John L. Gross, PhD, P.E. 

Therese P. McAllister, PhD, P.E.

Building and Fire Research Labora

National Institute of Standards and Techno

U.S. Department of Commerce

Federal Building and Fire Safety Inves

of the World Trade Center Disast

National Construction Safety T

Advisory Committee Meetin

Project 6 – Structural Fire Resp

Collapse Analysis

October 19, 2004
Project 6 Staff Project 6 Staff

NISTJohn GrossTherese McAllister Monica Starnes Long Phan Expert ContractorsEduardo KauselTeng & AssociatesDaniele VenezianoKaspar Willam

Simpson Gumpertz HeMehdi ZarghameeGlen BellAtis LiepensFrank KanRon HamburgerSaid BolourchiPedro SifreComputer Aided EnginSubcontractorPeter BarrettMichael Bak
Disclaimer Disclaimer

Certain commercial entities, equipment, producand non-commercial entities are identified in thin order to describe a procedure or concept adetrace the history of the procedures and practiceidentification is not intended to imply recommenendorsement, or implication that the entities, prmaterials, or equipment are necessarily the besthe purpose. Nor does such identification imply fault or negligence by the National Institute of STechnology.
Objectives Objectives

To determine the structural response of the WT

aircraft impact and internal fires and to identprobable structural collapse mechanisms.

 Task 1: Components and Subsystems Task 2: Global Analysis with Impact Damage Task 3: Evaluation of Collapse Hypotheses Task 4: Global Analysis without Impact Dama
Probabilistic Approach to Evaluate 

Probabilistic Approach to Evaluate 

in Global Capacity in Global Capacity

Global Reserve Capacity RC(t)

tco

RC(t

c)=0

Aircraft 

Impact

Capacity

Before 

Impact

RC(0)

Fire Growth and 

Structural Response

Capacity

After 

Impact

Events with sign

contribution to ch

global capac

Variability in 

analyses

0 10 20 30 ....
Analysis of Probable Collapse Seq

Analysis of Probable Collapse Seq

NIST developed and used a comprehensive approathe probable collapse sequences, from aircraft impainitiation. The approach:

 Combined mathematical modeling, well-establis

probability-based analysis methods, laboratory eanalysis of photographic and videographic evide

 Allowed for evaluation and comparison of possib

sequences based on different damage states, fistructural load redistribution paths.

 Accounted for variations in models, input param

and observed events.
Critical Analysis Inter Critical Analysis Inter

-Dependencie

Dependencie

Colla

Reference 

Structural 

Models 

SAP 2000 

SAP to ANSYS 

Conversion

SAP to LS-DYNA 

Conversion

Compartment Damage

Debris and Fuel 

Distribution

SFRM Damage

Structural 

Damage

Te

Tem

Resolution1-4 in.10-6

s

Aircraft Impact 

Damage

LS-DYNA

Fir

The

(FSI) 

Struc

and F

A

Time scale: 10 orders of magnitudeLength scale: 5 orders of magnitude

Baseline Performance Analysis
Observables from Photos, Videos, a

Observables from Photos, Videos, a

Aircraft Impact

 Impact damage to perimeter wall Engine exit location and speed Exit areas for debris Global stability after impact Aircraft impact conditions -velocity, location, orientation to  Stairwell damage

Fire/Thermal Analysis

 Fire in windows vs. location and time Smoke out windows vs. location and time Window breakage vs. location and time Metallurgical measurements of recovered steel

Structural Response

 Floors Draped in Windows Perimeter Column Bowing Rotation of Building above Impact Zone Time to Collapse
Inward Bowing of Perimeter Colum

Inward Bowing of Perimeter Colum

Minutes Prior to Collapse: WTC 1 S

Minutes Prior to Collapse: WTC 1 S
Inward Bowing of Perimeter Colum

Inward Bowing of Perimeter Colum

Minutes Prior to Collapse: WTC 2 E

Minutes Prior to Collapse: WTC 2 
WTC 1 Collapse WTC 1 Collapse

© 2001 Tim Main 

and Mike Ballou

Initiation of global collapse was first observed by the tilting of building sections above the impact regions of both WTC towers.WTC 1 tilted to the south in this view from the northeast.
WTC 2 Collapse WTC 2 Collapse

WTC 2 tilted to thtwisted in a counthis view from the 

© 2001 Luigi Cazzaniga© 2001 Luigi Cazzaniga
Determining the Probable Collapse 

Determining the Probable Collapse 

• Conduct extensive sensitivity analyses to determine most influential fac• Determine three sets of most influential factors for each analysis step: 

case, less severe case.

