NIST NCSTAR1-5G

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Fire Structure Interface and Thermal

Response of World Trade Center

Towers

Kuldeep Prasad

Howard Baum

National Institute of Standards and Technology • Technology Admlnisfrolion U.S. Department of Commen

NISTNCSTAR1-5G

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Fire Structure Interface and Thermal

Response of the World Trade Center

Towers

Kuldeep Prasad

Howard Baum

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, w/hichever 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 1-5G

Natl. Inst. Stand. Techno!. Natl. Constr. Sfty. Tm. Act Rpt. 1-5G, 338 pages (September 2005)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2005

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001
Abstract

The collapse of the World Trade Center (WTC) towers on September 11, 2001, resulting from a combination of aircraft impact damage and subsequent fires, was studied as part of the Federal Building

and Fire Safety Investigation of the WTC Disaster. This report documents the evolving thermal state of

the structure on the focus floors of each tower. A methodology was developed to couple the thermal

response of the towers to the fire dynamic simulations. Heat transfer to sub-grid scale structural elements was computed using a simple radiative transport model that assumes the compartment can be locally

divided into a hot, soot}' upper layer and a cool relatively clear lower layer. Properties of the two layers were extracted from temporal averages of the results of the fire simulations. The model predictions were

found to compare favorably with measurements from a series of large-scale experiments. Exploratory

studies were conducted to estimate the role of fireproofing thickness and sensitivity of the results to input

parameters. The methodology was subsequently used extensively in National Institute of Standards and

Technology 's (NIST"s) WTC Investigation to predict the thermally induced structural response to spatially and temporally developing fires. Finite element models were constructed for the steel structural elements and the fireproofing that covers the floor trusses, core beams, perimeter and core columns as well as the concrete slab on floors 92-99 of the North Tower and floors 78-83 of the South Tower.

Structural and fireproofing damage due to aircraft impact on the various floors of each tower was

incorporated into the models. The three dimensional time dependent thermal response of the structural elements was generated m a format that is consistent with the structural models and the finite element

analysis software. Four global simulations, two each for WTC 1 and WTC 2 are reported for prediction

of thermally induced structural response and collapse of the towers. Based on the resuhs of the global

simulations, fireproofing thickness and fireproofing damage due to aircraft impact was identified as the

single most important parameter that had the largest effect on steel temperature and on the thennally

induced structural response.

Keywords: Fire Structure Interface, fireproofing thickness, radiative heat transfer, World Trade Center.

NIST NCSTAR 1-5G. WTC Investigation iii
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iv NISTNCSTAR 1-5G, WTC Investigation
Table of Contents

Abstract iii

List of Figiires xi

List of Tables xxi

List of Acronyms and Abbreviations xxv

Preface xxvii

Acknowledgments xxxvii

Executive Summary xxxix

Chapter 1 Sub-Grid Scale Model for Radiative Heat Transfer 1

1.1 Introduction 1

1 .2 Radiative Transport Modeling 3

1 .3 Plane Layer Analysis 4

1 .4 Convective Heat Transfer 9

1.5 Summmary 9

Chapter 2 Model Validation and Accuracy Assessment 11

2. 1 Over\'iew of Experiments 1

2.1.1 Test Matrix 12

2.1.2 Steel Elements 13

2.1.3 Sprayed Coating of the Steel 13

2.2 Model Simulations 14

2.3 Results 17

2.3.1 Testl 17

2.3.2 Tests 20

2.3.3 Quantification of Agreement between Models and Measurements 24

2.3.4 Experimental Uncertainty and Model Sensitivity of the Steel Temperatures 25

2.4 Summary 28

Chapter 3 Exploratory Studies on Thermal Response of Structural Elements 29

3.1 Sensitivity Study on Core Columns 29

NIST NCSTAR 1-5G. WTC Investigation v
Table of Contents

3.1.1 Light Box Shape Core Columns 29

3.1 .2 Heavy Box Shape Core Columns 33

3.2 Statistical Variability in Fireproofmg Thickness 34

3.3 Selective Fireproofmg Damage on One Face of the Column 35

3.4 Sensitivity Study on Wide Flange Columns 36

3.5 Sensitivity of Thermal Response to Fireproofmg Geometry 37

Chapter 4

Perimeter Columns 55

4. 1 Typical Floor Plan 55

4.2 Finite Element Models of the Perimeter Column and Fireproofmg 55

4.3 Initial and Boundary Conditions 59

4.3.1 Radiative Flux 60

4.3.2 Convective Flux 60

4.3.3 Re-Radiation 61

4.3.4 Multi-floor Simulations 61

4.4 Incident Heat Flux 63

4.5 Thermal Response of Columns to Realistic Fires 65

4.5. 1 Temperature Contours 65

4.5.2 Fireproofmg and Steel Temperature 68

4.5.3 Time-Temperature Plots 68

4.6 Modeling Fireproofmg and Structural Damage on Perimeter Columns 71

4.6.1 Fireproofmg Damage 71

4.6.2 Structural Damage 73

4.7 Interface between Thennal and Structural Models 74

4.7. 1 Mapping of Thermal Data 75

4.7.2 Procedure for Mapping Thermal Data onto the Structural Model 76

4.7.3 Full Floor and Multiple Floor Mapping of Thermal Data 78

Chapter 5 Floor Trusses and Core Beams 81

5 . 1 Typical Floor Plan 81

5.2 Finite Element Model of the Floor Truss 82

5.2.1 Primary Truss 82

5.2.2 Connections 84

5.2.3 Bridging Truss 85

VI NISTNCSTAR 1-5G, WTC Investigation
Table of Contents

5.3 Structural and Fireproofing Damage 88

5.3.1 Incorporating Structural Damage on Floor Trusses or Core Beams 88

5.3.2 Incorporating Fireproofing Damage on Trusses or Core Beams 88

5.4 Initial and Boundary Conditions 89

5 . 5 Time-Temperature Profiles 91

5.6 Interface between Thermal and Structural Models 92

5.6.1 Mapping of Thermal Data 92

5.6.2 Procedure for Mapping Thermal Data 93

Chapter 6

Concrete Slabs 97

6. 1 Fmite Element Models of Concrete Slab 97

6.2 Initial and Boundary Conditions 99

6.3 Temperature and Temperature Gradient in Concrete Slab 103

6.3.1 Surface Temperature 103

6.3.2 Temperature Profiles through the Concrete Slab 104

6.3.3 Time-Temperature Plots 107

6.4 Structural Damage to the Concrete Slab 108

6.5 Mappmg of Thermal Data 109

Chapter?

Core Columns 111

7. 1 Finite Element Model for Core Columns 1 1

7.1.1 Wide Flange Columns Ill

7.1.2 Box Shaped Columns 112

7.1.3 Fireproofing Thickness for Core Columns 1 1

7.2 Initial and Boundary Conditions 1 14

7.3 Incident Radiative Flux 115

7.4 Core Column Temperature 117

7.4. 1 Temperature Contours 1 1

7.4.2 Time Temperature Profiles 118

7.5 Fireproofing and Structural Damage on Core Columns 123

7.6 Mapping of Thermal Data 124

7.6.1 Algorithm 124

7.6.2 Full Floor Mapping of Thermal Data 124

NISTNCSTAR 1-5G, WTC Investigation vii
Table of Contents

Chapters

Global Thermal Response of WTC 1: Case A 127

8.1 Overview 127

8.1.1 Aircraft Impact Analysis 128

8.1.2 Fire Dynamics Simulations 129

8.2 Fire Structure Interface 130

8.2.1 Floor 93, WTC 1 Case A 133

8.2.2 Floor 94, WTC 1 Case A 135

8.2.3 Floor 95, WTC 1 Case A 138

8.2.4 Floor 96, WTC 1 Case A 143

8.2.5 Floor 97, WTC 1 Case A 165

8.2.6 Floor 98, WTC 1 Case A 169

8.2.7 Floor 99, WTC 1 Case A 171

8.3 Data Transfer for Structural Analysis 171

Chapters

Global Thermal Response of WTC 1: Case B 179

9.1 Overview... 179

9.1.1 Aircraft Impact Analysis 1 79

9. 1 .2 Fire Dynamics Simulations 1 80

9.2 Fire Structure Interface 180

9.2.1 Floor 93, WTC 1 Case B 181

9.2.2 Floor 94, WTC 1 Case B 181

9.2.3 Floor 95, WTC 1 Case B 1 84

9.2.4 Floor 96, WTC 1 Case B 186

9.2.5 Floors 97, 98, and 99, WTC 1 Case B 188

9.3 Data Transfer for Structural Analysis 189

Chapter 10

Global Thermal Response of WTC 2 : Case C 199

10.1 Overview 199

10.1.1 Aircraft Impact Analysis 1 99

10.1.2 Fire Dynamics Simulations 200

10.2 Fire Structure Interface 201

10.2.1 Floor 79, WTC 2 Case C 204

10.2.2 Floor 80, WTC 2 Case C 207

vni NISTNCSTAR 1-5G, WTC Investigation
Table of Contents

10.2.3 Floor 81, WTC 2 Case C 210

10.2.4 Floor 82, WTC 2 Case C 216

10.2.5 Floor 83, WTC 2 Case C 218

10.3 Data Transfer for Structural Analysis 220

Chapter 11 Global Thermal Response of WTC 2 : Case D 229

11.1 Over\iew 229

11.1.1 Aircraft Impact Analysis 229

11 . 1 .2 Fire Dynamics Simulation 230

1 1 .2 Fire Structure Interface 231

11.2.1 Floor 79. WTC 2 Case D 231

11.2.2 Floor 80. WTC 2 Case D 234

11.2.3 Floor 8 1 . WTC 2 Case D 236

11.2.4 Floor 82, WTC 2 Case D 238

1 1 .2.5 Floor 83. WTC 2 Case D 240

1 1.3 Data Transfer for Strucmral Analysis 240

1 1.4 Extended Run Simulations 242

Chapter 12

Summary of Technical Results 249

12.1 Global Thermal Response of the World Trade Center Towers 250

12.2 Global Thermal Response of WTC 1 : Comparison of Case A and Case B 252

12.3 Global Thermal Response of WTC 2 : Comparison of Case C and Case D 261

Chapter 13

References 269

Appendix A

Thermo-physical Properties 271

Appendix B

Column Schedule 277

NISTNCSTAR 1-5G, WTC Investigation ix
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X NISTNCSTAR 1-5G, WTC Investigation
List of Figures

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

disaster xxix

Figure 1-1. Computed upper layer temperatures as predicted from an FDS simulation for World

Trade Center Tower 1, Floor 96. The temperature contours (in degree C) are shown

at 1,000 seconds after the impact of the airplane 3

Figure 1-2. Schematic of a two-layer model of a fire environment showing a hot layer at temperature Th and absorption coefficient 5

Figure 1-3. Dimensionless heat fluxes, Q, showing profiles as a function of dimensionless height,

Z, for a downward facing surface (bottom curve) and upward facing surface (top

curve) 7

Figure 1^. Dimensionless heat fluxes showing profiles as a function of dimensionless height, Z. The flux profiles correspond to values of dimensionless optical depth 8

Figure 2-1. Schematic drawing of compartment contents 12

Figure 2-2. Finite element model for the 3.05 m (10.0 ft) long steel bar, showing the elements

distributed uniformly along the length of the bar. Figure on the right exhibits a cross- sectional view of the bar showing mesh density 15

Figure 2-3. Finite element representation of the insulated steel truss (blue), the SFRM (violet),

and the ceiling (red) used in the thermal analysis of Test 5 16

Figure 2-A. Top view of the finite element model of the steel column with SFRM 16

Figure 2-5. Snapshot of the numerical simulation showing the fire and a contour plot of the gas

temperature along the compartment centerline, looking toward the north wall 17

Figure 2-6. Comparison of numerical simulations with measurements for the steel surface

temperature at four locations on Bar A in Test 1 18

Figure 2-7. Comparison of numerical simulations with measurements for the steel surface

temperature at four locations 3.70 m above the floor on Truss A in Test 1 19

Figure 2-8. Comparison of numerical simulations with measurements for the steel surface

temperature at four locations 3.69 m above the floor on the column in Test 1 19

Figure 2-9. Comparison of numerical simulations with measurements for the steel surface

temperature at four locations 2.13 m above the floor on the column in Test 1 20

Figure 2-10. Comparison of numerical simulations with measurements for the temperature of the

SFRM surface at four locations on the insulated bar in Test 5 21

Figure 2-11. Comparison of numerical simulations with measurements for the temperature of the

steel surface at four locations on the insulated bar in Test 5 21

NIST NCSTAR 1-5G. WTC Investigation xi
List of Figures

Figure 2-12. Comparison of numerical simulations with measurements for the temperature of the

SFRM surface at four locations, 3.69 m above the floor on the insulated steel column

in Tests 22

Figure 2-13. Comparison of numerical simulations with measurements for the temperature of the

steel surface at four locations, 3.69 m above the floor on the insulated steel coluinn in Test 5 22

Figure 2-14. Comparison of numerical simulations with measurements for the temperature of the

steel surface at four locations 3.70 m above the floor on Truss A in Test 5 23

Figure 2-15. Comparison of numerical simulations with measurements for the temperature of the

steel surface at four locations 3.70 m above the floor on Truss A in Test 5 23

Figure 3-1 . Finite element model of light box shape core column (left) and temperature contours

(Kelvin) at one instant in time 30

Figure 3-2. Non-dimensional yield strength as a function of temperature for typical steel used in WTC tower (NIST NCSTAR 1-3) 32

Figure 3-3. Finite element model of heavy box shape core column (left) and temperature

contours (Kelvin) at one instant in time 33

Figure 3-4. Effect of statistical variability in fireproofmg thickness on steel temperature

Temperature contours have been shown in Kelvin 35

Figure 3-5. Effect of fireproofmg damage on one face of a heavy core column. Temperature

contours have been shown in Kelvin 36

Figure 3-6. Example of variability in fireproofmg thickness and "gap" in fireproofmg on

diagonal member of a bridging floor truss 38

Figure 3-7. Model used to study effects of fireproofmg thickness and variability of thickness on

steel temperature: a) physical model used in analyses (points 1-5 are sensor locations

where temperature are monitored); b) finite element mesh used to represent physical

model 39

Figure 3-8. Finite element model to represent thickness of fireproofmg (purple). The elements in red represent a material that provides no resistance to heat flow (very high thermal

conductivity and very low thermal inertia) 40

Figure 3-9. Variation of steel temperature at Sensor Location #1 with time for different average

thickness of fireproofmg (shown as numbers on the curves). The fireproofmg

thickness is uniform over the entire length 41

Figure 3-10. Finite element model for studying the effect of gap in fireproofmg thickness a)

location of the gap on the plate b) temperature contours at one instant in time 42

Figure 3-11. Temperature at Sensor Location #1 plotted as a function of time for various

insulation thickness, standard deviation and gap size 44

Figure 3-12. Temperature at Sensor Location #2 plotted as a function of time for various

insulation thickness, standard deviation and gap size. 45

Figure 3-13. Temperature at Sensor Location #3 plotted as a function of time for various

insulation thickness, standard deviation and gap size. 46

xn NIST NCSTAR 1-5G, WTC Investigation
List of Figures

Figure 3-14. Temperature at Sensor Location #4 plotted as a function of time for various

msulation thickness, standard deviation and gap size 47

Figure 3-15. Temperature at Sensor Location #5 plotted as a function of time for various

insulation thickness, standard deviation and gap size 48

Figure 3-16. Temperature as function of length along the bar for various insulation thickness.

Results are shown at 30 min after application of radiative flux 50

Figure 3-17. Temperature as function of length along the bar for various insulation thickness.

Results are shown at 60 min after application of radiative flux 51

Figure 3-18. Temperature as fijnction of length along the bar for various insulation thickness.

Results are shown at 90 min after application of radiative flux 52

Figure 3-19. Temperature as function of length along the bar for various insulation thickness.

Results are shown at 120 min after application of radiative flux 53

Figure 3-20. Sensitivity of results to various input parameters 54

Figure 4-1. Representative floor plan of the exterior wall and column layout for WTC 1 and

WTC 2 56

Figure 4-2. Finite element model of a box shape perimeter column (Column Type 120) showing

the exterior faces. Cyan colored elements have material attributes of steel, while the

violet colored elements have material attributes of fireproofmg

(BLAZE-SHIELD DC/F) 57

Figure 4—3. Finite element model of box shape perimeter column (Column Type 120) showing

the interior faces. Cyan colored elements have material attributes of steel, violet

colored elements have material attributes of fireproofmg (BLAZE-SHIELD DC/F)

and pink colored elements have material attributes of Vermiculite plaster 58

Figure 4-4. Finite element mesh used to model a typical perimeter column. Figure shows the

density of the mesh elements in the column and spandrel beam 58

Figure 4-5. Finite element mesh used to model a typical perimeter column. Figure shows the

density of the mesh elements in the column 59

Figure 4-6. Heat flux boundary conditions were applied on the interior faces of the perimeter

column as indicated by the arrows. The intensity of the incident flux varies with time

and space 60

Figure 4-7. Convective flux boundary conditions indicated by arrows were applied to all the

exposed faces of the perimeter column 61

Figure 4—8. Arrows indicate the surfaces used for modeling re-radiation within the cavity of a perimeter column 61

Figure 4—9. Boundary conditions on a typical perimeter column 62

Figure 4—10. Heat flux incident on the inner faces of a perimeter column as a function of time.

Plots are shown for Column 145 on floor 96, WTC 1 at 1,000, 2,000, 3,000, 4,000

5,000 and 6,000 s after impact 64

Figure 4-1 1 . Incident heat flux at 6,000 s after impact for perimeter columns 145, 230, 330 and

430 on floor 96 (WTC 1 ). Partial results from one fire simulation have been used to

illustrate the nature of the coupling process 65

NISTNCSTAR 1-5G. WTC Investigation xiii
List of Figures

Figure 4-12. Temperature Contours (Kelvin) on perimeter column number 145 on floor 96

(WTC 1 ). The contours are shown at five different instants in time ranging from

10 min to 50 min at 10 min intervals. The contour scale ranges from 273 K to 948 K

(0°Cto 675 °C) 66

Figure 4-13. Temperature Contours (Kelvin) on perimeter column number 145 on floor 96

(WTC 1). The contours are shown at five different instants in time ranging from

60 min to 100 min at 10 min intervals. These contour plots supplement those shown

in Figure 4-12. The contour scale ranges from 273 K to 948 K (0°C to 675 C) 67

Figure 4-14. Temperature contours (Kelvin) through the cross-section of a perimeter column,

showing the temperature gradient between the interior and exterior faces 68

Figure 4-15. Time-temperature plot for perimeter Column 145, floor 96, WTC 1 , 69

Figure 4-16. Time-temperature plot for perimeter Column 230, floor 96, WTC 1 70

Figure 4-17. Time-temperature plot for perimeter Column 330, floor 96, WTC 1 70

Figure 4-18. Time temperature plot for perimeter Column 430, floor 96, WTC 1 71

Figure 4-19. Incorporating fireproofmg damage on the interior faces of a perimeter column.

Figure on the left shows fireproofmg damage on a part of the column below the coluinn under consideration. Figure on the right shows fireproofmg damage on part

of the column above the column under consideration, while the figure in the middle

shows fireproofmg damage on the column of interest 72

Figure 4-20. Incorporating fireproofmg damage on the interior and exterior faces of a perimeter

column. Material attributes of the fireproofmg elements were changed to model

damage to fireproofmg 73

Figure 4-21 . Incorporating structural damage caused by the aircraft impact on the steel columns.

The plot on the left shows damage to the steel column below the column under

consideration, while the figure on the right shows damage to the steel column above

and below the coluinn under consideration 74

Figure 4-22. Mapping between the thennal models (left) and the structural model (right) for a

typical perimeter column. Both temperature and temperature gradient information was transferred from the thermal model to the structural model for predicting the

thermally induced response of the column 76

Figure 4-23. Time temperature plot for perimeter column 145, floor 96, WTCl, indicating the

frequency at which the thermal data is plotted (small symbols) and the frequency at which it is transferred into the structural models (large symbols) 78

Figure 4-24. Perimeter columns on floor 82 of WTC 2. The perimeter columns on the south face

are damaged due to the impact of the aircraft, while many columns on the north and

east faces exhibit fireproofmg damage 79

Figure 4-25. Visualization of the temperatures and temperature gradient information transferred to the WTC 1 structural model (floor 92-99) at 6000 s after impact. The thermal data

is provided as a body load file 80

Figure 5-1 . Isometric view of a finite element model of a portion of the main truss 82

Figure 5-2. End view of the truss model showing upper and lower steel flanges (cyan colored)

covered with fireproofmg (violet colored) 83

XIV NISTNCSTAR 1-5G, WTC Investigation
List of Figures

Figure 5-3. Front and top views of the truss model 84

Figure 5-4. Finite element model of a primary truss showing truss construction details 84

Figure 5-5. Replications of the basic element of the truss model to create a 60' long truss. The

ends of the truss were modified to properly account for the connections to the core columns and perimeter columns 85

Figure 5-6. Basic unit of the primary and transverse truss 86

Figure 5-7. Rephcation of the basic truss unit to cover a small portion of the floor slab 87

Figure 5-8. Finite element model of the entire truss assembly (steel only) that supports the concrete slab on floor 96 of WTC 1 87

Figure 5-9. Incorporating structural damage due to aircraft impact on the truss model 88

Figure 5-10. Incorporating structural and fireproofmg damage on the truss assembly 89

Figure 5-11. Radiative flux boundary conditions on truss elements 90

Figure 5-12. Radiative fluxes coupled with realistic fire simulations are applied as surface loads on a truss model 90

Figure 5-13. Temperature profile as a function of time at four different locations on the steel truss. Sub-plot a and b are for locations on the north face, while sub-plots c and d are for locations on the south face of WTC 1, floor 96 91

Figure 5-14. Time temperature plot on the steel truss (north face, floor 96 WTC 1) indicating the

frequency at which the thermal data is plotted (small symbols) as well as the

frequency at which it is transferred into the structural models (large symbols) 92

Figure 5-15. Mapping of the thermal data between the thermal models and the structural models

showing element density 93

Figure 5-16. Mapping of the thermal data between the thermal models and the structural models

showing nodal density 94

Figure 5-17. Mapping of the thermal data for floor 96, WTC 1 onto the structural models. Thermal

data is provided as a body load file for the structural model. In this picture results are shown at 6000 s after impact 95

Figure 6-1 . Finite element model of a concrete slab (top view) showing element density 98

Figure 6-2. Finite element model of a concrete slab indicating element density through the

thickness of the slab 99

Figure 6-3. Variation of radiative heat flux incident on the surface of a concrete slab representing

the 96"' floor of the WTC 1 . Arrows pointing downwards indicate flux incident on

the floor from a fire on top of the concrete slab, while arrows pointing upwards

indicate flux incident on the ceiling of the concrete slab from a fire below 100

Figure 6-4. Radiative heat flux (WW) incident on the top of the 96"" floor concrete slab (due to

fires on the 96"^ floor) 101

Figure 6-5. Radiative heat flux (WW) incident on the bottom of the 96*'' floor concrete slab (due to fires on the 95'*' floor) 102

NISTNCSTAR 1-5G, WTC Investigation xv
List of Figures

Figure 6-6. Typical temperature contours (Kelvin) on the top and bottom faces of the concrete

slab (96"^ floor, WTC 1) at 6000 s after impact. A portion of the concrete slab on the

north face (top) was damaged by the impact of the aircraft 103

Figure 6-7. Temperature contours through the thickness of the concrete slab at two different

instants in time (1,000 s and 3,000 s after impact). The range of temperature

contours is the same as shown in Figure 6-6 104

Figure 6-8. Location of the four nodes on the concrete slab where temperature profiles through

the slab thickness have been plotted in Figure 6-9 through Figure 6-12 104

Figure 6-9. Temperature profiles tlirough the slab thickness at Location 1. The temperature

profiles have been plotted at six different instants in time at 1,000 s intervals 105

Figure 6-10. Temperature profiles through the slab thickness at Location 2. The temperature

profiles have been plotted at six different instants in time at 1,000 s intervals 106

Figure 6-11. Temperature profiles through the slab thickness at Location 3. The temperature

profiles have been plotted at six different instants in time at 1,000 s intervals 106

Figure 6-12. Temperature profiles through the slab thickness at Location 4. The temperature

profiles have been plotted at six different instants in time at 1,000 s intervals 107

Figure 6-13. Temperature is plotted as a function of time at four different locations (top and

bottom surfaces) on a concrete slab representing the 96"" floor of WTC 1 . These

results clearly demonstrate the transient nature of fire growth through the floors 108

Figure 6-14. Mapping of thermal data (temperature and temperature gradient) from the thermal

models to the structural model 109

Figure 6-15. Mapping of the thermal data (temperature and temperature gradient) from the themial

models to the structural model .109

Figure 7-1. Finite element models of wide flange core columns, including insulation developed

for thermal analysis of the World Trade Center Towers. Steel elements are colored

cyan and fireproofing elements violet 112

Figure 7-2. Finite element models of box columns including insulation used for thermal analysis

in the World Trade Center towers. Steel elements are colored cyan, and fireproofing

elements are violet 113

Figure 7-3. Isometric view of finite element models of the core columns showing mesh density

and column construction. The elements are color coded with material attributes. Cyan

colored elements have material attributes of steel while violet colored elements have

material attributes of fireproofing .114

Figure Radiative flux and convective flux boundary conditions (indicated by arrows)

imposed on wide flange and box shaped core columns 115

Figure 7-5. Radiative heat flux incident on column 501 at floor 96 (WTC 1, Case B) at six

different instants in time. (1,000-6,000 s at 1,000 s interval) 116

Figure 7-6. Temperature contours (Kelvin) for core columns in the World Trade Center towers. Both steel and fireproofing elements have been shown 117

Figure 7-7. Temperature contours (Kelvin) in the steel (Column 501, floor 96, WTC 1) as a function of time. Figure shows the temperature gradient in the steel and variability in temperature along the length of the columns 119

xvi NISTNCSTAR1-5G, WTC Investigation
List of Figures

Figure 7-8. Core column temperature for columns 501 through 508 on floor 96 of WTC 1 as a function of time 120

Figure 7-9. Core column temperature for columns 601 through 608 on floor 96 of WTC 1 as a function of time 120

Figure 7-10. Core column temperature for columns 701 through 708 on floor 96 of WTC 1 as a function of time 121

Figure 7-1 1 . Core column temperamre for columns 801 through 807 on floor 96 of WTC 1 as a function of time 121

Figure 7-12. Core column temperature for columns 901 through 908 on floor 96 of WTC 1 as a function of time 122

Figure 7-13. Core column temperature for columns 1001 through 1008 on floor 96 of WTC 1 as a function of time 122

Figure 7-14. Incorporating the effect of fireproofmg damage on columns by selectively changing

the material attributes of the underlying elements 123

Figure 7-15. Mapping of the thermal data from thermal models of the core column to the

structural models 125

Figure 7-16. Mapping of the thermal data for the core columns onto the structural model for the

96"'' floor of WTC 1 at 6,000 s after impact. The temperature data is provided as a body load and is generated in a format that is consistent with the structural model 126

Figure 7-17. Core column temperature for columns 501 through 508 on floor 96 of WTC 1, as a function of time indicating the frequency at which the thermal data is plotted (small

symbols) and the frequency at which data is transferred into the structural models

(large symbols) 126

Figure 8-1 . Sample plot of temperature contours superimposed on the steel structure showing the core beams and floor trusses that support a concrete slab. Perimeter and core columns

extend to the floor above and below the floor under consideration. The concrete slab

is included in the analysis, but has not been shown here for clarity 132

Figure 8-2. Thermal response of floor 93, WTC 1 Case A at different instants in time 134

Figure 8-3. Structural and fireproofmg damage on floor 94, WTC 1 Case A, 136

Figure 8-4. Thermal response of floor 94, WTC 1 Case A at different instants in time 137

Figure 8-5. Structural and fireproofmg damage on floor 95, WTC 1 Case A, 139

Figure 8-6. Upper layer temperature predicted by fire dynamics simulation for floor 95, WTC 1 Case A 140

Figure 8-7. Thermal response of floor 95, WTC 1 Case A at different instants in time, grid view. ... 141

Figure 8-8. Thermal response of floor 95, WTC 1 Case A at different instants in time 142

Figure 8-9. Structural and fireproofmg damage on floor 96, WTC 1 Case A, 144

Figure 8-10. Thermal response of floor 96, WTC 1 Case A at 600 s after impact 145

Figure 8-11. Thermal response of floor 96, WTC 1 Case A at 1200 s after impact 146

Figure 8-12. Thermal response of floor 96, WTC 1 Case A at 1800 s after impact 147

NISTNCSTAR 1-5G, WTC Investigation xvii
List of Figures

Figure 8-13. Thermal response of floor 96, WTC 1 Case A at 2400 s after impact 148

Figure 8-14. Thermal response of floor 96, WTC 1 Case A at 3000 s after impact 149

Figure 8-15. Thermal response of floor 96, WTC 1 Case A at 3600 s after impact 150

Figure 8-16. Thennal response of floor 96, WTC 1 Case A at 4200 s after impact 151

Figure 8-17. Thermal response of floor 96, WTC 1 Case A at 4800 s after impact 152

Figure 8-18. Thermal response of floor 96, WTC 1 Case A at 5400 s after impact 153

Figure 8-19. Thermal response of floor 96, WTC 1 Case A at 6000 s after impact 154

Figure 8-20. Thermal response of floor 96, WTC 1 Case A (slab and columns) at 600 s after impact 155

Figure 8-21. Thermal response of floor 96, WTC 1 Case A (slab and columns) at 1200 s after impact 156

Figure 8-22. Thermal response of floor 96, WTC 1 Case A (slab and columns) at 1800 s after impact 157

Figure 8-23. Thermal response of floor 96, WTC 1 Case A (slab and columns) at 2400 s after impact 158

Figure 8-24. Thermal response of floor 96, WTC 1 Case A (slab and columns) at 3000 s after impact 159

Figure 8-25. Thermal response of floor 96, WTC 1 Case A (slab and columns) at 3600 s after impact 160

Figure 8-26. Thermal response of floor 96, WTC 1 Case A (slab and columns) at 4200 s after impact 161

Figure 8-27. Thennal response of floor 96, WTC 1 Case A (slab and columns) at 4800 s after impact 162

Figure 8-28. Thermal response of floor 96, WTC 1 Case A (slab and columns) at 5400 s after impact 163

Figure 8-29. Thermal response of floor 96, WTC 1 Case A (slab and columns) at 6000 s after impact 164

Figure 8-30. Structural and fireproofing damage on floor 97 WTC 1 Case Ai 166

Figure 8-3 1 . Thermal response of floor 97, WTC 1 Case A at different instants in time, grid view. ... 167

Figure 8-32. Thermal response of floor 97, WTC 1 Case A at different instants in time 168

Figure 8-33. Structural and fireproofing damage on floor 98 WTC 1 Case A, 169

Figure 8-34. Thermal response of floor 98, WTC 1 Case A at different instants in time 170

Figure 8-35. Thermal response of floor 99, WTC 1 Case A at different instants in time 172

Figure 9-1. Thermal response of floor 93, WTC 1 Case B at different instants in time 182

Figure 9-2. Thermal response of floor 94, WTC 1 Case B at different instants in time 1 83

Figure 9-3. Structural and fireproofing damage on floor 95, WTC 1, Case B, 184

Figure 9-4. Thermal response of floor 95, WTC 1 Case B at different instants in time 185

xviii NISTNCSTAR 1-5G, WTC Investigation
List of Figures

Figure 9-5. Structural and fireproofing damage on floor 96, WTC 1, Case B, 186

Figure 9-6. Thermal response of floor 96, WTC 1 Case B at different instants in time 187

Figure 9-7. Structural and fireproofing damage on floor 97, WTC 1 , Case Bj 188

Figure 9-8. Structural and fireproofing damage on floor 98, WTC 1, Case B, 189

Figure 9-9. Thermal response of floor 97, WTC 1 Case B at different instants in time 190

Figure 9-10. Thermal response of floor 98, WTC 1 Case B at different instants in time 191

Figure 9-11. Thermal response of floor 99, WTC 1 Case B at different instants in time 192

Figure 10-1. Sample plot of temperature contours superimposed on the steel structure showing

core beams and floor trusses that support a concrete slab of a typical floor in WTC 2. Perimeter and core columns extend to the floor above and below the floor under

consideration. The concrete slab is included in the analysis, but has not been shown

here for clarity 203

Figure 10-2. Structural and fireproofing damage on floor 79 WTC 2, Case C, 205

Figure 10-3. Thermal response of floor 79 WTC 2 Case C at six different instants in time 206

Figure 10-4. Structural and fireproofing damage on floor 80, WTC 2 Case C, 208

Figure 10-5. Thermal response of floor 80, WTC 2 at different instants in time, Case C 209

Figure 10-6. Structural and fireproofing damage on floor 81, WTC 2, Case Cj 211

Figure 10-7. Thermal response of floor 81, WTC 2 Case C at 600 s after impact 211

Figure 10-8. Thermal response of floor 81, WTC 2 Case C at 1,200 s after impact 212

Figure 10-9. Thermal response of floor 81, WTC 2 Case C at 1,800 s after impact 212

Figure 10-10. Thermal response of floor 81, WTC 2 Case C at 2,400 s after impact 213

Figure 10-11. Thermal response of floor 81, WTC 2 Case C at 3,000 s after impact 213

Figure 10-12. Thermal response of floor 81, WTC 2 Case C at 3,600 s after impact 214

Figure 10-13. Thermal response of floor 81, WTC 2 Case C (concrete slab and columns) at different instants in time 215

Figure 10-14. Structural and fireproofing damage on floor 82, WTC 2, Case Ci 216

Figure 10-15. Thermal response of floor 82, WTC 2 Case C at different instants in time 217

Figure 10-16. Structural and fireproofing damage on floor 83, WTC 2, Case Cj 218

Figure 10-17. Thermal response of floor 83, WTC 2 Case C, at different instants in time 219

Figure 11-1. Structural and fireproofing damage on floor 79, WTC 2 Case D, 232

Figure 1 1-2. Thermal response of floor 79, WTC 2 Case D at different instants in time 233

Figure 1 1-3. Structural and fireproofing damage on floor 80, WTC 2, Case Dj 234

Figure 1 1-4. Thermal response of floor 80, WTC 2 Case D at different instants in time 235

Figure 1 1-5. Structural and fireproofing damage on floor 81, WTC 2, Case D, 236

NISTNCSTAR 1-5G. WTC Investigation xix
List of Figures

Figure 1 1-6. Thermal response of floor 81, WTC 2 Case D at different instants in time 237

Figure 1 1-7. Structural and fireproofmg damage on floor 82, WTC 2, Case Dj 238

Figure 1 1-8. Thermal response of floor 82, WTC 2 Case D at different instants in time 239

Figure 11-9. Structural and fireproofmg damage on floor 83, WTC 2, Case D, 240

Figure 1 1-10. Thermal response of floor 83, WTC 2 Case D, at different instants in time 241

Figure 1 1-1 1. Thermal response of floor trusses that support the concrete slab on floor 82, WTC 2

at 6,000 s after impact (extended simulation for Case D) 242

Figure 1 1-12. Typical time-temperature plot for floor trusses on the west side for the extended run, indicating rapid heating at 6000 s after impact followed by gradual cooling 243

Figure 12-1. Global thermal response of WTC 1, Case A 255

Figure 12-2. Global thermal response of WTC 1, Case B 256

Figure 12-3. Comparison of the final thennal state of perimeter columns for WTC 1, Case A, with

WTC 1, Case B 260

Figure 12^. Global thermal response of WTC 2, Case C 263

Figure 12-5. Global thermal response of WTC 2, Case D 264

Figure 12-6. Comparison of the final thermal state of perimeter columns for WTC 2, Case C with

WTC 2, Case D 268

XX NISTNCSTAR 1-5G, WTC Investigation
List of Tables

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

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

Table 2-1. Test Matnx for comparison with numerical simulations 12

Table 2-2. Percentage difference between peak values of the measured and the simulated steel surface temperatures at the hottest and coolest locations on various un-insulated

components in Test 1 24

Table 2-3. Percentage difference between peak values of the measured and the simulated steel temperamres for the hottest and coolest locations for various components with SFRM

in Test 5 25

Table 2-^. Percentage difference in the peak value of temperatures between the baseline

simulations and those with a 1 0 percent higher heat release rate at the hottest and

coolest locations on the steel surface of various un-insulated components in Test 1 27

Table 3-1 . Time (in seconds) to reach critical temperature for light box shape core columns 3

Table 3-2. Time (in seconds) to reach critical temperature for heavy box shape core columns 34

Table 3-3. Time (in seconds) to reach critical temperature for wide flange column 12WF133 37

Table 3^. Time (in seconds) to reach critical temperature for wide flange column 14WF730 37

Table 8-1 . Input parameters for WTC 1 global impact analyses (Base Case) 129

Table 8-2. Values of WTC 1 Case A fire simulation variables 129

Table 8-3. Input for global thermal response of WTC 1, Case A 130

Table 8^. Perimeter column damage status for WTC 1, Case A 173

Table 8-5. Core column damage status for WTC 1 , Case A 176

Table 9-1. Input parameters for WTC 1 global impact analyses (More Severe) 180

Table 9-2. Values of WTC fire simulation variables 180

Table 9-3. Input for global thermal response of WTC 1, Case B 181

Table 9^. Perimeter column damage status for WTC 1 , Case B 193

Table 9-5. Core dolumn damage status for WTC 1, Case B 196

Table 10-1 . Input parameters for WTC 2 global impact analyses (Base Case) 200

Table 10-2. Values of WTC fire simulation variables 201

NISTNCSTAR 1-5G. WTC Investigation xxi
List of Tables

Table 10-3. Input for global thermal response of WTC 2 202

Table 10-4. Perimeter column damage status for WTC 2 Case C 221

Table 10-5. Core column fireproofing damage status for WTC 2, Case C 224

Table 10-6. Core column structural damage status for WTC 2, Case C 226

Table 1 1-1. Input parameters for WTC 2 global impact analyses (more severe) 229

Table 11-2. Values of WTC fire simulation variables 230

Table 1 1-3. Input for global thermal response of WTC 2 231

Table 1 1-4. Perimeter Column Damage Status for WTC 2 Case D 244

Table 1 1-5. Core Column Damage Status for WTC 2, Case D 247

Table 12-1. Input parameters for WTC 1 global impact analyses 250

Table 12-2. Input parameters for WTC 2 global impact analyses 25

Table 12-3. Values of WTC fire simulation variables 251

Table 12-4. Input for global thermal response of WTC 1 and WTC 2 252

Table 12-5. Temperature range (°C) for perimeter columns on various floors at five instants in time, WTC 1, Case A 257

Table 12-6. Temperature range (°C) for perimeter columns on various floors at five instants in time, WTC l,CaseB 257

Table 12-7. Temperature range (°C) for core columns on various floors at five instants in time,

. WTC 1, Case A 258

Table 12-8. Temperature range (°C) for core columns on various floors at five instants in time,

WTC l,CaseB 258

Table 12-9. Temperature range (°C) for floor trusses on various floors at five instants in time,

WTC 1, Case A 259

Table 12-10. Temperature range (°C) for floor trusses on various floors at five instants in time,

WTC l,CaseB 259

Table 12-1 1. Temperature range (°C) for perimeter columns on various floors at six instants in time, WTC 2, Case C 265

Table 12-12. Temperature range (°C) for perimeter columns on various floors at six instants in time, WTC 2, Case D 265

Table 12-13. Temperature range (°C) for core columns on various floors at six instants in time,

WTC 2, Case C 266

Table 12-14. Temperature range (°C) for core columns on various floors at six instants in time,

WTC 2, Case D 266

Table 12-15. Temperature range (°C) for floor trusses on various floors at six instants in time,

WTC 2, Case C 267

XXll NISTNCSTAR 1-5G, WTC Investigation
List of Tables

Table 12-16. Temperature range (°C) for floor trusses on various floors at six instants in time,

WTC 2, Case D 267

NISTNCSTAR 1-5G. WTC Investigation

xxiii
List of Tables

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XXIV NISTNCSTAR 1-5G, WTC Investigation
List of Acronyms and Abbreviations

Acronyms

BPS Building Performance Study

FEMA Federal Emergency Management Agency

NIST National Institute of Standards and Technology

PANYNJ Port Authority of New York and New Jersey

SEAoNY Structural Engineers Association of New York

use United States Code

WTC World Trade Center

Abbreviations

°C degrees Celsius

°F degrees Fahrenheit

ft feet

in. inch

h hour

L liter K Kelvin

m meter

[im micrometer

min minute

s second

NISTNCSTAR 1-5G, WTC Investigation XXV
List of Acronyms and Abbreviations

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XXVI NISTNCSTAR 1^5G, 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 1-5G, WTC Investigation xxvii
Preface

NIST is a nonregulatory agency of the U.S. Department of Commerce's Technology Administration. The

purpose of NIST investigations is to improve the safety and structural integrity of buildings in the United

States, and the focus is on fact finding. NIST investigative teams are authorized to assess building

performance and emergency response and evacuation procedures in the wake of any building failure that has resulted in substantial loss of 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 building and fire safety investigation of the WTC disaster.

Technical Area and Project Leader Project Purpose

Analysis of Building and Fire Codes and

Practices; Project Leaders: Dr. H. S. Lew

and Mr. Richard W. Bukowski

Document and analyze the code provisions, procedures, and

practices used in the design, construction, operation, and

maintenance of the structural, passive fire protection, and

emergency access and evacuation systems of WTC L 2, and 7. Baseline Structural Performance and

Aircraft Impact Damage Analysis; Project

Leader: Dr. Fahim H. Sadek

Analyze the baseline performance of WTC 1 and WTC 2 under

design, service, and abnormal loads, and aircraft impact damage on

the structural, fire protection, and egress systems.

Mechanical and Metallurgical Analysis of

Structural Steel; Project Leader: Dr. Frank

W. Gayle

Determine and analyze the mechanical and metallurgical properties

and quality of steel, weldments, and connections from steel recovered from WTC 1, 2, and 7. Investigation of Active Fire Protection

Systems; Project Leader: Dr. David

D. Evans; Dr. William Grosshandler

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

Reconstruction of Thennal and Tenability

Environment; Project Leader: Dr. Richard

G. Gann

Reconstruct the time-evolving temperature, thennal 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.

xxvni NISTNCSTAR 1-5G, WTC Investigation
Preface

NIST WTC Investigation Projects

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

investigation of the WTC disaster.

National Construction Safety Team Advisory Committee

The NIST Director also established an advisory committee as mandated under the National Construction

Safety Team Act. The initial members of the committee were appointed following a public solicitation. These were:

Paul Fitzgerald, Executive Vice President (retired) FM Global, National Construction Safety

Team Advisory Committee Chair

John Barsom, President, Barsom Consulting, Ltd.

John Bryan, Professor Emeritus, University of Maryland

David Collins, President, The Preview Group, Inc.

Glenn Corbett, Professor, John Jay College of Criminal Justice

Philip DiNenno, President, Hughes Associates, Inc.

NISTNCSTAR 1-5G. WTC Investigation XXIX
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 infonnation 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.

XXX NISTNCSTAR 1-5G, WTC Investigation
Preface

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

Date Location Principal Agenda

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

August 21. 2002 Gaithersburg. MD Media briefing announcing the formal start of the Investigation.

Dprpmhpr Q ''007 VV a.Mlllig,lUll, i_/V_ ivicuid Driciing on release oi me riioiic upaaic ana inioi recjuesi for photographs and \ ideos. April 8. 2003 New York City, NY Joint public forum with Columbia University on first-person

inter\'iews.

April 29-30. 2003 Gaithersburg. MD NCST Advisory Committee meeting on plan for and progress on WTC Inx'esticration with a niihlir romiTipnt spssinn

May 7. 2003 New York City. NY Media briefing on release of Mm- 2003 Progress Report.

VJaJ llltl bUUI IVJLL/ NCST Ad\'isory 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.

jjecemuer -—o. _uuj Gaithersburg. MJJ NCST Ad\'isory Committee meeting on status and initial results and release of the Public Update with a public comment session. February 12. 2004 New York City. NY Public meeting on progress and preliminary findings with public

comments on issues to be considered in formulating final recommendations.

June 1 o. _UU4 i\ew I ork Lity. in y Media public briefing on release ot June 2004 Progress Report. June 22-23. 2004 Gaithersburg. MD NCST .Advisorv' Committee meeting on the status of and

preliminary findings from the WTC Investigation with a public

comment session. August 24. 2004 Northbrook. IL Pubhc viewing of standard fire resistance test of WTC floor system at Underwriters Laboratories, Inc. October 19-20. 2004 Gaithersburg, MD NCST Advisor)' Committee meeting on status and near complete

set of preliminary findings with a public comment session. No\ ember 22, 2004 Gaithersburg, MD NCST Advisory Committee discussion on draft annual report to Congress, a public comment session, and a closed session to discuss pre-draft recommendations for WTC Investigation.

April 5, 2005 New York City, NY Media and public briefing on release of the probable collapse sequence for the WTC towers and draft reports for the projects on

codes and practices, evacuation, and emergency response. June 23. 2005 New York City, NY Media and public briefing on release of all draft reports for the

VV 1 HJWtlO tllJU Liluil 1 C^^UlllillCllLlCllltJllo IKJl IJXAUiiK. V^JIl 111 It 11 L. September 12-13.

2005

Gaithersburg. MD NCST Advisory Committee meeting on disposifion 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 resulting from the WTC Investigation

and the R&D program, and (b) provide practical guidance and tools to better prepare facility owners, contractors, architects, engineers, emergency responders, and regulatory authorities

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

NISTNCSTAR 1-5G. WTC Investigation XXXI
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 hivestigation findings and the means by which these

technical results were achieved. As such, it is part of the archival record of this Invesfigation. The titles of the full set of Investigation publications 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. NISTNCSTAR 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 of the World Trade Center Disaster: Design and Construction ofStructural Systems.

NISTNCSTAR 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 I, 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

xxxu NISTNCSTAR 1-5G, WTC Investigation
Preface

York Cin- 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 ofthe 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 Safet}' 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 Technology.

Gaithersburg, MD, September.

Sadek, F. 2005. Federal Building and Fire Safet}' 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 1-5G. WTC Investigation xxxiii
Preface

Banovic, S. W. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Steel Inventoiy and Identification. NIST NCSTAR 1-3B. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Banovic, S. W., and T. Foecke. 2005. Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Damage and Failure Modes 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. Kuligowski, 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-Constmction 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

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 1-5 A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

xxxiv NIST NCSTAR 1-5G, 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 1-5G, WTC Investigation XXXV
Preface

T. P. McAllister. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Component, Connection, and Subsystem Structural Analysis. NISTNCSTAR 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 7. (Provisional). NIST NCSTAR 1-6E. National Institute of Standards and Technology.

Gaithersburg, MD.

Gilsanz, R., V. Arbitrio, C. Anders, D. Chlebus, K. Ezzeldin, W. Guo, P. Moloney, A. Montalva,

J. Oh, K. Rubenacker. 2006. Federal Building and Fire Safety Investigation 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). NISTNCSTAR 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). NISTNCSTAR 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. Prouk, 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 ofthe 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. NISTNCSTAR 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.

xxxvi NISTNCSTAR 1-5G, WTC Investigation
Acknowledgments

We thank the following individuals for their support and assistance during the development of the Fire Structure Interface:

James Filliben developed orthogonal design techniques for estimating model sensitivity.

Richard Gann helped during the review of FSI global simulation results and arranged for additional

software licenses during the compute intensive stages of the investigation.

Frank Gayle, Richard Fields, Bill Luecke and Tim Foecke performed tests on thermo-physical properties

of steel.

John Gross and Monica Stames provided data on shape and size of the structural components and

reviewed the models for accuracy.

William Grosshandler for technical discussions on thermal analysis.

Anthony Hamins, Alexander Maranghides and the entire staff at the NIST Large Fire Facility performed

experiments for FSI model validation and uncertainty analysis.

