National Institute of Standards and Techno

Technology Administration

U.S. Department of Commerce

NIST Response to the World Trade Center Di

Federal Building and Fire Safety Investiga

of 

the World Trade Center Disaster

Part IIB – Collapse Sequence

April 5, 2005


Determining the Probable Collapse Sequ

Determining the Probable Collapse Seq

• Performed extensive sensitivity analyses to determine mo

factors for each analysis step.

• Determined three sets of values for the parameters most 

the aircraft damage and the progress of the fires.

• Performed three aircraft impact and fire dynamics analyse

the expected aircraft and fire parameter sets that provide levels of damage.

• Conducted thermal and structural subsystem analyses for 

that reasonably matched observed impact damage and firprogression.

• Completed global structural collapse analyses for the cas

matched observed structural collapse data.
Progress in Determining Collapse Sequ

Progress in Determining Collapse Sequ

• Possible Collapse Hypotheses (May 2003) – not buildi

specific; key events not identified

• Working Collapse Hypothesis (June 2004) – single hyp

for both WTC towers; identifies chronological sequence oevents

• Leading Collapse Hypotheses (October 2004) – separa

hypothesis for each WTC tower; identifies building-specif

redistribution paths and damage scenarios in addition to chronological sequence of major events

• Probable Collapse Sequences (April 2005) – refined bu

specific collapse sequences with chronological sequence events, load redistribution paths, and damage scenarios.
Probable Collapse Sequence for WTC 1 Probable Collapse Sequence for WTC 1

1. Aircraft Impact Damage:

• Aircraft impact severed a number of exterior columns on the North wall from floo

the wall section above the impact zone moved downward. 

• After breaching the building’s perimeter, the aircraft continued to penetrate into 

severing floor framing and core columns at the North side of the core. Core col

damaged toward the center of the core and, to a limited extent on the South sidFireproofing was damaged from the impact area to the South perimeter wall, pr

the center of WTC 1 and at least over a third to a half of the core width.

• Aircraft impact severed a single exterior panel at the center of the South wall be

and 96.

• The impact damage to the exterior walls and to the core resulted in redistributio

column loads, mostly to the columns adjacent to the impact zones. The hat trus

downward movement of the North wall, and rotated about the East-West axis. 

• As a result of the aircraft impact damage, the North and South walls each carrie

percent less gravity loads after impact, and the East and West walls each carrie

percent more loads. The core carried about 1 percent more gravity loads after 
Probable Collapse Sequence for WTC 1 

Probable Collapse Sequence for WTC 1 

2. Effects of Subsequent Fires and Impact Damaged Fireproofing:

A. Thermal Weakening of the Core:

• The undamaged core columns developed high plastic and creep strains ove

building stood, since both temperatures and stresses were high in the core aand creep strains exceeded thermal expansion in the core columns. 

• The shortening of the core columns (due to plasticity and creep) was resiste

truss which unloaded the core over time and redistributed loads to perimeter 

• As a result of the thermal weakening (and subsequent to impact and prior to 

of the South wall), the North and South walls each carried about 10 percent loads, and the East and West walls each carried about 25 percent more load

carried about 20 percent less gravity loads after thermal weakening. 

B. Thermal Weakening of the Floors:

• Floors 95 to 99 weakened with increasing temperatures over time on the lon

and sagged. The floors sagged first and then contracted due to cooling on th

fires reached the South side later, the floors sagged, and the seat connectio

• Floor sagging induced inward pull forces on the South wall columns. 

• About 20 percent of the connections to the South perimeter wall on floors 97 

due to thermal weakening of the vertical supports.

C. Thermal Weakening of the South Wall: 

• South wall columns bowed inward as they were subjected to high temperatu

pull forces in addition to axial loads.

• Inward bowing of the South wall columns increased with time.
Probable Collapse Sequence for WTC 1 

Probable Collapse Sequence for WTC 1 

3. Collapse Initiation

• The inward bowing of the South wall induced column instability, which progres

horizontally across the entire South face.

• The South wall unloaded and tried to redistribute the loads via the hat truss to 

weakened core and via the spandrels to the adjacent East and West walls.

• The entire section of the building above the impact zone began tilting as a rigid 

faces; not only the bowed and buckled South face) to the South (at least about instability progressed rapidly from the South wall along the adjacent East and W

• The change in potential energy due to downward movement of building mass a

buckled columns exceeded the strain energy that could be absorbed by the str

collapse then ensued.
Probable Collapse Sequence for WTC 2 Probable Collapse Sequence for WTC 2

1. Aircraft Impact Damage:

• Aircraft impact severed a number of exterior columns on the South wall from flo

the wall section above the impact zone moved downward. 

