified thickness.

The adhesive strength of BLAZE-SHIELD DC/F to steel coated with primer paint (average value of

171 psf to 185 psf) was found to be a third to a half of the adhesive strength to steel that had not been

primed (average values of 450 psf to 666 psf). The SFRM products used in the WTC towers were applied

NISTNCSTAR 1-6, WTC Investigation xliii
Executive Summary

to steel components with primer paint. Cohesive strengths varied from average values of 367 psf to 610 psf.

Tests of Truss Floor Components and Subsystem

Review of available documents indicated that the fire performance of the composite floor system of the

WTC towers was an issue of concern to the Port Authority and its contractors during the original design

and throughout the service life of the buildings. NIST conducted a series of four standard fire tests to establish the baseline perfonnance of the floor system of the WTC towers as they were originally built, to

differentiate the factors that most influenced the response of the floors, and to study the procedures and

practices used to accept an innovative structural and fire protection system. The ASTM E 119 furnace

tests were performed on representative floor sections with SFRM for the as-specified thickness of 0.5 in. given in the design documents and the average as-built thickness of 0.75 in. that was applied before a program was established in the 1990's to upgrade the truss SFRM thickness to 1.5 in. The conditions in the standard test specified a prescribed temperature rise and duration until failure criteria were met; the

estimated fire conditions in the WTC towers imposed varied heating and cooling conditions as the fires grew and spread.

The tested floor assemblies were similar, though not identical, to steel-joist-supported concrete floors that are widely used in low rise construction. The test results provided valuable insight into the behavior of

these widely used assemblies and also identified issues that require farther study for other types of

structural components such as beams, girders, columns, trusses, etc.

The tests showed that the floors were capable of considerable sagging without collapse. The tests also

showed thermal damage to the bridging trusses and buckling of compression diagonals and the vertical

strut near the supports. No evidence of knuckle failures was seen in the tests.

The NIST tests have identified areas where further study related to the standard test method is warranted.

Among the issues related to the test method that NIST identified as requiring further study are:

• the scale of the test for prototype assemblies that are larger than the tested assemblies,

• the effect of restraint conditions on test results,

• the repeatability of test results (e.g., do multiple fire resistance tests conducted under the same conditions yield the same results?),

• effects of test scale, end restraint, and test repeatability on other types of structural components (beams, girders, columns, trusses, etc.), and

• the acceptance criteria to evaluate the load carrying capacity of the tested assemblies

(currently tests are stopped before the load carrying capacity of the assembly is reached

because other acceptance criteria are met or if the deflection becomes excessive and assembly

failure could damage the furnace).

xliv NISTNCSTAR 1-6, WTC Investigation
Executive Summary

Structural Response of Components and Detailed Subsystems to Assumed Damage

and Fire

Material Properties and Failure Criteria

The WTC towers were designed and constructed using 14 grades of steel and two types of concrete. Nominal properties for these materials were provided in the design documents. Additional information was required about the mechanical properties at room and elevated temperature for analysis of the towers' response to the impact and elevated temperature conditions.

The collapse analyses of the WTC towers concentrated on modeling failure mechanisms in steel rather

than concrete components, since the WTC towers were essentially steel structures; concrete was used

only for the floor slabs.

The two general types of steel that were used in the towers are typically described as carbon steels and

high strength steels. Carbon steels generally have lower strengths but are more ductile. The core columns, floor trusses, and beams and spandrel plates in the exterior wall were constructed with carbon

steels, ranging from 36 ksi to 50 ksi specified yield strengths. The exterior columns were designed with

various grades of high strength steels, ranging from 55 ksi to 100 ksi yield strength.

Normal weight concrete (150 pcf) was used in the core and mechanical floors, and lightweight concrete (110 pcf) was used in the floor system for the tenant spaces between the building core and exterior.

The mechanical properties of both steel and concrete are significantly affected by elevated temperatures.

Steel and concrete properties that are temperature sensitive include modulus of elasticity, instantaneous

coefficient of thermal expansion, tensile strength, and compressive strength. Additionally, creep strain rates for steel are also temperature dependent.

Mechanical properties of the various grades of steel used and normal and lightweight concrete, both at room tempcramre and throughout the expected temperature range, were determined. This information

provided the bases for describing the material models used in the finite element analyses. In addition to material models, failure criteria were also developed for concrete and steel components. Failure criteria defined the necessary conditions to characterize and quantify the expected failure modes or mechanisms,

including elastic or plastic buckling, yielding, or fracture. The state of component loads, material

properties, and temperature also affected the mode of failure.

In addition, the following observations can be made:

• Modulus of elasticity is reduced by 25 percent at 600 °C for steel and by 50 percent to 75 percent for concrete.

• Steel yield strength reduces to 20 percent of its initial (room temperature) value and ultimate

tensile strength is reduced to 40 percent of its initial value at 600 °C. Concrete compressive

strength is reduced to between 30 percent and 50 percent of its initial value. Concrete tensile

strength, which is already low, is also reduced to 30 percent.

• The instantaneous coefficient of thermal expansion for steel lies between that of lightweight

and normal weight concrete for a given temperature. If steel truss and lightweight concrete

NISTNCSTAR 1-6. WTC Investigation xlv
Executive Summary

components are at the same temperature, the steel components will thermally expand more

than the lightweight concrete. For steel beams and normal weight concrete in the core area, the normal weight concrete will expand more than the steel beams.

Floor Subsystem Analysis

The floors supported the occupants and furnishings and transferred these loads to the columns, acted as diaphragms to transfer loads between exterior faces when under wind loads, and provided lateral stability for columns. With damage to the SFRM on the floor trusses, fires caused thermal expansion and sagging

of the floors in the impact damage areas.

The analysis of floors progressed from individual components to major subsystems to global systems.

Three truss components were studied with detailed models using ANSYS, a general purpose finite element software package, before developing a model of a full floor subsystem:

• Shear connector between the truss and concrete slab,

• Truss seat connection to the columns,

• Composite section of a single floor truss and concrete slab that included the truss seats, knuckles, and section of the supporting exterior and core channel beam.

Shear connector tests conducted by the truss manufacturer, Laclede Steel, in the early 1 960s were

reviewed and modeled. The shear connector between the truss and the concrete slab was referred to as a knuckle, due to the bent bar configuration that extended past the top chord of the truss, instead of the

studs that are typically welded to the top chord. Detailed ANSYS models of the knuckle and concrete

slab were analyzed and compared to the measured transverse and longitudinal shear capacities of a knuckle. A reduced model of the knuckle for use in the single truss and ftill floor models was developed

that captured the dominant temperature-dependent behavior and failure modes.

Truss seats connected the trusses to the core and exterior columns. Truss seats were constructed with

standoff plates, seat angles, bolts, and welded gusset plates; details varied for each truss seat depending

upon its location within the floor plan. Truss seats were designed to carry floor gravity loads and small

horizontal loads, typically a few percent of the column capacity to which it was attached. Typical truss seats were analyzed to determine their failure modes and associated loading and thermal conditions. A

series of analyses were conducted to determine the truss seat response to thermal expansion of the floor

slab, floor sagging or deformation, and heating of the truss seat. A model of reduced complexity was

developed that captured the behavior and failure modes of the truss seats for use in the single truss and

full floor models.

With reduced models of the knuckle and truss seat, a composite section of a full single truss and concrete

slab was modeled to determine its behavior and failure modes for elevated temperatures and additional

debris loads. Steel components with damaged fire resistant coatings heated and softened within 10 to 15 minutes. The bottom surface of the concrete slab heated quickly, but the rate of heating through the

slab depth was slower, so that the slab response to fire lagged behind the steel response. Concrete

spalling was not included in the model. Analysis was conducted using uniform temperatures across the

truss and an imposed linear thermal gradient across the slab depth to study the floor section response.

xlvi NISTNCSTAR 1-6, WTC Investigation
Executive Summary

These conditions were assumed prior to completion of the fire and heat transfer analyses used for the full floor subsystem analysis. Two failure modes of interest were (1) floor component failures leading to sagging (i.e. buckling of truss components or knuckle separation from the concrete slab) and the truss pulling inw-ard on the columns and (2) failure of the truss seats. Analysis results were used to develop a model of reduced complexity with break elements that captured the behavior and failure modes of the floor section for use in the full floor model.

The fiill floor model included core columns and floor beams, exterior columns and spandrel beams, floor trusses and bridging trusses, and normal and lightweight concrete in the core and floor truss areas, respectively. The columns were extended one floor level above and below the floor subsystem and were

required to include the interaction between the floor subsystem and the core and exterior columns. The

full floor model contained a number of modifications from the model developed using the SAP2000

software of Floor 96 (NIST NSTAR 1-2) that reduced the number of finite elements and incorporated the

features for analyzing the structural response to thermal conditions.

Results of the floor system analyses showed that:

• Knuckle failures did not occur under gravity loading and elevated temperatures anticipated.

• Truss web diagonals buckled at loads and temperatures expected and, as a consequence, the

floor system sagged.

• Sagging of the floor system resulted in possible inward pull on the exterior columns, although

the magnitude of the force depended on fire conditions on surrounding floors.

• Truss seat connections could fail under elevated temperature conditions, and their behavior was included to accurately capture the overall performance of the floor system to impact and

fire conditions.

• Essential floor behavior, including buckhng of web diagonals and connection failures, could

be achieved with reduced models.

Core Column and Exterior Column and Panel Analysis

The primary function of the core columns was to carry the building gravity loads. The exterior columns

resisted wind loads and, in addition, carried approximately half of the gravity loads.

Preliminary analysis of the core and exterior columns considered their individual buckling behavior and

how it varied for uniform elevated temperatures. The columns were found to have sufficient capacity for tower gravity loads even under elevated temperatures and a loss of lateral support at several floors. This was also found in more detailed finite element models of the columns.

The core columns were studied to determine the most efficient way to reduce the complexity of the model

while still capturing buckling behavior at room and elevated temperatures.

Four exterior wall components were studied with detailed ANSYS models before developing a model of a nine-story by nine-column wall area:

NISTNCSTAR 1-6, WTC Investigation xlvii
Executive Summary

• Bolted connection for exterior columns

• Bolted connections for spandrels

• Single exterior columns with spandrel sections

• Single exterior wall panel, fabricated as a single unit for construction purposes with

three columns and three spandrels

The column and spandrel connections were analyzed to determine their failure modes and associated

loading and thermal conditions. A reduced model was developed that captured the connector behavior

and failure modes for use in exterior wall models.

The single column model with spandrel sections was loaded axially to determine its buckling load and

post-buckling behavior at room and elevated temperatures for one, two, three, and nine story column

heights.

The computer model of a single wall panel was validated against the reference structural models for the towers. The models were subjected to vertical and horizontal forces in the plane of the wall, representing

intended design behavior, and a horizontal force transverse to the wall, representing a possible floor load.

The exterior wall had three connections: the column splice, the spandrel splice, and the truss seat (for the

floors). The column splice had four bolts that connected columns through their end plates. The spandrel

connection had a splice plate to connect the two spandrel plates using high strength bolts. The spandrel

and column splices were represented in the nine by nine wall subsystem model and captured the spandrel

failure modes of bolt shear, tearing of the spandrel plate, and tearout of the spandrel plate at the bolt

holes.

The nine by nine wall model had a coarser mesh that used beam elements for the columns, shell elements

for the spandrels, and break elements for the connections. The wall model was subjected to axial loads

from above, lateral out-of-plane loads at the floor levels, and elevated temperature representative of fire conditions. The effect of missing floor supports was also evaluated.

Several analyses were run for a variety of temperature load cases and for various combinations of axial

load, disconnected floors simulating floor failure and loss of lateral column support, and inward pull

applied at one or more floor levels modeling floor sag due to elevated temperatures. Results showed that:

• Although spandrel plates experienced large distortions and high strains, column buckling did

not occur under the various temperature loadings applied when floors remained in place and

able to provide lateral support to the columns.

• Column buckling did not occur when lateral support was lost at three floors under the

expected gravity load that included dead plus service live loads.

• Column buckling did occur when lateral support was lost at three floors and the gravity load was increased to 150 percent of the expected gravity load simulating redistribution of load to the exterior wall.

xlviii NISTNCSTAR 1-6, WTC Investigation
Executive Summary

• Column buckling was found to occur when an inward lateral load (pull-in) of approximately

12 kips was applied to three adjacent floor levels. The inward deflection of the exterior wall

when it could no longer support the gravity load (i.e., at the buckling load) was

approximately 10 in.

Aircraft Impact Damage

The aircraft impact of the WTC towers caused extensive damage to the buildings' exterior, penetrated

into the interior causing further damage to the structural system, dislodged thermal insulation, and ignited

multi-floor fires. The structural damage to each tower resulting from the aircraft impact was estimated

using a transient finite element analysis. Results of this analysis were used to predict damage to the

structure, insulation, and partition walls in the path of the debris field.

The fire dynamics, thermal, and structural analyses all required input data derived from the aircraft impact

analyses. The fire dynamics analyses used estimates of damage to the floors and partition walls to describe ventilation paths and to identify the distribution of ftiel and debris immediately following impact.

The thermal analysis required estimation of the areas that had dislodged insulation on the structural components of the towers. For the structural analyses, elements that represented severed or heavily

damaged floors and columns were removed from the structural models of the towers.

The aircraft impact analyses considered three cases for each tower, where each case had a different set of

input parameter values, based upon sensitivity studies and detailed component analyses. The results for the three cases were compared to observations from photographs and videos. Damage to the exterior

walls predicted by the impact simulations matched reasonably well the exterior damage in photographic

and video records. The observed exterior damage was used in the structural analyses. The analysis results from two cases for each tower were found to match observations reasonably well and were selected for continued analysis by the fire dynamics, thermal, and structural analyses. The cases for each tower were

referred to as Case A and Case B for WTC 1 and Case C and Case D for WTC 2. However, prior to determining the final aircraft impact analysis results, earlier aircraft impact analyses produced an initial

set of aircraft impact cases for each tower. These initial cases, referred to as Case Aj and Case Bj for WTC 1 and Case Cj and Case Dj for WTC 2, were used to develop experience and gain understanding of

the fire spread and growth, the rate of structural component heating, and the structural response to damage

and elevated temperatures.

The final set of impact damage data for fire dynamics, thermal, and structural analyses was Cases A, B,

C, and D, with the exception of the full floor subsystem analyses which used initial damage Cases A, to

D|. The use of the aircraft impact data in the sequence of structural analyses was as follows:

1 . Full floor subsystem models were analyzed for all initial damage Cases Aj to Dj before the

final damage cases were available.

2. Full floor subsystem models were evaluated for changes in damage between final Cases A to D and initial Cases A , to D,. Changes in impact damage to the structural components and

insulation reflected in the two sets of Cases (i.e., initial and final) were found to have liftle

effect on the floor subsystem structural response. The ftill floor subsystem structural response for Cases A, to D, and Cases A to D were found to be equivalent.

NISTNCSTAR 1-6, WTC Investigation xlix
Executive Summary

3. Isolated core and exterior wall subsystem models were analyzed for Cases A, B, C, and D.

4. The global model of WTC 1 and WTC 2 were analyzed for Case B and D, respectively, based

upon the results of the subsystem analyses.

Four classifications of core column structural damage were established: severed, heavy damage, moderate

damage, and light damage. Classification criteria included plastic strain levels and lateral deformation

from the column centerline. Columns that were severed or heavily damaged were removed to simulate

impact damage in the global analysis of each tower. Two types of floor structural damage were identified

from the impact analysis results: (1) missing floor areas and (2) severely damaged floor areas incapable of

supporting loads.

Thermal insulation was assumed to be dislodged from core columns only if the columns were subject to direct debris impact that failed wall partitions in the immediate vicinity of the column'. For exterior

columns, the debris impact was required to be strong enough to damage or destroy room furnishings

(modular office workstations) adjacent to the columns. For floor trusses, the debris impact was required to be strong enough to damage or destroy room furnishings (modular office workstations) in the same area of the affected floor.

The structural damage in WTC 1 extended from the north exterior wall into the north side of the core. An

exterior panel was knocked out of the south wall by aircraft debris. Damage to the insulation from direct

debris impact extended over a larger region and extended to central regions of the south floor areas. Case

B predicted more damage to core columns and a larger extent of insulation damage to the south floor area than Case A, including damage to the south exterior wall insulation on the inside face, as shown in Fig. E-3.

The structural damage in WTC 2 extended from the south exterior wall to southeast region of the core. Exterior columns were severed by debris near the northeast comer. Damage to the insulation from direct

debris impact extended over a larger region and extended over most of the east floor area to the north

face. Case D predicted more damage to core columns than Case C, but the extent of the insulation

damage was similar, as shown in Fig. E-4.

' The Pentagon was impacted by an aircraft of similar size and at a similar speed as the WTC towers. The observed stripping of

the concrete cover from columns in similar circumstances provides an independent set of data that supports the criteria established for the removal of fireproofing materials subject to direct debris impact in the WTC towers.

1 NISTNCSTAR 1-6, WTC Investigation
Executive Summary

TCt W IflE 'tS tSt tS* tST t

Severe Floor Damiige

Floor fireproofing | |

Floor system

structural daniase

Floor system

removed

Column Damage

Severed

Hea^y damage

Moderate

damage

Light damage

O

O

O

N

\

Case A

dBIK7 a- M m M m 3X 30t

CaseB

Figure E-3. Plan view of WTC 1 cumulative damage for Floors 93 to 99.

NISTNCSTAR 1-6, WTC Investigation li
Executive Summary

Severe Floor Damage

Floor fireproofing

[ [

Floor system

structural damage

Floor system

renioved

Column Damage

Severed

Hea^'y damage

Moderate

damage

Light damage

O

O

N

t

Case A

Case B

Figure E-4. Plan view of WTC 2 cumulative damage for Floors 78 to 84.

lii NIST NCSTAR 1-6, WTC Investigation \
Executive Summary

Observations and Timeline of Structural Events

NIST assembled a collection of nearly 150 hours of video footage and over 7,000 photographs, which were reviewed for insights into the structural performance of the towers. A timeline of significant events

that characterized the weakening and eventual collapse of the WTC towers was developed with the photographs and x ideos that were time stamped. Quantitative information, such as the amount of inward

bowing obsen ed on the exterior walls of the buildings, was extracted from key photographs through

image enhancement and scaled measurements. Key observations and the timelines were used to guide the global collapse analyses.

Development of the probable collapse sequence for each tower was shaped by evidence gathered in the

investigation. Data about the events following the aircraft impact were primarily obtained from three sources:

• Photographic and videographic records that had been catalogued and time stamped for the

NIST Investigation (NIST NCSTAR 1-5A)

• Inter\'iews of individuals in the towers who survived and those who received telephone calls from individuals in the tower (NIST NCSTAR 1-7)

• Interviews of emergency response personnel and emergency communication records (NIST

NCSTAR 1-8)

Photographs and videos provided knowledge about aircraft impact damage to the exterior walls, fire growth and spread at the building exterior, inward bowing of an exterior wall in each tower, and the

direction of tilt for the building section above the impact and fire zone as the towers collapsed.

Changes in structural performance are generally difficult, if not impossible, to perceive until significant

deformation has taken place relative to the dimensions of the structure and depend on the detail and

resolution of the image being examined and the vantage point of the photographer. Observations of

structural performance for the WTC towers included severed components, local deflections or buckling,

possible sagging of floors, and relative alignment of columns or building sections.

Evidence was used in the analyses in three ways: (1) to determine input parameters, such as the aircraft speed and direction upon impact, (2) to impose time-related constraints upon an analysis, such as imposing observed broken windows over time to constrain the spread of fire, or (3) to vahdate analysis

results, such as global stability after impact and during thermal loading.

Observations of structural behavior were broken into two groups: key obsei-vatiom and noted

obsen'atiom. Key observ ations were significant structural events that were explicitly addressed in or used to validate the structural analyses. Noted observations were events that may have been a structural response, but could not be conclusively identified as a structural response.

Key observations were used to develop a timeline of structural events for each tower. Structural analyses were used to support development of the collapse hypothesis for each tower and to develop and refine

understanding of the probable sequence of events.

NIST NCSTAR 1-6, WTC Investigation liii
Executive Summary

WTC 1 key observations were:

• Inward bowing of the south exterior wall was first observed at 10:23 a.m., as shown in Fig. E-5.

• The time to collapse initiation was 102 minutes from the aircraft impact (9:46:30 a.m. until 10:28:22 a.m.).

• From exterior observations, tilting of the building section appeared to take place near

Floor 98. Column buckling was then observed to progress rapidly across the east and west

faces.

• The WTC 1 building section above the impact and fire area tilted to the south as the structural collapse initiated, as shown in Fig. E-6. A tilt to the south of at least 8 degrees occurred

before dust clouds obscured the view and the building section began to fall downward.

WTC 2 key observations were:

• Following the aircraft impact and fireballs, hanging objects were observed through the

windows of the east and north faces. The hanging objects suggest that there was structural damage to WTC 2 Floor 83 along the east face and to Floors 81 to 83 of the north face near

the northeast comer.

• Inward bowing of the east wall was first observed at 9:21 a.m. The inward bowing was

approximately 1 0 in. at Floor 80.

• An increase of the inward bowing of the east wall was observed at 9:53 a.m. The greatest

bowing was approximately 20 in.±1.0 in. at Floor 80 on the east face of WTC 1.

• Collapse initiated 56 minutes after the aircraft impact (9:02:59 a.m. to 9:58:59 a.m.).

• From a northeast viewpoint, initial downward motion was observed as columns moved

inward on the north side of the east face, as shown in Fig. E-7. Tilt of the building section

above the impact and fire area appeared to take place near Floor 82. Column buckling was

then seen to progress across the north face.

• The building section above the impact and fire area tilted to the east and south as the

structural collapse initiated as shown in Fig. E-8. There was approximately a 3 to 4 degree

tilt to the south and a 7 to 8 degree tilt to the east prior to significant downward movement of

the upper building section.

hv NISTNCSTAR 1-6, WTC Investigation
Executive Summary

1 . Measurements were based on calibration measures shown on

the west face

2. Foreshortening into depth of field across the south face of 1 7%

was included in the measurements

3. Measurement error was at least ± 6 inches

Figure E-5. WTC 1 exterior columns bowing inward across most of the south face

between Floors 95 to 98 at 10:23 a.m.

NISTNCSTAR 1-6, WTC Investigation Iv
Executive Summary

Figure E-6. WTC 1 building section above impact damage zone tilts to the south.

Figure E-7. View of WTC 2 buckling of east wall near northeast corner as collapse

Initiates from southeast.

Ivi NISTNCSTAR 1-6, WTC Investigation
Executive Summary

Figure E-8. View of upper building section of WTC 2 tilting to the east.

Structural Response of Major Tower Subsystems

Prior to conducting the analysis of the global structural response of each tower, major structural subsystems were analyzed to pro\ ide insight into their behavior within the WTC global system. The

three major structural subsystems, the core framing, a single exterior wall, and full tenant floors, were

analyzed separately for their response to impact damage and fire. The hat truss was not analyzed

separately as its structural behavior did not require significant reduction in the global analysis. The

component analyses provided a foundation for these large, nonlinear analyses with highly redundant load

paths by determining component behavior and failure modes and enabling a significant reduction in finite element model complexity and size. The major subsystem models used final estimates of impact damage

and elevated temperatures determined from the aircraft impact analysis and the fire dynamics and thermal

analyses.

The capacity of each subsystem to sustain loads for the imposed damage and elevated temperatures was

evaluated. The isolated subsystem models lacked the restraint and load paths to other subsystems found in the global analysis. Even so, the isolated subsystem response was useful for refining the global models

and interpreting subsystem behavior in the global system. For instance, when the column coimections to the hat truss in WTC 2 failed at the southeast comer of the core, the only load path available to carry those column loads was the floor system within the core structure. However, in the global structure, the

hat truss at the top of the core would transfer loads to other core columns or the exterior walls, assuming

the connections between the core columns and hat truss remained intact.

The subsystem models used modeling reductions from the component analyses, which kept the analysis

tractable while maintaining required nonlinear features. As previously noted, such reductions were

necessary to maintain a careful balance between model size and complexity as the model size increased.

Each of the major subsystem models used temperature histories for the towers. Elevated temperatures were applied to the models in 10 min intervals, where a temperature state was given for all structural components at a given time and linearly ramped to the next temperature state. Examination of structural

NISTNCSTAR 1-6. WTC Investigation Ivii
Executive Summary

temperature histories indicated that no significant fluctuations between temperature states occurred for the

10 min intervals selected for analysis.

Core Subsystem

The core subsystem models included temperature-dependent plasticity, creep, and plastic buckling

behavior in the core column elements. Core models extended from Floor 89 to Floor 106 for WTC 1 and

from Floor 73 to Floor 106 for WTC 2, and did not include the hat truss. The models included core columns and floor beams and slabs. Floor slabs were modeled as membrane elements with a relatively coarse mesh, which resulted in approximate slab openings for elevators and mechanical shafts. The

meshing did not affect the floor's ability to provide a load path between columns. For the purposes of the

isolated core model, only the floor beams with partial moment connections were included, as simple shear

connections were not capable of transferring significant loads between columns. Impact damage was

modeled by removing severed core columns and damaged floor areas. The core subsystem was analyzed

for stability under gravity loads. Temperature histories were then applied to the core structure.

By not including the hat truss, the primary load path for core column load redistribution was removed,

leaving the core floors, which typically provided a secondary load path. The WTC 1 isolated core subsystem was stable with Case A aircraft impact damage and gravity loads. To reach a stable solution for

Case C structural damage and gravity loads, the WTC 2 isolated core model required horizontal restraints

to be added in the east and south directions at each floor, representing the lateral restraint provided by the

office area floors. Without the horizontal restraints, the WTC 2 core model tilted significantly due to the

severed columns in the southeast comer of the core. The isolated core models did not converge for WTC

1 Case B and WTC 2 Case D structural impact damage, which had more severed columns than Cases A

and C. The core needed to redistribute loads to other areas in the global system for a stable solution with

Cases B and D structural damage.

Full Floor Subsystem

The full floor subsystem models included large deflection and temperature-dependent material properties

with plasticity for all steel components. Creep was not included in the fiill floor models, as this analysis

feature did not work with beam elements in version 8.0 of ANSYS (the detailed truss model had 3D finite strain elements that were changed to beam elements in the full floor model). Creep was included for

beam elements in ANSYS 8.1, and subsequent analyses of the core and exterior wall subsystems included

creep deformation. The floor slab was modeled as lightweight concrete across the entire floor (tenant and

core floor areas) with a bilinear stress-strain constitutive model that did not account for cracking,

crushing, or spalling. The concrete material model used the compressive strength as the yield point, with

the same yield strength in both tension and compression (the reinforcing steel was assumed to provide the

tensile capacity in the composite floor). Separate floor models were created from the Floor 96 structural model by imposing the different damage and temperature conditions for WTC 1 Floors 93 to 99 and

WTC 2 Floors 79 to 83. Structural components that were severed due to the aircraft impact were removed

from each floor model, based upon the four initial damage cases, WTC 1 Case A, and B, and WTC 2 Case

C, and D,. Each full floor model was analyzed for stability under floor gravity loads. No column loads were applied. Temperature histories were then applied to the floor structure.

The floor analysis results for Cases A, to Dj were used for Cases A to D in the exterior wall subsystem

and global analyses. Final damage Cases A, B, C, and D were completed after the initial set of floor

Iviii NISTNCSTAR 1-6, WTC Investigation
Executive Summary

analyses were conducted with Cases A;, Bj, C„ and D,. The full floor models were not rerun for Cases A

through D as comparisons showed that the structural temperature histories of the floors were nearly

identical for most floors and only slightly different for a few floors.

Exterior Wall Subsystem

The exterior wall subsystem models included temperature-dependent plasticity, creep strains, and plastic

buckling behavior in the exterior wall components. The exterior wall analyses extended over approximately 20 floors and were centered around the areas of impact and fire zone. The south face of

WTC 1 extended from Floor 89 to Floor 106 and the east face of WTC 2 extended from Floor 73 to Floor 90. The exterior panel that was severed during the aircraft impact and found south of the tower was

removed from the south face of WTC 1 . No structural damage to the panels was observed on the east wall of WTC 2. The analysis of a single exterior face provided insight into the conditions that would

resuh in the inward bowing of the south wall of WTC 1 and the east wall of WTC 2 observed in photographs. Conditions examined included pull-in forces resulting from sagging floors, disconnected

floors resulting from truss seat failure, additional vertical loads simulating load transfer to the exterior

wall, and elevated temperatures.

The exterior wall models were used to estimate the pull-in force magnitude and locations for each tower

that would produce the observed bowing of the exterior wall. The inward pull was caused by sagging of

the floors. Heating of the inside face of the exterior columns also contributed to inward bowing. Themial

expansion occurred as soon as steel temperatures began to rise; colmnn shortening occurred when creep

and plastic strains overcame thermal expansion strains, typically at temperatures greater than 500 °C to 600 °C with accompanying high stresses and duration of temperatures and stress levels. WTC 1 exterior wall analysis found that an inward pull force of 6 kips at each column at Floors 95 to 99,

starting 80 min after the aircraft impact, caused a maximum inward bowing of 31 in., shown in Fig. E-9. This inward deflection was smaller than the observed maximum bowing of 55 in. ±6 in., and the

wall was stable at 100 min. The magnitude of pull-in forces was expected to be less than 6 kip with the

addition of gravity loads from the core subsystem as it also weakened; therefore, pull-in forces of 4 kip to

5 kip were used in the global model analyses.

WTC 2 exterior wall analysis found that an inward pull force of 1.0 kip to 1 .5 kip and 4.0 kip to 5.0 kip on the south and north portions of the east wall, respectively, over Floors 79 to 83, caused a maximum

inward bowing of 9.5 in. at 20 min and 37 in. at 50 min, as shown in Fig. E-10. The observed deflections were 10 in. and 20 in., respectively, at corresponding times. Considering the possible increase in column

loads after impact for Case D conditions, a pull-in force of 1.0 kip on the south half and 4.0 kip on the

north half of the east wall were selected for the initial estimate for the WTC 2 global model analysis.

NISTNCSTAR 1-6, WTC Investigation lix
Executive Summary

NODAL SOLUTION ~:^n AIM

sTEP=2o COL359 ..--Tr::! ^ 42005

SUB =2668 ^ 11:04:52

TIME = 6000

I UY (AVG) RSYS = 0 DMX =31.451

SMN =-.477195

SMX =31.326 - ...... m

^

FL99

r^FL95

FL93

FL91

Figure E-9. Inward displacement of the WTC 1 south wall at 100 min of the Case B

temperatures with floor disconnections and 6 kip pull-in forces over five floors.

Ix NISTNCSTAR 1-6, WTC Investigation
Executive Summary

At 50 min

Figure E-10. Out-of-plane displacements of east wall of WTC 2 calculated with pull-in

forces of 1.5 kip on the south half and 5.0 kip on the north half.

NISTNCSTAR 1-6, WTC Investigation Ixi
Executive Summary

Structural Response of the WTC Towers

A separate global analysis of each tower helped determine the relative roles of impact damage and fires with respect to structural stability and sequential failures of components and subsystems and was used to determine the probable collapse initiation sequence.

Results of the major subsystem analyses were incorporated into the global models, reducing the

complexity of the modeling approach and/or level of detail where possible, while retaining sufficient

detail for nonlinear structural responses. The global models of the towers extended from several stories below the impact area to the top of the structure. WTC 1 was truncated at Floor 9 1 and WTC 2 was

truncated from Floor 77. The global models included the core subsystem, the exterior wall subsystem, the

hat truss, and an equivalent plate representation of the floor system. The core columns and exterior

columns and spandrels were modeled with elements and features similar to those used in the isolated core and exterior wall analyses. Column analysis features included the effects of thermal expansion, plastic,

and creep strains on column behavior within the global structural system. The full floor model was not included in the global models, as it would have made the models computationally too large. Instead,

office area and core floors were modeled with an equivalent floor slab thickness and modulus calculated

to match the in-plane stiffness of the composite floor system, including the concrete slab, floor trusses,

and the floor seats. Floor loads applied as concentrated loads at the column connections. These modeling

simplifications of the floor system were able to capture the floor behaviors observed in the full floor

subsystem analyses while keeping the analysis tractable.

Each global model was first evaluated for stability under gravity loads, with structural impact damage

modeled by removing severed and heavily damaged columns and floor areas. Temperature histories were

applied in 10 min intervals and linearly ramped to the next temperature state. Pull-in forces from sagging

floors were also applied during the appropriate 10 min intervals. The global analysis results provided a sequence of component and subsystem failures that led to the onset of global instability and collapse

initiation. WTC 1 Global Analysis Results

After the aircraft impact, gravity loads that were previously carried by severed columns were redistributed

to other columns. The north wall lost about 7 percent of its loads after impact. Most of the load was

transferred by the hat truss, and the rest was redistributed to the adjacent exterior walls by spandrels. Due

to the impact damage and the tilting of the building to the north after impact, the south wall also lost gravity loads, and about 7 percent was transferred by the hat truss. As a result, the east and west walls

and the core gained the redistributed loads through the hat truss.

In the early stages of the fire, structural temperatures in the core rose, and the thermal expansion of the core was greater than the thermal expansion of the exterior walls. The difference in the thermal

expansion increased the loads in the core columns at about 20 min. Thereafter, the core lost gravity loads

due to its thermal weakening and shortening until the south wall started to bow inward. At about

1 00 min, approximately 20 percent of the core loads were transferred by the hat truss to the exterior walls

due to thermal weakening of the core; the north and south walls each gained about 10 percent more loads,

and the east and west walls each gained about 25 percent more loads. Since the hat truss outriggers to the

east and west walls were stiffer then the outriggers to the north and south walls, they transferred more

loads to the east and west exterior walls.

Ixii NISTNCSTAR 1-6, WTC Investigation
Executive Summary

The inward bowing of the south wall caused failure of exterior column splices and spandrels, and induced

column instability. The instability progressed horizontally across the entire south face. The south wall

unloaded and redistributed its gravity loads to the thermally weakened core through the hat truss and to the east and west walls through the spandrels. The building section above the impact zone began tilting to the south as column instability progressed rapidly from the south wall along the adjacent east and west

walls, and increased the gravity' load on the core columns. The change in potential energy due to downward movement of building mass above the buckled columns exceeded the strain energy that could

have been absorbed by the structure. Global collapse then ensued.

WTC 2 Global Analysis Results

Before aircraft impact, the load distribution across the exterior walls and core was symmetric with respect

to the centerline of each exterior wall. After aircraft impact, the exterior column loads on the south side

of the east and west walls and on the east side of south wall increased. This was due to the leaning of the

building toward the southeast. After aircraft impact, the core carried 6 percent less loads. The north wall

loads reduced by 6 percent, and the east face loads increased by 24 percent. The south and west walls

carried 2 percent to 3 percent more load.

In contrast to the fires in WTC 1, which generally progressed from the north side to the south side over approximately an hour, the fires in WTC 2 were located on the east side of the core and floors for the

entire duration, with the fires spreading from south to north. With insulation dislodged over much of the same area, the structural temperatures became elevated in the core, floors, and exterior walls at similar

times. During early stages of the fires, columns with dislodged insulation elongated due to thermal

expansion. As the structural temperatures continued to rise, the thermal expansion was overcome by

plastic and creep deformations under compressive loads.

Vertical displacements of the south and east exterior columns were essentially constant after impact and

remained around 7.5 in. (over the severed columns) on the south face and about 3.5 in. on the east face

until the east wall became unstable at 43 min. The east wall, which had bowed inward to a total of

approximately 62 in., suddenly unloaded. The west wall also unloaded. Loads increased on the core and on the north and south walls. The core had weakened on the east side and shortened by 3.0 in. at the

southeast comer. At the same time, the northwest comer of the exterior wall displaced upward about

2.0 in., as the tower was tilting to the southeast around an axis passing through the southwest and

northeast comers.

The inward bowing of the east wall caused failure of exterior column splices and spandrels, and induced

column instability. The instability progressed horizontally across the entire east face. The east wall

unloaded and redistributed its gravity loads to the thermally weakened core through the hat tmss and to the north and south walls through the spandrels. The building section above the impact zone began tilting

to the east as column instability progressed rapidly from the east wall along the adjacent north and south

walls, and increased the gravity load on the weakened east core columns. The change in potential energy

due to downward movement of building mass above the buckled columns exceeded the strain energy that could have been absorbed by the structure. Global collapse then ensued.

NISTNCSTAR 1-6. WTC Investigation ixiii
Executive Summary

Structural Response ofthe WTC Towers to Fire Without Impact Damage

Whether the towers would have collapsed if subjected to the same fires with no aircraft impact damage

was considered as part of understanding the relative roles of the impact damage and fires. It was found

from the global analyses that both WTC 1 and WTC 2 were stable after the aircraft impact and that they

had considerable reserve capacity with structural impact damage. The global analyses also found that the

combined effect of structural and insulation impact damage with the ensuing fires caused both towers to collapse. The effect of the fires on the towers without structural or insulation damage was considered by

examining the subsystem and global analysis results for portions of the structures with intact insulation

that were subject to the fires.

The towers would likely not have collapsed under the combined effects of aircraft impact and the

subsequent multi-floor fires if the insulation had not been dislodged or had been only minimally

dislodged by aircraft impact. The existing condition of the insulation prior to aircraft impact and the

SFRM thickness on the WTC floor system did not play a significant role in initiating collapse of the towers.

Probable Collapse Sequences

To determine the probable collapse sequence for each tower, NIST adopted an approach that combined

mathematical modeling, statistical and probability based analysis methods, laboratory experiments, and

analysis of photographs and videos. The approach accounted for variations in models, input parameters,

analyses, and observed events. It included the evaluation and comparison of possible collapse hypotheses

based on different damage states, fire paths, and structural responses to determine the following:

• The probable sequence of events from the moment of aircraft impact until the initiation of

global building collapse;

• How and why WTC 1 stood nearly twice as long as WTC 2 before collapsing (102 min for

WTC 1 versus 56 min for WTC 2), though they were hit by virtually identical aircraft (Boeing 767-200ER);

• What factors, if any, could have delayed or prevented the collapse of the WTC towers.

Collapse hypotheses were developed over the course of the NIST Investigation. The first hypotheses were published in the May 2003 NIST Progress Report, and were updated in the June 2004 NIST

Progress Report and October 2004 Public Meeting at NIST. The Probable Collapse Sequence for each

tower was presented at the April 2005 Public Meeting in New York City. The stages of hypothesis

development are summarized as follows:

• Possible Collapse Hypotheses (May 2003) - not building specific; key events not identified

• Working Collapse Hypothesis (June 2004) - single hypothesis for both WTC towers;

identified chronological sequence of major events

ixiv NISTNCSTAR 1-6, WTC Investigation
Executive Summary

• Leading Collapse Hypotheses (October 2004) - separate hypothesis for each WTC tower;

identified building-specific load redistribution paths and damage scenarios in addition to chronological sequence of major events

• Probable Collapse Sequences (April 2005) - refined building specific collapse sequences

with chronological sequence of major events, load redistribution paths, and damage

scenarios.

To determine the probable collapse sequence for each tower, the following steps were required:

• identification of key observables, primarily from photographs and videos

• development of collapse hypotheses, which were updated periodically through the course of

the investigation with the acquisition of new data and analysis results

• sensitivity studies to identify influential parameters, through the application of a formal

statistical approach, orthogonal factorial design (OFD)

• development and refinement of mathematical modeling -fire dynamics simulation with

computational fluid dynamics and structural response to aircraft impact and fire with finite element analyses

• ev aluation of analysis results against observed and expected structural behavior, with

adoption of the event tree concept, and pruning and updating branches based upon

comparisons with observ ed data

These steps were applied to the degree needed for the sequence of analyses, from aircraft impact to structural response.