• The set of most influential factors is highly correlated for the different a

not independent). For example, the more severe case of aircraft impacmore severe case of fire dynamics, and they in turn lead to the more se

analysis, and together they led to the more severe case of structural re

• The first analysis sequence considers the set of factors for the realistic 

• A second analysis sequence is conducted to confirm the results for the 

• If the results for the realistic case suggest the possibility of more d

fire, the second analysis sequence considers the set of factors for each of the steps.

• If the results for the realistic case suggest the possibility of less da

the second analysis sequence considers the set of factors for the 

the steps.

• The analysis sequence is repeated with additional cases for the set of f

probable collapse sequence that best matches the observations.
Sensitivity Studies to Identify Influe

Sensitivity Studies to Identify Influe

Variables Variables

Numerical experiments with an Orthogonal Factorial Design were conducted for detailed models of components and subparameters that strongly influenced the analysis results:

 Only parameters whose values were not known with certainty we

(parameters that were known with certainty were set to the know

 Selected parameters were varied within a range of likely values: 

Realistic (0), More Severe (+)

 OFD approach allowed for identification of influential parameters 

of analysis runs

Examples of OFD Experiments:

 Wing Component Without Fuel Impacting Exterior Panel - 13 par

 Engine Impact on Structural Subassembly - 11 parameters

 Fire Growth and Spread in Compartments - 5 parameters

 Thermal Insulation Thickness - 5 parameters Floor System and Perimeter Wall Failure Criteria – 3 parameters 
Full Analysis Tree for Infl Full Analysis Tree for Influential Paramete

uential Paramet

Tower before Impact

FDS FDS FDS FSI FSI FSI

FSI FSI FSI

FSI FSI FSI

FDS FDS FDSFSI FSI FSI

FSI FSI FSI

FSI FSI FSI

FDFS

F

Realistic Impact Damage Combustibles, partitionsFireproofingStructural

MCFiSt

Less Severe Damage Combustibles, partitionsFireproofingStructural

(-) (0) 

(-) (0) (+) (-) (0) (+) (-

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)

STSTST

SSS

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

STRSTRSTR

(-)

(+)

(0)

(-)

(+)

(0)

(-)

(+)

(0)
Influential Parameters Determined from S

Influential Parameters Determined from S

Studies StudiesHighly Correlated to Aircraft Damage

 Aircraft Impact

• Aircraft Velocity and Location• Failure Criteria for High Strain Rates• Weights of Aircraft and Furnishings

 Fire Dynamics

• Combustible Load • Core Ventilation• Rubble

 Thermal Analysis/FSI

• SFRM damage

Independent of Aircraft Damage

 Structural Response to Temperature Dependent M

• Yield Strength• Modulus of Elasticity • Ultimate Strength
Bounding Computational Analyses Bounding Computational Analyses

Bounding computational analyses of subsystem

systems, for specific response behaviors thato be captured in the final analyses, provided insight into the collapse sequence.

These insights along with the sensitivity studies 

significant reduction of the number of scenaranalyzed from the full analysis tree.
Pruned Analysis Tree for Influentia

Pruned Analysis Tree for Influentia

Effects Effects

Tower before Impact

FDS

Realistic Impact DamageCombustibles, partitionsFireproofingStructural

Less Severe DamageCombustibles, partitionsFireproofingStructural

FDSSTR STR

FSI

STR

FSI 

STR 

1 

3a 

STR

Influential 

Parameters 

Highly 

Correlated by 

Aircraft Impact

Results

Influential 

Parameters

Independent 

of Aircraft

Impact Results 2 3a 3b
Leading Hypothesis for Collapse o

Leading Hypothesis for Collapse o

The following chronological sequence of major events led to the e1; specific load redistribution paths and damage scenarios are bethe probable collapse sequence for WTC 1: 

 Aircraft impact damage to perimeter columns, mainly on the 

redistribution of column loads, mostly to the adjacent perimelesser extent to the core columns.