• After breaching the building’s perimeter, the aircraft continued to penetrate into 

severing floor framing and core columns at the Southeast corner of the core. F

damaged from the impact area through the East half of the core up to the North 

perimeter walls. The floor truss seat connections over about 1/4 to 1/2 of the Eacore were severed on floors 80 and 81 and over about 1/3 of the East perimete

• Aircraft impact severed a few columns near the East corner of the North wall be

and 82.

• The impact damage to the exterior walls resulted in redistribution of severed co

mostly to the columns adjacent to the impact zones. The impact damage to the resulted in redistribution of severed column loads mostly to other intact core col

East exterior wall. The hat truss resisted the downward movement of the South about the East-West axis. 

• As a result of the aircraft impact damage, the core carried 6 percent less gravity 

impact and the North face carried 10 percent less loads. The East face carried 

gravity load, while the West face and the South face carried 3 percent and 2 pegravity load, respectively.

• After impact, the core was leaning toward the East and South perimeter walls. 

walls acted to restrain the core structure.
Probable Collapse Sequence for WTC 2 

Probable Collapse Sequence for WTC 2 

2. Effects of Subsequent Fires and Impact Damaged Fireproofing:

A. Thermal Weakening of the Core:

• Several of the undamaged core columns near the damaged and severed co

developed high plastic and creep strains over the duration the building stood

temperatures and stresses were high in the core area. The plastic and creeexceeded thermal expansion in the core columns. 

• The core continued to tilt toward the East and South due to the combination 

shortening (due to plasticity, creep, and buckling) and the failure of column struss in the Southeast corner.

• As a result of thermal weakening (and subsequent to impact), the East wall 

percent more gravity loads and the core carried about 2 percent less loads. 

walls carried between 0 and 3 percent less loads.

B. Thermal Weakening of the Floors:

• Floors 79 to 83 weakened with increasing temperatures over time on the lon

the East side and sagged. 

• Floor sagging induced inward pull forces on the East wall columns. 

• About an additional 1/3 of the connections to the East perimeter wall on floo

thermal weakening of the vertical supports.

C. Thermal Weakening of the East Wall: 

• East wall columns bowed inward as they were subjected to high temperature

pull forces in addition to axial loads.

• Inward bowing of the East wall columns increased with time.
Probable Collapse Sequence for WTC 2 

Probable Collapse Sequence for WTC 2 

3. Collapse Initiation

• The inward bowing of the East wall induced column instability, which progresse

horizontally across the entire East face.

• The East wall unloaded and tried to redistribute the loads via the hat truss to th

core and via the spandrels to the adjacent North and South walls.

• The entire section of the building above the impact zone began tilting as a rigid 

faces; not only the bowed and buckled East face) to the East (about 7o to 8o) an

3o to 4o) as column instability progressed rapidly from the East wall along the aand South walls. The building section above impact continued to rotate to the E

to fall downward, and rotated to at least 20 to 25 degrees.

• The change in potential energy due to downward movement of building mass a

buckled columns exceeded the strain energy that could be absorbed by the str

collapse then ensued.
Validation of Probable Collapse Sequen

Validation of Probable Collapse Sequen

• NIST evaluated the key factors related to:

• Innovative structural system• Aircraft impact and subsequent fires: How safe was each bu

immediately after aircraft impact but before fire weakened thstructures?

• Post-impact condition of fireproofing• Quality and properties of structural steel• Relative roles of the perimeter and core columns and the co

floor system, including connections

• Role of compartmentation (i.e. areas divided by fire-rated wa

• NIST made concerted efforts to validate results with key observa

obtained from its extensive collection of over 7,000 photographs 150 hours of videotape documenting the events at the World Traon September 11, 2001

• The probable collapse sequences are supported by the evidenc

NIST, including photographs and videos, recovered steel, eyewiaccounts, and emergency communication records
Time to Collapse Initiation Time to Collapse Initiation 

Estimated Time* 100 min 42 minActual Time 102 min 56 min

WTC 1 WTC 2

* The exact times are sensitive to the factors that control the 

inward bowing of the exterior columns. The sequence of events leading to collapse is not sensitive to these factors.
Inward Bowing of Perimeter Columns A

