Therese McAllister, Ph.D., P.E.

Building and Fire Research Laborato

National Institute of Standards and Techn

U.S. Department of Commerce

Therese.McAllister@nist.gov

Federal Building and Fire Safety Inves

of the World Trade Center Disast

Project 6: WTC 7 Structural Fire 

and Collapse Analysis 

June 22, 2004
Objectives Objectives

To determine the structural response of WTC 7 to d

internal fires and to identify the most probable str

mechanism.

 Task 1: Structural Response Analysis to Identify C Task 2: Structural Analysis of Possible Collapse In Task 3: Evaluation of Collapse Hypotheses
Task 1: Structural Response Analy

Task 1: Structural Response Analy

Identify Critical Components Identify Critical Components

Develop a nonlinear global structural model of WTC 

performance under design gravity loads

Identify credible failure sequences using the structu

the effect of component failures on the structura

Identify dominant failure modes for critical compone

for service loads and elevated structural tempera

Conduct parametric studies of critical subsystems to 

parameters

Develop approaches to simplify structural analyses 

and analyses 
Tasks 2 and 3: Structural Analysis 

Tasks 2 and 3: Structural Analysis 

Initiation Hypotheses Initiation Hypotheses

Refine the global model of WTC 7 to support nonline

analysis for building regions affected by fire

Analyze three (3) collapse initiation sequences selecConduct parametric studies of the global analyses to 

parameters

Identify the most probable collapse initiation sequenc

 Time-sequence of events Mode of failure or capacity reduction for each 

the sequence and associated temperatures

 Load redistribution during the sequence of eve Agreement between analysis and observed pe
Working Collapse Hypothesis Working Collapse Hypothesis

If it remains viable upon further analysis, the working colla

suggests that it was a classic progressive collapse, inc

An Initiating Event 

 An initial local failure at the lower floors (below Flo

building due to fire and/or debris induced structuracritical column (the initiating event), which supportfloor bay with an area of about 2,000 ft

2

A Vertical Progression at the East Side of the Building 

 Vertical progression of the initial local failure up to 

penthouse, as large floor bays were unable to redibringing down the interior structure below the east 

A Subsequent Horizontal Progression from the East to the 

 Horizontal progression of the failure across the low

region of Floors 5 and 7, that were much thicker thfloors), triggered by damage due to the vertical fai

Global Collapse 

 Events resulted in a disproportionate collapse of th
Building Description Building Description

Major Features Relative to the Working Collaps

Framing Differences Above/Below Floor Framing Relative to Con Ed SubstationTransfer System Between Floors 5 and 7Floor Plan and Built Up ColumnsRoof Layout
WTC 7 Framing WTC 7 Framing

Perimeter Moment Frame

Bra(Gr

Perimeter Belt Trusses (5th to 7th and 22nd to 24th)

7th

1st

22nd

47 Stories

Regular

Irregular

BrEa(G
WTC 7 and Con Edison Substation WTC 7 and Con Edison Substation

PB

Con-Edison Substation

WTC 7 3rd floor

North Face of BAbove

N

Promenade

Below 7th floor, braced frame constructed over Con E
Transfer System Between Floors 5 

Transfer System Between Floors 5 

Truss#1

Truss#2

Cantilever

Girders

Truss

Gir

Con-Edison Substation
7th Floor Plan Floor Plan – Built-up Columns up Columns

7th Floor Plan

Built-up Column Locations

Built-up Column

Non Builtup Column

Built-up C

Built-up C
Roof Layout Roof Layout

57 56 55 45 53 52 51 50 49 48 47 4

58 61 64 67 70 73

59

62 65 68 71 74

7677 

60 63 66 69 72 75 78 

WEST SCREEN WALL PENTHOUSE

COOLING TOWERS BELOW 

N 

WTC 7 was modified:

•Penthouse structures added to the roof•East half of floors 41 and 43 were removed and repla•HVAC and communication systems were added from •Fuel tanks and backup generators were added
Building Conditions Building Conditions

Data for building conditions has been expanded and 

through photographs, video, and interview record

 Debris Impact from WTC 1 and WTC 2 Observed and Reported Fire Locations and T External Signs of Collapse
Debris Damage from WTC 2 Debris Damage from WTC 2