 After breaching the building’s perimeter, the aircraft continue

building, damaging floor framing, core columns, and fireproo

damaged columns were redistributed to other intact core and mostly via the floor systems and to a lesser extent via the ha

 The subsequent fires, influenced by the impact damaged con

• Softened the core columns and caused them to shorten

displacement of the core relative to the perimeter which 

pulling the perimeter columns inward, and (2) transferriperimeter columns.

• Softened the perimeter columns on the South face and 

column loads to increase significantly due to restrained 
Leading Hypothesis for Collapse o

Leading Hypothesis for Collapse o

 Due to the combined effects of heating on the core and perim

perimeter wall bowed inward and highly stressed sections bu

 The section of the building above the impact zone began tilti

bowed South perimeter columns buckled. The instability rapihorizontally across the entire South face and then across the faces.

 The change in potential energy due to the downward movem

above the buckled columns exceeded the strain energy that 

the structure. Global collapse then ensued.
Leading Hypothesis for Collapse o

Leading Hypothesis for Collapse o

The following chronological sequence of major events led to the ev2; specific load redistribution paths and damage scenarios are beinthe probable collapse sequence for WTC 2: 

 Aircraft impact damage to perimeter columns mainly on the S

redistribution of column loads, mostly to the adjacent perimetlesser extent to the core columns.

 After breaching the building’s perimeter, the aircraft continued 

building, damaging floor framing, core columns, and fireproofdamaged columns were redistributed to other intact core and mostly via the floor systems and to a lesser extent via the hat 

 The subsequent fires, influenced by the impact damaged con

• Caused significant sagging of floors on the East side and 

pull the perimeter columns inward on the East face.

• Softened the core columns on the East side and caused 

transferred significant additional load to the perimeter coprimarily through the floor system and to a lesser extent 

• Softened some of the perimeter columns that were expo

towards the northern half of the East face.
Leading Hypothesis for Collapse o

Leading Hypothesis for Collapse o

 Due to the additional loads on the perimeter columns on the E

pulling of those columns, the East perimeter wall bowed inwasections buckled.

 The section of the building above the impact zone began tiltin

as both the East perimeter columns and the impact-damaged columns buckled. The instability rapidly progressed horizontaacross the North face.

 The change in potential energy due to the downward moveme

above the buckled columns exceeded the strain energy that cstructure. Global collapse then ensued.
Aircraft Impact Damage Aircraft Impact Damage to WTC 1 to WTC 1

Floor and Wall DaFireproofing and PartitionsFloors

More Severe More Severe Realistic Realistic
Aircraft Impact Damage Aircraft Impact Damage to WTC 2 to WTC 2

Realistic Realistic More Severe More Severe

Floor and Wall DaFireproofing and PartitionsFloors
Displaced Shape of WTC 1 After Air

Displaced Shape of WTC 1 After Air

(10x)

N Plane i
Axial Stress in North Face Compon

Axial Stress in North Face Compon

Aircraft Impact in WTC1 (10x) Aircraft Impact in WTC1 (10x)

North face looking from North
Stresses in Other Faces After Aircr

Stresses in Other Faces After Aircr

WTC1 (10x) WTC1 (10x)