Inward Bowing of Perimeter Columns A

Minutes Prior to Collapse: WTC 1 South 

Minutes Prior to Collapse: WTC 1 South 
South Face of WTC 1, 9:25 am. No inward 
South Face of WTC 1, 9:40 am. No inward b
South Face of WTC1

• Maximum = 55 inches

(uncertainty ~ +/- 6 inches)

• Time: 10:22 AM• Measurements of inward

bowing (inches)

• Floor locations approximate• Blue tinted region digitally

enhanced
Inward Bowing of Perimeter Columns A

Inward Bowing of Perimeter Columns A

Minutes Prior to Collapse: WTC 2 East 

Minutes Prior to Collapse: WTC 2 East 
WTC2: East Face

Time: 9:21:29 AM ~18 minutes post impact

Maximum inward bowing of columns approximately 10 inches
Map of Inward Bowing: East Face of W

Time: 9:52:54 AM to 9:53:04 AM

Pull-in (inches)

Estimated uncertainty: +/- 1 inch

Empty regions have no data (smoke, damaged aluminum, could not establish true 

159

10

95

07

131613

6

0

6

111412

7

059

1114

10

6

0

1620

17

9

38

14

058

14

0

5

9

12

06

13

6

08

151313

7

07

1213

13

5

0

5

1517

13

7

5

14141316

6

4

12

4

7

12

0

0

15

2

6

1014

1

8

13

5

0

7

14

28

111710

049

15

13

0

4

9

11

70

04

7

0

5

11

13

12

0

0

48

14

11

076

0

86

067

078

0

12

11

0

6

9

70

40

7

0

0

1616

12

840

Obscured in Photos
Photographic Evidence of Hanging Floo

Photographic Evidence of Hanging Floo

East Face of WTC 2. Image shows what appear to be a floor slab from the 83rd hanging across window opening over a large portion of the 82nd floor.

© 2001 Allen Murabayashi, 9:47:05 am ± 5 s
Photographic Evidence of Hanging Floo

Photographic Evidence of Hanging Floo

Amount and extent of floor sag increased over the 51 minute pe
Photographic Evidence of Hanging Floo

Photographic Evidence of Hanging Floo

North Face of the South Tower. Image shows what appear to be portions of several floor slabs hanging across window openings on Floors 80, 81, and 82
Tilting of Building Tilting of Building 

Sections Sections

Initiation of glowas first obsertilting of buildinabove the impaof both WTC to

WTC 1 tilted to the south; WTC 2 tilted to the east and south.

© 2001 Dean Riviere
WTC 1: First Responder Communication

WTC 1: First Responder Communication

• 10:06 am NYPD aviation unit advises everybody to evacuate the a

vicinity of Battery Park City and states that, about 15 flootop, it is totally glowing red on the inside and collapse waNYPD officer advises that it is isn’t going to take much lothe North tower comes down and to pull emergency vehfrom the building.

• 10:21 am NYPD aviation unit first reports that the top of the tower 

leaning, then confirms that it is buckling and leaning to th

NYPD aviation unit reports that the North tower is leanin

Southwest and appears to be buckling in the Southwest NYPD officer advises that all personnel close to the build

three blocks in every direction.

• 10:28 am NYPD aviation unit reports that the roof is going to come 

shortly.NYPD officer reports that the tower is collapsing.
WTC 2: First Responder Communication

WTC 2: First Responder Communication

• 9:36 am New York City 9-1-1 telephone operator receives a mess

occupant of WTC 2 that a floor had collapsed below them 

level.

• 9:41 am NYPD dispatcher advises units that floor 106* in WTC 2 

and that the message comes from someone on that floo

• 9:52 am NYPD aviation unit gives a radio report stating that “larg

falling from WTC 2.NYPD dispatcher advises that floor 106 of WTC 2 is cru

communications with victims trapped on the floor.

• 9:58 am NYPD aviation unit advises that the whole building is com

________________________________* The 9/11 Commission report suggests that the source of this message appears to be cited 911 call from a floor in the 90s.
WTC 1 Timeline WTC 1 Timeline

Tower begins to collapse – First exmovement is at floor 98. Rotation odegrees to the south occurs before section begins to fall vertically.

10:28:2010:22:59 95-99 S 308-326+ South face columns are bowing inw

~9:55 S 301-323 No bowing of South face columns.

Panel knocked out by nose or landiwindow 95-329.

94-96 S 329

WTC1 Aircraft Impact between floocolumns 109 to 152.

