John Gross, Ph.D., P.E. and Therese McAllister, 

Acknowledgements: Monica Starnes, Ph.D., Fahim Sade

William Fritz, Ph.D., Kuldeep Prasad, Ph.D.

Building and Fire Research Laborato

National Institute of Standards and Techn

U.S. Department of Commerce

John.Gross@nist.gov

Therese.McAllister@nist.gov

Federal Building and Fire Safety Inves

of the World Trade Center Disast

Project 6: Structural Fire Respo

Collapse Analysis 

December 2, 2003
Objectives Objectives

To determine the structural response of the WTC To

impact and internal fires and to identify the most structural collapse mechanisms.

 Task 1: Components and Subsystems

 Task 2: Global Analysis without Impact Dama Task 3: Global Analysis with Impact Damage Task 4: Evaluation of Collapse Hypotheses

To determine the structural response of WTC 7 to d

internal fires and to identify the most probable strmechanism.

 Task 1: Component, Subsystem and Global A Task 2: Global Analysis of Collapse Hypothes Task 3: Evaluation of Collapse Hypotheses
WTC Towers WTC Towers
Task 1: Components and Subsyst

Task 1: Components and Subsyst

Develop detailed nonlinear structural models of the 

exterior wall section and evaluate performance foand elevated structural temperatures. 

 Single exterior panel section for strength und

 Floor section behavior for strength under ther

(80-in wide section)

 Truss seat connection for strength under ther

 Full floor models of mechanical floor and typi Multi-panel exterior wall section with connect

Identify dominant failure modes and parameters tha

influence the analysis results for critical componesubsystems.

Develop approaches to simplify structural analyses 

modeling and analyses.
Task 2: Global Analysis Without Im

Task 2: Global Analysis Without Im

Determine the structural response to large fires with

damage

 Develop global model of one tower without im

nonlinear analysis of building regions affecte

 Analyze the structural response to ASTM sta

one to three (1-3) representative building fire 

Project 5. 

Improve WTC Tower models for analysis with impa

 Identify parameters that strongly influence an

 Develop approaches to simplify models for g
Task 3: Global Analysis With Impa

Task 3: Global Analysis With Impa

Develop global model of each WTC tower with impa

nonlinear analysis of building regions affected by 

Analyze three to five (3-5) building fire scenarios pro

for each tower and determine: 

 Time-sequence of events, 

 Mode of failure or capacity reduction for each 

the sequence and associated temperatures, 

 Load redistribution during the sequence of ev Agreement between analysis and observed p

Conduct parametric studies of the global analyses to 

influential parameters.

Repeat analyses for final fire scenarios. Identify mos

collapse initiation sequence. 
Task 4: Evaluation of Collapse Hyp

Task 4: Evaluation of Collapse Hyp

Identify candidate hypotheses for initiation and 

collapse.

Evaluate hypotheses for collapse initiation and 

including the role played by columns, floors, and hat truss.

Estimate variability of probable collapse initiatio

propagation mechanisms.

Identify most probable structural collapse sequ
Examples of Key Factors in Collaps

Examples of Key Factors in Collaps

Hypotheses Hypotheses

 Thermal sagging, expansion of floor system Loss of lateral support to columns  Load redistribution through hat truss Buckling/yielding of columns Core column failure in tension Failure of connections (e.g., truss seats, exte

bolts...)