Combined axial and bending stresses are: below 28 ksi (DCR 

≤ 0.3) in 

the South face  below 26 ksi (DCR 

≤ 0.2 ) in 

the East and West faces 

West Face 
Axial Stress in Core & Hat Truss M

Axial Stress in Core & Hat Truss M

Aircraft Impact Aircraft Impact

N
Axial Load Totals of Columns in W

Axial Load Totals of Columns in W

Between Floors 98 and 99 Between Floors 98 and 99

Σ = 38,300 kip

Change

=

-

1.7

%

Σ

Σ

=

7,

560

kip

N

• Compression is positive

Σ

=

10

,170 kip

Σ

=

10

,350 kip

Σ

Ch

ange

=

2.6%

Σ

=

7,

760

kip

Σ = 37,300 kip

Change = 2.7 %

Before aircraft impactRight after aircraft impact
Column Demand/Capacity Ratios B

Column Demand/Capacity Ratios B

Aircraft Impact in WTC 1 (Floors 93 

Aircraft Impact in WTC 1 (Floors 93 

501 

1001 

N

= 1.0

0.20

0.17

0.20

0.44 0.4

0.50
Column Demand/Capacity Ratios A

Column Demand/Capacity Ratios A

Impact in WTC 1 (Floors 93 to 98) Impact in WTC 1 (Floors 93 to 98)

501 

1001 

N

= 1.0

1.03

0.17

0.20 0.34

0.77

0.45 0.4
Findings for Global Analysis With Imp

Findings for Global Analysis With Imp

Structural Response to Aircraft Impac

Structural Response to Aircraft Impac

Structural and passive fire protection damage

 WTC 1 had 39 perimeter and 3 to 6 severed core colum WTC 1 damage was primarily centered through the nor

the core, and into the south floor area 

 WTC 2 had 29 perimeter and 5 to 10 severed core colu

 WTC 2 damage occurred primarily on the east side of t

The analysis shows that WTC 1 did not collapse following 

was observed

 Loads redistributed to adjacent core columns and East 

walls 

 Primary load redistribution path was the floor system, a

the hat truss

The analysis shows that WTC 2 did not collapse following 

was observed

 Loads redistributed to adjacent core columns and E

perimeter walls

 Primary load redistribution path was the floor system

extent by the hat truss on the east and south sides
Structural Response to Fires Subse

Structural Response to Fires Subse

Impact Damage Impact Damage

These key structural response are presented in 

order: 

Core column shorteningPerimeter column bowing and bucklingFloors sagging and pulling
Column Shortening in WTC 1 Meas

Column Shortening in WTC 1 Meas

of Plastic of Plastic

-to

-Elastic Strain (Ductility

Elastic Strain (Ductility

N

5011001 

12.8 3.1 0.9 5.8 2.9 0

10.6 6.4 0.8 0.2 2.1 310.4 10.5 1.6 0.4 1.8 0.3 

11.6 14.5 0.5 11.1 0.4 

1.0 31.9 0.4 5.0 21.0 1.

0.2 22.7 23.0 22.0 29.6 0

Creep and nonlinear buckling will increase cor
Buckling of Core Column 906 at Room Te

Buckling of Core Column 906 at Room Te

Due to Displacement Due to Displacement

-Induced Collapse A

Induced Collapse A

0

200000400000600000800000

100000012000001400000160000018000002000000

0 10 20 30 40 50 60 

Single Column Force vs. Displacement

Column Vertical Displacement, in.

Column Load, lbs
Findings for Global Analysis With Impa

Findings for Global Analysis With Impa

Structural Response of Core Columns 

Structural Response of Core Columns 

Combination of fireproofing damage and fires resulted in col

to 700 °C after 10 to 20 minutes of exposure.

At 500 °C to 700 °C, columns softened and shortened, throug

sufficiently to redistribute their loads to adjacent core cocolumns primarily through the floors. 

A large number of columns, more than half, redistributed the

shortening to remaining core columns. 

As the number of shortened core columns increased, the cor

downward relative to the perimeter, and the truss floors (horizontal pulling and vertical loads) to the perimeter co

Core column response to fire and damage can be generally d

 WTC 1 columns softened, leading to the greatest co

the south side. 