Steven Kirkpatrick from Applied Research Associates, Albuquerue, New Mexico, and his team

performed simulations of aircraft impact and estimated probable damage to structural systems.

Therese McAllister and Fahim Sadek estimated the fireproofmg damage due to debris field generated by

the aircraft impact on the towers.

Kevin McGrattan and Glenn Forney conducted FDS / Smokeview fire simulations for WTC 1 and

WTC 2.

Harold Nelson advised on fireproofing codes and standards and provided a link between modeling effort and current practice.

William Pitts provided information on structural damage to perimeter columns for WTC 1 and WTC 2.

Ronald Rehm conducted preliminary studies on fire spread in the towers and reviewed the final report.

Cheri Sawyer, John Eichner, Walter Soverow and Sydel Cavanaugh from SAIC, Gaithersburg, MD

provided administrative support and assistance with transferring the equations into a Word document.

Emil Simiu, Dat Duthinh, Long Phan and Michael Riley collaborated in the field of fire-thermalstructural engineering. Long Phan also provided the thermo-physical property data for concrete.

Dan Williams, Supervisor at the Critical Structures Building Inspections Section, Department of Public

Works & Environmental Services, Fairfax, VA, provided assistance in examining and measuring

fireproofing thickness in new and old construction buildings in Fairfax County.

NISTNCSTAR 1-5G, WTC Investigation xxxvn
Acknowledgments

Jiann Yang and Dale Bentz provided thermo-physical properties of fireproofing material.

Mehdi Zarghamee, Atis Liepins and Frank Kan from Simpson Gumpertz and Heger Inc., Boston, MA

provided the structural models for transfer of thermal data.

xxxvni NISTNCSTAR 1-5G, WTC Investigation
Executive Summary

The collapse of the World Trade Center (WTC) towers on September 11, 2001, left only limited direct

evidence to explain the collapse of each tower. In contrast, visual evidence, photographs, and videos, of

the damage initially inflicted by each aircraft, and the subsequent insult to each structure caused by the

fires, was probably more extensive than for any other disaster. Consequently, the National Institute of

Standards and Technology (NIST) investigation relied heavily on computer simulations, constrained by

the visual evidence, to try to explain the events of that morning.

The collapse of the WTC towers sparked an active debate on what was the maximum steel temperature

reached in each tower. Some speculated that the temperatures were high enough to melt the steel columns. The fire protection engineering community considered the differences in fireproofing thickness

in WTC 1 and WTC 2 and its effect on steel temperature. Lack of information on damage caused by the

aircraft impact complicated any analysis. Software tools that could simulate the thermally induced

structural response to spatially and temporally developing fires, over one or more floors of a WTC tower

damaged by the impact of the aircraft were needed to carry out the investigation. This report describes a methodology to couple the fire simulations with structural response for each tower. This methodology,

termed the Fire Structure Interface (FSI), uses the output of a fire simulation performed using the NIST

Fire Dynamics Simulator (FDS) together with aircraft impact analysis results to predict the thermal state of each tower. The temperature distribution is subsequently mapped in a form suitable for use in stress analysis of the load bearing structure.

Traditionally, structural response to extreme fires is estimated by subjecting the structure to a prescribed

time-temperamre curve. This approach de-couples the stress analysis in the load bearing structure from

the fire simulations that predict growth and spread of fires in a building. Direct coupling of the NIST

FDS with an appropriate structural analysis package (ANSYS) is an extremely difficult task. Enormous

differences in spatial and temporal length scales, differences in numerical techniques and complexity of

the software packages (both FDS and ANSYS) precludes a direct coupling of these codes at this time. The

FSI, developed during NIST's Investigation into the collapse of the WTC Towers, couples the fire simulations with a thermal analysis suitable for complex building geometries. The development of a methodology for predicting radiative fluxes from spatially evolving fires, to sub-grid scale structural elements (described in Chapter 1) is a critical element of the coupling process. FSI also links the thermal

analysis and structural analysis by creating thermal loading data files in a format that is consistent with

the structural models.

FSI was used extensively during NIST's Investigation into the collapse of the WTC towers. The

Investigation relied heavily on four major modeling efforts. The first was a detailed simulation of the impact of an aircraft on WTC 1 and WTC 2, to predict structural and fireproofing damage in the towers

(NIST NCSTAR 1-2).' Next, fire simulations were performed to predict the post-impact spread of fires on multiple floors of each tower (NIST NCSTAR 1-5F). FSI (third of four modeling tasks) couples the

results of fire simulations and aircraft impact with thermal analysis of a damaged structure and maps the

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 1-5G. WTC Investigation xxxix
Executive Summary

resulting temporally and spatially varying temperatures on to the structural models. Finally, global

structural analysis (NIST NCSTAR 1-6) of the towers was performed to predict the thermally induced

structural response and to identify the most probable collapse mechanism for each building.

The development of any computer model requires a complementary experimental program to furnish

large-scale test data with which to compare model predictions. A set of experiments with liquid

hydrocarbon fuel fires (2 MW and 3 MW) in a compartment with various structural components such as columns, trusses and bars was perfomied at the Large Fire Facility at NIST (NIST NCSTAR 1-5B). The

structural components were insulated in some tests and un-insulated in other tests to study the role of

fireproofmg. Experimental results were compared with model predictions to assess the accuracy of the

models and their sensitivity to changes in various input parameters. These experiments and model

predictions are summarized in Chapter 2.

Most of the structural steel in the WTC towers was "fireproofed" with sprayed fire-resistive material

(SFRM). The insulation presents some special problems that must be considered. The temperatures

reached in the steel in any fire simulation are extremely sensitive to the amount and spatial distribution of

the insulation. However, the amount of insulation sprayed on the trusses during construction is uncertain.

Moreover, it is impossible to spray insulation unifonnly on complex surfaces like the truss system.

Finally, any analysis of the thermal loading of the towers must consider the possibility of insulation

damage caused by the aircraft impact. Thus, any plausible model of the insulation must account for random variations in the thickness of the insulation, and the sensitivity of the results to variations caused

by insulation damage must be established. Exploratory studies described in Chapter 3 were performed to assess the role of fireproofing thickness and variability on the thermal response of structural elements.

Fireproofmg thickness and fireproofing damage due to aircraft impact is identified as the single most

important parameter in these simulations that has a direct impact on the thermally induced structural response.

The exploratory studies, experimental data, photographic evidence (NIST NCSTAR 1-5 A) as well as published literature, helped guide the development of models for various structural components

(perimeter and core columns, floor trusses, core beams and concrete slabs). A detailed thermal analysis

(coupled with a realistic fire) of each structural component is described in Chapter 4 through Chapter 7. A

sampling of the calculated results is shown to illustrate how the large-scale temperature distributions in the steel and concrete change with time. One of the most striking observations that emerge from these

results is the wide variation of "time - temperature" curves that hold at different points in the structure of

each tower. Equally striking is the lack of resemblance of these curves to the "standard" time -

temperature curves used in furnace tests of structural elements.

The component models were developed such that they could be readily replicated to cover an entire floor or extend to multiple floors of the WTC towers. Construction of the global models was guided by the

need to adequately resolve the underlying physics without resulting in computationally prohibitive

simulations. During the course of this Investigation, hundreds of full floor simulations were performed to understand the nature of the thermal insult. Four global simulations, two each for WTC 1 and WTC 2, are reported for prediction of thermally induced structural response and collapse of the towers. Each global

thennal simulation is coupled with a corresponding fire simulation and includes the aircraft impact

damage estimates. The simulations described in Chapter 8 through Chapter 1 1 represent an application of

FSI to simulate the global thermal response of WTC 1 and WTC 2. Results for the four cases are

xl NIST NCSTAR 1-5G, WTC Investigation
Executive Summary

compared and contrasted in Chapter 12. The figures included in these chapters are visual representations

of the thermal state of the towers at specific instants in time.

NISTNCSTAR 1-5G, WTC Investigation xli
Executive Summary

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xlii NISTNCSTAR 1-5G, WTC Investigation
Chapter 1 Sub-Grid Scale Model for Radiative Heat Transfer

Simulation of the effects of severe fires on the structural integrity of buildings requires a close coupling

between the gas phase energy release and transport phenomena and the stress analysis in the load bearing

materials. The connection between the tw'o is established primarily through the interaction of the

radiative heat transfer betu'een the solid and gas phases with the conduction of heat through the structural elements. This process is made difficult in large, geometrically complex buildings by the wide disparity

in length and time scales that must be accounted for in the simulations. A procedure for overcoming these

difficulties, used in the analysis of the collapse of the World Trade Center (WTC) towers, is presented in

this chapter. The large-scale temperature and other thermo-physical properties in the gas phase are predicted using the National Institute of Standards and Technology (NIST) Fire Dynamics Simulator

(FDS). Heat transfer to sub-grid scale structural elements is calculated using a simple radiative transport

model that assumes the compartment is locally divided into a hot, soot laden upper layer and a cool,

relatively clear lower layer. The properties of the two layers are extracted from temporal averages of the

results obtained from the FDS. Explicit formulae for the heat flux are obtained as a function of

temperature, hot layer depth, soot concentration, and orientation of each structural element. These

formulae are used to generate realistic thermal boundary conditions for a coupled transient three- dimensional finite element code. This code is used to generate solutions for the heating of complex

structural assemblies.

1.1 INTRODUCTION

The coupling between fire dynamics and structural analysis in building fires is largely due to radiative

heat transfer from combustion products to structural elements. A common assumption used in the

thermal analysis of structures is that the radiant heat flux, q, incident upon the surface of the element is related to the local gas temperature. . by the formula q = aT^. Here, cris the Stefan-Boltzmann constant

(5.67 X XQT^Wim'I^). Thus, given a spatially unifonn enclosure temperature and a 'lime-temperature

curve" the thermal environment of the enclosure is specified, and attention can be confined to the

calculation of the temperature and stress distribution in the structural elements.

However, this is tantamount to assuming that the radiation field is in local equilibrium with the gas, an unhkely scenario in most fires. In general, the radiation field must be determined from solutions of the

radiative transport equation, which relates the incident flux to the spatial distribution of temperature and

combustion products (most particularly the distribution of soot particulate) as well as the enclosure

geometry. Such calculations are typically performed as part of a computational fluid dynamics (CFD)

based simulation of the fire dynamics. However, the ability to couple such codes as the NIST FDS

(McGrattan et. al 2002) directly to a suitable structural analysis code does not yet exist. The enormous

differences in spatial and temporal length scales, differences in numerical techniques, and the complexity

of the computer codes makes the development of an efficient coupled analysis of fire-structure

interactions a daunting task. CFD codes like FDS assume that the radiative transport to a surface and the

thermal response of that structural element can be calculated as if the surface is locally one dimensional.

However, important elements of the WTC structure (e.g., trusses and perimeter columns) are inherently

NISTNCSTAR 1-5G. WTC Investigation 1
Chapter 1

three dimensional. Moreover, many of the critical segments of these structures lie below the resolution

limits of any CFD calculation that attempts to simulate the fire dynamics over the entire floor or multiple

floors of the WTC towers. Finally, the computed temperature distribution in the structure must be in a format accessible to the code used to perform structural analysis.

An interim approach to the calculation of the coupled heat transfer problem is presented here. It takes

advantage of the fact that the horizontal dimensions of each floor of the WTC towers are much greater

than the height of an individual floor. The ratio of these lengths, the geometric aspect ratio of the floor, is quite large. Each floor is about 63 m on a side and 4 m high. Thus, any floor can be thought of as a large

aspect ratio compartment with relatively few interior obstacles. A fire in a compartment configured in this manner tends to produce a stratified spatial distribution of temperature and soot particulate. The

temperature and soot concentration changes quite rapidly in the vertical direction, but much more slowly

in the two horizontal directions. Under these circumstances, the compartment can be thought of as containing a hot, sooty upper layer and a cool, relatively clear lower layer. The thickness, soot content,

and average temperature of each layer vary slowly in the horizontal directions. This idealization is very

similar to that used in "zone models" (Quintiere, 1989) of fire dynamics, which ignore any variation of

thermal properties in the horizontal directions.

The local spatial and temporal averages needed to define the properties of the layers are taken from the

output of simulations generated by FDS (Figure 1-1). Spatial averaging is defined by the grid size used in the FDS simulation. (For the WTC Investigation, the FDS resolution is 50 cm, and therefore, the spatial

averaging is perfomied over a 50 cm square area). The time intervals needed to generate the averages

used are chosen to be comparable to the time required for heat to diffuse through the smallest structural members of interest. The spatial averages replace the detailed vertical temperature and soot profiles with an effective "zone model" profile. It is important to note that the quantities averaged are 7^ and the

absorption coefficient, since these are the variables needed to define the radiative transport problem. The

result of this simplification is that the radiative transport equation takes the same form that it would have

in a plane layer geometry.

For this simplified geometry, the radiative transport equation can be solved exactly and explicit formulae

for the heat flux obtained as functions of the temperatures, hot layer depth, soot concentration, and

orientation of the structural element. The basic analysis is quite well known in the heat transfer

community (Siegel and Howell, 1992). These formulae can be used directly as input into finite element

computer codes widely used for time dependent three dimensional structural analysis. The output of such

codes yields the temperature distribution required for computing the thermal strains / stresses induced in the structural assembly when exposed to a fire. The higher temperature induced in the solid can also

degrade the strength of the material (yield strength) and affect the elastic-plastic stress strain relationship.

Thus, an approximate methodology capable of dealing with the heat transfer to realistic models of a building structure can be developed.

The objective of this chapter is to demonstrate how this approach can be applied to study the heating of a complex building structural geometry that is an idealization of an entire floor of the one of the World

Trade Center towers. The general solutions to the radiative transport equation and explicit results for radiative heat flux to vertical and horizontal planar surfaces are summarized in the next section.

2 NISTNCSTAR 1-5G, WTC Investigation
Sub-Grid Scale Model for Radiative Heat Transfer

Figure 1-1. Computed upper layer temperatures as predicted from an FDS simulation for

World Trade Center Tower 1, Floor 96. The temperature contours (in degree C) are shown

at 1 ,000 seconds after the impact of the airplane.

1.2 RADIATIVE TRANSPORT MODELING

Two major simplifications are introduced so that incident radiative fluxes that apply surface loads for thermal analysis of structures are obtained. First, we employ the concept of a grey gas, whose properties

are independent of frequency. If the spectral absorption coefficient, a:,, ,is replaced by some average

value to be defined later, then the radiative transport equation can be expressed in terms of the integrated

intensity, / [r, q].

-I (1)

(2)

The utility of this approximation depends strongly on the properties of the absorbing medium. For the

problems of interest here, soot particulate is the dominant absorber and emitter of thermal radiation. The

typical soot size distribution and temperature range are such that the spectral dependence of the soot

NISTNCSTAR 1-5G. WTC Investigation 3
Chapter 1

varies slowly compared with that of the Planck distribution. The result is that the soot absorption

coefficient can be approximated as follows:

K = (const) f^jT (cmy^

Here /„ is the soot volume fraction. Fomiulae of this type, with some variation in the numerical

coefficient, are widely used in the combustion and fire research literature (Quintiere, 1989). The quantity

( /f )~' based on representative values for the enclosure in question is the optical depth of the soot laden

gas.

The second simplification is that the enclosure geometry induces a vertically stratified distribution of

temperature and combustion products. Specifically, it is assumed that the vertical dimensions of each part

of the enclosure that can be considered as a separate entity are much smaller than either of the horizontal

dimensions. As a fire develops in such an enclosure, the hot layer that forms has spatial variations in these properties that vary much more rapidly in the vertical than in the horizontal directions (Fig. 2). If J

denotes the depth of a hot layer of roughly uniform thermal properties, and L denotes any of the

horizontal directions, then the analysis of the radiation field as a vertically stratified layer is internally

consistent if kL » 1 Physically, this means that the thermal radiation from remote points of the hot

layer cannot penetrate to affect the local radiation fields. In a typical commercial building environment

with ceiling heights of a few meters, we are interested in situations in which Kd ^ 1 . If Kd » 1, then the

radiation is in equilibrium with the matter at the local temperature, T, and the local heat flux is oT'^ . If Kd <K 1 , then the gas is locally transparent to thermal radiation.

The quantity of most interest in the analysis to follow is the heat flux incident on a material surface. Let n denote the unit nonnal pointing outward from an element of surface. Then the radiation heat flux to

that surface, ^„ , is determined by the formula:

^"="L.o A^^^)^-^'dQ. (4)

Clearly, the orientation of the surface, as well as the radiation field in the gas, plays a role in the analysis.

We will be particularly concerned with both vertically oriented (columns) and horizontally oriented

(floors, ceilings, trusses) surfaces.

1.3 PLANE LAYER ANALYSIS

Now consider the analysis of a plane heated layer of depth d. We define a Cartesian coordinate system

r = xi + yj + zk such that (x, v) are the horizontal coordinates and z is the vertical coordinate with 2 = 0

at the bottom of the hot layer. If the hot layer extends down to the floor, then the origin is at the floor. However, we always have 0 < z < d no matter how deep the layer is (Figure 1-2). The absorption

coefficient k — k:(z) ; similarly the temperature T=T {z) . The unit vector Q. can be represented in terms of a spherical polar coordinate system oriented so that the polar angle 6 is measured from the

positive z axis. n = sin ^ cos + sin 0 sin (j)j + cos Ok

4 NISTNCSTAR 1-5G, WTC Investigation
Sub-Grid Scale Model for Radiative Heat Transfer

In this system of coordinates, the integrated intensity, /, depends only on z and cos 0

transport equation becomes:

The radiative

cosfc' —= a:(z)

dz

-I

(6)

Considered as a function of z eq. (6), is in fact two equations. For values of cos ^ > 0 the radiation

emerges from the bottom of the hot layer. We denote the intensity of this radiation as t. For values of

cos ^ < 0 ; the radiation emerges from the ceiling. Let this radiant intensity be r. Boundary conditions

for r and F are obtained by assuming that radiation entering the hot layer from below is in equilibrium

with a "cool" floor at "ambienf temperature Ta, while radiation emitted by the "hot" ceiling is in equilibrium with the ceiling temperature T^ .

Figure 1-2. Schematic of a two-layer model of a fire environment showing a hot layer at temperature Th and absorption coefficient k„ .

The result of these assumptions is that eq. (6) must be solved subject to the following boundary

conditions:

ctT n n

(7)

The solutions can now be readily obtained in terms of local optical depths (z) and r (z) defined as follows:

(z) = K{z) dz r- (z) = j' k{z) dz

(8)

NISTNCSTAR 1-5G, WTC Investigation 5
Chapter 1

We are now in a position to evaluate the heat flux to a target surface. If the enclosure is completely

unobstructed, then only the heat flux to the floor and ceiling need be considered, and no further

assumptions are necessary. These results are well known, and will emerge as part of the analysis to follow. However, in order to calculate the heat flux to structural elements it is necessary to assume that each surface is unobstructed by others for the purpose of calculating the incident flux.

This is a reasonable assumption if the cross-sectional area of each element is small compared with the

unobstructed area in each plane normal to the axis of each of the elements. For example, the projected

floor area of a room containing several columns must be much larger than the cross-sectional area of all the columns. Similarly, a vertical plane crossing the enclosure should have an area much larger than that of all the trusses that penetrate the plane.

First consider horizontal surfaces. There are two possibilities: a ceiling-like or downward facing surface,

corresponding to n = -k , and a floor-like or upward facing surface, corresponding to n = k . The lower

surface of a truss would be an example of a downward facing surface, while the upper surface would be

an upward facing surface. Let q^{z) be the heat flux to a downward facing surface located at a height, z, and q, {z) be the corresponding flux to an upward facing surface. If we further assume that the upper layer

temperature, T, takes on the constant value T = Th and that the absorption coefficient k = k^ is also

constant, the following explicit results are obtained:

q^ iz) = 2(jT;E,{t^) + o-r^d - 2E,(r^)) ^0)

qj{z)^2GT^E,{T') + cjT'H{\-2E,(T-)) (H)

Here, E„ {z) is the Exponential Integral as defined in (Abramowitz and Stegun, 1964).

The flux to a downward facing surface (Figure 1-3) starts out from very low values because at the bottom

of the hot layer the surface only "sees" radiation from the "cold" lower surface. As the downward facing

surface is moved upward through the hot layer, the surface sees more and more of the hot layer radiation,

and eventually reaches a value corresponding to equilibrium radiation from an infinitely thick layer. The

flux to an upward facing surface typically starts from a somewhat higher value near the ceiling, since the

ceiling temperature is usually much hotter than the floor temperature. As the upward facing surface moves down through the hot layer, it also sees more and more radiation from the layer and ultimately the

flux again reaches a value corresponding to radiative equilibrium in the layer. In both cases, however, a

substantial fraction of the layer is far from radiative equilibrium with much lower fluxes to the surfaces.

6 NISTNCSTAR 1-5G, WTC Investigation
Sub-Grid Scale Model for Radiative Heat Transfer

Figure 1-3. Dimensionless heat fluxes, Q, showing profiles as a function of

dimensionless height, Z, for a downward facing surface (bottom curve) and upward

facing surface (top curve).

The heat flux to a vertical surface is not usually calculated within the context of the plane layer problem.

While the analysis is not difficult, it does not seem to be readily available. We consider this case next. Let the direction of the outv\'ard pointing normal to the surface be n = — i . Since the radiant intensity is independent of the azimuthal angle, (f) , any horizontal direction could be chosen, and this choice is convenient for the analysis. Under these circumstances, Q. ri = — sin 0 cos (f)

. Thus, we need to evaluate equation (4) over the domain Q<6<7T, — 7r/2<(j)<7T/2. Denoting the heat flux to the

vertical surface by q, (z), we have:

q^{z) = 2h''^~ /^sin^6'^/^+ j^^ r sin- 6 dO (12)

Equation (12) shows that the solution is a sum of two terms involving f and f respectively.

Denote the respective contributions of these terms as and q~ respectively. Then:

qAz) = q:{z) + q;{z). (13)

Again, letting the upper layer temperature and absorption coefficient be constant, the necessary

integrations can be carried out to yield:

^;(z) = ±</(r^) + -<g(r^)

7[ K

NISTNCSTAR 1-5G. WTCInvestigation 7
Chapter 1

g:(z)^^aT:fiT-) + ^aT^giT~)

^^^^

' f{z) = Ki,{z)-Ki,iz) (16)

g(z) = --Ki,(z)-Ki,{z)

Here, Ki, (z) is the integrated integral of the Modified Bessel function, Kq (r) (Abramowitz and Stegun,

1964a).

These results are illustrated in Figure 1^, which shows profiles of radiative flux to a vertical surface as a function of the dimensionless optical depth, /Cj^d . The floor and ceiling temperature ratios are respectively, Ta/Tn = 1/3 and T, /Th = 2/3. The most important observation is the extreme sensitivity of

the results to the optical depth of the hot layer. In the present example, the flux corresponding to radiative

equilibrium is not reached anywhere for any of the profiles. The maximum value of the flux is reached

near the center of the hot layer, since both the upward and downward moving radiation have had some

chance to be absorbed and re-emitted at the higher temperatures. However, unless the layer is either

much thicker or sootier than is the case for the present example, the equilibrium flux cannot be achieved.

Figure 1-4. Dimensionless heat fluxes showing profiles as a function of dimensionless

height, Z. The flux profiles correspond to values of dimensionless optical depth.

8 NISTNCSTAR 1-5G, WTC Investigation
Sub-Grid Scale Model for Radiative Heat Transfer

1.4 CONVECTIVE HEAT TRANSFER

Structural elements can heat up or cool down due to convective heating or cooling from the gas that surrounds the structure. Radiative heat transfer described in the earlier sections is the dominant mode of

heat transfer at high temperatures, while convective heat transfer is dominant at relative low temperatures.

The overall effect of convection is included through Newton's law of cooling:

Here, the heat-transfer rate. q. is related to the overall temperature difference between the wall and fluid and the surface area A. The quantity, h, is called the convection heat transfer coefficient and is a function

of the velocity of the fluid and its thermo-physical properties. The fluid temperature, , of the ambient

air is obtained from the results of the FDS simulations and is a function of the location of the structural element relative to the hot layer.

The coupling between fire dynamics and structural analysis due to radiative and convective heat transfer from the hot combustion products to the structural elements is studied. A simple radiative transport model

is developed, that permits the prediction of radiative flux incident on the surface as a function of the orientation of the structural element, temperature, hot layer depth, and soot concentration. The model is used to study radiative and convective heat transfer to structural elements of the WTC tower. Chapter 2

through Chapter 7 describe the results of this analysis at the component level, while Chapter 8 through

Chapter 1 1 present results of the global tower analysis.

q = hA(T,,-rj (18)

1.5 SUMMMARY

NISTNCSTAR 1-5G. WTC hvestigation 9
Chapter 1

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10 NISTNCSTAR 1-56, WTC Investigation
Chapter 2

Model Validation and Accuracy Assessment

Chapter 1 of this report described a methodology for coupling fire simulations with the thermal response

of structural elements. This methodology was used extensively in National Institute of Standards and

Technology's (NIST's) World Trade Center (WTC) Investigation to predict the thermally induced

structural response to spatially and temporally developing fires. To test the accuracy of the methodology,

a series of large-scale fire experiments was conducted in the NIST Large Fire Laboratory from March 13

to March 26. 2003. The experiments established a data set, which was used to test that the models were

accurately capturing the thermal response of the structural elements.

In this chapter, an application of Fire Structure Interface (FSI) to simulate these large-scale experiments

will be discussed. Model predictions were found to compare favorably with experimental results. The

accuracy of the FSI results and its sensitivity to changes in various input parameters is assessed.

Differences between numerical predictions and experimental data were attributed to uncertainty in physical input parameters, the most important of which was the variability in fireproofing thickness on a

structural component and to a lesser extent the heat release rate in fire simulations.

2.1 OVERVIEW OF EXPERIMENTS

A steel-frame compartment (3.60 m by 7.04 m by 3.82 m high) lined with 25 mm thick calcium silicate board (referred to as Marinite. manufactured by BNZ Materials, Inc.) was constructed. Several steel components (two trusses, one thin-walled tubular column, and a simple rod) were placed in this compartment. The components were either left bare or had a fibrous sprayed fire-resistive material

(SFRM) applied in two nominal thicknesses (17 mm and 34 mm).

The fire was generated using liquid hydrocarbon fuels introduced by a two-nozzle spray burner onto a

1 m by 2 m pan (0.1 m deep). The fire sizes tested were between 1.9 MW and 3.4 MW to assure that the

structural components were immersed in flames and hot gases. The fire pan was located on the floor of

the enclosure, 3.5 m from the front of the compartment (west side). Two fuels were used. A blend of

heptane isomers ("heptane") was selected as the baseline fuel. A mixture composed of 60 percent

heptane and 40 percent toluene by mass was selected as the second fuel, as it is representative of fires that yield elevated amounts of soot.

Figure 2-1 is a schematic drawing of the compartment and its contents. The compartment was equipped

with eight openings. There was no forced ventilation in the compartment, and the ventilation was solely

induced by the fire. The openings through which fresh air entered were located 1 m above the floor on the

west wall of the compartment. The openings through which heat and combustion products were emitted were located 2 m above the floor on the other end of the enclosure. The compartment was located so that the exhaust vents were under the 6 m by 6 m exhaust hood. Fire products flowed into a large exhaust

hood for measurement of the heat release rate and were then exhausted from the building. A vertical

NISTNCSTAR 1-5G, WTC Investigation 11
Chapter 2

baffle 1 m high, 2 m wide, and 0.15 m deep, was placed 2 m from the west wall of the compartment to

limit flame tiU. The experiments are described in detail in NIST NCSTAR l-SB."^

3.68 m

Inlet WEST

Outlet

EAST

Figure 2-1. Schematic drawing of compartment contents.

2.1.1 Test Matrix

The test matrix for which FSI simulations were performed and the experimental conditions for each test are presented in Table 2-1. The table lists the test number, the date the test was conducted, the nominal

fire heat release rate ( Q), the fuel type, the nominal thickness of the SFRM on the steel, and the

approximate test duration.

Table 2-1. Test Matrix for comparison with numerical simulations.

Nominal Q SFRM Nominal Duration

Test Date (MW) Fuel Thickness (mm) (min)

1 3/13/03 2.0 Heptane None 14.3

5 3/24/03 3.0 Heptane 17, 34 51.2

More than 350 channels of data were acquired for each of the experiments. Measurements were made of

the fuel flow, steel temperatures, heat release rate, local radiative and total heat flux to steel and SFRM,

gas phase temperature and location of upper and lower layers. Thermocouples were placed on the surface

of the walls and ceiling, within the walls, on the surface of the steel components, and at the surface of the

SFRM. Heat flux gauges were placed strategically around the compartment to measure the transport of

radiant energy. Bare-bead Type K 30-gauge thermocouples were used to measure the temperature

histories at the steel and SFRM surfaces. Thennocouple beads were spot welded to the bare steel surfaces.

In this report we will present FSI results for a few representative cases in Test 1 (un-insulated

components) and Test 5 (insulated components). A complete description of all the results, including the

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

to this report.

12 NIST NCSTAR 1-5G, WTC Investigation
Model Validation and Accuracy Assessment

instrumentation, measurement techniques, locations, and uncertainty associated with each measurement

has been presented in NIST NCSTAR 1-5B.

2.1.2 Steel Elements

The time varying temperatures of the steel components were measured to determine the thermal response

of structural steel components to quasi-steady controlled room fire conditions, including flame

immersion, and to establish a data set to validate predictions of the temperature rise of insulated and uninsulated structural steel components. The components were tested in an un-insulated state in Test 1 and

with an insulating coating of fireproofing or fibrous SFRM in Test 5. Steel components were selected to meet several criteria, including the following:

• Components were sized to be similar to those in WTC 1 and WTC 2.

• A bar was included as a simple reference case for analysis purposes.

• Various types of structural components were selected to measure the heating effect of fires on

horizontal and vertical steel components.

• The cross sectional thicknesses of the steel and the SFRM were selected based on

representative values used in the WTC towers.

Three t\'pes of steel components were selected for study: two bars, a hollow steel tubular column, and two

bar-joist trusses. The bars were nominally 3 m long and 25 irun in diameter. The columns were 0.26 m by

0.36 m tubular steel sections with a nominal 6 mm wall thickness. The trusses were 4.6 m long and 0.8 m

deep with 64 mm to 76 mm double angles for the top and bottom chords. The top and bottom chords of

the truss were 0.84 m apart and were bound by three web bars (25.6 mm thick), emanating from a single

location on the bottom chord every 1 .53 m. The top surface of the truss chords was located 15 cm from

the ceihng. Detailed drawings are given in NIST NCSTAR 1-5B. The steel components were constructed

of A572 steel, and the density was taken as 7860 kg/m^; its specific heat was 450 J/kg-K (McColskey and

Leucke 2005).

2.1.3 Sprayed Coating of the Steel

The SFRM product used in the WTC towers and these experiments was BLAZE-SHIELD DC/F, which was manufactured and supplied by Isolatek International. BLAZE-SHIELD DC/F is a spray applied

smgle-package factory controlled premix, based on a mixture of mineral wool fibers and cement binders.

The manufacturer reported average density is 208 kg/m^ (minimum), and its thermal conductivity is 0.042

W/m-K at 24 °C. The SFRM was applied by an experienced applicator from Isolatek. A nominal SFRM

thickness was specified for each steel component, either 17 mm or 34 mm. The applicator took

considerable care to apply an even coating of the specified thickness of SFRM to the steel components.

The first coating adhered to the bare steel, and the second coating was applied the following day to obtain

the required thickness. The SFRM was dried in place for four weeks, as required by the manufacturer,

before proceeding with thickness measurements, placement of thermocouple beads just under the SFRM

surface, and verifying the thermocouple locations and function prior to testing.

NIST NCSTAR 1-5G, WTC Investigation 13
Chapter 2

A pin thickness gauge specifically designed for SFRM products was used to measure the applied coating.

The gauge can measure to the nearest 1 mm; its measurement accuracy is 0.5 mm. A significant variation

in the thickness ofthe sprayedfire protective coating applied to the steel components was measured. The

SFRM mean thickness, //, was greater than the specified nominal thickness by 20 percent to 35 percent

topically, but ranged in valuefrom 5 percent to 42 percent greater. The standard deviation, a, assuming

a normal distribution ofthicknesses, rangedfrom 0.30 cm to 0.82 cm. The standard deviation was

influenced by the shape of the structural element as well as by the lumpy nature of the SFRM. A smooth

surface cannot be obtained when the product is spray applied. The coefficient of variation (COV), where

COV = o/|i, ranged from 0.17 to 0.27 for the bars, columns, and trusses, with the exception of the column

in Test 5 which had a COV=0.07. This lower COV occurred for the condition in which a thicker SFRM

coat was applied to a smooth steel surface. Lower COV values were measured for the columns; the

higher COV values occurred for the bars and angles.

We next describe the application of FSI and the methodology described in Chapter 1 for coupling the fire simulations with thermal response of structural elements and for simulating the experiments described in

this chapter. The experimental measurements are compared with model predictions for bare and

insulated steel components. Results are analyzed in terms of measurement uncertainty and model

sensitivity.

2.2 MODEL SIMULATIONS

The methodology for coupling fire dynamics with thermal response of structural components, described

in detail in Chapter 1 of this report, was employed to simulate the experiments and to predict the thermal

response of structural components placed in the compartment and subjected to realistic fires. The

methodology used here was analogous to that for the global analysis of the WTC towers and is discussed

extensively in Chapter 8 through Chapter 1 1

.

Finite element models of the structural components were developed using the ANSYS parametric design

language (Franssen, 1995) and coupled with fire simulations to predict their thermal response. Figure 2-2

is a finite element representation of the 3.05 m long steel bar. The bar was divided into 30 uniform

divisions along its length. The sub-figure on the right in Figure 2-2 shows the components through a cross-section of the steel bar and SFRM. The bar diameter was 2.54 cm. The thickness of the SFRM on

the various components tested in the experiments was reported in NIST NCSTAR 1-5B. In Test 5, the

SFRM had a mean thickness of 2.30 cm ± 0.55 cm. In the calculations, the SFRM thickness was assumed

to be uniform over the entire length of the component. A sensitivity analysis, presented below, was

performed to assess the validity of this assumption.

Figure 2-3 shows a finite element representation of the steel truss, the insulation, and the ceiling. The

elements have been color coded with the components in blue representing the steel and those in violet

representing the fire protective coating. Simulations were perfonned with approximately six to ten elements through the thickness of the fire resistive coating. Surface elements were used to model the re- radiation back to the ambient atmosphere, as well as to apply convective flux boundary conditions to the

structural components.

Figure 2-A shows a top view of the finite element model of the steel column with SFRM. The crosssectional view shows the steel column with a uniform SFRM wrapped around it. A portion of the ceiling

14 NIST NCSTAR 1-5G, WTC Investigation
Model Validation and Accuracy Assessment

and floor were also included in the model in an effort to capture the radiative exchange between the

column and these elements. Accuracy of the predicted thermal response of the structural steel components is discussed below in terms of an analysis of model sensitivity.

Figure 2-2. Finite element model for the 3.05 m (10.0 ft) long steel bar, showing the

elements distributed uniformly along the length of the bar. Figure on the right exhibits a cross-sectional view of the bar showing mesh density.

The compartment fire was modeled using the NIST FDS (McGrattan 2004). The geometry and

dimensions of the compartment conformed to a simple rectilinear grid. A uniform numerical grid whose

cells were 10 cm on a side was chosen based on a grid resolution study. Figure 2-5 is a snapshot of a simulation showing the isometric surface of stoichiometric mixture fraction, which is an adequate

representation of the fire. The major geometric features of the compartment as seen from the south wall

looking toward the north are seen in the figure, as are the inlets on the left (west) and the outlets on the

right (east). Figure 2-5 also shows the temperature profile through the centerline plane for one of the

simulations as seen fi-om the south wall looking north.

The fire heat release rate and the yields of the major combustion products were prescribed in the

simulation. No attempt was made to model the spray burner, and the distribution of the heat release rate was assumed to be uniform over a 1 m by 1 m area in the fuel pan, which was consistent with

observations of the hydrocarbon spray fire. A uniform numerical grid, with cells 10 cm on a side was

chosen based on a grid resolution study. The large-scale temperature and other thermo-physical properties

in the gas phase predicted using the CFD fire model were subsequently used in the finite element analysis

to apply a radiative flux on the surface of the structural elements and to predict the three-dimensional time

dependent thermal response (Prasad and Baum 2004).

Thermo-physical properties of steel and SFRM as a function of temperature, were needed for analysis.

These properties include density, specific heat, and thermal conductivity. Steel components were

constructed of A572 steel, which has thermo-physical properties very similar to A242 steel (McCloskey

and Fields 2005). Properties for BLAZE-SHIELD DC/F were estimate by Harmathy (1983). The

thermo-physical properties measured as part of the NIST Investigation were not available at the time

when these calculations were performed, hence the use of the data from Harmathy (1983). The Harmathy

NISTNCSTAR 1-5G. WTC Investigation 15
Chapter 2

data for thermo-physical properties of fireproofing was, in general, comparable to the one measured as

part of the NIST Investigation (NIST NCSTAR 1-6A). The Harmathy data, however, shows a slightly

lower thermal conductivity at high temperatures compared to the NIST data. Lower thermal conductivity can result in lower predicted steel temperatures. Thermo-physical properties for Marinite were obtained

from Taylor et al. (2003).

A detailed description of the application of initial and boundary conditions and the methodology for performed the analysis is presented in Chapter 4 through Chapter 7.

Figure 2-3. Finite element representation of the insulated steel truss (blue), the

SFRM (violet), and the ceiling (red) used in the thermal analysis of Test 5.

Figure 2-4. Top view of the finite element model of the steel column with SFRM.

16 NIST NCSTAR 1-5G, WTC Investigation
Model Validation and Accuracy Assessment

Figure 2-5. Snapshot of the numerical simulation showing the fire and a contour plot of

the gas temperature along the compartment centerline, looking toward the north wall.

2.3 RESULTS

In this Section, the experimental measurements and the numerical simulations of the steel temperatures

are presented. Comparisons of the simulation results with the measurements are considered in terms of

measurement uncertainty and a sensitivity analysis of the model results.

The simulation of the temperature of the steel was considered for Test 1 and Test 5, which were two

representative experiments. Test 1 was a 2 MW heptane fire with a 15 min duration in which the steel components were bare (uncoated). Test 5 was a 3 MW heptane fire with a 50 min duration in which the

steel components were insulated with various thicknesses of SFRM.

2.3.1 Test 1

Figure 2-6 compares the numerical simulations (symbols) with the measured steel surface temperatures

(solid lines) at four locations on Bar A during Test 1. The location of the origin was taken as the north

end of the bar (at the top of the bar in the figures). The difference between predicted results and

experimental data is -9 °C on the low side and +1 °C on the high side. The shape of the simulated timetemperature results was similar to the measurements. Both curves increased in a monotonic fashion from

ambient values at time equal to zero, to finite values, which obtained a maximum when the fuel was

stopped, and subsequently decreased, also in a monotonic manner. The general character of these results was representative of all of the tests. For most locations, the absolute difference between the numerical

predictions and the experimental data was less than 20 °C at any time. Temperature measurements and

the model simulations differed along the bar at some locations because the asymmetry of the fire plume was not predicted in the FDS simulations.

Figure 2-7 compares the numerical simulations and the temperature measurements at various locations on

the uppermost surface 3.70 m above the floor on Truss A during Test 1. The numerical results match the trends of the measurement profiles reasonably well. For most locations, the maximum difference between

the measurements and the simulations was less than 10 percent. The maximum temperatures on the steel occurred approximately when the fuel flow was stopped, which was at 860 s.

NISTNCSTAR 1-5G. WTC Investigation 17
Chapter 2

Figure 2-8 and Figure 2-9 compare the numerical simulations with measurements for the steel surface

temperature for locations on the north, south, east and west faces of the bare column during Test 1 at heights of 3.69 m and 2.13 m above the floor, respectively. The location of the column relative to the

compartment is shown in Figure 2-1. In Figure 2-8 and Figure 2-9, both the numerical predictions and

the experimental measurements have been color coded for the four faces of the column. (The west face of

the column faced the compartment inlet.) The highest predicted and measured temperatures occurred on

the south face of the column. At one location on the south face (3.69 m above the floor), the largest

difference between the simulations and measurements was immediately after the start of the test, when the

temperatures were still relatively low, but the rate of temperature change was large. The largest

temperature difference generally occurred just before the fuel flow was stopped. In Test 1 , this was 900 s after ignition. For most locations, the relative differences between the peak measured and simulated

temperatures was less than 5 percent, whereas for some locations the differences were as large as 10 percent.

18 NISTNCSTAR 1-5G, WTC Investigation
Model Validation and Accuracy Assessment

550

1000 1500

Time(s)

3000

Figure 2-7. Comparison of numerical simulations with measurements for the steel

surface temperature at four locations 3.70 m above the floor on Truss A in Test 1.

Figure 2-8. Comparison of numerical simulations with measurements for the steel

surface temperature at four locations 3.69 m above the floor on the column in Test 1.

NISTNCSTAR 1-5G, WTC Investigation 19
Chapter 2

500

450

400

350

O 300

3 250

§_200

E^ 150

100

50

0

'^%00 0 500 1000 1500 2000 2500 3000

Time(s)

Figure 2-9. Comparison of numerical simulations with measurements for the steel

surface temperature at four locations 2.13 m above the floor on the column in Test 1.

2.3.2 Test 5

Figure 2-10 through Figure 2-15 compare the simulations with measurements at locations on the steel surface and SFRM surface for the bar, column, and truss in Test 5. For the bar and the column, the

maximum difference between the measurements and the simulations was less than 15 percent, which

generally occurred shortly after the fuel flow was stopped. For Truss A, the maximum difference

between the measurements and the simulations was typically less than 20 percent. At one location,

however, the temperature difference was as large as approximately 30 percent. The reason for the

magnitude of this difference appears to be variability in fireproofing thickness, thermo-physical properties

of fireproofmg, and heat release rate associated with FDS simulations. Other reasons for the differences

between numerical simulations and temperature measurements are discussed below in terms of

uncertainty in the measurements and a sensitivity analysis of the model.

Experimental Data

North Face

i . West Face

J I I I I I I ] I \ Lj I I I Lj i lj I i \ l_j_J i i i l

20 NISTNCSTAR 1-5G, WTC Investigation
Model Validation and Accuracy Assessment

Figure 2-10. Comparison of numerical simulations with measurements for the

temperature of the SFRM surface at four locations on the insulated bar in Test 5.

Figure 2-11. Comparison of numerical simulations with measurements for the

temperature of the steel surface at four locations on the insulated bar in Test 5.

NISTNCSTAR 1-5G, WTC Investigation 21
Chapter 2

800

600

O

-*—» CO

0)Q.

400

t- 200

Experimental Data

North) Face

East Face

South Face

West Face

-1000 1000 2000 3000

Time (s) 4000 5000 6000

Figure 2-12. Comparison of numerical simulations with measurements for the

temperature of the SFRM surface at four locations, 3.69 m above the floor on the

insulated steel column in Test 5.

300

250

200

O

^ 150

•—

(0

1-Q

|-100 -

I- 50

-5' 000

Experimental Data

North Face

East Face

South Face

West Face

North Face

East Face

South Face

\Afest Face

1000 2000 3000

Time (s) 4000 5000 6000

Figure 2-13. Comparison of numerical simulations with measurements for the

temperature of the steel surface at four locations, 3.69 m above the floor on the

insulated steel column in Test 5.

22 NISTNCSTAR 1-5G, WTC Investigation
Model Validation and Accuracy Assessment

1200

1000 -

800 - O

I 600

.1) 400 -

200 -

0 -

Experimental Data

TU-1 _ TU-2

TU-3

TU-4

Numerical Results

TU-1

TU-2

TU-3

TU-4

1000 2000 3000

Time (s) 4000 5000 6000

Figure 2-14. Comparison of numerical simulations with measurements for the

temperature of the steel surface at four locations 3.70 m above the floor on

Truss A in Test 5.

600 -

a?

TO

Experimental Data

TU-1

TU-2

TU-3

TU-4

NuPDerical Results

TU-1

TU-2

TU-3

TU-4

± _L I 1000 2000 3000

Time (s) 4000 5000 6000

Figure 2-15. Comparison of numerical simulations with measurements for the

temperature of the steel surface at four locations 3.70 m above the floor on Truss A in Test 5.

NISTNCSTAR 1-5G. WTC Investigation 23
Chapter 2

2.3.3 Quantification of Agreement between Models and Measurements

The level of agreement between the calculated and the measured values of the peak temperature for the

various steel components are summarized in Table 2-2 and Table 2-3 for Tests 1 and 5, respectively.

The tables list the difference, for each location, between the peak values of the measured and the

stimulated steel surface temperatures, normalized by the average of those temperatures, with results listed for the hottest and the coolest temperature locations on each element. A negative percentage in the tables indicates that the numerical simulation was less than the measured value, whereas a positive percentage

indicates that the simulation was greater than the measured value. The average of the absolute values of

the percentages is given at the bottom of each table and the average of the percentages are shown in parenthesis.

Table 2-2 shows that the values were negative for Test 1 in most cases, indicating that the numerical

predictions consistently under predicted the measurements by a finite, but small percentage. The average

of the absolute value of the differences was less than 5 percent, although some individual differences were as large as 10 percent for some locations on some of the elements. Similar results were found in Test 5, although the agreement between models and simulations was somewhat less. Table 2-3 shows that the values were also generally negative for Test 5, indicating that the numerical predictions consistently

under predicted the measurements. The average of the absolute value of the differences was less than

20 percent, although some individual differences were as large as 33 percent for some of the locations on some of the elements.

The major difference between Test 1 and Test 5 is the presence of fireproofing on the structural components. The methodology for measuring temperature and for performing the FSI simulations did not change between Test 1 and Test 5. Agreement between model predictions and experimental data is somewhat less because of uncertainty associated with fireproofing thickness (variability in fireproofing

thickness along the length of the structural component) and its thermo-physical properties.

The uncertainty associated with a type K thermocouple measurement for near steady conditions is about

2 °C [Omega, 2000]. Because the magnitude of the disagreement shown in Table 2-2 and Table 2-3

could not be explained solely by uncertainty in this measurement, further analysis was necessary and is described below.

Table 2-2. Percentage difference between peak values of the measured and the

simulated steel surface temperatures at the hottest and coolest locations on various un

insulated components in Test 1. Element (Vertical Position) Hottest Location Coolest Location

Bar A -10% 3 %

BarB 8 % -2 %

Column (2.13 m above floor) 2 % -2 %

Column (3.69 m above floor) -3 % -6%

Truss A (2.89 m above floor) -4% 2 %

Truss A (3.29 m above floor) -6% -8%

Truss A (3.70 m above floor) 1 % -10%

Absolute Value of Average (Average) 4.9% (-1.7%) 4.7% (-3.3%)

24 NISTNCSTAR 1-5G, WTC Investigation
Model Validation and Accuracy Assessment

Table 2-3. Percentage difference between peak values of the measured and the

simulated steel temperatures for the hottest and coolest locations for various

com ponents with SFRM in Test 5. Element Hottest Location Coolest Location

Bar^ 8 % 3 %

Column (0.77 m above floor) -20 % 3 %

Column (2.13 m above floor) -11 % -13 %

Column (3.69 m above floor) -30 % 25 %

Truss A (2.89 m above floor) 15 % -15 %

Truss A (3.29 m above floor) -15 % -33 %

Truss A (3.70 m above floor) -10 % -30 %

Absolute Value of Average (Average) 16 %(-9 %) 20% (-11 %)

a. Onlv one bar was used in Test 5.

2.3.4 Experimental Uncertainty and Model Sensitivity of the Steel Temperatures

Model sensitivity was considered in an effort to understand the effect of uncertainty of the various input

parameters used in the NIST FSI models on the calculated thermal response of the structural steel components. This information is useful for understanding the differences between the simulated and

measured temperature behavior of the steel members in the experiments. The sensitivity study focused on

the effect of variability in fireproofmg thickness, the gas phase heat release rate, and the FDS grid on steel temperature.