After WTC 2 collapsed: 

 Some south face glass broken at lower floors Dust covered lobby areas at floors 1 and 3 Power on in building, phones working No fires observed
Debris Damage from WTC 1 Debris Damage from WTC 1

After WTC 1 collapsed: 

 Heavy debris on Vesey Street and WTC 7 Pr No heavy debris observed in lobby area, whit

 SW Corner Damage – floors 8 to 18 South face damage between two exterior colu

down 5 to 10 floors, extent not known

 South Face Damage –

• middle 1/4 -1/3 width south face, 10th floor to g

• large debris hole near center around 14th floor

• 1/4 width south face, above 5th floor, atrium gla

• 8th / 9th floor from inside, visible south wall gon

damage to west, 2 elevator cars dislodged into 
WTC 7 

Roof After WTC 1 Collapse Collapse

M
WTC 7 

SW Corner After WTC 1 Collapse Collapse
Vesey Street by Pedestrian Bridge Street by Pedestrian Bridge 

After WTC 1 Collapse After WTC 1 Collapse

Pede
Vesey Street by Promenade Street by Promenade 

After WTC 1 Collapse After WTC 1 Collapse

Third Story Promena

Standing with Debr

on Top Pedestrian Bridge
Estimated Extent of Debris Damag

Estimated Extent of Debris Damag

58 61 64 67 70 73

59

62 65 68 71 74

76 

80

60

N 63 66 69 72 75 81

TRUSS #1

TRUSS #3

78

77

TRUSS #2

78A

Cooling Towers 

Screenwall West Penthous

e

Approximate Region of Impact Damage by Large WTC1 Debris

Roof and Upper Level Debris Damage

Southwest Corner Debris Damage
Observed Fire Locations ( Observed Fire Locations (11:30

-2:3

2:3

General

 No diesel smells reported from the exterior, stairwe No signs of fire or smoke below floor 

6 from stairwe

 Fire reported at west wall of floor 

7 around 12:15 p

 In east stairwell, smoke was observed near floors 1

observed on floor 23

Looking from southwest corner to the south 

 Fire in SW corner near floors 10 or 11 Fire on floors 6, 7, 8, 21, 30 Multiple fires observed on floors numbered 20’s an Heavy black smoke coming out of south face gash

Looking from southeast corner to the south 

 Fire on floor 12;1 area above covered with smoke

 Fire on floors 11-121 moved to east face and progr

1 fires reported on floor 14, but photographs showed eas
Observed Fires Observed Fires

East Face Fires on Floors 11-12 near 2 PM 

North Face Fires on Flo
First 

Observation Observation of Collapse of Collapse 

East Penthouse Kink 

at Center of Roofline
2001 CBS News Archives Video Re

2001 CBS News Archives Video Re

Collapse from the North Collapse from the North

Insert video clip.
Failure Sequence Timeline Failure Sequence Timeline

Global collapse occurs as windows fail be

0.3 8.2 around column 55 

0.1 7.9 West penthouse submerged

Façade kink formed along column 46-47

0.5 7.7 East end of roof starts to move

0.2 7.2 West end of roof starts to move 

Movement of entire north face of WTC7 (v

3.0 7.0 North side of west penthouse moves

1.8 4.0 Windows break along column 46 at floors 

0.5 2.2 East penthouse completely submerged

0.4 1.7 3 windows break at floors 41 to 44

East penthouse submerged from view (no

0.3 1.2 4 windows fail at floor 40

2 windows at floor 40 fail between column

0.9 0.9 East penthouse kinks between columns 40.0 0.0 Movement of east penthouse roofline

Observation from CNN Net Dub 7 47.avi

Total Time 

(sec)

Time Interval 

(sec)
Structural Analysis Structural Analysis

Structural Response and Collapse Hypotheses is be

with the following models and analyses:

 Global structural analysis 

• design gravity and wind loads• service loads and debris damage

 Kinematic analysis of possible failure sequen Composite floor system analysis of larger spa Submodel of lowest 10 floors for analysis of f

mechanisms

 Thermal-structural analysis of critical compon
Working Collapse Hypothesis Working Collapse Hypothesis