 WTC 2 columns softened from east towards the cor

column shortening greatest on the east side
Perimeter Column Bowing in the South 

Perimeter Column Bowing in the South 

Floor 96

© 2001. New Yo

Departm

All rights re

Enhancement
Perimeter Wall System Under Typic

Perimeter Wall System Under Typic

Scenarios Scenarios

A Perimeter Wall Section was 

analyzed for two load conditions under typical fire scenarios:

 Lateral floor loads pulled 

inward on the wall until column buckling occurred

 Vertical loads pushed down 

on the upper wall section until column buckling occurred

Y X

Z

Vertical

Loads
0

100200300400500600700800900

0 600 1200 1800 2400 3000 

Time (second

Max. Temperature (

o

C)

Perimeter Wall System and Typical 

Perimeter Wall System and Typical 

Loading Condition Loading Condition

Y X

Z

158

150

Floor 91

Floor 99
Swall.avi

Gravity, Thermal and Pushdown Lo

Gravity, Thermal and Pushdown Lo

Floors Unrestrained by Floor Slabs Floors Unrestrained by Floor Slabs
19uy2.avi

Gravity and Thermal Load with 3 F

Gravity and Thermal Load with 3 F

Pulling Inward Pulling Inward

Y

Z

X

10X magnification
Inward column bowing was observed on the south f

east face of WTC 2 approximately 5 to 10 minutcollapse.

This behavior has been modeled with component an

analyses with the combined effects of 

 elevated temperatures,  additional loads transferred to the perimeter 

 inward pulling by floors.

The perimeter columns were loaded to 1/5 of their c

impact and were capable of large load transfers columns via the floors and hat truss 

Findings for Global Analysis With Imp

Findings for Global Analysis With Imp

Structural Response of Perimeter Colu

Structural Response of Perimeter Colu
Full Floor Model Full Floor Model

1

X Y

Z

WTC1 FL96 

SEP 3 2004

17:56:39

ELEMENTSSEC NUM

1

X Y

Z

WTC1 FL96 

ELEMENTSSEC NUM

With Slab Witho
Structural Damage to WTC 1 Floor 

Structural Damage to WTC 1 Floor 

Realistic Impact Analysis Realistic Impact Analysis

1

X Y

Z

WTC1 FL96 

SEP 2 2004

17:49:34

ELEMENTSTYPE NUM

Floor trusses with damage Core wi

North 

1

X Y

Z

WTC1 FL96 

ELEMENTSTYPE NUM
Temperature Distributions for Botto

Temperature Distributions for Bott

WTC 1 Floor 96 for Realistic Fires WTC 1 Floor 96 for Realistic Fires

600 s 600

1

XZ Y

0

100

200

300

400

500

600

700

800

900

950

SEP 3 2004

10:43:29

ELEMENTSTEMPERATURESTMIN=20TMAX=773.702

N

E 1

XZ Y

0

100

200

300

ELEMENTSTEMPERATURESTMIN=20TMAX=939.691
Vertical Displacement for WTC 1 Fl

Vertical Displacement for WTC 1 Fl

Realistic Fires Realistic Fires

1

MN

MX

X Y

Z

WTC1 FL96 - Base Line Temperature at 600 sec 

-23.31

-20.264

-17.219

-14.173

-11.127

-8.081

-5.036

-1.99

1.056

4.101

SEP 3 2004

17:57:02

NODAL SOLUTIONSTEP=2SUB =20TIME=600UZ (AVG)RSYS=0DMX =23.321SMN =-23.31SMX =4.101

600 s

Maximum vertical displacement = 23 in.

North

South West

East

1

MX

X Y

Z

WTC1 FL96 - Base Line Temperature at 60

-6.684

-5.477

-4.27

-3.064

NODAL SOLUTIONSTEP=11SUB =13TIME=6000UZ (AVG)RSYS=0DMX =7.314SMN =-6.684SMX =4.177

South 

East

60

Maximum vertical 
1

XZ Y

0

100

200

300

400

500

600

700

800

900

950

SEP 15 2004

19:53:29

ELEMENTSTEMPERATURESTMIN=20TMAX=771.928

600 s

N

E

S

Floor 96

1

XZ Y

0

100

200

300

4

ELEMENTSTEMPERATURESTMIN=20TMAX=941.11

6000 

S

Temperature Distributions for Bott

Temperature Distributions for Bott

of WTC 1 Floor 96 for More Severe 

of WTC 1 Floor 96 for More Severe 
600 s

Maximum vertical displacement = 28.6 in.