Sensitivity to the Thickness of the Sprayed Fire-Resistive Material (SFRM)

A major factor that influenced the thermal response of steel was fireproofmg thickness and its variability

along the length and perimeter of the structural components. The SFRM thickness was not uniform about

a strucmral element, even a simple symmetric element such as a bar. On the bars, for example, the SFRM

thickness was not uniform about all sides of the bar - even at a single location along the axis of the bar. The non-uniform thickness led to three-dimensional effects that impacted the thermal behavior of the

SFRM, effectively increasing the uncertainty in the simulated steel temperature underneath the SFRM.

The importance of this effect can be characterized by the coefficient of variation (GOV) of the SFRM

thickness (the ratio of the standard deviation to the mean thickness) and is bounded by the maximum and

minimum values of the SFRM thickness. The coefficient of variation ranged from 0.17 to 0.27 for the

bars, columns, and trusses.

The SFRM thickness in the calculations presented here was based on the mean value, which was assumed

to be uniform over the entire length of the bar. In Test 5 for example, the mean thickness of the SFRM on

the bars was measured as 2.3 1 cm, and the GOV was 0.24. Numerical simulation of the steel surface

temperature for Bar A in Test 5, assuming a 25 percent decrease in the thickness of the SFRM, led to a temperature increase of approximately 10 percent as compared to the baseline results. Locally, the SFRM

thickness on some sections of the bars was smaller than the mean by as much as 45 percent. This indicates

that the variation in the calculated steel surface temperature was as much as 20 percent simply from

NISTNCSTAR 1-5G, WTC Investigation 25
Chapter 2

uncertainty in the SFRM thickness. Sensitivity of resuhs to fireproofing thickness will be discussed in a systematic manner in Chapter 3 of this report.

The difference between Test 1 and Test 5 is the presence of fireproofing on the structural components.

The methodology for measuring temperature and for performing the FSI simulations did not change

between Test 1 and Test 5. Comparison between model predictions and experimental data is somewhat

less because of uncertainty associated with fireproofing thickness (variability in fireproofing thickness

along the length of the structural component) and its thermo-physical properties.

Sensitivity of the Steel Temperature to Thermo-physical Properties

Thenno-physical properties of steel and SFRM (used in this study) may have also had an influence on the

predicted steel temperature. The effect of uncertainty in the thermal conductivity of the SFRM will also

play a role in the computations performed using the finite element procedure. Typical uncertainty in the

determination of thermal conductivity using the ASTM standard method is not better than ±6 percent

(ASTM 2000). For insulafion materials like BLAZE-SHIELD DC/F SFRM, the measurement uncertainty

may be considerably larger, although it has not been quantified. According to the manufacturer, the

thermal conductivity for BLAZE-SHIELD DC/F was 0.042 W/m-K at 24 "C, which was quite different

from Harmathy's data (Harmathy, 1983), which was used in the baseline calculations reported here. The

Harmathy data exhibits slightly lower theraial conductivity at high temperature as compared with the

NIST data (NIST NCSTAR 1 -6A).

The sensifivity of the calculation results to the thenno-physical properties was investigated by considering an idealized situation where a 2.5 cm bar with 2.5 cm of insulation was exposed to a constant source of

heat flux. The results showed a 7 percent lower steel temperature for the calculations that used the

Hannathy data as compared to the calculations that used the NIST data. Lower thermal conductivity for

SFRM can result in lower steel temperatures. The sensitivity of FSI to the thermo-physical properties of

the SFRM analysis increased uncertainty in the simulation.

BLAZE-SHIELD DC/F has a significant amount of moisture that can evaporate under fire conditions.

The role of moisture was not accounted for in the simulations described in this chapter. However, the role of moisture was included in the simulations described in Chapter 4 through Chapter 1 1 by incorporating

the heat of evaporation in an enthalpy formulation.

The optical properties of the steel and the SFRM likely did not play an important role, as soot rapidly

coated the steel components upon ignition of the fire.

Sensitivity of the Steel Temperature to Heat Release Rate

The temperature of the steel elements in Test 1 were calculated using the methodology described in this report with heat release rate 10 percent higher than the baseline value (in the fire simulations). The

computed transient steel temperatures for the higher heat release rate fire were similar in shape to those

computed using the baseline heat release rate, but the rate of the temperature increase was always larger.

This led to simulated temperatures that were 5 percent to 2 1 percent higher than values in the baseline

case, depending on location, which either improved or worsened agreement with the measurements.

26 NIST NCSTAR 1-5G, WTC Investigation
Model Validation and Accuracy Assessment

The predicted steel surface temperature at two different heights on the column in Test 1 for a 10 percent

higher heat release rate than that used in the baseline simulations were paradoxically lower for the

increased gas phase heat release rate. Analysis showed that this occurred because the flame tended to bend away from the column for the simulated fire with the higher heat release rate. The simulated steel surface temperature on Truss A in Test 1 for a 10 percent higher heat release rate than that used in the

baseline calculations showed that the temperature of the steel on Truss A increased 4 percent to 29

percent for the higher heat release rate, depending on the exact location. Table 2-A summarizes the FSI

simulation results for the various steel elements. The table lists the percentage difference in the peak

value of the simulated temperature profiles between the baseline simulations and those with a 10 percent

higher heat release rate at the hottest and coolest locations on the steel surface of various un-insulated

components in Test 1 . The increased heat release rate and upper layer temperature led to changes in the

simulated steel surface temperature of 2 1 percent on average at the hottest locations on each of the

components, with a range from -38 percent to +29 percent. Table 2-4 shows that the percentage change

at the hottest locations was smaller on average than at the coolest locations.

Table 2-4. Percentage difference in the peak value of temperatures between the baseline

simulations and those with a 10 percent higher heat release rate at the hottest and

coolest locations on the steel surface of various un-insulated components in Test 1. Element (\ ertical Position) Hottest Location Coolest Location Bar A 5 % 5 %

BarB 21 % 14%

Column (3.69 m abo\e floor) -33 % -22 %

Column (2.13 m above floor) -38 % -26 %

Truss A (2.89 m above floor) 20% 5 %

Truss A (3.29 m above floor) 29 % 8%

Truss A (3.70 m abo\e floor) 4 % 4 %

Absolute Value of .Average 21 % 12%

The increase in the maximum calculated steel temperatures for locations on the bars, column, and truss should be compared with the change in the upper layer gas temperatures calculated by FDS, which

increased by 9 percent on average for the 10 percent increase in the heat release rate above baseline. The

change in the simulated steel surface temperature did not change linearly with the upper layer gas

temperature. The difference in the temperature increases on the steel can be attributed to changes in the

character of the simulated fire (captured by the FSI results), including its physical location, its size, and its extent, all of which impacted the calculated flux onto the structural steel components and, subsequently,

the calculated temperature distribution in the steel components.

Sensitivity to Grid Size

The predicted steel temperature for Bars A and B in Test 1 using coarse grid fire simulations were of

interest because the WTC calculations must be completed using a coarse grid due to large CPU time

required to completed the simulations. The computed steel temperatures using the coarse mesh were

similar in shape to those computed using the finer mesh, but the maximum calculated temperatures were

shifted by approximately 10 percent, to either larger or smaller values. The coarse mesh fire simulations

NISTNCSTAR 1-5G, WTC Investigation 27
Chapter 2

resulted in unifomi temperatures along the length of the bar, which was in contrast to the fine mesh

calculations, which led to finite temperature differences (5 percent) along the length of the bar. The

calculations show that FDS grid resolution has a finite effect on the structural analysis, although it is not

greater than the effect of model sensitivity to various model input parameters.

The difference between the measured and predicted steel surface temperatures (see Table 2-2) can be

explained through consideration of FSI model sensitivity to uncertainty in physical input parameters, the

most important of which was the variability in fireproofing thickness on a structural component and to a

lesser extent the heat release rate in fire simulations.

2.4 SUMMARY

A series of experiments was conducted to assess the models that were developed to predict the

temperature rise of stiuctural components within a compartment fire. The well-controlled large-scale

experiments described here were designed specifically to validate the NIST FSI, used to couple fire simulations with thermal behavior of structural components as applied to the investigation of the WTC

towers. Measurements were conducted to assess the accuracy of the model calculations.

On average, the numerical predictions of the steel surface temperature were within 5 percent of the

experimental measurements for the case of bare steel and within 20 percent for the steel components with

SFRM. For the bare steel, a series of calculations showed that these differences could be attributed to model sensitivity to the heat release rate in the fire model. For the insulated steel, the differences could be

attributed to model sensitivity to the variability in the SFRM coating thickness and thermo-physical

properties of fireproofing in addition to the uncertainties in fire modeling.

In conclusion, FSI couples the fires dynamics simulations with the thermal response of structural elements with an uncertainty that is small compared to the uncertainty in the measurements and the FDS

simulations. The study provides confidence in the application of the modeling methodology to the

investigation of the WTC disaster.

28 NISTNCSTAR 1-5G, WTC Investigation
Chapter 3

Exploratory Studies on Thermal Response of

Structural Elements

Modeling the global thermal response of World Trade Center (WTC) towers coupled with realistic fire simulations was a daunting task. Global analysis of each tower resulted in models that were

computationally intensive and required large memory for efficient calculations. At the same time, results of the global simulations were sometimes difficult to understand because of simultaneous variation in a number of parameters. Exploratory studies, that are computationally inexpensive, were performed during

the initial stages of the Investigation to understand the role of the important parameters on numerical

predictions. The objective of this chapter is to present these exploratory studies and to discuss the

sensitivity of the results to model parameters.

A large number of preliminar\' studies were conducted, to develop an understanding of how typical

structural elements behave under furnace conditions. Exploratory studies were limited to 2-D or axisymmetric representation of structural elements to help reduce computational costs. No attempt was made

to couple the analysis with fire simulations. Instead, the response of structural elements to a constant

radiative flux or gas temperature was simulated. The 2-D models allow the flexibility to conduct a large

number of parametric studies to estimate the time to reach a critical temperature under different

fireproofing scenarios and gas temperatures. The results of these exploratory studies have guided the

development of the component models (presented in Chapter 4 through Chapter 7) and the global analysis

discussed in Chapter 8 through Chapter 1 1

3.1 sensitivity study on core columns

The core columns were considered a critical structural element throughout the Investigation, and

sensitivity studies were conducted to predict the time required to reach a critical temperature under

different fireproofing thickness and damage scenarios as well as different incident flux (gas temperature).

Sensitivity studies on two box shape core columns are discussed in this section. The box columns were

chosen so that the predicted response for these two columns would cover the entire spectrum of responses

that are expected for the various core columns (each box shape core column in the affected floors of the

WTC tower was unique in its cross-sectional area and dimensions). The first of these two columns is a

light box shape core column, while the second core column is a heavy column.

3.1.1 Light Box Shape Core Columns

Figure 3-1 shows a finite element model of light box shape core column (15 by 17 by 1 3/16 in.) developed using the ANSYS structural analysis package finite element software. The steel is covered

with 1 1/8 in. of fireproofing (BLAZE-SHIELD DC/F). The thermo-physical properties for fireproofing

(used in the exploratory studies only and not in the main Investigation) were obtained from Harmathy

1983. In Figure 3-1, the elements have been color coded with the material attributes. Cyan colored

elements have material attributes of steel, while violet colored elements have attributes of fireproofing. As

NIST NCSTAR 1-5G. WTC Investigation 29
Chapter 3

mentioned earlier, exploratory studies were perfonned with 2-D models of core coluiruis. It was assumed

that the flux was constant over the entire length of the column. Heat conduction along the length of the

columns was neglected in this analysis.

Sensitivity studies described in this chapter de-couple the thennal analysis from fire simulations. Thermal

response of core columns was predicted by subjecting the columns to a constant radiative flux over their entire perimeter and for the duration of the simulation. There was no ramping of the incident flux. The

radiative flux q was related to gas temperature T by using the fonnula q = crT'^, where, cr is the StefanBoltzmann constant (5.67 x \Q~^WIm'I<^). The coluinn was not subjected to any convective flux from the

gases in these preliminary studies. (Convective heat transfer was included in the more detailed studies

presented in Chapter 4 through Chapter 11.) Radiation is the dominant mode of heat transfer at high

temperatures (1,100 °C gas temperature). Re-radiation to the ambient atmosphere and in the cavity

fonned by the four plates of the box column was modeled.

The finite element mesh in the steel and fireproofing has been shown with white lines superimposed on

the model shown in Figure 3-1. Because of the 2-D nature of the model, computational time (CPU time) was no longer a deciding factor, and the mesh density was varied to study its effect on the analysis. The

sub-figure on the right in Figure 3-1 shows typical temperature contours (Kelvin) through the steel and

fireproofing at one instant in time, ranging from 300 to 1,400 Kelvin. High surface temperature on the

surface of the fireproofing is predicted, and the energy gradually diffuses into the steel.

Figure 3-1. Finite element model of light box shape core column (left) and temperature

contours (Kelvin) at one instant in time.

Simulations were performed with different fireproofing thickness and gas temperatures to estimate the

time to reach a critical temperature. These results have been shown in Table 3-1. Fireproofing thickness was set at 0, Vi and 1 1/8 in. The first row shows gas temperature and critical temperature of the steel column. The first box for example shows that the gas temperature was set to 700 °C, and the critical temperature of steel was 573 °C. Gas temperature ranges from 700 °C to 1,100 °C. The boxes to the right

of fireproofing thickness show the time in seconds for the steel to reach the specified critical temperature.

For the conditions that are being simulated, we do not observe any significant temperature gradient in

steel (see right sub-figure in Figure 3-1).

Table 3-1 shows that it would take approximately 16,750 s (4.6 hours) for this light box shape core column (covered with 1 1/8 in. thick fireproofing) to reach a critical temperature of 700 °C when

subjected to a gas temperature of 1,100 °C (radiative flux of approximately 200 kW/m'). As the critical

30 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

temperature reduced from 700 °C to 538 °C, the time to reach the critical temperature also reduced to 2.9 hours.

The choice of critical temperature was based on the strength of material as a function of temperature.

Given enough time, any structural element, irrespective of its size, shape, and fireproofmg thickness will

reach the gas temperamre. One measure of the critical temperature, thus, can be the gas temperature itself Steel howe\ er. loses a large fraction of its strength before it reaches the gas temperature. Figure 3-2

shows non-dimensional yield strength of typical steel (NIST NCSTAR 1-3) as a function of temperature.

The figure shows that steel has only 10 percent of its strength at 700 °C, and that there is a rapid drop in strength in the 500-600 °C range. The choice of critical temperature in the 500-700 °C range was dictated

by this degradation in mechanical strength of steel.

Table 3-1. Time (in seconds) to reach critical temperature for light box shape core columns.

Gas Temperamre / Critical Temperature (°C)

700/538 700/650 900/538 900/600 900/700 1100/538 1100/600 1100/700

Bare 1100 2000 450 510 700 201 255 350

Thickness

1/2 inch

11750 21700 6550 8100 11545 4450 5300 7250

Thickness

1 1/8 inch

26424 36000 15191 18691 26691 10593 12593 16750

20%

damageHottest

location

12148 23648 7000 8750 12800 4800 6000 7850

20% damage

-Coolest

location

15000 28000 9312 11400 16300 5800 6900 9138

As expected, the time to reach critical temperature increases if the gas temperature reduces. If the gas

temperamre is 900 °C, then it takes the steel column 26,691 s (7.4 hours) to reach a critical temperature of

700 °C. If the gas temperamre is 700 °C, then it would take 10 hours to reach a critical temperature of

650 °C. Realistic fire simulations (NIST NCSTAR 1-5F) show that the fire moves from one location to another at 15-20 min intervals, depending on how quickly the combustibles are consumed. It is highly

unlikely that any column would see a continuous heat flux at the specified intensity for the duration of the

simulation.

NIST NCSTAR 1-5G, WTC Investigation 31
Chapter 3

0 200 400 600 800 1000

T (C)

Figure 3-2. Non-dimensional yield strength as a function of temperature for typical steel

used in WTC tower (NIST NCSTAR 1-3)

As the insulation thickness decreases from 1 1/8 in. to Yi in., the columns heats up quicker when subjected

to a constant radiative flux. At V2 in. the column takes approximately 7,250 s (2 hours) to reach a critical temperature of 700 °C with a specified gas temperature of 1,100 "C. If the column is completely bare (no

fireproofing) then its temperature increases very rapidly, and the critical temperature is reached within

350 s. For a bare column, the time to reach a critical temperature of 700 °C ranges between 350 to 2,000 s.

It is noted that the time to reach critical temperature for bare columns is less than one hour, the period

during which the buildings withstood intense fires. Core columns (similar to the one studied in this section) that have their fireproofing intact can not reach a critical temperature of 600 °C during the 1 or

1 Yi hour period. (Note that WTC 1 collapsed in approximately 1 Y2 hour, while WTC 2 collapsed in approximately 1 hour). This implies that if these core columns played any role in the final collapse, some

fireproofing damage, would be required to result in thermal degradation of it strength.

32 NIST NCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

3.1.2 Heavy Box Shape Core Columns

The comer columns of the core of the WTC towers had a heavy box shape construction. These columns were studied in a manner similar to the one for light box shape columns. Figure 3-3 shows a similar finite element model of a hea\y box column (22 by 22 by 3 by 2 15/16 in.). The sub-figure on the right shows

predicted temperature contours at one instant in time. Element density in the steel plates of heavy box

shaped core columns was varied to resolve any temperature gradient in the plates.

Table 3-2 shows time to reach critical temperature for heavy box shaped core columns for various

fireproofing thicknesses and gas temperatures. Results indicate that a heavy box column would take

approximately 14 hours to reach a critical temperature of 700 °C, when exposed to gas temperature of

1,100 "C. Reducing the fireproofing thickness to 0.5 in., results in more than 50 percent reduction in time

to reach a specified critical temperature. The heavy box core columns take a lot longer to heat up as compared with hght box columns because of the larger heat capacity associated with the thick section.

The trends with changing fireproofing thickness or gas temperature are similar to those for light box

columns.

m

a

n

Figure 3-3. Finite element model of heavy box shape core column (left) and temperature

contours (Kelvin) at one instant in time.

For bare columns, the time to reach a critical temperature of 700 °C is approximately 950 s when exposed

to a gas temperature of 1.100 °C for the entire duration. We conclude, that significant thermally induced

degradation of strength in such core columns would have been possible only if they had been bare, or if there was significant damage to the fireproofing on these columns.

NISTNCSTAR 1-5G, WTC Investigation 33
Chapter 3

Table 3-2. Time (in seconds) to reach critical temperature for heavy box shape core columns.

Gas Temperature / Critical Temperature (°C)

700/538 700/650 900/538 900/600 900/700 1100/538 1100/600 1 100/700

Bare 3200 5580 1250 1500 2100 550 800 950

Thickness

1/2 inch

>36000 >36000 20550 25500 37000 14000 17000 23000

Thickness

1 1/8 inch

>72000 >72000 46000 57000 84000 32040 38000 51040

20%

Hottest

location

>36000 >36000 22900 28401 42000 17500 21370 28972

20% damage

-Coolest

location

>36000 >36000 25901 31901 46000 17972 21472 28990

3.2 STATISTICAL VARIABILITY IN FIREPROOFING THICKNESS

The fireproofing was sprayed on to the various structural elements, and it is conceivable that the

fireproofing thickness varies from one location on the column to another. Variability in fireproofing

thickness for the World Trade Center Towers has been recorded in NIST NCSTAR 1-6A. The

experiments conducted at the large fire facility to study the response of insulated structural elements to 2-

3 MW fires also recorded a variability in fireproofing thickness (NIST NCSTAR 1-5B).

Statistical variability in fireproofing thickness was incorporated in the finite element model through a scheme based on the use of a random number generator. The random number generation scheme

identified a subset of elements (with material attributes of fireproofing). The material attributes of the

selected elements were modified, such that those elements offered no resistance to heat transfer. Figure 3-4 shows a finite element model of a thick box column with statistical variability in fireproofing

thickness. The fireproofing elements for which the material attributes were modified are shown in red. Because of the randomness of fireproofing damage the effective thickness of fireproofing becomes

smaller, which results in faster heating up of the colunm.

Table 3-1 and Table 3-2 show the effect of 20 percent random damage to fireproofing on the time to reach a certain critical temperature. Because of the variability in fireproofing thickness, small

temperature gradients can set up in the column. Two sets of number are recorded in Table 3-1 for the

hottest and coolest location on the column. The initial thickness of fireproofing was assumed to be the

specified thickness of 1 1/8 in., and 20 percent damage was subsequently included in the model, resulting

34 NIST NCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

in a smaller equivalent thickness. These results indicate that the time to reach a critical temperature of

700 °C is approximately 7.850 s when subjected to a gas temperature of 1,100 °C. As compared to the

time of 16,750 s for an un-damaged structure, statistical variability in fireproofmg thickness has resulted

in 50 percent reduction in heat up time. This is due to a combination of the reduction in equivalent

thickness and the variability in fireproofing thickness.

Figure 3-4. Effect of statistical variability in fireproofing thickness on steel temperature.

Temperature contours have been shown in Kelvin.

3.3 SELECTIVE FIREPROOFING DAMAGE ON ONE FACE OF THE COLUMN

One of the objectix es of this Investigation was to simulate the thermally induced response of a structure

that has been damaged by aircraft impact. The aircraft impact can knock fireproofmg from the columns.

In the previous section, the effect of complete fireproofmg damage (no fireproofmg over the entire

penmeter of the column) was studied. In this section we discuss the effect of selectively damaging

portions of the fireproofing on a column.

Figure 3-5 shows fireproofing damage on one face of a heavy core column (indicated by missing

fireproofing on the right side). The column is subjected to radiative flux on all its exposed faces. Temperature contours (Kelvin) shown in Figure 3-5 at 7,200 s after the application of radiative heating

indicate that the right side of the column heats up very quickly, while the left side stays relative cool,

resulting in a severe temperature gradient through the cross-section of the column. Temperature gradients

through a core column can result in differential thermal expansion and bowing of the column.

NISTNCSTAR 1-5G, WTC Investigation 35
Chapter 3

Figure 3-5. Effect of fireproofing damage on one face of a heavy core column.

Temperature contours have been shown in Kelvin.

Selective fireproofing damage on one or two faces can exist on core columns that are in the debris path of

an aircraft. This can result in a potentially severe thermal loading condition on the column. Although

fireproofing damage due to aircraft impact was incorporated in the global analysis (presented in Chapter 8

through Chapter 1

1 ), it was assumed that fireproofing was either completely damaged or fully intact. Selective fireproofing damage on one or two faces of any column was not accounted for in the main

investigation, due to lack of accurate techniques to estimate the extent of damage. These results are presented here to illustrate awareness of the extreme loading that can occur on the structure due to selective fireproofing damage and of its impact on thermally induced structural response.

3.4 SENSITIVITY STUDY ON WIDE FLANGE COLUMNS

A sensitivity study was conducted for wide flange columns in a manner identical to that for box columns

described in the earlier section. Two wide flange columns, one (12WF133) with a relatively light crosssection, and other (14WF730) with a heavy cross-section were selected for analysis. Table 3-3 and

Table 3—4 summarize the time to reach crifical temperature for wide flange columns 12WF133 and

14WF730, respectively. Results are presented for bare columns, columns with specified fireproofing

thickness of 2 3/16 in., columns with a 20 percent statistical variability in fireproofing, and columns with one face bare.

36 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

Table 3-3. Time (in seconds) to reach critical temperature for wide flange

column 12WF133.

Gas Temperanare / Critical Temperature (°C)

700/500 700/600 900/500 900/600 900/700 1100/500 1100/600 1100/700

Bare 580 900 220 320 450 100 150 180

Thickness

2 3/16 inch

>14400 > 14400 > 14400 >14400 > 14400 10200 12900 > 14400

20° 0 random

damage > 14400 > 14400 8200 10800 > 14400 4700 5900 7400

One face

bare

5000 8300 2650 3800 5900 1700 2500 3300

Table 3-4. Time (in seconds) to reach critical temperature for wide flange

column 14WF730.

Gas Temperature / Critical Temperature (°C)

700/500 700/600 900/500 900/600 900/700 1100/500 1100/600 1100/700

Bare 2260 3400 900 1250 1800 300 550 700

Thickness

1 3/16 inch

>14400 > 14400 > 14400 > 14400 > 14400 > 14400 > 14400 >14400

20% random

damage

> 14400 > 14400 14000 > 14400 > 14400 7200 9500 12200

One face

bare

11500 > 14400 5800 8400 12200 3500 5000 6700

3.5 SENSITIVITY OF THERMAL RESPONSE TO FIREPROOFING GEOMETRY

The fireproofing thickness has a great effect on the thermal response of the structural elements for a given

fire condition. UTiile others have considered the effect of thickness of fireproofing, the role of variation of

thickness along the length of a member or fireproofing damage (gap in fireproofing) in a portion of the

member, is not well known. A sensitivity study is described to investigate the effect of fireproofing

thickness, fireproofing damage, and variability in fireproofing thickness on thermal response of structural members.

NISTNCSTAR 1-5G, WTC Investigation 37
Chapter 3

Figure 3-6 shows a bridging truss of the WTC towers floor system. The picture shows the upper and

lower flange of the truss and the web diagonals that form the "M" shaped pattern. Fireproofmg thickness

varies along the length of web diagonal. The web diagonal on the far left of the picture, shows very thick

fireproofmg at the bottom and less fireproofmg on the top. This picture shows that fireproofmg was not applied evenly and that its thickness varied along the length of the diagonal. Fireproofing thickness also

varied from one diagonal to another.

The third diagonal from the left in Figure 3-6 shows fireproofmg damage (gap in fireproofmg). This

diagonal has fireproofmg intact at the top and bottom, and fireproofmg is completely missing in the

middle. Also note that fireproofmg at the top and bottom of the diagonal has variability in thickness. In

this section, the effect of variability in fireproofmg thickness and gap size (as observed in Figure 3-6) on

thermal response of structural member is presented.

Figure 3-6. Example of variability in fireproofing thickness and "gap" in fireproofing on

diagonal member of a bridging floor truss.

The simplified model that was used is shown in Figure 3-7 (a). A 1 in. thick, 60 in. long steel plate (cyan

color) was coated with fireproofing material (purple color) and subjected to the uniform radiative flux

arising from a 1,100 °C fire. In this figure the fireproofmg thickness is 2 in., but this thickness can change from one simulation to another simulation. A two-dimensional model using the ANSYS finite element software was developed to study the heat transfer in such a geometry. Figure 3-7 (b) shows the

finite element mesh on a blown up view of a portion of the steel plate and fireproofmg. As shown in Figure 3-7 (b), the fireproofing is modeled with a layer of finite elements (0.125 in. thick and 0.6 in. long) having the thermal properties of fireproofmg (purple). The steel plate is meshed with 10 elements

through its thickness, each element is 0.6 in. long.

A parametric study was conducted with average thickness of fireproofing varying from 0 in. to 2 in. in increments of % in. The effect of variability in thickness was modeled by imposing a normal probability

38 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

distribution to the fireproofing thickness along the length of the steel plate. The assumed standard

deviation varied from 0 in. (uniform thickness) to 1 in. A psuedo random number generator was employed

to determine the thickness at each cross section based on the assumed average thickness and standard

deviation. The randomness in fireproofmg thickness was incorporated in a finite element model by first constructing a model where fireproofing thickness was twice the average thickness and was meshed with

2-D finite elements. At any given location, the random number generator predicted a certain thickness.

Finite elements at that location, which lie above the predicted thickness were assigned a new material

attribute. The material attribute was chosen such that the elements did not provide any resistance to the

flow of heat. Finite elements that lie below the predicted thickness were assigned the material attributes of

fireproofing. Figure 3-8(a) shows an example of variable thickness fireproofing; in this case the average

thickness is 1 in. and the standard deviation is 3/8 in.

Figure 3-7. Model used to study effects of fireproofing thickness and variability of

thickness on steel temperature: a) physical model used in analyses (points 1-5 are sensor locations where temperature are monitored); b) finite element mesh used to

represent physical model.

When the model in Figure 3-7 is exposed to the thermal flux representing an 1,100 °C fire, the surface of

the insulation heats up quickly to the gas temperature. Numerical simulation was performed over a 2 h

period, and the steel temperature at five sensor locations were recorded at 30, 60, 90, and 120 min of

exposure. The temperature locations are 6 in. from each end and at 12 in. intervals, which are shown as numbers 1 to 5 in Figure 3-7 (a). The initial temperature of the model is 27 "C.

NISTNCSTAR 1-5G, WTC Investigation 39
Chapter 3

S 4 i :U M-Ofl

Figure 3-8. Finite element model to represent thickness of fireproofing (purple). The

elements in red represent a material that provides no resistance to heat flow (very high

thermal conductivity and very low thermal inertia).

Figure 3-8 shows temperature contours (Kelvin) through the fireproofing and steel at 60 min after initial exposure for the model shown in Figure 3-7 (a). The fireproofing surface temperature is close to the gas

temperature of 1,373 K, while the steel temperature is 31 1 K. If the fireproofing were of uniforai

thickness, the isotherms would be a series of lines parallel to the plate (since the flux is uniforai over the

entire plate). It is seen that, when the thickness of fireproofing is variable the isotherms follow the shape

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

local thickness of the fireproofing. We also observe that, if the fireproofing thickness is very small at any

location, then steel in that region heats up very quickly.

Figure 3-9 shows the steel temperature at the far sensor #1 (6 in. from the end) as a function of time for various insulation thicknesses ranging from 0 in. to 2 in. (the thickness is indicated by the numbers on the

curves). For the case in Figure 3-9 (a), the fireproofing is of uniform thickness, and for the cases in Figure 3-9 (b), the thickness varies with a standard deviation of 1 in. The time to reach a temperature of

600 °C is used as a measure of relative performance. It is seen that the presence of high variabihty in thickness has a detrimental effect on the protection provided by fireproofing. For example, for a uniform

thickness of 0.5 in., it takes about 60 min for the steel at point #1 to reach 600 °C; but when the standard

deviation of the thickness is 1 in., the average thickness has to be 1.75 in. for the same level of thermal

protection.

40 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

30 60 90 120

Time (Minutes)

Figure 3-9. Variation of steel temperature at Sensor Location #1 with time for different

average thickness of fireproofing (shown as numbers on the curves). The fireproofing

thickness is uniform over the entire length.

In addition to the effect of variation in thickness, it is important to understand the effect of missing

fireproofing over a portion of a member (Chang, Buchanan and Moss, 2005). As an example. Figure 3-10

models missing fireproofing from a diagonal of a bridging truss of the WTC towers floor system.

Figure 3—10 (a) shows an example of a numerical model with missing fireproofing. In this case, there are 12 in. of missing fireproofing on the steel plate, which is otherwise protected by 2 in. of uniform

thickness fireproofing. Figure 3-10 (b) shows isotherms in the steel and fireproofing. As expected, the

bare steel at the missing fireproofing is at the gas temperature, but more importantly the "gap" in fireproofing leads to a "leakage" of heat into the steel plate.

NISTNCSTAR 1-5G, WTC Investigation 41
Chapter 3

a)

t: 1 1

b)

14»

Figure 3-10. Finite element model for studying the effect of gap in fireproofing thickness

a) location of the gap on the plate b) temperature contours at one instant in time.

The combined effects of variation in thickness of the fireproofing and length of missing fireproofing were examined in a full factorial study with the following factors:

• Average thickness of fireproofing varying from 0 in. to 2.0 in. in % in. increments;

• Standard deviation of fireproofing thickness of 0 in., 0.25in., 0.5in., 0.75in., and 1.0 in; and

• Length of missing fireproofing varying from 0 in. to 30 in., in 6 in. increments.

The results of the sensitivity study can be summarized in a series of plot matrices as shown in Figure 3-1 1 through Figure 3-15 for sensor location # 1 through sensor location #5, respectively. Each

figure shows the time history of steel temperature for different combinations of gap length and variability

in fireproofing thickness. For example. Figure 3-1 1 shows the plot matrix (consisting of 25 x-y plots) for temperature history at sensor location #1 (6 in. from the end of the plate). Each x-y plot contains a series

of curves representing different average thickness of fireproofing, as in Figure 3-9. Each column of x-y

plots represents a constant value of thickness variability (defined by standard deviation), and each row

represents a constant gap length. The plot in the upper left comer represents the case of uniform thickness

of fireproofing and no gap, which is the same plot as Figure 3-9 (a). (Note that for the case of uniform

thickness and no gap, the steel temperature at any point in a cross section is the same along the length of

the plate. The upper left comer plots in Figure 3-1 1 through Figure 3-15 are the same as one shown in

Figure 3-9 (a).)

For gaps of 24 in. and 30 in., the temperature at sensor location #1 rises rapidly because there is no

fireproofing on the plate at that location. This explains the shapes of the curves in the two lower rows. It should be noted that for each x-y plot, data at five instants in time (0 min, 30 min, 60 min, 90 min and 120

42 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

min after the application of radiative flux) has been plotted. The data points have been connected with

straight lines. In reality the profile between any two points can be highly non-linear.

Moving from left to right in one of the top four rows, shows that as variability of thickness increases, the

time histories shift upward, thereby reducing the time to reach 600 °C. Moving from the top to the bottom

in any column shows the effect of increasing gap length. The effect of gap length depends, of course, on where the steel temperature is measured. Temperature rises quickly on points on the steel plate that are

bare. At points within the steel that are surrounded with fireproofing, the gap provides a path for heat

flow, as shown in Figure 3-9. As a result, points in the steel within the vicinity of the missing

fireproofing will experience higher temperatures, as indicated by the rising trend of the curves in going

downwards from the top of a column in Figure 3-11.

Figure 3-12 shows a matrix of plots similar to Figure 3-11, plotted at sensor location #2. Sensor location

#2 is shown in Figure 3-7 and is located 18 in. from the left end of the plate. Since this location is closer

to the center of the plate, the effect of gap size will have a bigger effect on the predicted steel temperature

at this location. The plots for a gap size of 24 in. or 30 in. with no variabihty in fireproofing thickness

shows a very rapid increase in steel temperature to a value close to the gas temperature.

Figure 3-14 shows a plot matrix at sensor location #4. Sensor location #2 and #4 are placed equidistance

from the center line. Note that the gap in fireproofing is always centered on the plate. The plot matrix for sensor locations #2 and #4 are not identical in general. The random number generator introduces

variability' in the fireproofing thickness distribution, and as a result there are visible differences in the x-y

plots shown in Figure 3-12 and Figure 3-14, especially for large values of standard deviation. Similarly

plots for sensor location #1 are in general different from those for sensor location #5. The only exception

is plots where the standard deviation is zero (no variability in fireproofing thickness). The first column of

plots in Figure 3-12 and Figure 3-14 are similar since there is no variability in thickness for this column.

NISTNCSTAR 1-5G, WTC Investigation 43
Chapter 3

44 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

NISTNCSTAR 1-5G. WTC Investigation 45
Chapter 3

46 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

NISTNCSTAR 1-5G. WTC Investigation 47
Chapter 3

48 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

When the standard de\"iation is zero, the steel temperature at any given time mcreases as the fireproofing

thickness decreases, but as the \'ariabihty in fireproofing thickness increases, this trend does not always

hold true. The first row of plots in Figure 3-15 shows that for a standard deviation of 1 in. and zero gap

thickness, the profile for 1.75 in. mean thickness shows lower temperature as compared to 2 in. thick

fireproofing. This is again due to the randomness of number generator. It is possible that if the experiment -

is carried out with a different initial seed value of the random generator, then the trend at this location

would reverse.

Figure 3-16 through Figure 3-20 show this effect in a clearer manner. Each figure shows a matrix of x-y

plots show^ing the steel temperature along the length of the steel plate for different combinations of gap

length and variability- in fireproofing thickness. The matrix in Figure 3-16 is for steel temperature at 30

min after application of radiative flux. Results have been showTi at 30 min intervals in Figure 3-16

through Figure 3-20. WTien there is no variability' in fireproofing thickness or gap in fireproofing, the

profiles are flat, indicating constant temperature along the length of the plate (no temperature gradient).

As the standard deviation increases, temperature profiles along the length of the plate indicate a temperature gradient. The temperature profiles do not indicate a monotonic increase in temperature as the

fireproofing thickness decreases (due to the randomness in fireproofing thickness).

Figure 3-20 summarizes all the results shown in Figure 3-1 1 through Figure 3-19 by plotting the

sensitivity' of \ arious input parameters on steel temperature. Results indicate that steel temperature

reduces with increasing thickness, as one would have expected. Variability in fireproofing thickness

results in higher steel temperature (higher standard deviation means higher temperature). Variability in fireproofing thickness reduces the local effective thickness of fireproofing and the results. Figure 3-20

shows that steel temperature increases with gap length and with time (duration of application of incident

radiative flux).

NISTNCSTAR 1-5G. WTC Investigation 49
Chapter 3

50 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

NISTNCSTAR 1-5G, WTC Investigation 51
Chapter 3

52 NISTNCSTAR 1-5G, WTC Investigation
Exploratory Studies on Thermal Response of Structural Elements

NISTNCSTAR 1-5G, WTC Investigation 53
54 NISTNCSTAR 1-5G, WTC Investigation
Chapter 4

Perimeter Columns

In Chapter 3, an exploratory study was presented for understanding the thermal response of 2-D or axisymmetric models of various structural components to furnace conditions that do not change with time.

This chapter is the first of four chapters that details how the major structural components (along with the

fireproofmg) in the World Trade Center (WTC) towers were represented with three-dimensional finite element models and also how the thermal response of these components was coupled to temporally and

spatially evolving realistic fires. The focus of this chapter is on perimeter columns. Starting with the basic

building block for a column, this chapter outlines a procedure for developing models that cover a single

floor or multiple floors. For each structural component, the approach for including fireproofmg and

structural damage is outlined. Typical radiative fluxes incident on a perimeter column and their dependence on fire growth and spread has been presented. Temperature contours on the structure are shows to help gain insight into the structural response of the column. Time-temperature profiles are discussed for various perimeter columns to help understand the spatial and temporal nature of the thermal

insult. The description applies to the fire floors, floors 92 through 99 in WTC 1 , and floors 78 through 83

in WTC 2. We have used results from one of the fire simulations to understand the coupled fire-thermal

response in this chapter. A full delineation of all the cases will be presented in Chapter 8 through

Chapter 1 1

4.1 TYPICAL FLOOR PLAN

An architectural design feature (NIST NCSTAR 1-1, NIST NCSTAR 1-1 A, NIST NCSTAR 1-lB) of

each World Trade Center tower was a series of closely spaced built-up box columns. At typical floors, a

total of 59 of these perimeter columns were present along each of the flat faces of the building. These

columns were built up by welding four plates together to form an approximately 14 in. square section,

spaced at 3 ft 4 in. on center. Adjacent perimeter columns were interconnected at each floor level by deep

spandrel plates, typically 52 in. in depth. In alternate stories, an additional column was present at the center of each of the chamfered building comers. The perimeter columns form an important structural component of the World Trade Center tower. This chapter describes a finite element analysis for studying

thermal response of a box column to the fires evolving on the various floors of the tower and for transferring the thermal data as body loads on to the structural models. The application of the ANSYS

finite element software (ANSYS 2005) to develop models for perimeter columns is discussed in this chapter.

4.2 FINITE ELEMENT MODELS OF THE PERIMETER COLUMN AND

FIREPROOFING

This section describes the development of finite element models to analyze the basic building block of a perimeter column. The model represents not only the steel but also the fireproofmg that protects the steel from thermal exposure. The finite element model was developed using the ANSYS parametric design

language to quickly and easily obtam a range of models with different thickness and dimensions. Finally,

we provide a brief explanation of the choice of elements and mesh density used in the thermal models.

NIST NCSTAR 1-5G, WTC Investigation 55
Chapter 4

Figure 4-1 shows a typical layout of a perimeter column on floor 96 of the WTC 1. Each perimeter

column was a box shape column, for which the plate thickness varied from one column to another. Each

perimeter column was identified as a specific coluinn type (e.g. column type 120, 121, ... 132). The

column types forWTC 1 (floors 92-99) are listed in Table B-1. Similarly the column types for WTC 2

(floors 78-83) are listed in Table B-2. As seen in these tables, the column type changed between floors,

and also between columns on the same floor. The plate dimensions (in meters) for the various column

types are shown in Table B-3. Here thickness tl refers to the flange plate thickness, t2 refers to the web

plate thickness and thickness t3 refers to the thickness of the spandrel plate (NIST NCSTAR 1-1,

NISTNCSTAR 1-2).

se—

<^

ici i«? ->06 m m 115 ue i2i i&i «?? laC' 36 136 !«? iff, 148 IS' ts* 15? 1S6

.701

sot

«02 603

702 KB

806 to?

901

1001 1004 100S

rrrnri

Figure 4-1. Representative floor plan of the exterior wall and column layout for WTC 1 and WTC 2 .

The fireproofing that covers the perimeters columns plays a very important role in the thermal response of

the perimeter columns. In this model, the fireproofing on the exterior faces of the columns has a thermally

equivalent thickness of 1 3/16 in. while the thickness on the exterior face of the spandrel plate was 0.5 in. The fireproofing material was identified as BLAZE-SHIELD DC/F (NIST NCSTAR 1-6A). The interior

faces of the perimeter columns were covered with a vermiculite aggregate. The thickness (thermally

equivalent) of the veraiiculite aggregate on the interior faces of the columns was 7/8 in. while that on the

spandrel beams was set to in. The thermo-physical properties of all the materials are discussed in Appendix A.

ANSYS parametric design language (ANSYS 2005) was used to construct finite element models of the

various perimeter columns. Figure 4-2 through Figure 4-5 each shows a solid finite element model of the

steel and the fireproofing that covers a typical perimeter column. Figure 4-2 shows the exterior faces of a perimeter column (Column type 120), while Figure 4-3 shows the interior face. Only the portion of the

perimeter column that lies in between two concrete slabs has been shown in Figure 4-2 and Figure 4-3.

Figure 4-^ shows the cross-sectional view of perimeter column type 1 20 along with the spandrel plate,

while Figure 4-5 shows the cross-sectional view of the column only (without the spandrel plate). The

56 NISTNCSTAR 1-5G, WTC Investigation
Perimeter Columns

elements are color coded with their assigned material attributes. Cyan colored elements have material

attributes of steel, violet colored elements have material attributes of fireproofmg (BLAZESHIELD DC/F) and pink colored elements have material attributes of Vermiculite plaster (See Figure 4-2

and Figure 4-3).

Figure 4-2. Finite element model of a box shape perimeter column (Column Type 120)

showing the exterior faces. Cyan colored elements have material attributes of steel,

while the violet colored elements have material attributes of fireproofing

(BLAZE-SHIELD DC/F).

NISTNCSTAR 1-5G, WTC Investigation 57
Chapter 4

Figure 4-3. Finite element model of box shape perimeter column (Column Type 120)

showing the interior faces. Cyan colored elements have material attributes of steel, violet

colored elements have material attributes of fireproofing (BLAZE-SHIELD DC/F) and pink

colored elements have material attributes of Vermiculite plaster.

Figure 4-4. Finite element mesh used to model a typical perimeter column. Figure shows

the density of the mesh elements in the column and spandrel beam.

58 NISTNCSTAR 1-5G, WTC Investigation
Perimeter Columns

Figure 4-5. Finite element mesh used to model a typical perimeter column. Figure shows

the density of the mesh elements in the column.

Brick elements with 3-D thermal conduction capability were used to mesh the steel and fireproofmg of

each perimeter column. The specific ANSYS element that was used for analysis is called "SOLID70".

The element has eight nodes with a single degree of freedom, temperature, at each node. The element is applicable to a 3-D, steady state or transient thermal analysis. Surface effect elements were superimposed on the brick elements to model convective and radiative heat transfer to the perimeter column. The mesh

density was dictated by the need to adequately resolve the physical processes of interest in this problem.

The radiative flux incident on the column can vary quite substantially over the length of the column

(Prasad and Baum 2004). It was found that dividing the length of the column into 20 equal increments

could capture the variation in the incident flux on a column without drastically increasing the

computational time of doing a global analysis (Chapter 8 through Chapter 11). Typical simulations of the

perimeter columns were performed with approximately 20 elements (Figure 4-2) along the length of the

column (the part between two concrete slabs). Sensitivity studies were performed by doubling the number

of elements (40 elements), and it was found to have a very small effect on steel temperature. Along the

circumference of the column, elements were placed roughly at 5 cm intervals (Figure 4-4). Usually, the

mesh density was controlled so that there were six elements through the thickness of the fireproofmg.

This was based on the need to adequately capture the propagation of the thermal wave through the fireproofmg. Again, sensitivity studies performed with a more refined finite element mesh did not result

in any significant change in the steel temperature. Once a finite element model of a perimeter column was constructed, the next task was to apply appropriate initial and boundary conditions on the column and

to couple the fire dynamics with the incident flux on the column.

4.3 INITIAL AND BOUNDARY CONDITIONS

The thermal response of the perimeter columns to spatially and temporally varying fires was simulated by

imposing a specific set of initial and boundary conditions. The initial temperature of the perimeter column

(steel and fireproofmg) was set at room temperature (27 °C).

NISTNCSTAR 1-5G, WTC Investigation 59
Chapter 4

4.3.1 Radiative Flux

Heat flux (Prasad and Baum 2004) boundaiy conditions (surface loads predicted from a plane layer

analysis) were applied to the inner web plates of the columns and the inner portion of the spandrel plates.

The flux is incident normally on the fireproofmg (if the fireproofing is intact) or on the steel (if the

fireproofmg is considered to be damaged). The flux varies as a function of space and time, depending on

the location of the perimeter column relative to the fire. Figure 4-6 shows the flux boundary conditions on a typical perimeter column. The radiative flux incident on the inner plates of the column varies along

the height of the column and changes with time. The incident flux is strongly coupled with the fire growth

and spread through the various floors of the World Trade Center towers as it is a function of the

instantaneous hot layer thickness, absorption coefficients, hot layer temperature, and ambient temperature

as predicted by the fire simulations.

Figure 4-6. Heat flux boundary conditions were applied on the interior faces of the

perimeter column as indicated by the arrows. The intensity of the incident flux varies

with time and space.

4.3.2 Convective Flux

Convective flux boundary conditions were applied on all the interior and exterior faces of the column and

spandrel plates as discussed in Chapter 1 . The heat transfer coefficient in general depends on the viscosity

and velocity of the local instantaneous flow field established in the vicinity of the columns. A convective

heat transfer coefficient value of 25 W/m'-K was assumed for computing the convective fluxes (NIST

NCSTAR 1-5F, Hollman et al.). Structural elements in the hot layer were subject to convective fluxes

with bulk temperature values equal to the local instantaneous value of the temperature in the hot layer.

Structural elements below the hot layer were subject to convective fluxes with bulk temperature equal to the local instantaneous value of the ambient temperature. The exterior faces of the column and spandrel

plate were subjected to convective fluxes with bulk temperature values equal to the room temperature (27

°C). The arrows in Figure 4-7 show the surfaces on which the convective flux boundary conditions were

applied for thermal analysis. The size and color of the arrows is proportional to the bulk temperature of

the surrounding gases. The small blue colored arrows on the exterior faces of the column represent a bulk

temperature value of 27 °C, while the red arrows on the interior faces indicate higher bulk temperature.

60 NIST NCSTAR 1-5G, WTC Investigation
Perimeter Columns

Figure 4-7. Convective flux boundary conditions indicated by arrows were applied to all the exposed faces of the perimeter column.

4.3.3 Re-Radlatlon

In order to model re-radiation from the surface of the perimeter column to the ambient atmosphere, the

perimeter columns were covered with surface effect elements (ANSYS 2005). An emissivity value of

0.95 was used based on fire simulations (NIST NCSTAR 1-5F).

Radiative heat transfer within the cavity formed by the four plates of the box shaped perimeter column is also modeled, as shown in Figure 4—8, to allow for heat exchange between the four plates. Re-radiation

between the four plates will help to equilibrate the temperature of the four plates as described in the

results section. It is assumed that the effect of natural convection within the cavity is much less than the

radiative exchange.

Figure 4-8. Arrows indicate the surfaces used for modeling re-radlation within the cavity

of a perimeter column.