A working collapse hypothesis has been developed 

 An initiating event  Vertical progression on the east side (northea Horizontal progression from east to west Global collapse

Initiating Event Vertical 

Collapse 

Progression

Horizontal 

Collapse 

Progression
Initiating Event & Vertical Progress

Initiating Event & Vertical Progress

Initiating Event

 First exterior sign of failure was at the east pe

aligned with interior columns 79, 80, and 81. initiating events include the failure of these co

Vertical Progression

 Columns 79, 80, and 81 supported large tribu

floor spans of approximately 50 ft. Failure of 81 would likely result in failure at the floor-coland would progress vertically up to the east p
Initiating Event: Thermal Initiating Event: Thermal

-Structural Resp

Structural Resp

Critical Components Critical Components

Column 79 at Floors 5-7

Possible Modes of 

 Squashing (Y Cover Plate W Failure of Co

Thermal Structural 

 Uniform and 

Temperatures

 Damaged and  Temperature 

Properties

 Thermal Softe Axial Therma Bowing from 

Temperature Distribution

P

W14 x 730

Cover plate

0.5-in Welds

Web plate

W14 x 730

0.313-in 

Welds

Columns 79 and 81 at Floors 5-7
Status of Initiating Event Analysis Status of Initiating Event Analysis

NIST continues to evaluate the factors that could have ca

80, or 81 to fail

Possible contributing factors include:

 Damage to components adjacent to truss #2 from  Damage to fireproofing from normal activities prior 

damage

 Unusual fuel loads (fuel lines, high density of build

Analysis to date indicates:

 Massive size of columns 79, 80, and 81 appears to 

fires and/or damaged fireproofing to initiate therma
Vertical Progression: Floor Respo

Vertical Progression: Floor Respo

of Column Support of Column Support

Col 79

Beam Hinges 

Æ

Å Slab Force

Col 80

Beam Hinges 

Æ

Å Slab Force

Col 81

Beam Hinges 

Æ

Å Slab Force
Initiating Event Initiating Event

& Vertical & Vertical Progression Progression

Exterior Observations:

 East Penthouse Kink  Window Breakage

East Penthouse Scree

Su

48th Fl40th Fl

30th Fl

13th Fl

VePr

5th Fl7th Fl

Fai
Horizontal Progression & Global C

Horizontal Progression & Global C

Horizontal Progression

 A vertical failure would pile debris on the east side 

damaging or severing transfer girders and trusses and 7.

 This secondary damage has been postulated to ca

progression of failure in the core columns at or nea

Global Collapse

 The global collapse occurred with few external sig

to have occurred with the failure of core columns
Truss #1 and Truss #2 Locations Truss #1 and Truss #2 Locations

N

6

5 68 71

74 77

Truss #2

78A

Truss #1

78

Truss 

#3

Braced 

Frames

Cantilever Transfers
Horizontal Progression Horizontal Progression 

– Truss #1 Truss #1

79 76 73 70 67 

5th

7th

9th

Colu

in Be

Truss #1

Splices Fail

in Bending
Horizontal Progression Horizontal Progression 

– Truss #2 Truss #2

80 77 74 71 68 

5th

7th9th

Truss #2

Columin Be
Horizontal Horizontal 

Collapse Collapse Progression Progression

Exterior Observations:

 East Penthouse Sinks  Further Window 

Breakage

 No Movement for 5 sec

East Penthouse 

Sinks into Building

Subs

48th Fl

40th Fl

30th Fl

13th Fl

10th Fl

5th Fl

7th Fl

Sky 
Global Collapse Global Collapse

Exterior Observations:

 Center and West 

Penthouses Sink 

 North Façade Kink at 

Column 76

 Window Breakage in 

Front of Truss #3

48th Fl

40th Fl

30th Fl

13th Fl

10th Fl

5th Fl

7th Fl

Global 

Collapse

Remainder of 

Building Sinks
Status

A Working Collapse Hypothesis has been The hypothesis is consistent with visual oComprehensive analysis is ongoing to co

remainder of Tasks 1, 2, and 3.