1

MN

MX

X Y

Z

WTC1 FL96 - Maximum Damage Case Temperature at 600 sec 

-28.551

-24.773

-20.994

-17.216

-13.437

-9.659

-5.88

-2.102

1.677

5.455

SEP 27 2004

09:12:35

NODAL SOLUTIONSTEP=2SUB =52TIME=600UZ (AVG)RSYS=0DMX =28.566SMN =-28.551SMX =5.455

North

South West

East

Vertical Displacement for WTC 1 Fl

Vertical Displacement for WTC 1 Fl

More Severe Fires More Severe Fires

6

Maximum vertical 

1

MN

MX

X Y

Z

WTC1 FL96 - Maximum Damage Case Temper

-22.431

-19.594

-16.756

-13.918

NODAL SOLUTIONSTEP=11SUB =8TIME=6000UZ (AVG)RSYS=0DMX =22.529SMN =-22.431SMX =3.108

South 

East
600 s

Floor 96

Temperature Distributions for Botto

Temperature Distributions for Bott

of WTC 2 Floor 81 for Realistic Fire

of WTC 2 Floor 81 for Realistic Fire

1

X Y

Z

0

100

200

300

400

500

600

700

800

900

950

SEP 12 2004

22:49:42

ELEMENTSTEMPERATURESTMIN=-24.371TMAX=787.126

North

South

West

East

1

X Y

Z

0

100

200

300

4

ELEMENTSTEMPERATURESTMIN=-24.371TMAX=787.126

North

East
Vertical Displacement for WTC 2 Flo

Vertical Displacement for WTC 2 Flo

Realistic Fires Realistic Fires

1

MN

MX

X Y

Z

WTC2 FL81 - Baseline Temperature at 600 sec 

-24.411

-21.579

-18.747

-15.915

-13.083

-10.251

-7.419

-4.587

-1.755

1.076

SEP 28 2004

20:00:00

NODAL SOLUTIONSTEP=2SUB =29TIME=600UZ (AVG)RSYS=0DMX =24.422SMN =-24.411SMX =1.076

600 s

Maximum vertical displacement = 24 in.

North

South

West

East

1

MX

X Y

Z

WTC2 FL81 - Baseline Temperature at 360

-30.768

-27.075

-23.382

-19.689

NODAL SOLUTIONSTEP=7SUB =8TIME=3600UZ (AVG)RSYS=0DMX =30.789SMN =-30.768SMX =2.467

North

East

36

Maximum vertical 
Truss temperatures exceeded 500 °C to 600 

fireproofing damage and fire exposure oc

Trusses were observed to cool as fires mareas, whereas concrete slab temperaturrise.

WTC 1 Floor 96

 Damage to truss fireproofing was extensive near th

the realistic fires, the maximum floor deflection wamore severe fires the maximum floor deflection wa

 Damage to truss fireproofing on the south side was 

the realistic fires, the maximum floor deflection wamore severe fires the maximum floor deflection wa

 As slab temperatures rose for both fire cases, the 

order of 2 to 4 in., and pushed outward on the peri

Findings for Global Analysis With Impact 

Findings for Global Analysis With Impact 

Structural Response of Floors to Fire (1) Structural Response of Floors to Fire (1)
WTC 2 Floor 81

 Damage to truss fireproofing was extensive near 

and across the east side. The maximum floor deffor realistic fires. 

 As slab temperatures rose, the slab expanded on 

in., and pushed outward on the perimeter columnwas the southeast corner where failed truss conn

led to the floor system pulling inward on the perim

The truss floor system provided a load redistrib

between the core columns and perimeter codisplacement relative to each other increasethe capacity of the seated connections.