E.3 Probable Collapse Sequence of WTC 1 and WTC 2

The specific factors in the collapse sequences relevant to both towers (the sequences vary in detail for

WTC 1 and WTC 2) are:

• Each aircraft severed exterior columns, damaged interior core columns, and knocked off

insulation from steel as the planes penetrated the buildings. The weight carried by the severed

columns was distributed to other columns.

• Subsequently, fires began that were initiated by the aircraft's jet fuel but were fed for the

most part by the building contents and the air supply resulting from breached walls and fire- induced window breakage.

NISTNCSTAR 1-6, WTC Investigation Ixv
Executive Summary

• These fires, in combination with the dislodged insulation, were responsible for a chain of

events in which the building core weakened and began losing its ability to carry loads.

• The floors weakened and sagged from the fires, pulling inward on the exterior columns.

• Floor sagging and exposure to high temperatures caused the exterior columns to bow inward

and buckle—a process that spread across the faces of the buildings.

• Collapse then ensued.

The sequences are supported by extensive computer modeling and the evidence held by NIST. The

probable collapse sequences for WTC 1 and WTC 2 are presented in Figs. E-1 1 and E-12, respectively.

Ixvi NISTNCSTAR 1-6, WTC Investigation
Executive Summary

1. Aircraft Impact Damage

• Aircraft impact se\'ered a number of exterior columns on the north wall from Floors 93 to 98,

and the wall section abov e the impact zone moved downward.

• After breaching the building's exterior, the aircraft continued to penetrate into the building,

severing floor framing and core columns at the north side of the core. Core columns were also damaged toward the center of the core and, to a limited extent, on the south side of the core. Fireproofmg was damaged from the impact area to the south exterior wall, primarily through

the center of WTC 1 and at least over a third to a half of the core width.

• Aircraft impact severed a single exterior panel at the center of the south wall between Floors 94

and 96.

• The impact damage to the exterior walls and to the core resulted in redistribution of severed

column loads, mostly to the columns adjacent to the impact zones. The hat truss resisted the downward movement of the north wall, and rotated about the east-west axis.

• As a result of the aircraft impact damage, the north and south walls each carried about 7 percent

less gravity loads after impact, and the east and west walls each carried about 7 percent more

loads. The core carried about 1 percent more gravity loads after impact.

2. Effects of Subsequent Fires and Impact Damaged Fireproofing

A. Thermal Weakening of the Core:

• The undamaged core columns developed high plastic and creep strains over the duration the building stood, since both temperatures and stresses were high in the core area. The plastic

and creep strains exceeded thermal expansion in the core columns.

• The shortening of the core columns (due to plasticity and creep) was resisted by the hat

truss, which unloaded the core over time and redistributed loads to exterior walls.

• As a resuh of the thermal weakening (subsequent to impact and prior to inward bowing of

the south wall), the north and south walls each carried about 10 percent more gravity loads,

and the east and west walls each carried about 25 percent more loads. The core carried

about 20 percent less gravity loads after thermal weakening.

B. Thermal Weakening of the Floors:

• Floors 95 to 99 weakened, with increasing temperatures over time on the long-span floors,

and sagged. The floors sagged first and then contracted due to cooling on the north side;

fires reached the south side later, the floors sagged, and the seat connections weakened.

• Floor sagging induced inward pull forces on the south wall columns.

• About 20 percent of the connections to the south exterior wall on Floors 97 and 98 failed

due to thermal weakening of the vertical supports.

C. Thermal Weakening of the South Wall:

• South wall columns bowed inward as they were subjected to high temperatures and inward

pull forces in addition to axial loads.

• Inward bowing of the south wall columns increased with time.

Figure E-11. WTC 1 probable collapse sequence.

NISTNCSTAR 1-6. WTC Investigation Ixvii
Executive Summary

3. Collapse Initiation

• The inward bowing of the south wall induced column instability, which progressed rapidly

horizontally across the entire south face.

• The south wall unloaded and tried to redistribute the loads via the hat truss to the thermally

weakened core and via the spandrels to the adjacent east and west walls.

• The entire section of the building above the impact zone began tilting as a rigid block (all four faces, not only the bowed and buckled south face) to the south (at least about 8°) as column instability progressed rapidly from the south wall along the adjacent east and west

walls.

• The change in potential energy due to downward movement of building mass above the

buckled columns exceeded the strain energy that could be absorbed by the structure. Global collapse then ensued.

Figure E-11. WTC 1 probable collapse sequence (cont).

1. Aircraft Impact Damage

• Aircraft impact severed a number of exterior columns on the south wall from Floors 78 to 84,

and the wall section above the impact zone moved downward.

• After breaching the building's exterior, the aircraft continued to penetrate into the building,

severing floor framing and core columns at the southeast comer of the core. Fireproofmg was damaged from the impact area through the east half of the core up to the north and east exterior walls. The floor truss seat connections over about one quarter to one half of the east side of the core were severed on Floors 80 and 81 and over about one third of the east exterior

wall on Floor 83.

• Aircraft impact severed a few columns near the east comer of the north wall between Floors

80 and 82.

• The impact damage to the exterior walls resulted in redistribution of severed column loads,

mostly to the columns adjacent to the impact zones. The impact damage to the core columns

resulted in redistribution of severed column loads mostly to other intact core columns and the

east exterior wall. The hat tmss resisted the downward movement of the south wall, and

rotated about the east-west axis.

• As a result of the aircraft impact damage, the core carried 6 percent less gravity loads after impact and the north face carried 10 percent less loads. The east face carried 24 percent more

gravity load, while the west face and the south face carried 3 percent and 2 percent more

gravity load, respectively.

• After impact, the core was leaning toward the east and south exterior walls. The exterior

walls acted to restrain the core stmcture.

Figure E-12. WTC 2 probable collapse sequence.

Ixviii NISTNCSTAR 1-6, WTC Investigation
Executive Summary

2. Effects of Subsequent Fires and Impact Damaged Fireproofing

A. Thermal Weakening of the Core:

• Several of the undamaged core columns near the damaged and severed core columns

developed high plastic and creep strains over the duration the building stood, since both

temperatures and stresses were high in the core area. The plastic and creep strains exceeded

thermal expansion in the core columns.

• The core continued to tilt toward the east and south due to the combination of column

shortening (due to plasticity, creep, and buckling) and the failure of column splices at the hat

truss in the southeast comer.

• As a result of thermal weakening (subsequent to impact), the east wall carried about 5 percent more gravity loads, and the core carried about 2 percent less loads. The other three walls

carried between 0 and 3 percent less loads.

B. Thermal Weakening of the Floors:

• Floors 79 to 83 weakened, with increasing temperatures over time on the long-span floors on

the east side, and sagged.

• Floor sagging induced inward pull forces on the east wall columns.

• About an additional one third of the connections to the east exterior wall on Floor 83 failed due to thermal weakening of the vertical supports.

C. Thermal weakening of the east wall:

• East wall columns bowed inward as they were subjected to high temperatures and inward pull

forces in addition to axial loads.

• Inward bowing of the east wall colunms increased with time.

3. Collapse Initiation

• The inward bowing of the east wall induced column instability, which progressed rapidly

horizontally across the entire east face. • The east wall unloaded and tried to redistribute the loads via the hat truss to the weakened

core and via the spandrels to the adjacent north and south walls.

• The entire section of the building above the impact zone began tilting as a rigid block (all four

faces, not only the bowed and buckled east face) to the east (about 7° to 8") and south (about

3° to 4°) as column instability progressed rapidly from the east wall along the adjacent north

and south walls. The building section above impact continued to rotate to the east as it began

to fall downward, and rotated to at least 20 to 25 degrees.

• The change in potential energy due to downward movement of building mass above the

buckled columns exceeded the strain energy that could be absorbed by the structure. Global

collapse then ensued.

Figure E-12. WTC 2 probable collapse sequence (cont).

NISTNCSTAR 1-6. WTC Investigation Ixix
Executive Summary

E.4 FACTORS THAT AFFECTED PERFORMANCE

• From the collective knowledge and insights gained through the Investigation of the collapse of

the WTC towers, the following factors were identified that enhanced the performance of both

towers on September 11, 2001 : The closely spaced columns, along with deep short spandrels,

allowed a redistribution of loads as a result of aircraft impact damage to the exterior wall.

• Because there was effectively no wind on the morning of September 11, 2001, the capacity of the

exterior wall provided to accommodate design wind loads was available to carry redistributed

gravity loads.

• The large dimensional size of the WTC towers helped the buildings withstand the aircraft impact.

• The composite floor system with primary and bridging trusses forming a 2-way grid, and the two

layers of welded wire fabric in the slab, acted to bridge over damaged areas without propagation

of collapse from areas of aircraft impact damage to other locations, thereby avoiding larger scale

floor collapse upon impact.

• The hat truss played a major role in the post-impact performance of the building. This was

accomplished through redistribution of the loads from the significant weakening of the core, due

to aircraft impact damage and subsequent thermal effects, by redistributing loads from the

damaged core columns to adjacent intact columns and, ultimately, by redistributing loads to the

exterior walls from the thermally weakened core columns that lost their ability to support the

buildings' weight.

The buildings would likely not have collapsed under the combined effects of aircraft impact and

the subsequent jet-fuel ignited multi-floor fires, if the insulation had not been dislodged or had

been only minimally dislodged by aircraft impact. The existing condition of the insulation prior to

aircraft impact and the SFRM thickness on the WTC floor system did not play a significant role

in initiating collapse on September 11, 2001.

The passive fire protection applied to the steel structural components in the WTC towers was investigated

to provide information on the in-place condition of the thermal insulation before and after aircraft impact.

The specified and "as applied" thicknesses, the variability in thickness, the condition of the insulation over a 30-year service life, and the effects that the variability and condition have on the structural behavior of insulated steel members were studied. The rationale behind the selection of the effective

thickness of thermal insulation for use in thermal analyses was presented. Additionally, the procedures

and practices used to provide the passive fire protection for the floor system of the WTC tower structures was documented.

kx NISTNCSTAR 1-6, WTC Investigation

E.5 FINDINGS

E.5.1 PASSIVE FIRE PROTECTION
Executive Summary

Building Code Requirements for Structural Fire Resistance

Finding 1: The WTC towers were classified as Class IB, as defined by the 1968 New York City

Building Code. This classification required a 3 h fire rating for columns and 2 h for floors. The towers

could have been classified as Class 1 A since both Class 1 A and IB permitted buildings of unlimited

height. Class lA required a 4 h fire resistance rating for columns and a 3 h rating for floors. In 1969, the Port Authority specified the 0.5 in. SFRM for all beams, spandrels, and trusses, to maintain the Class 1

A

Fire Rating of the New York City Building Code. A condition assessment conducted in 2000 reported

that the WTC towers were classified as Class IB—noncombustible, fire-protected, and retrofitted with

sprinklers in accordance with Local Law 5/1973.

Selection of Fire-Resistive Materials

Finding 2: The passive fire protection for the floor trusses was specified to be 0.5 in. of BLAZESHIELD Type D, although the technical basis for the selection of this product and required thickness

value is not known. After applying the Type D sprayed fire-resistive material to the lower 40 floors of

WTC 1 . the BLAZE-SHIELD insulating material was switched to Type D/CF (reported to meet or exceed

the insulating properties of Type D), which did not contain asbestos. In 1995, the Port Authority

conducted a study to establish the SFRM requirements for the floor trusses in areas undergoing major

tenant renov ation. The thickness required to achieve a 2 h fire rating was determined to be 1 .5 in. using

the BLAZE-SHIELD II product. At the time of the WTC disaster, SFRM had been upgraded on a number of floors in the WTC towers: 18 floors in WTC I, including all of the floors affected by the

aircraft impact and fires, and 13 floors in WTC 2, although none were directly affected by the aircraft impact and fires.

Equivalent thickness of SFRM

Finding 3: Based on analyses of SFRM thickness measurements and interpretation of photographs

showing the condition of the originally applied material, the average thickness of the original thermal

insulation on the floor trusses was estimated to be 0.75 in. with a standard deviation of 0.3 in. (coefficient

of variation of 0.40). The average thickness of the upgraded thermal insulation was estimated to be

2.5 in. with a standard deviation of 0.6 in. (coefficient of variation of 0.24). Based on finite-element

simulations, it was concluded that the original passive fire protection on the floor trusses was thermally

equivalent to a uniform thickness of 0.6 in., and the upgraded insulation was thermally equivalent to a uniform thickness of 2.2 in. These values were used in the thermal analyses for determining temperature

histories of structural components.

Finding 4: No information was available on in-place conditions of the thermal protection on the exterior

columns and spandrel beams, and little information was available on the conditions of fire-resistive material on core beams and columns. For thermal analyses of the towers, the SFRM on these elements was taken to have uniform thicknesses equal to the specified thickness. This assumption was supported

by the obsen- ation that measured average thickness tended to be greater than the specified thickness

while, due to variability, the effective thickness tended to be less than the average uniform thickness. The

specified thickness values were 0.5 in. for beams and spandrels, 2.06 in. (2 1/16 in.) for columns hghter

than 14WF228,and 1.19 in. (1 3/16 in.) for columns equal to or heavier than 14WF228.

NISTNCSTAR 1-6. WTC Investigation Ixxi
Executive Summary

Finding 5: The adhesive strength of BLAZE-SHIELD DC/F to primed steel was found to be a third to a half of the adhesive strength to steel that had not been coated with primer paint. The SFRM products used

in the WTC towers were applied to steel components with primer paint.

E.5.2 FIRE RESISTANCE TESTS

Four Standard Fire Tests (ASTM E 119) were conducted on floor assembhes constructed to duplicate, as closely as practical, the floor system used in the WTC towers. Full scale tests with a 35 ft span, and

having Va in. thick SFRM were tested; one in the restrained test condition and the other in the unrestrained

test condition. Tests of half-scale specimens, which spanned approximately 17 ft, were conducted using

SFRM conditions simulating the "as specified" condition (0.5 in. thick) and the "as-applied" condition

(0.75 in. thick). The following findings are based on this series of four tests and a comparison of their

results.

Structural Performance \.

Finding 6: Test assemblies, representative of the WTC floor system, exposed to the Standard Fire Test (ASTM E 119) conditions resulted in extensive spalling on the underside of the floor slab, thermal

damage to the bridging trusses, and buckling of compression diagonals and vertical struts of the main

trusses.

Finding 7: All four tests demonstrated that the floor assemblies were capable of sagging without failure. The unrestrained test, which had two 0.875 in. bolts fastening the main truss to the truss seats, did not sag

sufficiently to bear on the bolts. In the three restrained tests, the main truss ends were welded to the truss seats to provide the required restraint. The magnitude of the sagging observed in the tests was consistent

with that computed from finite element structural analyses. No evidence of knuckle failures was seen in the tests.

Finding 8: All four test assemblies supported their full design load under standard fire conditions for two

hours without collapse.

Fire Resistance Ratings

Finding 9: The 1968 New York City (NYC) Building Code—the code that the WTC towers were

intended but not required to meet when they were built—required a 2 h fire rating for the floor system.

Finding 10: The restrained floor system obtained a fire resistance rating of 1.5 h while the unrestrained

floor system achieved a 2 h rating. This finding was unexpected since the unrestrained rating is typically

less than the restrained rating.

Finding 11: The test of the 17 ft specimen with as-applied SFRM did not produce the same rating as the

35 ft test specimen, giving 2 h and 1.5 h, respectively. In both cases, the rating was established on the

basis of temperatures of the unexposed surface (top of concrete slab) and not on the ability of the

specimen to support the load.

Ixxii NISTNCSTAR 1-6, WTC Investigation
Executive Summary

Finding 12: The 45 min rating for the standard 17 ft test with the specified 0.5 in. SFRM did not meet

the 2 h requirement of the 1968 NYC Building Code. This test had no SFRM on the bridging trusses nor on the underside of the metal deck.

Finding 13: The 2 h rating for the standard 17 ft test with the as-applied average 0.75 in. SFRM met the

2 h requirement of the 1968 NYC Building Code. This test had half the SFRM thickness on the bridging

trusses (0.375 in.) and overspray on the underside of the metal deck.

Finding 14: The difference in test results for the two 17 ft specimens is due primarily to the concrete slab

performance (spalling and cracking) and the presence or lack of SFRM overspray on the metal deck and

not due to the SFRM thickness on the trusses. Differences in the degree of concrete spalling were

possibly due to differences in moisture content and the slab cracking.

E.5.3 RESPONSE OF STRUCTURAL COMPONENTS

The response of the structural components and their connections for the tenant floors and exterior walls was examined with detailed structural models. Results of the floor and exterior wall component and

connection analyses identified structural behaviors and failure modes that were required for inclusion in the global analyses.

Floor System

Finding 15: The interior truss seats had a greater vertical shear capacity than the exterior truss seats. The controlling failure mode for vertical shear was weld fracture. However, the vertical load at the truss connection of approximately 16 kip had to increase by a factor of two to six to reach failure (weld

fracture) for temperatures near 600 °C to 700 °C.

Finding 16: Detailed structural analysis of a single truss section of the composite floor system subjected

to elevated uniform temperatures was found to initially push out on the exterior columns as a result of the concrete slab thermal expansion and then pull inward as the web diagonals buckled and the truss sag

increased. The magnitude of the pull-in force was found to depend highly on the stiffness of the exterior

box column which, in turn, depended on expansion of floors above and below.

Finding 17: Detailed analysis of the knuckles (shear connectors in the floor system for composite action)

through test simulation and detailed truss analysis found that failure of the knuckles in the floor system was unlikely. This finding was also supported by the lack of any knuckle failures in the four standard fire resistance tests (ASTM El 19) of the floor truss assemblies with twice the floor load that was on the WTC

floors.

Exterior Wall System

Finding 18: Large inelastic deformations and buckling of the spandrels at elevated temperatures were

predicted, but were found not to significantly affect the stability of the exterior columns. Partial separations of the spandrel splices were also predicted at elevated temperatures, but were found not to

significantly affect the stability of the exterior columns.

NISTNCSTAR 1-6, WTC Investigation Ixxiii
Executive Summary

Finding 19: Analyses of bolted splices in the exterior columns found that the sphce may slide or open

when the exterior columns are bowing and subject to large lateral deflections. No column splice bolts were predicted to have failed.

Finding 20: An exterior wall section (9 columns wide and 9 floors high) was found to bow inward when

the floor connections applied an inward pull force. For the condition where three sequential floors were

disconnected, there was no bowing of the columns for five different elevated temperature conditions.

When the column section with three disconnected floors was subjected to increased axial column loads,

the wall section bowed outward over the unsupported column length.

E.5.4 FIRE PROTECTION AND PARTITION DAMAGE DUE TO AIRCRAFT IMPACT

The aircraft impacts into the WTC towers caused extensive damage to the buildings' exterior, penetrated

into the interior causing further damage to the structural system, dislodged insulation, and ignited multifloor fires. The structural damage to each tower resulting from the aircraft impact was estimated using a

transient finite element analysis. Results of this analysis were used to predict damage to the structure, insulation, and partition walls in the path of the debris field.

Finding 21: For WTC 1, partitions were damaged and insulation was dislodged by direct debris impact

over five floors (Floors 94, 95, 96, 97, and 98) and included most of the north floor areas between the

north face and the core, the core, and central regions of the south floor areas, and on some floors,

extended to the south wall. For WTC 2, partitions were damaged and insulation was dislodged by direct

debris impact over six floors (Floors 78, 79, 80, 81, 82, and 83) and included the south floor area between

the north face and the core, the central and east regions of the core, and most of the east floor area, and

extended to the north wall.

Finding 22: The insulation damage estimates were conservative as they ignored possibly damaged and

dislodged insulation in a much larger region that was not in the direct path of the debris but was subject to strong vibrations during and after the aircraft impact. Robust criteria to generate a coherent pattern of

vibration-induced dislodging could not be established to estimate the larger region of damaged insulation.

E.5.5 OBSERVATIONS AND TIMELINE

Thousands of photographs and hours of video records were reviewed for insights into the structural performance of the towers. A timeline of significant events that characterized the weakening and

eventual collapse of the WTC towers was developed with the photographs and videos that were timestamped. Quantitative information, such as the amount of inward bowing observed on the exterior walls

of the buildings, was extracted from key photographs through image enhancement and scaled

measurements. Key observations and the timelines were used to guide the global collapse analyses.

WTC 1

Finding 23: Inward bowing of the south exterior wall was first observed at 10:23 a.m. The bowing

appeared to extend between Floors 94 and 100 and Columns 305 and 359. The maximum bowing was

estimated from images to be 55 in.±6 in. at Floor 97 on the east side of the south face of WTC 1. The

central area in available images was obscured by smoke. The extent of fires observed on all faces of

Ixxiv NISTNCSTAR 1-6, WTC Investigation
Executive Summary

WTC 1 was similar, although somewhat more extensive on the east and west faces (where short span

floors were located) and similarly extensive on the north and south faces (where long span floors were

located). Inward bo\\'ing was observ ed only on the south face. The north face had extensive aircraft impact damage, and the damaged floors were not capable of imposing inward pull forces on the north

face.

Finding 24: The time to collapse initiation was 102 minutes from the aircraft impact (8:46:30 a.m. until 10:28:22 a.m.).

Finding 25: From exterior obser\'ations, tilting of the building section appeared to take place near Floor 98. Column buckling was then obser\'ed to progress rapidly across the east and west faces.

Finding 26: The WTC 1 building section above the impact and fire area tilted to the south as the

structural collapse initiated. The tilt was toward the side of the building that had long span floors. Video

records taken from east and west viewpoints showed that the upper building section tilted to the south. Video records taken from a north viewpoint showed no discemable east or west component in the tilt. A

tilt to the south of at least 8 degrees occurred before dust clouds obscured the view and the building

section began to fall downward.

WTC 2

Finding 27: On the east face and north face of WTC 2, draped objects were observed through the windows of Floor 82 on the east face and Floors 81 to 83 on the north face near the northeast comer. The

draped objects appeared to be hanging floors. The drape of these objects was observed to increase with

time and extend across approximately half of the east face.

Finding 28: Inward bowmg of the east wall was first observed at 9:21 a.m. The inward bowing was

approximately 10 in.±l in. at Floor 80 and extended between Floors 78 and 83 and Columns 304 and 344.

The remaining portion of the face to the south of Column 344 was not included in the image. The bowing

appeared to extend over a large fraction of the east face and to be greatest near the center of the face. Fires were more extensive along the east face (where long span floors were located) and at the east side of the

north and south faces (where short span floors were located). Fires were not observed on the west face

(where long span floors were located). Inward bowing was observed only on the east face. The south face

had extensive aircraft impact damage, and the damaged floors were not capable of imposing inward pull

forces on the south face. There was no impact damage or fire on the west floors to cause pull-in forces on

the west face.

Finding 29: An increase of the inward bowing of the east wall was observed at 9:53 a.m. The inward

bowing appeared to extend between Floors 78 and 84 and Columns 305 and 341. The remaining portion

of the face to the south of Column 344 was not included in the image. The maximum bowing was

estimated from images to be 20 in.±l in. at Floor 80 on the east face of WTC I.

Finding 30: The time to collapse initiation was 56 minutes after aircraft impact (9:02:59 a.m. to 9:58:59 a.m.).

Finding 31 : From exterior observations, tilt of the building section above the impact and fire area appeared to take place near Floor 82. Column buckling was then seen to progress across the north face.

NISTNCSTAR 1-6. WTC Investigation Ixxv
Executive Summary

Finding 32: The building section above the impact and fire area tihed to the east and south at the onset of structural collapse. The tilt occurred toward the east side with the long span floors. Estimates made

from photographs indicate that there was approximately a 3 degree to 4 degree tilt to the south and a 7 degree to 8 degree tilt to the east, prior to significant downward movement of the upper portion of the

building. The tilt to the south did not increase any further as the upper building section began to fall, but

the tilt to the east continued, reaching 20 degrees to 25 degrees before dust clouds obscured the view.

E.5.6 STRUCTURAL RESPONSE OF MAJOR TOWER SUBSYSTEMS

Prior to conducting the analysis of the global structural response of each tower, major structural subsystems were analyzed to provide insight into their behavior within the WTC global system. The

three major structural subsystems, the core framing, a single exterior wall, and full tenant floors, were

analyzed separately for their response to impact damage and fire. The hat truss was not analyzed

separately as its structural behavior did not require significant simplification in the global analysis. The

component analyses provided a foundation for these large, nonlinear analyses with highly redundant load

paths, and they enabled a significant reduction in fmite element model complexity and size. The major

subsystem models used final estimates of impact damage and elevated temperatures determined from the

aircraft impact analysis and the fire dynamics and thermal analyses.

isolated Core Subsystem

Finding 33: The WTC 1 isolated core subsystem analysis found that the core structure was most

weakened from impact and themial effects at the center of the south side of the core. Smaller

displacements occurred in the global model due to the constraints of the hat truss and floors.

Finding 34: The WTC 2 isolated core subsystem analysis found that the core structure was unstable for the estimated structural damage to core columns. The core was most weakened from impact and thermal

effects at the southeast comer and along the east side of the core. Larger displacements occurred in the

global model as the isolated core model had lateral restraints imposed that were somewhat stiffer than in the global model.

Full Floor Subsystem

Finding 35: Floor sagging was caused primarily by either buckling of truss web diagonals or disconnection of truss seats at the exterior wall or the core perimeter. Except for the truss seat failures near the southeast comer of the core in WTC 2 following the aircraft impact, web buckling or tmss seat

failure was caused primarily by elevated temperatures of the structural components.

Finding 36: Analysis results from both the detailed tmss model and the full floor models found that the

floors began to exert inward pull forces when floor sagging exceeded approximately 25 in. for the 60 ft floor span.

Finding 37: Sagging at the floor edge was due to loss of vertical support at the tmss seats. The loss of

vertical support was caused in most cases by the reduction in vertical shear capacity of the tmss seats due

to elevated steel temperatures.

Ixxvi NISTNCSTAR 1-6, WTC Investigation
Executive Summary

Isolated Exterior Wall Subsystem

Finding 38: Inward pull forces were required to produce inward bowing that was consistent with

displacements measured from photographs. The inward pull was caused by sagging of the floors. Heating

of the inside face of the exterior columns also contributed to inward bowing.

Finding 39: The observed inward bowing of the exterior wall indicated that most of the floor

connections were intact to cause the observed bowing.

Finding 40: The floors that were identified through analysis to be affected by the fires and the dislodged

insulation matched well with the floors that were observed to have participated in the inward bowing of

the exterior w alls.

Finding 41: The extent of floor sagging required at each floor was greater than that predicted by the full floor models. The estimates of the extent of sagging at each floor were governed by the combined effects of insulation damage and fire; insulation damage estimates were limited to areas subject to direct debris

impact. Other sources of floor and insulation damage from the aircraft impact and fires (e.g., insulation

damage due to shock and subsequent vibrations as a result of aircraft impact or concrete slab cracking and

spalhng as a resuh of thermal effects) were not included in the floor models.

E.5.7 STRUCTURAL RESPONSE TO AIRCRAFT IMPACT DAMAGE AND FIRE

Global analysis of WTC 1 and WTC 2 used final estimates of impact damage and elevated temperatures

to determine the structural response and sequence of component and subsystem failures that led to collapse initiation.

General Findings

Finding 42: The structural analyses of WTC 1 and WTC 2 found that the collapse of the towers was due

to the combined effects of structural and insulation damage from aircraft impact and the subsequent fires on the core, floor systems and exterior walls. The towers collapsed when the weakened core and exterior

columns could no longer redistribute or support the building loads with their reduced load carrying

capacity.

Finding 43: Impact damage alone did not cause collapse of the towers, as they were stable after the

aircraft impact. Global analyses showed that both towers had substantial reserve capacity after the

aircraft impact.

Finding 44: The multi-floor fires alone did not cause collapse of the towers. Without impact damage to the insulation, the structural steel temperatures would have been generally less than 200 °C to 300 °C,

with a few isolated locations of structural steel temperatures exceeding 400 °C in WTC 1 floors and

500 °C in WTC 2 floors. The core would not have weakened, the floor sag would have been insufficient

to pull inward on the exterior columns, and the exterior walls would not have bowed inward.

Finding 45: The towers would likely not have collapsed under the combined effects of aircraft impact

and the subsequent multi-floor fires if the insulation had not been dislodged or had been only minimally

dislodged by aircraft impact. Had insulation not been dislodged by the debris field, temperature rise of

NISTNCSTAR 1-6, WTC Investigation Ixxvii
Executive Summary

structural components would likely have been insufficient to induce global collapse. Structural

components that became thermally weakened were generally determined by impact of the debris field. The existing condition of the insulation prior to aircraft impact and the insulation thickness on the WTC

floor system did not play a role in initiating collapse of the towers.

Finding 46: Creep strain was significant in the core and exterior columns over the 56 min to 102 min

period of fire exposure in columns with temperatures greater than 500 °C to 600 °C and high stress. Columns with creep strains of sufficient magnitude to cause column shortening played a significant role

in the collapse initiation.

Finding 47: The faces of the buildings that exhibited inward bowing were associated with the long span

direction of the floor system. The primary direction of tilting at collapse initiation for WTC 1 and WTC 2 was in the direction of the bowed faces.

Performance with Intact Fire Protection

Finding 48: A detailed thermal-structural analysis, which did not include slab delamination/spalling

effects, showed that a full collapse of the WTC floor system would not occur even with a number of

failed trusses or connections.

Finding 49: Most of the horizontal and vertical capacity of the floor connections to the exterior and core columns significantly exceeded the demand under design load conditions.

E.5.8 PROBABLE COLLAPSE SEQUENCES

The results of structural analyses conducted in this study on components, subsystems, isolated exterior

walls, cores, and global models of WTC 1 and WTC 2 showed that the collapses of the towers were

initiated due to the combined effects of the structural and insulation damage from aircraft impact and the

subsequent intense fires. The probable collapse sequence for WTC 1 and WTC 2 are based upon the

collective consideration of structural analyses, statistical based methods, observations, and laboratory

testing.

Role of the Building Core

Finding 50: The core columns were weakened significantly by the aircraft impact damage and thermal

effects. Thennal effects dominated the weakening of WTC 1 . As the fires moved from the north to the

south side of the core, following the debris damage path, the core was weakened over time by significant

creep strains on the south side of the core. Aircraft impact damage dominated the weakening of WTC 2. Immediately after impact, the vertical displacement at the southeast comer of the core increased 6 in. (from 4 in. to 10 in.). With the impact damage, the core subsystem leaned to the southeast and was

supported by the south and east floors and exterior walls.

Finding 51: As the core was weakened from aircraft impact and thermal effects, it redistributed loads to the exterior walls primarily through the hat truss. Additional axial loads redistributed to the exterior

columns from the core were not significant (only about 20 percent to 25 percent on average) as the

exterior columns were loaded to approximately 20 percent of their capacity before the aircraft impact.

Ixxviii NISTNCSTAR 1-6, WTC Investigation
Executive Summary

Role of the Building Floors

Finding 52: The primary role of the floors in the collapse of the towers was to provide inward pull forces

that induced inward bowing of exterior columns (south face of WTC 1; east face of WTC 2).

Finding 53: Sagging floors continued to support floor loads as they pulled inward on the exterior

columns. There would have been no inward pull forces if many of the floor truss seats had failed and

disconnected.

Role of Exterior Frame-Tube

Finding 54: Column instability over an extended region of the exterior face ultimately triggered the global system failure as the loads could not be redistributed through the hat truss to the already weakened

building core. In the area of exterior column buckling, load transferred through the spandrels to adjacent

columns and adjacent exterior walls. As the exterior wall buckled (south face for WTC 1 and east face

for WTC 2), the column instability propagated to adjacent faces and caused the initiation of the building

collapse.

Finding 55: The exterior wall instability was induced by a combination of thennal weakening of the columns, inward pull forces from sagging floors, and to a lesser degree, additional axial loads

redistributed from the core.

Probable Collapse Sequences

Finding 56: Although the north face of WTC 1 had extensive impact damage, thermal weakening of the core columns on the south side of the core and inward bowing of the south face caused the building to tilt to the south at collapse initiation. The extent of fires observed on all faces of WTC 1 was similar,

although somewhat more extensive on the east and west faces (where short span floors were located) and

somewhat less extensive on the north and south faces (where long span floors were located). Thermal

weakening of exterior columns with floor sagging (which induced inward pull and occurred on the south

side) caused inward bowing of the south face and tilting in the south direction.

Finding 57: Although the south face of WTC 2 had extensive impact damage, thermal weakening of the core columns on the east side of the core and inward bowing of the east face caused the building to tilt more to the east and less to the south at collapse initiation. Fires were more extensive along the east face

and at the east side of the north and south faces. Thermal weakening of exterior columns with floor sagging (which induced inward pull and occurred on the east side) caused inward bowing of the east face

and primary tilting in that direction (with additional southward tihing due to the aircraft impact damage).

Finding 58: The time it took for each WTC tower to collapse was due primarily to the differences in

structural damage, the time it took the fires to travel from the impact area across the floors and core to

critical locations, and the time it took to weaken the core and exterior columns. WTC 2 had asymmetric

strucmral damage to the core, including the severing of a comer core column, and WTC 1 had more

symmetrical damage. The fires in WTC 2 reached the east side of the building more quickly, within 10 to 20 minutes, than the 50 min to 60 min it took the fires in WTC 1 to reach the south side.

NISTNCSTAR 1-6. WTC Investigation Ixxix I
Executive Summary

Finding 59: NIST found no corroborating evidence for alternative hypotheses suggesting that the WTC

towers were brought down by controlled demolition using explosives planted prior to September 1 1

,

2001. NIST also did not find any evidence that missiles were fired at or hit the towers. Instead,

photographs and videos from several angles clearly showed that the collapse initiated at the fire and

impact floors and that the collapse progressed from the initiating floors downward, until the dust clouds

obscured the view.

Ixxx NISTNCSTAR 1-6, WTC Investigation
Chapter 1 Introduction

1 .1 PROJECT OBJECTIVES

The National Institute of Standards and Technology (NIST) investigation into the collapse of the World

Trade Center (WTC) towers had eight interdependent projects. The purpose of each project is summarized in Table P-1, and the key interdependencies among the projects are illustrated in Fig. P-1,

found in the Preface to this report.

One of the four objectives of the technical investigation was to determine why and how the WTC towers

(WTC 1 and WTC 2) collapsed following the initial impacts of the aircraft. This objective is addressed in

this report. Both the north and south towers of the World Trade Center were severely damaged by the

impact of Boeing 767 aircraft, yet they remained standing for some time. The ensuing fires were observed

to move through both buildings and eventually, both buildings collapsed. The extent and relative

importance of the damage caused by the aircraft impact and subsequent weakening by fires were

inv estigated under this project. Structural Fire Response and Collapse Analysis. This report presents the

technical approach, modeling and testing methodologies, summary of results, and findings of the

structural response of the WTC towers to aircraft impact damage and ensuing fires.

In addition, this project contributes to another investigation objective by determining the procedures and

practices that were used in establishing the fire resistance ratings and providing passive fire protection to the components that made up the WTC tower structures.

The purpose of the project was to analyze the response of the WTC towers to fires - both with and

without aircraft damage - and to determine the probable sequence of structural collapse for each tower.

Specifically, this project attempted to:

• Determine the pre- and post-aircraft impact condition of the passive fire protection used to thermally insulate the structural members and provide resistance to fire damage,

• Conduct tests of structural components and systems under fire conditions to quantify their behavior,

• Evaluate the response of floor and column components and subsystems under fire conditions

to understand their response,

• Evaluate the response of the WTC towers under fire conditions, with and without aircraft impact damage, and

• Determine the probable sequence of structural collapse for each WTC tower.

The project relied primarily on a series of computer simulations to model the complete sequence of events leadmg to the mitiation of collapse of the WTC towers. The analyses included the damage to the towers

NISTNCSTAR 1-6. WTC Investigation 1
Chapter 1

resulting from aircraft impact, the spread of multi-floor fires ignited by jet-fuel, the heating and thermal

weakening of structural components, and the progression of local structural failures that led to the collapse of the buildings. Each of these models advanced the current state of the art and tested the limits of computational capabilities. The unprecedented complexity and sophistication of these analyses

required the use of various strategies for managing the computational demands while adequately

capturing the essential physics. The overall approach — from impact analysis to collapse initiation combined mathematical modeling, statistical and probability-based analysis, laboratory testing, and

analysis of photographic and videographic records.

1.2 TECHNICAL APPROACH AND TASKS

Events that played a significant role in the structural performance of the towers on September 1 1

, 200 1

,

were the aircraft impact, the fireballs immediately following the aircraft impact, and the ensuing fires across multiple floors in each tower. To estimate the structural response, detailed information was

required on the condition of the structural system and its passive fire protection system, both before and

after the aircraft impact. During the ensuing fires that resulted in elevated structural temperatures,

information on the degradation of the stiffness and strength of the structural system was also required.

Data was collected and reviewed from a number of sources. Such data included structural geometry,

details, and connections; thermal and mechanical (adhesion/cohesion) properties of fire resistant

materials; the thickness and condition of the passive fire protection in the towers; and recorded

obser\'ations of structural events subsequent to aircraft impact and prior to collapse. Information about

tower construction was obtained from original drawings and specifications, reports, and available records

from The Port Authority of New York and New Jersey (PANYNJ or Port Authority), Leslie E. Robertson

Associates (LERA), Silverstein Properties, and a number of contractors that had worked on the design,

construction, or modifications to the towers. Information about the events that occurred in each tower on

September 11, 2001, was obtained from analysis of available photographic and videographic records,

eyewitness accounts, and metallurgical analysis of recovered structural steel.

The analyses performed to determine the probable collapse sequence for each tower considered the as- built structural systems and their response to aircraft impact damage, temperature-dependent properties of

steel and concrete, growth and spread of the fires, and heating of structural components. The structural response analyses relied upon the following information:

• Reference global structural models of the WTC 1 and WTC 2 towers, and typical floor and

exterior wall subsystem models (NIST NCSTAR 1-2)"

• Extent of damage to the structural systems and interior contents of the WTC 1 and WTC 2

towers resulting from aircraft impact (NIST NCSTAR 1-2)

• Temperature-dependent mechanical properties of the steels, welds, and bolts used in the

construction of the towers, including elastic, plastic, and creep properties from 20 "C to 700 °C (NIST NCSTAR 1-3)

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

to this report.

NIST NCSTAR 1-6, WTC Investigation
Introduction

• Time-temperature histories for structural components and connections for both standard

fires (e.g.. ASTM E 1 19) and actual fires based on fire dynamics simulations

(NIST NCSTAR 1-6B).

• Photographic and videographic records with time stamps that documented the observed

sequence of events (NIST NCSTAR 1-5).

To simulate the effects of aircraft impact into the towers, the growth and spread of fires, and the

subsequent weakening of the structural system that ultimately led to collapse, a series of sophisticated

computer analyses was conducted. The resuhs of any computer analysis depend on the fidelity of the input data and the ability of the computer software to capture the fundamental physics that produce the

output response.