 Thermal effects (condition of fireproofing, the

cooling cycle)
Status (1) Status (1)

Contract awarded to Simpson Gumpertz & 

of Waltham, Massachusetts to develop csubsystem and global models and to constructural analyses

 Contract awarded October 30, 2003 Kickoff meeting held November 24, 2003

Fourth ASTM E119 test has been added to 

 35 ft span assembly, thermally restrained, 3⁄4- 35 ft span assembly, thermally unrestrained,  17 ft span assembly, thermally restrained, 3⁄4- 17 ft span assembly, thermally restrained, 1⁄2-
Status (2) Status (2)

For Task 1, work is underway (reported sub

develop finite element models of componsubsystems for nonlinear thermal-structu

 Single exterior panel section  Floor section behavior for strength (80-in wide  Truss seat connection for strength

For Task 2, work is underway to develop fin

models of floor systems and entire tower nonlinear thermal-structural analyses 

 Translate Reference Models to ANSYS Verify translated models against Reference M Refine models for thermal-structural analysis
Status (3) Status (3)

For Task 4, work is underway (reported sub

conduct simplified analyses:

a. Effect of thermal expansion of floor systemb. Effect of loss of floors on column failurec. Effect of fire and fireproofing condition on tim

steel columns 

d. Load redistribution due to failure of column i
Task 1 Progress: Detailed Models 

Task 1 Progress: Detailed Models 

Components and Subsystems Components and Subsystems

NIST has developed detailed finite ele

models of

 Exterior Column Panel Floor Assembly (80-in section width) Seated Truss Connection

And is evaluating performance under 

elevated temperature
Task 1 Progress: Exterior Column 

Task 1 Progress: Exterior Column 

Buckling Behavior Buckling Behavior

Comput

 Ge Ma

Prelimin

 Buc

percol
Task 1 Progress: Floor Section Be

Task 1 Progress: Floor Section Be

Computer 

 Geom Mater Unifor

servic

Prelimina

 Buckli

restra

 Effect 

• Unif• Rep

 Effect 

• Unr• Full• Par

stiff
Task 1 Progress: Seated Truss Co

Task 1 Progress: Seated Truss Co

Computer 

 Geome Materi

Preliminary 

 Vertica

bolt pu
Floor Truss System and Exterior Co

Floor Truss System and Exterior Co

Bolted cFloor truStrap at

Exterior 

Floor tr
Floor Truss System and Exterior Co

Floor Truss System and Exterior Co
Task 4a Progress: Simplified Assessmen

Task 4a Progress: Simplified Assessmen

Modes for Floor and Exterior Column Sys

Modes for Floor and Exterior Column Sy

Thermal expansion due to fire

Possible failure of connection of floor system to columns in tension

Possible buckling of floor system

P

B

P

T

P
Lateral Bracing of Exterior Column

Lateral Bracing of Exterior Column

Visual evidence that floor-to-exterior column co

 Possibilities: lateral capacity, vertical capacit

exceeded

©2001. Allen Murabayashi
Task 4a Progress: Lateral Bracing 

Task 4a Progress: Lateral Bracing 

Exterior Column (P Exterior Column (P

T

)

40 in

8

Strap

ConcreteAnchor

Gusset Plate Stud

Truss Top Chord

Plan View

Strap AttachmentPlate
Task 4a Progress: Lateral Bracing 

Task 4a Progress: Lateral Bracing 

Columns (P Columns (P

T

)

PT2

PT3 Failure by

Concrete breakout

Seat

Gusset Plate

PT1

PT = PT1 + PT2 + PT3
Task 4a Progress: Buckling of Floo

Task 4a Progress: Buckling of Floo

System (P System (P

B

)

Analytical (Euler) Solution

 Sensitive to end condition 

assumptions

ANSYS Computer Solution

 80 in wide panel (by symmetry) Geometric nonlinearities included Ambient temperature Preliminary results within bounds 

of simplified analytical solutions
Task 4a Progress: In Task 4a Progress: In

-Plane Failure 

Plane Failure 

Interim Findings of Simplified Analy

Interim Findings of Simplified Analy

Relative capacities (weakest to strongest) a

temperatures under lateral loads

 Exterior column yielding (P

Y

)

 Floor-to-column connection failure (P

T

)

 Floor system buckling (P

B

)

Capacities change with elevated temperatu

order does not change.