Findings for Global Analysis With Impact 

Findings for Global Analysis With Impact 

Structural Response of Floor System to F

Structural Response of Floor System to F
Example Analysis of Load Transfer Betwe

Example Analysis of Load Transfer Betwe

Perimeter Columns Perimeter Columns

 An additional horizontal force of 

7 to 8 kip pulled inward on the perimeter columns near the center of the face.

1

MX

X Y

Z

-12.403

-11.018

-9.633

-8.2

NODAL SOLUTIONSTEP=1SUB =2TIME=1UZ (AVG)RSYS=0DMX =12.403SMN =-12.403 A 12 in. downward displacement of SMX =.060412

core columns was imposed, producing the following results: 

 An additional vertical force of 7 to 8 kip on the perim

the center of the face. 

 A downward displacement of the core columns resul

pulling and vertical forces acting on the perimeter wato produce substantial forces at and above the impac
Final Analyses Currently Under Wa

Final Analyses Currently Under Wa

Phenomena governing stability (buckling) of columns 

 Large plastic and creep deformations (squash)

 Kinking induced by localized plastic deformations

Stability analysis approaches for compressive bucklin

columns

 Model actual geometrically and materially nonlinear 

behavior at the component and subsystem level

 Equivalent plastic strain to failure criterion for colum

global analysis combined with break elements

 Calibrate the equivalent plastic strain-to-failure criter

results from the detailed component and subsystem 

Complete analyses for WTC 1 and WTC 2 with above 

improvements

Global analysis without damage
Global Analysis Without Impact Dam

Global Analysis Without Impact Da

Determine the structural response of the WTC tower

without impact damage.

Based on what has been learned from Tasks 2 and 3

working hypothesis is: 

 A WTC tower with upgraded fireproofing on the floo

not have experienced significant heat-induced defolikely that burnout would have occurred without col

 A WTC tower with originally applied fireproofing in 

experienced some heat-induced deformations of this likely that burnout would have occurred without c

NIST expects to refine this working hypothesis based 

be completed soon.
Issues – Structural Systems Perfor

Structural Systems Perfor

Failure Analysis Failure Analysis

Availability of explicit standards, code provisions, methodo

design tools, and practical design guidance for designinresist progressive collapse in the event of abnormal loa

 Multihazard systems approach to structural design Coordination of ongoing federal and private sector 

Availability of analytical methodologies for prediction of co

phenomena of structural systems under abnormal load

 Proper identification of failure phenomenon to be a Physics-based/phenomenological models and expe Robust tools for routine analysis of such failures Magnitude of the problem that can be solved on ex

computational platforms
Issues – Fire Safety Design of Struc

Fire Safety Design of Struc

Availability of standards, codes, methodology, analyt

and practical design guidance to permit consideridesign condition for the structure as a whole syste

 Lack of standard methodology for evaluating t

vulnerability

 Creation of broad training opportunities for rig

computational fire dynamics and thermo-structools

 Method for rating the fire resistance of structu

barriers, for realistic design-basis fire scenario with furnace conditions)

 Structural principles education for fire protectio

fire protection principles education for structur
Issues – Passive Systems for Fire P

Passive Systems for Fire P

Availability of regulations that would adopt code provisions 

“structural frame” approach to fire resistance ratings, wstructural members (i.e., girders, beams, trusses, and sdirect connection to the columns and bracing members gravity loads) to be fire protected to the same rating as 

 Required by IBC 2000 Under consideration for adoption by NFPA 5000

Applied passive fire protection (such as spray-applied fire-

conformance to conditions in actual or equivalent tests fire resistance rating of the building component or asse

 Durability-related properties (under in-service expo

acceptance and quality control

 Inspection of fire-resistive materials after installatio

and electrical systems

 Criteria for required average thickness based on va

thickness

 Test and procedure to predict service life that acco

conditions (e.g., temperature, humidity)
Issues – General General

Availability of regulatory requirements for retention o

related to the design, construction, operation, mamodifications of buildings, including retention offs

 Maintenance and storage of documents Accessibility of building plans for emergency respo