The WTC towers were large, complex structural systems. To include all of the structural components and

connections and their associated behavior and failure mechanisms using refined finite element meshes

would have been prohibitive. The analysis approach used was a variant of the well-established substructuring approach, adapted for the analysis of structures with highly nonlinear behavior, that progressed fi'om individual components to major subsystems to global systems, as shown in Fig. 1-1. Extensive component analyses were conducted to identify critical behavior and failure mechanisms that contributed to the global structural response of each tower. Similarly, extensive subsystem analyses were

then performed. These analyses incorporated the behavior and failure mechanisms identified in the

component studies, with modifications to reduce the model size and complexity, thereby enhancing

computational performance, without adversely affecting the quality of the results. Whenever modeling

modifications were used, they were validated against the detailed component model results. The global

analyses incorporated critical behavior and failure mechanisms, determined from subsystem analyses,

while making necessary modifications in the level of modeling detail.

NIST NCSTAR 1-6. WTC Investigation 3
Chapter 1

Component

Shear knuckle

Truss seat connections

Single truss and concrete

slab

Full floor

Column splice connection

Single story column

Nine story column

Nine story-nine column

exterior wall panel

Detailed nonlinear analyses to detcnnine components

behavior and ftiilure mechanisms.

and sequential failure

mechanisms.

Figure 1-1. Structural Analysis Sequence.

As shown in Fig. 1-1, the structural response analyses began with the analysis of components,

connections, and subsystems to develop an understanding of their structural behavior at elevated

temperatures and associated failure mechanisms. Components, used herein, included single structural elements, such as a column or a truss web member. Subsystems were groups of components that had a major structural function, such as a floor system. Connections, such as a column splice or a floor truss

seat, transfer loads between components or subsystems. Results of the component and subsystem

analyses were used to develop the global models that were used to determine the global behavior and

sequential failure mechanisms.

The response of WTC 1 and WTC 2 global systems was estimated by (1) evaluating the response of floor

and column components, connections, and subsystems under thermal loading, (2) evaluating the response

of the WTC towers with and without aircraft impact damage under actual fire conditions, (3) conducting

tests of structural components and systems under fire conditions, and (4) developing and evaluating

collapse hypotheses for the WTC towers. The effort, including work performed outside the scope of this project, was divided into the following tasks:

1.2.1 Task A - Finite Element Reference Models

Develop finite element models based on reference models: Modeling the structure of the towers, whether

for the assessment of aircraft impact, the growth and spread of fires, or the structural response to those

fires, necessitated that the geometry, cross-sectional properties, and the material properties of the

structural components be a faithful representation of the actual structures. To that end, reference finite element models of both towers (since they were not identical) were developed. This was done under a

contract to Leslie E. Robertson Associates (LERA), the designers of the WTC towers, within the

Subsystems

WTC 1 • Isolated core

• South exterior face

• Full floors 93 to 99

WTC 2

• Isolated core

• East exterior face

• Full floors 79 to 83

Nonlinear analyses with

component behavior and

failure mechanism

simplifications to determine

major subsystem behavior

Global

WTC 1 WTC 2

Nonlinear analyses

to determine global

behavior and

sequential failure

mechanisms.

4 NISTNCSTAR 1-6, WTC Investigation
Introduction

framework of Project 2 of the Investigation. The models underwent a thorough review process, including an in-house NIST review and a third-party review by the firm of Skidmore, Owings& Merrill LLP

(SOM), also under contract to NIST, to test the accuracy of the models. The reviews included checking

the consistency of the models with the original design documents, verification and validation of the

models (including assumptions and level of detail), and testing the accuracy of the models under various

loading conditions. The development of the reference models is described in NIST NCSTAR 1-2. These

reference models became the basis for all subsequent finite element analyses.

1.2.2 Task B - Material Properties

Develop the constitutive relationships for the materials used in the construction of the towers: Properties

of the structural steels used in the construction of the towers were part of the fundamental data needed for the development of models. Mechanical and chemical properties were determined for steel specimens

recovered from the WTC site to assure that the materials used were in conformance with those specified

in the original design. Further, the behavior of the structural steels used was characterized to determine

the mechanical properties at high loading rates for the aircraft impact analyses and at elevated

temperatures (from room temperamre to 800 °C) for the thermal-structural response analyses. Properties

of structural materials are given in NIST NCSTAR 1-3 and summarized in Chapter 4 of this report.

1.2.3 Task C - Passive Fire Protection

Characterize the passive fire protection applied to the structural steel: The type of SFRM materials and

required thickness were specified in correspondence between the Port Authority and SFRM contractor. Estimates of the characteristics of SFRM materials were deemed essential for the thermal-structural

modeling of the towers. Of primary importance was the condition of the sprayed fire-resistive material

(SFRM) used since the type of material (hence its thermal insulating properties), its average applied

thickness, and variation in application thickness all had an effect on the temperatures developed in the

structural elements as a result of exposure to fire. Since upgrading of the SFRM was begun in the 1990s,

and the upgraded thickness was greater than that originally applied, it was important to determine: (1) the areas in the buildings where upgrading had been completed, and (2) the average thickness and its associated variability. Chapter 2 of this report addresses the passive fire protection and its application, the

determination of thermal properties, and calculation of an equivalent uniform thickness of material used

for the thermal and structural finite element analyses. Detailed information and a complete description of

the procedures and practices used in the selection of fire protection for the WTC project is covered in NIST NCSTAR 1-6A.

1.2.4 Task D - Standard Fire Resistance Tests

Conduct standard fire resistance tests of composite truss floor svstem: Tests were conducted to:

(1) establish the baseline fire resistance rating of the composite truss floor system used in the WTC

towers, (2) understand the influence of thermal restraint by testing the floor system under both thermally

unrestrained and fully restrained conditions, and (3) provide experimental data to validate and provide

guidance to the development of the floor models and to the interpretation of analyses resuhs. The

standard ASTM E 1 1 9 fire test was selected since it represents a fairly severe condition both in the fire exposure and specimen loading and provides a frame of reference with respect to the historical

performance of alternative floor systems. Four tests, representing both full-scale and reduced-scale

NIST NCSTAR 1-6, WTC Investigation 5
Chapter 1

specimens, are covered in Chapter 3 of this report and are reported in more detail in NIST

NCSTAR 1-6B.

1.2.5 Task E - Aircraft Impact Damage

Establish the damage to the structure, insulation, and partition walls as a result of aircraft impact: The

damage induced by the impact of a Boeing 767 aircraft into each tower had significant influence on many

facets of the analytical investigation into how and why the towers collapsed. First, the aircraft impact

resuhed in significant damage not only to the exterior of the buildings, but also to the floors and core

structures inside the buildings and as a consequence, weakened the structures to some degree. Second,

the jet fuel dispersed inside the towers ignited the building contents and furnishings, and the damage to the buildings' facades as well as damage to the interiors influenced the amount of oxygen reaching the

fires and, therefore, the speed at which the fires grew and moved throughout the affected floors. Third,

the impacts of the jet aircraft were of sufficient force to dislodge significant portions of the all-important

SFRM in the impact and fire-affected regions. The finite element analyses required to predict the extent of damage due to aircraft impact are presented in NIST NCSTAR 1-2. This infonnation was then used to estimate the extent of the SFRM damage based on the results of impact simulations, including the paths

of the debris field and damage to interior partitions and furnishings. Chapter 5 summarizes the results of

the impact damage and the approach used to estimate the extent of insulation damage.

1.2.6 Task F - Observations and Timeline

Document observations and data for structural events: NIST has made concerted efforts to validate

analysis results with key observations obtained from its extensive collection of over 7,000 photographs

and over 150 hours of videotape (NIST NCSTAR 1-5 A) documenting the events at the World Trade

Center on September 11, 2001. Development of the probable collapse sequence for each tower was

shaped by evidence gathered from these photographs and videos, along with eyewitness accounts. The

photographs and videos provided knowledge about aircraft impact damage to the exterior walls, fire growth and spread at the building exterior, inward bowing of an exterior wall in each tower, and the

direction of tilt for the building section above the impact and fire zone as the towers collapsed. Evidence was used in the analyses in three ways: (1) to determine input parameters, such as the aircraft speed and

trajectory upon impact, (2) to impose time-related constraints upon an analysis, such as imposing

observed broken windows over time thereby affecting the spread of fire, or (3) to validate analysis results,

such as global stability after impact and during thermal loading. A timeline of impact, fire, and structural events was developed for each tower, primarily from photographs and videos. Analyses were used to develop and refine the probable sequence of events between timeline observations. The quality of the

results compared to the visual and physical evidence supports NIST's view that the significant

phenomena relevant to the probable collapse sequence have been adequately captured. Details of this task

are provided in Chapter 6 of this report.

1.2.7 Task G - Temperatures of Structural Components

Compute temperature histories for structural components subjected to fires: After the aircraft impacted

each building, fires started on multiple floors, ignited by the rapid spread of burning jet fuel. To

determine how the towers were affected by the high temperatures resulting from the fires, estimates of the

growth and spread of fires over time were developed using fire dynamics simulations. These were based

6 NIST NCSTAR 1-6, WTC Investigation
Introduction

on sophisticated computational fluid d}Tiamics (CFD) modeling as described inNTSTNCSTAR 1-5. These computations relied on: (1) the fire loads on each floor, (2) estimated ventilation as determined

from the aircraft im.pact analyses (Task E), and (3) window breakage resulting from the fires as determined from photographic interpretation (Task F). Temperamres of the steel structural components

and concrete floor slabs were predicted using accurate models of the structures (Task A), thermal

properties of the steel and concrete (Task B), thermal properties of the insulation applied to protect the

steel and its equivalent uniform thickness (Task C), and the time and spatially varying temperature fields predicted from the fire dynamics calculations as described above. The thermal analyses conducted to estimate realistic temperatures in the steel and concrete are covered in NIST NCSTAR 1-5.

1.2.8 Task H - Component and Subsystem Analyses

Conduct component and subsvstem analyses: The purpose of these analyses was to provide a basic

understanding of the behavior of the various structural components and subsystems of the towers under

gravity' and thermal loading and to develop reduced models that could be reliably used in the global

models. The subsystems considered in this phase of the study included: (1) typical floor subsystem with

its associated components: (a) the shear knuckles, (b) truss seats, and (c) a single truss and concrete slab; and (2) a nine-story by nine-column exterior wall subsystem with its associated components (a) bolted

connection between exterior columns, (b) bolted connection between spandrels, (c) single exterior column

with spandrel sections, and (d) single exterior wall panel with three columns and three spandrels. The

floor and exterior wall subsystems included modeling reductions as developed from the component

models. The models were based on the reference models developed in Task A, material properties

estimated in Task B. and SFRM thickness and properties determined from Task C. The floor

components and subsystem models were verified using the standard fire test results (Task D). Chapter 4

of this report describes the development of the component and subsystem models

1.2.9 Task I - Major Subsystem Analyses

Conduct analvses of major subsystems: Analyses of three major subsystems - the isolated core framing

subsystem, an exterior wall subsystem, and the composite floor subsystems - were analyzed to determine

their abihty to resist and redistribute loads after impact damage and elevated temperatures. These major

subsystem models used final estimates of impact damage and elevated temperamres determined from the

aircraft impact analysis (Task E) and the fire dynamics and thermal analyses (Task G). The subsystem

models used modifications from the component analyses, which kept the analysis solution times

reasonable while maintaining required nonlinear features and failure modes. These analyses were crucial

for determining critical structural behaviors of the towers, including floor sagging under thermal loading,

the resulting pull-in forces, and the inward bowing of the exterior walls. The subsystem analyses used the

reference models (Task A), material properties (Task B), SFRM properties (Task C), and results and

simplifications from component analyses (Task I). The major subsystem analyses were verified using

photographs and videos (Task F). Details of these analyses are described in Chapter 7 of this report.

1.2.10 Task J - Global Structural Analyses

Conduct a separate global analysis for each tower: The purpose of these analyses was to determine the

relative roles of impact damage and fires with respect to structural stability and sequential failures of

components and subsystems and to determine the probable collapse initiation sequence. Results of the

NIST NCSTAR 1-6, WTC Investigation 7
Chapter 1

major subsystem analyses were incorporated into the global models, simplifying the modeling approach

and/or level of detail where possible, while retaining sufficient detail for the nonlinear structural responses, including creep and buckling effects on columns. Each global model was first evaluated for

stability under gravity loads with structural impact damage modeled by removing severed core and

exterior columns, failed spandrels, and damaged floor areas. Temperature time-histories based upon the

fire dynamics and thermal analyses were applied in 10 min intervals and linearly ramped to the next temperature state. Pull-in forces from sagging floors were also applied during the appropriate 10 min

intervals. The global analysis results provided a sequence of component and subsystem failures that led to the onset of global instability and collapse initiation. The global analyses used the output from various

tasks (A, B, C, E, G, and I) and were verified using photos and the timeline (Task F). The global tower

analyses are described in Chapter 8 of this report.

The question of how the WTC towers would have responded to the same fires without the aircraft impact

damage was considered to determine the general vulnerability of the towers to fire-initiated collapse. The

structural response of the WTC towers to large fires without impact damage considered if collapse was

possible, or under what conditions collapse may have occurred, without the aircraft impact damage. The

analyses of the major subsystems (Task I) and each global analysis (Task J) provided sufficient data for addressing this issue. This analysis considered the role of fire in the towers with respect to structural

stability, sequential failures of components and subsystems, and collapse initiation for the towers without

impact damage. This analysis is presented in Chapter 8 of this report.

1.2.11 Task K - Probable Collapse Sequence

Determine the probable collapse sequence for each tower: The following steps were taken:

• Identify key observed events,

• Refine collapse hypotheses,

• Conduct sensitivity studies to identify influential parameters,

• Conduct analyses with these parameters, and

• Evaluate the collapse hypotheses and analysis results against key observables.

Sensitivity analyses were performed to determine most influential parameters that affect the response of

components, subsystems, and connections for the aircraft impact, fire dynamics, and thermal analyses.

Three sets of values for the parameters most influential to the aircraft damage and the progress of the fires were determined from the sensitivity studies. The three sets of parameters represented a range of severity

levels, including a base case, a more severe case, and a less severe case. Three aircraft impact (Task E)

and fire dynamics (Task F) analyses were performed for correlated sets of expected aircraft and fire parameters that provided different levels of damage. The analysis results were compared to the key

observables. Thermal and structural subsystem analyses (Task I) were conducted for the selected cases

that reasonably matched observed impact damage and fire progression. The cases that reasonably

matched the evidence were identified for global structural response analyses (Task J). The probable

collapse sequence is presented in Chapter 9 of this report.

8 NISTNCSTAR 1-6, WTC Investigation
Introduction

Task D was conducted in collaboration with experts from Underwriter's Laboratories (UL), under

contract to NIST. Further details are provided in NIST NCSTAR 1-6B.

Tasks H, I and J were conducted in collaboration with experts from Simpson Gumpertz & Heger Inc. (SGH). under contract to NIST. Further details are provided in NIST NCSTAR 1-6C and NIST

NCSTAR 1-6D.

1.3 CHALLENGES

Estimating the structural response of the WTC towers to impact damage and the ensuing fires presented a number of significant challenges:

• The towers had steel and concrete materials that exhibited nonlinear, temperature-dependent

behavior. The structural behavior and failure mechanisms required for the analyses included

mechanical properties (stress-strain) at room temperature and at elevated temperatures,

thermal expansion, plasticity, creep, large deformations, and plastic buckling. The

constitutive relationships for the materials included in the models were based on tests conducted by NIST on steels recovered from the collapse site, certified mill test reports found

in historical project records, and data available in the technical literature.

• The WTC towers were large, complex structural systems. To include all of the structural components and connections and their associated behavior and failure mechanisms using

refined finite element meshes would have been prohibitive. As a result, increasingly coarser meshes were used in the subsystem and global analyses. The models, thus, used a reduced

number of elements while still capturing the nonlinear, complex behavior of the tower

components. A number of component and connection analyses were conducted separately to develop a basis for the reduced models used in the global analyses that captured essential

temperature-dependent behavior and failure mechanisms.

• This investigation required analyses of the structural response of components and subsystems

of the WTC the towers, such as temperature-dependent properties, creep and post-buckling

strength of columns, that required software tools not typically employed in structural analysis or design. This study necessitated the use of sophisticated analysis methodologies at the

limits of structural engineering experience and training.

• The thermal loadings (temperature histories) used in the analyses of the various components

and subsystems, were derived from thermal analyses which, in turn, were derived from fire dynamics simulations. The mapping of the output from the fire dynamics simulations to the

thermal models, and the mapping of the temperatures derived from the thermal analyses to the structural models, were complex and challenging tasks due to the vastly different

dimensional scales, time increments, element types, and software used in the various

analyses.

• For the subsystem and global models, with thousands of degrees of freedom, numerical

convergence of the structural analyses that encountered local failures (such as connection

failure), large deflections, plastic (inelastic) buckling, creep effects, etc., presented many

problems that resulted in prematurely hahing the simulation. Overcoming the convergence

NIST NCSTAR 1-6. WTC Investigation 9
Chapter 1

issues and successive failures of thousands of elements, required considerable effort and

innovative approaches. Some convergence problems were solved through the use of dynamic

analysis with appropriate damping, and in some instances explicit dynamics formulation

solvers were required to capture, for example, concrete crushing.

• The structural response analyses were subject to uncertainties in the input parameters, such as the extent of impact damage to the structure, the temperature histories of structural components (based upon the post-impact insulation condition and the fire growth and spread),

and material properties at elevated temperatures. The aircraft impact damage and component

temperature histories provided a range of inputs for the subsystem and global analyses that captured the uncertainty in these inputs. With the uncertainty of the aircraft impact and fires captured in the input data files, the primary parameters required for the structural response

analyses were related to structural behavior and failure mechanisms. The uncertainty in the

temperature-dependent material properties increased with increasing temperature. However,

the effect of increasing temperature on the structural behavior and failure mechanisms

influenced the results more than variability of the material properties.

• Testing of structural components under static or dynamic (time-varying) loadings is challenging at best. Testing of loaded structural components under fire conditions,

particularly at the scale that it was done, was of even greater difficulty and pushed the limits

of fire testing capabilities in the United States.

1.4 REPORT ORGANIZATION

Chapter 2 describes the as-built properties of the passive fire protection systems used for the structural components of the towers. The two types of passive fire protection systems used in the WTC towers were SFRM products and gypsum enclosures and partitions. The tests and data used to determine the thermal

and mechanical properties of these systems as a function of temperature are presented. Also presented is the average thickness of the SFRM along with the variation in the thickness.

Chapter 3 describes the fire resistance tests of the composite floor truss components and subsystem

under standard fire conditions. A series of four tests was conducted to establish the baseline performance

of the WTC floor system under thermal loading as it was originally designed, differentiate factors that most influenced the response of the floors to fires, and study the procedures and practices used to accept an innovative structural and fire protection system.

Chapter 4 describes the component and detailed subsystem analyses. The chapter outlines the structural material properties and failure criteria used for the concrete, steel, welds, and bolts used in the WTC

towers. The series of analyses conducted included detailed models of knuckle and truss seat connections,

a single truss section of the floor, and a model of an entire floor subsystem. The analyses also included a nine-story by nine-column exterior wall subsystem along with its associated components, such as the

bolted connection between exterior columns and between spandrels, a single exterior column, and a single

exterior wall panel with three columns and three spandrels. Each analysis description includes details of

the model, applied loads, and structural results. Modifications to models to reduce the model size and

detail while retaining essential behaviors are described, and comparisons are made of the response of the

modified model relative to the original detailed model.

10 NISTNCSTAR 1-6, WTC Investigation
Introduction

Chapter 5 summarizes the results of the analysis of aircraft impact damage to the WTC towers. The

chapter describes how the structural damage due to impact was imposed on the subsystem and global

models used in this smdy and outlines the methodology used to estimate the damage to insulation as a

result of aircraft impact.

Chapter 6 describes the observations and timeline of structural events. The observations and timeline are based primarily on photographic and videographic records. Key observations were used to help validate

the probable collapse sequence for each tower.

Chapter 7 describes the analyses conducted for three major tower subsystems to aircraft impact damage

and fire. These analyses included full floors, core columns, and exterior walls of WTC 1 and WTC 2 in the impact and fire zones. These analyses were conducted prior to the global analysis for each tower. Each analysis description includes details of the model, applied loads, and structural results.

Chapter 8 describes the global structural response of each WTC tower to aircraft impact damage and fire conditions. The model for each tower is described, including aircraft impact damage and temperaturehistories for the observed fires. Modeling simplifications based on the previous analyses are described,

and the resuhs of the analyses are presented. The chapter also evaluates the structural response of the

WTC towers to fire conditions without aircraft impact damage. A separate global analysis was not conducted for this analysis. Instead, the results from the analyses conducted in previous chapters are used

as the basis for evaluating this hypothetical condition.

Chapter 9 presents the probable collapse sequence for each tower, based on the analysis results presented

in Chapter 8 and the key observations presented in Chapter 6.

Chapter 10 presents the findings of the study.

NISTNCSTAR 1-6. WTC Investigation 11
Chapter 1

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12 NISTNCSTAR 1-6, WTC Investigation
Chapter 2

Passive Fire Protection

Many structural materials are adversely affected by high temperatures resulting from an uncontrolled

building fire where compartment temperatures can reach 1,100 °C (2,000 T). Generally, some means of

protecting, or insulating, the structural components is required to provide an acceptable level of

performance in fire. Steel, for example, loses both its strength and stiffness at the elevated temperatures

associated with building fires, and an insulating barrier is required to slow or prevent damage to structural

steel components.

The structural steel in the World Trade Center (WTC) towers was protected with sprayed fire-resistive material (SFRM) or rigid fire-rated gypsum panels. SFRMs are a combination of fibrous material and

cementitious binder that, when mixed with water, can be spray-applied to the steel (Gewain et al. 2003).

With time, the cementitious materials harden and the excess water evaporates, and when dry, SFRMs

provide an insulation barrier to limit excessive temperature rise in the steel during a fire. Similarly, fire- rated g>'psum wallboard was used to enclose some structural steel core columns to provide the required

level of fire protection.

The structural analysis of the WTC towers focused on the response of the two towers damaged by the

aircraft impact and exposed to the subsequent fires. To reduce the uncertainties in the calculated

temperature histories of various structural elements, the thermal properties and condition of the passive

fire protection as it existed on September 1 1, 2001, was estimated as accurately as possible. In addition,

reasonable estimates of the extent of SFRM dislodged by the aircraft impact and flying debris were made

(see Chapter 5).

NIST NCSTAR 1-6A reports on many aspects of the passive fire protection in the WTC towers,

beginning with an overview of U. S. building regulations that are intended to provide structural fire resistance. The report continues with a chronicle of the procedures and practices used in the selection and

application of the SFRMs used in the construction of the WTC towers. The variability of SFRM

application on the uncertainty in estimating the steel temperatures is covered. Finally, the thermal and

mechanical properties of the SFRM materials are reported. This chapter summarizes the salient material

covered in NIST NCSTAR 1 -6A.

2.1 FIRE RESISTANCE OF STRUCTURAL ELEMENTS

Building codes require that elements that support loads are to be protected to achieve a specified fire resistance rating", expressed in hours. The fire rating of structural materials and assemblies is generally

determined through testing, and in the United States, such testing is frequently conducted in accordance

with the ASTM International standard, ASTM E 119, "Standard Test Methods for Fire Tests of Building

Construction and Materials" (ASTM 2000).

^ The term "fire resistance rating" (or simply "fire rating") is variously called in the ASTM E 119 Standard, "period of

resistance," "performance," "exposure" or classification."

NIST NCSTAR 1-6, WTC Investigation 13
Chapter 2

Building codes generally require the highest fire resistance rating for columns and other elements

supporting multiple floors, and a somewhat lower resistance rating for columns and other elements

supporting single floors, and for floors. The required fire resistance ratings have been reduced in recent years when fire sprinklers are installed in high-rise and other commercial buildings. In the past, high-rise

buildings generally required a 4 h rating for columns; this has been reduced to 3 h in recent model codes,

and can be as low as 2 h in current model codes, based on the additional mandatory requirement for

sprinklers. Some codes allow a reduction in fire-resistance rating for high-rise buildings that have been

retrofitted with sprinklers.

2.2 HISTORICAL REVIEW RELATED TO PASSIVE FIRE PROTECTION

2.2.1 Building Code Requirements for the Design of the WTC Towers

As an interstate compact. The Port Authority of New York and New Jersey (Port Authority or PANYNJ)

was not required to comply with the New York City Building Code (NYCBC 1968), or any other building

code, for the design and construction of the WTC towers. The Port Authority, however, made explicit statements that it would comply with the New York City Building Code. In a letter dated May 15, 1963'*,

the Port Authority instructed its consulting engineers and architects to comply with the New York City

Building Code. In areas where the Code was not explicit or where technological advances made portions

of the Code obsolete, it directed that design could be based on acceptable engineering practice. At that time, the 1938 edition of the New York City Building Code was in effect, and a revised code was being

drafted. In a directive dated September 29, 1965^ the Port Authority instructed its consultants to revise

the WTC design plans to comply with the second and third drafts of the Code revision. The revised New

York City Building Code became effective in December 1968.

2.2.2 New York City Building Code Requirements

Application of the 1968 New York City Building Code (NYCBC) provisions affected the assigned

building classification and, thus, the required fire rating of the WTC towers and their structural members.

The WTC towers were classified as Occupancy Group E—Business. The 1968 NYCBC identified two

construction groups: Noncombustible Construction (Group 1) and Combustible Construction (Group 2). The WTC towers were classified as Construction Group 1 because their walls, exit ways, shafts, structural members, floors, and roofs were constructed of noncombustible materials. At the time of design and

construction, the towers were not sprinklered.

The 1968 New York City Building Code defined five Classes within Construction Group 1. For Business

occupancy, each Class required a fire endurance rating as follows:

• Class lA: 4-hour protected

• Class IB: 3-hour protected

Letter dated May 15, 1963 from Malcolm P. Levy (Chief, Planning Division, World Trade Department) to Minoru Yamasaki

(Minoru Yamasaki & Associates) - see NIST NCSTAR 1-6A, Appendix Figure A-1.

^ Letter dated September 29, 1965 from Malcolm P. Levy (Chief, Planning Division, World Trade Department) to Minoru

Yamasaki (Minoru Yamasaki & Associates) - see NIST NCSTAR 1-6A, Appendix Figure A-2.

14 NIST NCSTAR 1-6, WTC Investigation
Passive Fire Protection

• Class IC: 2-hour protected

• Class ID: 1 -hour protected

• Class IE: unprotected

Construction Classes 1 A and IB permitted buildings of unlimited height. Thus, the WTC towers could

have been designed to meet either Class 1 A or Class IB requirements.

2.2.3 Classification of WTC Towers

It was the practice at the time, and continues to be the practice, for the architect to establish the building

classification, fire rating of members and systems, and thermal protection requirements. The review of

documents uncovered during the investigation indicated a discrepancy in the classification, and therefore

the fire ratings, to be used in the design of the towers. Documents issued in the early stages of the design

appear to indicate that the towers were classified as Class lA^. With the directive in 1965 to comply with

the 1968 New York City Building Code, it appears that the towers were classified ultimately as Class IB''.

According to the 1968 New York City Building Code, construction classification IB provided, in part, the

following fire protection requirements:

• Columns, girders, trusses, other than roof trusses, and framing supporting more than one floor

shall have 3-hour fire endurance;

• Columns, girders, trusses, other than roof trusses, and framing supporting one floor shall have

2-hour fire endurance

• Floor construction including beams shall have 2-hour fire endurance.

• Enclosure of vertical shafts, exits, passage-ways, and hoistways shall have 2-hour fire endurance;

and

• Roof construction including beams, trusses, and framing including arches, domes, shells, cable

supported roofs, and roof decks (for buildings over one story in height) shall have 2-hour fire endurance.

Thus, the columns were required to have a 3-hour fire endurance rating and the floor system was required

to a ha\ e a 2-hour rating when tested in accordance with ASTM E 1 19 (ASTM 1961).

* Letter dated October 30, 1969 from Robert J. Linn (Manager, Project Planning, The World Trade Center) to Mr. Louis

Di Bono. Mario & Di Bono Plastering Co., Inc. - see NIST NCSTAR 1-6A, Appendix Figure A-23.

^ Memorandum dated January 15, 1987 from Lester S. Field (Chief Structural Engineer, World Trade Department) to Robert J. Linn (Deputy Director for Physical Facilities, World Trade Department) - see NIST NCSTAR 1 -6A, Appendix Figure A-7.

NIST NCSTAR 1-6, WTC Investigation 15
Chapter 2

2.2 A Response to Local Law 5/1973

In 1973, New York City Local Law No. 5 amended the New York City Building Code (effective

January 18, 1973). Local Law No. 5 required, in part, the retrofit of existing unsprinklered office

buildings 100 ft or higher. The New York City Department of Buildings pennitted either:

• Subdividing the floor area into compartments of specified square footage by fire separations (1-h or 2-h fire rated depending on the size of the compartment), or

• Providing sprinkler protection.

A code compliance evaluation of the towers, conducted in 1997, indicated that that all tenant floors in the

two towers had been retrofitted with sprinklers (sprinklered) with the exception of four floors in WTC 1. In a 1999 update by the Port Authority, it was noted that all tenant floors had been sprinklered, and work

was underway to complete sprinklering of the sky lobbies. In 2000, a property condition assessment

report** stated that the WTC towers were classified as "Class IB - noncombustible, fire-protected,

retrofitted with sprinklers in accordance with New York City Local Law 5/1973."

2.2.5 Selection of Fire-Resistive Materials

Classification of a building leads to its overall fire endurance rating and ratings of the various structural components. The New York City Building Code, however, does not prescribe how the required fire resistance rating is to be achieved. The Port Authority chose to protect main structural components such

as columns, spandrel beams, and floor trusses with sprayed fire-resistive material. This thermal

protection technique was an established method for protecting columns, beams, and walls. In the 1960s,

however, composite steel truss floor systems were usually protected using "lath and plaster" enclosures or

fire-rated ceiling tiles. Figure 2-1 shows a mock-up of the steel truss system that supported the concrete

floors in the World Trade Center towers, illustrating the thin steel rods that form the truss diagonals.

Property Condition Assessment of World Trade Center Portfolio, prepared for the Port Authority by Merritt & Harris, Inc., December 2000 - see NIST NCSTAR 1-6A, Appendix Figure A-41.

16 NISTNCSTAR 1-6, WTC Investigation
Passive Fire Protection

Source: (Photograph from about 1967) provided by Laclede Steel Co. Figure 2-1. Mock up of floor truss system.

Since application of sprayed fire-resistive material to floor trusses was an innovative fire protection

method, the Port Authority arranged for demonstrations to establish its feasibility for the World Trade

Center (see Fig. 2-2). The demonstrations were successful, and in November 1968, the Port Authority

awarded the contract for "spray fireproofing" of the interior portions (floor system and core) of the WTC

towers to Mario & Di Bono Plastering Co., Inc. The fire protection of the exterior colurrms was included

in the contract for the exterior aluminum cladding.

NISTNCSTAR 1-6, WTC Investigation 17
Chapter 2

Source: Photograph provided by Laclede Steel Co. Figure 2-2. Demonstration of application of Monokote sprayed fire-resistive

material to floor trusses.

Several materials were considered for the sprayed thermal insulation. The exterior columns required

insulation not only for fire protection but also to control column temperatures under service conditions.

Alcoa recommended for the exterior columns the use of a sprayed material produced by U.S. Mineral

Products, Co. known as BLAZE-SHIELD D. The same material was also selected for the floor trusses and core beams and columns. This product, however, contained asbestos fibers. On April 13, 1970, New

York City issued restrictions on the application of sprayed thermal insulation containing asbestos. The

use of BLAZE-SHIELD D was discontinued in 1970 at the 38th floor"^ of WTC 1. The asbestoscontaining material was subsequently encapsulated with a sprayed material that provided a hard coating.

A green dye was added to the encapsulating material so that the asbestos-containing SFRM could be

identified. Thermal protection of the remaining floors of WTC 1 and all of WTC 2 was carried out using

BLAZE-SHIELD DC/F, a product that contained mineral wool (glassy fibers) in place of the crystalline

asbestos fibers. On the basis of tests, it was reported that the thermal properties of BLAZE-SHIELD

DC/F were equal to or "slightly better" than those of BLAZE-SHIELD D'^

2.2.6 Specified Thickness of Fire-Resistive Material

The thickness of fire-resistive material necessary to achieve the required fire endurance was being

assessed in 1965, more than three years before the award of the thermal protection contract. At that time,

Various floor levels at which the asbestos-containing SFRM was reported to have been discontinued have been found in the documents reviewed for the investigation. Floor 38 is reported here but the exact floor is of no consequence in the investigation into the response of the towers to impact and fire. Letter dated April 24, 1970 from S.W. Bell (Assistant Engineer, Fire Protection Department, Underwriters" Laboratories, Inc.) to R. Monti (Construction Manager, World Trade Center, Port of New York Authority) - see NIST NCSTAR 1 -6A, Appendix

Figure A-29.

18 NIST NCSTAR 1-6, WTC Investigation
Passive Fire Protection

Emory Roth & Sons (ER&S), the Architect-of-Record, recommended 1 in. of thermal protection for the

top and bottom chords of the floor trusses and 2 in. for other members of the trusses". WTC project

specifications for sprayed fire-resistive material did not provide required material thickness or hourly

ratings. In October 1969, the manager of project planning of WTC provided the following instructions to the contractor applying the sprayed insulation'":

"...Tower 'A' columns that are less than 14WF228 will require 2 3/16"

thick of 'Cafco Glaze [sic]-Shield 'Type D" spray-on fireproofing. All

Tower columns equal to or greater than 14WF228 will require 1 3/16" of

fireproofing...

All Tower beams, spandrels and bar joists requiring spray-on

fireproofing are to have a 1/2" covering of 'Cafco.'

The above requirements must be adhered to in order to maintain the

Class 1-A Fire Rating of the New York City Building Code"

NIST's review of available documents has not uncovered the reasons for selecting BLAZE-SHIELD fire- resistive material or the technical basis for specifying Vz in. thickness of thermal insulation for the floor

trusses. The last sentence in the above excerpt indicates that, in October 1969, the towers were

considered a Class 1 A construction.

In February 1975, a fire broke out in WTC 1, spreading from the 9th to the 19th floor. After the fire, the Port Authority contracted Skilling, Helle, Christiansen, Robertson (SHCR), the Structural Engineer of

Record for the design of the WTC towers, to assess the resulting structural damage and to report, in general, on the fire resistivity of the floor system. In its report to the Port Authority dated April 1, 1975'"\

SHCR stated,

"The fire of February, while reported in the press to have been very hot,

did not damage a single primary, fireproofed element. Some top chord

members (not needed for structural integrity [sic], some bridging

members (used to reduce floor tremor and the like) and some deck

support angles (used only as construction devices) were buckled an the

fire - all were unfireproofed steel."

The 1975 post-fire report by SCHR stated further that thermal protection of the top chords of the floor

trusses was not necessary, except for the comers of the buildings where the floor acted as a two-way

system in bending. Additionally, it was stated that protection of the bridging trusses was not required

because the bridging trusses were "not required as a part of the structural system." This information was

used to guide the selection of the passive protection for the fire resistance tests conducted under Project 6

and discussed in Chapter 3.

" Letter dated December 23, 1965 from Julian Roth (ER&S) to Malcolm P. Levy (The Port of New York Authority) - see NIST

NCSTAR 1-6A, Appendix Figure A-21.

Letter dated October 30, 1969 from Robert J. Linn (Manager, Project Planning, The World Trade Center) to Mr. Louis DiBono

(Mario & Di Bono Plastering Co., Inc.) - see NIST NCSTAR 1-6A, Appendix Figure A-23. Report on WTC Fire dated April 1, 1975 from Skillmg Helle Christiansen Robertson (SHCR) to the Port Authority - see NIST

NCSTAR 1-6A, Appendix Figure A-30.

NIST NCSTAR 1-6. WTC Investigation 19
Chapter 2

2.2.7 Upgrading SFRM on Floor Trusses

In 1995, the Port Authority performed a study'"* to estabhsh requirements for retrofit of SFRM to the floor

trusses during major new construction or renovations when tenants vacated spaces in the towers. The

study estimated the thermal protection requirements based on "the fireproofing requirements" for Design

No. G805 contained in the Fire Resistance Directory published by Underwriters Laboratories

(Underwriters Laboratories 2002). The study concluded that "a two hour fire rating for the steel floor

joist trusses can be achieved by applying a 1 Vi inch thickness of spray-on mineral fiber fire protection

material directly to the steel truss chords and webs." In the years between 1995 and 2001, thermal

protection was upgraded on a number of floors and some of these were affected by the fires on

September 1 1, 2001. In WTC 1, floors 92 through 100 and 102 had been upgraded, and in WTC 2, floors 77, 78, 88, 89, 92, 96 and 97 had been upgraded.

In 1999, the Port Authority established "guidelines regarding fireproofing repairs, replacement, and

upgrades" for the towers'". The guidelines for tenant spaces may be summarized as follows:

• For full floors undergoing new construction or renovation, the floor trusses should be protected

with 1 Yi in. of sprayed mineral fiber fire-resistive material. Retrofit of thermal protection

requires removal of existing material and controlled inspection.

• For "tenant spaces less than a full floor undergoing either new construction or renovation," the

floor trusses "need only meet the original construction standard. Fireproofing shall be inspected

and patched as required to the greater of 3/4 in. or to match existing" if it has already been

upgraded to 1 Vi in.

In July 2000, Buro Happold, an engineering consultant, commissioned by the Port Authority to conduct a fire-engineering assessment of the insulation of the floor trusses, issued a report on the requirements of

the fire resistance of the floor system of the towers'*^. This report stated that BLAZE-SHIELD DC/F was

used on the majority of the floor trusses. Based on calculations and a risk assessment, the consultant

concluded that:

• "The structural design has sufficient inherent fire performance to ensure that the fire condition is never the critical condition with respect to loading allowances.

• A single coat application is possible.

• Significant savings are possible.

White paper titled "Fireproofing Requirements for World Trade Center Tenant Floor Joist Construction that Requires

Installation Due to Asbestos Removal or Local Removal to Facilitate Construction" transmitted by way of memorandum from

Joseph Englot (Chief Structural Engineer, Port Authority) to Peter Sweeney (Engineering Program Manager, Port Authority) on August 18, 1995 - see NISTNCSTAR 1-6A, Appendix Figure A-34. Memorandum dated March 24, 1999 from Alan L. Reiss (Director, World Trade Department) to John Castaldo and Kent Piatt (Port Authority) - see NIST NCSTAR l-6A,Appendix Fig. A-36. World Trade Center: Fire Engineering of Steelwork - Phase 1 Report, Buro Happold Consulting Engineers PC, February 2000 - see NIST NCSTAR Appendix Figure A-40.

20 NISTNCSTAR 1-6, WTC Investigation
Passive Fire Protection

• The target reduction of fiber content and increased long term durability can be achieved.

• Alternative materials should be considered."

As quoted, the report states that significant savings were possible by reducing the fiber content and

considering alternative materials. The report suggested that the thickness of the SFRM could be reduced

to Vi in. if the material properties at ambient temperamre are applicable at higher temperature. The report

recognized the lack of available temperature-dependent material data for BLAZE-SHIELD DC/F. Thus,

considering the uncertainties in the material properties and having the understanding of material

degradation with temperature and time, it was recommended that 1.3 in. of fire-resistive material be used

for the floor trusses.

Later, in December 2000, the final draft of a report on Property Condition Assessment of World Trade

Center Portfolio' stated that, based on existing conditions "The rating of the structural fireproofmg in the

Towers and subgrade has been judged to be an adequate 1-hour rating considering the fact that all Tower

floors are now sprinklered." The report also noted the ongoing Port Authority program to upgrade the

fire-resistive material thickness to 1 Yi in. in order to achieve a 2-hour fire rating.