4.3.4 Multi-floor Simulations

Specifying boundary conditions at the top and bottom of a perimeter column is a challenging task. Since

the column extends all the way up to the 1 10* floor, one could construct finite element models of

NIST NCSTAR 1-5G, WTC hvestigation 61
Chapter 4

perimeter columns that are 1 10 floors tall. From a computational point of view, it would have been

prohibitive to solve such a model. In the present Investigation, the length of the column was extended half

a floor above and half a floor below the floor under consideration to account for the effect of muhiple

floor fires. As an example, consider a part of the column that lies between floor 96 and floor 97 as the

column under study (See Figure 4-9). The part of the column between floor 96 and floor 97 is subjected

to fires on floor 96, the portion of the column above floor 97 is subjected to fires on floor 97, while

portion of the column below floor 96 is subjected to fires on floor 95. Zero flux boundary conditions were imposed at the top and bottom edge of the column. By extending the column above and below the floor

under consideration, the effect of fires on the 95"*^ and 97'*' floor can be incorporated on the column under

study.

Only the portion of the column between the 96'*^ and 97'"^ floor is used for structural analysis. The portions

of the column above floor 97 and below floor 96 are not used in the analysis. By extending the length of

the column, boundary conditions were applied further away from the region of interest. The procedure

also incorporates fireproofing and structural damage to the column above and to the column below the

column under consideration, into the thermal analysis.

1.83 m

Subjected to tires on

the 97^^ floor

Subjected to fires on

3.65 m y the 96th fl^^r

1.83 m

Subjected to fires on

the 95^^ floor

Figure 4-9. Boundary conditions on a typical perimeter column

62 NISTNCSTAR 1-5G, WTC Investigation
Perimeter Columns

4.4 INCIDENT HEAT FLUX

Simulating the effects of severe fires on the structural integrity of buildings requires a close coupling

betw een the gas phase energy release and transport phenomena and the stress analysis in the load bearing

materials. The connection between the two is established primarily through the interaction of the radiative

heat transfer between the solid and gas phases with the conduction of heat through the structural elements.

Heat transfer to sub-grid scale structural elements is calculated using a simple radiative transport model

that assumes that the compartment is divided into a hot. soot-laden upper layer and a cool, relatively clear lower layer. The properties of the two layers are extracted from temporal averages of the results obtained

from the Fire Dynamics Simulator. The incident flux is a function of temperature, hot layer depth, soot

concentration, and location and orientation of each structural element. In the remainder of this section, the nature of the incident radiative flux on typical perimeter columns when subjected to realistic fires is described. Results for a few perimeter columns are presented to understand the temporal and spatial

variation of the incident flux.

Figure 4-10 shows the heat flux contours incident on a perimeter column. Plots are shown for column

number 145 on floor 96 (WTC 1) at 1,000 s intervals. Note that contour levels are shown at the right of

each figure and are different from one time to another. At 1 ,000 s after impact, the flux ranges between a maximum value of 130,000 W/m" to a minimum value of 80,000 W/m". At 3,000 s after impact, the flux

ranges between a maximum value of 19,000 W/m' to a minimum value of 10,000 W/m". At 6,000 s after impact, the flux ranges between 2,200 W/m' and 2,700 W/m". The flux varies along the height of the

column and reaches a peak value somewhere close to the middle of the hot layer. Figure 4-10 illustrates the transient nature of the fire and the resulting flux incident on column 145 of floor 96, WTC 1 . It should

be noted that there is a horizontal variation in radiative flux, but this variation is relatively small

compared to the variation along length of the column.

NISTNCSTAR 1-5G, WTC Investigation 63
Chapter 4

Time= 1000 s Time =2000 s Time = 3000 s pi0f m.

C3

80000

ssooo

30Q0G

j'SCOS

nsoco

lioos

13000

'iSOOD leooQ

;7000 18000 19Q00

Time = 4000 s Time =5000 s Time = 6000 s

6000

52CK5 6400

asao i(m

?200 MOO

'?S00

; HO, ^lOC

320C 530D

5400

3SOt5 iCCC 3700 3800

5300 4oeo

WiD — .?so

1 iOO — ISO

:,00

.—, -603 iid 2650

Figure 4-10. Heat flux incident on the inner faces of a perimeter column as a function of

time. Plots are shown for Column 145 on floor 96, WTC 1 at 1,000, 2,000, 3,000, 4,000

5,000 and 6,000 s after impact.

The radiative flux incident on a column not only varies in time (Figure 4—10) but also varies from one column to another as shown in Figure 4-11. This figure shows the incident heat flux at 6,000 s (100 min)

after impact, for perimeter columns 145, 230, 330 and 430 on floor 96 of WTC 1 . As for Figure 4-10, we

have presented results for a few columns at specific instants in time, coupled to one of the fire simulations

to illustrate the fire - thermal coupling. The contour levels are shown at the right of each figure and are

different from one column to another. For perimeter column 330, the flux ranges between 48,000 W/m"

and 77,000 W/m", while for perimeter column 145 the flux ranges between 2,200 W/m" and 2,700 W/m".

This illustrates that the incident flux is not only a function of time, but also varies spatially from one column to another column. Since the fires on the various floors evolve and spread at different rates, it can

be stated that the incident radiative flux on a column varies from one floor to another floor (depending on

fire growth and spread on those floors).

64 NISTNCSTAR 1-5G, WTC Investigation
Perimeter Columns

mm5^ j?-'Gs

Column 145 Column 230 Column 330 Column 430

Figure 4-11. Incident heat flux at 6,000 s after impact for perimeter columns 145, 230,

330 and 430 on floor 96 (WTC 1). Partial results from one fire simulation have been used

to illustrate the nature of the coupling process.

4.5 THERMAL RESPONSE OF COLUMNS TO REALISTIC FIRES

In the previous section, we have demonstrated the temporal and spatial variation of the incident flux on a perimeter column. It was shown that the flux varies not only from one column to another column, but also

varies along the length of any one column. The temporally and spatially varying fluxes incident on the

perimeter column sets up some interesting temperature contours in the steel column. This section will

discuss the temperature contours and temperature gradients that arise in a typical perimeter column.

4,5.1 Temperature Contours

Figure 4-12 and Figure 4—13 show the temperature contours on perimeter column 145 of floor 96

(WTC 1). Contours are shown at 10 different instants in time ranging from 600 s to 6,000 s and are spaced at 600 s intervals. Temperature scales ranges from 0 °C to 675 °C. The temperature contours are a

direct result of the incident flux, the geometry of the column, the fireproofmg thickness, and the boundary

conditions imposed on the perimeter column.

Numerical results indicate that the spandrel plate heats up faster than the perimeter column. This is due to smaller fireproofmg thickness on the spandrel plate as compared to the column. The portion of the

spandrel plate furthest away from the column shows highest temperature, and heat gradually diffuses into

the rest of the columns. Note that only a small portion of the column that lies between two concrete slabs has been shown in these figures. Depending on the thickness of the hot layer and the heat flux contours (shown in Figure 4-6), the upper half of the column heats up faster. The exterior faces of the column are being cooled convectively, and the temperature contours are consistent with these imposed boundary

conditions. The radiative flux on column 145 reaches a peak value at approximately 3,000 s. Correspondingly, the temperature contours also exhibit a peak value at 3,000 s, and the column gradually

cools down after this point. The temperature contours are also influenced by the fires on floors 95 and 97

(floors above and below floor 96) as energy gradually conducts through the steel.

NISTNCSTAR 1-5G. WTC Investigation 65
Chapter 4

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Perimeter Columns

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NISTNCSTAR 1-5G, WTC Investigation 67
Chapter 4

4.5.2 Fireproofing and Steel Temperature

Figure 4-14 shows the cross-section view of temperature contours through a perimeter column (Column

number 145). In this figure temperature contours in the steel and the fireproofing have been included. The

temperature scale ranges between 300 K and 1200 K. The figure exhibits a very high temperature on the

surface of the fireproofing and gradual diffusion of heat through the fireproofing into the steel column.

The figure also shows a temperature gradient in the perimeter column between the inner web plate and the outer web plate. This temperature gradient is also visible in Figure 4-12 and Figure 4-13. The

temperature gradient through the cross-section of a box column is important because it can result in differential thermal expansion and can influence the structural response of a coluinn.

Figure 4-14. Temperature contours (Kelvin) through the cross-section of a perimeter

column, showing the temperature gradient between the interior and exterior faces.

4.5.3 Time-Temperature Plots

The nature of combustibles and the ventilation patterns are some of the parameters that affect fire growth

and spread on a typical floor of the WTC towers (NIST NCSTAR 1-5F). As the combustibles are consumed, the fire activity also dies down. The heat flux incident (see Figure 4-10 and Figure 4—11) on a perimeter column varies in space and time depending on the local fire activity. The temperature contours

(Figure 4-12 and Figure 4-13) in a perimeter column depend on the geometry, fireproofing status, and

boundary conditions. In this section, the detailed time temperature history of various nodes on a perimeter

column is discussed.

Figure 4-15 through Figure 4-18 show the time - temperature history for perimeter columns 145, 230,

330 and 430 respectively, on floor 96 of WTC 1. In each figure, profiles have been shown at four

different nodal locations. Red symbols show temperature on the inner web plate. The nodal location is in the center of the web plate, half the distance between the floor and the ceiling. Similarly, green symbols

68 NIST NCSTAR 1-5G. WTC Investigation
Perimeter Columns

denote the temperature history of the flange plates, and blue symbols show the temperature of the outer web plate. Again the nodes are located in the center of the plate, half the distance between the floor and

the ceiling. Black symbols are used to indicate the temperature of the spandrel plate. The nodal location

used for the spandrel plate is half the distance between two perimeter columns.

650

600

550 r InnGr WGb

Flangg

OutGr WGb

SpandrGi PlatG

1000 2000 3000 4 000

Time (s)

5000 6000

Figure 4-15. Time-temperature plot for perimeter Column 145, floor 96, WTC 1.

For perimeter column 145, Figure 4-15 shows a maximum inner web plate temperature of 200 °C while

the spandrel plate shows a maximum temperature of 325 °C at approximately 3,000 s after impact. After

3,000 s the temperature of the inner web plate and spandrel plate gradually decreases to 150 °C and

250°C, respectively. The temperatures of the flange plate and the outer web plate continue to increase

monotonically for the entire duration of the simulation. The flange plate shows higher temperature than

the outer web plate. This picture is consistent with the fire activity in this region. Note that for the purpose

of illustration, we have presented results for a few columns from one fire simulation. Global cases are discussed in Chapter 8 through Chapter 11. Perimeter columns 145, 230, and 430 have no fireproofing

damage and show similar time-temperature plots. However because the fire activity in the vicinity of

these columns peaks at different times, these columns show peak temperature at correspondingly different

times.

Perimeter column 330 (Figure 4-17) shows different behavior due to fireproofing damage on the interior plates of the column. Peak temperatures of approximately 450 °C are predicted, and the resulting time

temperature profile depends heavily on the local fire activity. Numerical simulations for perimeter

column 330 do not indicate any significant temperature gradient between the inner web and the outer web

plate. Note that the fireproofing on the exterior faces of the column is assumed to be intact for this simulation.

NISTNCSTAR 1-5G, WTC Investigation 69
Chapter 4

650

600

_550

500

Inner Web

FlangG

Outer Web

I Plate

1000 2000 3000 40f)0

Time (s)

000 6000

Figure 4-16. Time-temperature plot for perimeter Column 230, floor 96, WTC 1,

70 NISTNCSTAR 1-5G, WTC Investigation
Perimeter Columns

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100 -

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1000 2000 3000 4000 5000 6000

Time (s)

Figure 4-18. Time temperature plot for perimeter Column 430, floor 96, WTC 1.

4.6 MODELING FIREPROOFING AND STRUCTURAL DAMAGE ON

PERIMETER COLUMNS

One objective of this Investigation was to simulate the thermally induced structural response of the towers

damaged by the impact of the aircraft. Aircraft impact into the towers and the resulting debris field moving through the various floors can sever perimeter columns (NIST NCSTAR 1-2). A perimeter

column that has been severed by aircraft impact changes the boundary conditions and affects the heat

capacity of the column. The debris field can also damage the fireproofing that covers a perimeter column.

Damaged fireproofing can drastically affect the thermal response of the column (Figure 4-17). Since

fireproofing and structural damage can have a large effect on the thermally induced structural response, it was necessary to include this damage in our analysis. This section describes a methodology for incorporating fireproofing and structural damage on a perimeter column.

4.6.1 Fireproofing Damage

Each element in a finite element model is assigned certain material attributes, which, in turn estabhshes

the elements material properties. Fireproofing damage on a column was modeled by changing the

material attributes of the element. The new material attributes are such that they offer negligibly small

resistance to heat flow. Such an element would have extremely low heat capacity and extremely high

thermal conductivity.

NIST NCSTAR 1-5G. WTC Investigation 71
Chapter 4

The following procedure was followed to include fireproofing damage on a perimeter column.

1 . Identify all the elements that have material attributes of fireproofing.

2. Select the subset of these elements that are damaged due to the impact of the aircraft.

3. Define a new material ("Non Material") that has extremely high thermal conductivity and very

low density and heat capacity. The assumed properties of this "Non Material" are described in 0.

4. Change the material attributes of the selected elements to that of the new material defined in step 3.

Figure 4-19 shows a perimeter column that has fireproofing damage. The damage elements are assigned

different material attributes as indicated by the blue color. Extending the column above and below the

column under consideration provides a method to include the results of fireproofing damage on the

columns above and below the column of interest. In Figure 4-19 (middle figure) fireproofing damage is limited to the column under consideration. The figure on the extreme right has fireproofing damage on the

column above the 9T^ floor, while the figure on the extreme left has fireproofing damage below the 96*

floor.

Figure 4-19. Incorporating fireproofing damage on the interior faces of a perimeter

column. Figure on the left shows fireproofing damage on a part of the column below the

column under consideration. Figure on the right shows fireproofing damage on part of

the column above the column under consideration, while the figure in the middle shows

fireproofing damage on the column of interest.

72 NISTNCSTAR 1-5G, WTC Investigation
Perimeter Columns

Fireproofing damage may not be only limited to the interior faces of the column. Figure 4-20 shows

fireproofmg damage on the exterior and interior faces of the column under consideration.

Figure 4-20. Incorporating fireproofing damage on the interior and exterior faces of a perimeter column. Material attributes of the fireproofing elements were changed to

model damage to fireproofing.

4.6.2 Structural Damage

Modeling a structure that is damaged by the impact of the aircraft (NIST NCSTAR 1-2) and studying the

role of structural damage was an important objective of this Investigation. Structural damage can reduce

both the load bearing capacity and the heat carrying capacity of the structure and can also affect the

boundary conditions on a perimeter column. If a perimeter column above or below the column under

consideration is severed by the impact of the aircraft, then energy can no longer be conducted up or down

the steel column. Reduction in heat loss due to the damage can increase the temperature of the portion of

the column of interest over what it would have been without the damage.

Figure 4-2 1 shows the methodology for incorporating structural damage on the perimeter column. The

plot on the left shows damage to the steel column below the column under consideration, while the figure on the right shows damage to the steel column above and below the column under consideration. The

following steps outline the methodology for incorporating structural damage.

1. Construct a finite element model of a perimeter column.

2. Identify portions of the column which have structural damage.

NIST NCSTAR 1-5G, WTC Investigation 73
Chapter 4

3. Identify and select the nodes and elements in the damaged column with material attributes of

steel.

4. Delete the selected elements and nodes, or alternatively, change the material attributes to that of

fireproofmg.

Figure 4-21. Incorporating structural damage caused by the aircraft impact on the steel

columns. The plot on the left shows damage to the steel column below the column under

consideration, while the figure on the right shows damage to the steel column above and

below the column under consideration.

The themial analysis predicts the spatially and temporally evolving temperature fields in all the perimeter

columns (structural components) of interest. The output of the thermal analysis is in the form of a timetemperature curve at each and every node in the thermal model. The thermal models (database file and the

results files containing the time-temperature histories) can now be subjected to gravity loading to perform

a structural analysis (stress analysis) in a sequential manner. Three major difficulties prevent us from

following such an approach:

1 . Non-linear structural analysis performed with solid models can be computationally prohibitive.

Structural analysis for large, complex building assemblies is normally perfonned with beam or

shell elements. Beam elements are either linear (2 node) or quadratic (3 node) and can have six

degrees of freedom. Beam elements are suitable for linear, large rotation and/or large strain nonlinear applications. Thermal analysis, on the other hand, requires solid elements as opposed to

74 NISTNCSTAR 1-5G, WTC Investigation

4.7 INTERFACE BETWEEN THERMAL AND STRUCTURAL MODELS.
Perimeter Columns

beam or shell elements for analysis to accurately model both the radiative flux incident on the surface and heat conduction.

2. Thermal models of steel columns include fireproofmg. A large fraction of the nodes and elements

of a thermal model have material attributes of fireproofmg. Using the thermal models directly in a

structural analysis would un-necessarily complicate the analysis due to the presence of

fireproofing elements.

3. Structural analysis can be speciahzed to account for buckling of the columns or may include the

effect of failure of bolts or connections. These effects are generally not included in a thermal

analysis. The thermal and structural models are resolving different physical processes, and the meshing criteria may not be identical in the two models.

For the reasons mentioned above, it was necessary to construct two different models, one for the thermal

analysis and another for structural analysis and then to transfer the thermal data between the two models.

This section describes a methodology for transferring the data between the thermal and the structural models.

4.7.1 Mapping of Thermal Data

The transfer of temperature data between the thermal and structural models for a perimeter column has

been shown schematically in Figure 4-22.

The figure on the left shows the nodes and elements used for studying the thermal response of a perimeter

column. The steel column and spandrel plates are modeled with solid elements. The figure in the middle

shows the elements of a structural model for a perimeter column. The column is made up of beam

elements, while the spandrel plate is made of shell elements. The figure on the right shows the nodal

locations for the elements of the structural model. The goal is to transfer the temperature and temperature

gradient data from the thermal to the structural model. The detailed step by step procedure for transferring

the thermal data between the two models is described in the following section.

NISTNCSTAR 1-5G, WTC Investigation 75
Chapter 4

Transfer of temperature

and temperature gradient

data between thermal

and structural model

Thennal model of a perimeter column

and spandrel plate showing element

density.

Element nlot Node location

Structural model of a perimeter column

and spandrel plate showing element

density (color) and node locations

Figure 4-22. Mapping between the thermal models (left) and the structural model (right)

for a typical perimeter column. Both temperature and temperature gradient information

was transferred from the thermal model to the structural model for predicting the

thermally induced response of the column.

4.7.2 Procedure for Mapping Thermal Data onto the Structural Model

• Open the structural model database file and select the perimeter column for which thermal data is required.

• Identify a portion of the perimeter column that is modeled with beam elements. Temperature

gradient data is provided for columns that were modeled with beam elements. For each beam

element write the element number and co-ordinate location (x, y and z) of the centroid of the

element. The location of the elements is written to a file, with respect to a global co-ordinate

system.

• Identify a portion of the structural model that is modeled with shell elements. The spandrel plates were commonly modeled with shell elements. For these elements only temperature data was

76 NISTNCSTAR 1-5G, WTC Investigation
Perimeter Columns

provided at the structural nodes. Identify and select the nodes of the shell elements. Write the

node number and the spatial location (x, y and z location) of the nodes to a file. The location of . the node is in reference to a global co-ordinate system.

• Open the thermal model database file. Load the solution data files at a specific time where the

thermal loading data file has to be created.

• Read the element number and the co-ordinate location (of structural model) for temperature

gradient data. Compute the temperature gradient at the nodes closest to the co-ordinate location

specified by the structural model. Write the thermal gradient information in a thermal loading

data file.

• Repeat step 5 for all the structural elements that require temperature gradient information.

• Read the node number and co-ordinate location for temperature data. Compute the temperature at the node closest to the structural node. Write the temperature data in a thermal loading data file.

• Repeat step 7 for all the structural elements that require temperature data.

• Repeat steps 4-8 for each time step where a thermal loading data file is need.

• Close the thermal models, and open the structural database.

• Read the thermal loading data files to apply body loads on the structural model.

Thermal data were provided as a set of thermal load files (text files) for structural analysis. Thermal load

files for all perimeter columns in each tower were created at each time step where thermal data were

required. The text files were formatted for the ANSYS finite element analysis software and structural models. For WTC 1. ten thermal load files were created at 10 min intervals ranging from 10 min to 100

min. For WTC 2, six thermal load files were generated at 10 min intervals ranging from 10 min to 60 min.

Figure 4-23 shows the time vs. temperature plot for perimeter column 145, floor 96, WTC 1 . In this figure, the small symbols indicate the frequency at which the thermal data are plotted (200 s), while the

large symbols indicate the frequency at which the thermal data were mapped onto the structural models

for analysis (600 s). Perimeter column temperatures were found to vary slowly relative to the gas

temperature fluctuations; the 200 s interval in the figure represents the thermal response of the column to the fires without missing significant peaks or fluctuations. The 10 min time intervals captured the

computed thermal profiles reasonably well, as indicated in the figure. The structural response analysis

used linear interpolation between two time intervals for modeling changes in temperatures of the

structure.

NISTNCSTAR 1-5G, WTC Investigation 11
Chapter 4

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600

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E 200

CD

I— 150

100

50

0

InnGr VVob

FlangG

OutGr WGb

SpandrGl PlatG

3000 4000

Time (s)

Figure 4-23. Time temperature plot for perimeter column 145, floor 96, WTC1, indicating

the frequency at which the thermal data is plotted (small symbols) and the frequency at which it is transferred into the structural models (large symbols).

4.7.3 Full Floor and Multiple Floor Mapping of Thermal Data

Figure 4-24 shows the mapping of thermal data, based on the algorithm described above, on to a full floor

structural model (NIST NCSTAR 1-6D) for WTC 2, floor 82. Thermal mapping has been shown at one

specific instant in time. The mapping includes the temperature on the spandrel plates and temperature

gradients for all the perimeter columns on a floor. Figure 4—25 shows a similar mapping for floors 92-99

of WTC 1, at one specific instant in time.

The approach presented in this chapter for predicting the thermal response of perimeter columns and

transferring or mapping the thermal data on a structural model was used extensively throughout the

Investigation. The perimeter columns shown in Chapter 8 through Chapter 1 1 (global response of the

towers) show results from a similar mapping for four specific scenarios, two each for WTC 1 and WTC 2. The figures presented in those chapters are visual representations of the thermal state of the perimeter

columns at specific instants in time. This information is provided in the fonn of a text file to apply body

loads on the structural model and to subsequently perform thermally induced stress analysis.

78 NIST NCSTAR 1-5G, WTC Investigation
Perimeter Columns

0 150 530 450 600

75 225 375 525 675

Figure 4-24. Perimeter columns on floor 82 of WTC 2 . The perimeter columns on the

south face are damaged due to the impact of the aircraft, while many columns on the

north and east faces exhibit fireproofing damage.

NISTNCSTAR 1-5G. \An'C Investigation 79
Chapter 4

Figure 4-25. Visualization of the temperatures and temperature gradient Information

transferred to the WTC 1 structural model (floor 92-99) at 6000 s after Impact. The

thermal data is provided as a body load file.

80 NISTNCSTAR 1-5G, WTC Investigation
Chapter 5

Floor Trusses and Core Beams

This chapter is the second is a series of four chapters that details how the major structural components

(along with their fireproofmg) in the building were represented with finite element models. The first of

those four chapters (Chapter 4) deah with perimeter columns. The focus of this chapter in on the modeling of floor trusses and core beams that support the concrete slab of each floor of World Trade

Center (WTC) 1 and WTC 2. A detailed description of finite element models for the basic building block

of primarN' and bridging truss is provided. The method for coupling with fire simulations and transferring

the thermal data on to the structural models is discussed. The approach for applying boundary conditions

and mcludmg fireproofing and structural damage is outlined. Time-temperature profiles and temperature

contours are provided to understand the spatial and temporal nature of fire damage. The description in this chapter applies for floors 92-99 of WTC 1 and floors 78-83 of WTC 2. As for perimeter columns, partial results from one of the fire simulations are utilized to understand the coupled fire-thennal response. A full delineation of all the cases will be presented in Chapter 8 through Chapter 1 1

5.1 TYPICAL FLOOR PLAN

In the WTC towers, floor construction typically consisted of 4 in. of lightweight concrete on 1.5 in., 22

gauge non-composite steel deck. In the core area, slab thickness was 5 in. Outside the central core, the

floor deck was supported by a series of composite floor trusses that sparmed between the central core and

exterior wall. Composite behavior with the floor slab was achieved by extending the truss diagonals

above the top chord to form knuckles that acted much like shear studs. Detailing of the trusses was similar

to that employed m open-web joist fabrication. However, the floor system design was not typical of openweb-joist floor systems. It was considerably more redundant and was well braced with transverse bridging

truss members. Trusses were placed in pairs, with spacing of 6 ft 8 in. and spans of approximately 60 ft to the sides and 35 ft at the ends of the central core. The metal deck spanned parallel to the main trusses and was directly supported by continuous transverse bridging trusses spaced at 13 ft 4 in. and intermediate

deck support angles spaced at 6 ft 8 in. from the transverse trusses. The combination of main trusses, transverse trusses, and deck support enabled the floor system to act as a grillage to distribute load to the various columns.

At the exterior wall, truss top chords were supported in bearing seats extending from the spandrels at alternate columns. Welded plate connection, with an estimated ultimate capacity of 90 kips tied the pairs

of trusses to the exterior wall for out-of-plane forces. At the central core, trusses were supported on seats off a channel section that ran continuously past and was supported by the core columns. Floors were

designed for a uniform live load of 100 pounds per square foot over any 200-square foot area with

allowable live load reduction taken over larger areas.

Since the trusses that support the concrete slab were considered as important structural elements

(Usmani et al. 2001), it was necessary to study their thermally induced structural response. Finite element

models were developed for trusses that cover an entire floor of the WTC Towers. This chapter describes

NISTNCSTAR 1-5G, WTC Investigation 81
Chapter 5

the development of the finite element models and the methodology for coupling the thermal response of a

truss to the temperature flow field determined by fire dynamic simulations.

5.2 FINITE ELEMENT MODEL OF THE FLOOR TRUSS

This section describes the development of finite element models of the basic building block of a truss element using the ANSYS parametric design language. The basic building block was replicated to cover an entire floor of the World Trade Center towers. In this section, the challenges that were faced as these

models were developed, mesh requirements, choice of elements for analysis, and the approximations that are buih into the analysis is described.

5.2.1 Primary Truss

Figure 5-1 shows an isometric view of a portion of the main truss (primary truss). In order to predict the

thermal response of the truss, a solid model was constructed for the steel and fireproofing that covers the

truss model. The truss model consists of an upper flange and a lower flange held together by web

diagonals. The upper flange and the lower flange each consist of L shaped angles. Note that the

dimensions of the L shaped angles are different for the upper flange and the lower flange. In these

pictures, the elements have been color coded with the material attributes. Cyan colored elements have

material attributes of steel, while violet colored elements have attributes of fireproofing

(BLAZE-SHIELD DC/F).

Figure 5-1. Isometric view of a finite element model of a portion of the main truss.

82 NISTNCSTAR 1-5G, WTC Investigation
Floor Trusses and Core Beams

Figure 5-2 shows the end view of the tmss model showing the finite element construction of the upper

and lower flange. The plot on the left shows a model which is covered with 2.2 in. of fireproofmg. This

model was adopted for modeling the trusses in WTC 1 . The figure on right shows a model of a truss, covered with 0.6 in. of fireproofmg. This model was adopted for the trusses in WTC 2 (Choi, Burgess and

Plank, 2003, Liew and Chen, 2004).

The thermo-physical properties of all the materials are discussed in Appendix A.

a) iihsulation 1 hitkncss 2.2 in. b) Insulation Thickness 0.6 in. Figure 5-2. End view of the truss model showing upper and lower steel flanges

(cyan colored) covered with fireproofing (violet colored).

Figure 5-3 shows the front and top views of the finite element model of the primary truss, indicating the

element density used for the simulations. The truss upper and lower flanges were constructed using a system of keypoints, areas, and volumes. The volumes were subsequently meshed to create elements and

nodes for finite element analysis. Since the truss model covers an entire floor, it was clear that it would

require a very large number of elements to cover the entire floor. The computational time scales

approximately as N' where, N is the number of nodes in the model. Increasing the resolution in the model

by a factor of two or more, for example, can quickly result in calculations that would not be

computationally feasible (large CPU time). Another major limitation was the RAM available on the fast personal computers used for the investigation. It was necessary to limit the size of the model such that it would not be larger than the RAM available for the simulation (to avoid memory swapping). To avoid

prohibitively large CPU costs and memory swapping (which results in very slow computational speed), it was necessary to control the number of elements and nodes used in the simulation. The truss model was

designed such that it would results in the minimum number of volumes that required meshing. Since we

wanted to control the mesh density in the trusses (to reduce computation costs), it was also necessary to have regular shaped elements.

NISTNCSTAR 1-5G, WTC Investigation 83
Chapter 5

Top View

Figure 5-3. Front and top views of the truss model.

5.2.2 Connections

Figure 5-4 shows the connectivity between the steel and fireproofing in the upper and lower flanges. The

model was designed such that the fireproofing does not result in cuts in the L shaped steel flanges, and

similarly, the steel flanges do not introduce additional cuts in the fireproofing.

Figure 5-4. Finite element model of a primary truss showing truss construction details.

84 NISTNCSTAR 1-5G, WTC Investigation
Floor Trusses and Core Beams

Attaching the web diagonals to the upper and lower flanges can result in additional cuts through the

flanges. To avoid these cuts in the model, the diagonals were attached to the flanges with link elements.

Use of link elements allows the diagonals to be meshed independently of the flanges and resuhs in the

minimum possible volumes for the meshing.

Link elements were also used to model heat transfer between the upper flange and the concrete slab. The

cross-sectional area of the link element was chosen to be equal to the contact surface between the upper

flange and concrete slab.

The basic finite element model of the truss component and insulation, shown in Figure 5-1 through

Figure 5^, was replicated to cover a 60 ft long truss shown in Figure 5-5. The end connections for the

trusses were modified on an individual basis to account for the connections with the perimeter and core columns.

Figure 5-5. Replications of the basic element of the truss model to create a 60' long

truss. The ends of the truss were modified to properly account for the connections to the core columns and perimeter columns.

5.2.3 Bridging Truss

The next task in building a finite element model for the entire floor was to construct the bridging trusses (transverse trusses). Figure 5-6 shows several views of the basic unit of the primary and bridging truss system. Again, intersecting solid geometries can result in cuts in the model and will result in a very large

number of elements. Link elements were used to model the heat conduction between the primary and

transverse trusses. The procedure for creating the link elements can be summarized as follows:

1 . Identify the two surfaces that are in contact (e.g. lower part of the upper flange in the main truss and the upper part of the upper flange in the transverse truss).

NISTNCSTAR 1-5G. WTC Investigation 85
Chapter 5

2. Identify the nodes and elements with material attributes of steel on each surface.

3. Define link element attributes by dividing the cross-sectional area by the number of links.

4. Identify a pair of nodes (one on each surface) that will be linked. The nodes are chosen such that the length of the link element is as small as possible. This results in a link between two nodes that are closest to each other.

5. Create a link element between the two nodes.

6. Repeat steps 4 and 5 for all the nodes on the two contact surfaces.

7. Repeat steps 1 through 6 for the various contact surfaces.

Figure 5-6. Basic unit of the primary and transverse truss.

The basic unit of the primary and transverse truss (shown in Figure 5-6) can be replicated several times to cover a portion of the slab as shown in Figure 5-7 or to cover the entire floor slab as shown in Figure 5-8.

86 NISTNCSTAR 1-5G, WTC Investigation
Floor Trusses and Core Beams

Figure 5-7. Replication of the basic truss unit to cover a small portion of the floor slab.

Figure 5-8. Finite element model of the entire truss assembly (steel only) that supports

the concrete slab on floor 96 of WTC 1.

NISTNCSTAR 1-5G. WTC Investigation 87
Chapter 5

5.3 STRUCTURAL AND FIREPROOFING DAMAGE

An important concern throughout the Investigation was to simulate the thermally induced structural response of the towers damaged by impact of the aircraft. Aircraft impact can damage the floor trusses and core beams that support a concrete slab. The damage can take two forms (as discussed in Chapter 4):

• Structural damage, where the steel has been severed. This can result in changes in boundary

conditions for thermal analysis and the overall heat capacity of the structure.

• Fireproofing damage can result in rapid heating or cooling of the structure, which in turn can have a large impact on the structural response.

In this section we will briefly describe the methodology for including structural or fireproofing damage

on the steel trusses and/or core beams.

5.3.1 Incorporating Structural Damage on Floor Trusses or Core Beams

Figure 5-9 shows structural damage on a portion of the steel truss that covers an entire floor of the WTC

tower. Structural damage on the trusses was incorporated by first constructing a model of a floor truss with no damage. Subsequently, portions of the steel truss with structural damage were identified, and the

nodes and elements in that portion were selected. The nodes and elements were deleted from the database.

Figure 5-9. Incorporating structural damage due to aircraft impact on the truss model.

5.3.2 Incorporating Fireproofing Damage on Trusses or Core Beams

Fireproofing damage on the floor trusses was modeled by changing the material attributes of the elements

such that the element offers no resistance to heat flow. Extremely low heat capacity and very high thermal

conductivity is a requirement for such materials. Figure 5-10 shows fireproofing and structural damage

on a portion of the floor truss. The procedure is very similar to the one described for perimeter columns

in Chapter 4.

88 NISTNCSTAR 1-5G, WTC Investigation
Floor Trusses and Core Beams

Figure 5-10. Incorporating structural and fireproofing damage on the truss assembly.

5.4 INITIAL AND BOUNDARY CONDITIONS

As for perimeter columns described in Chapter 4. the thermal responses of floor trusses and core beams

depend on a specific set of initial and boundary conditions and surface loads that are applied on the full floor model. In this section, we will point out the peculiarities of applying boundary conditions that are specific to floor trusses and core beams.

Heat flux boundary conditions (surface loads predicted from a plane layer analysis) were applied to the

exposed faces of the floor truss. The flux is incident normally on the fireproofing (if the fireproofing is intact) or on the steel (if the fireproofing is considered to be damaged). The flux varies as a function of

space and time, depending on the location of the truss element relative to the fire.

Figure 5-1 1 and Figure 5-12 show the flux boundary condition on a portion of the truss element. The

finite element model for floor trusses has surfaces that can be classified as either horizontal or vertical surfaces. Among the horizontal surfaces, some are facing upwards while others are facing downwards.

The upward and downward facing surfaces of the truss were identified and appropriate radiative fluxes

obtained from a plane layer analysis were applied. Figure 5-1 1 shows that upward facing surfaces were

subjected to a flux Qc, while downward facing surfaces were subjected to flux Qf The truss model also

has vertical surfaces and these surfaces were subjected to flux Qs as shown in Figure 5-11.

The incident flux is strongly coupled with the fire growth and spread through the various floors of the

World Trade Center Towers. The flux is a function of the instantaneous hot layer thickness, absorption

coefficients, hot layer temperature, and ambient temperature as predicted by the fire simulations. The

radiative flux also varies depending on the location and orientation of the structural element. Figure 5-12

shows the actual values of the flux incident on the various elements of a truss model at one instant in time. As the fire evolves and spreads on the floor, the flux incident on the surface also changes with time.

NISTNCSTAR 1-5G. WTC Investigation 89
Chapter 5

90 NISTNCSTAR 1-5G, WTC Investigation
Floor Trusses and Core Beams

5.5 TIME-TEMPERATURE PROFILES

The fire growth is dependent on the combustibles and the ventilation pattern on the various floors of the

WTC towers. As the combustibles are consumed, or if all the oxygen is consumed, the fire activity can

die down. The flux incident on the truss elements varied from one location to another and is a function of

the local, instantaneous properties of the hot layer as well as the location and orientation of the element.

In this section time-temperature history on a few nodes of the steel trusses is presented to understand the nature of the thermal insult.

Four locations were chosen on the model of the steel trusse that supports floor 96, WTC 1. Two locations were on the north face of WTC 1, while the remaining two were on the south face. Figure 5-13 shows the

temperature time history for the four locations. The temperature range is between 0-800 "C. Results are presented for the duration of the simulation (6300 s). The sub-figures a and b are for nodes located on the

north face, while sub-figures c and d are for nodes located on the south face. It should be noted that there

is a v^'ide variation of time-temperature curves that hold at different points in the structure and that these curve do not resemble those from a "standard time temperature curve" used in furnace tests.

800

O

-600

CO

q5 400

Q.

E

Q^ 200

a)

2000 4000

Time (s) 6000

800

o

-BOO

o 400

Q.

E

0) ^ 200

c)

2000 4000

Time (s) 5000

800

0) 600

^400

-I

E

200

b)

2000 4000

TimG (s) 6000

800

O

-600

Toa 400

Q_

E

I- 200

d)

2000 4000

Time (s) 6000

Figure 5-13. Temperature profile as a function of time at four different locations on the

steel truss. Sub-plot a and b are for locations on the north face, while sub-plots c and d

are for locations on the south face of WTC 1, floor 96.

NISTNCSTAR 1-5G, WTC Investigation 91
Chapter 5

Time (s)

Figure 5-14. Time temperature plot on the steel truss (north face, floor 96 WTC 1)

indicating the frequency at which the thermal data is plotted (small symbols) as well as

the frequency at which it is transferred into the structural models (large symbols).

5.6 INTERFACE BETWEEN THERMAL AND STRUCTURAL MODELS

Thermal analysis predicts the time temperature cui^ve at each and every node of a thermal model. Some of

these time-temperature curves were discussed in the previous section. As discussed in Chapter 4, the

thermal and structural models are two entirely different models, consisting of different basic element

types and nodal locations. The thermal data, computed as part of the thermal analysis, needs to be

transfen'cd on to the structural model to apply body loads as a function of time. The procedure is very

similar to that for perimeter columns described in Chapter 4. In this section, the special requirements and

peculiarities of the mapping procedure for floor trusses are discussed. Although the focus is on floor

trusses, the analysis is equally valid for core beams.

5.6.1 Mapping of Thermal Data

The transfer of temperature data between thermal and structural models for full floor trusses is shown

schematically in Figure 5-15 and Figure 5-16. In Figure 5-15, the sub-figure on the left shows a portion

of full floor truss model used in the theraial analysis. In this case, elements in the steel and fireproofmg

have been shown. The sub-figure on the right shows the elements of a corresponding structural model.

Each element is represented by a different color. Similarly, Figure 5-16 shows mapping of thermal data

from the nodes of the thermal model (left sub-figure) to the nodes of the structural model (right subfigure). The structural models are meshed with beam elements (NIST NCSTAR 1-6D), which are capable

of handling both a temperature and temperature gradient. However, thermal analysis indicates that the

92 NIST NCSTAR 1-5G, WTC Investigation
Floor Trusses and Core Beams

temperature gradient through the upper flange cross-section (or the lower flange or web diagonal) is relatively small. This is due to the fact that the upper flange, lower flange, and web diagonals have

relatively small thickness. Temperature data was transferred from the thermal model to the structural models only, at discreet intervals in time, as discussed below.

5.6.2 Procedure for Mapping Thermal Data

The following algorithm summarizes the procedure for transferring temperature data between thermal and

structural models.

• Open the structviral model database file.

• Identify and select the elements that form the entire floor truss.

• Identify and select the nodes associated with the elements.

• Write node number and spatial location (x, y and z location) of the nodes. The location of the node is in reference to a global co-ordinate system.

• Open the thermal database file. Load the solution data files at a specific time where the thermal

loading data file has to be created.

• Read the node number and co-ordinate locations for temperature data. Compute the temperature

at the node closest to the structural node. Write the temperature data in a thermal loading data

file.

• Repeat for all nodes in the model, on various floors and at various time steps where thermal

loading data is needed.

Structural model of a floor

Thermal Model of a truss. Figure 5-15. Mapping of the thermal data between the thermal models and the

structural models showing element density.

NISTNCSTAR 1-5G, WTC Investigation 93
Chapter 5

Thermal Model of a truss showing nodal density Structural Model of a truss showing node density

Transfer of thermal data

between thermal and

structural models

35621

.1297

.1300

.35455

.38622

.22828

.90005

Figure 5-16. Mapping of the thermal data between the thermal models and the

structural models showing nodal density.

Full Floor Mapping of Thermal Data Figure 5-17 shows the results of mapping of thermal data (using the

procedure described in the previous sub-section) onto a full floor truss model (NIST NCSTAR 1-6D).

Temperature data was transferred for the trusses and core beams from the thermal model to the structural model. For each floor and for each time step, a thermal loading data file was constructed that contains the

temperature data. The themial loading data file is in a format that is consistent with structural models and

can be easily read in to apply body loads on the structural model. Thermal loading data files were

constructed at each time step and were transferred to the structural analysis. Figure 5-17 is a visual

representation of the data contained in the thermal loading data file. Partial results from one floor fire are used for demonstration; a full description of all the cases is in Chapter 8 through Chapter 11.

The approach outlined in this chapter for constructing thennal models for floor trusses and core beams,

for coupling the fire simulations with the thermal response and for mapping the thermal data between the

thermal and the structural models was used extensively in the WTC Investigation, as described in the

global tower results presented in Chapter 8 through Chapter 1 1

.

94 NIST NCSTAR 1-5G. WTC Investigation
Floor Trusses and Core Beams

North Tower : 96th Floor

Figure 5-17. Mapping of the thermal data for floor 96, WTC 1 onto the structural models.

Thermal data is provided as a body load file for the structural model. In this picture

results are shown at 6000 s after impact.

NISTNCSTAR 1-5G, WTC Investigation 95
Chapter 5

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96 NISTNCSTAR 1-5G, WTC Investigation
Chapter 6

Concrete Slabs

This chapter is the third in a series of four chapters (Chapter 4 through Chapter 7) that details how the major structural components in the building were represented with finite element models and the method

for coupling the thermal response to multiple floor fires in the World Trade Center (WTC) towers

evolving in space and time. The focus of this chapter is on concrete slabs. Finite element models of

concrete slabs along with mesh restrictions and approximations are discussed. The thermal response to an

incident radiative flux on a concrete slab from fires evolving on two floors is described. Temperature

plots through thickness of the slab at different instants in time are provided to understand the thermal

loading on concrete. Results from one of the fire simulations are used to understand the coupled response. The description applies to the fire floors, floors 93-99 of WTC 1 and floors 79-83 in WTC 2. As

discussed before, global response of the towers is presented in Chapter 8 through Chapter 11. Finally, the

approach and the challenges of mapping body loads from thermal models to structural models are discussed.

6.1 FINITE ELEMENT MODELS OF CONCRETE SLAB

In the WTC towers, floor construction typically consisted of 4 in. of lightweight concrete on 1 Vi in., 22

gauge non-composite steel deck. In the core area, the concrete slab thickness was 5 in. (NIST

NCSTAR 1-1). The concrete slab was one of the heaviest structural elements of the WTC towers. It was

necessary to perform a thermal analysis of the concrete slab to understand its thermally induced structural response to fire growth and spread. This section describes the development of a finite element model,

including mesh requirements and challenges in resolving the thermal waves that can move during fire induced heating of the a concrete slab (Ahmed and Hurst, 1995, Bailey and Moore, 2000, Buchanan 2001,

Lament, Usmani and Drysdale, 2001).

Figure 6-1 shows the top view of a concrete slab that was used to predict the thermal response. The

concrete slab was meshed with solid brick elements with 3-D thermal conduction capability (as discussed

in Chapter 4). Surface effect elements were superimposed on the brick elements to model convective and

re-radiative heat transfer to the slab. The length and width of the slab (60 m), shown in Figure 6-1, was

chosen to be consistent with the fire simulations (NIST NCSTAR 1-5F). The mapping algorithm

discussed at the end of this chapter accounts for the appropriate scaling between the thermal and structural models.

Simulations were performed with approximately 120 equally spaced elements along the length and width

of the concrete slab. The choice of mesh density was based on a number of considerations. The mesh

density was chosen such that it was finer than the resolution of the structural models (which take the

thermal data as an input) as well as the fire dynamics simulation. This ensured that thermal analysis did not add to the uncertainty in the fire simulafions or the structural calculations. Increasing the number of

elements along the length or width of the slab results in higher computafional time and can significantly

slow down the mapping procedure (search algorithm associated with the interpolation routine). A

parametric study was performed to study the effect of mesh density. Doubling the number of elements

NIST NCSTAR 1-5G. WTC Investigation 97
Chapter 6

along the length and width of the slab was found to have only a minor effect on the predicted

temperatures in the concrete slab.

The theraio-physical properties (NIST NCSTAR 1-3, NIST NCSTAR 1-6A, Phan 2003) required for thermal analysis of all the materials are discussed in Appendix A.

Figure 6-1. Finite element model of a concrete slab (top view) showing element density.

Figure 6-2 shows a finite element model of a comer of the concrete slab, again showing the element

density along the length and width of the slab relative to the density through the thickness of the slab. The

finite element models (the thermal and the structural models) of the concrete slab do not account for the

22 gauge non-composite steel deck. The structural analysis discussed in NIST NCSTAR 1-6 prescribed an equivalent thickness of 4.35 in. to account for both the concrete and steel deck. This equivalent

thickness was used in all the calculations and is consistent with the structural models.

The element density through the thickness of the concrete slab was dictated by the need to adequately

resolve the both the thermal wave propagating through the slab and the steep temperature gradients that exist at the surface. Temperature gradients can lead to differential thermal expansion, which can have a

significant effect on the structural response. Simulations were perfomied with approximately 16 elements

through the thickness of concrete slab to resolve the thermal wave. Again, this resolution was

significantly higher than the resolution of the structural models. Note that in the structural models, the

slab was modeled with a single shell element with four layers and a total thickness of 4.35 in. It should be

pointed out that increasing the number of elements through the thickness of the slab has a significant

98 NIST NCSTAR 1-5G, WTC Investigation
Concrete Slabs

effect on computational time and effort. Parametric studies were conducted to understand the role of mesh

densit>' on the computed results. Based on these parametric studies, resolution in the structural models,

and the computational time associated with increasing mesh density, it was decided that 16 elements was

an optimum number of elements through the slab thickness.

The number of elements through the thickness of the slab could not be changed arbitrarily, but was

constrained by the mesh element size in the horizontal directions, so as to maintain a reasonable aspect

ratio for the thermal elements.

Figure 6-2. Finite element model of a concrete slab indicating element density through

the thickness of the slab.

6.2 INITIAL AND BOUNDARY CONDITIONS

The initial temperature of the concrete slab was assumed to be 27 °C. Radiative flux from the fire was

applied as a surface loads on the top and bottom surface of the slab, while the edges of the slab were assumed to be adiabatic (zero flux boundary condition). This section describes the transient and spatially

varying heat flux incident on a concrete slab. A concrete slab is subjected to radiative fluxes on its top and bottom surfaces due to fire activity above

and below the slab. Fire on the floor below the concrete slab provides radiative flux to the bottom face of

the concrete slab. Radiation from the fires above the slab provides radiative flux to the top surface of the

slab. Figure 6-3 shows the concrete slab on the 96'*' floor, WTC 1. It is subjected to radiative flux from

fires on the 95'*^ floor on its bottom face and a flux corresponding to fires on the 96'*^ floor on its top face. The lengths of the arrows are proportional to the intensity of the radiative flux incident on the surface. Since the fire evolves and grows from one part of the floor to another, the radiative flux incident on the

NISTNCSTAR 1-5G, WTC Investigation 99
Chapter 6

concrete slab varies with location and with time. The top and bottom surfaces of the concrete slab were

covered with surface effect elements to model convective and radiate heat flux to the slab. Convective

flux on the bottom surface of the slab is from fires on the 95**^ floor while that on the top surface is from

fires on the 96* floor. Chapter 4 gave a detailed description of the method for applying convective and re- radiative fluxes on perimeter columns and that discussion is equally valid for concrete slabs.