8:46:26 93-99 N 109-152

Time Floors Face Columns Event Description
WTC 2 Timeline WTC 2 Timeline

WTC2 begins to collapse. 9:58:59East face columns are bowing inw

9:53:04 ~78-82 E -318-334+9:21:29 ~78-82 E ~318-338 East face columns are bowing inw

Floor edge appears to be draped twindows in ‘cold spot’ toward floopile. Column 81-253 is severed.

9:10:01 79-82 N

Floor 83 edge appears to be drape82 windows between columns 310 

9:03:42 83 E 310-342

Impact followed by fireballs on N, with brief period of intense burninAirplane strikes WTC 2 between fl85, columns 404 to 443. 9:02:59 77-85 S 404-443

Time Floors Face Columns Event Description
Tilting of WTC 2 at Collapse Initiation Tilting of WTC 2 at Collapse Initiation
Critical Analysis Inter Critical Analysis Inter

-Dependencies Dependencies

Collapse Seq

Reference 

Structural 

Models 

SAP 2000 

SAP to ANSYS 

Conversion

SAP to LS-DYNA 

Conversion

Compartment Damage

Debris and Fuel 

Distribution

Fireproofing

Damage

Structural 

Damage

Gas Temperat

Time-Historie

(FSI)

Structural

Temperature T

Histories

Resolution1-4 in.10-6 s

Aircraft Impact 

Damage Analysis

LS-DYNA

Fire Dynam

Analysis (FD

Thermal Ana

ANSYS v.8

Structural Res

and Failure An

ANSYS v.8

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

Baseline Performance Analysis
WTC 1 Damage: Composite Summa

WTC 1 Damage: Composite Summa

for Floors 93 to 98 for Floors 93 to 98

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Damage by Floor WTC 1 Damage by Floor

Floor 94 

Floor 96 Floor 97 

Floors 93 to 98 Floor 93

Cumulative Damage

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Damage for Severe Case WTC 2 Damage for Severe Case

Composite Summary for Floors 78 to 83 Composite Summary for Floors 78 to 83

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Severe Damage WTC 2 Severe Damage

Floor 79 F

Floor 81 Floor 82 F

Floors 78-83 Floor 78

Cumulative Damage

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
Comparison of Aircraft Damage to Core Comparison of Aircraft Damage to Core 

Columns with Prior Studies Columns with Prior Studies

MIT study reported in Chapter 4 of a collection of essays by MIT researchers “The Towers Lost and BWeidlinger study prepared for Silverstein Associates as part of insurance litigation involving the WTC 

3 Failed

Plus 2 Heavily D

1 Failed

Plus 2 Heavily Damaged

NIST Less Severe 

Impact Analysis

10 Failed

Plus 1 Heavily D

6 Failed

Plus 3 Heavily Damaged

NIST More Severe 

Impact Analysis

5 Failed

Plus 4 Heavily D

3 Failed

Plus 4 Heavily Damaged

NIST Base Case 

Impact Analysis

20 Failed 5 Failed WAI Collapse Analysis

14 failed and significa

Plus 10 dama

23 failed & significantly damaged

Plus 5 Damaged

WAI

Impact Analysis

4-12 Failed 7-20 Faile

MIT Impact Analysis

WTC Impact Study WTC 1 Core Column Damage WTC 2 Core Colum
WTC 1 Column Demand to Capacity Ratio Befo

WTC 1 Column Demand to Capacity Ratio Befo

Aircraft Impact (Maximum over Floors 93 to 98

Aircraft Impact (Maximum over Floors 93 to 98

501 508

1001 1008

= 1.0

0.110.230.12 0.11

0.12

0.49

0.610.43

0.26

0.27

0.13 0.14

0.14 0.13

0.23

Demand to Capacity Ratio (DCR) = (Dead + Service Live Loads)/Loa
WTC 1 Column Demand to Capacity Ratios Aft

WTC 1 Column Demand to Capacity Ratios Aft

Aircraft Impact (Maximum over Floors 93 to 98

Aircraft Impact (Maximum over Floors 93 to 98

501 508

1001 1008

= 1.0

1.20

0.46

1.15

0.140.250.11 0.10

0.150.25

1.12 1.07

0.37

Demand to Capacity Ratio (DCR) = (Dead + Service Live Loads)/Loa
WTC 2 Column Demand to Capacity Ratio Befo