2.2.8 Need for Fire Resistance Tests

The fire protection of a truss-supported floor system by directly applying sprayed fire-resistive material to the trusses was innovative at the time the WTC towers were designed and constructed. While the benefits

of conducting fire endurance tests were realized by individuals involved in the 1967 demonstrations of the

application of SFRM, apparently no tests were conducted on the floor system used in the WTC towers. The Architect-of-Record, in a letter to the Port Authority'** addressing issues that . .might not conform to the New York City Building Code...", dated July 25, 1966, stated,

"Obviously, with so many penetrations of the floor system the fire rating

of the floor construction is of an indeterminate value unless tested. It is doubtful if it will meet a 3-hour test."

In the 1975 post-fire report to the Port Authority'^, the Structural Engineer-of-Record stated,

"These special floor assemblies would best be fire tested—since actual

testing is the only known, reliable method known [sic] to assure compliance with fire testing requirements."

Communication from the Port Authority in 2003'" confirms that there is no record of fire endurance

testing of assemblies representing the thermally protected floor system.

Property Condition Assessment of World Trade Center Portfolio, prepared for the Port Authority by Merritt & Harris, Inc., December 2000 - see NISI NCSTAR 1 -6A, Appendix Figure A-4 1

Letter dated July 25, 1966 from Harry J. Harman (ER&S) to Malcolm P. Levy (Port of New York Authority) - See NISI

NCSTAR 1-6A, Appendix Figure A-26. " Report on WTC Fire dated April 1 , 1975 from Skilling Helle Christiansen Robertson (SHCR) to the Port Authority - see NIST

NCSTAR 1-6A, Appendix Figure A-30 - see NIST NCSTAR 1-6A, Figure A-43.

^° See NIST NCSTAR 1 -6A, Appendix Figure A-3 1

NIST NCSTAR 1-6, WTCInvestigation 21
Chapter 2

To address some of these open issues, standard fire resistance tests of the floor system used in the WTC

towers were conducted as part of this investigation. Results of four tests and the fire resistance ratings

determined from these tests are presented in Chapter 3 of this report and in NIST NCSTAR 1-6B.

2.2.9 Maintenance of SFRM in Elevator Shafts

Throughout the hfe of the WTC towers, the structural members that required the largest amount of

inspection and maintenance within the core were the exposed columns and beams within the elevator

shafts. Except for the floors, these columns and beams were the only accessible fire-protected elements in the buildings. Adhesion failures were common, likely because of the exposed conditions of the columns

and the inherently low bond strength of the SFRM.

Inspections of the shafts and accessible columns were reported as early as 1971. Problems were noted in the form of fallen insulation or with the over-spray material used to provide a harder surface. In 1993, the Port Authority commissioned Leslie E. Robertson Associates (LERA) to carry out a continuing Structural

Integrity Inspection Program to appraise the condition of the accessible columns located in the core of the

towers (see NIST NCSTAR 1-lC). The columns were inspected visually for signs of rusting, cracking,

bowing, and loss of thermal insulation. During the first inspection, carried out in 1993, particular shafts were chosen based on the quantity and types of accessible columns, and the convenience to the Port

Authority. Subsequent inspections involved sampling of the structural components and assemblies,

which were more important to the structural integrity of the towers, and at locations with a relatively

higher potential for defects and problems. The Structural Integrity Inspection Report"' stated that the

accessible columns in selected elevator shafts in WTC 1 and WTC 2 were "generally in good condition, no structural deficiencies such as cracking or bowing were found, the most common irregularities

observed were missing fireproofing and light surface rusting of the exposed steel." Based on the

inspections, LERA recommended "that remedial action to be taken where spray fireproofing is damaged,

deteriorated or missing and where there is corrosion of the column base due to water leaks at elevator

pits."

2.3 AS-APPLIED THICKNESS OF SFRM

2.3.1 1994 Measurements from WTC 1 Floors 23 and 24

In its search of documents, NIST found no information related to measurements of the thickness of

thermal insulation taken during construction. Reviewed documents, however, indicate that thickness

appears to have been checked during construction. Recorded information on the in-place condition of the

sprayed thermal insulation for the floor system first appeared in 1990 in the form of "Sample Area Data

Sheets," which provided qualitative comments on the state of the in-place SFRM. Information regarding

quantitative inspection of existing fire-resistive material appeared in documentation from 1 994. That

year, the Port Authority perfomied a series of thickness measurements of the existing SFRM on

floors 23 and 24 of WTC 1 (see NIST NCSTAR Appendix A Figure A-58). Six measurements were

taken from "both flanges and web" of each of 16 randomly chosen trusses on each floor (see Table 2-1).

Measured average thickness varied between 0.52 in. and 1.17 in. For the 32 measurements (16 on each

floor), the overall average was 0.74 in., and the standard deviation of these averages was 0.16 in. Four of

Structural Integrity Inspection Report dated 14 April 1995 by LERA - see NIST NCSTAR 1-6A, Appendix Figure A-57.

22 NIST NCSTAR 1-6, WTC Investigation
Passive Fire Protection

the 32 floor trusses had average thicknesses between 0.52 in. and 0.56 in. These measurements suggest

that the minimum average thickness exceeded Vi in. Analysis of the reported mean thicknesses data

indicated that a lognormal distribution gave a better representation of the distribution than did a nornial

distribution.

Table 2-1. Average SFRM thickness from six measurements taken in 1994

on each of 16 random floor trusses on floors 23 and 24 of WTC 1. SFRM Thickness (in.)

r lOOr r lour m

\J.VV 0 lf\

KJ.JJ u.ou

u. / u

\J. 1 D 0 79

U.oo n 8Q n 80

0 83 0.68

1.17 0.65

0.88 0.67

0.71 0.77

0.82 0.96

0.52 0.66

0.69 0.65

0.52 1.11

0.64 0.95

0.52 0.56

Source: Data provided by The Port Authority of New York and New Jersey.

2.3.2 Analysis of Photographs

Additional SFRM thickness data were developed by evaluating photographs of floor trusses taken during

inspections. Two groups of photographs were used. The first group included images of floor trusses from

WTC 1 (floors 22, 23, and 27). These photographs were taken in the mid-1990s and illustrated conditions

prior to the upgrade carried out by the Port Authority. Thus, SFRM thickness on the photographed

trusses would be expected to be at least Vi in. The second group of photographs, taken in 1998, illustrated conditions after initiation of the upgrade program that began in 1995. These photographs were of trusses for floor 3 1 and below in WTC 1 . Selection of the photographs to be used to estimate thickness of SFRM

was based on clarity of SFRM edges and the presence of a feature of known dimension to provide a reference measurement. Figure 2-3 shows one such photograph used for the estimation of SFRM

thickness. "Reference" points to a known dimension of the steel member which connects the damper to the bottom chord of the truss.

NISTNCSTAR 1-6, WTC Investigation 23
Chapter 2

For the floors that had not been upgraded, it was observed that the estimated thickness of SFRM on the

webs of the main tmsses tended to be greater than that on either the diagonal struts or on the webs of the

bridging trusses. Hence, estimates of SFRM thickness for non-upgraded floors were divided into three

groups: webs of main trusses, webs of bridging trusses, and diagonal struts at the exterior wall end of the

truss.

It was not possible to estimate the thickness of the SFRM on any truss element except the round bars

Consequently, for the upgraded floors in WTC 1 that were included in the second group of photographs,

only estimates of the thickness on the webs of the main trusses were made. The average, standard

deviation, and coefficient of variation were computed for the total number of measurements in each of

these groups. The results are summarized as follows:

• Main trusses before upgrade (85 measurements): Average thickness 0.6 in., standard deviation = 0.3 in., and coefficient of variation = 0.5.

• Bridging trusses before upgrade (52 measurements): Average thickness 0.4 in., standard deviation ^ 0.25 in., and coefficient of variation = 0.6.

• Diagonal struts before upgrade (26 measurements): Average thickness 0.4 in., standard deviation = 0.2 in., and coefficient of variation = 0.5.

• Main trusses after upgrade (52 measurements): Average thickness 1.7 in., standard deviation = 0.4 in., and coefficient of variation = 0.2.

24 NISTNCSTAR 1-6, WTC Investigation
Passive Fire Protection

2.3.3 Port Authority Data on Upgraded SFRM on Trusses

In the 1990s, the thermal protection for some floor trusses was upgraded to a specified thickness of

1 Vz in. as tenants vacated their spaces. According to the Port Authority"", 18 floors of WTC 1 and

13 floors of WTC 2 were upgraded. The Port Authority also stated that: "The entire impact zone for Tower 1 (92-99) was upgraded with 1 Vi" spray-on fireproofmg. Only the 78'*^ floor was upgraded with

the 1 V2" spray-on fireproofmg within the impact zone in Tower 2 (78-84)." The Port Authority provided

Construction Audit Reports to the NIST Investigation that included the density, average thickness, and

strength characteristics of the upgraded SFRM (BLAZE-SHIELD II) as of 2000. In 2004, the Port Authority provided NIST reports of the individual measurements for many of the average thicknesses

recorded in the Construction Audit Reports. These individual measurements permitted analysis of the variation of thickness at a cross section of a truss member and the variation in average thickness from

truss to truss. A total of 18 data sets for WTC 1 (including floors 93, 95, 98, 99, and 100) and 14 data sets for WTC 2 (including floors 77, 78, 88, 89, and 92) were analyzed.

Data analysis indicated that the thickness measurements from the two towers represented similar

distributions, and so the data were combined. It was also found that the distribution of thickness values

could be approximated by a lognormal distribution.

The overall average thickness determined from the 256 individual measurements was found to be 2.5 in. with a standard deviation of 0.6 in. Thus, the average SFRM thickness on the upgraded upper floors appears to be greater than that estimated from photographs taken on the upgraded lower floors.

The overall standard deviation of 0.6 in. includes two contributions: (1) the variation of thickness at a

cross section (within-truss variability), and (2) the variation of average thickness between trusses (between-truss variability). From analysis of variance, it was found that the within-truss standard

deviation was 0.4 in., and the between-truss standard deviation was also 0.4 in. The within-truss standard

deviation of 0.4 in. is similar to the standard deviation of the estimated individual thicknesses obtained

from analysis of the photographs of upgraded main trusses.

2.4 EFFECT OF THE VARIABILITY OF SFRM THICKNESS ON THERMAL

RESPONSE

As would be expected, and as confirmed by analyses of available data, the thickness of thermal insulation can have high variability. The effects of thickness variation on thermal response of a member are not

well known. A sensitivity study using finite element analyses to simulate heat transfer was conducted to investigate the sensitivity of steel temperature rise to the variability in SFRM thickness.

2.4.1 Effects of Thickness Variability and Gaps in SFRM

A finite element model for thermal analysis was developed for a plate protected on both faces with SFRM

of variable thickness. A random number generator was used to assign a lognormally distributed random

thicknesses of insulation along the length of the plate, and the plate was subjected to a thermal flux representative of a 1,100 °C fire. A parametric study was conducted with average thickness of fire-

" Structural Integrity Inspection Report dated 14 April 1995 by LERA - see NIST NCSTAR 1-6A, Appendix Figure A-57.

NISTNCSTAR 1-6, WTC Investigation 25
Chapter 2

resistive material varying from 0 in. to 2 in. in increments of 1/4 in. and a standard deviation varying from

0 in. to 1 in. Steel temperatures at five locations in the plate were recorded at 30 min, 60 min, 90 min,

and 120 min of exposure to the thermal flux. For more details, refer to NIST NCSTAR 1-6A

The simulations showed that when the SFRM thickness is variable, the isothenns in the steel follow the

shape of the SFRM surface contour. Thus, the temperature history at any point in the steel depends on the

local thickness of the insulation. It was shown that an increase in thickness variability reduced the time to reach a critical temperature. Conversely, for a given time to reach a specific temperature, the required

average thickness of thermal insulation increased with increasing variability in thickness of SFRM.

In addition to the effect of variation in thickness, the effect of missing SFRM over a portion of a member

was studied. Figure 2-4 shows an example of a "gap" in fire-resistive material on a diagonal member of a bridging truss. As expected, thermal analysis results indicated that the bare steel where the insulation was

missing reached the gas temperature quickly, which led to a transmission of heat into the interior steel.

The combined effects of variation in insulation thickness and extent of missing material were examined

by a full factorial design study with the following factors:

• Average thickness of insulation varying from 0 in. to 2.0 in. in 1/4 in. increments;

• Standard deviation of insulation thickness of 0 in., 0.25 in., 0.5 in., 0.75 in., and 1.0 in.; and

• Length of missing insulation varying from 0 in. to 30 in., in 6 in. increments.

Figure 2-4. Example of "gap" in fire-resistive material on diagonal member

of a bridging floor truss.

26 NIST NCSTAR 1-6, WTC Investigation
Passive Fire Protection

The results were summarized by a series of temperature-time plots representing the response for different

combinations of the three factors (see NIST NCSTAR 1-6A). As expected, increasing the variability of

insulation thickness or gap length reduced the time to reach a given critical temperature. Because there was not sufficient information to determine the frequency of occurrence of these gaps or their typical

locations, gaps in insulation were not considered in the thermal modeling.

2.4.2 Thermally equivalent thickness of SFRM

A sensitivity study, reported in NIST NCSTAR 1-6A, indicated that increased variation in thickness

reduced the "effective thickness" of the SFRM. It would be impractical to attempt to account for the

variation in SFRM thickness in the thermal modeling of the WTC towers by introducing variable

thickness insulation material in the finite-element models. As an alternative, a "thermally equivalent

uniform thickness" was determined that would result in the same thermo-mechanical response of a member as the \ ariable thickness thermal protection. In the analyses, an insulated 1 in. diameter by 60 in. long steel bar was subjected to the heat flux arising from a 1,100 °C fire. The temperature history along

the length of the bar was computed and was used to calculate the length change of the unrestrained bar

under a tensile stress of 12,500 psi. The bar was assumed to be similar to the steel used in the WTC floor

trusses, and the temperature dependence of the coefficient of thermal expansion and the modulus of

elasticity was based on NIST measurements.

The average SFRM thickness and variability in thickness used in the models were based on the

measurements (summarized in NIST NCSTAR 1-6A) for the web bars of the main trusses with both the

original insulation and the upgraded insulation. The following values were investigated:

• Original conditions: Average thickness = 0.75 in., standard deviation = 0.3 in., lognormal

distribution.

• Upgraded conditions: Average thickness = 2.5 in., standard deviation = 0.6 in., lognonnal

distribution.

Three sets of random data were generated for each condition. When the randomly selected thicknesses of

each element were applied to the bar, abrupt changes in insulation thickness along the length of the bar

resulted. This "rough" surface texture was not representative of actual conditions. As an alternative,

five-point averaging was used to reduce the roughness of the insulation profile and produce a profile that was consistent with photographic evidence. Care was taken to ensure the "smoothed" profiles maintained

the required variability (i.e.. mean and standard deviation).

The calculated temperature histories of the bar elements were used to calculate the unrestrained length

changes of the bar due to thermal expansion and the applied stress. For comparison, elongations of the

bar with different uniform thicknesses of thermal insulation were calculated. The "thermally equivalent

thickness" was taken as the uniform thickness that resulted in approximately the same elongation of the

bar as produced with the variable thickness insulation. Figure 2-5 shows a plot of the thermo-mechanical

response of 1 in. diameter bar with both rough and smooth random thickness SFRM and thermally

equi\ alent uniform thickness SFRM.

NIST NCSTAR 1-6. WTC Investigation 27
Chapter 2

Time (min)

Figure 2-5. Thermo-mechanical response of 1 in. bar compared with

uniform thickness SFRM.

On the basis of these analyses, it was concluded that SFRM with a uniform thickness of 0.6 in. provides

thermally equivalent protection to an average thickness of 0.75 in. and a standard deviation of 0.3 in. (see

the solid green line in Fig. 2-5). Similarly, 2.2 in. of uniform thickness insulation is thermally equivalent

to an average SFRM thickness of 2.5 in. with a standard deviation of 0.6 in.

2.4.3 Thickness of SFRM Used for Thermal Analyses

Analyses of available data on SFRM thickness and thermal modeling revealed the following:

• From measurements of SFRM thickness, the average values exceeded the specified thickness.

• SFRM thickness was variable, and the distribution of thickness in the floor trusses was best

described by a lognormal distribution.

• The standard deviation of SFRM thickness on the trusses varied between about 0.3 in. and 0.6 in.

• The standard deviation of SFRM thickness on columns and beams from the core tended to be

lower, with a value of 0.2 in. obtained from the available data.

• No information was available on the SFRM thickness on the exterior columns and spandrel

beams.

• Variation in thickness reduces the effectiveness of SFRM, and the thermally equivalent uniform

thickness, based on thickness measurements, exceeded the specified thickness.

28 NISTNCSTAR 1-6, WTC Investigation
Passive Fire Protection

Based on the findings given above, the following unifomi thicknesses for the undamaged SFRM were

used in the thermal analyses to determine temperature histories of the WTC towers under various fire scenarios:

• Original SFRM thickness on all floor trusses (primary and bridging): 0.6 in.

• Upgraded SFRM thickness on all floor trusses (primary and bridging): 2.2 in.

• Thermal protection on other elements: the specified thickness.

The choice of specified thickness for those members lacking data is justified by the following offsetting

factors: (1) measured average thicknesses were found to exceed specified values, and (2) variation in thickness reduces the effectiveness of the SFRM.

2.5 SUMMARY OF SFRM THICKNESS

Establishing the specified SFRM material and thickness for the protection of the steel trusses, columns

and beams in the WTC towers required the review of documents, correspondence, photographs, and

videos that chronicled the decisions made in selection of the passive fire protection in the towers. Since

the towers were destroyed completely when they collapsed on September 11, 2001, establishing the in- place thicknesses and condition of the SFRM required analysis of available thickness measurement data

and interpretation of available photographs. Lastly, the determination of appropriate thickness of the

SRFM for use in thermal calculations required finite element thermal analyses to establish a thermally

equivalent uniform thickness. Table 2-2 summarizes the specified thickness, in-place (or as-applied)

thickness, and thermally equivalent thickness for the structural elements that were fire-protected using

SFRM.

Table 2-2. Summary of specified, in-place and thermally equivalent thickness of SFRM

Building Component Material

Thickness (in.)

Specified"* In-place

Thermally

Equivalent

FLOOR SYSTEM

Original

Main trusses BLAZE-SHIELD DC/F 1/2 0.75 0.6 Main truss diagonal Strut BLAZE-SHIELD DC/F 1/2 0.75'-' 0.6

Bridging trusses - One-Way BLAZE-SHIELD DC/F 0.38<'' (5)

Bridging trusses - Two-Way BLAZE-SHIELD DC/F 0.75<'» 0.6<^'

Metal deck (7) (8) (9) Upgraded

Main trusses BLAZE-SHIELD 11 1 1/2 2.5 2.2 Main truss diagonal Strut BLAZE-SHIELD II 1 1/2 2.5 2.2

Bridging trusses BLAZE-SHIELD II 1 1/2 2.5 2.2

Metal deck (7) (8) (9)

NISTNCSTAR 1-6, WTC Investigation 29
Chapter 2

Thickness (in.)

Building Component Material Specified"* In-place

Thermally

Equivalent

EXTERIOR WALL PANEL

Box columns

Exterior faces (plates 1 & 2) BLAZE-SHIELD DC/F 1 3/16 (8) 1.2

Interior face (plate 3) Vermiculite plaster 7/8 (8) 0.8 Spandrels (plate 4)

Exterior face BLAZE-SHIELD DC/F 1/2 (8) 0.5

Interior face Vemiiculite plaster 1/2 (8) 0.5 CORE COLUMNS

Wide flange columns

<WF14x228 BLAZE-SHIELD DC/F 2 3/16 (8) 2.2 >WF14x228 BLAZE-SHIELD DC/F 1 3/16 (8) 1.2 Box columns

< 228 lb/ft BLAZE-SHIELD DC/F (6) (8) > 228 lb/ft BLAZE-SHIELD DC/F (6) (81

-)(10) CORE BEAMS

All BLAZE-SHIELD DC/F 1/2 (8) 0.5

In no case was a specified SFRM material or thickness found on contract documents. Rather, the term "specified" means material and thicknesses determined from correspondence among various parties. Anecdotal and photographic evidence suggests that there may have been, in general, less than 0.75 in. thick SFRM on

the diagonal struts, but there is insufficient evidence to estimate what that value should be. Thickness of SFRM on bridging trusses was not expressly specified (only "trusses'" were mentioned), and anecdotal and photographic evidence as well as written documentation, suggests that the one-way and two-way floor areas were

treated differently; specifically SFRM was required for the two-way floor areas and, while not required, was also applied in the one-way areas. Analysis of photographs of originally applied SFRM indicates that the thickness on the bridging trusses was

approximately one half that on the main trusses. A thermally equivalent thickness was not calculated for this condition and a value of 0.6 in. was used for the thermal

analyses. 1975 report by LERA indicates bridging taisses in two-way areas were fire protected. Not specified.

Unknown or not able to be determined. Not included in analyses. Since no information regarding specified thickness was found and installed thickness could not be determined, the same thickness specified for the wide flange columns (based on weight of section per foot) was used for thermal

analyses.

2.6 THERMOPHYSICAL PROPERTIES

To provide thermophysical property data for modeling the fire-structure interaction of the towers, the

thermal conductivity, specific heat capacity, and density of the SFRMs used in the WTC towers were

determined as a function of temperature up to 1,200 °C (2,190 °F). Since there are no ASTM test methods for characterizing the thermophysical properties of SFRMs as a function of temperature, ASTM

test methods developed for other materials were used. Samples were prepared by the manufacturers of

the fire-resistive materials, which included BLAZE-SHIELD DC/F, which was originally applied,

BLAZE-SHIELD II, used in the recent upgrade, and Monokote MK-5, assumed to be similar to the

30 NISTNCSTAR 1-6, WTC Investigation
Passive Fire Protection

vermiculite plaster applied to the inside surface of the exterior columns. Since Monokote MK-5 is no

longer produced, the samples were manufactured specially for this study according to the original MK-5

formulation.

2.6.1 Thermal Conductivity

The thermal conductivit}' measurements were performed according to ASTM C 1113, Standard Test Method for Thermal Conductivity of Refractories by Hot Wire (Platinum Resistance Thermometer

Technique) (ASTM 1 999). The room temperature values were in general agreement with the

manufacmrer's published values for BLAZE-SHIELD DC/F and BLAZE-SHIELD II. No published

values were available for Monokote MK-5. The thermal conductivities increased with temperature as shown in Table 2-3.

Table 2-3. Measured thermal conductivity as a function of temperature.

Temperature (°C)

Thermal Conductivity (W/(m • K))*

BLAZE-SHIELD DC/F BLAZE-SHIELD H Monokote MK-5

25 0.0460 0.0534 0.0954

50 0.0687 0.0745 0.0926

100 0.0628 0.0921 0.1252

200 0.0810 0.0895 0.0919

300 0.1106 0.1057 0.1214

400 0.1286 0.1362 0.1352

500 0.1651 0.1689 0.1504

600 0.2142 0.2156 0.1622

800 0.3380 0.2763 0.1895

1000 0.5010 0.3708 0.2618

1200 0.5329 0.4081

* Results are presented in SI units because this system was used to make the measurements. To convert to BTU • in / (h ft2 • °F) divide by 0.1442279.

2.6.2 Specific Heat Capacity

Specific heat capacity determinations were made with the same instrument as for thermal conductivity

with a slight modification. A thermocouple was added to the system, which permitted determination of

the thermal diffusivity of the material. Knowing the thermal conductivity, the thermal diffusivity, and the

density obtained from other tests, the specific heat capacity was calculated. The inherently indirect nature of the technique used precluded the direct measurements of specific heat capacity peaks associated with

chemical reactions.

To examine the chemical reactions associated with heating of SFRMs, differential scanning calorimetry

(DSC) measurements were made in accordance with ASTM E 1269, Standard Test Method for Determining Specific Heat Capacity by Differential Scanning Calorimetry (ASTM 2001). Differential

thermal analysis (DTA) which is a "fingerprinting" technique that provides information on the chemical

reactions, phase transformations, and structural changes that occur in a specimen during a heating or

NISTNCSTAR 1-6. WTC Investigation 31
Chapter 2

cooling cycle. These tests revealed large peaks in the specific heat capacities in the range of 1 25 °C to 140 °C, which were accounted for in the thermal analyses conducted in the course of this investigation.

2.6.3 Density

Bulk densities of the SFRMs were not measured directly (except at room temperature) but were

calculated from thermal gravimetric analysis (TGA) and thermal expansion measurements. The TGA

tests to measure mass loss were performed according to ASTM E 1131, Standard Test Method for Compositional Analysis by Thermogravimetry (ASTM 1998). Thermal expansion measurements were

performed according to ASTM E 228, Standard Test Method for Linear Thermal Expansion of Solid

Materials (ASTM 1995). Since the materials were not isotropic, separate measurements were performed

in the plane of the SFRM sample and perpendicular to the free surface of the sample. Consequently,

measurements were performed both in the plane of deposition and perpendicular to the plane of

deposition. The density values were calculated from the results of the thermal gravimetric analysis and

thermal expansion. The room temperature densities were found to be 15.7 pcf for BLAZE-SHIELD

DC/F, 20.8 pcf for BLAZE-SHIELD II, and 19.4 pcf for Monokote MK-5.

2.6.4 Thermophysical Properties of Gypsum Panels

Thermophysical properties of four representative types of commercially available gypsum panels were

examined. The materials were:

• 5/8 in. thick gypsum panel A,

• Vi in. thick gypsum panel,

• 5/8 in. thick gypsum panel B, and

• 1 in. thick gypsum liner panel.

Thermal conductivity was measured using the heated probe technique described in ASTM D 5334,

Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal

Needle Probe Procedure (ASTM 2000b). In general, the thermal conductivity initially decreased as the

temperature increased to 200 °C and then increased with increasing temperature above 300 °C.

Specific heat capacities of the cores of the four gypsum panel samples were measured using a differential scanning calorimeter according to ASTM E 1269, Standard Test Method for Determining Specific Heat

Capacity by Differential Scanning Calorimetry (ASTM 2001). The four panels had similar specific heat

capacities as a function of temperature, with a high peak at about 150 °C and a smaller peak at about

250 °C.

Densities were calculated from the thermogravimetric analysis and linear thermal expansion

measurements. All four materials showed the same trend as a function of temperature. The variation in density with temperature is associated with the mass loss and the change in volume of the gypsum

material.

32 NISTNCSTAR 1-6, WTC Investigation
Passive Fire Protection

2.7 ESTIMATION OF SFRM DISLODGED BY AIRCRAFT IMPACT

To analyze the thermo-structural response of the WTC towers during the fires after the aircraft impacts, it was necessary to estimate the extent of dislodged thermal insulation on structural members.

Dislodgement could occur as a result of direct impact by debris or due to inertial forces resulting from

aircraft impact. The dislodgement due to flying debris was estimated from results of the aircraft impact

analyses that predicted damage to the structure (columns, beams and floors), partitions, and furnishings. In addition, a study was conducted to estimate dislodgement due to inertial forces. NIST established

conservative estimates for the extent of dislodged SFRM. However, since NIST was not able to establish

robust criteria to predict the extent of vibration-induced dislodgement, insulation dislodged by inertial effects other than that dislodged by direct debris impact was ignored and not included in the analyses.

The methodology and criteria for estimating the extent of damage and dislodgement of SFRM from

results of the aircraft impact analysis are presented in Chapter 5.

2.7.1 In-place Density and Bond Strength

The magnitude of the inertial forces resulting from shock and vibration is dependent on the density and

thickness of the thermal insulation. The insulation would dislodge if the stresses resulting from inertial forces exceed the bonding, or adhesive/cohesive strength of the insulation.

The Port Authority provided data on in-place density and bond strength characteristics of the thermal

insulation (BLAZE-SHIELD II) applied to the floor trusses during tenant alterations. According to the

manufacturer, BLAZE-SHIELD II is about 20 percent denser and has about 20 percent greater

adhesive/cohesive strength than BLAZE-SHIELD DC/F. The Port Authority test reports indicate that bond strength was determined in accordance with ASTM E 736, Standard Test Method for Cohesion/Adhesion of Sprayed Fire-Resistive Materials Applied to Structural Members (ASTM 2000a).

The method involves gluing ajar screw cap to the surface of the thermal insulation, and after the glue has

cured, pulling on the cap until failure of the SFRM occurs. The force required to pull off the cap is divided by the area of the cap, and reported as the "cohesive/adhesive strength." Failure is described as "cohesive" if it occurs within the insulation and is defined as "adhesive" if it occurs at the interface with

the substrate.

Analysis of the reported density values indicated no statistically significant differences between the

average SFRM densities in the two towers. The overall average density was 18.9 pcf with a standard

deviation of 3.2 pcf, giving a coefficient of variation of 16 percent.

Analysis of the bond strength values indicated that there were statistically significant differences between

the average bond strengths for the different floors, but there was no statistically significant difference

between the average bond strengths for the two towers. The overall average bond strength was 302 psf,

with a standard deviation of 91 psf, giving a coefficient of variation of 30 percent. This average value is

less than the "tested performance" value of 360 psf indicated in the manufacturer's catalogs, but this published value is for tests under controlled conditions and may not be representative of field strengths.

The manfacturer's product literature dated February 2002 refers to average bond strength of 150 psf as "standard performance," and the same value is used in its guide specification for BLAZE-SHIELD II.

NISTNCSTAR 1-6, WTC Investigation 33
Chapter 2

2.7.2 Specimen Preparation and Test Procedures

While the in-place bond strength data of BLAZE-SHIELD II reported by the Port Authority appear to indicate acceptable perfomiance, ASTM E 736 tests do not provide sufficient information for predicting

whether insulation would be dislodged from structural members under various impact conditions. The

standard test does not provide unambiguous values of cohesive and adhesive strengths, and it does not provide tensile strength in a direction parallel to the surface, that is, the in-plane cohesive strength. Thus,

tests were conducted by NIST to determine different tensile strength properties of sprayed thermal

insulation. BLAZE-SHIELD DC/F was used because the Port Authority data did not include tests of this material.

Test specimens were made by applying the SFRM to % in. steel plates measuring 8 in. by 16 in. One half

of the plates were coated with primer paint. Nominal SFRM thicknesses of y4 in. and 1 Vi in. were

applied. Thickness was built up in several passes of the spray nozzle. Gentle hand rubbing was used to remove local high spots and produce reasonably uniform thicknesses. The plate specimens were allowed

to dry for over five months in the laboratory before testing. Companion specimens were weighed

periodically to determine loss of water, and it was found that the 1 Vi in. thick specimen reached

equilibrium in about one month.

Tests were devised to determine adhesive strength, cohesive strength normal to the surface, and cohesive

strength parallel to the surface of the SFRM. The first two properties were detennined by adapting the

pull-off test method described in ASTM C 1583, Standard Test Method for Tensile Strength of Concrete

Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct

Tension (Pull-off Method) (ASTM 2004). The SFRM layer was cut carefully in two orthogonal

directions to create a prismatic test specimen, and a 3/8 in. by 2.7 in. by 2.7 in. aluminum plate was glued

to the surface. The advantages of this approach over the ASTM 736 technique are that the resisting area

is easily determined and it offers the ability to measure both adhesive and cohesive strengths.

From each plate, three specimens were prepared for measuring both density and in-plane cohesive

strength, and two specimens were prepared for measuring adhesive strength and cohesive strength normal

to the surface. Prismatic specimens were prepared by carefully removing strips of SFRM from the steel plates and sanding them to obtain uniform thickness. These specimens were weighed to determine their

densities. Then the specimens were glued to a steel plate, and a small plate was glued to the other end for

applying a tensile load.

The adhesive strength and cohesive strength normal to the surface were obtained using the modified pulloff procedure. An aluminum plate was bonded to the top surface of the SFRM using a fast curing, twocomponent urethane foam adhesive. After the adhesive had cured, the SFRM layer was cut to produce a

prismatic test specimen. A hook was screwed into the aluminum plate, and a load was applied by hand

using a 50-lb. digital force gauge. The average length and width of the failure area were measured and

used to compute the adhesive or cohesive strength. After the first test, the specimen was repaired with the same polyurethane adhesive, and the test was repeated. If the first test resulted in an adhesive failure, the

second test of the repaired specimen measured cohesive strength of the bulk SFRM. If the first test resulted in cohesive failure, specimens were repaired and retested until an adhesive failure was obtained.

34 NISTNCSTAR 1-6, WTC Investigation
Passive Fire Protection

2.7.3 Test Results

Table 2—4 summarizes the results of the test described in the previous section.

Table 2-4. Summary of physical characteristics of BLAZE-SHIELD DC/F specimens

tested at NIST

Property

Bare Steel Primed Steel

Va in. 1 Vi in. Va in. 1 V2 in.

Densitv' (pcf) 27.2(0.8)* 29.7(1.3)

In-plane cohesive strength (psf) 1120(390) 1740(540)

Adhesive strength (psf) 450 (63) 666 (151) 185 (96) 17r(196)

Cohesive strength normal to surface (psf) 433 (99) 610 (142) 367 (79) 595 (163)

* First number is the average and the number in parentheses is the standard deviation. Based on testing selected samples.

The densities of the BLAZE-SHIELD DC/F measured in this study were higher than published values in the manufacturer's catalogs and higher than the in-place average density of 18.9 pcf reported in Port Authority test reports for BLAZE-SHIELD II. The difference in average densities of the two thicknesses was statistically significant. The higher values in this study are attributed to the specimen preparation

procedures, which tended to result in denser test specimens than would be representative of field application.

The difference in average adhesive strength for the two SFRM thicknesses is statistically significant. The

relative strengths are consistent with the difference in density for the two thicknesses.

The presence of primer reduced the adhesive strength, especially for the 1 V2 in. thick specimens. Twothirds of the specimens with the thicker SFRM had no adhesion to the coated steel plates.

Analysis of the cohesive strength normal to the surface indicated that there was no statistically significant

effect due to the presence or absence of primer. This is logical because the condition of the steel surface

is not expected to influence the properties of the bulk SFRM. There was a statistically significant

difference in the average strengths for the two thicknesses, with the 1 V2 in. SFRM having higher strength.

For comparison with the measured cohesive strength normal to the surface, two tests were conducted in accordance with ASTM E 736. The results of the two tests were in agreement with those obtained by the

pull-off technique. This suggests that the ASTM E 736 procedure probably provides a measure of

cohesive strength.

A comparison was made of the adhesive strength and cohesive strength normal to the surface for the

plates made with bare steel. An analysis of variance indicated that there is an 8 percent probability that the difference could be the result of randomness. Generally, if this probability is greater than 5 percent, it can be concluded that the difference is not statistically significant. Thus, for the case of good adhesion,

the test results do not contradict the assumption that the adhesive strength and cohesive strength normal to

the surface are equal. If this assumption is accepted, the average of the adhesive and cohesive strengths was found to be 409 psf for the Va in. SFRM, and the average is 622 psf for the 1 Yi in. SFRM. These

values are considerably greater than the manufacturer's published strength of 295 psf, obtained using the

ASTM E 736 method under laboratory conditions.

NISTNCSTAR 1-6, WTC Investigation 35
Chapter 2

2.8 REFERENCES

ASTM 1961. Standard Test Methods for Fire Tests of Building Construction and Materials.

ASTM E 1 1 9-6 1, ASTM International, West Conshohocken, PA.

ASTM 1995. Standard Test Method for Linear Thermal Expansion of Solid Materials With a Vitreous

Silica Dilatometer, ASTM E 228-95, ASTM International, West Conshohocken, PA.

ASTM 1998. Standard Test Method for Compositional Analysis by Thermogravimetry, ASTM E 1131-

98, ASTM International, West Conshohocken, PA.

ASTM 1999. Standard Test Method for Thermal Conductivity of Refractories by Hot Wire (Platinum

Resistance Thermometer Technique), ASTM C 1 1 13-99. ASTM International, West Conshohocken,

PA.

ASTM 2000. Standard Test Methods for Fire Tests of Building Construction and Materials,

ASTM E 1 19-OOa. ASTM International, West Conshohocken, PA.

ASTM 2000a. Standard Test Method for Cohesion/Adhesion of Sprayed Fire-Resistive Materials

Applied to Structural Members, ASTM E 736-00, ASTM International, West Conshohocken, PA.

ASTM 2000b. Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock

by Thermal Needle Probe Procedure, ASTM D 5334-00, ASTM International, West Conshohocken,

PA.

ASTM 2001. Standard Test Method for Determining Specific Heat Capacity by Differential Scanning

Calorimetry, ASTM E 1269-01, ASTM International, West Conshohocken, PA.

ASTM 2004. Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-off Method),

ASTM C 1583-04. ASTM International, West Conshohocken, PA.

Gewain, R.G., Iwankiw, N.R. and Alfawakhiri, F. 2003. Facts for Steel Buildings—Fire, American

Institute of Steel Construction, Chicago, IL, October 2003. 51 pp. NYCBC 1968. Buildmg Code—Local Law No. 76 of the City of New York. New York, NY.

Underwriters Laboratories Inc. (2002). Fire Resistance Directory - Volume 1 , Northbrook, IL.

36 NISTNCSTAR 1-6, WTC Investigation
Chapter 3

Fire Resistance Tests

3.1 BACKGROUND

3.1.1 Motivation for Conducting Standard Fire Tests

National Institute of Standards and Technology (NIST) review of available documents indicated that the

fire performance of the composite floor system of the World Trade Center (WTC) towers was an issue of

concern to the building owners and designers from the original design and throughout the service life of

the buildings (NIST NCSTAR 1-6A). NIST found no evidence regarding the technical basis for the

selection of the SFRM for the WTC floor trusses and for the SFRM thickness to achieve a 2 h rating.

Further, NIST no evidence was found that fire resistance tests of the WTC floor system were conducted.

Review of the documents did not identify a similar concern for other structural components of the WTC

towers.

NIST NCSTAR 1-6B reports on the motivation for testing full- and reduced-scale assemblies representing

the floor system used in the World Trade Center towers, and the purpose of the Standard Fire Test. The

test variables are given and the construction of the floor test assemblies, test set-up, and loading are described. Results (fire resistance ratmgs) of the four tests are presented and discussed. This chapter

summarizes the salient material covered in NIST NCSTAR I-6B.

3.1 .2 Purpose of the Standard Fire Tests

NIST conducted a series of four standard fire tests of the WTC floor system:

• to establish the baseline performance of the floor system of the WTC towers under thermal

loading as they were originally designed,

• to differentiate the factors that most influenced the response of the WTC floors to fires as they may relate to normal building and fire safety considerations and those unique to the events of September 1 1

, 200 1

,

• to determine whether there was an adequate technical basis for the original SFRM

specification, and

• to study the procedures and practices used to accept an innovative structural and fire protection system.

NIST NCSTAR 1-6, WTC Investigation 37
Chapter 3

3.2 DESCRIPTION OF WTC FLOOR SYSTEM

3.2.1 Structural System

The floor system design for the World Trade Center consisted of a lightweight concrete floor slab supported by steel trusses bridging between the core and exterior wall"^. The main composite trusses, which were used in pairs, were spaced at 6 ft 8 in. on center (o.c.) and had a nominal clear span of either

60 ft or 35 ft. The trusses were fabricated using double-angles for the top and bottom chords and round

bars for the webs. The web members protruded above the top chord in the fonn of a "knuckle," which was embedded in the concrete slab to develop composite action. Additionally, the floor system included

bridging trusses (perpendicular to main trusses) spaced 13 ft 4 in o.c. In the comers of the towers, the

bridging trusses acted with the main tmsses to provide "two-way" slab action, i.e., bending moments

existed in both principal directions. Figure 3-1 is a cut-away of the floor system showing the main

trusses, bridging trusses, metal deck, and concrete floor slab. Figure 3-2 shows a cross-sectional view of

the basic configuration of the floor system.