Figure 6^ shows contours of radiative flux incident on the top surface of a concrete slab representing the

96"" floor of WTC 1. This flux is due to fires on the 96* floor. The contour maps have been shown at six

different instants in time, ranging from 1,000 s to 6,000 s at 1,000 s intei'vals. The heat flux contours range from 0 to 140,000 W/m" (140 kW/m"). Figure 6-5 similarly shows contours of radiative flux

incident on the bottom surface of the concrete slab. This flux is due to fires on the 95* floor. Again, the contours maps have been shown at the same six instants in time. For both Figure 6-4 and Figure 6-5, the

north face of floor 96 is the top while the east face is on the right. Since the fires on floor 95 and floor 96

evolve in a different manner and at a different pace, the flux contours on the top and bottom surface are not in sync with each other.

Figure 6-A shows intense fire activity on the north and west faces at 1,000 s after impact, while at 2,000 s, the fire is located in the southwest comer and the northeast comer. This is followed by gradual cooling of

the fires on 96* floor (NIST NCSTAR 1-5F). Fire activity on the 95* floor (Figure 6-5) results in high

radiative flux to the concrete slab on the north and east faces (1,000 - 3,000 s) followed by hot spots in

the south face at 4,000-6,000 s after impact. The hot spots indicate a peak flux level of 140 kW/m"^. In these simulations, a fraction of all the results from one typical fire simulation are presented to indicate the

transient nature of the fires and the resulting fluxes on the concrete slab. A full discussion of all the global

calculations is provided in Chapter 8 through Chapter 1 1

.

Figure 6-4 and Figure 6-5 demonstrate two points: (1) the radiative fluxes on a slab varies as a function

of both space and time, depending on the local fire behavior, and (2) the flux on the top and bottom faces

are almost completely independent of each other.

Radiative tliix

tiom fnefi on 96^'^'

floor

Radiative tlux

fioin fiiet: on 95^^

tlooi"

Figure 6-3. Variation of radiative heat flux incident on the surface of a concrete slab

representing the 96*^ floor of the WTC 1 . Arrows pointing downwards indicate flux

incident on the floor from a fire on top of the concrete slab, while arrows pointing

upwards indicate flux incident on the ceiling of the concrete slab from a fire below.

96^*^ tloor

concrete slab

100 NIST NCSTAR 1-5G, WTC Investigation
Concrete Slabs
Chapter 6
Concrete Slabs

6.3 TEMPERATURE AND TEMPERATURE GRADIENT IN CONCRETE SLAB

The temperature of the slab responds to the spatially and temporally varying fires and the resulting flux incident on the concrete slab. High temperatures and temperature gradients can result in differential thermal expansion in the concrete slab and can influence its structural response. This section describes

ty pical temperature contours on the slab surface as well as temperatures inside the slab. Temperature

profiles through the thickness of the slab are plotted as a function of time to understand the nature of the

thermal insult to the slab.

6.3.1 Surface Temperature

Figure 6-6 shows temperatures (Kelvin) of the top and bottom faces of a concrete slab at 1 00 min

(6.000 s) after impact. A portion of the concrete slab on the north face (top) was damaged by the impact

of the aircraft and that portion has been removed from the visual. The temperature scale ranges from

273 K to 1 ,273 K (0 °C to 1 ,000 °C).

a) Top surface b) Bottom surface

Figure 6-6. Typical temperature contours (Kelvin) on the top and bottom faces of the

concrete slab (96*^ floor, WTC 1) at 6000 s after impact. A portion of the concrete slab on

the north face (top) was damaged by the impact of the aircraft.

Figure 6-7 shows temperature contours through the thickness of a concrete slab, at two different instants

of time. The temperature contour scale is the same as that shown in Figure 6-6 and it ranges from 273 K

to 1 ,273 K. As the top and bottom surfaces heat up, energy gradually conducts throughout the slab and

this can result in a very steep temperature gradient at the surface. The temperatures on the top and bottom

faces are different since they are subjected to different fire growth and spread patterns. The results shown

in this picture were obtained with 16 elements through the thickness of the slab. Increasing the element

density can result in steeper temperature gradients at the surface and slightly lower surface temperatures,

while decreasing the element density can give rise to higher surface temperature and milder temperature

gradients at the surface.

NISTNCSTAR 1-5G, WTC Investigation 103
Chapter 6

Bottom Suilace

> Top Sill face

Figure 6-7. Temperature contours through the thickness of the concrete slab at two

different instants in time (1,000 s and 3,000 s after impact). The range of temperature

contours is the same as shown in Figure 6-6.

6.3.2 Temperature Profiles through the Concrete Slab

This section describes the temperature profiles through slab thickness as a function of time. Four locations on a floor were chosen for analysis. These locations have been marked with red dots in Figure 6—8.

"Location 1" is in the middle of the north face and is half the distance between the north wall and the

core. "Location 2" is in the middle of the south face and is half the distance between the South Wall and

the core. "Location 3" is in the middle of the west face and is half the distance between the West Wall and

the core, while "Location 4" is in the middle of the east face and is half the distance between the east wall

and the core. Temperature profiles tlirough the slab thickness as a function of time at these four locations

are discussed in this section.

The profiles presented in this section qualitatively describe a vast spectrum of the thermal data that is presented in Chapter 8 through Chapter 1 1 as part of the global thermal response of WTC l and WTC 2.

Location 1

« MS If '(s; m

Location 3

Location 4

Location 2

Figure 6-8. Location of the four nodes on the concrete slab where temperature profiles

through the slab thickness have been plotted in Figure 6-9 through Figure 6-12 .

104 NISTNCSTAR 1-5G, WTC Investigation
Concrete Slabs

Figure 6-9 shows temperature profiles through the slab thickness at "Location 1". The temperature

profiles have been plotted at six different instants in time ranging from 1,000 s to 6,000 s at 1 ,000 s inter\'als. The slab thickness is 4.35 in. (0.12 m) and is plotted on the x axis. "Location 1" shows very

high temperamres at the top and bottom surface at 1,000 s, followed by gradual cooling. As the top and

bottom surfaces cool down, heat gradually conducts into the middle of the slab (2,000 s), which results in a small increase in temperature even though the fire has moved away. At 6,000 s after impact, the slab center shows higher temperature than the top and bottom surfaces due to convective and radiative cooling

of the surface. The temperature profiles at different instants in time are approximately symmetric about

the center of the slab for this location.

800

^ 600

o

Q)=J

CD 400 v_

QJCLE

200

"0 0.02 0.04 0.06 0.08 0.1 0.12 Slab thickness (m)

Figure 6-9. Temperature profiles through the slab thickness at Location 1. The

temperature profiles have been plotted at six different instants in time at 1,000 s

intervals.

Figure 6-10 shows profiles through slab thickness at "Location 2". This location, close to the south wall,

shows that the slab is gradually heating as a whole during the entire duration of the event. The

temperatures at the top and bottom faces as well the slab center show a gradual increase with time. At

6,000 s the slab center shows higher temperature than at 1,000 s or 2,000 s after impact, due to gradual

heating of the slab. The temperature profiles at different instants in time are not symmetric about the slab

center. The temperature profiles are consistent with fire activity in this area on the QS"' and 96"" floor.

1000 s 2000 s

J_l__] I I I I I

' ' 1 I I I : l_l I I I I

NISTNCSTAR 1-5G, WTC Investigation 105
Chapter 6

1000 s 2000 s

0 0.02 0.04 0.06 0.08 0.1 0,12

Slab thickness (m)

Figure 6-10. Temperature profiles through the slab thickness at Location 2. The

temperature profiles have been plotted at six different instants in time at 1,000 s

intervals.

Figure 6-1 1 reflects intense fire activity on the 95"^ floor resulting in very high surface temperatures

(1,000 s and 2,000 s) at the bottom of the slab (Location 3). Over time the fires on 95"^ floor die down,

resulting in lower slab temperatures. Very little fire activity on 96'*' floor at 1,000 s results in relatively

low temperatures at the top face of the slab. This low activity period is followed by gradual heating at Location 3 over time. It should be noted that the temperature profiles are not symmetric about the center of the slab.

1000 s 2000 s

Q I

I

' ' ' '

' '

I

' I l_l I l_l I I I ' ' ' I 0 0.02 0.04 0.06 0.08 0.1 0.12 Slab thickness (m)

Figure 6-11. Temperature profiles through the slab thickness at Location 3. The temperature profiles have been plotted at six different instants in time at 1,000 s intervals.

106 NISTNCSTAR 1-5G, WTC Investigation
Concrete Slabs

Temperature profiles at "Location 4" show results that are quite different from that at "Location 1". Figure 6-12 indicates relatively low surface temperamres at 1,000 s, followed by rapid heating at 3,000 s. Temperature at the slab center reaches a peak value at 5,000 s after impact. The profiles are in general not symmetric about the center line of the slab.

1000 s 2000 s 3000 s

0.04 G.Cb 0.08 Slab thickness (m)

0.1 0.12

Figure 6-12. Temperature profiles through the slab thickness at Location 4. The

temperature profiles have been plotted at six different instants in time at 1,000 s

intervals.

6.3.3 Time-Temperature Plots

Top and bottom face temperatures at four locations described in Figure 6-13 are plotted as a function of

time to indicate the transient nature of the fires. Symbols in this figure are plotted at 200 s intervals. Top

face of "Location 1" shows a peak temperature at 500 s after impact, while "Location 2" shows a peak

temperature just before collapse (6.000 s). Top face of "Location 3" and "Location 4" show peak values

at approximately 1,500 s and 3,000 s respectively. Figure 6-13 also indicates that the temperatures at the

top and bottom faces do not reach peak values at the same time. This figure further illustrates the transient nature of the fires as they develop on the various floors resulting in slab surface temperatures that vary

drastically over time. This figure illustrates that the temperature on the top and bottom faces are almost

completely independent of each other. Also note, that small symbols in this plot indicate the frequency at which data is plotted, while large symbols indicate the frequency at which data is transferred into the

structural models for analysis.

NISTNCSTAR 1-5G. WTC Investigation 107
Chapter 6

000

800

1^

/ B=?-

Loc 1 - Top

Loc 1 - Bol

Loc 2- Top

" Loc 2- Bol

- Loc 3

- Top

Loc 3 - Bot

Loc 4- Top

Loc 4- Bol

2000 3000 4000

TlfTlG (S.)

Figure 6-13. Temperature is plotted as a function of time at four different locations (top

and bottom surfaces) on a concrete slab representing the 96*^ floor of WTC 1. These

results clearly demonstrate the transient nature of fire growth through the floors.

6.4 STRUCTURAL DAMAGE TO THE CONCRETE SLAB

An objective of the NIST Investigation was to simulate the theraially induced response of the towers

damaged by aircraft impact. Aircraft impact modeling results indicated severe damage to the concrete

slab (NIST NCSTAR 1-2). It was necessary to include this damage in the thermal and structural models

to accurately predict collapse of the towers. Damage to the concrete slab was simulated by the following

three step process:

1. Construct a pristine structure (concrete slab) with no structural damage.

2. Identify the elements where the concrete slab is damaged by the impact of the aircraft.

3. Delete the elements where the structure was damaged by aircraft impact.

It was assumed that the concrete slab was not sprayed with fireproofmg material and that there was no

over spraying of the fireproofmg from the trusses onto the concrete slab.

108 NIST NCSTAR 1-5G, WTC Investigation
Concrete Slabs

6.5 MAPPING OF THERMAL DATA

As discussed in this chapter, the thermal analysis was performed with sohd brick elements, while the

structural analysis was performed with shell elements (NIST NCSTAR 1-6). In addition, the two models

required different levels of resolutions for the physical processes that are being simulated. Therefore, the temperature data and the temperature gradient information must be transferred from the thermal models to the structural models. This data transfer is the form of a series of thermal loading data files (NIST

NCSTAR 1-6D). The thermal loading data files must be compatible with the structural models so that they can be readily used in the analysis. The methodology and algorithm for transferring the thermal data was discussed in Chapter 4. In this section, the peculiarities of the mapping (interpolation) process for concrete slabs, that were not clarified in Chapter 4. are discussed.

Transfer of thermal data between thermal and structural models for concrete slabs has been shown

schematically in Figure 6-14 and Figure 6-15. Figure 6-14 shows the thermal model (brick elements) on

the left and structural model (shell elements) of a concrete slab on the right.

Tliei mal Ivlodel Stnictui al Klodel

Mafipitig of tempemtufe and

teraperatuie giaidient

Figure 6-14. Mapping of thermal data (temperature and temperature gradient) from the

thermal models to the structural model.

Trarisfer of tliermal data bet'wen Ihermal and

stmrtural model

Figure 6-15. Mapping of the thermal data (temperature and temperature gradient) from

the thermal models to the structural model.

Figure 6-15 illustrates the number of elements through the thickness of the thermal model. The shell element consists of four layers (ANSYS 2005) that w ere equally space through the thickness of the slab. Thermal data was provided at the interfaces between the layers and at the top and bottom face. The data

transfer was done for shell elements using the methodology described in this chapter.

NIST NCSTAR 1-5G. WTC Investigation 109
Chapter 6

The approach presented in this chapter for predicting the thermal response of the concrete slab and

transferring or mapping the thermal data on a structural model was used extensively throughout the

investigation. The concrete slabs shown in Chapter 8 through Chapter 1 1 (Global response of the towers)

present results from a similar mapping for four specific scenarios, two each for WTC 1 and WTC 2 . The

results for the concrete slab, shown in the figures included in Chapter 8 through Chapter 1 1, is a visual

representation of their themial state.

110 NISTNCSTAR 1-5G, WTC Investigation
Chapter 7

Core Columns

This chapter is the last of four chapters (Chapter 4 through Chapter 7) that detail how the major structural components in the building were represented with finite element models. In addition, the method for coupling the thermal response of the structural components to multiple floor fires in the World Trade

Center (WTC) towers is described. The focus of this chapter is on core columns. The thermal analysis of

core columns and coupling of the models with fire simulations and structural models was in general very

similar to the one described for perimeter columns in Chapter 4. In this section we will only describe the

peculiarities of the core columns (modeling and analysis) that are different from those for perimeter

columns. These differences are mainly in model construction and application of boundary conditions.

Typical incident radiative flux on core columns and its dependence on fire spread has been presented.

Time-temperature profiles are discussed to understand the nature of thermal insult. As for perimeter

columns, partial results from one of the fire simulations are utilized to understand the coupled response. The description applies to the fire floors, floors 92-99 of WTC 1 and floors 78-83 of WTC 2. As noted

earher, global response of the towers is presented in Chapter 8 through Chapter 1 1

7.1 FINITE ELEMENT MODEL FOR CORE COLUMNS

Figure 4-1 1 shows a t>'pical layout of the core columns on floor 96 of the WTC 1. The core was

supported by a combination of wide flange and box section columns. Some of these columns were very

large, w ith cross-sections measuring 14 in. wide by 36 in. deep (NIST NCSTAR 1-1). Since plasticity and

creep of the core columns were an important part of the structural analysis (NIST NCSTAR 1-6C),

thermal models were constructed of the steel column and the fireproofing to predict the thermally induced

structural response. This section describes the finite element models that were developed for core column

analysis.

On a t\'pical floor of the WTC tower, there were a total of 47 core columns. Like the perimeter columns

the core columns were not identical in general. Some of these columns were wide flange columns while

others had a box shape cross-section.

7.1.1 Wide Flange Columns

All wide flange core columns in the WTC towers were not identical. The thickness of the web plates,

flange plates, and overall column dimensions varied from one column to another. Figure7-1 shows finite element models of two of the wide flange columns. The columns in Figure 7-1 have been chosen to

illustrate the differences in the size of two wide flange columns. The elements have been color coded with

the assigned material attributes. Cyan colored elements have material attributes of steel, and violet

colored elements have material attributes of fireproofing.

NIST NCSTAR 1-5G. WTC Investigation 111
Chapter 7

7.1.2 Box Shaped Columns

Figure 7-2 shows finite element models of two box columns, illustrating the differences in coluirm size. The thickness of the flange plate, web plate, and overall dimensions varied from one box shaped column

to another. The shape and size of the core columns also differed from one floor to another. The elements

have been color coded with the assigned material attributes. Cyan colored elements have material

attributes of steel, and violet colored elements have material attributes of fireproofing.

Column 50 1 , Floor 96 North Tower Column 704, Floor 96 North Tower

Figure 7-1. Finite element models of wide flange core columns, including insulation

developed for thermal analysis of the World Trade Center Towers. Steel elements are colored cyan and fireproofing elements violet.

Since the thennal response of the column is dependent on the cross-sectional area, the circumference and

the insulation, finite element models using the ANSYS parametric design language were constructed to include the variability in shape, size, material properties, and fireproofing of the core columns. The

structural descriptions of the core columns on floors 92-99 of WTC land floors 78-83 of WTC 2 are the

only ones of interest here and are given in Table B-^ through Table B-8. Table lists the column

types for floor 92. Table B-5 lists the column types for floors 93-95, and Table B-6 lists the column types

for floors 96-99. Table B-7 and Table B-8 list the coluinn type for floors 78-80 and floors 81-83 of

WTC 2 respectively. For box columns, the dimensions of the columns have been noted while for the wide

flange columns the dimensions were obtained from the Steel Construction Handbook. The thermal

analysis of the core columns is limited to floors 92-99 in WTC 1 and floor 78-83 in WTC 2.

The thermo-physical properties of all the structural materials used in this analysis are discussed in Appendix A.

112 NISTNCSTAR 1-5G, WTC Investigation
Core Columns

7.1.3 Fireproofing Thickness for Core Columns

The fireproofing material used on the core column was BLAZE-SHIELD DC/F (NIST NCSTAR 1-6A).

The fireproofing thickness was a fiinction of the core column size. In general there were two different

fireproofing thicknesses used on the core columns. The lighter core columns were covered with 2.2 in. of

fireproofing while the heavier core columns had smaller thickness of 1.2 in.

Column 701, Floor 95, WTC 1 Column 501, Floor 78, South Tower

Figure 7-2. Finite element models of box columns including insulation used for thermal

analysis in the World Trade Center towers. Steel elements are colored cyan, and

fireproofing elements are violet.

Figure 7-2, the elements have been color coded with the assigned material attributes. Typically there were approximately 20 elements along the length of a core column and six elements through the thickness

of fireproofing. Along the perimeter the elements were spaced at roughly 6 cm intervals. Mesh density

along the length of the column was governed by the requirement to accurately capture the variation in radiative fiux from the hot layer on the column (Chapter 1). The mesh density through the thickness of the

fireproofing was dictated by the need to adequately resolve the thermal wave through the fireproofing,

and to reduce computational time (Figure 7-3). Exploratory studies described in Chapter 3 also played an

important role in the choosing the mesh density. Lightweight columns had only one element through the

thickness of the steel plate, while heavy column had two or three elements. Sensitivity studies were

performed with a more refined finite element mesh, but the calculations did not result in any significant

change in steel temperature.

NIST NCSTAR 1-5G, WTC Investigation 113
Chapter 7

iili

iili lili II

if

\

Colm-nn 1001, floor 96 WTC 1 Column 701, floor 95 WTC 1

Figure 7-3. Isometric view of finite element models of the core columns showing mesh

density and column construction. The elements are color coded with material attributes.

Cyan colored elements have material attributes of steel while violet colored elements

have material attributes of fireproofing.

The thermal response of core columns to spatially and temporally developing fires (NIST NCSTAR 1-5F) was simulated subject to a set of initial and boundary conditions. The approach for specifying initial and

boundary conditions is again very similar to that for perimeter columns described in Chapter 4. Difference

in the application of boundary conditions between core columns and perimeter columns is described in

this section.

For perimeter columns, radiative and convective fluxes were applied on the interior faces, while the

exterior faces were assumed to be convectively cooled by the ambient atmosphere. Core columns were

subjected to radiative and convective fluxes over the entire perimeter. The radiative or convective fluxes

incident on wide flange or box columns are indicated with arrows in Figure 7-4. The length and color of

the arrows indicates the intensity of the incident flux. As for perimeter columns, the radiative fluxes for core columns were coupled with fire simulations and were obtained by simplifying the radiative transport

equation under the plane layer approximation, as described in Chapter 1 . The radiative flux obtained from

the plane layer analysis varied along the height of the columns and changed with time.

Re-radiation from the surface of the column to ambient atmosphere was modeled with surface effect elements (as for perimeter columns described in Chapter 4). Radiative heat transfer within the cavity of

box column was also modeled to allow for heat exchange between the four plates.

114 NIST NCSTAR 1-5G, WTC Investigation

7.2 INITIAL AND BOUNDARY CONDITIONS
Core Columns

7.3 INCIDENT RADIATIVE FLUX

In this section, the nature of the incident radiative flux on a typical core column (wide flange column),

when subjected to realistic fires is discussed. These resuhs are based on one typical fire simulafions and

are provided here to understand the temporal and spatial variation of the incident flux.

Figure 7-4. Radiative flux and convective flux boundary conditions (Indicated by arrows)

imposed on wide flange and box shaped core columns.

Figure 7—4 exhibits radiative fluxes incident on core column 50 1 at six different instants in time. Contours

have been shown at 1000 s intervals ranging between 1000 - 6000 s. The column under consideration is on the 96* floor, but the model has been extended into both the 97* floor and the 95* floor, as noted

above, to account for fires above and below the 96* floor. The portion of the column below floor 96 is subjected to fires on the 95* floor. The part between the 96* and 97* floor is subjected to fires on floor

96. while the portion of the column above the 97* floor is subjected to fires on floor 97. Since the fires on

the various floors (95, 96 and 97) evolve independently of each other, the radiative flux incident on the

column is not continuous at the slab boundaries.

The radiative flux contours range from 0 to 130 kW/m". At 1,000 s after impact of the aircraft, we find

that there are significant fires on 96* floor resuhing in high fluxes to the column. Fire intensity on the 97*

floor is relatively small, while fire intensity on floor 95 is extremely small resulting in very low fluxes on

the column below the 96* floor. Because of the presence of partitions on any floor, the fire activity

around the column is not necessarily uniform, and as a result, the flux incident on the column can vary

over the perimeter at any given height. At 3,000 s after impact, column 501 experiences relatively low

heat flux on floors 95-97. Beyond 4,000 s after impact, the fires in the vicinity of column 501 on floor 96

and 97 have died down, while floor 95 shows intense fire activity.

Figure 7-5 illustrates the variability in flux on a typical core column due to spatially and temporally

evolving fires on the 95*, 96* and 97* floors. It should be noted that there is also a horizontal variation in radiative fluxes, but this variation is quite small.

NISTNCSTAR 1-5G, WTC Investigation 115
Chapter 7

116 NISTNCSTAR 1-5G, WTC Investigation
Core Columns

7.4 CORE COLUMN TEMPERATURE

This section presents temperature contours and time-temperature profiles for columns in the WTC towers. Temperature of a core column depends on several important factors. They include

• fireproofing stams of the column,

• size and shape of the column (heavy or light column),

• fire intensity in the vicinity of the column. Incident flux varies from one column to another and is also a function of time, and

• fireproofing and structural damage status of columns above and below the column under

consideration.

7.4.1 Temperature Contours

Figure 7-6 shows temperature contours (Kelvin) for typical core columns in the WTC towers. The subfigure on the left is a wide flange column, while the sub-figure on the right is a heavy box shape column.

Temperature contours have been shown at one instant in time through the steel and fireproofing element.

The contours range from 300 K to 900 K. The surface of the fireproofing shows very high temperature,

and energy gradually diffuses into the steel.

Column 1001, floor 96, WTC 1 Column 501, floor 78, WTC 2

Figure 7-6. Temperature contours (Kelvin) for core columns in the World Trade Center

towers. Both steel and fireproofing elements have been shown.

Figure 7-7 shows temperature contours on typical wide flange core column (Column 501, floor 96,

WTC 1 ). Only a portion of the column between floor 96 and floor 97 is plotted. Temperature contours at

six different instants in time, ranging from 1,000 - 6,000s at 1,000 s intervals are presented to understand

the nature of thermal insult on core columns. Temperature contours range between 300 K and 450 K.

NISTNCSTAR 1-5G, WTC Investigation 117
Chapter 7

Column 501 has its fireproofing intact on floor 96, and is a relatively heavy column. The temperature

contours in Figure 7-7 indicate that the mean temperature of the entire column increases gradually over

time. Predicted temperatures are slightly higher in the upper half of the column due to higher radiative

flux from the high temperatures in the upper layer. There is a small temperature gradient along the length

of the column and along the cross section of the column.

7.4.2 Time Temperature Profiles

Figure 7-8 through Figure 7-13 shows time-temperature profiles for all the core columns on floor 96,

WTC 1. Figure 7-8 shows column temperature for the "Series 5"' column, while Figure 7-13 shows the

"Series 10" column. The temperature ranges from 0 °C to 750 °C and the time ranges from 0 to 6,300 s. Temperature is plotted at 200 s intervals, but the symbols are placed at every 400 s.

The time-temperature profiles for each core column is quite different from other core columns in terms of

its peak temperature value, time to peak value and the overall trend. The core column temperature

depends on its fireproofing status, size, shape, and fire intensity in the immediate vicinity of the column.

As indicated earlier in this chapter the specified fireproofing thickness also varies from one column to another. Finally the fireproofing status of the column above and below the column under study can also

have an effect on its temperature. The time-temperature profiles shown in Figure 7-13 depend on the

value of all of these parameters.

The temperature axis has a range from 0-750 "C. Some of the core columns show temperature values that are higher than 750 "C, which results in a break in the time-temperature profile. Temperature range was

restricted to 750 °C since steel has very little strength at these temperatures.

Finally, it should be noted that the time-temperature curves presented in Figure 7-13 do not resemble the

"standard" time-temperature curves used in furnace tests of structural elements.

118 NISTNCSTAR 1-5G, WTC Investigation
Core Columns

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NISTNCSTAR 1-5G, WTC Investigation 119
Chapter 7

501

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' ' ' I ' ' I' I ' ' ' ' I ' ' ' ' I ' ' ' ' I'

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' ' I L_J. 0 1000 2000 3000 4000 5000 6000

Time (s)

Figure 7-8. Core column temperature for columns 501 through 508 on floor 96 of WTC 1 as a function of time.

0 1000 2000 3000 4000 5000 6000

Time (s)

Figure 7-9. Core column temperature for columns 601 through 608 on floor 96 of WTC 1 as a function of time.

120 NISTNCSTAR 1-5G, WTC Investigation
Core Columns

'''' I '' '

I ' ' I l__L 0 1000 2000 3000 4000 5000 6000

Time (s)

Figure 7-10. Core column temperature for columns 701 through 708 on floor 96 of WTC 1 as a function of time.

J l_l I ' ' 'I I I I L_l I l_l I I I I I 1—I—I 1 I I I—I 1 L 0 1000 2000 3000 4000 5000 6000

Time (s)

Figure 7-11. Core column temperature for columns 801 through 807 on floor 96 of WTC 1 as a function of time.

NISTNCSTAR 1-5G, WTC Investigation 121
Chapter 7

0 1000 2000 3000 4000 5000 6000

Time (s)

Figure 7-12. Core column temperature for columns 901 through 908 on floor 96 of WTC 1 as a function of time.

'

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I ' ' I ' ' ' ' I ' —I l__L 0 1000 2000 3000 4000 5000 6000

Time (s)

Figure 7-13. Core column temperature for columns 1001 through 1008 on floor 96 of WTC

1 as a function of time.

122 NISTNCSTAR 1-5G. WTC Investigation
Core Columns

7.5 FIREPROOFING AND STRUCTURAL DAMAGE ON CORE COLUMNS

Fireproofing and structural damage on core columns is predicted as a result of aircraft impact (NIST

NCSTAR 1-2). Some of the debris field generated as a result of aircraft impact can reach the core columns and can damage them. The damage to these columns can be classified under two categories:

1 . Structural damage, where the column is severed or heavily damaged. This affects the boundary

conditions and heat capacity of the column.

2. Fireproofing damage.

Both strucmral and fireproofing damage ha\'e a large impact on the thermally induced structural response.

The methodology for incorporating fireproofing and structural damage has been illustrated schematically

in Figure 7-14 for t^'pical wide flange columns. The approach is very similar to the one described for

perimeter columns in Chapter 4.

Figure 7-14. Incorporating the effect of fireproofing damage on columns by selectively

changing the material attributes of the underlying elements.

Structural damage was incorporated by deleting the elements and nodes associated with the steel column.

Fireproofing damage was simulated by changing the material attributes of the damaged fireproofing

elements so as to offer negligibly small resistance to heat flow. The methodology is similar to that of

perimeter columns, and was described in Chapter 4.

NIST NCSTAR 1-5G. WTC Investigation 123
Chapter 7

7.6 MAPPING OF THERMAL DATA

The thennal analysis for core columns was performed with solid brick elements, while the structural analysis was performed with beam elements. There is also a significant difference in node and element

distributions between the two models, and this difference required a mapping of the temperature data

between the thennal and structural models. In general, the mapping procedure for core columns is very

similar to that of perimeter columns, described in Chapter 4. The peculiarities of the mapping procedure,

specific to core columns, are presented in this section.

7.6.1 Algorithm

The transfer of data between the thermal and structural models for a column is shown schematically in Figure 7-15. The sub-figure on the left represents a brick model for thermal analysis of core columns

(part between two concrete slabs). The plot on the right shows the nodal locations of a corresponding

structural model. The line in the middle shows the beam elements (color coded with element number) in the structural model. The complete algorithm for the mapping procedure can be summarized as follows:

1. Open the structural model database file and select the core column for which thermal data is required.

2. Identify and select core columns that are modeled with beam elements. Core columns modeled

with beam elements are prescribed with temperature gradient data. For each beam element write

the element number and co-ordinate location (x, y and z) of the centroid of the element. The

location of the elements is written with respect to a global co-ordinate system.

3. Open the thermal model database file. Load the solution data files at a specific time where the

load step file has to be created.

4. Read the element number and the co-ordinate location of the structural model for which

temperature gradient data is required. Compute the temperature gradient at the nodes closest to the co-ordinate location specified by the structural model. Write the thermal gradient information

in thennal loading data file.

5. Repeat step 4 for all the elements in a core column that require temperature gradient information

and for all the core columns of interest.

6. Repeat step 3 through 5 for each time step where a thermal loading data file is required.

7.6.2 Full Floor Mapping of Thermal Data

Figure 7-16 shows the mapping between the thermal and structural models for core columns between

floor 96 and floor 97 of WTC 1. For each floor, thermal data were provided as a set of thermal load files (text files) for structural analysis. Thermal load files for every floor of each towers were created at each

time step where thennal data were required. The text files were formatted for the ANSYS finite element

software and structural models. For WTC 1, ten thermal loading files were created at 10 min intervals

ranging from 10 min to 100 min. For WTC 2, six thermal loading files were created at 10 min intervals

ranging from 10 min to 60 min. Figure 7-17 shows time temperature plots for core columns 501 through

124 NISTNCSTAR 1-5G. WTC Investigation
Core Columns

508 on floor 96 of WTC 1 . In this figure, the small symbols indicate the frequency at which the thermal

data are plotted (200 s), while the large symbols indicate the frequency at which the thennal data were mapped onto the structural models for analysis (600 s). Core column temperatures were found to vary

slowly relative to the gas temperature fluctuations; the 200 s interval in the figure represents the thermal

response of the column to the fires without missing significant peaks or fluctuations. The 10 min time

intervals captured the computed thermal profiles reasonably well, as indicated in the figure. The

structural response analysis used linear interpolation between two time intervals for modeling changes in temperatures of the strucmre.

Figure 7-16 shows the thermal data on the core columns at 100 min after impact. This figure is a visual

representation of the information contained in the thermal loading data files. The approach presented in

this chapter for predicting the thermal response of core columns and the transfer of this data to the

structural models was used extensively throughout the Investigation. Specifically, the response of the core column to both thermal and structural loading required to analyze the global response of each tower,

presented in Chapter 8 through Chapter 11, relied completely upon the mapping described in this chapter.

The figures for core columns shown in those chapters are visual representations of the thermal state of the

columns at specific instants in time.

Transfer of temperature

and temperature gradient

data between thermal

and structural model

Thermal Model of a Core Column

showing element density.

Elements Nodes

Structural Model of a Core Column showing

element density (colors) and node location.

Figure 7-15. Mapping of the thermal data from thermal models of the core column to the

structural models.

NISTNCSTAR 1-5G, WTC Investigation 125
Chapter 7

0 150 300 450 600

75 225 375 525 675 Figure 7-16. Mapping of the thermal data for the core columns onto the structural model

for the 96*^ floor of WTC 1 at 6,000 s after impact. The temperature data is provided as a body load and is generated in a format that is consistent with the structural model.

bO(

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400 Z)

D.

E

200

J. 000 2000 3000 4000

Time (s)

5000 6000

Figure 7-17. Core column temperature for columns 501 through 508 on floor 96 of WTC 1, as a function of time indicating the frequency at which the thermal data is plotted (small

symbols) and the frequency at which data is transferred into the structural models

(large symbols).

126 NISTNCSTAR 1-5G, WTC Investigation
Chapter 8

Global Thermal Response of WTC 1 : Case A

During the course of the Investigation, hundreds of preliminary calculations were performed to study the

thermal response of individual structural components and full floor systems. Some of these calculations were discussed in Chapter 2 through Chapter 7. Predicted results were compared with experimental data

to assess the accuracy of the models and sensitivity to input parameters. The preliminary studies,

photographic and visual evidence and published literature, helped guide the development of global

models. Four such global simulations, two each for World Trade Center (WTC) 1 and WTC 2, are presented in this report to predict the global thermally induced structural response of WTC 1 and WTC 2. This chapter is the first of four chapters that describe the resuhs of these four cases.

This chapter describes an application of the Fire Structure Interface (FSI) algorithm to predict the time

dependent global thermal response of WTC 1. The aim of these calculations was to estimate the

temperature of the entire structure (steel and concrete) on floors 92-99 of WTC 1 for prediction of the

thermally induced structural response. The period of time that is simulated is 105 min and it ranges, from

the point immediately after impact of the aircraft until collapse.

The focus of this chapter is on the global response of WTC 1 for a specific set of aircraft impact damage

and fire dynamics simulation. The results of the thermal calculations (presented in visual form in this chapter) were provided to the structural analysis group in a form that is compatible with the structural models and can be readily used to apply a temporally and spatially varying body force (theraial loading) on the structure. A floor by floor description (along with appropriate visuals) is presented to understand

the namre of the thermal insult on the structure. In this chapter the detailed construction of an individual

column, truss or concrete slab is not considered, nor the methodology for transferring temperature data

onto the structural models. Finite element models at the component level and the methodology for linking the thermal and structural models were presented in Chapter 4 through Chapter 7. The focus of

this chapter is on full single floor or multiple floor level global results at discrete instants in time.

8.1 OVERVIEW

The thermally induced structural response and collapse of each tower depends on several analyses and

sub tasks that feed their results either directly or indirectly into the collapse analysis. The aircraft impact

analysis predicts the structural and fireproofing damage caused by the aircraft to the various structural components. These results directly feed into the thermal analysis of the structure. The aircraft impact

damage also feeds the damage data into the fire simulations and the results of the fire simulations were

subsequently utilized in the FSI algorithm to predict the coupled thermal response of the structural assembly. The final collapse analysis depends on four major analyses namely:

1 . Aircraft impact analysis

2. Fire simulations

NISTNCSTAR 1-5G, WTC Investigation 127
Chapter 8

3. Fire structure interface and thennal analysis

4. Structural collapse analysis

Preliminary calculations were performed during the course of this Investigation to study the thermally

induced response of structural components and sub-systems. The objective of these preliminary

simulations was to:

1 . assess the sensitivity of the many input parameters in the various analyses,

2. test the robustness of the numerical models during their development, and

3. gain insight into the fire induced collapse of the WTC towers.

The set of simulations ternied "WTC 1, Case A" represent one set of global simulations where base

settings (see Table 8-1, Table 8-2 and Table 8-3) were employed for the four major analyses to predict

the thermally induced structural response of WTC 1. In this chapter, we present a floor by floor

description along with visuals (Figure 8-2 through Figure 8-35) for the thennal response of WTC 1. The

thermal data shown in these visuals was provided to the structural analysis group as thermal loading data

files (text files) for analyzing the structural response and collapse induced by impact damage and thermal

loading.

8.1.1 Aircraft Impact Analysis

Aircraft impact analysis was performed with base case settings shown in Table 8-1, to predict the

structural and fireproofing damage on floors 92-99 of WTC 1. The main objective of the impact analysis was to estimate the damage to structural systems, including the exterior wall, floor systems, and interior core columns. Table 8-4 shows the fireproofing and stmctural damage predicted for perimeter columns on floors 92-99 (WTC 1, Case A). In this table a "0" indicates that the column and its fireproofing is

intact, a "1" indicates fireproofing damage on the interior faces and a "2" indicates that the column has

been severed due to aircraft impact. For WTCl Case A, none of the perimeter columns were completely

stripped of fireproofing on all four sides. The methodology for incorporating fireproofing and structural damage on perimeter columns was discussed extensively in Chapter 4. Table 8-5 shows fireproofing and

structural damage predicted for core columns on floor 92-99. In this table, a

"0'" indicates that the column

and its fireproofing is intact, a "1" indicates fireproofing damage on one side, a

"'2" indicates that the

column has been severed by aircraft impact, while a "3" indicates fireproofing damage on all the faces. For WTC 1 Case A, the core columns had either no fireproofing damage or complete fireproofing damage

on all the faces.Aircraft impact damage also results in structural and fireproofing damage to the floor

systems. For each floor, a map was prepared that describes the contours of the region where fireproofing

and structural damage was predicted. This damage for floors 94, 95, 96, 97 and 98 have been shown in Figure 8-3, Figure 8-5, Figure 8-9, Figure 8-30 and Figure 8-33 respectively (NIST NCSTAR 1-2). In these figures the area marked by green rectangles indicate structural damage, while the area marked by

blue rectangles show fireproofing damage only (steel structure is intact). Structural damage was

incorporated into the analysis, by removing the concrete slab, trusses, or core beams located in the area marked by the green rectangles. The method for incorporating structural and fireproofing damage was

discussed extensively in Chapter 5 and Chapter 6 of this report.

128 NIST NCSTAR 1-50, WTC Investigation
Global Thermal Response of WTC 1: Case A

Table 8-1. I nput parameters for WTC 1 global impact analyses (Base Case)

Analysis Parameters Base Case

Flight Parameters Impact speed 443 mph

Trajectory - pitch 10.6°

Trajectory - yaw 0.0°

Orientation - pitch 8.6°

Orientation - yaw 0.0°

Aircraft Parameters Weight 100 percent

Failure Strain 100 percent

Tower

Parameters

Failure Strain 1 00 percent

Live Load Weight" 25 percent

8.1.2

a. Live load weight expressed as a percentage if the design live load.

Fire Dynamics Simulations

Fire simulations were performed on the focus floors of WTC 1 with base case settings for the various

input parameters summarized in Table 8-2 using the FDS software. Window breaking times were

prescribed in the fire model as determined from the photographic and video recording. It was found that the distribution and condition of the furnishings and the damage to the core walls / shafts had the greatest

influence on the model outcome. Fire activity around the building exterior served as observable to assess

the accuracy of the fire model.

Properties of the hot layer were extracted from results of the fire simulations. Upper and lower layer gas

temperatures, depth of the smoke layer, and absorption coefficients were computed at the grid locations

and written to a text file at 100 s intervals. The text file also contains information on the underlying grid

used for fire simulations as well as the specific instants in time where the data is provided. The text file was subsequently read into the ANSYS finite element analysis software. Figure 8-6 shows the typical

upper layer gas temperatures obtained from fire simulations for floor 95, WTC 1 Case A (NIST

NCSTAR 1-5F).

Table 8-2. Values of WTC1 Case A fire simulation variables.

Variable WTC 1 Case A

Fuel load 20 kg/m- (4 lb/ft-)

Distribution of

disturbed

combustibles

Even

Condition of

combustibles

Undamaged except in impact zone

Representation of

impacted core

walls

Fully removed

NIST NCSTAR 1-5G, WTC Investigation 129
Chapter 8

8.2 FIRE STRUCTURE INTERFACE

The Fire Structure Interface (FSI) uses the output of a fire simulation performed using the NIST Fire

Dynamics Simulator (FDS) together with aircraft impact analysis results to predict the evolving thermal

state of WTC 1 . Table 8-3 summarizes the data that was used as an input for these calculations. FSI also

linked the theraial analysis and structural analysis by creating thermal loading data files in a forniat that is consistent with the structural models. The global structural response and collapse analysis of WTC 1 under base case conditions described in NIST NCSTAR 1-6C was performed using the thermal data

presented in visual fomi, in this chapter.

Table 8-3. I nput for global thermal response of WTC 1, Case A.

Input WTC 1 Case A

Structural damage,

NIST NCSTAR 1-2 NIST NCSTAR 1-6

Case A

(Base Case)

Fireproofing damage,

NIST NCSTAR 1-6 Case A

Fire Simulations,

NIST NCSTAR 1-5F

Case A

Exploratory studies, photographic evidence and published literature in this subject guided the

development of the global models (Quintiere, Di Marzo and Becker 2002; Hori 2004; Abboud et al. 2003;

Usmani, Chung and Torero 2003; Rehm et al. 2005.; Hori 2004). Figure 8-1 is a sample plot that shows

the thermal data generated with FSI and subsequently transferred to the structural models for analysis.

Temperature contours are superimposed on the steel structure of a typical floor of WTC 1 . The figure

shows the core beams and floor trusses that support a concrete slab. The concrete slab has not been

included in the figure for clarity, but is included as part of the analysis. Portions of the core beams and

floor trusses were damaged by the impact of the aircraft and this portion of the structure is not shown in the figure nor was it included in the thennal analysis. The figure also shows the perimeter and core columns extending to the floor above and below the floor under consideration. Perimeter and core columns that were severed by the impact of the aircraft are not shown in this figure. The structural damage varies from one floor to another and has been summarized in Table 8^ and Table 8-5. As has

been discussed in detail in Chapter 4 through Chapter 7, the thermal and structural finite element models

were constructed from inforaiation obtained from the LERA database (NIST NCSTAR 1-2) and

summarized in Appendix B. The type and location of the fireproofing thickness (not damaged by the

aircraft impact) is summarized in Table B-9.

For WTC 1, thermal analysis was performed to cover the period of approximately 6,300 s, immediately

following impact and extending all the way to collapse of the tower. The minimum time step for the

thermal analysis is 1 ms while the maximum time step was limited to 50 s. For WTC 1, thermal data at ten instants in time, spaced at 10 min (600 s) intervals was provided for the structural analysis. For each

step, a set of thermal loading data files was generated to completely specify the thermal state of the tower. The thermal loading data files were in a format that was consistent with the structural models and could

be readily read in with the ANSYS finite element software (NIST NCSTAR 1-6D) to specify body loads

(temperature and temperature gradient information) on the entire structure. The thermal loading data files

130 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

were provided at 10, 20, 30, 40, 50 60, 70, 80, 90, and 100 min after impact of the aircraft. No thermal

loading data files were provided at zero seconds after impact. It was assumed that the structure is initially

at room temperature. The final load step file was at 100 min after impact, and it contains the final thermal

state of the tower extending all the way to collapse. ANSYS perfomis a linear interpolation between two

consecutive load step files for convergence and stability of the numerical procedure.

Under the plane layer approximation, a simplified form of the radiative transport equation was solved to predict the radiative fluxes incident on the sub-grid scale structural elements. The radiative fluxes are functions of space and time and depend on the location and orientation of the structural elements. Besides

the radiati\ e fluxes are related to the local instantaneous properties of the hot layer as defined by the

upper and lower layer temperature, absorption coefficient and depth of the hot layer.

The fire simulations and the thermal analysis were performed for floors 92 through 99. However, in the

following sections, discussion is limited to floors 93 through 99 only. Thermal mapping was not performed for floor 92. This is due to a combination of the nature of the structural model and the mapping

process (discussed in Chapter 4 through Chapter 7) between the thermal and structural model. The floor

model (NIST NCSTAR 1-6, NIST NCSTAR 1-6D) has a single slab and floor truss system, but the

columns extend one floor above and below the floor. A mapping for floor 92 would require us to perform

a thermal analysis of the columns on floor 91. Thermal analysis of columns on floor 91 was not performed. Besides the slab on floor 92 is subjected to radiative fluxes fi^om fires on floor 91 on its bottom face and fires on floor 92 on its top face. Since fire simulations were not performed on floor 91

thermal analysis of the slab on floor 91 could not be completed. Due to these limitations, the theraial

mapping was not performed for floor 92.

In the following sections, FSI results for WTC 1 Case A are described on a floor by floor basis. The

figures in each of the following sections are a visual representation of the thermal state of that floor at a

specific instant in time. The data shown in the various plots were transferred to the structural analysis

group for stress analysis. The transfer of data was in the form of thermal loading data files (text files) and was in a format that is consistent with the structural models.

NIST NCSTAR 1-5G, WTC Investigation 131
Chapter 8 O I. o a>

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132 NISTNCSTAR 1-5G. WTC Investigation
Global Thermal Response of WTC 1: Case A

8.2.1 Floor93, WTC 1 Case A

Figure 8-2 shows the thermal response of the 93"^ floor WTC 1 Case A at nine different instants of time

ranging from 10 min (600 s) to 100 min (6000 s) at 10 min (600 s) intervals. The thennal state of the

iloors at 60 min (3600 s) after impact of the aircraft is not shown due to space limitation on one page. Each sub-figure is similar to the sample plot shown in Figure 8-1, and shows the truss assembly and core beams that support the 93"^ floor. The floor trusses and core beams are subjected to fires on the 92"'^ floor. Perimeter and core columns above the 93^^* floor concrete slab are subjected to fires on the 93"^ floor,

while portion of columns below the 93"^^ floor slab are subjected to fires on the 92"'' floor. The first subfigure shows the north face and the east face of the building for orientation.

Aircraft impact analysis results show no structural or fireproofing damage on the trusses or the core beams. Table 8-4 shows the fireproofing and structural damage predicted for perimeter columns on

floors 92-99 (WTC 1, Case A). Aircraft impact analysis resulted in no fireproofing or structural damage

for any perimeter column on floor 92 or floor 93. Table 8-5 shows fireproofing and structural damage

predicted for core columns on floors 92-99. Columns 504 and 605 are damaged structurally on floors 92

and 93, while column 705 is damaged on floor 93 only. There is no fireproofing damage predicted for the core columns on these floors.

Since there is no fireproofing damage, the trusses and columns on Floor 93 shown in Figure 8-2 stay

relatively cool. The floor trusses have a mean fireproofing thickness of 2.2 in. which can delay the heating

of the entire floor system. The core beams are covered with 0.5 in. fireproofing and as a result, the core beams heat up faster. This is illustrated by significant heating of the core beams at 1800 s after impact.

The heating of the core beams is consistent with fire activity in this region. As the fire spread towards the

south faces of WTC 1, the core beams gradually cooled down as seen by relatively lower temperatures at 6.000 s after impact.

NISTNCSTAR 1-5G, WTC Investigation 133
Chapter 8
Global Thermal Response of WTC 1: Case A

8.2.2 Floor 94, WTC 1 Case A

Figure 8-4 shows the thermal response of the 94* floor, WTC I Case A at nine different instants in time

ranging from 600 s to 6,000 s at 600 s intervals. As for the 93"* floor, the thermal state of the floors at 5.600 s has not been shown due to space limitation on one page. Each sub-figure shows the truss assembly and core beams that support the 94'*' floor. Perimeter and core columns above the 94"" floor concrete slab are subjected to fires on the 94''' floor while columns below the 94"" floor slab are subjected

to fires on the 93"* floor. The north and east faces of the floor have been marked on the first sub-figure for orientation.

The structural and fireproofing damage due to the aircraft impact is shown in Figure 8-3. The sub-figure on the left shows structural damage (contours in green) to floor 94, while the sub-figures on the right

shows fireproofing damage (contours in blue) to floor trusses and core beams that support floor 94. Structural damage was incorporated by removing the concrete slab, trusses or core beams in the area marked by the green rectangles in Figure 8-3.