P

B

P

T

P

Y

P

Y
Task 4b Progress: Simplified Analy

Task 4b Progress: Simplified Analy

Column Failure Column Failure

Failure of core and exterior columns

 Effect of floor removal Effect of elevated

temperatures

 Increased loads

L

FCR

F

Y

Squash Inelastic Elastic (Euler)

0 15 30 120 -150 200

Decreasing FCR with

Increasing L
Task 4b Progress: Effect of Floor R

Task 4b Progress: Effect of Floor R

Column Failure Column Failure 

– A Typical Examp

A Typical Examp

0

102030405060

012

Number of Flo

Fcr (ksi) 

Room Temperat

600 oC

Average for Exterior ColumnsWTC 1 North Face Floor 95 underService Loads
Task 4c Progress: Estimated Time

Task 4c Progress: Estimated Time

Columns to Reach 600 Columns to Reach 600 

o

C

Typical Core Column12WF133 Floor 92-95(2-3/16-in fireproofing)

Bare Steel

Uniform Thickness

20% Variable 

Thickness

01234

700 9

Gas T

Hours to Reach 60 Hours to Reach 600 oC

Bare 

One Face

Bare Steel

Time>4 hrs

Gas Tempe

One Face Bare Steel
Task 4d Progress: Simplified Analy

Task 4d Progress: Simplified Analy

Redistribution Effects on Column 

Redistribution Effects on Column 

Loads from severed or damaged columns are redistribute

to the remaining columns. Columns will continue to redistribute loads as they fail during fires

As column loads increase, less damage (fire and loss of 

floors) is needed to fail columns

Removed Column Support

0

102030405060

012345

Number of Floors Removed

Fcr (ksi) 

Increasing 

Loads

Room 

Temperature

600 oC
WTC 7
Objectives Objectives

To determine the structural response of WTC 7 to d

internal fires and to identify the most probable str

mechanism.

 Task 1: Component, Subsystem and Global A

 Task 2: Global Analysis of Collapse Hypothes Task 3: Evaluation of Collapse Hypotheses
WTC 7 Structural Features WTC 7 Structural Features
Significant Features of WTC 7 Significant Features of WTC 7

Differences to note from the WTC Towers

 Lack of evidence of 

significant damage to structural, fireproofing, or fuel supply systems 

 Fuel distribution 

systems were located between floors 1 to 9

©2001. Steven Sp
Significant Features of WTC 7 Significant Features of WTC 7

Floor 5 is a 14-in slab reinforced with T-beamsFloor 7 is an 8-in reinforced slab
WTC 7 Transfer Trusses WTC 7 Transfer Trusses

Transfer trusses between floors 5 to 7 were large s(600-1000 plf)
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 selec

Conduct 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
Status (1) Status (1)

Contract has been awarded to Gilsanz, Murray, and 

of New York City to develop component, subsystmodels and to conduct thermal-structural analyse

 Contract awarded October 27, 2003 Kickoff Meeting November 7, 2003

Data collection

 Final design drawings  Complete set of specifications Design and service loads Mechanical properties of steel (Project 3) Fireproofing materials (Projects 5 and 6) Fuel distribution system, tanks, and generato
Status (2) Status (2)

Structural temperature time-histories for standard fires (e.g., ASTM E-119 and ASTM E-1529) and representative fires (Project 5). Observables for collapse hypotheses (Project 5) 

 Initial damage to WTC 7 

structure from collapse of towers

 Timeline and locations of fire and 

smoke

 Timeline and location of exterior 

signs of collapse (penthouse, windows, fault-line, etc) ©2001. Nicolas Cianc
Structural Fire Response and Colla

Structural Fire Response and Colla

Analysis of WTC 7 Analysis of WTC 7

Expected Results 

 Determine the structural response of compon

to fires that contributed to the collapse of the 

 Determine the effect of component failures on 

redistribution and sequence of failures in the 

 Estimate the margin of safety for each critical 

function of time.

 Identify the most probable collapse sequence 

with the observed failure sequence in video a

records and other available observations.