Figure 3-1 . Floor system of the WTC towers.

All information and data related to the design and construction of the WTC floor system were obtained from contract drawings

provided to NISI by The Port Authority of New York and New Jersey. Refer to NIST NCSTAR 1-2A for a complete

description of the WTC structural system and index of all structural drawings.

38 NIST NCSTAR 1-6, WTC Investigation
Fire Resistance Tests

Figure 3-2. Schematic of floor system viewed along the main steel trusses.

(not to scale)

3.2.2 Sprayed Fire-Resistive Material (SFRM) Thickness

As noted in Chapter 2, the average thickness of SFRM as originally installed was approximately Va in. The thicknesses of Vi in. representing the specified thickness, and Va in. representing average applied

thickness, were used in the standard fire resistance tests described here.

3.3 FIRE RESISTANCE TESTING

3.3.1 General Description

The fire rating of structural materials and assemblies is generally determined through testing, and in the

United States, such testing is frequently conducted in accordance with the ASTM International standard,

ASTM E 1 19. "Standard Test Methods for Fire Tests of Building Construction and Materials." This

standard was first published in 1917 as a tentative standard, ASTM C 19, and was first adopted as ASTM

E 1 19 in 1933. Since its introduction, the test method has been modified and updated, although its essential character has remained unchanged. ASTM E 119 prescribes a standard fire exposure for comparing the test resuhs of building construction assemblies. For the tests of floors and roofs, a test assembly is structurally loaded, and the standard fire exposure is applied to the underside of the specimen.

The assembly is evaluated for its ability to contain a fire by limiting passage of flame or hot gases, and

limiting heating of the unexposed surface, while maintaining the applied load. The assembly is given a

rating, expressed in hours, based on these acceptance, or end-point, criteria.

3.3.2 Test Restraint Conditions and Ratings

ASTM E 1 19 Standard in 1971 introduced the concept of fire endurance classifications based on two

conditions of support: restrained and unrestrained. A restrained condition""* is "one in which expansion at the support of a load carrying element resulting from the effects of fire is resisted by forces external to the

element." In an unrestrained condition, the element is free to expand and rotate at its supports.

According to Appendix A4 of ASTM E 1 1 9-73.

NISTNCSTAR 1-6, WTC Investigation 39
Chapter 3

The current standard describes a means to establish both restrained and unrestrained ratings from

assembhes tested in the restrained condition. The conditions of acceptance are based on limiting passage

of flame or hot gases, limiting temperatures on the unexposed surface of the slab, and failure to support

the applied load. In addition, temperature limitations are placed on the main structural members. The

location of temperature measurements on the structural members is specified in the Standard.

In addition, the Standard describes a means to establish unrestrained ratings from unrestrained test conditions. For tests of assemblies not restrained against thermal expansion, the fire resistance rating is based on limiting passage of flame and hot gases, exceeding temperatures on the unexposed surface of the

slab, and failure to sustain the applied load; however, there are no limiting temperatures for the steel structural members.

Prior to 1970, restrained and unrestrained ratings were not defined in ASTM E 1 19. Ratings were

determined based upon the requirements for restrained assemblies except that no temperature limitations were placed on the structural steel members.

In practice, a floor assembly such as that used in the WTC towers is neither restrained nor unrestrained

against thennal expansion, but is likely somewhere in between. Testing under both restraint conditions

bounds expected performance under standard fire exposure.

3.3.3 Scale of Tests

For floor and roof assemblies, ASTM E 1 19 requires that the area exposed to fire be a minimum of 180 ft" with neither dimension of the furnace less than 12 ft. Furnaces available in 2002 in the United States for

conducting standardized fire resistance tests of floor and roof assemblies had a maximum span less than

18 ft (NIST GCR 02-843). Traditionally, relatively small scale assemblies have been tested, and results have been scaled to practical floor system spans.

The Underwriters Laboratories (UL) fire testing facility in Toronto, Canada has a furnace with nominal

dimensions of 35 ft by 14 ft. Thus, full- or large-scale floor assemblies could be tested in this furnace.

Availability of the 35 ft furnace in UL's Toronto facility, in addition to the 17 ft furnace at its Northbrook, Illinois, facility allowed NIST to conduct tests to compare the effect of scale.

3.3.4 Test Variables

To limit the number of tests and obtain information of greatest value to meet the investigation objectives

discussed above, NIST studied three factors: SFRM thickness, scale of the test, and test restraint • conditions. To this end, four tests were conducted as follows:

• Test #1: Full-scale (35 ft span), restrained test condition, Va in. thick SFRM.

Test #2 Full-scale (35 ft span), unrestrained test condition, in. thick SFRM.

Test #3 Reduced-scale (17 ft span), restrained conditions, in. thick SFRM.

Test #4 Reduced-scale (17 ft span), restrained conditions, V2 in. thick SFRM.

40 NISTNCSTAR 1-6, WTC Investigation
Fire Resistance Tests

The objective of the full-scale restrained test with in. thick SFRM, Test 1 , was to determine the

baseline fire resistance of the floor system with average as-applied SFRM thickness. This test also

demonstrated whether the fire resistance of such a system was significantly different from that of a system

with the specified SFRM thickness of Vi in.

The test conditions for Test 2, full-scale unrestrained test with Va in. thick SFRM, were the same as those

for Test 1 except that the specimen was supported to allow thermal expansion and, therefore, represented

the unrestrained test condition. Results of this test allowed a determination of the unrestrained rating by

test and, by comparing with the results of Test 1 , a comparison of unrestrained ratings from both a restrained and unrestrained assembly test.

Test 3 was a reduced-scale test which, other than scale, was the same as Test 1 . Thus, a comparison of

the results of these two tests allowed an examination of whether test results are independent of test assembly scale.

Test assembhes for Tests 1 , 2 and 3 were fire protected in the same manner, with in. thick SFRM

representing the average SFRM thickness in the impact and fire affected floors of WTC 2. Measurements

taken from photographs of the originally applied SFRM indicated that, while the SFRM thickness on main the trusses was approximately Va in., the thickness on the bridging trusses was approximately half

that value (see NIST NCSTAR 1-6A). Also, photographs indicated that the metal deck was sometimes

sprayed and sometimes not. For these three tests (Tests 1, 2, and 3), then, the main trusses were protected

with Va in. thick SFRM. the bridging trusses with Vs in. thick SFRM, and the metal deck was not

intentionally sprayed but was also not masked from overspray and thereby had, in most instances, at least a light covering of SFRM. These conditions best represented the thickness of the SFRM as it was

originally applied in the one-way slab areas.

The objective of the test with the Yi in. SFRM (Test 4) was to determine whether or not there was

adequate technical basis for the original SFRM specification. As explained by the designer, it was not necessary to fire protect the bridging trusses in the one-way areas nor was it necessary to spray the metal

deck (see NIST NCSTAR 1-6A). Consequently, the Test 4 specimen had Vi in. thick SFRM applied to the main trusses and no SFRM applied to either the bridging trusses or the underside of the metal deck.

Both the bridgmg trusses and metal deck were masked to prevent overspray as well. These conditions best

represented the SFRM that was necessary, in the opinion of the designer, to provide the required level of

passive fire protection.

3.4 PREPARATION OF TEST ASSEMBLIES

Original shop drawings by Laclede Steel were used for the design of the 35 ft 0 in. span and 17 ft SYi in. span test assemblies. Figure 3-3 shows a drawing for the trusses used in the 35 ft span assemblies. The

steel trusses faithfully duplicated the geometry of the original design. Since equipment for making the

resistance welds is not available in the United States, metal inert gas (MIG) welding was used, and the

welds were designed per the American Institute of Steel Construction (AISC) weld criteria to develop the

web capacities in tension or compression. This strength requirement was based on test data from

Laclede's files indicating that weld capacities exceeded design loads by a factor of 2.

NIST NCSTAR 1-6, WTC Investigation 41
Chapter 3

Itam Descriplton Laclede MK. Type of Steel Size Length

1 Top Chord 6 A572 2x1-1/2x0.25 in 35 fl, - 0 in.

2 Bottom Chord 7 A572 2x 1-1/2x0 25 in. 28 tt. - 2 in.

3 Main Web 3 A529 0 92 In Diam Rod As Required

4 Mair^ Web 3A A529 0 98ir Diam Rod As ReQUired

5 Vertical Strut 2 A529 0 92 in Diam Rod 2tt -4 7/8 in

6 Diagonal End Strut 1 A529 0.92 in. Diam Rod As Required

7 End Stiffener V3 A529 0.92 in. Diam Rod As Required

8 End Stiflener VI A529 0.92 in Diam. Rod 4 3/8 in.

9 Bearing Angle 9 A572 2x1-1/2x0.25 In, 9 5/16 in.

10 Vertide Strut Core Side 5 A529 0,98 in Diam. Rod As Required

11 Diagonal Strut 10 A529 0.98 in Diam Rod As Required

12 End Slitfener V2 A529 0,98 In Diam. Rod 7 3/8 In.

13 End Stiffener VI A529 0.98 In, Diam Rod As Required

Figure 3-3. Drawing of 35 ft truss and end detail.

In addition, the steel angles, round bars, reinforcing steel, welded wire fabric (WWF), metal deck,

lightweight concrete, and primer paint were all matched as closely as practical. Figure 3-4 shows the

steel trusses being fit-up and welded in the test frame for one of the 35ft span test assemblies. NIST

NCSTAR 1-6B provides a complete description of the construction of the test assembhes, including

materials used, sprayed fire-resistive material, and instrumentation.

42 NIST NCSTAR 1-6, WTC Investigation
Fire Resistance Tests

Source: NIST. Figure 3-4. Trusses being fit-up in test frame of 35 ft test assembly.

3.5 DESCRIPTION OF TESTS AND LOADING

3.5.1 Full-scale Tests (35 ft span)

Full-scale (35 ft span) tests were conducted at the UL furnace facility in Toronto, Canada. Loading of the

floor slab, to "simulate a maximum load condition" as required by ASTM E 1 19, was accomplished

through a combination of concrete block and steel containers filled with water. The water containers were restrained using steel cables to prevent them from falling into the furnace and causing damage to the

fire brick and instrumentation in the event of a catastrophic failure of the floor system. Figure 3-5 shows

the water containers being placed on the concrete slab.

Source: NIST. Figure 3-5. 35 ft span test assembly loaded with concrete

blocks and water-filled containers.

NISTNCSTAR 1-6. WTC Investigation 43
Chapter 3

3.5.2 Reduced-scale Tests (17 ft span)

The reduced-scale tests were designed to extract as much information as practicably possible considering

that the purpose of the Standard Fire Resistance Test is to measure the ability of the test assembly to contain a fire and is, therefore, based on integrity (passage of hot gases), thermal insulation (heating of the

unexposed surface) and load bearing capacity (support of the applied load). For the reduced-scale tests, the sizes of the steel members and the slab thickness were the same as in the full-scale tests. Otherwise,

the length and depth of the trusses were scaled by one half. The spacing between the trusses was the same as in the full-scale test. The scaled length and depth, coupled with the original member sizes, slab

thickness, and truss spacing, required an increase in the loading to produce the same stress levels as in the

full-scale specimen. The loading represented the maximum calculated load condition as required by the

ASTM El 19 Standard.

The superimposed uniform load was applied through a combination of concrete blocks, water-filled

containers, and hydraulic actuators located along the trusses. Figure 3-6 shows the assembly of blocks,

water containers, and hydraulic actuators.

Source: NIST. Figure 3-6. 17 ft span test assembly loaded with concrete

blocks, water-filled containers and hydraulic actuators.

3.6 TEST RESULTS

3.6.1 General Discussion of Tests

Specimen No. 1: 35ft span restrained specimen with 3/4in. SFRM

In the test of Specimen No. 1 , loud noises (reports) were heard beginning roughly fifteen minutes after the

start of the test. These loud, but somewhat muffled, reports continued and were often accompanied by

noticeable bulging of the metal deck and the dislodging of deck SFRM. It is believed that these loud

44 NISTNCSTAR 1-6, WTC Investigation
Fire Resistance Tests

noises were associated with spalling of the concrete on the underside of the slab. The slab was later sectioned to determine the depth of delamination resulting from the spalling.

At about 50 min, a very loud report was heard, associated with a noticeable sudden downward deflection

of the specimen. When access to the top of the slab was possible after completion of the test, it was found

that significant cracking and dislocation of the slab had occurred near the comers of the slab. Several of

these events were recorded, and post-test inspection revealed that concrete failure had occurred in all four comers of the slab as shown in Fig. 3-7. It is believed that the very loud report and abmpt deflection was

a result of the sudden concrete cracking. The test was stopped at 11 6 min when, after another sudden

drop, the center span deflection reached about 1 5 in.

Source: NISI. Figure 3-7. Unexposed surface of Assembly No. 1 after

loading equipment was removed.

The specimen supported the applied load for the duration of the test. Post-test inspection showed that the comers of the slab had not deflected; rather, all of the slab deflection occurred beginning roughly 5 ft from the ends as seen in Fig. 3-7. It is believed that the thermal expansion of the concrete and the

resistance provided by the test frame, put the slab in compression and "wedged" the slab so that no

deflections could occur at the ends of the slab. The high compressive stresses that developed hkely

contributed to the failures noted above.

NISTNCSTAR 1-6. WTC Investigation 45
Chapter 3

The test was conducted beyond the El 19 end point criteria to obtain as much information as possible.

This test, as well as the other three, were continued as long as practical. The tests were stopped, in general, when there was a risk of imminent failure or when deflections were so large as to affect instrumentation readings and, as a consequence, safe conduct of the test. The restrained and unrestrained

ratings obtained from this test are given in Section 3.6.2.

Specimen No. 2: 35ft span unrestrained specimen with 3/4in. SFRM

Reports were heard during the test of the 35 ft unrestrained Specimen No. 2 but were not as loud as those

observed during the previous test. A significant difference was that no loud reports or attendant sudden

increase in downward deflection were observed for the entire test period, which was 146 minutes, almost

two and a half hours, and post-test inspection of the top of the concrete slab showed very little cracking as seen in Fig. 3-8. Since there was a gap between the concrete slab and test frame, thermal expansion of

the slab did not produce a state of stress that resulted in significant and sudden cracking and crushing

similar to that observed in Specimen No. 1. Additionally, the slab in this test was not wedged at the ends

and, unlike Specimen No. 1, the slab deflected over its entire length.

Source: NISI. Figure 3-8. Unexposed surface of Assembly No. 2 after

loading equipment was removed.

46 NISTNCSTAR 1-6, WTC Investigation
Fire Resistance Tests

The center-of-slab deflection at the end of the test was more than 13 in. The test had to be stopped when

the slab deflections affected the instrumentation, and readings could not be obtained. The unrestrained

rating for Specimen 2 obtained from this test is given in Section 3.6.1.

Specimen No. 3: 17ft span restrained specimen with 3/4in. SFRM

As in the first two tests, the metal deck began bulging due to spalling of concrete on the underside of the

slab within the first 15 min. At 82 min into the test, a very loud report was heard, and pieces of concrete flew in the air signifying explosive spalling at the north end of the slab as shown in Fig. 3-9. A slight

increase in downward deflection at the center of the slab was recorded at 82 min. The test was continued

for 210 min and was stopped when the deflections were so large as to affect instrumentation.

Source: NIST

Figure 3-9. Unexposed surface of Assembly No. 3 after loading

equipment was removed.

Specimen No. 4: 17ft span restrained specimen with 1/2 in. SFRM

Specimen No. 4 behaved similarly to the other three specimens with regard to bulging of the metal deck on the underside of the slab beginning in the early stages of the test. At 55 minutes, a very loud report was heard, and a significant section of the slab spalled explosively, sending concrete fragments high in the air. As seen in Fig. 3-10, the resultmg hole in the slab (left side of the photograph) measured over 2 ft

in length and had to be covered with insulating material to safely continue the test as hot gases were coming through the hole.

NISTNCSTAR 1-6, WTC Investigation 47
Chapter 3

Source: NIST. Figure 3-10. Unexposed surface of Assembly No. 4 after loading

equipment was removed.

3.6.2 Fire Resistance Ratings

As noted above, prior to 1971, the ASTM E 1 19 Standard did not differentiate between testing and

classifying thermally restrained and unrestrained floor assemblies. The 1961 revision of ASTM E 119,

the revision referenced in the 1968 New York City Building Code, is used for reporting the Standard Fire Test rating (no distinction was made for an unrestrained rating). The year 2000 revision of the Standard

is used for reporting restrained and unrestrained ratings.

Table 3-1 shows results for all four tests giving the times (in minutes) to reach the acceptance, or endpoint, criteria and the Standard Fire Test rating (in hours) for both the 1961 and 2000 revisions of

ASTM E 1 19. Note that in none of the tests did the floor assembly fail to support the superimposed load.

48 NISTNCSTAR 1-6, WTC Investigation
Fire Resistance Tests

Table 3-1. Results of ASTM E119 Standard Fire Tests.

Test Description

Times to Reach End-Point Criteria (min)

Test Terminated (min)

Standard Fire Test Rating (hr)

Temperature on

I'nexposed Surface

Steel Temperatures

Failure to Support Load

ASTM

E 119-61 ASTM El 19-00

A>erage

(Ambient

+250"F)

Maximum

(Ambient+

325"F)

.Average (1100°F) Maximum

(l.WF)

Rating

Restrained Rating

Unrestrained Rating

1 35 ft, restrained.

3 4 in SFRM

1 1 1 66 62 (3) 116(1) 1 V2 1 Vi 1

1 35 ft, unrestrained,

J 4 in ShKJvl 76 62 (3) 146(2) 2 2

1 7 ft. restrained. 3 '4 in SFRM

180 157 86 76 (3) 210(2)

-) 1

1

4

17 ft. restrained.

1 2 in SFR^1

58 66 58 (3) 120(1) 'A

( 1 ) Imminent collapse

(2) Vertical displacement exceeded capability' to measure accurately

(3) Did not occur

3.6.3 General Observations

Buckling of Trusses

A photograph of the un(jerside of the 35 ft, restrained test specimen after almost two hours of fire exposure is shown in Fig. 3-1 1 . Buckling of the compression diagonals can be seen as well as sagging of

the metal deck between supports. Note that, upon cooling, the test specimen recovered at least half of the

deflection achieved during the test so deflections seen in Fig. 3-1 1 are considerably less than the

deflections at the end of the test.

Source: NIST. Figure 3-1 1 . Fire exposure side of the 35 ft restrained test

assembly after almost 2 hours of fire exposure

NISTNCSTAR 1-6, WTC Investigation 49
Chapter 3

Spalling of Concrete

The sagging of the metal deck was asumed to be a resuh of spalHng of the underside of the concrete slab. This was confirmed when, after the tests, concrete cores were removed and sections cut to determine the

condition of the concrete as shown in Fig. 3-12. The depth of the delamination varied but the depth of

the remaining slab was on the order of 2 in. to 3 in., essentially up to the double layer of welded wire

fabric as seen in Fig. 3-13.

Source: NISI. Figure 3-12. Sections cut through concrete slab to confirm extent

and depth of spalling.

Source: NIST. Figure 3-13. Measurement of remaining slab thickness after spalling.

50 NISTNCSTAR 1-6, WTC Investigation
Fire Resistance Tests

Concrete Slab Failure

The three specimens that were restrained showed significant cracking and explosive spalhng at the top

surface of the concrete slab. The 35 ft restrained specimen had significant cracking and crushing of

concrete near the ends and the comers as seen in Fig. 3-7 and Fig. 3-14. These major concrete failures were accompanied by sudden increases in deflection of the floor as seen in Fig. 3-15. The unrestrained

35ft specimen showed little cracking and no crushing or explosive spalling at the top surface of the slab

unlike the restrained specimen. The restrained 17 ft specimen with Vi in. of SFRM exhibited significant

explosive spalling in which concrete fragments flew in the air and left a hole in the slab about 2 ft long

through which hot gases escaped.

Source: NISI. Figure 3-14. Detail of spalling concrete at east end of Assembly No. 1.

3.6.4 Floor Deflections and Temperatures

Deflections of floor assembly

The following plots show the vertical deflection measured at the center of each floor assembly.

Figure 3-15 shows the deflection while Fig. 3-16 is a plot of the deflection normalized by the span

length. It is seen that Specimen No.l experienced a significant increase in vertical deflection at 49 min

which corresponded directly to a loud report and visible increase in deflection noted during the test. A

small increase in vertical deflection for Specimen No. 3 was seen to occur at 82 min, the time when the

slab spalled explosively at one end.

NISTNCSTAR 1-6, WTC Investigation 51
Chapter 3

Fire Resistance Testing of WTC Floor System Unexposed Surface Deflection -Center of Assembly12

Time (minutes)

Figure 3-15. Deflection measured at the center of each assembly.

Fire Resistance Testing of WTC Floor System Unexposed Surface Deflection/Span

-Center of Assembly52 NISTNCSTAR 1-6, WTC Investigation
Fire Resistance Tests

Unexposed surface temperatures

The temperature of the top (unexposed) surface of the floor assemblies is plotted in Fig 3 -17. It is observed that the unexposed surface temperatures of all four test assemblies were similar prior to the onset of significant concrete crushing and spalling at around 50 min. In Test 4, the surface-mounted

thermocouple on the west edge near the center of the span was affected by the explosive failure of the

slab and recorded hot gas temperatures.

Fire Resistance Testing of WTC Floor System Unexposed Average Surface Temperature Comparison

450 T

50

0 50 100 150 200 Time (minutes)

Figure 3-1 7. Average temperature of the unexposed surface for all four tests.

Steel temperatures

Steel temperatures were recorded at several locations on the main and bridging trusses. Figure 3-18

shows a comparison of the average temperature of the bottom chord for the three tests in which the

thickness of the SFRM was Va in. Temperatures were seen to be very comparable up to about 75 min,

which was around the time when SFRM began to dislodge. The location and extent of dislodged SFRM

could not be ascertained through visual observation during the tests.

NISTNCSTAR 1-6, WTC Investigation 53
Chapter 3

2000

Fire Resistance Testing of WTC Floor System

Bottom Chord Average Temperature Comparison

Section C

1800

1600 1 .

ULC Restrained Bottom Chord Average (Test 1)

- UL Restrained Bottom Chord Average (Test 3)

ULC Unrestrained Bottom Chord Average (Test 2)

50 100 150 200 Time (minutes)

Figure 3-18. Average temperatures of the bottom chord for Test Nos. 1, 2 and 3

(3/4 in. thick SFRM).

3.6.5 Observations and Comparisons

Several observations can be made from the results presented and the summary table of hourly ratings

(Table 3-1). The reader is referred to NIST NCSTAR 1-6B for a complete description of the fire resistance tests and the results obtained.

• The test assemblies were able to withstand standard fire conditions for between Va h and 2 h

without exceeding the limits prescribed by ASTM E 1 19.

• Test specimens protected with Va in. thick spray applied fire-resistive material were able to sustain the maximum design load for approximately two hours (the minimum was 1 1 6 min)

with no structural failure; in the 35 ft, unrestrained test, the load was maintained for 3'/2 h

(210 min).

• The restrained 35 ft floor system obtained a fire resistance rating of 1 Vi h while the

unrestrained 35 ft floor system achieved a 2 h rating. Past experience with the ASTM E 1 19

test method would lead investigators to expect that the unrestrained floor assembly would not perform as well as the restrained assembly, and therefore, would receive a lower fire rating.

• A fire rating of 2 h was determined from the 1 7 ft restrained test with the average applied

SFRM thickness of y4 in. while a fire rating of 1 V2 h was determined from the 35 ft restrained

test with the same SFRM thickness.

• A fire rating of Va h was determined from the 1 7 ft restrained test with the specified SFRM

thickness of Yi in.

54 NIST NCSTAR 1-6, WTC Investigation
Fire Resistance Tests

3.7 SUMMARY

The tested floor assemblies were similar, though not identical, to steel-joist-supported concrete floors that are widely used in low rise construction. The test results provided valuable insight into the behavior of

these widely used assemblies and also identified issues that require further study.

The fire resistance tests showed that the floors were capable of considerable sagging without collapse.

The tests also showed fire damage to the bridging trusses and buckling of compression diagonals and the

vertical strut near the supports. No evidence of knuckle failures was seen in the tests.

The standard test method has been used for many decades and has for the most part served its intended

purpose well when taken together with the fire rating requirements. This is supported by historical fire loss data for more than half a century for different high-rise building occupancies. In addition, there is extensive data and experience that has been developed using the test method.

The NIST tests have identified areas where further study related to the standard test method may be

warranted. Among the issues related to the test method that NIST identified as requiring further study

are:

• Criteria for determining structural limit states, including failure, and means for measurement,

• Scale of test assembly versus protot>'pe application,

• Effect of end restraint conditions (restrained and unrestrained) on test results, including the

influence of stiffness,

• Strucmral connections (not currently addressed in ASTM E 119),

• Combination of loading and exposure (temperature profile) adequately represent expected

conditions,

• Procedures to analyze and evaluate data from fire resistance tests of other building

components and assemblies to qualify an untested building element,

• Repeatability of test results (single test currently defines rating for system),

• Reproducibility of heat flux environment between different furnaces and laboratories, and

• Relationships between prescriptive ratings and performance of the assembly in realistic building fires.

NISTNCSTAR 1-6, WTC Investigation 55
Chapter 3

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56 NISTNCSTAR 1-6, WTC Investigation
Chapter 4

Structural Response of Components, Connections

AND Subsystems

In this chapter, the structural analysis of components, connections, and detailed analysis for the

development of two subsystems -- a floor subsystem and an exterior wall subsystem — are covered"^ The

subsystem response to impact damage and fire is addressed in Chapter 7. This work was conducted to provide guidance for the development of the global finite element models with respect to element types

and sizes, appropriate constitutive models, and failure criteria for any given structural component. The

subsystem models were also used to correlate the results of the fine mesh component analyses with the coarser mesh global analyses.

Work reported herein includes the following:

• Evaluation of the structural response of components, connections and subsystems to service

loads due to gravity (dead and live loads) and elevated structural temperatures.

• Identification of the failure modes and failure sequences, the associated temperatures at

failure, and where temperature histories were used, times to failure.

• Identification of the changes in mechanical properties or geometry at initiation of component

and subsystems failure.

• Identification of modifications for the global structural models to reduce complexity and size

while maintaining the quality of analysis results.

This chapter covers the mechanical properties of concrete and steel at elevated temperatures, analysis of

components and connections for the floors and exterior wall, and the development of models for a full floor and portion of an exterior wall. The reader is referred to NIST NCSTAR 1-6C for a complete

description of component, connection, and subsystem structural analyses.

4.1 MECHANICAL PROPERTIES OF CONCRETE AND STEEL

The mechanical properties of both steel and concrete are affected significantly by temperature. In the

following sections, the material properties used in this study are specified as a function of temperature.

For finite element analysis (FEA) of components, subsystems, and global models of the World Trade

Center (WTC) towers, a material properties catalog was developed. Each material model was idenfified

with a number in ANSYS"^'; steels were Material ID 1 through Material ID 29, and concretes were

All information and data related to the design and construction of the WTC floors and exterior walls were obtained from

contract drawings provided to NIST by The Port Authority or New York and New Jersey. Refer to NIST NCSTAR 1-2A for a complete description of the WTC structural system and an index of all structural drawings.

ANSYS (ANSYS, Inc., Cannonsburg, PA) is the structural analysis software used for nonlinear finite element analyses.

NIST NCSTAR 1-6, WTC Investigation 57
Chapter 4

Material ID 5 1 through Material ID 83. The details of concrete and steel materials are discussed

separately in this section.

4.1.1 Concrete Properties and Failure Criteria

Two types of concrete were used for the floors of the World Trade Center towers: lightweight concrete in the tenant office areas and normal-weight concrete in the core area. Thermal properties of normal-weight

concrete depend on the type of aggregate. Petrographic inspection of several samples of lightweight concrete taken from the debris showed siliceous sand in the lightweight concrete. Because coarse and

fine aggregates are usually from the same source for a construction site, it was assumed that the normalweight concrete had siliceous aggregate.

The specified design strength for lightweight concrete was 3,000 psi and either 3,000 psi or 4,000 psi for nomial-weight concrete, depending upon the floor location within the buildings. The actual strength of

concrete at room temperature is greater than that measured from cylinders poured for testing during

construction, referred to as 28-day cylinder strength, as concrete continues to strengthen with age. Methods for estimating changes in concrete strength with age are specified by the American Concrete

Institute (ACI) 209. The actual compressive strength of WTC concrete slabs was estimated to be

38 percent greater than the specified design strengths: 5,500 psi for 4,000 psi normal-weight concrete and

4,100 psi for 3,000 psi normal-weight and lightweight concretes (see NIST NCSTAR 1-6C).

Normal-weight and lightweight concrete had similar design strengths of 3,000 psi to 4,000 psi, but

respond differently to elevated temperatures. Temperature dependent properties of concrete are modulus

of elasticity, instantaneous coefficient of thermal expansion, compressive strength, and tensile strength.

The effects of elevated temperature on concrete mechanical properties, plotted in Fig. 4-1, are based upon

Phan (1996) and Phan and Carino (2003). Lightweight concrete shows less degradation in modulus of

elasticity and a constant instantaneous coefficient of thermal expansion as temperatures increase.

Lightweight concrete heats more slowly than does normal weight concrete. Tensile strength is identical

for lightweight and normal weight concretes with the same compressive strength, since tensile strength

depends upon the formation of cracks. Compressive strength for lightweight concrete shows no

degradation until 300 °C, whereas normal weight concretes begin degrading as temperatures exceed room

temperature.

58 NIST NCSTAR 1-6, WTC Investigation
structural Response of Components

1000'

0 200 400 600 0 200 400 600 Temperature (°C) Temperature (°C) Normal-weight (3000psi) Normal-weight (SOOOpsi)

Normal-weight (4000psi) Normal-weight (4000psi)

Light-weight (3000psi) Light-weight (3000psi)

(c) Compressive strength (d) Tensile strength

Figure 4-1. Temperature-dependent concrete properties.

Figure 4-2 shows concrete stress-strain curves at room and elevated temperatures, where compressive

stresses and strains are negative and tensile stresses and strains are positive. Tensile stress in concrete

increases linearly up to the tensile strength. When strained beyond this point, the concrete begins to crack

and the stress across the section will drop. However, this drop in the stress-strain relationship caused

significant numerical instability problems during structural analysis. Numerical instabilities were avoided

by assuming that the reinforced concrete slab became plastic in tension as the reinforcement carried the

tensile load.

NISTNCSTAR 1-6. WTC Investigation 59
Chapter 4

-0.015 -0.01 -0.005 0 "0.015 -Q.Ol "0.005 0

Strain (in/in) Strain (in/in) — T-RT T-RT

..... T=300°C T=300°C

T-600°C T-600X

(a) Normal-weight concrete (SOOOpsi) (b) Light-weight concrete Figure 4-2. Concrete stress-strain curves.

The assumption of concrete plasticity after the onset of cracking is valid for balanced reinforcement in concrete. Although the reinforcement ratio in the WTC concrete slabs was smaller than the balanced

reinforcement ratio, the inaccuracy was not significant.

Compressive failure of the concrete slabs was modeled with the von Mises yield criterion.

4.1.2 Steel Properties and Failure Criteria

Steels used in WTC 1 and WTC 2 are listed in Table 4-1. For each steel, described by its design strength,

the table lists the estimated yield and uhimate strengths, CyRT and (Turt , respectively. Figure 4-3 shows

the mechanical properties of steel that are affected by elevated temperatures: (a) modulus of elasticity;

(b) Poisson's ratio; (c) yield strength reduction factor; (d) tensile strength reduction factor; and

(e) instantaneous coefficient of thermal expansion. A single line is plotted for the properties of all steels

in Table 4-1, since the reduction factors for elevated temperature effects on steel mechanical properties were assumed to be the same for all steels, except for the yield and tensile strength reduction factors for bolt steels. See NIST NSTAR 1-3C for a complete description of the development of the steel properties

at elevated temperatures.

When compared to concrete properties, the following observations can be made:

• Modulus of elasticity is reduced by 25 percent at 600 °C for steel and by 50 percent to 75 percent for concrete.

• Poisson's ratio increases for steel but remains constant for concrete at 0.17. However, the values for steel up to 600 °C are close to the rounded value of 0.3 that is often assumed for design purposes at room temperature.

60 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Components

Table 4-1. Steel types used in WTC 1 and WTC 2.

Material ID Description

^yRT

(psi) (psi)

1 All 36 ksi core box columns, plates, straps^ 36,720 64,470

') All 36 ksi core WT. channels, and tubes 36 ksi large area and large

inertia "rigid" beams in SAP2000 model"

37,000 63,450

J All 42 ksi box columns (1<=0.75 in.) 51,400 79,200 4 All 42 ksi box columns (0.75 in. < t <= 1 .5 in.) 47,000 74.800

5 All 42 ksi box columns (t> 1.5 in.) 42,600 70.400

6 42 ksi or 45 ksi Group 3 WF core columns 53,800 74,400

7 42 ksi or 45 ksi Group 3 WF core columns 49,000 71,040

8 42 ksi Group 4&5 WF core columns 44,200 66,640

9 45 ksi Group 4&5 WF core columns 47.800 71,074

10 AW 36 ksi Plates 1. 2. and 4 in perimeter columns 35.630 61,170

1

1

All (42, 45, or 46) ksi Plates 1, 2, and 4 in. perimeter columns 53,051 74,864

12 All 50 ksi Plates 1 . 2, and 4 in. perimeter columns. All 50 ksi channels and plates"53,991 75,618

13 All 55 ksi Plates 1. 2. and 4 with t<=1.5 m. in perimeter columns 60,8 1

7

82,558

14 All 60 ksi Plates 1. 2. and 4 w ith t<=1.25 in. in perimeter columns 62,027 87,250

15 All 65 ksi Plates 1, 2, and 4 with t<=0.5 in. in perimeter columns'' 69,642 90,442

16 All 70 ksi Plates 1. 2. and 4 in. perimeter columns 76,735 91,951

17 All 75 ksi Plates 1. 2. and 4 in perimeter columns 82,469 96,821

18 All 80 ksi perimeter columns steels, regardless of plate 91,517 99.442

19 All (85. 90. 100) ksi perimeter column steels, regardless of plate 104.783 1 15,983 20 Laclede truss web bar rounds specified as A36 38,067 59,567

21 Laclede truss chord angels (regardless of ASTM Spec) and all rounds specified as A24255.332 74,050

22 A325 bolts' 104,783 1 15,983

23 All 42 ksi Plate 3 in perimeter columns 42,600 67,216 24 All 45 ksi Plate 3 in perimeter columns 45,900 69,831

25 All 50 ksi Plate 3 in perimeter columns 51,400 74,188

26 All 55 ksi Plate 3 in perimeter columns 56,900 78,546

All 60 ksi Plate 3 in perimeter columns 62.400 83,903 28 All 65 ksi Plate 3 in perimeter columns 67.900 87,261

29 All 70 ksi and 75 ksi Plate 3 in perimeter columns 78,900 95,976

a. Steels in the following members are assumed to have the properties shown in the table: 36 ksi plates and straps (Material 1). 36 ksi channels, tubes, and '"rigid" beams (Material 2). 50 ksi channels and plates (Material 12).

b. 65 ksi steels in perimeter columns with t>0.5 in. are assumed to have the same properties as those in Material 15.

c. In the column model, stress-strain relationships of bolts are used. Note: Bolt properties are assumed to be the same as those in Material 19.

• Steel yield strength reduces to 20 percent of its initial value and its ultimate tensile strength is reduced to 40 percent at 600 °C. Concrete compressive strength is reduced to 30 percent to 50 percent of its initial value. Concrete tensile strength, which is already low, is also reduced

to 30 percent.

NISTNCSTAR 1-6. WTC Investigation 61
Chapter 4

• The instantaneous coefficient of thermal expansion for steel lies between the curves for

lightweight and normal weight concrete. If steel truss and lightweight concrete components

are at the same temperature, the steel components will thermally expand more than the

lightweight concrete. For steel beams and normal weight concrete in the core area, the

nornial weight concrete will expand more than the steel beams.

To illustrate the effect of elevated temperature reduction factors on steel strength. Fig. 4-4 shows

stress-strain curves for Material ID 1 (see Table 4-1) at room and elevated temperatures. Figure 4-4

(a) shows the low strain range and Fig. 4^ (b) shows strain levels up to 0.3. These figures show elastic and plastic strains, but for temperatures above 350 °C, the stress-strain curve beyond the elastic limit may

also include creep strains. The presence of significant creep strains relative to plastic strains depends

upon the combination of three factors: temperature, stress level, and time duration at the temperature and

stress level. Creep strain behavior for steel was based upon the creep model by Fields and Fields (1991).

The creep model was vahdated against experimental data (NIST NCSTAR 1-3D). Figure 4-5 illustrates behavior of steel elastic, plastic, and creep strains at elevated temperatures for Material ID 1. Figure 4-5 (a) shows the effect of creep strain rate for various temperatures, and Fig. 4-5 (b) compares

elastic, plastic, creep, elastic plus plastic, and total strains at T= 400 °C and constant loading for an

1,800 s duration.

The elastic-plastic behavior of steels was modeled with ANSYS "Multi-linear isotropic hardening von

Mises plasticity" material model. Creep behavior was modeled using the ANSYS time hardening implicit

creep model for nonlinear beam elements (BEAM 188 and BEAM 189) in the full floor model. For

BEAM 24 in the global models, an explicit primary creep model was used. BEAM 188 and BEAM

1 89 elements include thermal expansion, creep, and temperature-dependent material properties. When

the elements were used in the global model, numerical difficulties occurred for creep and post-buckling

behaviors which were resolved by using BEAM 24 elements.

The failure criteria for steel were defined in terms of plastic strains. The multiaxial fracture strain criterion for true stress and true strain was evaluated under a uniaxial stress condition. For element sizes

ranging from 0.025 in. to 0.75 in., a relationship between element size and equivalent uniaxial fracture

plastic strain was established. The process was repeated for steel at temperatures 20 °C, 100 "C, 300 °C,

500 °C, and 700 °C leading to a fracture criteria for various mesh sizes of components. Figure 4-6 (a)

shows the ratio of the maximum plastic strain in the direction of applied displacement-to-uniaxial plastic

strain vs. element size at various temperatures. The finite element analysis results were extrapolated to an element size of 50 in. based on the linear curve fit to the analysis results shown in Fig. 4-6 (a). Plastic

strain shown in Fig. 4-6 (b) was used as the fracture criterion for the corresponding element size in subsequent finite element analyses.

Weld properties at all temperatures were assumed to have essentially the same material properties as the

base metal of the same ultimate tensile strength (see NIST NCSTAR 1-3). High temperature properties of

weld metals were not found in the literature, however, most observed fractures in the exterior columns

were in the base metal and not the welds.

ASTM A325 bolts were used in the perimeter column, spandrel, and floor truss connections. In addition

to accounting for shear strength, the analyses included load elongation curves developed for tensile

loading of bolts.