Figure 8-3 shows no fireproofing damage and limited structural damage due to aircraft impact on

floor 94." Table 8-^ shows no fireproofing or structural damage for any perimeter column on floor 93. However there is extensive structural damage on the perimeter columns above floor 94 especially on the

north face (as evidenced in the photographic observations). Only one perimeter column panel on floor 94 was predicted to have structural damage on the south face (NIST NCSTAR 1-5 A). Table 8-5 shows

fireproofing and structural damage for core columns on floors 93 and 94. There is no fireproofing damage

predicted for core columns on floor 93. Columns 504, 604, and 706 were damaged structurally on

floors 93 and 94. There is also extensive fireproofing damage for coluiuns 503-508, 603-607, 703-707,

803-806, 904-906, and 1004-1006 on floor 94 (NIST NCSTAR 1-2).

Figure 8^ shows the thermal response of the 94'*' floor, WTC 1 Case A at nine different instants of time

ranging from 600 s to 6,000 s at 600 s intervals. The perimeter and core columns that have been damaged

by the impact of the aircraft have not been shown in the pictures. Since there is no fireproofing damage on

the trusses that support the slab on the 94'*' floor. Figure 8-4 shows that the trusses stay relatively cool. The trusses have a mean fireproofing thickness of 2.2 in., which results in only a very gradual heat up of

the floor system. Peak temperatures on the order 100 °C in the trusses are predicted, in the northeast comer at 2400 s after impact of the aircraft and peak temperatures of approximately 150°C during the course of the simulation. The core beams are covered with 0.5 in. fireproofing and as a resuh, the core beams heat up faster. This is illustrated by significant heating of the core beams at 1,800 s and 2,400 s after impact (northeast comer) followed by a cooling phase (3,000 s and 4,200 s). This is consistent with

fire activity in this region. As the fire moves around the core and spreads toward the south face of WTC 1

heating of the core beams on the west face is predicted, due to fire activity on the 93'^'' floor (Note that the core beams are subjected to fires on the 93'^'^ floor).

The thermal response of the perimeter and core columns is highly dependent on the state of the

fireproofing and to a lesser extent on fire growth and spread pattem. The size and shape of the column

(cross-sectional area and circumference) also determine the thermal response of the column. As discussed

^ The damage graphics used in the thermal analysis are discussed in detail in NTST NCSTAR 1-6 and have been presented here

for reference only.

NIST NCSTAR 1-5G. WTC Investigation 135
Chapter 8

in Chapter 4 and Chapter 7 the thermal state of perimeter columns on the 92" and the 95'^ floors also

have an effect on the themial response of the perimeter columns on the 93'^'^ and 94"^ floors. Since there is no fireproofmg damage for perimeter columns on the 93'^'' or the 94^'' floor, a moderate increase in perimeter column temperature is predicted. Maximum perimeter column temperature at 6,000 s after impact is approximately 200 °C to 300 °C depending on its location.

The core colunms above floor 94 exhibit a vast variation in steel temperature. Columns 501, 601, 701,

801, 901, and 1,001 stay relatively cool throughout the duration of the event despite the fire activity in

this region. These columns have no damage to their fireproofing. Column 607 shows a very high

temperature at 600 s after impact. At 1,200 s after impact, column 607 has become cooler and continues

to cool but shows an increase in column temperature at 3,000 s. Since column 607 has fireproofing and

has a relatively lighter cross-section, the theraial response of this column is closely coupled and consistent

with the fire activity in the vicinity of this column (NIST NCSTAR 1-5F). Fire simulations indicate upper

layer temperatures on the order of 1 ,000 °C in the northeast comer at 1 5 minutes into the simulation,

followed by a cool down phase.

All core columns that have lost their fireproofing do not heat up at the same rate. Columns 1004, 1005,

and 1,006 (no fireproofing on any of the columns) heat up at different rates. This is due to differences in the size of the columns and differences in fire exposure. (Note that because of the presence of partitions,

the fire exposure on four sides of a core column could be different.) These columns exhibit significantly

high temperatures in the 500 °C to 600 °C range from 1,800 s until the end of the simulation. This is due

to constant fire activity in the area surrounding these columns on floor 94.

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Fireproofing damage on floor trusses and core beams tliat support floor 94

Structural Damage Fireproofing Damage

Figure 8-3. Structural and fireproofing damage on floor 94, WTC 1 Case Aj,

136 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

NISTNCSTAR 1-5G, WTC Investigation 137
Chapter 8

8.2.3 Floor 95, WTC 1 Case A

Figure 8-7 and Figure 8-8 shows the thennal response of the 95"^ floor, WTC 1 Case A at nine different

instants in time ranging from 600 s to 6,000 s at 600 s intervals. The figure shows the truss assembly and

core beams that support the 95* floor. Perimeter and core columns above the 95* floor concrete slab are subjected to fires on the 95* floor, while columns below the 95* floor are subjected to fires on the 94*

floor. The north and east faces have been marked for orientation.

The structural and fireproofing damage caused by the aircraft impact is shown in Figure 8-5. This figure

shows structural damage to floor 95 and fireproofing damage to the trusses and core beams that support

floor 95. Figure indicates extensive fireproofing and structural damage on the north face for floor trusses, core beams and concrete slabs on floor 95. Table 8^ shows no fireproofing damage for any of the

perimeter columns on floor 94 or floor 95. However, there is extensive structural damage on the perimeter

columns on these floors, especially on the north face (as evidenced in the photographic observations NIST

NCSTAR 1-5 A). Only one perimeter column panel on floor 94 and 95 had structural damage on the south

face (again observed in photographic evidence). Table 8-5 shows fireproofing and structural damage for core columns. Columns 504, 604, and 706 are damaged structurally on floors 94 and 95. There is also

extensive fireproofing damage for columns 503-508, 603-607, 703-707, 803-806, 904-906, and 1004-

1006 on floor 94 (below the 95* floor concrete slab). On floor 95 fireproofing damage is indicated for

columns 503-506, 603-606, 703-708, 803-806, 904-906, and 1004-1006.

Figure 8-6 shows upper layer temperatures for floor 95, WTC 1 Case A predicted from the fire dynamics

simulations at six different instants in time (NIST NCSTAR 1-5F). These upper layer temperatures are

directly responsible for heating the perimeter and core columns above floor 95 and the trusses that support

floor 96. The bottom face of the concrete slab on floor 96, and the top face of the concrete slab on floor

95 was subjected to radiative fluxes from fire growth on floor 95.

Figure 8-7 and Figure 8—8 show the thennal response of the 95* floor WTC 1 Case A at nine different

instants of time ranging from 600 s to 6,000 s at 600 s intervals. The perimeter and core columns, core beams and trusses severed by the impact of the aircraft have not been included. Since there is extensive

fireproofing damage on the trusses that support the slab on the 95* floor. Figure 8-7 shows that the

trusses in the north face heat up rapidly. There is significant fire activity in the northeast comer on floor

94 and this fact coupled with lack of fireproofing thickness results in very high temperatures in the trusses (sub-figures for the 600 s -1 800 s duration). As the fires spread towards the south face of the building, the

steel trusses in the northeast comer loose heat convectively and radiatively and this results in gradual

cooling of the trass elements (4200 s). The rest of the trass (other than the one with no fireproofing) has a mean fireproofing thickness of 2.2 in., which can result in only a very gradual heat up of the floor system.

Peak temperatures of the order 100 °C are predicted in the trasses on the west face at 2,400 s after impact

of the aircraft and peak temperatures of approximately 1 50 °C during the course of the simulation.

The thermal response of the perimeter and core columns is highly dependent on the state of the

fireproofing and to a lesser extent on fire growth and spread pattern. The size and shape of the column

(cross-sectional area and circumference) also determine the thermal response of the column. As discussed

in Chapter 4 and Chapter 7 the thermal state of perimeter columns on the 93'^'' floor and the 96* floor also

have an effect on the thermal response of the perimeter columns on 94* and 95* floor. Lack of

fireproofing damage for perimeter columns on the 94* floor or the 95* floor, can result in moderate

138 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

increase in perimeter column temperature. The core columns exhibit a \ ast \ ariation m steel temperature

as opposed to the perimeter columns. The west side of the core continues to stay cool, because of lack of

fireproofing damage on those columns. Since the fireprooflng damage is in the center of the core and

towards the east side, the columns in this area show rapid changes in temperature depending on fire intensit\- and column size. Column 508 at 3000 s shows high temperature below the slab and low

temperatures above the concrete slab. This is due to change in fireproofmg status of this column across the concrete slab. Toward the end of the simulation, heating of the core columns on the south side is predicted, as there is significant fire activit}- on the south face on the 94* and 95* floor. At 6.000 s after impact, the core columns on north side of the core are relative cooler as compared w hh core columns in the south side of the core.

Structural damage for floor 95

Structural Damage | |

Fireproofmg damage on floor trusses and core beams that support floor 95

Fireproofmg Damage

Figure 8-5. Structural and fireproofing damage on floor 95, WTC 1 Case Ai.

NISTNCSTAR 1-5G. WTC Investigation 139
Chapter 8

1:15:00.0 1;42;00.0

Figure 8-6. Upper layer temperature predicted by fire dynamics simulation for floor 95,

WTC 1 Case A.

140 NISTNCSTAR 1-5G, WTC Investigation
NISTNCSTAR 1-50. WTC Investigation 141
Chapter 8
Global Thermal Response of WTC 1: Case A

8.2.4 Floor 96, WTC 1 Case A

The thermal state of the 96* floor WTC 1 Case A is shown in Figure 8-10 through Figure 8-29 at nine

different instants in time ranging from 600 s to 6,000 s at 600 s intervals. Each figure shows the truss assembly and core beams that support the 96''' floor. Perimeter and core columns above the 96"" floor concrete slab are subjected to fires on the 96"^ floor while columns below the 96'*' floor slab are subjected

to fires on the 95* floor. The north and east faces have been marked in Figure 8-10 for orientation

purposes.

The aircraft impact analysis and fireproofing damage results have been shown in Figure 8-9. The figure

shows extensive fireproofing and structural damage on the north face. Table 8-4 shows the fireproofing

and structural damage status for perimeter columns on floors 95 and 96. (WTC 1 Case A). There is no

fireproofing damage for perimeter columns on floor 95, and only a single panel on the south face on floor

96 is assumed to have fireproofing damage. This panel is directly above the panel that was damaged

(structurally) by the impact of the aircraft on the south face of WTC 1 . There is also extensive structural damage on the perimeter columns on floor 95 and 96, especially on the north face (as evidenced in the

photographic observations NIST NCSTAR 1-5 A). Only one perimeter column panel on floor 95 was

assumed to have structural damage on the south face (again observed in photographic evidence).

Table 8-5 shows fireproofing and structural damage for core columns on floors 95 and floor 96.

Columns 504. 604 and 706 on floor 95, and column 504 and 604 on floor 96 were damaged structurally.

Fireproofing damage is predicted for column 503-506, 603-606, 703-708, 803-806, 904-906 and 1004-

1006 below the concrete slab on 96'*' floor. Above the slab, fireproofing damage on core columns is restricted to 502-506, 602-606, 702-705, 803-805, 902-905 and 1002-1005. Floor 96 has the maximum

number of core columns that have their fireproofing damaged due to the impact of the aircraft.

Figure 8-10 through Figure 8-29 show the thermal response of the 96* floor, WTC 1 Case A at nine

different instants in time ranging from 600s to 6000s at 600 s intervals. The perimeter and core columns

damaged by the impact of the aircraft are not included in the pictures. Also the core beams and trusses severed by the impact have not been included (Figure 8-10 through Figure 8-29). Since there is extensive

fireproofing damage on the trusses that support the slab on the 96'*' floor. Figure 8-10 shows that the truss elements in the north face (fireproofing damaged) heat up rapidly. There is significant fire activity on the

north face on floor 95 and this fact coupled with lack of fireproofing thickness results in very high

temperatures in the trusses (600 s-1,800 s). As the fires spread towards the south face of the building, the

steel trusses in this region looses heat convectively and radiatively, and this results in gradual cooling of

the truss elements (4,200 s). The rest of the truss (other than the one with no fireproofing) has a mean

fireproofing thickness of 2.2 in. which results in only a very gradual heat up of the floor system. Peak

temperamres are of the order of 100 °C in the trusses on the west face at 2,400 s after impact of the

aircraft, and peak temperatures of approximately 150°C are predicted during the course of the simulation.

In the area where fireproofing is intact, the fire activity has very little impact on the thermal response of

the trusses.

NIST NCSTAR 1-5G, WTC Investigation 143
Chapter 8

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Figure 8-9. Structural and fireproofing damage on floor 96, WTC 1 Case Aj.

Since there is no fireproofing damage for perimeter columns on floor 95, moderate increase in perimeter

column temperature is predicted. The core columns exhibited a vast variation in steel temperature as opposed to the perimeter columns. The west side of the core (as discussed in previous sections) continues

to stay cool, because of lack of fireproofing damage on those columns. The fireproofing damage is in the center of the core but is shaded towards the west side. The columns in this area show rapid changes in temperature depending on fire intensity and column size. Columns 502, 602, 702, 802, 902, and 1002

show significant heating on the 96"^ floor (because of lack of fireproofing) and are relatively cool on the

95**^ floor (columns on this floor are fireproofed).

144 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

NISTNCSTAR 1-5G, WTC Investigation 145
Chapter 8
Global Thermal Response of WTC 1: Case A

NISTNCSTAR 1-5G, WTC Investigation 147
Chapter 8
Global Thermal Response of WTC 1: Case A

NISTNCSTAR 1-5G, WTC Investigation 149
Chapter 8
Global Thermal Response of WTC 1: Case A

NISTNCSTAR 1-5G, WTC Investigation 151
Chapter 8
Global Thermal Response of WTC 1: Case A

NISTNCSTAR 1-5G. WTC Investigation 153
Chapter 8
Global Thermal Response of WTC 1: Case A

NISTNCSTAR 1-5G, WTC Investigation 155
Chapter 8

156 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

NISTNCSTAR 1-5G, WTC Investigation 157
Chapter 8
Global Thermal Response of WTC 1: Case A
Chapter 8

160 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A
Chapter 8
Global Thermal Response of WTC 1: Case A
Chapter 8
Global Thermal Response of WTC 1: Case A

8.2.5 Floor 97, WTC 1 Case A

Figure 8-3 1 and Figure 8-32 shows the thermal response of the 97^^ floor WTC 1 Case A at nine different

instants of time ranging from 600s to 6000s at 600 s intervals. The thermal state of the floors at 3600 s has not been shown due to space limitation. The figure shows the truss assembly and core beams that support the 97'*' floor. Perimeter and core columns above the 97* floor concrete slab are subjected to fires on the 97'^ floor while columns below the 97'*' floor slab are subjected to fires on the 96'*' floor.

The structural and fireproofing damage due to aircraft impact is shown in Figure 8-30. This figure shows

structural damage to floor 97 and fireproofing damage to floor trusses and core beams supporting the

floor. The figure shows extensive fireproofing damage on the north and south faces and limited structural damage. Table 8^ shows no fireproofing damage to perimeter columns on floor 97. A single panel on the

south face on floor 96 is assumed to have fireproofing damage. This panel is directly above the panel that was damage (structurally) by the impact of the aircraft on the south face of WTC 1. There is also

extensive structural damage on the perimeter columns on floor 96, especially on the north face (as

evidenced in the photographic observations) and damage to a lesser extent on floor 97. Table 8-5 shows

fireproofing and structural damage assumed for core columns on floors 96 and 97. All the core columns

on floor 97 are assumed to be intact, while columns 504 and 604 on floor 96 are structurally damaged.

Fireproofing damage is assumed for columns 501-504, 601-604, 702-705, 802-803, 903-904, and 1003

above the concrete slab on 97"" floor. Below the slab, fireproofing damage on core columns is restricted to 502-506, 602-606, 702-705, 803-805, 902-905, and 1002-1005.

Figure 8-31 and Figure 8-32 shows the thermal response of the 97* floor WTC 1 Case A at nine different

instants of time ranging from 600s to 6000s at 600 s intervals. The damaged structural elements are not included in the pictures. The west side of the core on floor 97 has suffered extensive fireproofing damage

and this results in heating up of the core columns. Core columns on the east side of the core have their

fireproofing intact and as a result stay relatively cool.

NISTNCSTAR 1-5G, WTC Investigation 165
Chapter 8

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Structural damage for floor 97 Fireproofing damage on floor trusses and core beams that support floor 97

Structural Damage Fireproofing Damage

Figure 8-30. Structural and fireproofing damage on floor 97 WTC 1 Case Aj.

166 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

NISTNCSTAR 1-5G, WTC Investigation

167
Chapter 8

168 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

8.2.6 Floor 98, WTC 1 Case A

The thermal state of floor 98, WTC 1 Case A is shown in Figure 8-34. The portion below the concrete

slab is subjected to fires on the 97* floor, while structural elements above floor 98 are subjected to fires on the 98'*' floor. Fireproofing damage and structural damage results for this floor are shown in Figure 8-33. Fireproofing and structural damage for perimeter and core columns have been provided in tabular format. The thermal response of floor 98 is consistent with the fire activity on this floor and with

the predicted fireproofing and structural damage. For the core columns, numerical simulations predict

higher temperatures on the west side, because of fireproofing damage and fire activity in this region.

Some core columns indicate very high temperatures below the concrete slab and relatively low

temperatures above the slab. This is due to differences in fireproofing status and fire activity in the

immediate vicinity of these columns on floors 97 and 98.

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Structural damaoe for floor 98 Fireproofing damage on floor trusses and core beams that support floor 98

Structuml damugt Flrepraofing Damage

Figure 8-33. Structural and fireproofing damage on floor 98 WTC 1 Case Aj.

NISTNCSTAR 1-5G, WTC Investigation 169
Chapter 8

170 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

8.2.7 Floor 99, WTC 1 Case A

Figure 8-35 shows the thermal response of the 99th floor WTC 1 Case A at nine different instants of time

ranging from 600 s to 6,000 s at 600 s intervals. The thermal state of the floors at 3600 s has not been

shown due to space limitation. Aircraft impact analysis results show no structural or fireproofmg damage

to the trusses or the core beams. There is no fireproofing or structural damage for any perimeter column

on floor 98 or floor 99. There is no fireproofing or structural damage assumed on floor 98 and 99. Since

there is no fireproofing damage, the trusses and columns in Figure 8-35 stay relatively cool. The core columns have their fireproofing intact on these floors and do not reach any significantly high temperature.

8.3 DATA TRANSFER FOR STRUCTURAL ANALYSIS

The thermal results presented in this chapter were passed on to the structural analysis group (NIST

NCSTAR 1-6D) for stress analysis in the load bearing elements. The data was provided on a floor by

floor basis ranging from floor 93 through 99. For each floor, the thermal results were provided at ten

instants in time, spaced at 10 min interval. For each instant in time a thermal loading data file was

generated to completely specify the thermal state of the floor. The thermal loading data files were

generated at 10 min. 20 min. 30 min, 40 min. 50 min, 60 min, 70 min, 80 min, 90 min, and 100 min after

aircraft impact. Each file was generated in a format that is consistent with the structural models and could

be readily read in with the ANSYS software to specify body loads (temperature or temperature gradient) on the structure. The files were transferred using a file transfer protocol (ftp) utility.

The results shown in Figure 8-2 through Figure 8-35 are visual renderings of the information contained

in the thermal loading data files.

NIST NCSTAR 1-5G, WTC Investigation 111
Chapter 8
Global Thermal Response of WTC 1: Case A

Table 8-4. Perimeter column damage status for WTC 1, Case A.

Column Floor 92 Floor 93 Floor 94 Floor 95 Floor 96 Floor 97 Floor 98 Floor 99

101 0 0 0 0 0 0 0 0

103 0 0 0 0 0 0 0 0

106 0 0 0 0 0 0 0 0

109 0 0 0 0 0 0 0 0

1 12 0 0 0 0 0 0 0 0

115 0 0 0 0 0 0 0 0

118 0 0 0 0 0 2 0 0

121 0 0 0 0 2 0 0 0

124 0 0 0 2

1 0 0 0

127 0 0 2 2 2 2 0 0

130 0 0 0 2 2 0 0 0

1 0 0 2 2 2 2 0 0

136 0 0 2 2 0 0 0 0

139 0 0 2 0 0 0 0 0

142 0 0 2 0 0 0 0 0

145 0 0 0 2 0 0 0 0

148 0 0 0 0 0 0 0 0

151 0 0 0 0 0 0 0 0

154 0 0 0 0 0 0 0 0

157 0 0 0 0 0 0 0 0

201 0 0 0 0 0 0 0 0

203 0 0 0 0 0 0 0 0

206 0 0 0 0 0 0 0 0

209 0 0 0 0 0 0 0 0

212 0 0 0 0 0 0 0 0

215 0 0 0 0 0 0 0 0

218 0 0 0 0 0 0 0 0

221 0 0 0 0 0 0 0 0

224 0 0 0 0 0 0 0 0

227 0 0 0 0 0 0 0 0

230 0 0 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 173
Chapter 8

233 0 0 0 0 0 0 0 0

236 0 0 0 0 0 0 0 0

239 0 0 0 0 0 0 0 0

242 0 0 0 0 0 0 0 0

245 0 0 0 0 0 0 0 0

248 0 0 0 0 0 0 0 0

251 0 0 0 0 0 0 0 0

254 0 0 0 0 0 0 0 0

257 0 0 0 0 0 0 0 0

301 0 0 0 0 0 0 0 0

303 0 0 0 0 0 0 0 0

306 0 0 0 0 0 0 0 0

309 0 0 0 0 0 0 0 0

312 0 0 0 0 0 0 0 0

315 0 0 0 0 0 0 0 0

318 0 0 0 0 0 0 0 0

321 0 0 0 0 0 0 0 0

324 0 0 0 0 0 0 0 0

327 0 0 0 0 0 0 0 0

330 0 0 2 2 1 0 0 0

333 0 0 0 0 0 0 0 0

336 0 0 0 0 0 0 0 0

339 0 0 0 0 0 0 0 0

342 0 0 0 0 0 0 0 0

345 0 0 0 0 0 0 0 0

348 0 0 0 0 0 0 0 0

351 0 0 0 0 0 0 0 0

354 0 0 0 0 0 0 0 0

357 0 0 0 0 0 0 0 0

401 0 0 0 0 0 0 0 0

403 0 0 0 0 0 0 0 0

174 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

406 0 0 0 0 0 0 0 0

409 0 0 0 0 0 0 0 0

412 0 0 0 0 0 0 0 0

415 0 0 0 0 0 0 0 0

418 0 0 0 0 0 0 0 0

421 0 0 0 0 0 0 0 0

424 0 0 0 0 0 0 0 0 42" 0 0 0 0 0 0 0 0

430 0 0 0 0 0 0 0 0

433 0 0 0 0 0 0 0 0

436 0 0 0 0 0 0 0 0

439 0 0 0 0 0 0 0 0

442 0 0 0 0 0 0 0 0

445 0 0 0 0 0 0 0 0

448 0 0 0 0 0 0 0 0

451 0 0 0 0 0 0 0 0

454 0 0 0 0 0 0 0 0

457 0 0 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 175
Chapter 8

Table 8-5. Core column damage status for WTC 1 , Case A

Column Floor 92 Floor 93 Floor 94 Floor 95 Floor 96 Floor 97 Floor 98 Floor 99

501 0 0 0 0 0 3 0 0

502 0 0 0 0 3 3 0 0

503 0 0 3 3 3 3 0 0

504 2 2 2 2 2 3 0 0

505 0 0 3 3 3 0 0 0

506 0 0 3 3 3 0 0 0

507 0 0 3 0 0 0 0 0

508 0 0 3 0 0 0 0 0

601 0 0 0 0 0 3 0 0

602 0 0 0 0 3 3 0 0

603 0 0 3 3 3 3 0 0

604 2 2 2 2 2 3 0 0

605 0 0 3 3 3 0 0 0

606 0 0 3 3 3 0 0 0

607 0 0 3 0 0 0 0 0

608 0 0 0 0 0 0 0 0

701 0 0 0 0 0 0 0 0

702 0 0 0 0 3 3 0 0

703 0 0 3 3 3

-> 0 0

704 0 0 3 3 3 3 0 0

705 0 0 3 3

-* 3 0 0

706 0 2 2 2 0 0 0 0

707 0 0 3 3 0 0 0 0

708 0 0 0 0 0 0 0 0

801 0 0 0 0 0 0 0 0

802 0 0 0 0 3 3 0 0

803 0 0 3 3 3 3 0 0

804 0 0 3 3 3 0 0 0

176 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case A

805 0 0 3 3 0 0 0 0

806 0 0 3 0 0 0 0 0

807 0 0 0 0 0 0 0 0

901 0 0 0 0 0 0 0 0

902 0 0 0 0 3 0 0 0

903 0 0 0 0 3 3 0 0

904 0 0 3 3 3 3 0 0

905 0 0 3 3 3 0 0 0

906 0 0 3 3 0 0 0 0

907 0 0 0 0 0 0 0 0

908 0 0 0 0 0 0 0 0

1001 0 0 0 0 0 0 0 0

1002 0 0 0 0 3 0 0 0

1003 0 0 0 0 3 3 0 0

1004 0 0 3 3 3 0 0 0

1005 0 0 3 3 3 0 0 0

1006 0 0 3 3 0 0 0 0

1007 0 0 0 0 0 0 0 0

1008 0 0 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 111
Chapter 8

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178 NISTNCSTAR 1-5G, WTC Investigation
Chapter 9

Global Thermal Response of WTC 1 : Case B

9.1 OVERVIEW

Four major simulations are presented in this report to predict the global themially induced structural response of World Trade Center (WTC) 1 and WTC 2. This chapter is the second of four chapters that describe the results of the four cases. The focus of this chapter is on a set of simulations termed "WTC 1 Case B."

Following the completion of the Case A simulation of WTC 1 (described in Chapter 8), the calculation was rerun with changes made to several important input parameters. The methodology for performing the

simulations is identical to that for Case A. In this chapter we focus on the model inputs and simulation

results for WTC 1 Case B. The differences in model inputs and simulations resuUs between WTC 1 Case

A and WTC 1 Case B are summarized in Chapter 12. By analyzing the thermal and structural results for Case A and Case B, sensitivity of our results to changes in model input can be assessed.

9.1.1 Aircraft Impact Analysis

As in Case A. structural and fireproofing damage to the columns, trusses, and floor slabs were obtained

from an aircraft impact analysis. The impact analysis was performed with more severe settings

summarized in Table 9-1. The severe settings resulted in more structural and fireproofing damage to the

various structural elements (NIST NCSTAR 1-2). The predicted damage for Case B is more than that for Case A. but the damage for Case B does not necessarily include the damage for Case A. In other words,

the damage for Case A was not a subset of the damage for Case B.

Table 9-4 (presented later) shows the fireproofing and structural damage predicted for perimeter columns

on floors 92-99 (Case B). In this table a "0" indicates that the column and its fireproofing are intact, a 'T'

indicates fireproofing damage to the interior faces and a "2" indicates a column that has been severed by

the aircraft impact. Table 9-5 (presented later) shows fireproofing and structural damage predicted for core columns on floor 92-99 (NIST NCSTAR 1-5 A). In this table a "0" indicates that the column and its fireproofing is intact, a "1" indicates fireproofing damage on one face, a "2" indicates a column that has

been severed by the aircraft impact while a "3" indicates fireproofing damage on all the faces. For WTC 1 Case B, core columns had their fireproofing intact or the fireproofing was damaged on all the faces.

Aircraft impact damage (NIST NCSTAR 1-2) also results in structural and fireproofing damage on the

floor systems. For each floor, a map was prepared that describes the contours of the region where

fireproofing and structural damage was predicted. This damage for floors 95 through 98 is provided as contour maps in Figure 9-3 through Figure 9-8 (later in this chapter).

NIST NCSTAR 1-5G, WTC Investigation 179
Chapter 9

Table 9-1. Input parameters for WTC 1 global impact

analyses (More Severe).

Analysis Parameters More Severe

Flight

Parameters

Impact speed 47') innVi

Trajectory - pitch 7 f,°

Trajectory - yaw

Orientation - pitch 5.6°

Orientation - yaw 0.0°

Aircraft

Parameters

Weight 105 percent

Failure Strain 125 percent

Tower

Parameters

Failure Strain 80 percent

Live Load Weight" 20 percent

a. Live load weight expressed as a percentage of the design live load.

9.1.2 Fire Dynamics Simulations

Fire simulations were perforaied as in Case A but with different input parameters to predict contours of

upper layer temperature, absorption coefficient, layer depth, and ambient temperatures as a fiinction of

space and time on each floor of the WTC 1 . For Case B the area of the core identified in the impact study

to have sustained major damage had only partially open walls. By opening the walls partially, oxygen

could still reach the fire, but the hot gases were trapped resulting in higher temperatures. The input

parameters for fire simulations are listed in Table 9-2. The fire simulation results for Case B were not

qualitatively very different from Case A (NIST NCSTAR 1-5F).

Table 9-2. Values of WTC fire

simulation variables.

Variable WTC 1 Case B

Fuel load 25 kg/m- (5 lb/ft-)

Distribution of

disturbed

combustibles

Weighted toward

the core

Condition of

combustibles

Displaced furniture rubblized

Representation of

impacted core walls

Soffit remained

9.2 FIRE STRUCTURE INTERFACE

The Fire Structure Interface couples the thermal response to the fire simulations for a specific set of

structural and fireproofing damage scenario. Changes in fire dynamics between Case A and Case B result

in a new set of thermal results. Fireproofing damage and structural damage caused by the aircraft impact

analysis also has a large impact on the thermal results. Table 9-3 summarizes the input for global thermal

response of WTC 1 Case B. In the next section, thermal results for WTC 1 Case B are presented, on a

floor by floor basis. As in Chapter 8, the goal in this chapter is to understand the nature of the thermal

180 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case B

insult on the structure for each floor of the WTC 1 . The differences in model inputs and simulations

results between Case A and Case B are summarized in Chapter 12. The thermal results were provided for

analysis in a manner that is suitable for the structural analysis package that was employed for collapse

analysis.

Figure 9-1 through Figure 9-1 1 shows the thermal state of floors 93 through 99 in WTC 1, respectively,

at nine different instants in time ranging from 10 min to 100 min at 10 min interval. The thermal state at 60 min after impact has not been shown due to space limitation on each page. Each sub-figure shows the

truss assembly, the core beams, and perimeter columns and core coluinns above and below the concrete

slab. Color contours are superimposed on the structural elements ranging from 0 to 675 °C.

Table 9-3. Input for global thermal response

of WTC 1, Case B.

Input WTC 1 Case B

Structural damage,

NISTNCSTAR 1-2 NIST NCSTAR 1-6

Case B

(More severe)

Fireproofing damage,

NISTNCSTAR 1-6 Case B

Fire Simulations,

NISTNCSTAR 1-5F

Case B

9.2.1 Floor 93, WTC 1 Case B

The truss system that supports floor 93 shows slightly higher temperatures for "Case A" as compared to "Case B". Although "Case B" has more severe fires, the intensity of the fires varied from one floor to another. Core beams in the northwest comer show higher temperature for "Case B" at 1 00 min after impact. The temperature of the core beams for "Case A" cooled down by the end of the simulation.

Neither case shows fireproofing or structural damage for floor systems due to aircraft impact.

9.2.2 Floor 94, WTC 1 Case B

Core columns (1005-1007) in "Case B" show higher temperatures compared to Case A at 20-30 min after impact. Core columns on the east side of the core indicate higher temperature for Case B. The thermal

state of the perimeter columns and floor trusses is not significantly different. The core beams exhibit

slightly higher temperature in Case B as compared to Case A at 1 00 min after impact.

NISTNCSTAR 1-5G, WTC Investigation 181
Chapter 9

182 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case B
Chapter 9

9.2.3 Floor 95, WTC 1 Case B

The structural damage on truss elements that support floor 95 is shifted toward the east face for Case B.'' Fireproofing damage on the trusses extends south of the core, which results in higher thermal loading on

floor trusses throughout the simulation. Perimeter columns in the south face indicate higher temperature

for Case B due to more extensive fireproofing damage. A larger fraction of core columns in Case B

exliibits higher temperature as compared to Case A. The peak temperatures in the two cases were not very

different, but in Case B, a larger fraction of the structural elements shows thennal heating.

Stmctural damage for floor 95 Fireproofing damage on floor trusses and core beams that support floor 95

Sti-uctural Damage I I Fireproofing Damage EZ]

Figure 9-3. Structural and fireproofing damage on floor 95, WTC 1, Case Bj.

The damage graphics used in the thermal analysis are discussed in detail in NIST NCSTAR 1-6 and have been presented here

for reference only.

184 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case B

NISTNCSTAR 1-5G, WTC Investigation 185
Chapter 9

9.2.4 Floor 96, WTC 1 Case B

Case B shows higher temperature on trusses in the region south of the core. Fireproofing damage on the

truss elements is more extensive for Case B. Truss elements south of the core show a temperature of

600 °C at 20 min after impact. Perimeter columns on the south face also exhibit higher temperature due to more extensive fireproofing damage. There is more structural damage to core columns and a larger

fraction of the core columns exliibit fire induced damage.

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Fireproofing damage on floor trasses and core beams that support floor 96

Structurai Damage Fireproofing Damage

Figure 9-5. Structural and fireproofing damage on floor 96, WTC 1, Case Bj.

186 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case B
Chapter 9

9.2.5 Floors 97, 98, and 99, WTC 1 Case B

Floor 97 shows similar differences in thermal response as observed on floor 96. The damage patterns are skewed towards the west face on floor 97. Very extensive heating of the truss elements, south of the core

is observed for Case B. Core columns on the west face show more fire induced heating in Case B as compared to Case A. The differences between Case A and Case B for floor 98 are similar to those

described for floor 96 and 97. Floor 99 in both cases stays relatively cool during the entire simulation.

The truss elements do not heat up significantly, despite fire activity in this region, because of the larger

2.2 in. equivalent fireproofing thickness on the trusses.

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Structural damage Fireproofing Damage

Figure 9-7. Structural and fireproofing damage on floor 97, WTC 1, Case Bj.

188 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case B

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Figure 9-8. Structural and fireproofing damage on floor 98, WTC 1, Case Bj.

9.3 DATA TRANSFER FOR STRUCTURAL ANALYSIS

The data transfer is performed in a manner that is identical to that described in the previous chapter for WTC 1 Case A (see Section 8.3).

NISTNCSTAR 1-5G, WTC Investigation 189
Chapter 9
Global Thermal Response of WTC 1: Case B

NISTNCSTAR 1-5G, WTC Investigation 191
Chapter 9
Global Thermal Response of WTC 1: Case B

Table 9-4. Perimeter column damage status for WTC 1, Case B.

Column Floor 92 Floor 93 Floor 94 Floor 95 Floor 96 Floor 97 Floor 98 Floor 99

101 0 0 0 0 0 0 0 0

103 0 0 0 0 0 0 0 0

106 0 0 0 0 0 0 0 0

109 0 0 0 0 0 0 0 0

112 0 0 0 0 0 0 0 0

115 0 0 0 0 0 0 0 0

118 0 0 0 0 0 2 0 0

121 0 0 0 0 2 0 0 0

124 0 0 0 2 2 0 0 0

127 0 0 2 2 2 2 0 0

130 0 0 0 2 2 0 0 0

133 0 0 2 2 2 2 0 0

136 0 0 2 2 0 0 0 0

139 0 0 2 0 0 0 0 0

142 0 0 2 0 0 0 0 0

145 0 0 0 2 0 0 0 0

148 0 0 0 0 0 0 0 0

151 0 0 0 0 0 0 0 0

154 0 0 0 0 0 0 0 0

157 0 0 0 0 0 0 0 0

201 0 0 0 0 0 0 0 0

203 0 0 0 0 0 0 0 0

206 0 0 0 0 0 0 0 0

209 0 0 0 0 0 0 0 0

212 0 0 0 0 0 0 0 0

215 0 0 0 0 0 0 0 0

218 0 0 0 0 0 0 0 0

221 0 0 0 0 0 0 0 0

224 0 0 0 0 0 0 0 0

227 0 0 0 0 0 0 0 0

230 0 0 0 0 0 0 0 0

233 0 0 0 0 0 0 0 0

236 0 0 0 0 0 0 0 0

239 0 0 0 0 0 0 0 0

242 0 0 0 0 0 0 0 0

245 0 0 0 0 0 0 0 0

248 0 0 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 193
Chapter 9

Column Floor 92 Floor 93 Floor 94 Floor 95 Floor 96 Floor 97 Floor 98 Floor 99

251 0 0 0 0 0 0 0 0

254 0 0 0 0 0 0 0 0

257 0 0 0 0 0 0 0 0

301 0 0 0 0 0 0 0 0

303 0 0 0 0 0 0 0 0

306 0 0 0 0 0 0 0 0

309 0 0 0 0 0 0 0 0

312 0 0 0 0 0 0 0 0

315 0 0 0 0 0 0 0 0

318 0 0 0 0 0 0 0 0

321 0 0 0 0 0 0 0 0

324 0 0 0 1 0 0 0 0

327 0 0 0 1 "I 0 0 0

330 0 0 2 2 0 0 0

333 0 0 0 1 0 0 0

336 0 0 0 1 0 0 0

339 0 0 0 0 0 0 0

342 0 0 0 0 0 0 0

345 0 0 0 0 0 0 0

348 0 0 0 0 0 0 0 0

351 0 0 0 0 0 0 0 0

354 0 0 0 0 0 0 0 0

357 0 0 0 0 0 0 0 0

401 0 0 0 0 0 0 0 0

403 0 0 0 0 0 0 0 0

406 0 0 0 0 0 0 0 0

409 0 0 0 0 0 0 0 0

412 0 0 0 0 0 0 0 0

415 0 0 0 0 0 0 0 0

418 0 0 0 0 0 0 0 0

421 0 0 0 0 0 0 0 0

424 0 0 0 0 0 0 0 0

427 0 0 0 0 0 0 0 0

430 0 0 0 0 0 0 0 0

433 0 0 0 0 0 0 0 0

436 0 0 0 0 0 0 0 0

439 0 0 0 0 0 0 0 0

194 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case B

Column Floor 92 Floor 93 Floor 94 Floor 95 Floor 96 Floor 97 Floor 98 Floor 99

442 0 0 0 0 0 0 0 0

445 0 0 0 0 0 0 0 0

448 0 0 0 0 0 0 0 0

451 0 0 0 0 0 0 0 0

454 0 0 0 0 0 0 0 0

457 0 0 0 0 0 0 0 0

459 0 0 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 195
Chapter 9

Table 9-5. Core dolumn d amage status for W1rc 1, Case B.

Column Floor 92 Floor 93 Floor 94 Floor 95 Floor 96 Floor 97 Floor 98 Floor 99

501 0 0 0 0 0 0 0 0

502 0 0 0 3 3 3 0 0

503 0 0 3 2 2 3 0 0

504 2 2 2 2 2 3 0 0

505 0 0 3 2 3 0 0 0

506 0 0 3 3 0 0 0 0

507 0 0 3 0 0 0 0 0

508 0 0 3 0 0 0 0 0

601 0 0 0 0 0 0 0 0

602 0 0 0 3 3 3 0 0

603 0 0 3 3 3 3 0 0

604 2 2 2 2 2 3 0 0

605 0 0 3 3 3 0 0 0

606 0 0 3 3 0 0 0 0

607 0 0 3 0 0 0 0 0

608 0 0 3 0 0 0 0 0

701 0 0 0 0 0 0 0 0

702 0 0 0 0 3 3 0 0

703 0 0 3 3 3 3 0 0

704 2 2 2 2 2 3 0 0

705 0 0 3 3 3 0 0 0

706 0 2 2 2 3 0 0 0

707 0 0 3 0 0 0 0 0

708 0 0 3 0 0 0 0 0

801 0 0 0 0 0 0 0 0

802 0 0 0 0 3 3 0 0

803 0 0 3 3 3 3 0 0

804 0 0 3 3 3 0 0 0

805 0 0 3 3 3 0 0 0

806 0 0 3 0 0 0 0 0

807 0 0 0 0 0 0 0 0

901 0 0 0 0 0 0 0 0

902 0 0 0 0 3 3 0 0

903 0 0 0 3 3 3 0 0

904 0 0 3 3 3 3 0 0

905 0 0 3 3 3 0 0 0

906 0 0 3 3 0 0 0 0

196 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case B

Column Floor 92 Floor 93 Floor 94 Floor 95 Floor 96 Floor 97 Floor 98 Floor 99

907 0 0 3 0 0 0 0 0

908 0 0 0 0 0 0 0 0

1001 0 0 0 0 0 0 0 0

1002 0 0 0 0 3 3 0 0

1003 0 0 0 0 3 3 0 0

1004 0 0 3 3 3 3 0 0

1005 0 0 3 3 3 0 0 0

1006 0 0 3 3 0 0 0 0

1007 0 0 3 0 0 0 0 0

1008 0 0 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 197
Chapter 9

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198 NISTNCSTAR 1-5G, WTC Investigation
Chapter 10

Global Thermal Response of WTC 2 : Case C

Four global simulations, two each for World Trade Center (WTC) 1 and WTC 2, are presented in this report to predict the thermally induced structural response of the World Trade Center towers. This chapter

is the third of four chapters (Chapter 8 through Chapter 11) that describe the resuhs of the four cases. Chapter 8 and Chapter 9 describe results for WTC 1 Case A and Case B respectively, while this chapter

and Chapter 1 1 focus on WTC 2.

The focus of this chapter is on the global thermal response of WTC 2 for a specific set of aircraft impact

damage and fire dynamics simulation. The temporally and spatially varying temperatures on floors 78

through 83 of WTC 2 were estimated for all the structural elements including the concrete slab. These

temperamres were subsequently used for prediction of thermally induced structural response including

collapse of WTC 2. Floors below floor 78 and above floor 83 were assumed to be convectively cooled by

air at room temperamre. The period of time simulated is 60 min, and it ranges from point of impact to collapse of the WTC 2 tower. The results of the thermal analysis (presented in visual form in this chapter) were provided for structural analysis in a format that is consistent with the structural models. A floor by

floor description of each floor is presented to understand the nature of the thermal insult on WTC 2.

10.1 OVERVIEW

During the course of this Investigation, preliminary calculations were performed to study the thermally

induced response at the component or single floor level. The objective of these preliminary simulations was to assess the sensitivity of the input parameters, test the robustness of the numerical models and gain

insight. The set of simulations termed "WTC 2 Case C" represent one such set of simulations where base

settings (see Table 10-1, Table 10-2 and Table 10-3) were employed for the various input parameters to predict the thermally induced structural response of WTC 2. In this chapter, a floor by floor description

along with \ isuals (Figure 10-2 through Figure 10-7) is presented to understand the thermal loading

caused by the flres. The thermal data was provided in the form of thermal loading data files for structural analysis and for predicting collapse induced by heating of the structure.

10.1.1 Aircraft Impact Analysis

Aircraft impact analysis was performed for WTC 2 with base settings to predict the structural and

fireproofing damage on floors 78 through 83 of WTC 2. The input parameters for the impact analysis are summarized in Table 10-1. The main objective of this analysis was to predict the damage to columns,

trusses, and floor slabs due to aircraft impact (NIST NCSTAR 1-2).

Table 10-4 shows the structural and fireproofing damage observed (NIST NCSTAR 1-5A) on perimeter

columns for WTC 2 (floors 78 through 83). In this table a "0" indicates that the column and its fireproofing is intact, "1" indicates fireproofing damage on the interior face only and a "2" indicates a column that has been severed by aircraft impact. The methodology for incorporating fireproofing and

structural damage for perimeter columns was discussed in Chapter 4.

NIST NCSTAR 1-5G. WTC Investigation 199
Chapter 10

Table 10-5 shows fireproofmg damage predicted for core columns in WTC 2 (floors 78-83). In this table

a "0" indicates that the column and its fireproofmg is intact, a "2" indicates fireproofmg damage on one

face, and a "3" indicates fireproofmg damage on all the faces. Core columns in WTC 2 Case C were

either complete stipped of their fireproofmg or their fireproofmg was fully intact. Table 10-6 shows

structural damage predicted for core columns due to aircraft impact in WTC 2 (floors 78-83). In this table a "0" indicates that the column is intact while a "2" indicates that the columns has been severed (NIST

NCSTAR 1-2). -

Aircraft impact can also result in structural and fireproofmg damage to the floor system. For each floor, a map was prepared showing contours of the region where fireproofmg and structural damage was

predicted. This damage for floors 79, 80, 81, 82, and 83 is shown in Figure 10-2, Figure 10^,

Figure 10-6, Figure 10-14, Figure 10-16, respectively (NIST NCSTAR 1-2). In these figures, the regions

marked by green rectangles indicate structural damage, while the region marked by blue rectangles shows

fireproofmg damage only (steel stmcture is intact). Structural damage was incorporated in the analysis by

removing the concrete slab, trusses, or core beams in the area marked by the green rectangles.

Fireproofing damage was included in the finite element model by changing the material attributes of the

element. The method for incorporating structural and fireproofing damage has been discussed extensively

in Chapter 5 and Chapter 6 of this report.

Table 10-1. Input parameters for WTC 2 global

impact analyses (Base Case).

Analysis Parameters Base Case

Flight

Parameters

Impact Velocity 546 mph

Trajectory - pitch 6.0°

Trajectory - yaw 13.0°

Orientation - pitch 5.0°

Orientation - yaw 10.0°

Aircraft

Parameters

Weight 100 percent

Failure Strain 1 00 percent

Tower

Parameters

Contents Strength 100 percent

Failure Strain ] 00 percent

Live Load Weight" 25 percent

a. Live load weight expressed as a percentage of the design live load.

10.1.2 Fire Dynamics Simulations

Fire activity in WTC 2 was more difficult to simulate than that in WTC 1 due to the lack of a detailed

floor by floor layout and due to the nature of the impact damage on the various floors. Fire simulations were performed using the NIST Fire Dynamics Simulator (NIST NCSTAR 1-5F). Table 10-2 shows the

input parameters for the fire simulations.

200 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

Table 10-2. Values of WTC fire simulation variables.

Variable CaseC

Fuel load 20 kg/m- (4 lb/ft-) OiQtnhiitiAn ijlL iUUHKJil CiV KJl. disturbed

combustibles

concentrated in the northeast comer

Condition of

combustibles

All rubblized

Representation of

impacted core walls

Fully removed

In the fire simulations, workstations were damaged throughout most of the six focus floors of WTC 2 included in the simulations. For each floor, temporally averaged properties that describe the hot layer at 100 s intervals were recorded at each grid point in the fire simulation. The properties of the hot layer

include upper layer temperature, absorption coefficients, height of the hot layer, and ambient temperature.

The text file also contains information on spatial locations (underlying FDS grid) where the properties were recorded. Such text files were created for floors 78-83 of WTC 2 and were provided as input to the

ANSYS finite element analysis software.

10.2 FIRE STRUCTURE INTERFACE

The Fire Structure Interface (FSI) uses the output of the fire simulations together with the aircraft impact

damage results to predict the evolving thermal state of WTC 2. The important inputs that are required for FSI are listed in Table 1 0-3 . The thermal data computed was subsequently mapped on the structural elements and was provided in a format that was consistent with the finite element software as well as the

model used for performing structural analysis. The structural response of WTC 2 and collapse analysis

described in NIST NCSTAR 1-6C was performed using the thermal data presented in this chapter.

Figure 10-1 is a sample plot that shows the thermal data generated with FSI and subsequently transferred

to the structural models for analysis. Temperature contours are superimposed on the steel structure of a

typical floor of WTC 2. The figure shows the core beams and floor trusses that support a concrete slab. The concrete slab has not been included in the figure for clarity, but is included as part of the analysis.

Portions of the core beams and floor trusses were damaged by the impact of the aircraft and this portion

of the structure is not shown in the figure nor was it included in the thermal analysis. The figure also

shows the perimeter and core columns extending to the floor above and below the floor under

consideration. Perimeter and core columns that were severed by the impact of the aircraft are not shown

in this figure. The stnactural damage varies from one floor to another and has been summarized in Table

10-4 through Table 10-6. As has been discussed in detail in Chapter 4 through Chapter 7, the thermal and

structural finite element models were constructed from the data discussed in NIST NCSTAR 1-2 and

summarized in Appendix B. The type and location of the fireproofing thickness (not damaged by the

aircraft impact) is summarized in Table B-9.