WTC 2 Column Demand to Capacity Ratio Befo

Aircraft Impact (Maximum over Floors 78 to 84

Aircraft Impact (Maximum over Floors 78 to 84

501 508

1001 1008

= 1.0

0.22

0.22

0.20 0.20

0.15

0.15

0.44

0.50

0.40

0.42

0.38

Demand to Capacity Ratio (DCR) = (Dead + Service Live Loads)/Loa
WTC 2 Column Demand to Capacity Ratio Afte

WTC 2 Column Demand to Capacity Ratio Afte

Aircraft Impact (Maximum over Floors 78 to 84

Aircraft Impact (Maximum over Floors 78 to 84

501 508

1001 1008

= 1.0

0.23

0.28

0.23

>1.2

0.27

1.2

0.6

>1.2

0.8

1.0 0.8

0.32

0.35

0.4

Demand to Capacity Ratio (DCR) = (Dead + Service Live Loads)/Loa

• The two WTC towers withstood the initial impact of virtually identical aircraft (Boeing 767-20

terrorist attacks of September 11, 2001. 

• The robustness of the perimeter frame-tube system and large dimensional size of the WTC 

buildings withstand the aircraft impact. 

• The WTC towers displayed significant reserve capacity, vibrating immediately following imp

amplitudes that were more than about 1/3 of the design wind sway from the baseline analy

oscillation period nearly equal to the average measured for the undamaged building.

• Calculations of demand to capacity ratios due to aircraft impact damage showed tha

affected by the aircraft impacts, the majority of the core and perimeter columns in bo

continued to carry their loads after the impact. The loads from damaged and severe

redistributed to nearby undamaged columns.

• The above finding supports the fact that the WTC towers withstood the initial aircraf

finding that they would have continued to remain standing indefinitely but for anothe

event such as the subsequent fires.

Aircraft Impact Damage to WTC Tower St

Aircraft Impact Damage to WTC Tower St
Relative Roles of Aircraft Impact and Fir

Relative Roles of Aircraft Impact and Fir

• Fires played a major role in further reducing the structural capacity o

buildings, initiating collapse. While aircraft impact damage did not, b

initiate building collapse, it contributed greatly to the subsequent firethermal response of the structures by:

• Compromising the sprinkler and water supply systems;• Dispersing jet fuel and igniting building contents over large areas;• Creating large accumulations of combustible matter containing aircraft 

building contents;

• Increasing the air supply into the damaged buildings that permitted sig

higher energy release rates than would normally be seen in ventilation building fires, allowing the fires to spread rapidly on multiple floors; 

• Damaging and dislodging fireproofing from structural components in th

of the debris and due to the strong vibrations generated by aircraft imp

• Damaging ceilings that enabled “unabated” heat transport over the floo

partition walls and to structural components.
Relative Roles of Aircraft Impact and Fir

Relative Roles of Aircraft Impact and Fir

• The jet fuel, which ignited the fires, was mostly consumed w

first few minutes after impact. The fires that burned for almoentire time that the buildings remained standing were due mburning building contents and, to a lesser extent, aircraft cojet fuel.

• Typical office furnishings were able to sustain intense fires f

an hour on a given WTC floor. No structural component, howevsubject to intense fires for the entire period of burning. The duraintense burning impacting any specific component was controlled 

• The availability of combustible materials• Fuel gases released by those combustibles• Combustion air in the specific area

• The typical floor had on average about 4 psf of combustible 

on floors. Mass of aircraft solid combustibles was significant in immediate impact region of both WTC towers.
Effect of Initial Fire Ball Effect of Initial Fire Ball

 The overpressure on WTC floors associated with i

internal fire ball (deflagration) is estimated to be 

• maximum pressure roughly 2 psi to 3 psi• duration roughly 0.5 sec to 2 sec

 The natural frequency of the composite floor syste

approximately 3.7 Hz (period approx. 0.27 sec)

 The pressure pulse duration is sufficiently greater t

natural frequency of the floor such that dynamic efcan be neglected

 Based on failure of the truss seat connections, the 

capacity of the floor is calculated to be

• 4.8 psi against uplift pressure • 4.4 psi against downward pressure 
Base Case More Sever

Reconstruction of Fires (WTC 1, 97th Flo
Base Case More Severe

Combustible load more critical in WTC 2

Reconstruction of Fires (WTC 2, 81st Fl
Spread of Jet Spread of Jet-Fuel Ignited Multi Fuel Ignited Multi-Floor Fi

Floor Fi

• Consistent with available photographic and videographic evidence, NIST compute

capture the broad patterns of fire movement around the floors, with flames in a giv

lasting for about 20 min before spreading to adjacent, yet unburned combust

observed instances where fires persisted longer in regions with accumulated comb

other instances of sudden or interrupted fire spread. 