62 NIST NCSTAR 1-6, WTC Investigation
structural Response of Components

3-10

>• 2.510

= 210

1.5-10 200 400 600 Temperature (°C)

(a) Modulus of elasticity

1000

0.36

0.34

c 0.32

200 400 600 Temperature (°C)

(b) Poisson's ratio

800 1000

400 600 Temperature (°C) Construction Steel

Bolt Steel

1000

(c) Yield strength reduction factor

400 600 Temperature {°C) Construction Steel

Bolt Steel

000

(d) Tensile strength reduction factor

1.710

1.610

li 1.510

1.4 10

o 1.310

<DOO

1.210

200 400 600 800 1000 Temperature (°C)

(e) Instantaneous coefficient of thermal expansion

Figure 4-3. Temperature-dependent properties for all steels.

NISTNCSTAR 1-6, WTC Investigation 63
Chapter 4

0 0.005 0.01 0.015 0.02 0.025 0.03

Elastic + Plastic Strain (in/in)

T=300X

T=500X

T=700°C

(a) Strain < 0.03

0 0.05 0.1 0.15 0.2 0.25 0.3 Elastic + Plastic Strain (in/in) T=RT

T=300°C

T=500°C

T=700°C

(b) Strain < 0.30

Figure 4-4. Stress-strain relationships for Material ID 1 steel.

64 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Components

T=500°C Creep T=600°C — Elastic * Plaslic Tolal

(a) Creep strain rate (b) Components of strain at T-400 °C and t= 1,800 s Figure 4-5. Strain behaviors at elevated temperatures for Material ID 1 steel.

0.01

' —— — ^—J 0.0 -I ^ • 1 0.01 0.10 1.00 10.00 100.00 0 100 200 300 400 500 600 700 Element Size (in) Temperature ("C) (a) FEA Results and Extrapolation (b) Recommended Limiting Plastic Strain

Figure 4-6. Maximum plastic strain from the finite element analysis

and limiting plastic strain.

4.2 FLOOR SUBSYSTEM ANALYSIS

The floors played an important role in the structural response of the World Trade Center towers to the

aircraft impact and ensuing fires, and were identified as a critical subsystem requiring study prior to the

development of global models for each tower. The floor system in the office area, outside the core, was a composite system consisting of a lightweight concrete slab supported by steel trusses. Within the core

area, a composite steel beam and normal weight concrete slab floor system was used.

The floor subsystem analysis included: (1) the translation, validation, and modification of ANSYS models

to incorporate nonlinear behavior, (2) evaluation of structural response under dead and live loads and

elevated structural temperatures, (3) identification of failure modes and associated temperatures and times

to failure, and (4) reduction in complexity of detailed component models for inclusion in the floor model.

NISTNCSTAR 1-6, WTC Investigation 65
Chapter 4

Component analyses included the truss seat connections, shear connectors between the truss and

lightweight concrete slab (referred to as knuckles), and a single truss and concrete slab section.

The failure modes and the failure loads for different components of the full floor subsystem were

evaluated through analysis of detailed models of those components, using either hand calculations or

finite element analyses. Models with a reduced size and/or complexity that captured the failure loads and

failure modes were then developed for each component. These modified models of component behavior were incorporated in the full floor subsystem model.

4.2.1 Description of Floor Subsystem

The WTC towers had two types of floors above the plaza and mezzanine areas, tenant floors and

mechanical floors. The structural layout and features of the tenant floors were similar throughout the

towers, with minor differences in component dimensions and core framing layouts. Core framing

changes were made to accommodate stairs, vents, and other features that varied in their locations between

floors. Most of the floors in the towers were tenant floors. Mechanical floors were located at the

skylobby levels and near the roof level of the towers (floors 7, 9, 41, 43, 75, 77, 107, 108, and 110). The

aircraft impact and ensuing fires did not directly affect any mechanical floors.

Figures 4-7 (a) and (b) illustrate the structural layout and features of Floor 96 in WTC 1 . The core area contained the elevators, stairways, mechanical shafts, restrooms, and storage areas. Office space was

generally located outside the core where the floors were supported by truss framing. Above the

77th floor, a portion of the core was used for office space since there were fewer elevator shafts and the

additional floor space could be used for tenant occupancy.

The floor system for WTC 1 and WTC 2 consisted of a lightweight concrete floor slab supported by steel trusses that spanned between the core and perimeter walls and a normal weight concrete floor slab

supported by steel beams in the core area. There were three "types" of trusses comprising the floor

system as illustrated in Fig. 4-7 (b). The trusses that spanned from the core to the exterior wall were

referred to simply as "trusses" or were sometimes called "primary" or "main" trusses, or on some contract drawings, "C32" trusses. Trusses which ran peipendicular to the primary trusses were called "bridging

trusses." At the comers of the floor areas, special trusses referred to as "transfer trusses" supported the

end reactions of several primary trusses. This section will focus on the load-carrying trusses which will

be referred to as "primary trusses."

Figures 4-8 and 4-9 illustrate a primary truss section and connection details. The primary trusses, which

were installed in pairs, were spaced 6 ft-8 in. on center and had a nominal clear span of either 60 ft- for the

long span direction or 35 ft for the short span. The trusses, commonly referred to as steel bar joists, were

fabricated using double-angles for the top and bottom chords, and round bars for the webs. The web

members protruded above the top chord in the form of a "knuckle" which was embedded in the concrete

slab and provided composite action between the trusses and the slab. Additionally, the floor system

included bridging trusses (perpendicular to the primary trusses) spaced 13 ft-4 in. on center. In the comers of the towers, the bridging tmsses acted with the primary trusses to provide two-way floor action,

where loads were transferred to both perimeter walls near the comers and the transfer tmss that ran between the core comer and the exterior wall.

66 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

(a) Single floor structural system (without slab)

0

101 100 106 1<B 112 IIS lis 121 124 127 130 133 136 136 142 14S 148 151 154 157 15S

Straps

Primary Trusses 60 ft Span

Bridging — «-

Tnjsses

Pnmary

Trusses 36 ft Span 42v

Transfer ^

Truss

Two-Way

Zone

(Shaded Area)

too.y

1 1 ..i.. .1 .i 1 ..1 . 1 .

1

I —i

——>

B—

'

501 502

b 503 504 S05i506 507 508>

fiOl 608>

602 603 604 605 606 607 Core

• 701 m 7oa«

.

—

1

702 703 704 705 706 707

• 801 B03 804 W5 806 807 —i •if—-Core Columns

901 •am

m

ariA BOS Bn7

908

"

——< 1001 1«» 1003 10O4 1 • 005 1C 06 1007

• 1008>

—

1

— I

<

—

H

7

/ <

1

400 rv 1 \V V f I 'I I 1 'I I r r

35» 357 354 3St 3U

One-Way Zone

342 330 336 333^ 3X 327 324 321 31< 315 312 30e aoe 303 am

Perimeter Columns

(b) Floor plan

Figure 4-7. Floor structural subsystem (WTC 1, Floor 96).

NISTNCSTAR 1-6. WTC Investigation 67
Chapter 4

Exterior Seat

Knuckle Top chor«iriterior Seat

Bottom chord

Truss Section

Figure 4-8. Primary truss components.

68 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

Figure 4-9. Plan view of truss seat connection, straps, and horizontal studs.

The primary trusses were supported by a seat angle at the exterior wall and a stiffened seat at the interior ends. The trusses framed into every other perimeter column (the odd numbered columns). The interior stiffened seat was welded to a steel channel that ran continuously around the core area. The steel channel was supported by the perimeter core columns through stub beam members. Each truss was attached to the

seat with a 5/8 in. bolt through a truss bearing angle. The exterior seat angles and interior stiffened seat had 1 Va in. slotted holes. At the exterior wall connection, there were three additional components that

tied the floor subsystem to the perimeter columns. A gusset plate was welded to the spandrel with a complete penetration groove weld and to the top chord of the two primary trusses with a fillet weld.

Additionally, a pair of straps with shear studs tied the primary trusses to the intermediate columns and a shear stud, welded to the spandrels between each column, anchored the concrete slab.

The lightweight concrete slab was supported by a 22 gauge, 1 Yi in. deep "Type B" steel deck. The steel deck was supported by the top chord of the bridging trusses (which were 1 Yi in. below the primary truss top chord) and by deck support angles. The concrete slab had two layers of welded wire fabric

reinforcement, and steel reinforcing bars at the perimeter of the floor, perpendicular to the primary trusses

at the knuckles, over the trench headers (conduits for electrical wiring in the floor) and at the interface

between the lightweight and normal weight concrete slabs to maintain slab continuity.

4.2.2 Truss Seats

The truss seat connections transferred floor gravity loads to the exterior and core columns and provided

lateral bracing for the columns. All seat connections were similar in design, but varied in their

dimensions and w eld sizes. For Floor 96 of WTC 1, there were seven types of interior truss seats and

eight types of exterior truss seats. The different types of interior truss seats were identified with Detail

Numbers 15, 17, 20, 21, 22, 23, and 226A; and the exterior truss seats with Detail Numbers 1013, 1111,

1212, 1311, 1313, 1411, 1511, and 1611, as shown in Fig. 4-10.

NISTNCSTAR 1-6, WTC Investigation 69
Chapter 4

f) (Tff) (!Sp (isp (JFJi

i-?'-7' L-?*-f! ui»lz!. L'*'-^' LiJ-""

Figure 4-10. Truss seat detail location on northeast quadrant of Floor 96 of WTC 1. Reproduced with permission of The Port Authority of New York and New Jersey.

Failure Modes

The failure modes and associated load and temperature conditions were identified using a detailed finite element model. Each truss seat, strap, and spandrel stud was then represented by a submodel that captured failure of these components.

Figure 4-1 1 shows a finite element model of an exterior seat connection modeled using symmetry. The

finite element analysis showed that vertical shear force was carried primarily by the stand-off plates,

while the bending moment was resisted by tensile force in the gusset plate and compressive force in the

70 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

stand off plates (see Fig. 4-8). The seat restrained the moment until the horizontal force in the connection

caused slip between the seat angle and bearing angle. Fillet welds, connecting the stand-off plates to the

spandrel, resist shear, bending, and compression, and controlled the seat capacity. The controlling failure mode was fracture of the fillet welds at this connection, which resulted in loss of vertical support.

5/8 in. Diameter

bolt

Figure 4-1 1 . Finite element model of exterior seat.

The failure modes for the truss seats were identified for vertical force (shear), horizontal tensile force,

horizontal compressive force, and combined vertical and horizontal force. Failure modes and sequences

of failure were computed for each truss seat type. The capacity of, for example, an exterior seat due to horizontal tensile (pull-in) force was determined by considering: (1) fracture of the groove weld between

the gusset plate and spandrel. (2) fracture of the fillet weld between the gusset plate and the truss top

chord, (3) tensile fracture of the gusset plate, and (4) shear failure of the boUed connection by bolt shear,

bolt bearing, tear-out, and block shear. For calculation purposes, the bolts were assumed to be centered in the slotted holes. The typical failure sequence of the truss seat was determined to be: yield failure of the

gusset plate, yielding followed by fracture of the gusset plate groove weld, truss deformation leading to bolt bearing against the slotted hole, bolt shear failure, and finally the truss slipping or "walking off the

seat. The travel distance for the truss to walk off the seat was computed to be 4 5/8 in. This failure sequence is illustrated in Fig. 4-12 as path (A) and shown in Fig. 4-13, where the relationship between

the tensile force resistance from the seat connection and the truss travel distance is plotted. In this plot,

frictional resistance between the seat angle and bearing angle was not included.

NISTNCSTAR 1-6. WTC Investigation 71
Chapter 4

Under combined vertical and horizontal forces, the failure modes were a combination of the individual

failure modes for vertical and horizontal forces. A typical interaction relationship for the capacity of an interior seat against combined vertical and

horizontal tensile forces is shown in Fig. 4-14.

Yielding of gusset

plate

(A Fracture of

groove weld

between

gusset plate

and spandrel

Fracture of

gusset plate

(C)

Fracture of

fillet weld

between

gusset plate

and top chord

Shear failure of bolts

Fracture of

fillet weld

between

gusset plate

and top chord

Truss walkoff the seat

(A) Seat details 131 1, 14 11. 151 1, and 1611 at all temperatures.

(B) Seat detail 1111 at all temperatures.

(C) Seat detail 1013 at temperatures below 100 "C.

(D) Seat details 1212 and 1313 at all temperatures, and detail 1013 at temperatures more than or equal to 100 '^C.

Figure 4-12. Failure sequence of the exterior seats against tensile force.

72 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Components

140

20

0

20T

V- 600 C

J

/— 700T

Groove weld fraetures

At travel distance 4-5/8 in. truss walks off support

Bolt shears off

Slip resistance from

bolt connection —

,

OT - 2( 300T

4000 "C

SOOT

600 T

700"C -1

(O'C

)0

0 0.2 0.4 0.6 0.8 Truss travel distance (in)

Figure 4-13. Capacity of exterior seat against tensile force (Detail 1411 in Fig. 4-10).

200

50

Horizontal tensile strength (kip)

Figure 4-14. Capacity of interior seat against vertical and horizontal force

(Detail 22 in Fig. 4-10).

NISTNCSTAR 1-6, WTC Investigation 11>
Chapter 4

Loads

Truss seat failures were analyzed for vertical and horizontal loads. The floor system gravity loads v^ere approximately 80 psf for dead load and service live load. The dead load is the weight of the floor system

and the service live loads are the loads due to occupancy that are supported by the floor, which are generally on the order of 25 percent of the design live loads. The load from the pair of trusses that each

truss seat connection supported was approximately 16 kips for the 80 psf service gravity loads (where 16

kips = Vi X 60 ft span x 6.67 ft width x 80 psf).

Computation Results

Tables 4-2 through 4-5 show the computed capacity of each truss seat detail as a function of steel temperature and loading direction. Truss seat capacities against vertical and horizontal force are presented graphically in Figs. 4-15 and 4-16, respectively. In general, the exterior seat had a greater

horizontal tensile capacity, and the interior seat had a greater vertical shear capacity. Even though the

controlling failure mode for vertical shear was weld fracture, truss seat connection failure from vertical

loads was less likely, given the capacities listed in Tables 4-2 and 4-3. The vertical load at the truss comiection of approximately 1 6 kips would have had to increase by a factor of 2 to 6 to reach failure (weld fracture) for temperatures near 600 °C to 700 °C.

able 4-2. Interior seat capacity against vertical force. Temp.

(°C)

Connection Capacity Against Vertical Force (kip)

#15 #17 #20 #21 #22 #23 #226A

20 233 233 274 229 194 194 395

100 233 233 274 229 194 194 395

200 232 232 273 228 194 194 393

300 226 226 267 223 189 189 384

400 207 207 244 204 173 173 352

500 164 164 194 162 137 137 279

600 101 101 119 100 85 85 172

700 46 46 54 45 39 39 78

800 23 23 27 22 19 19 38

900 19 19 22 18 16 16 32

1000 19 19 22 18 16 16 32

74 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

Tab e 4-3. Exterior seat capacity against vertical force.

Temp. Connection Capacity against \'ertical Force (kip)

#1013 #1111 #1212 #1311 #1313 #1411 #1511 #1611

20 94 94 111 94 94 140 193 207

100 94 94 111 94 94 140 193 207

200 93 93 110 93 93 139 192 206

300 91 91 108 91 91 136 187 201

400 84 84 100 84 84 126 172 184

500 69 69 81 69 69 102 136 146

600 45 58 53 60 45 78 84 90

700 29 26 34 27 29 35 38 41

800 14 13 17 13 14 17 19 20

900 12 11 14 11 12 14 16 17 1000 12 11 14 11 12 14 15 17

Table 4-4. Interior seat capacity against horizontal tensile force

Temp.

m

Capacit} (kip)

Shear Failure of

Bolts 20 44

100 44

200 44

300 42

400 34

500 21

600 9

700 4

800 4

900 4

1000 4

NIST NCSTAR 1-6. WTC Investigation 15
Chapter 4

Table 4-5. Exterior seat capacity against horizontal tensile force.

Temp

(°C)

Connection Detail Capacity for Horizontal Tensile Force (kip)

#1013 #1111 #1212 #1311 #1313 #1411 #1511 #1611

Fracture of Fillet Weld/

Shear

Failure of

Bolts

Fracture of Gusset

Plate

Shear

Failure of

Bolts

Fracture of Groove

Weld

Shear

Failure of

Bolts

fracture of Groove

Weld

Fracture of Groove

Weld

Fracture of Groove

Weld

20 100 104 182 126 182 126 126 126

100 138 104 181 126 181 126 126 126

200 135 103 180 126 180 126 126 126

300 130 101 174 123 174 123 123 123

400 115 93 156 113 156 113 113 113

500 84 75 117 91 117 91 91 91

600 42 49 67 58 67 58 58 58

700 20 25 32 30 32 30 30 30

800 14 16 19 18 19 18 18 18

900 13 14 17 16 17 16 16 16

1000 13 14 17 16 17 16 16 16

76 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Components

1in 1 n 20C 400C 600C 700C #1411

38 JZL

20C 400C 600C 700C #1511 n 20C 400C 600C 700C #1311

jkiaHWWapiiiJa-W la—la^ M »< w ^-m m M- m w lfl-»i--w—w m

226A,

^17 .

17 '<

p. 1 4 1111

JMII

tT.. f r

t 'v.-t 1114

15 i "J ill?' 4'

"1

101

46 n

#20

20C 400C 600C 700C #226A20C 400C 600C 700C #15 and #17

9" 84

90

20C 400C 600C 700C #1611

• lin

20C 400C 600C 700C #1111

Figure 4-15. Truss seat capacity against vertical force. Reproduced with permission of

The Port Authority of New York and New Jersey. Enhancements by NIST.

NISTNCSTAR 1-6. WTC Investigation 77
Chapter 4

Figure 4-16. Truss seat capacity against horizontal force. Reproduced with permission

of The Port Authority of New York and New Jersey. Enhancements by NIST.

Reduced Complexity Model

Break elements ( ANSYS user-defined element) were developed for use in the full floor model that captured the temperature-dependent failure modes of the truss seat connections, strap anchors, and

spandrel studs. These elements replaced the detailed 3-D solid finite element model used for the truss connection analysis, allowing a reduction in model size. A break element is a unidirectional spring element that can simulate connection failure by disconnecting

two "active nodes" when the relative displacement between two "control nodes" exceeds a specified

threshold. Temperature dependence was achieved by coupling the active nodes to a beam element with

specified thermal expansion characteristics. Failure modes that required multiple connection failures

78 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

were created with break elements using parallel and series constructs. For example, Fig. 4-17 illustrates the sequence of events that occurs for an interior truss seat under a) horizontal tensile loading and

b) vertical shear loading.

Figure 4-17 a) shows the results from analysis where the interior truss seat was subjected to a constant

vertical load and horizontal displacement increments at 500 °C. Failure of a truss seat subjected to a large

horizontal tension and small vertical shear was by boh shearing off followed by truss walking off the seat. The shear strength of the bolts controlled the truss seat horizontal tension capacity. The bolt shear by

itself did not cause the truss to lose its vertical support, but was the prerequisite for truss walking off the

seat. The travel distance for a truss to walk off a truss seat was 4 in. for an interior seat.

Figure 4-17 b) shows the results from analysis where the interior truss seat was subjected to a constant

horizontal load and vertical displacement increments at 500 °C. In this case, seat failure was governed by

fracture of the fillet welds between the vertical plate stiffeners and the channel beam resulting in loss of

both \ ertical and horizontal support.

4.2.3 Knuckles

The "knuckle" is formed by the extension of the truss diagonals into the concrete slab and provides for

composite action of the steel truss and concrete slab. The composite action is due to the shear transfer between the knuckle and the concrete slab both in the truss transverse and longitudinal directions. The

purpose of this analysis was to predict the knuckle capacity when the truss and concrete slab act compositely and to develop a reduced model of the knuckle behavior for the full floor model.

Failure Modes

Failure modes for the knuckles included: (1) horizontal shear failure by crushing of concrete over a small

region adjacent to the knuckle and (2) vertical tensile failure where the knuckle pulls a conical section of

concrete out of the slab.

Experimental Data

As part of the original truss design, Laclede Steel Company in Saint Louis, Missouri, conducted

experiments in 1967 to determine the transverse and longitudinal shear capacities of the knuckle.

Knuckles were cast into two reinforced concrete blocks confined at the comers by angles, as shown in Figure 4-18, and loaded to determine the knuckle shear capacity in both the transverse and longimdinal

directions.

The transverse tests were conducted when the lightweight concrete was 6 and 27 days old. The average

shear capacity measured was 16.9 kips per knuckle when concrete shear failure occurred. A comparable

value of 35 kips per knuckle for the WTC floor system was determined after adjusting for the strength of

in-place, mature, lightweight concrete.

NISTNCSTAR 1-6. WTC Investigation 79
Bolts shear off — Horizontal Resistance

- Vertical Resistance

Seat continues to provide vertical resistance until truss

v\/alks off the seat

2 3

Truss Travel Distance (in.)

a) failure from horizontal force

Truss walks off the seat

160

140

Q.

o

0)u

c

(0 *>

v> "w

0)

120 + /

; ; ; /

100 -

80

60

40

20

0

Horizontal Resistance

Vertical Resistance

-7"

Shear vertical shear failure

Loss of horizontal

resistance due to seat vertical shear

failure

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Applied Vertical Load / Vertical Shear Capacity

b) failure fronn vertical shear force

Figure 4-17. Results of interior truss seat model at 500 °C.

NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

The longitudinal tests, which used nonnal weight concrete, were conducted when the concrete was 28 and

96 days old. Of the five tests conducted, three reported a weld failure in the rebar loading the knuckles,

and two reported a failure in the concrete. The average shear capacity measured was 28.3 kips per

knuckle. A value of 3 1 kips per knuckle for the WTC floor system was determined after adjusting for the

strength of in-place, mature, lightv\'eight concrete.

Analysis and Results

Finite element analyses of the Laclede knuckle test set-up were conducted to compare analysis results to the measured transverse and longitudinal shear capacities. The bond between the concrete and steel was

varied in the analysis; the bars were assumed to be either full bonded or frictionless, which had a significant effect on the results.

rfCTlON ^-A Reproduced with permission of Laclede Steel, (a) Transverse shear test.

£'2 Is iW'*^<i.eoH

Reproduced with permission of Laclede Steel, (b) Longitudinal shear test. Figure 4-18. Laclede Steel Company shear tests of a knuckle.

Finite Element Analysis of Tests

The results of the finite element analyses modeling the Laclede tests are shown in Figs. 4-19

through 4-21 . Analyses were conducted to establish both the longitudinal capacity and the transverse capacity of the knuckle. The results showed significant dependence on the interface characteristics

between the steel and concrete. A plot of compressive stresses for the longitudinal shear model is shown

in Fig. 4-19. Results of the analyses for longitudinal shear, with steel-to-concrete interface either fully

bonded or completely unbonded (frictionless), are shown in Fig. 4-20. Results showed that each knuckle

had strength in the range of 15 kip to 35 kip, depending on the interface condition. When the analysis

results were compared to the test results, the fully bonded case showed better agreement.

Compressive stresses for the transverse shear model are plotted in Fig. 4-21. The small crushed regions

indicate that a pair of knuckles can be expected to behave nearly independently of each other, and,

therefore, have nearly double the capacity of a single knuckle. The transverse shear results (Fig. 4-22)

showed that transverse knuckle strength was about 24 kip for the frictionless condition with 2,500 psi

concrete, which corresponds to 39 kip for 4,100 psi concrete. For the full bonded case, the analysis was

terminated at 20 kip per knuckle before reaching the ultimate strength.

NISTNCSTAR 1-6, WTC Investigation 81
Chapter 4

Although the analysis showed the sensitivity of the results to the steel-concrete interface assumptions, it supported the shear capacities determined from test results.

'in

mi —108T? tm -!36.«33

AN

Figure 4-19. Compressive stresses in longitudinal shear finite element model

(4,100 psi concrete).

50000

o

LL.

_ ^ * V -

f ^.^^^

u

r A' / —/." h

li —

If/ #^ —^ 0 Friction Contact I

• Fully Bonded m

.00 0.02 0.04 0.06 0.08 0.10 0.12 Displacement (in.)

Figure 4-20. Shear force versus displacement from finite element model

for longitudinal shear of two knuckles (4,100 psi concrete).

82 NISTNCSTAR 1-6. WTC Investigation
structural Response of Components

Figure 4-21. Compressive stresses in transverse shear finite element model

(2,500 psi concrete).

Reduced Model

Based upon the analysis results, a reduced model with beam and break elements was developed for the

composite floor section with a single truss and the full floor models. The temperature-dependent knuckle

behavior was represented with 1 5 break elements and five beam elements.

Based on the results of the Laclede tests and the finite element analyses, the knuckle capacity for the

reduced model was estimated and is shown inTable 4-6. Steel temperatures in Table 4-6 were assumed

with a corresponding reduction in concrete temperature immediately around the knuckle of 75 °C to 150

°C. Concrete has a lower coefficient of conductivity and does not respond as rapidly to rising

temperatures as steel.

NISTNCSTAR 1-6. WTC Investigation 83
Chapter 4

Table 4-6. Knuckle shear capacity reduction for elevated temperatures.

Steel temperature

(°C)

Concrete temperature

CO

Knuckle shear capacity

(kip)

20 - 375 20- 300 30

550 450 24

725 600 19 900 750 15

4.2.4 Single Truss and Concrete Slab Section

A single composite truss with concrete slab was modeled to study failure modes and sequences of failures

under gravity and thennal loads. The thermal loading approximated a uniform heating condition, not a

fire exposure from the WTC towers. The purpose of these analyses was to determine the relative

importance of the truss and slab components and their failure modes. These results were used to develop

a reduced truss model for the full floor model that captured essential behaviors while reducing the level of

model complexity.

Failure Modes

Two possible deformation/failure modes were identified for the floor-truss section:

• Sagging of the floor section due to yielding or buckling of truss components or failures of the

knuckle/concrete interface,

• Loss of truss seat support due to combinations of vertical and horizontal loads and thermal

weakening that result in bolt shear and truss walk-off or stand-off plate weld failure at the

spandrel.

Truss weld failures were not included as a failure mode. From data gathered from the truss manufacturer,

the resistance welds between the web and chord members were found to have a greater capacity than the

members they connected.

Finite Element Model

Figure 4-23 illustrates the composite truss and concrete slab model. Symmetry allowed modeling one of

the two trusses and one half of the 80 in. composite concrete slab. The model included two perimeter

coluinns, the spandrel, the truss seat, and strap attachments. Each coluinn extended one floor above and

one floor below the floor section to account for interaction between the exterior wall and the floor section.

Each coluirm was attributed with half of its area and bending properties to account for the symmetry

reduction of the floor section. The interior and exterior truss seat connections were also included. The

truss was restrained at bridging truss locations to simulate the lateral bracing that would be provided at the bottom chord.

84 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Components

288 in.

Perimeter

Columns

/

Spandrel

/ Exterior

T

^ Truss Seat

Interior Truss Seat

\/\/\/\/\/\/\/\/\/\/\/ \T^/

Span = 713 in.

(a) Entire model

Lightweight

Concrete Slab

40 in.

29 in.

Bottom Chord

Knuckle

Top Chord

(12x1. 5x0.25 angles)

Bottom Chord

(13x2x0.37 angles)

(h) Cross section of truss and slab Cc) Tod and bottom chords and web

Figure 4-23. Composite truss and concrete slab model.

The concrete slab was modeled as an isotropic plate with a thickness of 4.3 in. (the average thickness of a 4 in. slab on 1 V2 in. metal deck) using eight-node solid elements. The Hjelm plasticity model was used

with the solid elements to allow different yield strengths in tension and compression. Slab cracking was

simulated by assuming tensile plasticity after the onset of cracking, where the reinforcement carried the

tensile load.

The top and bottom chords of the truss were modeled with quadratic finite strain beam elements with

temperature dependent elastic, plastic, and creep material properties. The chords had four elements

between panel points (a panel point is the intersection of the web diagonal and chord).

NISTNCSTAR 1-6, WTC Investigation 85
Chapter 4

Loading

Loading of the truss model consisted of gravity dead and live loads and temperature histories for the truss, truss seats, and concrete slab. The gravity loads included the weight of the structure, 8 psf superimposed

dead load (including nonstructural dead loads due to architectural items and fixed service equipment), and

13.75 psf of live load equal to 25 percent of design live load of 55 psf. The temperature was ramped from

20 °C to 700 °C in the steel members; from 20 °C to 700 °C at the bottom surface of the slab and from

20 °C to 300 °C at the top surface of the slab over a period of 1,800 s. Thereafter, the truss and concrete temperatures were linearly increased by 200 °C at 2,400 s. A linear gradient through the thickness of the

slab was assumed. Temperature loading was not applied to the columns.

Effects of construction sequence were included by applying the ANSYS "element birth and death" feature

to the concrete slab. This feature allows elements to be treated as either acting or not acting, as defined

by the user. In the first step, the self-weight of the truss members and concrete slab was applied to the

truss without the concrete slab acting (i.e., fresh concrete has no strength). In the second step, the concrete slab was reactivated (i.e., cured concrete now able to carry load), and superimposed dead load

and live load were applied.

To determine the effect of debris load on the truss behavior, the gravity load was increased until the

analysis failed to converge, signaling collapse.

Analysis and Results

Under gravity load to simulate casting of the concrete slab, the maximum calculated vertical deflection was found to be 1.7 in. downward. Note that the design camber ranged from 1 in. to 2 in. to accommodate this deflection, resulting in a slab of uniform thickness. When the superimposed dead load

and the live load were applied to the truss and concrete slab, the maximum calculated vertical deflection was 2.0 in. The maximum stress in the top chord, bottom chord, diagonal, and end diagonal strut were

14.8 ksi, 11.6 ksi, 6.7 ksi, and 15.7 ksi, respectively.

For gravity and thermal loading, the analysis was carried out statically until the solution failed to converge at which point the analysis was switched to dynamic mode with 5 percent Rayleigh damping to overcome convergence difficulties. The analysis proceeded to a temperature of 727 °C. Figure 4-24

shows the vertical displacement contour at 700 ''C. Figure 4-25 presents plots of displacement versus temperature where Fig. 4-25 (a) is the horizontal displacement at Column 143, and Fig. 4-25 (b) is the

vertical displacement at midspan after the self-weight is applied. A positive horizontal displacement

indicates that the exterior columns were pushed out, and negative vertical displacement indicates that the

truss deflected downward. For the assumed thermal loading, the analysis indicated that, at 445 °C, the

horizontal displacement at the exterior column started to decrease, and at 565 °C, the exterior columns

began to pull inward.

The plot of the deflected shape shown in Fig. 4-24, shows that compression diagonals at the core end of

the truss have buckled, and the floor system has deflected approximately 42 in.

86 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Components

My

Figure 4-24. Vertical displacement at 700 "C.

0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 Temperature (°C) Temperature (°C)

(a) Horizontal displacement at column 143 (b) Vertical displacement at midspan

Figure 4-25. Displacement versus temperature.

The truss behavior under the gravity and uniform thermal loading where the temperature was ramped up

to 727 "C can be summarized as follows:

• The stud on the spandrel and studs on the strap anchor failed in shear below 275 °C.

• The first knuckle from the interior end failed in vertical tension at around 100 °C, and the

second and third knuckles from the interior end failed in the horizontal shear at 566 °C.

• Top chords yielded above 300 °C due to the difference in coefficients of thermal expansion of

steel and lightweight concrete.

• Four compression web diagonals buckled due to high axial compressive force at 565 °C.

• The interior truss seat bolts sheared off at 566 °C.

• The gusset plate fractured and the exterior truss seat bolts sheared off at around 680 °C.

• The truss walked off the exterior truss seat at 730 °C.

NISTNCSTAR 1-6, WTC Investigation 87
Chapter 4

Reduced Model

For the full floor subsystem model, the trusses were reduced in size. The reduced model captured

essential behavior: (1) the same total horizontal reaction force under the themial loading and (2) the same

vertical deflection at midspan under the thermal loading. The reduced truss model had the following

features:

• The geometry of the truss was preserved.

• A pair of trusses was merged into one truss. Areas of truss members were doubled.

• The top and bottom chords and diagonals were modeled by 3-D linear finite strain beam

elements (BEAM 188). A member between two panel points was modeled by one element

only.

• Break elements (ANSYS user-defined elements) were used to model the following failure modes: (a) seat bolt shear failure; (b) gusset plate fracture; (c) truss walk-off; (d) diagonal

buckling/resistance weld failure; (e) failure of studs on the spandrel; and (f) weld failure between strap anchors and top chords.

• Steel had temperature-dependent elastic and plastic properties. Creep was not included.

• The concrete slab was modeled by SHELL 1 8 1 elements with temperature-dependent elastic properties.

Figure 4-26 shows the comparison between the detailed and reduced truss models. Figure 4-26 (a)

compares the vertical deflection at midspan while Fig. 4-26 (b) compares the horizontal reaction at exterior columns. Although the reduced truss model predicted buckling of diagonals at roughly 530 °C,

which is about 35 °C lower than the temperature at which diagonal buckling occurred in the detailed truss model, overall truss behavior under the uniform themial loading was found to be in good agreement

between the two models.

0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 Temperature ('C) Temperature ("C)

(a) Midspan vertical deflection (b) Horizontal reacfion force at exterior columns

Figure 4-26. Comparison of detailed and reduced truss models.

88 NISTNCSTAR 1-6. WTC Investigation
Structural Response of Components

4.2.5 Floor Subsystem Analysis

Analysis of a full floor of the World Trade Center towers involved:

• the translation, validation, and modification of finite element models in ANSYS to incorporate nonlinear behavior,

• evaluation of structural response under dead and live loads and elevated structural temperatures,

• identification of failure modes and associated temperatures and times to failure, and

• reduction of detailed component models for inclusion in the floor model.

Failure Modes

Possible deformation/failure modes of the floor subsystem that were investigated are as follows:

• Floor sagging betM'een edge supports resulting from:

- loss of stiffness and weakening of steel truss and/or concrete slab at high temperature,

- change in floor behavior from flexure to catenary action due to yielding or buckhng of

diagonal web members required for truss action, or

- loss of composite action from floor slab-knuckle failures.

Note that floor sagging betu'cen supports may cause tensile failure of the truss seats, or development of tensile forces that pull columns inward.

• Floor sagging al edge supports resulting from failure of truss seat connections at either the

interior or exterior supports.

Floor sagging at the exterior edge was obser\'ed in photographs of the east exterior wall of

WTC 2, near the impact zone. Floor sagging along one edge would have a tendency to reduce the buckling strength of columns supported by that floor and would increase demand

on other components of the floor.

• Abrupt failure of the floor truss supports due to:

- vertical shear failure resulting from debris and/or impact load of the dropping floor above,

- vertical and/or horizontal shear failure resulting from slab expansion acting on truss support seats, or

- tension failure of column truss seats from inward pull forces

NISTNCSTAR 1-6, WTC Investigation 89

1
Chapter 4

Finite Element Model

A review of floors in the impact zones of WTC 1 and WTC 2 found that floor 96 in WTC 1 was

representative of the floors of interest (see NIST NCSTAR 1-2A), and it was used for the floor analysis.

Reference structural models of Floor 96 were developed in SAP2000 for traceability to a verified data set (see NIST NCSTAR 1-2) and translated into ANSYS models (see NIST NCSTAR 1-6C). The ANSYS

floor model was used for the structural response analyses. Figure 4-27 shows an overall view of the

converted ANSYS model of Floor 96. Figure 4-28 and Fig. 4-29 show a close-up of the truss floor and

core framing, respectively.

ms mm

m

MRR 22 2094

14:41:28 Vim m. 1

Figure 4-27. Converted ANSYS model for Floor 96 of WTC 1.

90 NIST NCSTAR 1-6, WTC Investigation
structural Response of Components

Figure 4-29. Core floor beams and columns of converted ANSYS model for

Floor 96 of WTC 1.

NISTNCSTAR 1-6. WTC Investigation 91
Chapter 4

Analyses were conducted to validate the converted ANSYS floor model against the verified SAP2000

model as follows:

• Static analysis with gravity loads.

• Modal analysis using structural mass only.

Comparisons of the SAP2000 and ANSYS results for the gravity load case are given in Table 4-7. The

total reactions for the SAP2000 and ANSYS models were within 0.1 percent of each other. The

maximum slab displacement predicted by the ANSYS model was 3.2 percent smaller than that obtained

from the SAP2000 model.

Table 4-7. Comparison of SAP2000 and ANSYS results for gravity load case. SAP2000 ANSYS (BEAM 188)

Total reaction, kip 2,212.81 2.210.85 (-0.09 %)

Maximum slab displacement, in. 0.718 0.695 (-3.2 %)

Table 4-8 summarizes the comparison of the SAP2000 and ANSYS results for the modal analysis. The

total masses of the SAP2000 and ANSYS models were within 0.02 percent of each other. The dominant

natural frequency of the floor predicted by the ANSYS model was 2.5 percent higher than that obtained

from the SAP2000 model, which is consistent with the discrepancy observed for gravity displacement.

Table 4-8. Comparison of SAP2000 and ANSYS Modal Analysis Results.

SAP2000 ANSYS (BEAM 188)

Total mass. Ib sec'/in. 5448.7 5447.7 (-0.018 %)

Dominant natural frequency of floor. Hz 4.32 4.43 (+2.5 %)

Reduced Model

The converted ANSYS model was modified to incorporate the nonlinear behaviors of the components and

to reduce model complexity to achieve computation efficiency while retaining essential behaviors. The

final model used for analyzing the floor response to gravity loads and elevated temperatures included the

following modifications:

• Two adjacent trusses were combined into a single truss. The elements in the truss model had

twice the areas and moments of inertia of elements in each single truss.

• Spandrels defined as beam sections in SAP2000 model were replaced with four-node finite strain shell (SHELL 181) elements (eight elements between two columns and four elements

along the height). This modification eliminated the need for defining panel zone stiffness.

• Elastic column elements were changed to 3-D quadratic finite strain beam (BEAM 189)

elements with user-defined composite sections and nonlinear material properties.

92 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

• Section offsets of exterior columns were removed, and nodes were placed at centroids of their cross sections.

• Spandrel plates were cormected to exterior columns using rigid beam elements.

• The entire core was remeshed to produce a more uniform element size.

• Section offsets of core beams were removed to eliminate the end bending moment due to eccentricity. Core beams were placed at their centroids and connected to the slab by rigid

beams.

• Beam elements of the top chord between panel points were merged into one element to prevent the top chord from buckling and penetrating the slab.

• Web diagonals were modeled by 3-D linear beam (BEAM 188) elements.

• Coincident nodes were provided for break elements.

• Break elements were incorporated into the model to represent:

- buckling of diagonals;

- truss seat failure;

- failure of connections between primary and bridging trusses;

- failure of connections between long-span and transfer trusses;

- failure of studs connecting the slab and the spandrel;

- failure of welds between strap anchors and top chords.

The full floor model as shown in Fig. 4-30 included the following structural members:

• Exterior and core columns extending from one floor below to one floor above

• Spandrels on the floor of interest

• Concrete floor slab

• Steel floor trusses including primary and bridging trusses

• Strap anchors

• Core beams

• Deck support angles

NISTNCSTAR 1-6, WTC Investigation 93
Chapter 4

Figure 4-30. Full floor model before impact damage is included

(without concrete floor slab).

Some components were found to fail in early stages of thermal loading, causing numerical solution

difficulties, and were removed from the model. Removal of these components did not control the stability or failure mode of the full floor system under thermal loading. The removed members were:

• Deck support angles

• Shear studs on strap anchors and welds between strap anchors and truss top chords

• Bridging trusses in the one-way zone, and extending to the comer of the core

• Shear studs connecting the slab and the spandrel

• Strap anchors

The concrete slab was attached to the tmsses at the knuckle nodes. Break elements were not used to represent knuckle failure as the detailed truss and slab analyses showed that web diagonal buckling, rather

than knuckle failure, caused floors to sag. Concrete slab and trusses were always connected in the

analysis.