The total simulation time for WTC 2 is approximately 60 min (3,600 s) and it ranges from the point of

aircraft impact until collapse. The time steps for thermal analysis ranged from 1 ms to 50 s. Thermal data

at six instants in time, spaced at 10 min (600 s) intervals was provided for structural analysis. For each

NIST NCSTAR 1-5G. WTC Investigation 201
Chapter 10

time step, a set of thermal loading data files were created to completely specify the themial state of each

floor. The thermal loading data files were in a format consistent with the structural models and could be

easily read in with the ANSYS finite element software to specify body loads on the structure. Thennal

loading data files were created at 10 min, 20 min, 30 min, 40 min, 50 min, and 60 min after impact. The

structure was assumed to be initially at room temperature. The final load step file was at 60 min (3,600 s) and it contained the final thermal state of the tower.

Table 10-3. Input for global thermal

response of WTC 2. Input WTC 2 Case C

Structural damage,

NISTNCSTAR 1-2 Base Case

Fireproofing damage,

NISTNCSTAR 1-2 Base Case

Fire simulations,

NIST NCSTAR 1-5F

Case C

Fire simulations and thermal analyses were performed for WTC 2 on floors 78 through 83. The full floor

structural models (NIST NCSTAR 1-6D) had a single slab and floor truss system with the columns

extending on one floor above and one floor below the slab. A mapping for floor 78 would have required

fire simulations on floor 77 and thermal analysis on the columns between floor 77 and 78 to completely

specify the thennal state of floor 78. Since no significant fires were observed on floor 77, fire simulations were not perfonned for that floor. As a consequence thermal mapping was not perfonned for floor 78.

In the following sections, the thennal mapping results for floors 79 through 83 is described. The thermal

data provided for structural analysis as body load files is presented in visual format on a floor by floor

basis, preceded by a brief discussion of the significant features.

202 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 1: Case C

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NISTNCSTAR 1-5G, WTC Investigation 203
Chapter 10

10.2.1 Floor 79, WTC 2 Case C

The aircraft impact analysis and fireproofing damage results for floor 79, WTC 2 Case C are shown in Figure 10-2 indicating extensive fireproofing damage on the south face and in the core area (southeast

comer)"\ There is also limited structural damage on the trusses and core beams.

Table 10-4 shows the fireproofing and structural damage predicted for perimeter columns on floors 78

and 79. There is no fireproofing damage on any of the perimeter columns on floor 78 or 79 (NIST

NCSTAR 1-2). However several perimeter columns on floors 78 and 79 were severed due to aircraft impact, especially on the south face (as evidenced in the photographic observations, NIST

NCSTAR 1-5 A). Finite element models of perimeter columns and the methodology for incorporating

structural damage were discussed extensively in Chapter 4.

Table 1 0-5 and Table 1 0-6 show fireproofing and structural damage predicted for core columns on floors 78 and 79. The tables indicate extensive fireproofing damage to core columns on the east side of the core on floors 78 and 79. Column 903 and 1001 are severed (damaged) by the aircraft impact on floor 78,

while columns 901, 903, 1001, and 1002 are severed on floor 79. The methodology for incorporating

structural and fireproofing damage was discussed extensively in Chapter 7.

Figure 1 0-3 shows the thermal response of the 79"^ floor of WTC 2 Case C at six different instants in time, ranging from 10 min to 60 min at 10 min intervals. The figure shows the truss assembly and core beams that support the 79'"^ floor. Perimeter and core coluiuns above the 79'*^ floor concrete slab are subjected to fires on the 79"" floor while columns below 79^"" floor slab are subjected to fires on the 78**^

floor. The perimeter and core columns damaged by impact of the aircraft are not been shown in the

pictures. Also the core beams and trusses severed by impact of the aircraft have not shown in the pictures

(Figure 10-3).

Inspite of extensive fireproofing damage on the trusses that supports the slab on the 79* floor,

Figure 1 0-3 shows that trusses in the south face do not heat up significantly. Fire simulations indicate that there was only light fire activity on the 78* floor due to a small amount ofjet fuel released on this floor

during impact. As a result trusses and core beams that were subjected to fires on the 78* floor did not show significant temperature increase (Figure 10-3). The perimeter and core columns between floor 78

and floor 79 also remained relatively cool through out the simulation, due to insufficient fire activity on

the floor. Core columns on the east side (1000 series) above floor 79 indicate heating due to a combination of fireproofing damage and fire activity on the 79* floor. Core column 1008 has fireproofing

damage but shows gradual heating because of its high thermal inertia (thicker, heavier cross-section).

" The damage graphics used in the thermal analysis are discussed in detail in NIST NCSTAR 1 -6 and have been presented here

for reference only.

204 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

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Fireproofing Damage

Figure 10-2. Structural and fireproofing damage on floor 79 WTC 2, Case Cj

NISTNCSTAR 1-5G, WTC Investigation 205
Chapter 10

Figure 10-3. Thermal response of floor 79 WTC 2 Case C at six different instants in time.

206 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

1 0.2.2 Floor 80, WTC 2 Case C

Structural and fireproofing damage due to aircraft impact on floor 80 (floor trusses and slabs) is shown in Figure 10-4. The figure indicates extensive fireproofing damage in the south face of WTC 2 and the

southeast comer of the core. There is also limited amount of structural damage for trusses, core beams,

and concrete slabs. Structural damage was incorporated into the analysis by removing the concrete slab, trusses or core beams in the area marked by the green rectangles. Fireproofing damage was included in the simulations by changing the material attributes of the elements to those of a perfect conductor. The

method for incorporating structural and fireproofing damage on truss and floor slabs was discussed

extensively in Chapter 5 and Chapter 6.

There is no fireproofing damage on any of the perimeter columns below floor 80, but fireproofing damage

was predicted (NIST NCSTAR 1-2) for perimeter columns 251-259 above floor 80, on the north face of

WTC 2 (Table 10-4). The fireproofing damage on these columns is limited to the interior faces of the

column. Several perimeter columns on floors 79 and 80 were severed due to aircraft impact, especially on

the south face (NIST NCSTAR 1-5A). Table 10-5 and Table 10-6 indicate extensive fireproofing

damage to core columns on east side of the core on floors 79 and 80, especially in the 9 and 10 series columns.

Figure 1 0-5 shows the thermal response of the 80'" floor of WTC 2, Case C at six different instants in time ranging from 10 min to 60 min at 10 min inters'als. The figure shows the truss assembly and core beams that support floor 80. subjected to fires on floor 79. Perimeter and core columns above the 80*

floor slab are subjected to fires on 80'*^ floor, while columns below the 80"^ floor slab are subjected to fires on the 79* floor. The perimeter and core columns, core beams, truss, and slab damaged by the aircraft have been removed from the pictures.

Trusses and core beams that have fireproofing damage show higher temperatures in the areas where there

is intense fire activity (10 min and 20 min after impact). There are portions of the floor truss that do not

heat up as quickly (even though they have lost their fireproofing) due to lack of fire activity in this area. Fire simulations indicate little fire activity in the south face and moderate activity in the north face. Some

fire activity is predicted in the east and west faces. Truss elements in WTC 2 are covered with 0.6 in. equivalent thickness of BLAZE-SHIELD DC/F product. These elements show thennal heating in the

northeast comer and in the east face. Simulation resuhs predict heating of the tmss element on the west

face, due to fire activity in this area (NIST NCSTAR 1-5F). It should be noted that visual observations

(window breaking) do not indicate the predicted level of fire activity; as a result, heating of the tmss

elements in this area might have been significantly lower.

Perimeter columns in the north face of WTC 2, between floor 80 and floor 81 show significant heating

due to lack of fireproofing damage on the columns and intense fire activity. As a result perimeter

columns in the northeast comer show temperatures on the order of 500 °C to 600 °C. Numerical

simulations predict heating of the perimeter column on the west face of floor 79. Heating of core columns was predicted on the east side on floor 79, but relatively cool core columns were predicted above

floor 80. This is due to lack of high upper layer temperatures in the core on floor 80 as indicated by the

fire simulations (NIST NCSTAR 1-5F).

NIST NCSTAR 1-5G, WTC Investigation 207
Chapter 10

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Stmctural damage for floor 80

Structural Diimaoe

Fireproofing damage on floor tmsses

and core beams that support floor 80

Fireproofing Damage

Figure 10-4. Structural and fireproofing damage on floor 80, WTC 2 Case Cj

208 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

Figure 10-5. Thermal response of floor 80, WTC 2 at different instants in time, Case C.

NISTNCSTAR 1-5G, WTC Investigation 209
Chapter 10

10.2.3 Floor 81, WTC 2 Case C

Aircraft impact analysis and fireproofing damage contour maps for floor 81 are shown in Figure 10-6.

These maps apply to floor trusses and core beams that support the slab on floor 8 1 . Considerable damage

is predicted along a path from the south central to northeast comer of the floor. Extensive fireproofing

damage is predicted for perimeter columns above floor 8 1 ranging from column 25 1 -35 1 . Below floor

81, fireproofing damage is on the north face, toward the northeast corner. Fireproofing damage is due to the predicted path of the aircraft debris (NIST NCSTAR 1-2). Structural damage is limited to the south

face of WTC 2 (Table 10^). Fireproofing damage for core columns is quite extensive above and below

floor 81 (Table 10-5).

Figure 10-7 through Figure 10-13 show contour maps of temperature superimposed on the structural elements on floor 81, WTC 2 Case C at various instants in time. Trusses and core beams that have

fireproofing damage show higher temperatures in the area where there is intense fire activity. Figure 10-7

shows significant heating of the trusses, 10 min after impact, but the trusses indicate coohng down at 20 min and 30 min after impact. Some heating of the trusses in the northwest comer is observed at 50 min

to 60 min after impact. Although there is no fireproofing damage to the tmsses in this region, the tmss

elements are covered with 0.6 in. of fireproofing, which results in gradual heating of the tmsses in the

region where there is fire activity. Fire simulations indicate less activity in the northeast comer on floor

80 as compared to floors 81 and 82.

Perimeter columns above floor 81 show significant heating, due to lack of fireproofing and intense fire activity over the entire east face and northeast comer. Temperature greater than 600 °C is predicted for these perimeter columns at 30 min after impact. The rest of the perimeter columns stay relatively cool. Core column heating is observed for the 10 series columns above floor 81 due to a combination of

fireproofing damage and fire activity in this area.

210 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

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Structural Damage Fireproofing Damage

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South Tower : Flexor 81

Time = 600 s

Figure 10-7. Thermal response of floor 81, WTC 2 Case C at 600 s after impact.

NISTNCSTAR 1-5G, WTC Investigation 211
Chapter 10

0 150 300 450 600

75: 225 375 525 575 Figure 10-8. Thermal response of floor 81, WTC 2 Case C at 1,200 s after impact.

0 150 3u0 450 tO'J 75 225 375 525 675 Figure 10-9. Thermal response of floor 81, WTC 2 Case C at 1,800 s after impact.

212 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

NISTNCSTAR 1-5G. WTC Investigation 213
Chapter 10

0 150 300 450 600 75 225 375 525 675 Figure 10-12. Thermal response of floor 81, WTC 2 Case C at 3,600 s after impact.

214 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

Figure 10-13. Thermal response of floor 81, WTC 2 Case C (concrete slab and columns)

at different instants in time.

NISTNCSTAR 1-5G, WTC Investigation 215
Chapter 10

10.2.4 Floor 82, WTC 2 Case C

Extensive fireproofing damage is predicted on the floor trusses and core beams in the east face of the

tower as shown in Figure 10-14. This results in significant heating of the trusses for the entire duration of

the simulation (see Figure 10-15). Temperatures in the truss elements above 675 °C are predicted. Some

heating is also observed in the southwest comer at 60 min after impact (due to the small thickness of

fireproofing). Perimeter columns above floor 82 showed high temperatures at 40 min to 60 min after impact on the east face, and core columns on the east side of the core also experience extreme heating.

The west side of the core stays relative cool, and there is very little activity in the northwest comer.

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Stmctural Damage Fireproofing Damage

Figure 10-14. Structural and fireproofing damage on floor 82, WTC 2, Case Cj.

216 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

Figure 10-15. Thermal response of floor 82, WTC 2 Case C at different instants in time.

NISTNCSTAR 1-5G, WTC Investigation 211
Chapter 10

10.2.5 Floor 83, WTC 2 Case C

The thennal response of floor 83 is consistent with the fireproofing damage shown in Figure 10-16 and

the predicted fire activity on floors 82 and 83. Thermal mapping results for floor 83 at six different

instants in time have been shown in Figure 10-17.

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Structural Damage Fireproofing Damage

Figure 10-16. Structural and fireproofing damage on floor 83, WTC 2, Case Ci.

218 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

Figure 10-17, Thermal response of floor 83, WTC 2 Case C, at different instants in time.

NISTNCSTAR 1-5G, WTC Investigation 219
Chapter 10

1 0.3 DATA TRANSFER FOR STRUCTURAL ANALYSIS

The thermal resuUs presented in this chapter were passed on for structural analysis (NIST NCSTAR 1-6D,

NIST NCSTAR 1-6) in the load bearing elements. As for Case A and Case B, the data was provided on a

floor by floor basis ranging from floor 79 through floor 83. For each floor, the thermal results were

provided at six instants in time, spaced at 10 min interval. For each instant in time a thennal loading data

file was generated to completely specify the thermal state of the floor. The thermal loading data files were generated at 10 min, 20 min, 30 min, 40 min, 50 min, and 60 min after aircraft impact. Each file was generated in a format that is consistent with the structural models and could be readily read in, to specify body loads (temperature or temperature gradient) on the structure. The files were transferred

using a file transfer protocol (ftp) utility.

The thermal results shown in Figure 10-2 through Figure 10-17 Figure 10-17 are visual renderings of the

information contained in the thermal loading data files provided for stress analysis.

220 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

Table 10-4. Perimeter column damage status for WTC 2 Case C.

Column # Floor 78 Floor 79 Floor K1 FInnr 87 Floor

101 0 0 0 0 0 0

103 0 0 0 0 0 0

106 0 0 0 0 0 0

109 0 0 0 0 0 0

112 0 0 0 0 0 0

115 0 0 0 0 0 0

118 0 0 0 0 0 0

121 0 0 0 0 0 0

124 0 0 0 0 0 0

127 0 0 0 0 0 0

130 0 0 0 0 0 0

133 0 0 0 0 0 0

136 0 0 0 0 0 0

139 0 0 0 0 0 0

142 0 0 0 0 0 0

145 0 0 0 0 0 0

148 0 0 0 0 0 0

151 0 0 0 0 0 0

154 0 0 0 0 0 0

157 0 0 0 0 0 0

201 0 0 0 0 0 0

203 0 0 0 0 0 0

206 0 0 0 0 0 0

209 0 0 0 0 0 0

212 0 0 0 0 0 0

215 0 0 0 0 0 0

218 0 0 0 0 0 0

221 0 0 0 0 0 0

224 0 0 0 0 0 0

227 0 0 0 0 0 0

230 0 0 0 0 0 0

233 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 221
Chapter 10

236 0 0 0 0 0 0

239 0 0 0 0 0 0

242 0 0 0 0 0 0

245 0 0 0 0 0 0

248 0 0 0 0 0 0

251 0 0 1 1 0 0

254 0 0 1 1 0 0

257 0 0 1 1 0 0

301 0 0 0 1 0 0

303 0 0 0 1 0 0

306 0 0 0 1 0 0

309 0 0 0 0 0

312 0 0 0 1 0 0

315 0 0 0 1 0 0

318 0 0 0 1 0 0

321 0 0 0 1 0 0

324 0 0 0 1 1 0

327 0 0 0 1 1 0

330 0 0 0 1 0

333 0 0 0 1 1 0

336 0 0 0 1 1 0

339 0 0 0 1 1 0

342 0 0 0 1 1 0

345 0 0 0 1 1 0

348 0 0 0 1 1 0

351 0 0 0 1 1 0

354 0 0 0 0 1 0

357 0 0 0 0 1 0

401 0 0 0 0 0 0

403 0 0 0 0 0 0

406 0 0 0 0 0 0

409 0 0 0 0 0 0

412 0 0 0 0 2 2

222 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

415 0 0 2 2 2 2

418 0 2 2 2 2 0

421 0 0 2 2 0 0

424 0 2 2 2 0 0

427 0 2 0 0 0 0

430 2 2 0 0 0 0

433 2 2 0 0 0 0

436 2 0 0 0 0 0

439 2 0 0 0 0 0

442 0 0 0 0 0 0

445 0 0 0 0 0 0

448 0 0 0 0 0 0

451 0 0 0 0 0 0

454 0 0 0 0 0 0

457 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 223
Chapter 10

Table 10-5. Core column fireproofing damage status for WTC 2, Case C.

Column No. Floor 78 Floor 79 Floor 80 Floor 81 Floor 82 Floor 83

501 0 0 0 0 0 0

502 0 0 0 0 0 0

503 0 0 0 0 0 0

504 0 0 0 0 0 0

505 0 0 0 0 0 0

506 0 0 0 0 0 0

507 0 0 0 0 0 0

508 0 0 0 0 0 0

601 0 3 0 0 0 0

602 0 3 0 0 0 0

603 0 3 0 0 0 0

604 0 0 0 0 0 0

605 0 0 0 0 0 0

606 0 0 0 0 0 0

607 0 0 0 0 0 0

608 0 0 0 0 0 0

701 3 3 3 0 0 0

702 3 3 3 0 0 0

703 3 3 3 0 0 0

704 3 3 3 0 0 0

705 3 3 0 0 0 0

706 3 3 0 0 0 0

707 0 3 0 0 0 0

708 0 0 0 0 0 0

801 3 3 3 3 0 0

802 3 3 3 0 0 0

803 3 3 3 0 0 0

804 3 3 3 0 0 0

805 3 3 3 0 0 0

806 0 3 0 0 0 0

807 0 0 0 0 0 0

224 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

901 3 3 3 3 3 0

902 3 3 3 3 3 0

903 3 3 3 3 0 0

904 3 3 3 3 0 0

905 3 3 3 3 0 0

906 0 3 3 3 0 0

907 0 3 0 3 0 0

908 0 3 0 3 0 0

1001 3 3 3 3 3 0

1002 3 3 3 3 3 0

1003 3 3 3 3 3 0

1004 3 3 3 3 3 0

1005 0 3 0 3 3 0

1006 0 3 0 3 3 0

1007 0 3 0 3 3 0

1008 0 3 0 3 3 0

NISTNCSTAR 1-5G, WTC Investigation 225
Chapter 10

Table 10-6. Core column structural damage status for WTC 2, Case C.

Column

No. Floor 78 Floor 79 Floor 80 Floor 81 Floor 82 Floor 83

501 0 0 0 0 0 0

502 0 0 0 0 0 0

503 0 0 0 0 0 0

504 0 0 0 0 0 0

505 0 0 0 0 0 0

506 0 0 0 0 0 0

507 0 0 0 0 0 0

508 0 0 0 0 0 0

601 0 0 0 0 0 0

602 0 0 0 0 0 0

603 0 0 0 0 0 0

604 0 0 0 0 0 0

605 0 0 0 0 0 0

606 0 0 0 0 0 0

607 0 0 0 0 0 0

608 0 0 0 0 0 0

701 0 0 0 0 0 0

702 0 0 0 0 0 0

703 0 0 0 0 0 0

704 0 0 0 0 0 0

705 0 0 0 0 0 0

706 0 0 0 0 0 0

707 0 0 0 0 0 0

708 0 0 0 0 0 0

801 0 0 0 0 0 0

802 0 0 0 0 0 0

803 0 0 0 0 0 0

804 0 0 0 0 0 0

805 0 0 0 0 0 0

806 0 0 0 0 0 0

807 0 0 0 0 0 0

226 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case C

901 0 2 2 2 2 0

902 0 0 0 0 0 0

903 2 2 2 2 2 2

904 0 0 0 0 0 0

905 0 0 0 0 0 0

906 0 0 0 0 0 0

907 0 0 0 0 0 0

908 0 0 0 0 0 0

1001 2 2 2 2 2 2

1002 0 2 2 2 0 0

1003 0 0 2 0 0 0

1004 0 0 0 0 0 0

1005 0 0 0 0 0 0

1006 0 0 0 0 0 0

1007 0 0 0 0 0 0

1008 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 227
Chapter 10

;1

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228 NISTNCSTAR 1-5G, WTC Investigation
Chapter 11

Global Thermal Response of WTC 2 : Case D

11.1 overview

This chapter is the last of four chapters that describe the resuhs of the four global simulations (two each

for WTC 1 and WTC 2). The focus of this chapter is on WTC 2 Case D.

Following the completion of the Case C analysis of WTC 2 (described in Chapter 10), the calculations were re-run with more severe settings for the input parameters (described in Table 1 1-1, Table 1 1-2 and

Table 1 1-3). The methodology for performing the simulations is identical to that for Case C. In this chapter, the focus is on the model inputs and simulation results for WTC 2 Case D. Analysis of the

differences and similarities in thermal and structural resuhs for Case C and Case D, presented in Chapter

12 will help to assess the sensitivity of the results to changes in input parameters.

11.1.1 Aircraft Impact Analysis

Aircraft impact analysis was performed for WTC 2 with more severe settings for the various input

parameters. The input parameters used in the analysis are summarized in Table 1 1-1. Structural and

fireproofing damage to columns, trusses, and floor slabs was obtained from the impact analysis. The

predicted structural and fireproofing damage for Case D is different and in general more extensive than

that for Case C (NIST NCSTAR 1-2).

Table 11-1. Input parameters for WTC 2 global impact

analyses (more severe).

Analysis Parameters More Severe

Flight

Parameters

Impact Velocity 570 mph

Trajectory - pitch 5.0°

Trajectory - yaw 13.0°

Orientation - pitch 4.0°

Orientation - yaw 10.0°

Aircraft

Parameters

\A eight 105 percent

Failure Strain 1 15 percent

Tower

Parameters

Contents Strength 80 percent

Failure Strain 90 percent

Live Load Weight' 20 percent

Live load weight expressed as a percentage of the design live load.

The main objective of the impact analysis was to estimate the damage to structural systems, including the

exterior wall, floor systems and interior core columns. Table 1 1-4 shows the fireproofing and stmctural

damage predicted for perimeter columns on floors 78 through 83, WTC 2 Case D (NIST NCSTAR 1-5 A).

NIST NCSTAR 1-5G, WTC Investigation 229
Chapter 1

In this table a "0" indicates that the column and its fireproofing is intact, a "1" indicates fireproofing

damage on the interior face, and a "2" indicates that a column has been severed by the aircraft impact.

Table 1 1-5 shows fireproofing and structural damage predicted for core coluinns on floor 78-83 In this table a "0" indicates that the column and its fireproofing are intact, a "1" indicates fireproofing damage on

one face, a "2" indicates a column that has been severed by the aircraft impact while a

"3'" indicates

fireproofing damage on all the faces. For WTC 2 Case D,core columns had their fireproofing intact or the

fireproofing was damaged on all the faces.

Aircraft impact damage also results in structural and fireproofing damage on the floor systems. For each

floor, a map was prepared that describes the contours of the region where fireproofing and structural damage was predicted. This damage for floors 79 through 83 is displayed as contour maps in the aircraft impact analysis report (NIST NCSTAR 1 -2).

11.1.2 Fire Dynamics Simulation

Fire simulations were performed as in Case C to predict contours of upper layer temperature, absorption

coefficient, layer depth, and ambient temperatures as a function of space and time on each floor of WTC

2. For Case D, the combustible load for fire simulations was kept at 20 kg/m', the same as Case C, but

the aircraft debris and "rubble" was spread out over a wider area. In Case C the debris pile was

concentrated in the northeast comer, while in Case D it was less concentrated. The input parameters for WTC 2 Case D fire simulations are summarized in Table 1 1-2.

Fire simulations for Case D indicated high temperatures over greater areas. This was due to increased

burning rate of the furnishing and large number of broken windows (NIST NCSTAR 1-5F). The fires in WTC 2 were fuel-limited, unlike those in WTC 1, which were oxygen hmited. Since the fires in WTC 2 were fuel limited, the results for WTC 2 Case C are in some instances substantially different from those

for WTC 2 Case D.

Table 1 1-2. Values of WTC fire

simulation variables.

Variable WTC 2 Case D

Fuel load 20 kg/m- (5 lb/ft")

Distribution of

disturbed

combustibles

Moderately

concentrated in the northeast comer

Condition of

combustibles

Undamaged except in impact zone

Representation of

impacted core walls

Soffit remained

230 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case D

1 1 .2 FIRE STRUCTURE INTERFACE

Changes in fire dynamics between Case C and Case D affect the thermal response of the structural elements. Fireproofing damage and structural damage caused by the aircraft impact analysis also has a

large impact on the temperatures of the structure. FSI simulations were performed with different input

conditions to predict the time evolving thermal state of WTC 2. The input parameters are listed in Table 11-3.

In the next section, the thermal resuhs for WTC 2 Case D are presented on a floor by floor basis. As in Chapter 10 our goal in this chapter is to understand the nature of the thermal insuh on the structure for each floor in WTC 2.

Figure 1 1-2 through Figure 1 1-10 show the thermal state of floors 79 through 83 in WTC 2, respectively,

at six different instants in time ranging from 10 min to 60 min at 10 min interval. Each sub-figure shows

the truss assembly, the core beams, perimeter columns and core columns above and below the concrete

slab. Color contours are superimposed on the structural elements ranging from 0 to 675 °C.

Table 11-3. Input for global thermal response

of WTC 2. Input WTC 2 Case D

Structural damage,

NISTNCSTAR 1-2 NISI NCSTAR 1-6

Case D

More severe

Fireproofing damage,

NISTNCSTAR 1-6 Case D

Fire Simulations,

NISTNCSTAR 1-5F

Case D

1 1 .2.1 Floor 79, WTC 2 Case D

The truss system that supports floor 79 has extensive fireproofing damage to trusses and core beams (see

Figure 1 1-1 ).^ Structural damage due to aircraft impact is not very significant.

The truss system that supports floor 79 shows slightly higher temperatures for Case D as compared to Case C, especially in the southeast comer at 20 min after impact. This is due to continuous fire activity on

floor 78 in this area. The thermal state of floor 79 at 10 min intervals is shown in Figure 1 1-2.

* The damage graphics used in the thermal analysis are discussed in detail in NISI NCSTAR 1 -6 and have been presented here

for reference only.

NIST NCSTAR 1-5G, WTC Investigation 23
Chapter 1

2i« aai sffii

s« s« Tot tat m lopa

zae 22e »t£ 2*;

706 &

DM low

SM 704 tSM no «I3

«a3 R

MB4£r ^ *B' MS 44ft «S «3e 433 43C C7 <1S 4» 41? ^tE «K

Structural damage for floor 79

2fit aas iSK sxi -zjs m S'ib 2!?! asi

1 1 j.

12

?w set a« i£!M

»8 site

M V

r—f rn V"! vi

4» Me 'Mb 443 4» tie <K 4& 4»r «M «^ 4^ 4U 413 'SB «6 -ta «r

Fireproofing damage for floor tmsses

and core beams that support floor 79

structural Daniiige CH Fireproofing Damage CZI

Figure 11-1. Structural and fireproofing damage on floor 79, WTC 2 Case Dj.

232 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case D
Chapter 1

1 1 .2.2 Floor 80, WTC 2 Case D

The east side of the truss system that supports floor 80 shows high temperatures due to continuous fire activity on floor 79. The heating of the truss system was more extensive than that for Case C. Core

columns showed more extensive structural damage in the southeast comer as compared to Case C.

Fireproofmg damage was also more extensive in Case D, especially in the east side of the core. Fireproofmg damage results in higher temperature on the core columns in this case. Thermal states of

floor 80 at 10 min intervals are shown in Figure 1 1-4.

ZS*- 2& 3X ir.'* -iii i'9 it

&i m £x m m im lot i^s M Sii^ :&! Xti

tun Kit . > va imj

fit? BM

fm am wa 10

ok W 804 9& I «2 KM 9D4 1 04

«3 «a£J «f7 4^6

Structural damage for floor 80 Fireproofmg damage for floor trusses and core beams that support floor 80

Sti-uctural Damage Fireproofmg Damage

Figure 11-3. Structural and fireproofing damage on floor 80, WTC 2, Case Dj.

234 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case D

Figure 11-4. Thermal response of floor 80, WTC 2 Case D at different Instants In time.

NISTNCSTAR 1-5G, WTC Investigation 235
Chapter 1

1 1 .2.3 Floor 81 , WTC 2 Case D

The truss system that supports floor 8 1 shows intense heating on the east side and in the northeast comer.

Perimeter columns in the northeast comer, as well as core columns, show significant heating due to a combination of fireproofmg damage and fire activity in this area. Structural damage due to aircraft impact

in Case D was more extensive on the floor systems and core coluirms. Fireproofing damage was also more extensive on this floor as compared to Case C. The thermal state of floor 81 at 10 min intervals is shown in Figure 1 1-6.

251 SOS Its 2» is.) SK) 2«

Structural damage for floor 81 Fireproofing damage on floor tmsses

and core beams that support floor 81

Structural Damage Fireproofing Damage

Figure 11-5. Structural and fireproofing damage on floor 81, WTC 2, Case Dj.

236 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case D

Figure 11-6. Thermal response of floor 81, WTC 2 Case D at different instants in time.

NISTNCSTAR 1-5G, WTC Investigation Til
Chapter 1

11.2.4 Floor 82, WTC 2 Case D

Floor 82 shows large regions on the east side of the core that suffered fire damage as indicated by very

high temperatures in the east face and northeast comer. This floor is one of the most severely damaged

floors in our simulations. Fire simulations predict burnout in the northeast comer at 50 min to 60 min

after impact, and the simulation results do not match with the visual and photographic observations,

which indicate continuous fire activity until collapse. The lack of predicted fire activity may have resulted

in prediction of lower temperatures for floor tmsses and columns in the northeast comer. The thennal

state of floor 82 at 10 min intei^vals is shown in Figure 1 1-8.

mes sm m m m

'J.

«fc s» as* ill] 2« 2« K\ 2S? iefi

—'—

'» ' ' ' '

' - Sm TH 607 SM last •

KT tool

»3 MS

I

m *« mrrrr \ \ \ x m m ist! m ^ ^1 m ^af-

\ \ \ \ \ m 4tS «« 4CE! «-!

Stmctural damage for floor 82

Structural Damag*

Fireproofmg damage on floor trusses and core beams that support floor 82

Fireproofing Damage

Figure 11-7. Structural and fireproofing damage on floor 82, WTC 2, Case Dj.

238 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case D

6?r> Figure 11-8. Thermal response of floor 82, WTC 2 Case D at different instants in time.

NISTNCSTAR 1-5G, WTC Investigation 239
Chapter 1

11.2.5 Floor 83, WTC 2 Case D

Like floor 82, floor 83 also indicates very intense fire activity throughout the simulation resuhing in extreme fire induced damage on the east side of the building. The thermal insult on this floor is predicted

for the entire duration of the simulation. The theimal state of floor 83 at 10 min intervals is shown in Figure 1 1-10.

Structural damage for floor 83 Fireproofmg damage on floor trusses and core beams that support floor 83

Structural Damage Fireproofing Damage

Figure 11-9. Structural and fireproofing damage on floor 83, WTC 2, Case Dj.

1 1 .3 DATA TRANSFER FOR STRUCTURAL ANALYSIS

The data transfer for WTC 2 Case D is identical to that described in the previous chapter for WTC 2

Case C (see Section 10.3).

240 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case D
Chapter 1

11.4 EXTENDED RUN SIMULATIONS

The thermal response analysis of WTC 2 described above was limited to 60 min (1 h) for both Case C and

Case D, as described in Chapter 10 and Chapter 11.

In order to assess the possible outcome of a severe fire in a tower for which no fireproofmg damage due

to aircraft impact had occuired, a simulation was perfonned in which the Case D fires were allowed to bum an additional 2 h (see NIST NCSTAR 1-5F). This simulation assumed that all the windows were

broken at the 1 h point. This resulted in intense fire activity in the west side of WTC 2.

Thermal results indicated significant heating of the floor trusses on the west side (where the insulated was

undamaged), with peak temperatures of approximately 760 °C. Temperatures of 700 °C to 760 °C were

reached over approximately 15 percent of the west floor area for less than 10 min. Approximately

60 percent of the floor steel had temperatures between 600 °C and 700 °C for about 15 min.

Approximately 70 percent of the floor steel had temperatures that exceeded 500 °C for about 45 min (see

Fig. 1 1-11). After approximately 2 h, all the combustibles on the west side were consumed and this resulted in a gradual cool down of the steel temperature (see Fig. 1 1-12).

South Tower : Floor 82

Time = 6000 s

75

150

225 300

375

450

525

600

675

Figure 11-11. Thermal response of floor trusses that support the concrete slab on

floor 82, WTC 2 at 6,000 s after impact (extended simulation for Case D).

242 NIST NCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case D

Terrperature (C)

800-

700-

600

500

400

300

200

100

0

\

180 360 540 720 900

90 270 450 630 810 990 Tiine (s)

(xlO**l) 1080

Figure 11-12. Typical time-temperature plot for floor trusses on the west side for the

extended run, indicating rapid heating at 6000 s after impact followed by gradual cooling.

NISTNCSTAR 1-5G. WTC Investigation 243
Chapter 1

able 11-4. Perimeter Column Damage Status for WTC 2 Case D.

Column # Floor 78 Floor 79 Floor 80 Floor 81 Floor 82 Floor 83

101 0 0 0 0 0 0

103 0 0 0 0 0 0

106 0 0 0 0 0 0

109 0 0 0 0 0 0

112 0 0 0 0 0 0

115 0 0 0 0 0 0

118 0 0 0 0 0 0

121 0 0 0 0 0 0

124 0 0 0 0 0 0

127 0 0 0 0 0 0

130 0 0 0 0 0 0

133 0 0 0 0 0 0

136 0 0 0 0 0 0

139 0 0 0 0 0 0

142 0 0 0 0 0 0

145 0 0 0 0 0 0

148 0 0 0 0 0 0

151 0 0 0 0 0 0

154 0 0 0 0 0 0

157 0 0 0 0 0 0

201 0 0 0 0 0 0

203 0 0 0 0 0 0

206 0 0 0 0 0 0

209 0 0 0 0 0 0

212 0 0 0 0 0 0

215 0 0 0 0 0 0

218 0 0 0 0 0 0

221 0 0 0 0 0 0

224 0 0 0 0 0 0

227 0 0 0 0 0 0

230 0 0 0 0 0 0

244 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case D

233 0 0 0 0 0 0

236 0 0 0 0 0 0

239 0 0 0 0 0 0

242 0 0 0 1 1 0

245 0 0 1 1 1 0

248 0 0 1 1 1 0

251 0 0 1 1 1 0

254 0 0 1 1 1 0

257 0 0 0 1 1 0

301 0 0 0 1 1 0

303 0 0 0 1 1 0

306 0 0 0 1 1 0

309 0 0 0 1 1 0

312 0 0 0 1 1 0

315 0 0 0 1 1 0

318 0 0 0 1 1 1

321 0 0 0 1 1 1 324 0 0 0 1 1 1 327 0 0 0 1 1 1 330 0 0 0 1 1 1

333 0 0 0 1 1 1 336 0 0 0 1 1 1 339 0 0 0 1 1 1 342 0 0 0 1 1 1 345 0 0 0 1 1 1

348 0 0 0 1 1 1

351 0 0 0 1 1 1 354 0 0 0 0 1 1

357 0 0 0 0 1 1

401 0 0 0 0 0 0

403 0 0 0 0 0 0

406 0 0 0 0 0 0

NISTNCSTAR 1-5G, WTC Investigation 245
Chapter 1

409 0 0 0 0 0 0

412 0 0 0 0 2 2

415 0 0 2 2 2 2

418 0 2 2 2 2 0

421 0 0 2 2 0 0

424 0 2 2 2 0 0

427 0 2 0 0 0 0

430 2 2 0 0 0 0

433 2 2 0 0 0 0

436 2 0 0 0 0 0

439 2 0 0 0 0 0

442 0 0 0 0 0 0

445 0 0 0 0 0 0

448 0 0 0 0 0 0

451 0 0 0 0 0 0

454 0 0 0 0 0 0

457 0 0 0 0 0 0

246 NISTNCSTAR 1-5G, WTC Investigation
Global Thermal Response of WTC 2: Case D

Table 1 1-5. Core Column Damage Status for WTC 2, Case D. Column

No. Floor 78 Floor 79 Floor 80 Floor 81 Floor 82 Floor 83

501 0 0 0 0 0 0

502 0 0 0 0 0 0

503 0 0 0 0 0 0

504 0 0 0 0 0 0

505 0 0 0 0 0 0

506 0 0 0 0 0 0

507 0 0 0 0 0 0

508 0 0 0 0 0 0

601 3 0 0 0 0 0

602 3 0 0 0 0 0

603 3 0 0 0 0 0

604 3 0 0 0 0 0

605 3 0 0 0 0 0

606 3 0 0 0 0 0

607 3 0 0 0 0 0

608 3 0 0 0 0 0

701 3 2 2 0 0 0

702 3 3 3 0 0 0

703 3 3 3 0 0 0

704 3 3 3 0 0 0

705 3 3 3 0 0 0

706 3 3 0 0 0 0

707 3 3 0 0 0 0

708 3 3 0 0 0 0

801 3 2 2 3 3 0

802 2 2 2 0 3 0

803 2 2 2 0 3 0

804 3 3 3 0 3 0

805 3 3 0 0 3 0

806 3 3 0 0 3 0

NISTNCSTAR 1-5G, WTC Investigation 247
Chapter 1

807 3 3 0 0 0 0

901 3 3 2 2 2 0

902 3 3 3 3 3 0

903 2 2 2 2 2 2

904 3 3 3 3 3 0

905 3 3 3 3 3 0

906 3 3 3 0 3 0

907 3 3 3 0 3 0

908 3 3 3 0 0 0

1001 2 2 2 2 2 2

1002 3 2 2 2 2 2

1003 3 2 2 2 2 2

1004 3 2 2 2 2 2

1005 3 3 3 3 3 0

1006 3 3 3 3 3 0

1007 3 3 3 3 3 0

1008 3 3 3 3 3 0

248 NISTNCSTAR 1-5G, WTC Investigation
Chapter 12

Summary of Technical Results

The attack on the World Trade Center (WTC) Towers and their subsequent collapse on

September 11, 2001, sparked an active debate in the engineering community regarding what was the

maximum temperature reached within the steel structure and how the heating of the steel led to structural collapse. Software tools that could simulate the thermally induced structural response to spatially and

temporally developing fires, on one or more floors of each WTC tower damaged by the impact of the

aircraft, were needed to carry out the investigation.

This report presents numerical simulation results for the evolving thermal state of all the structural components on focus floors of the WTC towers. A methodology to couple the fire simulations with

structural response for each tower is described. This methodology, termed the Fire Structure Interface

(FSI). utilizes the output of a fire dynamics simulation together with aircraft impact analysis results to produce a representation of the temperature distributions in the steel and concrete stracture. A simple

radiative transport model was developed, that permits the prediction of radiative flux incident on the

surface as a function of orientation of the structural element, temperature, hot layer depth and soot concentration. The resulting thermal data was generated in a format that is consistent with the structural models and the software that was used to perform structural analysis.

FSI links the fire simulations and damage predicted by the aircraft debris with stress analysis in the load

bearing structure. This methodology was used extensively throughout investigation. To test the accuracy

of the methodology, a series of large-scale experiments were conducted in the NIST Large Fire

Laboratory. The experiments established a dataset, which was used to test that the models were accurately

capturing the thermal response of the structural elements. Results indicate that FSI can reliably predict the

temperature in the structural components and that it does not add any more uncertainty in the overall

analysis than what is inherent in the aircraft impact analysis and fire dynamic simulations.

A large number of exploratory studies were conducted to understand the nature of the thermal insult caused by the fire on the structure. Sensitivity studies were performed to estimate the role of fireproofing on steel temperature. These studies included the effect of fireproofing thickness, its statistical variability

and damage or gaps in fireproofing thickness. Fireproofing thickness and fireproofing damage due to aircraft impact is identified as the single most important parameter in the simulations that has a direct

effect on steel temperature.

The exploratory studies, photographic evidence and published literature in this subject guided the

development of three-dimensional finite element models for global thermal analysis of each WTC tower. The construction of these models was constrained by the need to adequately resolve the physical

processes in a computationally efficient manner. Four global simulation results, two each for WTC 1 and

WTC 2 are presented in this report. Each global thermal simulation was coupled with a corresponding

fire simulation and includes aircraft impact damage estimates. The figures included in Chapter 8 through

Chapter 1 1 similar to the sample plot shown in Figure 8-1 of this report are visual representations of the

NISTNCSTAR 1-5G, WTC Investigation 249
Chapter 12

thermal state of the towers at specific instants in time. These figures provide a valuable insight into the

thermal response of the structural components for each tower.

In the remainder of this chapter, the important differences in input parameters and the results of the

theiTnal analysis for the four global simulations are summarized.

12.1 GLOBAL THERMAL RESPONSE OF THE WORLD TRADE CENTER

TOWERS

The methodology described in this report was used extensively throughout the World Trade Center

Investigation to predict the evolving thermal state of each tower. Four global simulations were performed,

two each for WTC 1 and WTC 2. Each of the global simulations was coupled with a corresponding fire simulation and the finite element models included the aircraft impact damage estimates.

The four global simulations were different in terms of the input parameters that were used to perform the

aircraft impact analysis for each tower. Table 12-1 lists the input parameters for "Base Case" and "More

Severe" case used in the impact analysis for WTC 1. Similarly Table 12-2 lists the input parameters used

in the impact analysis for WTC 2.

Table 12-1. Input parameters for V\lrrc 1 global impact analyses.

Analysis Parameters Base Case More Severe

Flight Parameters Impact speed 443 mph 472 mph

Trajectory - pitch 10.6° 7.6°

Trajectory - yaw 0.0° 0.0°

Orientation - pitch 8.6° 5.6°

Orientation - yaw 0.0° 0.0°

Aircraft

Parameters

Weight 1 00 percent 105 percent

Failure Strain 100 percent 125 percent

Tower

Parameters

Failure Strain 100 percent 80 percent

Live Load Weight" 25 percent 20 percent

a. Live load weight expressed as a percentage of the design live load.

The "More Severe" case for each tower had higher impact speed, assumed weight and failure strain for the aircraft. On the other hand, the tower parameters for the "More Severe" case were less than the "Base

Case". The impact analysis predicted the structural and fireproofing damage to the various structural components for each tower. As a direct result of the choice of the input parameters used in the

simulations, the "More Severe" case resulted in more structural and fireproofing damage to the structural components. The structural and fireproofing damage data as predicted by the impact analysis was

subsequently used in the FSI simulations for predicting the global thermal response of each tower. The

methodology for incorporating the structural and fireproofing damage in the finite element models was

discussed in Chapter 4 through Chapter 7.

250 NISTNCSTAR 1-5G, WTC Investigation
Summary of Technical Results

able 12-2. Input parameters for WTC 2 global impact analyses . Analysis Parameters

Flight

Parameters

Impact speed 546 mph 570 mph

Trajectory - pitch AO 6.0 C AO 5.0 Trajectory - yaw 13.0° 13.0° Orientation - pitch C AO

5. (J A AO 4,0

Orientation - yaw 1 A AO 10.0 1 A AO 10.0 Aircraft

Parameters

Weight 1 UVJ pel LCiU 1 (IS n^^T"^*f^t^t

I UJ> pclLCllL

Failure Strain 100 percent 1 1 5 percent

Tower

Parameters

Contents Strength 100 percent 80 percent

Failure Strain 100 percent 90 percent

Live Load Weight' 25 percent 20 percent

a. Live load weight expressed as a percentage of the design live load.

The four global simulations were also different from each other in terms of the input parameters used for the fire simulations. Table 12-3 hsts the input parameters for the four global simulations. Changes in the

fuel loading, distribution and condition of the combustibles and representation of impacted core walls

affected the fire growth and spread on the various floors of each tower. The large scale temperature and

other thermo-physical properties in the gas phase as predicted by the FDS simulations were used in the

FSI methodology for predicting the radiative flux to the sub-grid scale structural elements.

Table 12-3. Values of WTC fire simulation variables.

Variable

\A TC 1 W TC 2 Case A Case B CaseC Case D

Fuel load 20 kg m- (4 lb ft-) 25 kg/m-(5 lb ft-) 20 kg/m- (4 lb/ft-) 20 kg/m- (5 lb/ft-)

Distribution of

disturbed

combustibles

Even Weighted toward

the core

Heavily

concentrated in the northeast comer

Moderately

concentrated in the northeast comer

Condition of

combustibles

Undamaged except

in impact zone

Displaced furniture rubblized

All rubblized Undamaged except

in impact zone

Representation of

impacted core walls

Fully removed Soffit remained Fully removed Soffit remained

Global thermal response of each tower was computed using the FSI methodology. Each global thermal

simulation was coupled with a corresponding fire simulation and included the structural and fireproofing

damage estimates due to aircraft impact. Table 12^ lists the various input parameters for the four global

simulations described in detail in Chapter 8 through Chapter 11. As an example, the global thermal

response of WTC 1 Case B, used the aircraft damage estimates predicted with "More Severe" input

parameters (Table 12-1). used the fire simulation results for Case B (Table 12-3) and used the column

schedule and fireproofing thickness data as described in Appendix B.

NISTNCSTAR 1-5G, WTC Investigation 251
Chapter 12

Table 12-4. Input for global thermal response of WTC 1 and WTC 2. WTC 1 WTC 2 Case A Case B i^ase c case u

Input (Chapter 8) (Chapter 9) (Chapter 10) (Chapter 11)

Structural damage,

NISTNCSTAR 1-2 NISTNCSTAR 1-6

Case A Case B Case C Case D

(Base Case) (More severe) (Base Case) (More severe)

Fireproofing damage,

NISTNCSTAR 1-6 Case A Case B Case C Case D

Fire simulations,

NISTNCSTAR I-5F Case A CaseB Case C Case D

Column schedule Tahlf R 1 1 dUlC -D 1 , Table B^,

Table B-5,

Table B-6

Table B-1,

Table B-4,

Table B-5,

Table B-6

Table B-2,

Table B-7,

Table B-8

Table B-2,

Table B-7,

Table B-8

Fireproofing thickness Table B-9 Table B-9 Table B 9 Table B-9

Thermo-physical

properties Appendix A Appendix A Appendix A Appendix A

As the input data for the FSI simulations changes, the thermal response predicted for the various

structural elements on the focus floors of each tower also changed. In the next two sections the

differences in the global thermal response due to differences in input parameters for WTC 1 and WTC 2

is summarized. The global simulation results indicate that the fireproofing thickness and the damage to the fireproofing due to aircraft impact was the single most important parameter that has the largest effect on the predicted thermal response of the structural elements.

12.2 GLOBAL THERMAL RESPONSE OF WTC 1 : COMPARISON OF CASE A

AND CASE B

Global thermal response of WTC 1 Case A was described in detail in Chapter 8, while thermal analysis

for WTC 1 Case B was discussed in Chapter 9. For each case a detailed floor by floor analysis was

provided along with visuals, to describe the evolving thermal state of the various structural elements such

as floor trusses, core beams, concrete slabs perimeter and core columns. In this section, a comparison is made between the global thermal results of WTC 1 Case A and WTC 1 Case B.

Figure 12-1 and Figure 12-2 summarize the thermal response for WTC 1 Case A and WTC 1 Case B,

respectively. Each figure consists of six sub-figures. Each sub-figure shows all the core columns and

every third perimeter column. A column is represented by a square symbol. If a column is damaged at any

location on a floor, it is not included in the corresponding sub-figure. The color of the square symbol is related to the temperature of the column. The temperature value for a column shown in the plot is the

maximum temperature of that column and could occur any where between floor 92 through floor 99. Temperature range has been selected to lie between 0-800 "C. Each sub-figure shows the thermal results

at a specific instant in time. Results over a period ranging from 1000 s to 6000 s at 1000 s interval are presented.