94 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

Four different types of break elements were used in the full floor model. Their features are summarized

in Table 4-9. There were a total of 2.028 break elements used to capture failure of the web diagonals,

truss seats and truss-to-truss connections, welds, and studs. Figure 4-31 shows a summary of break

element locations in the floor model.

Table 4-9. Types of break elements.

T\pe D.O.F. Capacities to be defined

Stiffness to be defined Description Usage in the floor model

102 UX

UY

UZ

ROTX

ROTY

ROTZ

Positive FX

Negative FX

Positive FY

Negative FY

Positive FZ

Negati\ e FZ

MX

MY

MZ

Initial and post-failure

stiffness for UX, UY,

and UZ

Initial and post-failure

stiffness for ROTX,

ROTY, and ROTZ

All force and moment

components are checked

with corresponding

capacities.

Failure of seats. Fracture of gusset

plates. Failure of connections

between primary and

bridging trusses. Failure of connections

between long-span and

transfer trusses.

103 UX

UY

UZ

Positive F

Negative F

Initial and post-failure snffness for UX. UY,

ana uz.

SRSS* of three force components is checked with

the capacity. The sign of

force is determined by the

direction specified by the

user.

Failure of strap anchor

welds.

104 UX

UY

UZ

Positive FX

Negative FX

Positive FY

Negative FY

Positive FZ

Negative FZ

Initial and post-failure

stiffness for UX. UY.

and UZ

All force components are checked with corresponding

capacities.

Failure of studs connecting the spandrel and the slab.

105 UX

UY

UZ

ROTX

ROTZ

ROTZ

Positive F

Negative F

Initial and post-failure

stiffness for UX. UY,

and UZ

Initial and post-failure

stiffness for ROTX,

ROTY. and ROTZ

SRSS of three force components is checked with

the capacity. The sign of

force is determined by the

direction specified by the

user.

Buckling of web

diagonals.

Failure of resistance weld between web

diagonals and chords.

*SRSS: square-root-of-sum-of-square

NISTNCSTAR 1-6. WTC Investigation 95

i
Chapter 4

ELEMENTS AN

SEC NUM SEP 10 2004 09:12:21

Diagonals: green

Seats: red Primary and bridging truss connections: blue

Lonc-span and transfer truss connections: Dink

WTCl FL96

Figure 4-31. Break element locations in the floor model (Floor 96, WTC1).

Floor Analysis Results

The results of the full floor analyses are given in Chapter 7, Structural Response of Major Tower

Subsystems. The floor models were analyzed for their response to impact damage and elevated

temperatures from the fires for each floor in WTC 1 and WTC 2.

Just as the floors played an important role in the response of the World Trade Center towers to both

aircraft impact and the ensuing fires, so did the exterior columns. Indeed, photographic and video

evidence shows bowing of large sections and eventual buckling of an exterior wall of each tower at the

time of collapse.

The exterior walls of the towers were made up of closely-spaced steel box columns and deep spandrel

plates. For constnaction, three story high panels, consisting of tliree columns and three spandrels, were

shop fabricated, lifted into position, and bolted together.

Component analyses were conducted to enable capturing all of the relevant failure modes for: ( 1 ) spandrel

splices, (2) bolted column splices, and (3) a single column of one, two, or three stories. A section of an exterior wall was analyzed that was three panels wide and three panels high and included

the column and spandrel splices. Thus, the model had nine columns and nine spandrel plates. The

objective of the exterior wall section model was to study the performance of the wall under the combined

96 N1ST NCSTAR 1-6. WTC Investigation

4.3 EXTERIOR WALL SUBSYSTEM
Structural Response of Components

effects of gravity and thermal loads for several conditions of lateral support. The wall section analyses

included (1) the translation and validation of a single-panel ANSYS model, (2) development of the wall

section model and (3) evaluation of structural response under gravity loads and pull-in forces resulting

from floor sag and elevated structural temperatures.

4.3.1 Exterior Wall Subsystem Description

The exterior wall of the towers was comprised of prefabricated wall panels, referred to hereafter as panels. Typical panels contained three-column segments spanning three stories with portions of the

spandrels extending one half-span past the outer columns. The panels were typically arranged such that spandrel splices between panels aligned vertically and column splices between panels offset each other by

one story.

The wall panel section selected for study was located on the north face of WTC 1 toward the east side and

included nine columns, extending \ ertically from the column splice located below Floor 91 to the column

sphce above Floor 99. and nme spandrels, extending horizontally from the spandrel splice located at midspan between Columns 149 and 150 to the spandrel splice at mid-span between Columns 158 and 159.

This exterior wall subsystem model included seven full panels and portions of four other panels.

Figure 4-32 shows the exterior wall section, and Fig. 4-33 is a schematic of an exterior box column

showing the colimin plate notation. Tables 4-10 through 4-12 give the dimensions of the column and

spandrel plates and their splice coimections.

The odd-numbered columns supported floor trusses. Pairs of strap anchors extended diagonally from the

top chords of truss pairs to the e\ en-numbered columns. The trusses and the straps braced the exterior

columns out-of-plane of the exterior wall.

NISTNCSTAR 1-6, WTC Investigation 97
Chapter 4

Figure 4-32. Exterior wall section model.

14"

Exterior Column

Reference Line"""""

Plate 1

Plate 2

4,5"

13.5"

Plate 1

Plate 3

6.5"

Spandrel

Figure 4-33. Schematic of exterior column cross-section.

98 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Components

Ta ble 4-10. Column section pro perties.

Column

Type

Plate 1 length by

thickness

(in. \ in.)

Plate 2 length by

thickness

(in. \ in.)

Plate 3

length by

thickness

(in. \ in.) 120 13.5 X 0.25 13.5x0.25 15.75x0.25

121 13.5x0.3125 13.375 x 0.25 15.75 X 0.25

122 13.5x0.375 13.25 X 0.25 15.75 X 0.25

123 13.5 x 0.4375 13.125x0.25 15.75 X 0.25

124 13.5x0.5 13 X 0.25 15.75 X 0.25

125 13.5 x 0.5625 12.875 x 0.25 15.75 X 0.25

All spandrels in exterior wall subsystem model are 52 in. deep x 3/8 in. thick.

Table 4-11. Spandre Splice Details

Spandrel

Splice

Type

Number of Bolts/Row

Total Number

Of Rows

Bolt Spacing

(No. of bolts@

spacing)

(in.)

Gage

(in.)

Overall Splice

Plate Dimensions

(in. X in. x in.)

Bolt to Centerline

of Splice

(in.)

Gap

Between

Spandrels

(in.)

Spandrel

Splice ID

101 6 2 5(a9 49x6.75x.25 1.875 0.75 101

102 8 2 3,6,3@^9,6,3 49x6.75x.25 1.875 0.75 102

111 6 4 5(S9 3 49xl2.75x.25 1.875 0.75 111

112 8 4 3,6.3(gi9,6,3 3 49xl2.75x.25 1.875 0.75 112

'Holes in spandrel are 1/4 in. larger than bolts; holes in plates are bolt + 1/16 in. or option to match spandrel holes.

Table 4-12. Column Splice Details.

Column Butt Plate Number Bolt Diameter Gage Bolt Spacing Column

Splice Type Thickness of Bolts (in.) (in.) (in.) Splice ID

(in.)

411 1.375 4 0.875 3.5 6 411

421 1.625 4 0.875 3.5 6 421

431 1.875 4 1 3.5 6 431

Bun plates have specified yield strength of 50 ksi. -Bolts are A325.

NISTNCSTAR 1-6, WTC Investigation 99
Chapter 4

4.3.2 One, Two, and Three-Story High Columns

Figure 4-34 shows the finite element model of a one-story high exterior column representing Column 151

(see Fig. 4-32). Shell elements were used to model the plates comprising the box column and the

spandrels. Rigid elements connected the center of gravity of the column to its component plates and the

spandrel at both the top and the bottom of the model. The column was simply supported in three

directions at the bottom and simply supported in the horizontal directions at the top. hicrements of axial

displacement were applied at the top of the model.

Figure 4-35 shows the variation of axial load with imposed axial displacement and resulting lateral deflection for two assumed unifonn temperature conditions, room temperature and 700 °C. Figure 4-35

also presents results of standard handbook calculations at room temperature and at 700 °C for (1) local

buckling of plates 2 and 3, (2) uniform yielding of the column, and (3) gravity load demand.

Figure 4-36 shows the local buckling defomiation of Plate 2 and Plate 3 at the maximum, load level (approximately 1,050 kip) at room temperature. Figure 4-37 shows a plastic hinge at mid-height of the

column for an axial displacement of 2 in. Figure 4-38 shows the presence of local buckling in Plate 2 and

Plate 3 at 700 °C and the maximum load (approximately 250 kip).

It can be seen from Fig. 4-35 that, at room temperature, local buckling occurs at a load that is less than

the maximum column load, but that at 700°C the column yields before it buckles locally. This figure also

shows that the column demand load of 175 kip is substantially lower than the local buckling load at room

temperature and the column yield load at 700'"C. At room temperature, the post-buckling strength

decreased rapidly; however, the reduction in the strength was much more gradual in the post-buckling

regime at 700 °C.

Axial load-displacement behaviors of two- and three-story models are also examined, and the results are shown in Fig. 4-35. As can be seen, for longer unsupported lengths and higher temperatures, the slope of

the axial load-deflection curve in the post-buckhng regime became less steep.

100 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

ANSYS

MAR 29 2004

13: 49; 28

Figure 4-34. One-story exterior column model.

1225

1050

Axial yield load of column at RT =1177 Kip

Elastic local buckling load for 1-story high column at RT =1004 Kip T

• 1 -Story (RT)

1 - story (700C)

•2-Story (RT)

2-Story (400C)

3-Story (RT)

-X— 3-Story (400C)

0.0 0.5 1.0 1.5 2.0

Vertical Displacement (in)

2.5

Figure 4-35. Load-deflection of column at room temperature (RT) and 700 °C.

NISTNCSTAR 1-6. WTC Investigation 101
Chapter 4

DISPLACEMENT

STEP=3

SUB =34 TIME = 1. 346 DMX =. 3 6137

ANSTSI

MAR 2 2 00 4 14:27: 10

Figure 4-36. Local buckling of column at room temperature.

DISPLACEMENT

STEP=4

SUB =200 TIME=3

DMX =9.169

ANSYSI

MA.R 2 2004 14:31:48

DISPL. UP TO 2in

102

Figure 4-37. Plastic hinge in column at room temperature.

NIST NCSTAR 1-6, WTC Investigation
Structural Response of Components

DISPLACEMENT ANSYSI

STEP=4

SUB =234

TIME=2. 173 DMX =.234252

MAR 29 2004

13: 45: 08

1

Figure 4-38. Deformed shape of column at maximum axial load at 700 °C.

Analysis of a section of the exterior wall of the World Trade Center towers involved:

• the translation and validation of a single panel finite element model to ANSYS and

modification to include nonlinear behavior,

• development of the exterior wall section model,

• evaluation of structural response under dead and live loads and elevated structural temperatures, and

• determination of loads that cause buckling (instability).

4.3.3 Exterior Wall Section Analysis

NISTNCSTAR 1-6. WTC Investigation 103
Chapter 4

Failure Modes

The exterior wall section model captured the following failure modes:

• Column buckling from large lateral deformations,

• Column buckling from loss of support at floor truss seats and diagonal straps,

• Failure of column splice bolts, and

• Failure of spandrel splice bolts or tearing of spandrel or splice plates at bolt holes.

Finite Element Model of Single Panel

A finite element model of a single exterior wall panel using both beam and shell elements was developed

to reduce the model size. This model was validated against a finite element shell model of a single

exterior wall panel developed by LERA (NIST NCSTAR 1-2A) by comparing the stiffnesses for a variety

of loading conditions.

Figure 4-39 shows the SAP2000 shell model of a typical prefabricated panel at Floors 79 to 82, and

Fig. 4-40 shows the reduced ANSYS model which had fewer degrees of freedom. In the ANSYS panel

model, beam elements replaced shell elements to model the columns, while shell elements were used to model the spandrels, and beam elements attached the center of gravity of the columns to the mid-plane of

its coiTcsponding spandrel component at each shell element through the depth of the spandrel.

Each of the models was subjected to the following loads at room temperature as shown in Fig. 4^1

:

• A vertical force (FZ) at the top of one of the outside columns.

• A horizontal force in the plane of the wall (FX) at the top of one of the outside columns. The

stiff beam elements distributed this shear between the tops of all three columns.

• A transverse force (FY) on the middle column at Floor 81 (middle floor).

Figure 4-42 shows the deflected shape of the panel for both the SAP2000 and ANSYS models for the case of 100 kip lateral load at the top of the panel. Table 4-13 presents the results for this and the other

two loading conditions.

Lateral and vertical displacements were found to be within 7 percent while the out-of-plane displacement

for the ANSYS model was 13 percent less than that obtained from the SAP2000 model.

104 NIST NCSTAR 1-6, WTC Investigation
structural Response of Components

Figure 4-39. SAP2000 shell model of prefabricated panel.

NISTNCSTAR 1-6. WTC Investigation 105
Chapter 4

46 in.

52 in.

92 in.

52 in.

92 in.

• Rigid truss members to transfer horizontal forces

t = 0 4375 !n., typ

10 elements between spandrels

9 elements deep, 4 elements

between columns

t = 0-4370 in. at columns,

2 elements wide

20 in.—" ^40 iir^

Figure 4-40. ANSYS model of prefabricated panel showing finite element mesh.

lateral (100 k)

vertical (10 k)

UY supports at top, typ.

UX,UY,UZ supports at base, typ.

Figure 4-41. ANSYS model of prefabricated panel showing

boundary conditions and loading.

106 NISTNCSTAR 1-6. WTC Investigation
Structural Response of Components

SAP2000 Deflected Shape ANSYS Deflected Shape

Figure 4-42. Deflection of prefabricated panel under 100 kip lateral load.

Table 4-13. Prefabricated panel validation results.

Loading Condition

SAP2000/ANSYS Difference Range

Reaction Displacements'

Lateral FX RX: -2%to+l% UX: 7%

Transverse FY RY: -6% to +7% UY: -13%

Vertical FZ RZ: -l%to+2% UZ: -7%

1 . Displacements considered at tops of columns for FX and FZ, and at points of

load application for FY.

NISTNCSTAR 1-6. WTC Investigation 107
Chapter 4

Spandrel Splice Model

Figure 4-43 shows a typical layout of the spandrel splices in the model. User-defined break elements

model the interior spandrel splice connections, and nodal couples model the exterior spandrel splice

connections. Figure 4-44 shows the modeling of an interior spandrel splice. User-defined break

elements at each node through the depth of a spandrel allow the model to capture connection failure modes including ( 1

) bolt shear, (2) tearing of the spandrel plate, and (3) tearing of the splice plates at the

bolt holes. The break elements transfer forces and moments between nodes according to the initial stiffness values until the element reaches capacity in one direction. Upon reaching one of the capacities,

the stiffness of the element in all directions changes to the corresponding failure stiffness, and the element

sheds load through other load paths.

150 151 152 153 154 155 156 157 158

Nodal Couples Break Elements Nodal Couples 1

W = west splice

Wl = west interior splice X

E = east splice

El = east interior splice

Figure 4-43. Typical spandrel splice layout for exterior wall section model.

Spandrel \ v— Splice plate

j— Spandrel ^ - ^ Jcontinues

1 ^40

1

continues

Coincident nodes and

break elements

Figure 4-44. Modeling of a typical interior spandrel splice in the

exterior wall section model.

108 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

Column Splice Model

Bolted column splices were modeled using beam elements for each of the four bolts, four pairs of contact elements at the faying (contact) surfaces between column ends (butt plates) and stiff beam elements

connecting the tops of the bolts to the contact elements. Fracture of the column bolts was included and was based on test data (NIST NCSTAR 1-3). Figure 4-45 shows a schematic view of the column splice

model. The contact elements use a coefficient of friction of 0.35. The 7/8 in. diameter column splice

bolts are pretensioned to 36.05 kip at 20 °C (AISC 1964).

Column above vjttb COMBIN37 break elements that connect column to spfice

Stiff BEAM4 elements

CONTA178 contact element

BEAM189 for bolt tension

BEAM44 for shear stability

Column Cross Section

Not to Scale

Figure 4-45. Column Splice Model used in Exterior Wall Model.

Finite Element Model of Exterior Wall Section

The single panel model was used to form a nine column by nine spandrel wall section model.

Fig. 4-46 shows the model in elevation, and Fig. 4^7 shows a typical finite element mesh for a portion

of the model. The colors of the elements illustrated in Fig. 4-47 represent the various element properties

assigned. Element properties include large deflections, plastic deformation, and creep at elevated

temperatures.

NIST NCSTAR 1-6. WTC Investigation 109
Chapter 4

Floor 99

40 in. typ. .

Li+dttt" UY supports at top, typ.

144 in. typ

Y

X

Floor 91

ROTY and ROTZ

supports at spandrel

edges, typ.

UY support at seats - each

column, each floor, typ. unless

floors specifically unrestrained

UY,UZ supports, typ. —^

^— UX,UY,UZ support at middle column

Figure 4-46. Exterior wall subsystem model with boundary conditions.

10 elements

through depth

4 elements each side of splice

BEAM189

elements for columns

2 elements at columns

4 elements

Ijetween columns, typ.

17.33 in. (gusse! plate)

25-5 in (center of stand-off)

1 element at spandrel

splice

Figure 4-47. Typical meshing of exterior wall model components.

110 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

Loading

The exterior wall section was subjected to several loading conditions as described here. Gravity loads

corresponding to dead load plus 25 percent of the design live load were applied to simulate the axial

forces in the columns and floor loads applied to the truss seats. Five thermal load conditions were

considered (see NIST NCSTAR 1-5G) that represented fire exposures and thermal insulation conditions

at several locations in the towers. The most severe of these conditions was used for subsequent analyses.

The five thermal loading conditions were labeled Cases D, E and F, DBARE (representing absence of

insulation); and El 19 (corresponding to the standard ASTM E 1 19 thermal loading). Table 4-14 presents

the various thermal load conditions, and Fig. 4^8 shows how the maximum temperature anywhere in the model varied with time for each thermal loading condition. Thermal loading condition DBARE was

selected as the most severe exposure from this group. (The designation of thermal load condition D used

here should not be confused with Cases D introduced later on in this report.)

Table 4-14. Therma loading conditions used in the exterior wall model.

Thermal

Loading

Condition

Building and

Location Columns Floors Insulation

Time

Duration

Maximum

Temperature

"C

D WTCl

South face towards West

340-348 91-99 as specified 5400 s 537 T

DBARE WTCl

Same as D

340 - 348 91 -99 none 5400 s 598 "C

E WTCl

East face towards North

221 - 229 91-99 as specified 5400 s 871 "C

E119 WTCl as specified 5400 s 418 °C

F WTC2

North face East comer

250-258 76-84 as specified 3600 s 382 "C

NIST NCSTAR 1-6. WTC Investigation 111
Chapter 4

900

800 --

700 -

rr 600

a 500

400

E 300

200

100 -

2400 3000 Time (seconds)

-DBARE

4200 4800 5400

-E119

•

Figure 4-48. Exterior wall model temperature time-histories.

Table 4-15 shows the nine cases that were analyzed for the exterior wall model that included the thermal

load cases, creep effects, and floor support conditions. Several combinations of disconnected floors were

analyzed for the effects of loss of lateral support if a floor sagged or failed and the consequent increase in un-braced length of the columns.

Two cases were analyzed to investigate the stability of the exterior wall section. In one case, forces were

applied to simulate pull-in from sagging floors to the point of instability. When trusses sag extensively,

they pull the columns inward. Results of tmss component analyses indicated approximately 14 kips of

inward pull per truss. The strap anchors helped distribute this pull to the columns that did not support

trusses. A 15 kip inward pull force was applied at each column that was laterally-unsupported, and in the

second case, with three disconnected floors, a "push-down" analysis was conducted to simulate additional

column loads being redistributed from the core. The top of the wall model was displaced downward until

instability was reached.

The loads on the model were applied in stages in the following order:

• Self weight of the exterior wall components,

• Column splice bolt preload,

• Dead load of floor construction, including superimposed dead loads,

• 25 percent of floor design live loads,

• Temperatures of fire scenarios, and

112 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Components

• Transverse pull loading from sagging trusses, or

• Imposed vertical displacements at top of columns.

Table 4-15. Analysis cases for exterior wall section model.

Analysis

Case

Thermal

Load

Condition

Bolt

Temperatures

Creep

Effects

Floor Supports Pull-in Force

PushDown

Force

D No Yes All

"> DBARE No Yes All

E Yes Yes All

4 E119 No Yes All

F I c>

6 DBARE Yes Yes All but 95 and 96

7 DBARE Yes No All but 95, 96. and 97

8 DBARE Yes No All but 95, 96. and 97

X

9 DBARE Yes No All but 95, 96. and 97

X

Exterior Wall Analysis Results

Columns Laterally Supported at All Floors (Case 1 - Case 5)

The analysis results for the five thermal load conditions in Cases 1 to 5 indicated the following:

• Spandrels had the maximum stress (strains are plastic)

• Spandrels experienced large lateral distortions and separated partially; no spandrel splice

separated completely in any of the five Cases.

• Lateral (out-of-plane) deflections of the columns were less than 1.0 in. and were due

primarily to differential thermal expansion between the columns and spandrels.

• Column and spandrel thermal expansion was unrestrained. The columns elongated between 1 and 3 in.

• General instabihty (buckling) of exterior wall columns did not occur.

Deformations observed in the spandrel plates are illustrated in Fig. 4-49.

NISTNCSTAR 1-6. WTC Investigation 113
Chapter 4

Fire Scenario D (Case 1) - Floor 94 Fire scenario F (Case 5) - Floor 99

Figure 4-49. Spandrel plate deformations.

Columns Not Laterally Supported at Two or Three Consecutive Floors (Cases 6 and 7)

The analysis results for Case 6 and Case 7 are similar and the following observations can be made for both conditions:

• Lateral deflections of the columns were less than 1.0 in. for the thermal loads.

• No plastic strain in the columns and spandrels.

• All column splices remained in contact.

• Spandrel splices separated partially on several floors; no spandrel splice separated completely

• The structures remained stable.

The out-of-plane deformations for the two cases are show in Fig. 4-50. Deformations for the case with

three floors removed were somewhat greater than for the case with two floors removed, although the

maximum deformations in both cases was less than 1.0 in.

I

"TrrTTTT~

ifiiii'ff!

«Ki

est -5. Jse

H ••COW 11 -ifSiS,

Two Floors Removed (Case 6) Three Floors Removed (Case 7)

Figure 4-50. Lateral deflections for Case 6 and Case 7.

114 NISTNCSTAR 1-6, WTC Investigation
structural Response of Components

Columns Not Laterally Supported and Pulled-In at Three Floors (Case 8)

The analysis results for Case 8 indicated the following:

• Column instability (buckling) was reached with a transverse load of 12.6 kips per column

• For a lateral load of 12.6 kips, the inward deflection of the exterior wall section was 10.2 in.

• The maximum column tensile stress of 77.2 ksi was at Floor 94 where the lateral deflection was 10.2 in.

• Column splices experienced shp or opened up at several column locations; no column splice

bohs fractured

Figure 4-5 1 shows the deflected shape of the exterior wall subsystem at the point of instability due to inward pull. The status of the column splice contact elements is shown in Fig. 4-52 and the column

splice bolt stresses are shown in Fig. 4-53.

Figure 4-51. Structural response (out-of-plane deformations) for

temperature time history DBARE and pulled-in at three disconnected floors for

Case 8. (10X displacement magnification).

NISTNCSTAR 1-6, WTC Investigation 115
Chapter 4

Floor 97

Floor 94

r r.

b b

b b' b

b 3 ^ m '. . > Blue (b)- open

Green (g)- sliding

SHALL -IS Red (r) - sticking

ANSYS 8.1

JUL Zl 2004

11: li: 58 ELEHEHT SOLUTION

STEP=31

SUB =22 TIHE=5401

HHISl

TOP

I'lIX =10. 391 SHH =1 SHX =3

1•'•-222 1.444

1.667

1 .889

Z .111

2 . 333

2 . S56

2 .778

Note: 1 contacl element pair also sliding at Col. 155 b/w Floors 98 and 99

Figure 4-52. Column splice contact element status for temperature time history and

DBARE and pulled at three disconnected floors for Case 8.

Figure 4-53. Column splice bolt stresses for temperature time history and DBARE and

pulled at three disconnected floors for Case 8.

116 NIST NCSTAR 1-6, WTC Investigation
Structural Response of Components

Columns Not Laterally Supported at Three Floors and Columns Pushed Down at Top (Case 9)

Figure 4-54 shows the deflected shape of the exterior wall subsystem due to push-down. The out-ofplane deformation as a function of push-down displacement is shown in Fig. 4-55.

The total vertical reaction (sum of the gravity and push-down loads) vs. imposed vertical displacement is plotted in Fig. 4-56. Plastic buckling was found to occur with a vertical apphed displacement of

approximately 1 .2 in. which occurred, as can be seen in Fig. 4-56, at the point at which the total vertical reaction began to decrease. It is seen from Fig. 4-56 that, for the given thermal loading condition, the

maximum total vertical reaction was approximately 2,700 kips, or an average of 300 kips on an individual

column. Individual coluinn forces are shown in Fig. 4-57 and are seen to range from approximately

250 kips to 350 kips. The gravity load on an individual coluinn was approximately 200 kips.

fCDM, scurricK

SCB -1113 TM>S405

OSDM (AW3) KSVS-0 EM< -14.785 SMX -14.785

,JUL 2$ 20D4

0

SVRLI.--9

3. 256 6.571

Figure 4-54. Structural response (out-of-plane deformations) for temperature time

history DBARE and pushdown with three disconnected floors for Case 9 (10X

displacement magnification).

Figure 4-55. Out-of-plane deformation as a function of pushdown displacement after

application of temperature DBARE with three disconnected floors for Case 9.

NISTNCSTAR 1-6. WTC Investigation 117
Chapter 4

Applied UZ at Tops of Columns (in.)

Figure 4-56. Total reaction at column base resulting from pushdown with temperature

DBARE and three disconnected floors for Case 9.

0.5 1 Applied UZ at Tops of Columns (In.)

-col 150 -

-col 157 -

-col 151 -

-col 158

Figure 4-57. Individual column reaction during pushdown with temperature DBARE and

three disconnected floors for Case 9.

The analysis results for Case 9 indicated the following:

• The maximum total reaction force resulting from self weight, column and floor loads, and

pushdown force was 4,580 kip for nine columns; the maximum individual reaction force was

approximately 570 kip.

• The maximum pushdown force was 2,710 kip; the maximum push-down force at a column was approximately 350 kip (Fig. 4-56).

• The push-down deflection was 1 .2 in. for the maximum column forces (Fig. 4-56).

• The lateral deflection at a push-down deflection of 1 .2 in. was 5.2 in. (Fig. 4-55).

118 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Components

• For a downward deflection of 2.0 in. (the maximum downward deflection analyzed) the pushdown force decreased from 2,710 kips to approximately 2,000 kip (Fig. 4-56).

• At a downward deflection 2.0 in., the lateral deflection increased from 5.2 in. to 14.7 in. (Fig. 4-55).

• Plastic buckling occurred at a downward deflection of 1 .2 in. when the pushdown force was

about 150 percent of the column gravity loads.

• At lower temperatures, thermal expansion of the inside face was insufficient to result in inward bowing of the entire exterior column. At higher temperatures, outward bowing

resulted from the combined effects of reduced steel strength on the heated inside face, which

shortened first under column gravity loads, and the lack of lateral restraint from the floors.

4.3.4 Summary

The analyses of the exterior wall section model support the following modifications for modeling of the

exterior walls:

1 . Large inelastic deformations and buckling of the spandrels at elevated temperatures could be

expected, but would not significantly affect the stability of the exterior columns and need not be accurately modeled in the global models, except at specific locations where large shear

forces were expected to exist.

2. Partial separations of the spandrel splices could occur at elevated temperatures, but would not

significantly affect the stability of the exterior columns and need not be accurately modeled

in the global models, except at specific locations where large shear forces were expected to

exist.

3. Exterior column splices could fail by sliding or opening when floors applied a pull-in force to the exterior wall that results in lateral deflection and had to be accurately modeled in the

global models.

4. Instability of an exterior wall subsystem could occur when at least three floors were

disconnected (i.e. the floor does not restrain the exterior wall subsystem) and the exterior wall

subsystem was subjected to additional vertical or lateral loads.

5. Plastic buckling by kinking with rapid reduction of load in the post-buckling regime of

columns could occur at high column loads and room temperatures.

4.4 REFERENCES

Phan, L. T. (1996), Fire Performance of High-Strength Concrete: A Report of the State-of-the-Art,

NISTIR 5934, National Institute of Standards and Technology, Gaithersburg, MD.

Phan, L. T. and Carino, N. J. (2003), Code provisions for high strength concrete strength-temperature

relationship at elevated temperatures, Materials and Structures, v 36, issue 256, p 91-98.

NISTNCSTAR 1-6, WTC Investigation 119
Chapter 4

Voce, E. 1948. "The relationship between stress and strain for homogeneous deformation," J. Inst. Metals,

V 74, p. 537.

120 NISTNCSTAR 1-6, WTC Investigation
Chapter 5

Aircraft Impact Damage

5.1 INTRODUCTION

The structural damage to each tower resuUing from the aircraft impact was estimated using a transient

finite element analysis of a Boeing 767 aircraft model crashing into a global model of the tower as described in NIST NCSTAR 1-2 and NIST NCSTAR 1-2B. Results of the impact damage analyses were

used in the fire dynamics analysis and thermal analysis (NIST NCSTAR 1-5) and the structural response

analysis (this report).

The analysis of aircraft impact was conducted using the LS-DYNA software. A range of damage

estimates was produced for each tower by varying the values of input parameters that were found to strongly influence the analysis results. An experimental design approach, using the method of orthogonal

factorial design, was used to determine the parameters that had the greatest effect on the estimated

damage. The parameter ranges were selected on the basis of the uncertainty associated with

1) interpretation of photographs and videos of the impact events, 2) material properties that were obtained

by testing or from the technical literature, and 3) live loads on the floors of the towers. Three global

aircraft impact analyses were conducted for each tower, where the input parameters were selected to represent a base case, a more severe case and a less severe case of damage estimates. Analysis results were compared with observations of damage at the exterior walls and the location of debris that exited the

buildings. Two of the three analyses, those associated with the base case and the severe case, were found

to ha\ e a reasonable match to key observations. The less severe case was not used in the subsequent fire dynamics, thermal, and structural analyses as it did not reasonably match key observables. These analyses

are fully described in NIST NCSTAR 1-2 and NIST NCSTAR 1-2B.

The fire dynamics analysis, thermal analysis, and structural response analysis all used the impact damage

results associated with the base case and the more severe case for each tower to detennine which case most closely matched key post-impact observations up to the time of collapse of each tower. For World

Trade Center (WTC) 1, these cases are referred to as Case A and Case B for the base case and the more

severe case, respectively, and for WTC 2, Case C and Case D for the base case and the more severe case, respectively.

Prior to producing the final aircraft impact analysis resuUs, initial cases referred to as Case A, and Case Bj

for WTC 1 and Case C, and Case Di for WTC 2, were run to develop experience and gain understanding

of the fire spread and growth, the rate of structural component heating, and the structural response to damage and elevated temperatures. These initial cases were used in the ftall floor subsystem analyses

described in Chapter 7. The fires and fioor slab temperatures were the same for an initial and final Case

(i.e., such as Aj and A); the thermal insulation damage estimated for the columns and floor framing

differed to a moderate degree. Floor 97 in WTC 1 had the largest change in insulation damage to the

floor trusses, where the insulation damage over 1 1 trusses increased from just beyond the core to two

thirds of the floor span. Structural analysis showed that the differences in the floor framing insulation

damage would cause little difference in the floor temperatures or in the structural behavior.

NIST NCSTAR 1-6, WTC Investigation 121
Chapter 5

1

This Chapter presents the methodology and criteria for developing input data from the aircraft impact

analysis results for the fire dynamics, thermal, and stmctural analyses, and summarizes the aircraft impact

damage data for Cases A, B, C, and D. A brief discussion of how aircraft impact affected the concrete columns in the Pentagon shows how direct debris impact dislodged the columns' concrete cover, which

has a much higher bond strength than the SFRM and gypsum materials for thermal insulation of the

structural steel. Data sets for structural analyses with Cases Aj, B„ C,, and D, are presented in Appendix

A and Cases A, B, C, and D are presented in Appendix B.

5.2 METHODOLOGY AND CRITERIA FOR DEVELOPING INPUT DATA FROM

AIRCRAFT IMPACT ANALYSIS RESULTS

The fire dynamics, thermal, and structural analyses all required input data derived from the aircraft impact

analyses. The fire dynamics analyses used estimates of damage to the floors and partition walls to describe ventilation paths and to identify the distribution ofjet fiiel and debris immediately following

impact. The thermal analysis required estimation of the areas that had dislodged insulation on the

structural components of the towers. For the structural analyses, elements that represented severed or heavily damaged floors and columns were removed from the structural models of the towers.

Damage to the exterior walls in the structural models was based on photographic and video records,

which matched reasonably well the exterior damage predicted by the impact simulations (see NIST

NCSTAR 1-2). Figures 5-1 and 5-2 show a comparison between the observed and predicted aircraft impact damage to the exterior walls for WTC 1 and WTC 2. The observed exterior damage was used in the structural analyses.

Figure 5-1. Validation of Aircraft Impact Analysis Prediction With Observations for

WTC 1 North Exterior Wall Damage.

122 NIST NCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

[DIILIDIDDIDIIDIDIID

f - - - [DIDiriDZDIZDinDIDIIDZffilJal

[DlfBLZDIMIDIZDZnillDISp

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T

i

M

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LI

_| # Correct moae ar.c rr.sgr.ituae Correct mQcl#, rnagtiitijcle itisufflclsjif 1 # Incorrect mode

# Obscured on images

Figure 5-2. Validation of Aircraft Impact Analysis Prediction With Observations for

WTC 2 South Exterior Wall Damage.

5.2.1 Core Column Damage

The damage predicted by the impact simulations was classified into four levels as shown graphically in Figure 5-3 (the colors represent plastic strain magnitude with undamaged sections in blue and strains at or above 5 percent shown in red). The classification levels were light damage, moderate damage, heavy

damage. Z-wd failed (or severed). The light damage level had low level plastic strains but no significant

structural deformations. The moderate damage level had visible local deformations of the column cross

section (e.g. local flange bending) but without lateral displacements of the column centerline. The heavy

damage level had significant global deformations that resuUed in a permanent deflection of the column

centerline. The failed columns were completely severed and could not carry any load. For details, refer to NIST NCSTAR 1-2.

The column damage criteria were applied to the aircraft impact analysis results. Tables 5-1 to 5-4 show

the column damage classifications obtained from each analysis. Refer to Fig. 4-7 for core column

numbering. Figure 4-7 shows the WTC 1 column layout with the 100 series exterior columns on the

north side; WTC 2 column layout is the same except that the 200 series exterior columns face north. WTC 1 was estimated to have 3 severed core columns and 4 heavily damaged columns for Case A, and

6 severed core columns and 3 heavily damaged columns for Case B. The WTC 1 severed and heavily

damaged columns were located at the center of the north side of the core. WTC 2 was estimated to have

NIST NCSTAR 1-6. WTC Investigation 123
Chapter 5

5 severed core columns and 4 heavily damaged columns for Case C, and 10 severed core columns and a heavily damaged column for Case D. The WTC 2 severed and heavily damaged columns were located at the southeast comer of the core. The core column damage was used in the major subsystem and global

analyses for each tower. The misalignment of the heavily damaged columns dramatically reduced their load carrying capacity. In the structural models described in this report, elements corresponding to the

heavily damaged and severed columns were removed, while those corresponding to moderately or lightly

damaged columns were retained without modifications.

Light

Damage

Moderate

Damage

Heavy

Damage

Severed

Fringe Levels

1.000e-01

9.000e-02 _|

8.000e-Q2 _l

7.000e-fl2 _

6.000e-02 _

5.000e-02 _

4.000e-02 _

3.000e-fl2 _

2.000e-02

1.000e-02 _l O.OOOe+00 I

Plastic Strains

Figure 5-3. Core column damage levels.

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

Column 506 Floors 93-94 Heavy 10 Column 604 Floors 92-96 Severed

Column 605 Floors 94-95 Moderate

Column 702 Floor 96 Moderate

Column 703 Floor 96 Moderate

Column 704 Floor 94 Heavy 18 Column 705 Floor 95 Moderate

Column 706 Floors 93-95 Severed

Column 802 Floor 96 Moderate

Column 805 Floor 94 Moderate

124 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

Table 5-2. WTC 1 Case B core column damage

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 Hea>y 19 Column 905 Floor 95 Light

Table 5-3. WTC 2 Case C core column damage

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

NISTNCSTAR 1-6. WTC Investigation 125
Chapter 5

Table 5-4. WTC 2 Case D core column damage.

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

5.2.2 Structural Damage to Floor Slabs, Core Beams, and Floor Trusses

Two types of floor damage were identified from the impact analysis results: (1) missing floor areas and

(2) severely damaged floor areas incapable of supporting loads. Figure 5-4 illustrates the floor damage

computed by the aircraft impact analysis, with WTC 1 Structural Floor 95 Case A as an example. The

damage to floor framing (i.e., trusses and beams) ranged from being severed and bent out of alignment to having localized damage to a component of the framing. Concrete slab damage ranged from crushed

areas (failure of both concrete slab and metal decking) to permanent plastic strains (failure of concrete

slab, but not metal decking). The concrete slab was failed in the red regions shown in Figure 5-4(c),

which indicated a 2 percent plastic strain or greater. At these strain levels, the concrete slab was assumed

to be severely damaged and likely exposed the supporting metal decking.

126 NISTNCSTAR 1-6. WTC Investigation
Aircraft Impact Damage

a. Floor trusses

b. Core floor beams

Impact

Column

113

c. Floor slab

Figure 5-4. Impact damage to WTC 1 Floor 95 for Case A (plan view).

NISTNCSTAR 1-6, WTC Investigation 127
Chapter 5

For the fire dynamics analysis, missing floor areas were important since they created new ventilation

paths between floors. Ventilation is a critical parameter for fire growth and spread. For the structural analyses, floor areas were removed where the intended structural function of the floor was severely

impaired or no longer available. For instance, if truss end comiections were severed, the floor framing

could not transfer its loads to the column at that point. The condition of the concrete slab was also

important as the concrete slab and floor framing were designed to act compositely. Severe damage to the concrete over more than half of a truss or a beam length was considered to cause a severely impaired floor

area.

Figure 5-5 shows an example of how the damage in Figure 5-4 was evaluated and summarized for the

structural analysis. Areas with severed floor framing and crushed concrete (indicated by red zones) were

outlined with a dashed line, indicating where floor areas were to be removed or considered missing.