252 NISTNCSTAR 1-5G, WTC Investigation
Summary of Technical Results

The size of the square symbol representing a perimeter or a core column is proportional to the product of

the cross-sectional area of that column and its yield strength. The product of the cross-sectional area and

yield strength is a measure of the load can-} ing capacity of a column. The yield strength is a function of

temperature, decreasing with increasing temperature (See Figure 3-2). As the temperature of the column

increases or decreases with time, the size of the symbol decreases or increases. Changes in the size of the

symbol indicate a change in the load carrying capacity of that column.

The size of the square symbol is related to the column cross-sectional area and since each column has a

different cross-sectional area (Appendix B). the size of the square symbols can be different from one

coliuim to another. e\ en if all the columns are at room temperature. Thus at a point in time just before the

aircraft impact when all the coltmins are at room temperature, the load carrying capacity' of the hea\'ier

columns is more than those of light columns. The heavier columns such as the comer core columns are therefore represented with a larger square symbol as compared to a lighter core column located in the center of the core.

The most notable difference in the thermal response of perimeter columns for Case A and Case B was for columns located in the middle of the south face of WTC 1. These perimeter columns show temperatures

of approximately 350 °C for Case A and temperatures of approximately 800 °C for Case B. Significantly

higher temperatures for Case B was due to fireproofing damage on the south face of WTC 1 as predicted

by the aircraft impact analysis. Note that at a temperature of 800 °C, the yield strength of the column is a small fraction of its room temperature value. As a result the cross-sectional area of the square symbol

representing a column that has reached a temperature of 800 °C is extremely small.

The thermal response of perimeter columns located on the north face, east face and west face was similar

for Case A and Case B. These columns do not heat up significantly because of lack of fireproofmg

damage. This was also obser\'ed in Figure 12-3, where the final thermal state of perimeter columns at 6000 s after impact has been shown for Case A and Case B.

Core column 501 shows significant heating in Case A and reaches a peak temperature of approximately

700 °C at 2000 s after impact. This column subsequently cools down to 400 °C at 6000 s after impact. For

Case B, core column 501 does not show significant heating and peak temperatures of 200 °C w ere

predicted. This difference in heating pattern is due to the fireproofmg status of the column in the two

cases. Column 501 was a \ er>' heaw comer column and the thermal response of this column could have a large impact on the strucmral stability of WTC 1. The thermal response of the other comer columns, 508,

1001 and 1008 was in general very similar for the two cases. Note that structural damage was not

predicted for any of the comer columns in Case A or Case B.

Column 502 shows significant heating at 2000 and 3000 s after impact in Case B. but only moderate

temperature were attained in Case A. This column had fireproofing damage in both cases, and the

differences in heating pattem was due to differences in fire activity in the vicinity of this column in the

Uvo cases. The aircraft impact analysis for Case B predicted more stmctural damage to core column as compared to Case A. Columns 503, 504 and 505 were damaged in Case B, while only core column 504 was damaged in Case A. The thermal response of core columns 506 and 507 was similar for Case A and

Case B.

NISTNCSTAR 1-5G. WTC Investigation 253
Chapter 12

Core coluirais on the east side of the core (Column 608 and 708) attain very high temperatures for Case B

and are relatively cool for Case A. The remaining columns on the east side of the core (Column 808 and

908) have similar thermal response for Case A and Case B.

FSI simulations predict similar temperatures for columns 1002, 1003, 1004, 1005 and 1006 at 1000, 2000

and 3000s after impact for Case A and Case B. Beyond 3000 s, these columns cool down for Case A, but

in Case B, these columns continue to stay hot. This difference in heating pattern is due to differences in

fire activity in the vicinity of these columns. Column 1007 remains relative cool during the entire duration

of the simulation for Case A. On the other hand column 1007 in Case B reaches very high temperatures of

800 °C, due to fireproofmg damage on this column. Differences in the thennal response of the 1000 series

columns between Case A and Case B was expected to have a large impact on the corresponding structural analysis and stability of WTC 1.

Core columns 701 , 801 and 901 on the west side of the core indicate similar thermal response for the two

cases. Column 601 has fireproofmg damage for Case A and no fireproofing damage for Case 6, which

explains the differences in the predicted thermal response.

Table 1 2-5 and Table 1 2-6 compare the predicted temperature range for perimeter columns for WTC 1 Case A and Case B. Similar comparison for core columns is shown in Table 12-7 and Table 12-8, and for floor trusses in Table 12-9 and Table 12-10. In each table, the minimum and maximum temperature are noted for floors 93-99 at five different instants in time. These tables clearly illustrate the differences in the

thermal response of WTC 1 for Case A and Case B. Floor trusses in WTC 1 were covered with an

equivalent fireproofmg thickness of 2.2" and the fireproofmg in some places was damaged extensively

due to the impact of the aircraft. Temperature range of 27 - 921 °C was predicted for floor trusses in WTC 1, Case A. Floor trusses on floor 95, 96 and 98 show significantly higher temperature in Case B as compared to Case A. Numerical simulations predict a gradual monotonic increase in maximum

temperature on floors 93, 94 and 99 in Case B. On floors 95-98, the maximum temperature is consistently above 700 °C (temperature at which steel looses most of its yield strength).

Overall, the perimeter and core column temperature depends on several important factors. They include:

1 . state of fireproofmg,

2. intensity and duration of fires in the vicinity of the column.

3. size and shape of the column.

In addition the thermal state of a column on a given floor can also depend on the structural and

fireproofmg damage status of the column above or below the floor under consideration, as well as the fire growth and spread pattern on those floors. The global simulation results indicate that the fireproofing

thickness and the damage to the fireproofing due to aircraft impact was the single most important

parameter that had the largest effect on the predicted thermal response of the structural elements in WTC 1.

254 NISTNCSTAR 1-5G, WTC Investigation
Summary of Technical Results

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NISTNCSTAR 1-5G, WTC Investigation 255
Chapter 12

256 NISTNCSTAR 1-5G, WTC Investigation
Summary of Technical Results

Table 12-5. Temperature range (°C) for perimeter columns on various floors at five

instants in time, WTC 1, Case A.

20 min 40 min 60 min 80 min 100 min

Floor 93 25-210 25 -278 25 -322 30-339 39-352

Floor 94 25-214 25 - 303 27-346 27 - 327 28 - 332

Floor 95 25-210 25 - 252 27-331 37-362 34 - 350

Floor 96 25 -225 25 -301 28 - 304 34-297 40 - 454

Floor 97 25-205 25-310 26-322 30-418 35 -473

Floor 98 25 -241 25 -331 28 - 344 32-352 46 - 320

Floor 99 25-210 25 - 305 28 - 340 36-332 45-316

Table 12-6. Temperature range (°C) for perimeter columns on various floors at five

instants in time, WTC 1, Case B.

20 min 40 min 60 min 80 min 100 min

Floor 93 25-213 25 -264 25 -328 30-337 44 - 360

Floor 94 25-355 26-292 27-334 27-369 29 - 448

Floor 95 26-582 25 - 706 29-758 36-669 46-717

Floor 96 26 - 464 26-599 28-661 34- 656 39 - 796

Floor 97 25-413 25-430 25 -539 26-553 35 - 786

Floor 98 25 - 247 25-315 27-352 34-361 45 - 351

Floor 99 25-179 25 -294 28-341 33-317 43 - 347

NISTNCSTAR 1-5G, WTC Investigation 257
Chapter 12

Table 12-7. Temperature range (°C) for core columns on various floors at five instants in

time, WTC 1, Case A.

20 min 40 min 60 min 80 min 100 min

Floor 93 27-65 27-98 28-121 28-156 28-198

Floor 94 27 - 832 27- 820 28 -788 28-749 28 - 590

Floor 95 27- 565 29-823 31-871 33 - 822 35 -696

Floor 96 27-913 27 - 888 28 -707 28 -687 29 - 748

Floor 97 27 -773 30-775 35-713 39-556 46-636

Floor 98 27-49 29-97 32-144 36-203 41 -252

Floor 99 27-50 29-81 31-119 33-148 36- 175

Table 12-8. Temperature range (°C) for core columns on various floors at five instants in

time, WTC 1, Case B.

20 min 40 min 60 min 80 min 100 min

Floor 93 27-63 27-108 27-126 27-150 28- 193

Floor 94 27- 870 27-738 28 -741 29-861 31 -904

Floor 95 28 -553 31-819 36-822 44 - 804 52 - 761

Floor 96 27 - 896 27-909 28-763 28 - 745 29 - 780

Floor 97 27 - 846 30 - 938 34-916 39-817 45 - 901

Floor 98 27-52 29-101 32-156 37-191 41 -227

Floor 99 27-49 29-84 31 - 129 36-161 41 - 192

258 NISTNCSTAR 1-5G, WTC Investigation
Summary of Technical Results

Table 12-9. Temperature range (°C) for floor trusses on various floors at five instants In time, WTC 1, Case A.

20 min 40 min 60 min 80 min 100 min

Floor 93 29-58 35-165 47-283 73-319 92-325

Floor 94 28-61 31 - 146 37-222 47-328 60 - 408

Floor 95 27- 882 30-653 33 -494 40 - 440 48 -389

Floor 96 28-875 36-782 46 - 734 61-516 79-427

Floor 97 28-921 33 -887 40 - 873 51-897 70 - 897

Floor 98 27-919 29-907 33 -706 39-468 45 - 469

Floor 99 28-51 32-129 41-265 58-406 79-449

Table 12-10. Temperature range (°C) for floor trusses on various floors at five instants in time, WTC 1, Case B. 20 rain 40 min 60 min 80 min 100 min

Floor 93 27-56 27-142 28-263 29-349 30-386

Floor 94 28-62 31 - 154 37-212 44-346 50-411

Floor 95 27-798 30-760 34 - 664 41-881 48-861

Floor 96 23-911 36-883 48-831 63 -839 82 - 902

Floor 97 27 -886 32-888 40-852 51-777 66-927

Floor 98 27-913 29-914 33 -896 41-876 51-926

Floor 99 28-54 32-143 41-275 57-420 78 -461

NISTNCSTAR 1-5G, WTC Investigation 259
Chapter 12

0 150 300 450 600 75 225 375 525 675

0 150 300 450 600

75 225 375 525 675

Figure 12-3. Comparison of the final thermal state of perimeter columns for WTC 1,

Case A, with WTC 1, Case B.

260 NISTNCSTAR 1-5G, WTC Investigation
Summary of Technical Results

12.3 GLOBAL THERMAL RESPONSE OF WTC 2 : COMPARISON OF CASE C

AND CASE D

Global thermal response of WTC 2 Case C and Case D were discussed on a floor by floor basis in Chapter 1 0 and Chapter 1 1 . For each case, the time evolving thermal state of the various structural elements such as floor trusses, core beams, concrete slabs perimeter and core columns was presented in a

visual format. In this section, a comparison is made between the global thermal results of WTC 2 Case C

and WTC 2 Case D.

Figure 1 2-4 and Figure 1 2-5 summarize the thermal response for WTC 2 Case C and WTC 2 Case D,

respectively. These figures are similar to Figure 12-1 and Figure 12-2 discussed earlier for WTC 1. Each

figure consists of six sub-figures. Each sub-figure shows all the core columns and every third perimeter

column. A column is represented by a square symbol. If a column is damaged at any location on a floor, it

is not included in the corresponding sub-figure. The color of the square symbol is related to the

temperature of the column. The temperature value for a column shown in the plot is the maximum

temperature of that column and could occur any where between floor 78 through floor 83. Temperature

range has been selected to lie between 0-800 °C. Each sub-figure shows the thermal results at a specific

instant in time. Results over a period ranging from 600 s to 3600 s at 600 s interval are presented.

The size of the square symbol representing a perimeter or a core column is proportional to the product of

the cross-sectional area of that column and its yield strength. The product of the cross-sectional area and

yield strength is a measure of the load carrying capacity of a column. The yield strength is a function of

temperamre, decreasing with increasing temperature (See Figure 3-2). As the temperature of the column

increases or decreases with time, the size of the symbol decreases or increases. Changes in the size of the

symbol indicate a change in the load carrying capacity of that column.

The size of the square symbol is related to the column cross-sectional area and since each column has a

different cross-sectional area (Appendix B), the size of the square symbols can be different from one column to another, even if all the columns are at room temperature. Thus at a point in time just before the

aircraft impact when all the columns are at room temperature, the load carrying capacity of the heavier

columns is more than those of light columns. The heavier colunms such as the comer core columns are therefore represented with a larger square symbol as compared to a lighter core column located in the center of the core.

Table 12-1 1 and Table 12-12 compare the predicted temperature range for perimeter columns for WTC 2

Case C and Case D. Similar comparison for core columns is shown in Table 12-13 and Table 12-14 and

for floor trusses in Table 12-15 and Table 12-16. In each table, the minimum and maximum temperature

are noted for floors 79-83 at five different instants in time. These tables clearly illustrate the differences in the thermal response of WTC 2 for Case C and Case D.

The thermal response of perimeter columns located on the east face of WTC 2 and in the north-east comer

is quite different in Case C as compared to Case D. For WTC 2 Case C, perimeter columns in the northeast comer heat up rapidly and reach a temperature of approximately 800 °C at 1200 s after impact. These

colunms stay at this high temperature for the duration of the simulation. Approximately half of the

perimeter columns on the east wall reach a peak temperature value of 800 °C. These columns have their fireproofing damaged due to the impact of the aircraft. The remaining columns on the east face reach a peak temperature of approximately 350-400 °C.

NISTNCSTAR 1-5G, WTC Investigation 261
Chapter 12

On the other hand, perimeter columns on the entire east wall of WTC 2 Case D, have their fireproofing

damaged and as a result quickly heat up to temperatures of the order of 800 °C at 1,800 s and 2,400 s after impact of the aircraft. Beyond 2,400 s, numerical simulations predict gradual cooling of these columns to the 400-500 °C range at 3600 s after impact. Similar cooling is predicted for perimeter columns in the

north-east comer for WTC 2 Case D. The fmal thermal state of perimeter columns on floor 78-83 are compared for WTC 2 Case C and Case D in Figure 12-6.

Structural damage to core columns is significantly more in Case D as compared to Case C. Both Case C

and Case D predict structural damage to column 1 00 1 and to other core columns in the south-east comer. The themial response of 500, 600, 700, 800 and 900 series core columns is similar for Case C and Case

D. Core columns on the west side of the core stay relatively cool due to lack of fireproofing damage on

these columns. Some differences in heating rates are observed for core columns 1005, 1006, and 1007.

Core column 1008 is a relatively heavy column and its themial response for Case C is similar to that for Case D. Although there is intense fire activity in the vicinity of column 1008, the column does not heat up

since its fireproofing is intact and since it is a relatively heavy column.

As for WTC 1 . the perimeter and core coluinn temperature for WTC 2 depends on the state of

fireproofing, intensity and duration of fires in the vicinity of the column and the size and shape of the

column. In addition the thermal state of a column on a given floor can also depend on the stmctural and

fireproofing damage status of the column above or below the floor under consideration, as well as the fire growth and spread pattern on those floors. The WTC 2 global simulation results indicate that the damage

to the fireproofing due to aircraft impact was the single most important parameter that had the largest effect on the predicted thermal response of the structural elements.

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264 NISTNCSTAR 1-5G, WTC Investigation
Summary of Technical Results

Table 12-11. Temperature range (°C) for perimeter columns on various floors at six

instants in time, WTC 2, Case C.

10 min 20 min 30 min 40 min 50 min 60 min

Floor 78 27- 143 27- 146 27- 173 27- 198 27 - 206 27 - 203

Floor 79 27- 143 27-189 27-229 27-262 27-266 27 - 264

Floor 80 27 - 566 27-585 27-514 27- 523 27-505 27 - 505

Floor 81 27-684 27- 809 27- 832 27 - 846 27- 855 27 - 861

Floor 82 25 - 663 25-796 25-816 25 - 828 25 - 836 25 - 841

Floor 83 27-435 27 - 702 27- 778 27-791 27-801 27 - 807

Table 12-12. Temperature range (°C) for perimeter columns on various floors at six

instants in time, WTC 2, Case D.

10 min 20 min 30 min 40 min 50 min 60 min

Floor 78 27 - 115 27- 165 27-217 27 - 243 27 - 227 27 - 242

Floor 79 27-117 27 - 21

27 - 270 27 - 295 27 - 305 27 - 300

Floor 80 27 - 325 27 - 393 27-401 27 -431 27 - 394 27 - 300

Floor 81 27 - 750 27 - 813 27 - 831 27 - 845 27 - 740 27 - 638

Floor 82 27 - 720 27 - 804 27 - 819 27 - 830 27 - 782 27 - 673

Floor 83 27 - 696 27 - 796 27-810 27 - 817 27 - 771 27 - 669

NISTNCSTAR 1-5G. WTC Investigation 265
Chapter 12

Table 12-13. Temperature range ("C) for core columns on various floors at six instants in

time, WTC 2, Case C.

10 min 20 min 30 min 40 min 50 mm 60 min

Floor 78 27 - 282 27-426 27-453 27-473 27-482 27 - 484

Floor 79 27 - 329 27-454 27-472 27-488 28-496 28 - 497

Floor 80 25 -267 27-325 27 - 322 27 - 313 27 - 307 28 - 303

Floor 81 27 - 317 27-411 27 - 464 27 - 516 27 - 563 27 - 603

Floor 82 27 - 256 27 - 493 27 - 569 27 - 582 27 - 638 27 - 687

Floor 83 27-31 27-49 27-70 27-88 27-103 27- 122

Table 12-14. Temperature range (°C) for core columns on various floors at six instants in

time, WTC 2, Case D.

10 min 20 min 30 min 40 min 50 min 60 min

Floor 78 27-288 27- 534 27- 574 27-555 27-511 27-482

Floor 79 27-318 27 -552 27-586 27-557 27 - 604 27 - 567

Floor 80 27-299 27 - 444 27-625 27-712 27 - 726 27 - 690

Floor 81 27-356 27 - 576 27 - 708 27- 791 27 - 767 27 - 703

Floor 82 27-319 27-590 27-766 27-817 27-726 27 - 666

Floor 83 27-32 27-49 27-72 27-98 27-125 27 - 146

266 NISTNCSTAR 1-5G, WTC Investigation
Summary of Technical Results

Table 12-15. Temperature range (°C) for floor trusses on various floors at six instants in

time, WTC 2, Case C.

10 min 20 min 30 min 40 min 50 min 60 min

Floor 79 27-385 28 - 194 28-184 29-173 29-163 30-154

Floor 80 28 - 825 29-790 30-620 31-583 33 -580 34 - 597

Floor 81 28-787 29 - 694 30-530 30 - 559 31-580 31-603

Floor 82 30- 834 35-910 42 - 898 48 - 900 53-913 58-911

Floor 83 30- 764 37- 891 43 - 905 50-911 58-912 65-914

Table 12-16. Temperature range (°C) for floor trusses on various floors at six instants in

time, WTC 2, Case D.

10 min 20 min 30 min 40 min 50 min 60 min

Floor 79 28 -348 28-432 29 - 549 30-610 30-606 31 -551

Floor 80 28 - 821 28- 859 30-860 3 1 - 647 32 - 606 34 - 636

Floor 81 28-654 28-656 29 - 763 29-778 30-860 32 - 856

Floor 82 28 - 865 30-916 33 -921 37-917 40-789 46-852

Floor 83 29-877 33-910 41 -917 46-913 52 - 899 58 - 820

NISTNCSTAR 1-5G, WTC Investigation 267
Chapter 12

0 150 300 450 600

75 225 375 525 675

D 150 300 450 600

75 225 375 525 675

Figure 12-6. Comparison of the final thermal state of perimeter columns for WTC 2,

Case C with WTC 2, Case D.

268 NISTNCSTAR 1-5G, WTC Investigation
Chapter 13

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Rehm, R.G., Pitts, W. M., Baum, H.R., Evans, D.D., Prasad, K., McGrattan, K.B., and Forney, G. P.,

Initial Model for Fires in the World Trade Center Tower, NISTIR 6879, 2005.

Siegel, R. and Howell, J. R.., Thermal Radiation Heat Transfer, Third Ed., Hemisphere, Washington,

D.Cpp. 699-722, (1992).

Sterner, E. and Wickstrom, U., TASEF - Temperature Analysis of Structures Exposed to Fire, SP Report

1990:05, Swedish National Testing and Research Institute, Boras, 1990.

Tien, C.L., Lee, K.Y., and Stretton, A. J., "Radiation Heat Transfer", The SFPE Handbook of Fire

Protection Engineering, Third, Ed., Nat. Fire Prot. Ass'n, Quincy, MA pp. 1-73 - 1-90, (2002).

Usmani, A. S., Rotter, J. M., Lamont, S., Sanad, A. M. and Gillie, M., Fundamental principles of

structural behavior under thermal effects. Fire Safety Journal, 36 (8): 721-744, 2001.

Usmani, A.S., Chung, Y.C. And Torero, J.L., "How did the WTC Towers Collapse: A New Theory," Fire

Safety Journal, vol. 38, no. 6, Oct. 2003, pp. 501-533.

Zhang, W., Carpenter, D.J., Roby, R.J., Viehe, D. and Hamer, A., "The Prediction of Thermal Loading

for Structural Analysis under Fire Exposure," Designing Structures for Fire, Society of Fire

Protection Engineers and Structural Engineering Institute, Sept. 30-Oct. 1, 2003, Baltimore, MD.

270 NISTNCSTAR 1-5G, WTC Investigation
Appendix A

Thermo-physical Properties

This appendix presents the thermo-physical properties of various materials used in the World Trade

Center Investigation. Thermo-physical properties include

1. Specific Heat

2. Density

3. Thermal Conductivity

The materials for which thermo-physical properties were needed include

1. A36 steel

2. A 242 steel

3. BLAZE-SHIELD DC/F

4. Monokote

5. Lightweight Concrete

6. Non material. This material was used to incorporate fireproofing damage. This material provides

very little resistance to heat flow.

Each thermo-physical property is in general a function of temperature. Plots are presented to indicate this variation of the property with temperature. The data were determined as part of the investigation and is reported in companion reports (NIST NCSTAR 1-3, NIST NCSTAR 1-6A, Phan 2003).

NISTNCSTAR 1-5G, WTC Investigation 271
Appendix A

SPECIFIC HEAT

a) A 36 Steel

- i 1 -

.: !

i

4^-

900

850

800

750-

700

650

600

550'

500

450

400 Pejjperatur© (Kelvin)

475 673 873 1073

373 573 773 973 Temperature (Kelvin)

o

CDuH

%AH

O

(U

C/3

1520

1440

1360

1280

1200

1120

1040

960

880

800

720

Blazeshield D C/F

A

1600

5550

M-iO

1280

1200

11^0

W40

"ifiO

S80

j73 473 673 873 1073 1275 1473 373 573 775 973 1173 1373 Temperature (Kelvin)

73 67; 'i'fi 1073 1273 1473 Vn TB 973 ll'B IJB Teirperature (Kelvin)

ices e) Liehtweieht Concrete

/"

1 .16

i.m

l.Ol'

f) Non Material

- -4-

_ „.

~ —r

1— —

\

reTTperature {Kelvin)

5.2} 773 1025 1273 Tenperature (Kelvin)

Figure A-1. Specific heats of various materials used in the World Trade Center

Investigation plotted as a function of temperature.

272 NISTNCSTAR 1-5G, WTC Investigation
Thermo-Physical Properties

DENSITY

C

oa

> a) A 36 Steel

*Xlt>

-:4k

425

.—. 4!X n

500

Teiperature (Kelvin)

8400

8300

8200

8100

8000

7900

1800 noo

1600

1500

1400

A 242 Steel

213 473 673 873 1013

313 573 773 913 Temperature (Kelvin)

420

400

380

360

340

320

300

280

260

240

273; 413 67- 373 i73 r:. Tenperature (Kelvin)

13 1273 1«1S 220

1173 1373

d 1 Monokote

"

-t1

213 413 613 813 1013 1213 1413

313 513 113 913 1113 1313 Tenperature (Kelvin)

i3iO

2130

05CQ

Tenperature (Kelvin)

el Liehtweight Concrete

..^ r

DFake Material I,

0-

Ci

1

i

»

Tenperature (Kelvin)

Figure A-2. Density of various materials used in the World Trade Center Investigation,

plotted as a function of temperature.

NISTNCSTAR 1-5G, WTC Investigation 273
Appendix A

THERMAL CONDUCTIVITY

i'ereperature (Kelvin)

Blazeshield D C/F

<

m

t.4S '

' SSe IHS TemperatiJre (Kelvin)

e^l Li ehtweieht Concrete

,,.

•'si TesRoerature (Kelvin

)

A 242 Steel

V

Tenperature (Kelvin)

d) Monokote p. A

-

/V

Temperature (Kelvin)

t) hake Material

1 —)-— -•

J

^.j. J ^

1

Temperature (Kelvin)

Figure A-3. Thermal conductivity of various materials used in the World Trade Center

Investigation plotted as a function of temperature.

274 NISTNCSTAR 1-5G, WTC Investigation
Thermo-Physical Properties

ENTHALPY

SeSOT i

n-is 14*; »'i -m in-s Terperarure (Kelvin) Tenperature (Kelvin)

Figure A-4. Enthalpy of fireproofing used in the World Trade Center Investigation plotted as a function of temperature.

REFERENCES

Phan, Long T.. 2003. "Properties of Concrete at Elevated Temperatures," NIST Reference Docviment,

Building and Fire Research Laboratory.

NISTNCSTAR 1-5G, WTC Investigation 275
Appendix A

This page intentionally left blank.

276 NISTNCSTAR 1-5G, WTC Investigation
Appendix B

Column Schedule

Table B-1. Exterior Column Type Schedule for WTC 1, Floor 92-99

Column # Floor 92 Floor 93 Floor 94 Floor 95 FInnr 96 FInnr 97 FInnr 9fi FInnr 99

101 120 120 120 120 120 120 120 120

103 120 120 120 120 120 120 120 120

106 123 122 122 122 121 121 121 120

109 123 123 123 122 122 122 121 121

112 124 124 123 123 23 121 121 121

115 125 123 123 123 122 122 122 120

118 124 124 124 122 122 122 121 121

121 124 124 123 123 123 121 121 121

124 124 123 123 123 122 122 122 120

127 124 124 124 122 122 122 121 121

130 125 125 123 123 123 122 122 122

133 125 125 125 123 123 123 122 122

136 126 124 124 124 123 123 123 121

139 125 125 124 124 124 122 122 122

142 124 124 124 123 123 123 121 121

145 125 123 123 123 122 122 122 120

148 124 124 123 123 123 121 121 121

151 123 123 123 122 122 122 121 121

154 123 122 122 122 121 121 121 120

157 120 120 120 120 120 120 120 120

201 120 120 120 120 120 120 120 120

203 120 120 120 120 120 120 120 120

206 120 120 120 120 120 120 120 120

209 120 120 120 120 120 120 120 120

212 120 120 120 120 120 120 120 120

215 120 120 120 120 120 120 120 120

218 120 120 120 120 120 120 120 120

NISTNCSTAR 1-5G, WTC Investigation 111
Appendix B

Column # Floor 92 Floor 93 Floor 94 Floor 95 Floor 96 Floor 97 Floor 98 Floor 99

221 120 120 120 120 120 120 120 120

224 120 120 120 120 120 120 120 120

227 120 120 120 120 120 120 120 120

230 120 120 120 120 120 120 120 120

233 120 120 120 120 120 120 120 120

236 120 120 120 120 120 120 120 120

239 120 120 120 120 120 120 120 120

242 120 120 120 120 120 120 120 120

245 120 120 120 120 120 120 120 120

248 120 120 120 120 120 120 120 120

251 120 120 120 120 120 120 120 120

254 120 120 120 120 120 120 120 120

257 120 120 120 120 120 120 120 120

301 120 120 120 120 120 120 120 120

303 120 120 120 120 120 120 120 120

306 123 122 122 122 121 121 121 120

309 123 123 123 122 122 122 121 121

312 124 124 123 123 123 121 121 121

315 125 123 123 123 122 122 122 120

318 124 124 124 122 122 122 121 121

321 124 124 123 123 123 121 121 121

324 125 123 123 123 122 122 122 120

327 124 124 124 122 122 122 121 121

330 125 125 123 123 123 122 122 122

333 125 125 125 123 123 123 122 122

336 126 124 124 124 123 123 123 121

339 125 125 124 124 124 122 122 122

342 124 124 124 123 123 123 121 121

345 125 123 123 123 122 122 122 120

348 124 124 123 123 123 121 121 121

351 123 123 123 122 122 122 121 121

354 123 122 122 122 121 121 121 120

278 NISTNCSTAR 1-5G, WTC Investigation
Column Schedule

Column # Floor 92 Floor 93 Floor 94 Floor 95 Floor 96 Floor 97 Floor 98 Floor 99

357 120 120 120 120 120 120 120 120

401 120 120 120 120 120 120 120 120

403 120 120 120 120 120 120 120 120

406 120 120 120 120 120 120 120 120

409 120 120 120 120 120 120 120 120

412 120 120 120 120 120 120 120 120

415 120 120 120 120 120 120 120 120

418 120 120 120 120 120 120 120 120

421 120 120 120 120 120 120 120 120

424 120 120 120 120 120 120 120 120

427 120 120 120 120 120 120 120 120

430 120 120 120 120 120 120 120 120

433 120 120 120 120 120 120 120 120

436 120 120 120 120 120 120 120 120

439 120 120 120 120 120 120 120 120

442 120 120 120 120 120 120 120 120

445 120 120 120 120 120 120 120 120

448 120 120 120 120 120 120 120 120

451 120 120 120 120 120 120 120 120

454 120 120 120 120 120 120 120 120

457 120 120 120 120 120 120 120 120

a. Column types and dimensions are described in detail in NIST NCSTAR 1-2A.

NISTNCSTAR 1-5G, WTC Investigation 279
Appendix B

Table B-2. Exterior column type schedule for WTC 2 Floor 78-83. Floor 78 Floor 79 Floor 80 Floor 81 Floor 82 Floor 83

101 126 126 124 124 123 123

103 126 124 124 124 123 123

106 129 129 129 127 127 127

109 131 131 129 129 129 128

112 132 129 129 129 129 129

115 132 132 132 129 129 129

118 132 132 129 129 129 128

121 132 129 129 129 129 129

124 132 132 132 129 129 129

127 132 132 129 129 129 128

130 132 130 130 130 129 129

133 132 132 129 129 129 129

136 132 132 132 129 129 129

139 132 130 130 130 129 129

142 132 132 129 129 129 128

145 132 132 132 129 129 129

148 132 129 129 129 129 129

151 131 131 129 129 129 128

154 129 129 129 127 127 127

157 126 124 124 124 123 123

201 124 122 122 122 122 121

203 121 120 120 120 120 120

206 122 122 122 120 120 120

209 122 122 120 120 120 120

212 123 121 121 121 120 120

215 125 125 125 123 123 123

218 126 126 124 124 124 123

221 126 124 124 124 123 123

224 125 125 125 123 123 123

227 125 125 123 123 123 122

230 125 124 124 124 123 123

280 NISTNCSTAR 1-5G, WTC Investigation
Column Schedule

Floor 78 Floor 79 Floor 80 Floor 81 Floor 82 Floor 83

233 125 125 123 123 123 122

236 125 125 125 123 123 123

239 126 124 124 124 123 123

242 126 126 124 124 124 123

245 125 125 125 123 123 123

248 123 121 121 121 120 120

251 122 122 120 120 120 120

254 122 122 122 120 120 120

257 121 120 120 120 120 120

301 126 126 124 124 123 123

303 126 124 124 124 123 123

306 129 129 129 127 127 127

309 131 131 129 129 129 128

312 132 129 129 129 129 129

315 132 132 132 129 129 129

318 132 132 129 129 129 128

321 132 129 129 129 129 129

324 132 132 132 129 129 129

327 132 132 129 129 129 128

330 132 130 130 130 129 129

333 132 132 129 129 129 129

336 132 132 132 129 129 129

339 132 130 130 130 129 129

342 132 132 129 129 129 128

345 132 132 132 129 129 129

348 132 129 129 129 129 129

351 131 131 129 129 129 128

354 129 129 129 127 127 127

357 126 124 124 124 123 123

401 124 122 122 122 122 121

403 121 120 120 120 120 120

NISTNCSTAR 1 -5G. WTC Investigation 281
Appendix B

Floor 78 Floor 79 Floor 80 Floor 81 Floor 82 Floor 83

406 122 122 122 120 120 120

409 122 122 120 120 120 120

412 123 121 121 121 120 120

415 125 125 125 123 123 123

418 126 126 124 124 124 123

421 126 124 124 124 123 123

424 125 125 125 123 123 123

427 125 125 123 123 123 122

430 125 124 124 124 123 123

433 125 125 123 123 123 122

436 125 125 125 123 123 123

439 126 124 124 124 123 123

442 126 126 124 124 124 123

445 125 125 125 123 123 123

448 123 121 121 121 120 120

451 122 122 120 120 120 120

454 122 122 122 120 120 120

457 121 120 120 120 120 120

282 NISTNCSTAR 1-5G, WTC Investigation
Column Schedule

Table B-3. Plate dimensions (m) for the various

exterior column types.

PLATE r PLATE 2^ PLATE 3' Column t1 -meters t2-meters t3-meters

120 0.0064 0.0064 0.0064

121 0.0079 0 0064 0.0064

122 0 0095 0.0064 0.0064

123 0.01 1

0.0064 0.0064

124 0.0127 0.0064 0.0064

125 0.0143 0.0064 0 0064

126 0.0159 0.0064 0.0064

127 0.0175 0.0064 0.0064

128 0.0191 0.0064 0.0064

129 0.0206 0.0079 0.0079

130 0.0222 0.0079 0.0079

131 0.0238 0.0079 0.0079

132 0.0254 0.0095 0.0095

a. Plate 1 is the flange component that are perpendicular to the building face.

b. Plate 2 is the outside web component parallel to the building face.

c. Plate 3 is the inside web component (adjacent to the spandrel) parallel to the building face.

NISTNCSTAR 1-5G, WTC Investigation 283
Appendix B

Ta ble B-4. Core Column Schedule for WTC 1 , Floor 92.

Column no. Shape

B

(width in.) D(Depth in.)

t1 (in.)t2

(in.)

t3

(in.) Rotation

501 14WF550 1

502 14WF370 0

503 14WF370 0

504 14WF342 0

505 14WF287 0

506 14WF398 0

507 14WF370 0

508 14WF605 1

601 12WF161 0

602 14VVF193 0

603 14WF202 0

604 12WF161 0

605 12WF161 0

606 14WF202 0

607 14WF184 0

608 12WF161 0

701 15 14 1.125 1.125 1 1

702 14WF237 0

703 14WF142 0

704 14WF68 1

705 14WF53 0

706 14WF142 0

707 14WF219 0

708 15 14 1.250 1.188 1 1

801 15 16 1.188 1.188 1 1

802 14WF264 0

803 15 17 0.813 0.813 1 1

804 12WF106 0

284 NISTNCSTAR 1-5G, WTC Investigation
Column Schedule

805 14WF136 0

806 14WF211 0

807 14WF211 0

901 12WF190 0

902 14WF202 0

903 14WF237 0

904 12WF133 0

905 12WF133 0

906 14WF184 0

907 14WF202 0

908 12WF161 0

1001 14WF605 1

1002 14WF370 0

1003 14WF455 0

1004 14WF287 0

1005 14WF287 0

1006 14WF426 0

1007 14WF370 0

1008 14WF605 1

0 indicates that the column web is oriented in a north-south direction

1 indicated that the column web is oriented in an east-west direction

NISTNCSTAR 1-5G, WTC Investigation 285
Appendix B

Table B-5. Core Column Schedule for WTC 1, Floors 93, 94 and 95

Column No. Shape

B

(width in.) D

(Depth in.)

t1 (in.)t2

(in.)

t3

(in.) Rotation

501 14WF500 1 502 14WF314 0

503 14WF314 0

504 14WF287 0

505 14WF264 0

506 14WF342 0

507 14WF314 0

508 14WF500 1

601 12WF133 0

602 14WF176 0

603 14WF184 0

604 14WF120 0

605 14WF133 0

606 14WF176 0

607 14WF167 0

608 12WF133 0

701 15 14 0.938 0.875 1 1 702 14WF211 0

703 14WF127 0

704 14WF61 1 705 14WF48 0

706 14WF142 0

707 14WF219 0

708 12WF190 0

801 15 16 1.063 1.063 1 1 802 14WF228 0

803 15 17 0.750 0.688 1 1

804 12WF92 0

286 NISTNCSTAR 1-5G, WTC Investigation
Column Schedule

Column No. Shape

B

(width in.) D

(Deoth in )

t1 fin )t2 Cm )t3

fin ) Rotation

805 14WF119 0

806 14WF184 0

807 14WF193 0

901 12WF161 0

902 14WF184 0

903 14WF211 0

904 12WF120 0

905 12WF120 0

906 14WF167 0

907 14WF176 0

908 12WF133 0

1001 14WF550 1

1002 14WF314 0

1003 14WF398 0

1004 14WF264 0

1005 14WF237 0

1006 14WF370 0

1007 14WF314 0

1008 14WF550 1

0 indicates that the column web is oriented in a north-south direction

1 indicated that the column web is oriented in an east-west direction

NISTNCSTAR 1-5G, WTC Investigation 287
Appendix B

Table B-6. Core Column Schedule for Floors 96 - 99.

Column no. Floor 96 Floor 96 Floor 97 Floor 97 Floor 98 Floor 98 Floor 99 Floor 99

Shape Rotation Shape Rotation Shape Rotation Shape Rotation

501 14WF426 1 14WF426 1 14WF426 1

A A \ Pi 1 f\ A 14WF342 1

502 14V\/F264 0 14\/\/F264 0 14WF264 0 14WF21

0

503 14WF264 0 14WF264 0 14WF264 0 14WF228 0

504 14WF246 0 14WF246 0 14WF246 0 14WF193 0

505 14WF219 0 14WF219 0 14WF219 0 14WF184 0

506 14WF287 0 14WF287 0 14WF287 0 14WF237 0

507 14WF264 0 14WF264 0 14WF264 0 14WF210

508 14WF426 1 14WF426 1 14WF426 1 14WF342 1

601 12WF106 0 12WF106 0 12WF106 0 12WF92 0

602 14\/VF150 0 14WF150 0 14\/VF150 0 14WF127 0

603 14WF158 0 14WF158 0 14WF158 0 14WF127 0

604 12WF106 0 12WF106 0 12WF106 0 12V\/F85 0

605 12WF120 0 12WF120 0 12WF120 0 12WF92 0

606 14WF150 0 14WF150 0 14WF150 0 14WF127 0

607 14WF142 0 14WF142 0 14WF142 0 14WF119 0

608 12WF106 0 12WF106 0 12WF106 0 12WF92 0

701 12WF161 0 12WF161 0 12V\/F161 0 12WF120 0

702 14WF176 0 14WF176 0 14WF176 0 14WF150 0

703 14WF103 0 14WF103 0 14WF103 0 14WF84 0

704 14WF53 1 14WF53 1 14WF53 1 14WF43 1

705 14WF43 0 14WF43 0 14WF43 0 14WF43 0

706 14WF111 0 14WF111 0 14WF111 0 14WF95 0

707 14WF167 0 14\/VF167 0 14WF167 0 14WF150 0

708 12WF190 0 12WF190 0 12WF190 0 12WF161 0

801 12WF161 0 12WF161 0 12WF161 0 12WF161 0

802 14WF176 0 14WF176 0 14WF176 0 14WF158 0

803 12WF133 0 12WF133 0 12WF133 0 12WF120 0

804 12WF79 0 12WF79 0 12WF79 0 12WF65 0

288 NISTNCSTAR 1-5G, WTC Investigation
Column Schedule

Column no. Floor 96 Floor 96 Floor 97 Floor 97 Floor 98 Floor 98 Floor 99 Floor 99

805 14WF111 0 14WF111 0 14WF111 0 14WF95 0

806 14WF167 0 14WF167 0 14WF167 0 14WF142 0

807 14WF167 0 14WF167 0 14WF167 0 14WF136 0

901 12WF120 0 12WF120 0 12WF120 0 12WF99 0

902 14WF150 0 14WF150 0 14WF150 0 14WF127 0

903 14WF193 0 14WF193 0 14WF193 0 14WF167 0

904 12WF92 0 12WF92 0 12WF92 0 12WF79 0

905 12WF99 0 12WF99 0 12WF99 0 12WF79 0

906 14WF142 0 14WF142 0 14WF142 0 14WF127 0

907 14WF150 0 14WF150 0 14WF150 0 14WF127 0

908 12WF120 0 12WF120 0 12WF120 0 12WF99 0

1001 14WF426 1 14WF426 1 14\A/F426 1 14WF370 1

1002 14WF264 0 14WF264 0 14WF264 0 14WF219 0

1003 14WF342 0 14WF342 0 14WF342 0 14WF287 0

1004 14WF219 0 14WF219 0 14WF219 0 14WF193 0

1005 14WF202 0 14WF202 0 14WF202 0 14WF176 0

1006 14WF314 0 14WF314 0 14WF314 0 14WF237 0

1007 14WF287 0 14WF287 0 14WF287 0 14WF228 0

1008 14WF426 1 14WF426 1 14WF426 1 14WF370 1

0. Indicates that the column web is oriented in a north-south direction.

1 . Indicated that the column web is oriented in an east-west direction.

NISTNCSTAR 1-5G. WTC Investigation 289
Appendix B

Table B-7. WTC 2 Core Column Schedule for floor 78-80.

Column no. Shape

B

(width in.) D

(Depth in.)

t1

(in.)

t2

(in.)

t3

(in.) w1

(in.) Rotation

501 22 34 2.25 2.25 1.5 1 502 22 28 1.69 1.69 1.5 0

503 22 28 1.63 1.63 1.5 0

504 22 28 1.31 1.31 1.5 0

505 18 28 1.38 1.38 1.5 0

506 22 28 1.81 1.81 1.5 0

507 22 28 1.63 1.63 1.5 0

508 22 34 2.25 2.25 1.5 1

601 14 36 0.81 0.81 1.5 0

602 16 36 0.94 0.88 1.5 0

603 16 36 0.81 0.81 1.5 0

604 14 36 0.81 0.81 1.5 0

605 14 36 0.81 0.81 1.5 0

606 16 36 0.88 0.81 1.5 0

607 16 36 0.88 0.88 1.5 0

608 14 36 0.81 0.81 1.5 0

701 15 14 1.88 1.81 1 0

702 14WF314 1 703 14WF193 1 704 14V\/F84 0

705 14WF68 1 706 14WF184 1 707 14WF287 1 708 15 14 1.75 1.69 1 0

801 15 16 1.44 1.44 1 0

802 14WF370 1 803 15 17 1 1 1 0

804 12WF161 1 805 14WF158 1

290 NISTNCSTAR 1-5G, WTC Investigation
Column Schedule

Column no. Shanp

B

^wiHth I VV 1 in ^ VJ LI 1 M 1 . 1 D t1 t2

(\n.)

t3 Cm \ w1

Rritatinn 806 14WF287 1 807 15 16 1.50 1.50 1 0

901 12WF190 0.31 14 1 902 14WF287 1 903 16 21 1.19 1.19 1.5 0

904 12WF190 1 905 14 14 1 1 1 0

906 14WF237 1 907 14WF287 1 908 12WF190 0.31 14 1

1001 22 22 3 2.94 1.5 1 1002 14WF500 1 1003 14WF665 1 1004 18 31 1.19 1.19 1.5 0

1005 22 28 1.06 1.06 1.5 0

1006 14WF605 1 1007 14WF500 1 1008 22 22 3 2.94 1.5 1

0. Indicates that the column web is oriented in an east-west direction.

1 . Indicates that the column is oriented in a north-south direction.

NISTNCSTAR 1-5G, WTC Investigation 291
Appendix B

Fable B-8. WTC 2 Core Column Schedule for floor iM-83.

Column no. Shape

B

(width in.) D(Depth in.)

t1 (in.)t2

(in.)

t3

(in.) w1(in.) Rotation

501 22 28 2.50 2.50 1.5 1 502 22 25 1.69 1.63 1.5 0

503 22 25 1.69 1.63 1.5 0

504 22 25 1.38 1.31 1.5 0

505 18 25 1.44 1.38 1.5 0

506 22 25 1.81 1.75 1.5 0

507 22 25 1.63 1.56 1.5 0

508 22 28 2.50 2.50 1.5 1

601 12WF190 0.31 14 1 602 14WF264 1 603 14WF264 1 604 12WF190 1 605 12WF190 0.31 14 1 606 14WF287 1 607 14WF246 1 608 12WF190 0.31 14 1

701 15 14 1.63 1.63 1 0

702 14WF314 1 703 14WF193 1 704 14WF84 0

705 14WF68 1 706 14WF184 1 707 14WF314 1 708 15 14 1.75 1.75 1 0

801 15 16 1.50 1.50 1 0

802 14WF314 1 803 15 17 1.06 1.00 1 0

804 12WF161 1 805 14WF158 1 806 14WF287 1

292 X.

' NISTNCSTAR 1-5G, WTC Investigation
Column Schedule

Column no. Shape

B

(width in.) D

(Deoth in )

t1 (in V"))t2

(\n )

t3

(in ) V'"-/

w1

(in ) Rotation

807 15 16 1.50 1.50 1 0

901 12WF190 0.31 14 1 902 14WF287 1 903 14WF314 1 904 12WF190 1 905 14 14 1 1 1 0

906 14WF237 1 907 14WF287 1 908 12WF190 0.31 14 1

1001 14WF730 0

1002 14WF500 1 1003 14WF665 1 1004 18 28 1.31 1.25 1.5 0

1005 22 25 1.19 1.19 1.5 0

1006 14WF605 1 1007 14WF500 1 1008 14WF730 0

0. Indicates that the column web is oriented in an east-west direction.

1 . Indicates that the column is oriented in a north-south direction.

NISTNCSTAR 1-5G. WTC Investigation 293
Appendix B

Table B-9. Types and locations of sprayed i fire-resistive material on focus floors.

Building Component Material

Thickness (in.)

Specified'' Installed Used in Analysis''

FLOOR SYSTEM

Original

Main trusses and diagonal struts BLAZE-SHIELD DC/F 0.5 0.75 0.6

Bridging trusses (one-way zone)*" BLAZE-SHIELD DC/F 0.5 0.38'' 0.3

Bridging trusses (two-way zone)' BLAZE-SHIELD DC/F 0.5 0.38^ 0.6 Upgraded

Main trusses BLAZE-SHIELD 11 1.5 2.5 2.2 Main truss diagonal struts BLAZE-SHIELD II 1.5 2.5 2.2

Bridging trusses BLAZE-SHIELD II 1.5 2.5 2.2 EXTERIOR WALL PANEL

Box columns

Exterior face BLAZE-SHIELD DC/F 1 3/16 (e) 1.2

Interior face Venniculite plaster 7/8 (e) 0.8

Spandrels

Exterior face BLAZE-SHIELD DC/F 0.5 (e) 0.5

Interior face Venniculite plaster 0.5 (e) 0.5 CORE COLUMNS

Wide flange columns

Light BLAZE-SHIELD DC/F 2 3/16 (e) 2.2 Heavy BLAZE-SHIELD DC/F 1 3/16 (e) 1.2 Box columns

Light BLAZE-SHIELD DC/F (f) (e) Heavy BLAZE-SHIELD DC/F (0 (e) 1.2"*^' CORE BEAMS BLAZE-SHIELD DC/F 0.5 (e) 0.5

a. "Specified" means material and thicknesses determined from correspondence among various parties.

b. The analysis is described in NIST NCSTAR 1-6A.

c. Not expressly specified. SFRM was required for the areas where the main trusses ran in both directions and, while not required. was also applied in the areas where they ran in one direction only.

d. Analysis of photographs indicated that the thickness was approximately one half that on the main trusses.

e. Not able to determine.

f. Not specified.

g. Thickness assumed equal to wide flange columns of comparable weight per foot.

294 NIST NCSTAR 1-5G, WTC Investigation