Areas with misaligned framing or loss of composite action with the slab (due to damaged concrete) over more than half the member length were marked with green shading as damaged floor areas. In this example, the dashed outline and green overlay cover nearly the same area, though this is not generally the case for all floors. For analysis purposes, only severe damage was considered; isolated member damage

or small areas of concrete damage were ignored as they were considered localized damage. Damage

ranges in Figure 5-5 were delineated by boxed areas as there was insufficient data to develop criteria for

irregular boundaries.

siK !«> m !(« >«s- m w m m iia m m »« >5! !S« •» jse

Severe Floor Damage

Floor system structural i i damage '—

'

Floor system removed

701

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sgot 9CS

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707

maoe

1006

7m4

j> ^ ati MS aB M» «trTiTTTTTl as la; g? a< »• 3ie s't ifi ses ws as

I

in

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Figure 5-5. Damage to WTC 1 Floor 95 framing and slab for Case A.

128 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

5.2.3 Damage to Fire Protection for Structural Steel

The aircraft impact simulation models included not only the structural components of the towers and

aircraft, but also representations of the partition walls and building contents and fiamishings (modular

office workstations). The resuhs of the analyses included damage to the partition walls, workstations, and

structural elements. Such damage estimates were crucial for the estimation of areas with dislodged

insulation as explained in this section.

Estimates of the post-impact condition of the fire protection was based on criteria that considered damage

to structural components, building partitions, and furnishings along with the debris field as calculated

fi-om the aircraft impact analyses. Estimates for the extent of dislodged insulation considered insulation

damage to structural components only in the direct path of debris, as follows:

• Core columns had sprayed fire-resistant material (SFRM), gypsum wallboard enclosures, or a combination of both. Insulation was assumed to be dislodged from the coluiruis if they were

subject to direct debris impact that could fail wall partitions in the immediate vicinity. The

representative bending strength of building partitions in the impact simulations was 500 psi

(NIST NCSTAR 1-2). while the representative adhesive and cohesive strength of SFRM

measured in the laboratory by NIST was generally less than 12 psi (NIST NCSTAR 1-6A).

Gypsum column enclosures were also assumed to have a lesser representative strength than wall

partitions.

To consider that insulation on core columns was damaged, the predicted debris impact had to be

sufficient to fail building partitions immediately in front of the columns. If the wall partitions

remained intact in the core area after interaction with the debris field, then the insulation on core columns behind these partitions was assumed to remain intact. If wall partitions were damaged or destroyed by the debris field, then insulation on core columns behind these partitions was

assumed to be dislodged over that floor height.

• To consider that insulation on exterior columns was damaged, the debris impact had to damage or destroy office furnishings (modular office workstations) adjacent to the coluinns. If the office

furnishings remained intact after interaction with the debris field, then the insulation on the inside

face of the exterior columns behind these furnishings was assumed to remain intact. If the room

furnishings were damaged or destroyed after interaction with the debris field, then the insulation on the inside face of the exterior columns in the same vicinity was assumed to be dislodged over

that floor height. The other three faces of the exterior columns were protected by the windows

and/or aluminum cladding and were assumed to have no insulation damage.

• To consider that SFRM on floor trusses was damaged, the debris impact had to be sufficient to damage or destroy room furnishings (modular office workstations) in the same area of the

affected floor. If the room furnishings remained intact, then the insulation on the steel trusses above these furnishings was assumed to remain intact. If the room furnishings were damaged or destroyed by the debris field, then the insulation on the steel trusses above these furnishings was

assumed to be dislodged.

The insulation damage estimates were conservative as they ignored possibly damaged and dislodged

insulation in a much larger region that was not in the direct path of the debris but was subject to strong

NIST NCSTAR 1-6, WTC Investigation 129
Chapter 5

vibrations during and after the aircraft impact. A robust criteria to generate a coherent pattern of

vibration-induced dislodging could not be established due to (1) the numerical noise inherent in the

acceleration time-histories on structural components obtained from the aircraft impact analyses, and (2)

lack of data on the strength of insulation materials under such a high rate of loading with sharp peaks in a very short duration. However, there were indications that insulation damage occurred over a larger region

than that estimated. Photographic evidence showed insulation dislodged from exterior columns not

directly impacted by debris (NIST NCSTAR 1-3C). The towers underwent a period of strong impact

loading for about 0.6 to 0.7 s. Further, video analysis showed that WTC 2 vibrated for over 4 minutes

after aircraft impact with amphtudes in excess of 20 inches at the rooftop (NIST NCSTAR 1-5 A). First- person interviews of building occupants indicated that building vibrations due to aircraft impact were

strong enough to dislodge ceihng tiles and collapse walls throughout the height of both WTC towers and

to cause nearly all elevators to stop functioning (NIST NCSTAR 1-7). Due to lack of experimental data,

estimates of insulation damage ignored the possibility that the impact ofjet fuel might also result in dislodging insulation. The global structural analyses used to determine the probable collapse sequence

included some variation in the extent of dislodged insulation.

Figure 5-6 shows an elevation view of WTC 1 during the aircraft impact for Case B. As impact debris

traveled across multiple floors, it tended to fill the space between the two floor slabs. Figure 5-7 shows more detail with specific damage to framing, partitions, and furnishings on Floor 95. The floor-to-floor dispersal pattern led to the assumption that when the insulation was dislodged from direct debris impact,

it was dislodged over the full floor height. This assumption was consistent with the level of modeling

detail (i.e. insulation was not included in the aircraft impact model) and with expected thermal behavior

of a steel component if substantial portions of the insulation were removed.

As an example, Figs. 5-8 and 5-9 show plan views of WTC 1 Floor 95 for Case B impact damage.

Figure 5-8 shows the location of floor furnishings and partitions prior to impact. The extent of

furnishings and partitions in the impact models was limited to areas where interaction with the aircraft was expected due to computational limits on the number of nodes and elements in the model. Figure 5-9

shows the extent of damage to Floor 95. The area of dislodged insulation for columns and floor framing

between floor slabs 95 and 96 is indicated by the shaded overlay. Where partition walls and furnishings

remained intact, the insulation was also assumed to remain intact. Where the debris extended to the

exterior wall, the insulation on the inside surface of the exterior columns and spandrels was assumed to be

dislodged over the full floor height.

130 NIST NCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

Figure 5-6. Debris and fuel field in WTC 1 Case B analysis of aircraft impact.

Figure 5-7. WTC 1 Case B aircraft impact damage to framing, partitions, and furnishings

on Floor 95.

NISTNCSTAR 1-6. WTC Investigation 131
Chapter 5

Figure 5-8. WTC 1 Case B Floor 95 partitions and furnishings layout prior to impact

(layout provided in model only where interaction with aircraft was expected).

Debris damage limited

to extent of lavout

508 1008

Debris field limited

to extent of damage

to furnishings and

partitions

Figure 5-9. WTC 1 Case B Floor 95 partitions and furnishings layout after impact with

overlay showing extent of dislodged insulation from direct debris impact.

132 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

5.2.4 Summary of Aircraft Impact Damage

The use of the aircraft impact results by the fire dynamics, thermal, and structural analyses required

presentation of debris damage data in two formats, referred to here as 'occupancy floor' and 'structural

floor' formats. Figure 5-10 illustrates the terms occupancy floor (e.g. the elevator floor number) and

structural floor. Damage to columns, partitions, and insulation between floor slabs are presented in the

occupancy floor format, as shown in Fig. 5-1 1 . Structural damage to the composite floor (i.e. truss, beams, and floor slab) is presented in the structural floor format, as shown in Fig. 5-12. The aircraft impact, thermal, and structural analyses used both formats. The fire dynamics analyses used the

occupancy floor format, as the floor slabs provided natural boundaries for fire.

Structural Floor 96

Structural Floor 95

Slab 96

Truss 96

Slab 95 ^

Occupancy

Floor 95

Truss 95

Figure 5-10. Definition of structural floor and occupancy floor.

tOI m I-3B ion 112

Severe Floor Dninage

Floor fu eproorin«

| |

Cohiinn D;iin;ige

Severed

Hea^•^ dnmaoe

Moderate

damage

Lisht damage

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Figure 5-11. Plan view of WTC 1 Case B insulation and column damage for Occupancy

Floor 95.

NISTNCSTAR 1-6. WTC Investigation 133
Chapter 5

i3 tM -m ts^ t«

Severe Floor Damage

Floor system

structural damage

Floor system

removed

Floor 95

Figure 5-12. Plan view of WTC 1 Case B damage to Structural Floors 95.

5.3 STRUCTURAL AND FIRE PROTECTION DAMAGE TO WTC 1

This section presents summary graphics of structural and fire protection damage that were developed

using results of the aircraft impact simulations. Figures 5-13 and 5-16 show plan views of the

cumulative damage over Floors 93 to 99 for Case A and Case B, respectively. The impact damage at each floor level is shown in Figures 5-14 and 5-15 for Case A and 5-17 and 5-18 for Case B with

occupancy and structural formats. The damage graphics for Cases A and B are also presented at a larger

scale in Appendix B.

The stmctural damage in WTC 1 extended from the north exterior wall, through the central region of the

north floor area and through to the south side of the core. An exterior panel was knocked out of the south

wall by aircraft debris. Damage to the insulation from direct debris impact extended over a larger region

and included most of the north floor areas, the core, and central regions of the south floor areas. Case B

predicted more damage to core columns and a larger extent of insulation damage to the south floor area than Case A, including damage to the south exterior wall insulation on the inside face.

134 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

101 10S ins 108 11? 115 «» «! tJ4 127 1» 136 135 1« 14S )« 151 ISt 1i7 ISS

Severe Floor Dnninge

Floor flreproofing

| [

Floor sA steni |

. structural damage I I

Floor s^'stem " ' * removed '• ' "

Column Damage

Severed

Hea^y damage

Moderate

damage

Light damage

o

o

o

N

!

Figure 5-13. Plan view of WTC 1 Case A cumulative damage for Floors 93 to 98.

NISTNCSTAR 1-6. WTC Investigation 135
Chapter 5

lfi!|lllitHt|j|)jlfi!"

|jliiHllijjt|||!illi,|jil|ii,i|.:

Floor 93

-V

Floor 94

* .\

mm —

Floor 95 Floor 96

Floor 97 Floor 98

Figure 5-14. Plan view of WTC 1 Case A insulation and column damage to Occupancy

Floors 93 to 98.

136 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

-tliliiHtlt|ltl1itni|

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Floor 95

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Figure 5-15. Plan view of WTC 1 Case A damage to Structural Floors 93 to 98.

NISTNCSTAR 1-6. WTC Investigation 137
Chapter 5

Severe Floor Damage

Floor fireproofing

| |

Floor system |—

,

structural damage I I

Floor system

removed

Column Damage

Severed

Hea\y damage

Moderate

damage

Light damage

O

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I

336 333 330 327 324 331 3ie 316 812 309 306 3C3 301 Figure 5-16. Plan view of WTC 1 Case B cumulative damage from Floors 93 to 98.

138 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

Floor 97 Floor 98

Figure 5-17. Plan view of WTC 1 Case B insulation and column damage to Occupancy

Floors 93 to 98.

NISTNCSTAR 1-6. WTC Investigation 139
Chapter 5

Floor 93

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Floor 97 Floor 98

Figure 5-18. Plan view of WTC 1 Case B damage to Structural Floors 93 to 98.

140 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

5.4 STRUCTURAL AND FIRE PROTECTION DAMAGE TO WTC 2

This section presents summary graphics of structural and insulation damage that were developed using

results of the aircraft impact simulations. Figures 5-19 and 5-22 show plan views of the cumulative

damage over floors 78 to 84 for Case C and Case D, respectively. The impact damage at each floor level

is shown in Figures 5-20 and 5-21 for Case C and 5-23 and 5-24 for Case D with occupancy and

structural formats. The damage graphics for Cases C and D are also presented at a larger scale in Appendix B.

The structural damage in WTC 2 extended from the south exterior wall, through the south and east floor areas and the southeast region of the core, to the north side of the east floor area. Exterior columns were

severed by debris near the northeast comer. Damage to the insulation from direct debris impact extended

over a larger region, and included the south floor area, the central and east regions of the core, and most

of the east floor area. Case D predicted more damage to core columns than Case A, but the extent of the

insulation damage was similar.

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Se^^e^e Floor Damage

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removed

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Severed

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damage

Light damage

o

o

o

N

t

4ab4Sf 40* 44e 44& 442 «3S tiM <33 43t <r <ie 4t5 4I£ <tO& «0S 433 «i

Figure 5-19. Plan view of WTC 2 Case C cumulative damage from Floors 78 to 83.

NISTNCSTAR 1-6, WTC Investigation 141
Chapter 5

.» 8 S t J

1:1

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4. s

Floor 78

Floor 80

3« *

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Floor 82

Floor 79

4r A

J

4. i&

Floor81

Floor 83

Figure 5-20. Plan view of WTC 2 Case C insulation and column damage to Occupancy

Floors 78 to 83.

142 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

Floor 78

Floor 80

Floor 79

N

\

- " W

-V; I

Floor 81

W MH « Floor 82 Floor 83

Figure 5-21. Plan view of WTC 2 Case C damage to Structural Floors 78 to 83.

NISTNCSTAR 1-6. WTC Investigation 143
Chapter 5

Figure 5-22. Plan view of WTC 2 Case D cumulative damage from Floors 78 to 83.

144 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

Floor 78 Floor 79

N

1

Floor 80 Floor 81

!Pi<^ m m m 9» m im. m- .»r

Floor 82 Floor 83

Figure 5-23. Plan view of WTC 2 Case D insulation and column damage to Occupancy

Floors 78 to 83.

NISTNCSTAR 1-6. WTC Investigation 145
Chapter 5

If* *

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Floor 81

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Floor 82 Floor 83

Figure 5-24. Plan view of WTC 2 Case D damage to Structural Floors 78 to 83.

146 NISTNCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

5.5 OBSERVATIONS OF AIRCRAFT IMPACT DAMAGE TO THE PENTAGON

The Pentagon was impacted by an aircraft of similar size (Boeing 767) and at a speed similar to those of

the WTC towers (ASCE 2003). The impact speed at the Pentagon was about 530 mph (460 knots or 780

fps), which is similar to the aircraft impact speeds of 443 mph and 542 mph for WTC 1 and WTC 2, respectively. The aircraft completely entered the lower floors of the Pentagon and traveled along a diagonal path for approximately 230 ft. The Pentagon was constructed with a reinforced concrete frame

with columns spaced at regular inter\'als. Figure 5-25 shows a graphic depiction of the damage that was

documented after the impact and subsequent fire. An overlay of the WTC tower footprint was added to the original graphic to provide a sense of scale between the two building footprints. Figure 5-25 shows

column damage classifications similar to those described in Section 5.2.1 —severed, heavy damage

(permanently deformed laterally from the column centerline), moderate damage (some impairment of

function), and light damage (concrete cracking and spalling but no impairment of function).

It is the light damage that is of interest relative to insulation damage from aircraft impact. Reinforced

concrete columns have a concrete cover of at least one inch, and typically more, over the steel reinforcement. Figures 5-26 and 5-27 show two examples of columns with their concrete cover stripped

by the debris field douTi to the spiral reinforcement over most of the column height. It was assumed that

if the debris field could strip away concrete cover then a similar debris field would strip SFRM from steel components.

NISTNCSTAR 1-6, WTC Investigation \A1
Tests Performed to Support Design Innovations

148 NISTNCSTAR 1-1A, WTC Investigation
Aircraft Impact Damage

Source: Reproduced from "The Pentagon Building Performance Report" (2003) and use with permission of The American Society of Civil Engineers.

Figure 5-26. Typical damage to spirally reinforced columns in the Pentagon impacted

and bent by large debris (ASCE, 2003).

NISTNCSTAR 1-6. WTC Investigation 149
Chapter 5

Source: Reproduced from "The Pentagon Building Performance Report" (2003) and use with permission of The American Society of Civil Engineers.

Figure 5-27, Typical damage to spirally reinforced columns in the Pentagon not impacted

or bent by large debris (ASCE, 2003).

5.6 SUMMARY

The methodology and criteria for developing input data from the aircraft impact analysis results was

summarized for Cases A, B, C, and D. The approach for identifying severe stmctural damage to columns

and floors and insulation damage was described.

A four category classification of core column structural damage into four categories was established:

severed, heavy damage, moderate damage, and light damage. Classification criteria included plastic

strain levels and lateral deformation from the column centerline. Two types of floor structural damage

were identified from the impact analysis results: ( 1 ) missing floor areas and (2) severely damaged floor areas incapable of supporting loads.

Insulation was assumed to be dislodged from core columns only if the columns were subject to direct

debris impact that failed wall partitions in the immediate vicinity of the column. For exterior columns, the

debris impact was required to be strong enough to damage or destroy room furnishings (modular office

workstations) adjacent to the columns. For floor trusses, the debris impact was required to be strong

enough to damage or destroy room furnishings (modular office workstations) in the same area of the

affected floor.

150 N1ST NCSTAR 1-6, WTC Investigation
Aircraft Impact Damage

The insulation damage estimates were conservative as they ignored possibly damaged and dislodged

insulation in a much larger region that was not in the direct path of the debris but was subject to strong

\ ibrations during and after the aircraft impact. A robust criteria to generate a coherent pattern of

vibration-induced dislodging could not be established to estimate the larger region of damaged insulation.

The strucmral damage in WTC 1 extended from the north exterior wall, through the central region of the

north floor area, into the north side of the core. An exterior panel was knocked out of the south wall by

aircraft debris. Damage to the insulation from direct debris impact extended over a larger region, and

included most of the north floor areas, the core, and central regions of the south floor areas. Case B

predicted more damage to core columns and a larger extent of insulation damage to the south floor area than Case A. including damage to the south exterior wall insulation on the inside face.

The structural damage in WTC 2 extended from the south exterior wall, through the south and east floor areas and the southeast region of the core. Exterior columns were severed by debris near the northeast comer. Damage to the insulation from direct debris impact extended over a larger region and included the

south floor area, the central and east regions of the core, and most of the east floor area. Case D predicted more damage to core columns than Case C, but the extent of the insulation damage was similar.

5.7 REFERENCES

ASCE (2003) "The Pentagon Building Performance Report", American Society of Civil Engineers,

Reston, VA, 20191.

NISTNCSTAR 1-6. WTC Investigation 151
Chapter 5

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152 NISTNCSTAR 1-6, WTC Investigation
Chapter 6

Observations and Timeline of Structural Events

6.1 introduction

Development of the probable collapse sequence for each tower was shaped by evidence gathered in the

investigation, from photographs and videos, design and maintenance documents, and eyewitness

accounts. Data about the events following the aircraft impact were primarily obtained from three sources:

• Photographic and videographic records that had been catalogued and time stamped for the

NIST Investigation (NIST NCSTAR 1-5 A)

• Interviews of individuals in the towers who survived and individuals outside the towers who

received telephone calls from individuals in the towers (NIST NCSTAR 1-7)

• Interviews of emergency response personnel and emergency communication records (NIST

NCSTAR 1-8)

Changes in structural performance are generally difficult, if not impossible, to perceive until significant

deformation has taken place relative to the dimensions of the structure. The ability to perceive structural changes depends on the detail and resolution of the image being examined and the vantage point of the

photographer. Observations of structural performance for the WTC towers include severed components,

local deflections or buckling, possible sagging of floors, and relative alignment of columns or building

sections.

Photographic and videographic records were reviewed to identify structurally-related events. Where

possible, all four faces of a building were examined for a given event or time period to provide complete

understanding of the building response. Observations from a single vantage point can be misleading and

may result in incorrect interpretation of events. For instance, photographic and videographic records taken

from due north of the WTC 1 collapse appeared to indicate that the antenna was sinking into the roof

(McAllister 2002). When records from east and west vantage points were viewed, it was apparent that the

building section above the impact area tilted to the south as the building collapsed.

Photographs and videos provided information about events at or near the exterior walls of the towers. Events that occurred in the building interior were predicted through analytical simulations validated by

exterior observations of aircraft impact, fire dynamics, and structural response.

Evidence was used in the analyses in three ways: ( 1 ) to determine input parameters, such as the aircraft speed and direction upon impact or floor sagging at exterior windows, (2) to impose time-related

constraints upon an analysis, such as imposing observed broken windows over time to constrain the spread of fire or the extent of inward bowing of an exterior wall, or (3) to validate analysis results, such as global stability after impact and during thermal loading.

NIST NCSTAR 1-6. WTC Investigation 153
Chapter 6

Observations of structural behavior were broken into two groups: key obsen^ations and }wted

obsei-vatiom. Key observations were significant structural events that were explicitly addressed in or used to validate the structural analyses. Noted observations were events that may have been a structural response but could not be conclusively identified as to their significance to the structural response.

Key observations were used to develop a timeline of structural events for each tower. Structural analyses were used to support development of the probable collapse sequence for each tower and to develop and

refine understanding of the probable collapse sequence of events between observations.

6.2 OBSERVATIONS OF STRUCTURAL EVENTS

The following key observations were obtained primarily from photographic and videographic records and

are shown in the structural events timeline developed for each tower. Some of the observations may not have directly reflected the structural condition of the towers, but they contributed to a determination of

the extent of damage or the duration of fires in damaged areas. Other observations reported here were

derived from testing of materials recovered from the collapse site.

Aircraft Impact:

• Aircraft impact conditions - aircraft velocity, location, orientation to building

• Stmctural damage to the exterior columns and spandrels

• Stmctural stability of each tower after the aircraft impact

• Areas of debris accumulation near the exterior walls

• Locations where debris exited the buildings

• Stairwell damage

• Damage to WTC 2 east and north face floor systems (observed as draped, hanging objects in windows)

• Damage to fireproofmg on the exterior sides of the exterior columns

Fire and Thermal Analysis:

• Duration and location of fires and smoke near the perimeter of the floors

• Locations and times of window breakage

Structural Materials:

• Mechanical properties of all steel types from recovered steel

• Concrete composition from concrete samples

154 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

• Damage and fracture patterns in recovered steel

Structural Response:

• Additional damage to floor systems on the east and north sides of WTC 2 (observed as draped, hanging objects in windows)

• In\\ ard bowing of an exterior wall on the south face of WTC 1 and the east face of WTC 2

• Tilting of the building section above the impact areas as the structural collapse initiated (WTC 1 tiked to the south, WTC 2 tilted primarily to the east and somewhat to the south)

• Time to collapse initiation

• Observ ed component and subsystem failures and building movements at collapse initiation

The specific events and timelines for each tower are given in the following sections.

6.2.1 WTC 1 Structural Response Observations

Table 6-1 summarizes the timeline of structural events for WTC 1. Column 6 of the table refers to the

figure (Figs. 6-1 through 6-11) that illustrates the described event.

Key Observations

• Inward bowing of the south exterior wall was first observed at 10:23 a.m., as shown in Fig. 6-6. The bowing appeared to extend between Floors 94 to 100 and columns 305 to 359.

The maximum bowing was estimated from images to be 55 in.±6 in. at Floor 97 on the east side of the south face of WTC 1. The central area in available images was obscured by

smoke. The extent of fires observed on all faces of WTC 1 was similar, although somewhat

more extensive on the east and west faces (where short span floors were located) and similar

in extent on the north and south faces (where long span floors were located). Inward bowing was observed only on the south face.

• The time to collapse initiation was 102 minutes from the aircraft impact (9:46:30 a.m. until 10:28:22 a.m.)

• From exterior obser\'ations, tilting of the building section appeared to take place near Floor 98. Column buckling was then observed to progress rapidly across the east and west

faces

• The WTC 1 building section above the impact and fire area tilted to the south as the structural collapse initiated, as shown in Fig. 6-7. The tih was toward the side of the building that had

long span floors. Video records taken from east and west viewpoints showed that the upper

building section tilted to the south. Video records taken from a north viewpoint showed no

discemable east or west component in the tilt. A tih to the south of at least 8 degrees occurred

before dust clouds obscured the view and the building section began to fall downwards.

NISTNCSTAR 1-6. WTC Investigation 155
Chapter 6

Noted Observations

At 10:18 a.m., smoke was observed to be suddenly expelled on the north and west faces:

• North face - Floor 92, Floor 94 on the east side, and Floors 95 to 98 on west side

• West face - Floors 95 and 98 on north side; a lower floor on south side

• Smoke puffs were observed, but they occurred less frequently than in WTC 2.

• The first exterior sign of collapse (downward movement of building) was observed at Floor 98. From a northwest viewpoint (there were no useful south views), large amounts of

smoke and dust were first expelled from Floor 98 across the north and west faces. Smoke and

dust were also observed being expelled from the east face.

Table 6-1. WTC 1 Timeline of Observed Structural and Fire Events.

Start Time Floors Face Columns Figures Event Description

1 8:46:26 93-99 N 109-152 Fig. 6-1 WTC 1 was impacted by a Boeing 767 between Floors 93 to 99 and Columns 109 to 152. Fig. 6-1 (taken

nearly an hour after impact) shows Columns 120 to 159.

2 94-96 S 329 Fig. 6-2 A perimeter wall panel face was knocked out by the

aircraft nose or landing gear at the center of the south

between Floors 94 to 96

3 92-95 N 130-151 Fig. 6-3 SFRM was knocked off the exterior sides of perimeter

columns; the pattern of damage was irregular

4 9:25:28 Fig. 6-4 Fire was observed only on the west side of the south

face (note debris under missing panel)

5 9:40 S 301-323 Fig. 6-5 No inward bowing of perimeter columns was visible

6 10:22:59 95-99 s 308-

326+

Fig. 6-6 Inward bowing of the south perimeter wall was visible from Floor 95 to about Floor 99, with a maximum

inward bowing of ~ 55 in. at Column 315 and Floor

97

7 10:28:18 Fig. 6-7

Fig. 6-8

Fig. 6-9

Pressure pulses of smoke were pushed out the west

face at its north edge and center; Smoke and debris clouds were pushed out the north, east, west faces at Floor 98; Fire came out windows on the north, east, west, and south faces between Floors 92 to 98 and

Floor 104

10:28:20 Fig. 6-10

Fig. 6-11

WTC 1 began to collapse. The first exterior movement

was at Floor 98. Rotation of the building section above

the impact and fire zone to at least 8 degrees to the south occurred before the building section began to fall vertically.

156 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

Figure 6-1. Initial aircraft impact damage on WTC 1 north face.

NISTNCSTAR 1-6, WTCInvestigation 157
Chapter 6

Figure 6-2. Initial aircraft impact damage on WTC 1 west and south faces minutes after

impact (exact time of image is unknown).

158 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

150 145 140 135

Figure 6-3. SFRM knocked off north exterior columns. Arrows show where fireproofing

was damaged or missing.

Figure 6-4. South face of WTC 1 with fire visible only on the west side at 9:25. Arrow

shows region where debris pile under missing panel was observed.

NISTNCSTAR 1-6. WTC Investigation 159
Chapter 6
Observations and Timeline of Structural Events

Measurements were based on calibration measures shown on the w est face Foreshortening into depth of field across the south face of 1 7% was included in the measurements Measurement error w as at least +/- 6 inches Figure 6-6. WTC 1 exterior columns bowing inward across most of the south face

between Floors 95 to 97 (or 98) at 10:22:59 a.m. Note buckled panel at SW corner.

NISTNCSTAR 1-6, WTC Investigation 161
Chapter 6

Fire expelled on

east face

: ,1 t

I Tim Mr r; . Mik e BaJlou -'^ 2001"

I 111 BIHllil 1 II

1 II 1—

Smoke and debris

expelled on north face at Floor 98

in Mam I Mtke BaWou 200

WTC 1 building

section above impact

damage zone tilts to the south

/Tim Ma-r Mske^a.fou 2ij6i

"

Figure 6-7. Expulsion of smoke and debris at WTC 1 Floor 98 on the east, north, and

west faces.

162 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

NISTNCSTAR 1-6. WTC Investigation 163
Chapter 6
Observations and Timeline of Structural Events

Figure 6-10. Rotation of WTC 1 building section above the aircraft impact zone toward

the south as viewed from due north. Note that there is no tilt in the east or west

directions.

NISTNCSTAR 1-6. WTC Investigation 165
Chapter 6

Figure 6-11. WTC 1 tilt to the south of approximately 8 degrees was measured before

smoke and debris obscured view. Note view is from west and tilt is directly south.

166 NISTNCSTAR 1-6, WTC Investigation
Obseivations and Timeline of Structural Events

6.2.2 WTC 2 Structural Response Observations

Table 6-2 summarizes the timeline of structural events for WTC 1 . Column 6 of the table refers to the

figures and table (Figs. 6-12 through 6-26 and Table 6-3) that illustrate the described event.

Key Observations

• Following the aircraft impact and fireballs, hanging objects were observed through the

windows of the east and north faces. These objects appeared to be floor slabs and were

observ'ed to change shape and/or length over time and extend across approximately half of the

east face. The hanging objects suggest that there was structural damage to WTC 2 Floor 83

along the east face and to Floors 81 to 83 of the north face near the northeast comer.

• Inward bowing of the east wall was first observed at 9:21 a.m. The inward bowing was

approximately 10 in. at Floor 80 and extended between Floors 78 and 83 and Columns 304

and 344. The remaining portion of the face to the south of Column 344 was not included in the image. The bowing appeared to extend over a large fraction of the east face and to be

greatest near the center of the face. Fires were more extensive along the east face (where long

span floors were located) and at the east side of the north and south faces (where short span

floors were located). Fires were not observed on the west face (where long span floors were

located). Inward bowing was observed only on the east face.

• An increase of the inward bowing of the east wall was observed at 9:53 a.m. The inward

bowing appeared to extend between Floors 78 and 84 and Columns 305 and 341. The

remaining portion of the face to the south of Column 344 was not included in the image. The

greatest bowing was approximately 20 in. ±1.0 in. at Floor 80 on the east face of WTC 1.

• Collapse initiated 56 minutes after the aircraft impact (9:02:59 a.m. to 9:58:59 a.m.).

• From a northeast viewpoint, initial downward motion was observed at several columns as they moved inward on the north side of the east face. From exterior observations, tilt of the

building section above the impact and fire area appeared to take place near Floor 82. Column

buckling was then seen to progress across the north face.

• The building section above the impact and fire area tilted to the east and south as the

structural collapse initiated. Estimates from photographs indicated that there was

approximately a 3 to 4 degree tilt to the south and a 7 to 8 degree tilt to the east prior to

significant downward movement of the upper building section. The tilt to the south did not

increase any further as the upper building section began to fall, but the tilt to the east continued up to 20 to 25 degrees before dust clouds obscured the view.

Noted Observations

• A fireball on the east face was observed coming from Floor 82. Fireballs on the north face were observed coming from Floors 79 to 82. The deflagration prior to the fireballs may have

caused a pressure pulse to act on floors above and below.

NISTNCSTAR 1-6. WTC Investigation 167
Chapter 6

• A 'cold spot' on the north face, where little or no fires were observed, may indicate that Floors 82 and 81 had disconnected and dropped over a 12 window span along the north face.

• Molten material pouring from the northeast comer indicated that Floor 8 1 on the east side of

the north face may be shifting. If the substance was molten aluminum, that would have

required temperatures on the order of 500 °C or higher.

• Numerous puffs of smoke may indicate internal changes in architectural or structural features.

• Outward bowing of the spandrel near the center of the north face on Floor 79 was observed

near columns 237 and 238.

• As the portion of the building above the impact area tilted to the south and east:

1 . A kink formed at the southeast comer near Floor 1 06 as the upper building section

tilted and collapse initiated.

2. The southeast comer also kinked approximately midway between the impact and fire zone and the kink near Floor 106 as the upper building section tilted.

Hanging Objects

The hanging objects observed on the east face and north face of WTC 2 (listed in the Key Observations)

appeared to be the exterior edges of the floor slabs.

The slab thickness was nominally 4 inches over a fluted deck with 1.5 in. ribbing. Scaling of the object in the windows found the depth to be approximately 4 in. to 6 in. The concrete slab was reinforced with two

layers of welded wire fabric and had a flexural stiffness that was greater than other items that might have

been draped in the windows, such as ductwork. The drape of the object was consistent with a floor

flexural stiffness. There was ductwork between the damper at the lower chord of the tmss and the floor

slab at the exterior wall. However, for the ductwork to be draped in the windows, all the dampers would

have had to fail while the floor slab would have had to remain in place. Such a sequence of events is unlikely. Also, such ductwork would be light and hang with a deeper drape. From these observations, it appears that the hanging objects were the exterior edges of floor slabs.

168 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

Table 6-2. WTC 2 Timeline of Observed Structural and Fire Events

Start TimeFloors Face f^oliiinns Figures

and

Tables

HiVciii ucscripilun

I 9:02:59 77-85 s 404-443 Fie 6-12 vv 1 Z, Was lllipaL ICU Uy a DUClIlu /U/ UClWCCll Floors 77 and 85 and Columns 404 and 443.

2

0 1 h1 A r\ '2/10 rig. D-li

Tbl. 6-3

Ine edge or Moor 83 appeared to be draped in Floor 82 windows between Columns 310 and

342.

3 910:01 79-83 N.E Fig. 6-15 Hanging object in windows of Floor 79 that appears to be the edge of Floor 80. 4 9:1 1:14 79-82 N Fig. 6-14

Tbl. 6-3

Debris piles were visible at windows where

fires were burning at Floor 79, Columns 23 1 to 241, and Floors 81 to 82 at the northeast comer. Hanging objects noted with arrows.

5 9:14:03 79-82 N 237-254 Fig. 6-16 Missing SFRM on several columns.

6 9:21:29 78-82 E 302-342 Fig. 6-17

Fig. 6-18

Inward bowing of east face, maximum

deflections of 10 in. at Floor 80.

7 9:53:04 -78-82 E 318-334+ Fig. 6-19

Fig. 6-20

Bowing in of columns, maximum deflections of

20 in. at Floor 80.

8 9:55:04 83 E 310-342 Tbl. 6-3 Floor edge is draped in Floor 82 windows

between columns 310 and 342.

9 9:58:55 E Fig. 6-21 Perimeter columns bowing inward on east face.

10 9:58:59 WTC2 begins to collapse.

1

1

9:58:59 78-83 E 324-359 Fig. 6-22

Fig. 6-23

Columns spring back from bowing as collapse

initiates on east face near NE comer (every 3'^''

panel).

9:58:59 E,N,W Smoke and debris clouds are expelled from

Floor 81 on E,N,W faces of the building.

9:58:59 S Fig. 6-24

Fig. 6-25

Building section above the impact area tilted to the east and south. Tilting appears to take place

around Floor 82. Rotation of approximately 4

to 5 deg to the south and 20 to 25 deg to the east occurred before the building section begins

to fall vertically.

9:59:02 - 106 SE + E Fig. 6-26 Kink (and offset) about Floor 106 which

propagates across the east face where degrades

into a gentle curve on the northeast corner; indicates that the kink did not precede the

initiation of the global collapse.

NISTNCSTAR 1-6. WTC Investigation 169
Chapter 6

Table 6-3. Possible floor damage observed in photos of WTC 2 windows.

East Face

Floor Estimated Left

Intact Column

Connection

Left Visible Floor

Location

Right Visible Floor

Location

Estimated Right

Intact Column

Connection

9:03:42

83 343 340 321 317

9:38:22

83 317 317 311 308

9:55:04

83 346 343 310 309

North Face

Floor Estimated Left

Intact Column

C onnection

Left Visible Floor

Location

Right Visible Floor

Location

Estimated Right

Intact Column

Connection

9:10:01

82

81 251 248 241 241

80

9:14

83

82 247 243 238 237

81 251 248 241 237

255 254 252 251

80

9:58:37

83 259 259 250 248

82 254 251 247 235

81 251 249 245 235

80 234 234 229 226

258 255 252 249

*Floor is not visible beyond this point, separation from wall was truncated at the closest intact point where there appeared to be no damage beyond the burning debris pile on Floor 79.

170 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

Figure 6-12. Aircraft Impact into WTC 2 and fireball, view from the east.

NISTNCSTAR 1-6. WTC Investigation 171
Chapter 6

Figure 6-13. Hanging object (noted by arrows) in east windows of Floor 82 appears to be

edge of Floor 83.

Figure 6-14. Debris piles at windows where fires are burning at the northeast corner.

172 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

I

Figure 6-15. Hanging object (noted by arrows) in north windows of Floor 79 appears to

] be edge of Floor 80.

j

NISTNCSTAR 1-6, WTC Investigation 173
Chapter 6

3^J5 301 © 2001 Lyie Owerko/Wonderlust

Figure 6-16. Image showing damage to fireproofing on east face of WTC 2 due to

internal impact. Red arrows highlight areas where fireproofing has been damaged. The

blowup to the right shows a column where red Tnemec primer paint is visible.

174 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

© 2001 Allen Murabayashi

Figure 6-17. WTC 2 exterior columns bowing inward across north side of the east face

between Floors 77 and 83 at 9:21 a.m.

3S9 355 350 MS 320 315 310 305 301 0 0 0

83 0 III It llllllilll

82 6 5 6 7 6 d

81 6 5 5 4 6 7

til 9 6 8 9 5 4

80 4 4 4 4 4 9

HI 9 9 8

III 8

III 8 5

79 5 2 0 2 2 2 2 4 5 2 6 5 6 7 7 8 7 8 7 5 7 3 3 4 3 2 2 2

78 4 Q 0 0 0 0 0 3 3 3 3 3 ? 3 3 4 2 3 3 3 3 0 0 u (1 0 d d

77 1) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 !

0 u 0 d u 0 d 0 d - ... -: .: "J

83

82

81

80

79

78

77

Image blocked by

another building

1. East face of WTC2 inward deflection (in.)

2. Estimated uncertainty: +/- 1 inch

3. Empty regions have no data (smoke, damaged aluminum, could not

establish true vertical, etc.)

Figure 6-18. Inward Bowing of east Face of WTC 2 at 9:21 a.m.

NISTNCSTAR 1-6. WTC Investigation 175
Chapter 6

Figure 6-19. Inward Bowing of east Face of WTC 2 at 9:53 a.m.

176 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

3S 355 305 301

83

82

81

80

79

78

77

r; 0 0 0 0 0 0 0

„, 6 7 4 7 4 5 0 0 0 0 0 0 0

3

] II ( 7 5

5' 7 7 8 (> 6 9 7 -

-r

-• - -

I) III

-

-J - -

III

- -

-;-

-

- - r

,-

- 5 0 0 d

-

- -

- -~6

1'0 7 8 9 9 8 9 9 - _- 9 8 7

.-

9 6 6 3 -5 3 0 0

8 6 6 5 5 5 S 5

6^ 5 6 8 8 s 4 4 4 4 3 3 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 T 0 0 0 r 0 5 4 4 3 T 0

=:

U 2 {) 0 0 0 U 0 0 0 0 0 0 0

'SI -

r. z

83

82

81

80

79

78

77

Image blocked by

another building

1 . East face of WTC 2 inward deflection (in.)

2. Estimated uncertainty: +/- 1 inch

3. Empty regions have no data (smoke, damaged aluminum,

could not establish true vertical, etc.)

Figure 6-20. Inward Bowing of east Face of WTC 2 at 9:53 a.m.

NISTNCSTAR 1-6, WTC Investigation 111
Chapter 6

Figure 6-21. WTC 2 exterior columns bowing inward across the east face between

Floors 77 and 83 at 9:58:55 a.m.

178 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events

Figure 6-22. View of WTC 2 buckling of east wall near northeast corner as collapse

initiates from northeast.

NISTNCSTAR 1-6, WTC Investigation 179
Chapter 6
Observations and Timeline of Structural Events
Chapter 6

(e) (f)

Figure 6-25. View of upper building section of WTC 2 tilting to the east from the

northeast.

182 NISTNCSTAR 1-6, WTC Investigation
Observations and Timeline of Structural Events
Chapter 6

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184 NISTNCSTAR 1-6, WTC Investigation
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