• The affected floors in the WTC towers had an open floor plan—with a modest 

perimeter offices and conference rooms and an occasional special purpose area. had two tenants, and those spaces, like the core areas, were partitioned (slab to 

slab). Photographic and videographic evidence confirms that even non-tenant spa

(such as those that divided spaces to provide corner conference rooms) provided 

resistance to fire spread in the affected floors. 

• For the time that the fires were active prior to building collapse, the presence of u

fire-rated compartments may have assisted in mitigating fire spread and conthermal weakening of structural components.

• The 1968 NYC Building Code required buildings like the WTC towers to have 1 h f

separations, but the code did not impose any minimum compartmentation requirem

12,000 ft2) to mitigate the spread of fire in large open floor plan buildings. 
Results of Thermal Analysis for WTC 1 C

Base Case Severe Case

Shows maximum temperature reached by each column.
Results of Thermal Analysis for WTC 2 C

Base Case Severe Case

Shows maximum temperature reached by each column.
WTC 1 Severe Case: Maximum Elastic, Plastic, 

WTC 1 Severe Case: Maximum Elastic, Plastic, 

Creep Strain in Columns between Floors 93 an

Creep Strain in Columns between Floors 93 an

Severed or Heavily Damaged

Elastic + Plastic + Creep Strain

7.3

1001

501 

1.39

1001

501 508

1008

600 s 6000 s

Maximum strain 
Vertical Displacement of WTC 1 Core at 

Vertical Displacement of WTC 1 Core at 

1

MN

MX

Y

X Z

WTC1 - Severe Temp at 6000s w/5kip pull 

-8.571

-7.616

-6.661

-5.706

-4.752

-3.797

-2.842

-1.887

-.931941

.022969

MAR 30 2005

10:24:23

NODAL SOLUTIONSTEP=33SUB =1437TIME=150UZ (AVG)RSYS=0DMX =27.32SMN =-8.571SMX =.022969

N

FL93FL95FL97FL99
Summary of Column Loads at Floor 98 o

Summary of Column Loads at Floor 98 o

WTC 1

North East South West 

Before Impact 10,974 8,545 11,025 8,572 

After Impact 10,137 9,071 10,356 9,146 

10 min 9,796 8,490 9,848 8,536 20 min 10,437 9,108 9,900 9,202 30 min 10,913 10,034 10,420 9,715 40 min 11,068 10,599 11,004 10,178 50 min 11,149 10,908 11,192 10,458 60 min 11,205 11,168 11,285 10,716 70 min 11,286 11,366 11,343 10,939 80 min 11,376 11,555 11,409 11,119 90 min 10,916 11,991 9,949 11,657 

100 min 10,828 12,249 9,638 11,905 
Summary of Column Loads at Floor 105 

Summary of Column Loads at Floor 105 

WTC 1

North East South West 

Before Impact 8,026 6,562 8,092 6,604 

After Impact 7,294 7,028 7,488 7,076 

10 min 6,944 6,461 6,981 6,469 20 min 7,551 7,075 7,057 7,158 30 min 8,020 7,998 7,569 7,685 40 min 8,193 8,571 8,129 8,147 50 min 8,285 8,878 8,315 8,428 60 min 8,351 9,130 8,414 8,687 70 min 8,435 9,319 8,481 8,914 80 min 8,528 9,497 8,551 9,097 90 min 8,096 9,847 7,327 9,506 

100 min 8,023 10,076 7,066 9,720 
Changes in Total Load in WTC 1 Subsys

Changes in Total Load in WTC 1 Subsys

between 80 min and 100 min between 80 min and 100 min

105 -504 +579 -1,485 +623 

-44 +115 -285 +163 

Change between 

Floor 98 and Floor 

105

98 -548 +694 -1,771 +786 

West Wall

(kip)

South 

Wall

(kip)

East Wall

(kip) 

North 

Wall 

(kip)

Floor No.
WTC 2 Severe Case: Maximum Elastic, Plastic, 

WTC 2 Severe Case: Maximum Elastic, Plastic, 

Creep Strain in Columns between Floors 78 an

Creep Strain in Columns between Floors 78 an

Compression is taken as positive, strain values are in %

6.7

1001

501 508

1008

N

6.9

1001

501 508

100

N

~2.6

~0.09

~0.03

~3.20

~0.80

~0.40

~0.30

~1.1 ~2.3

2400 s 2540 s
N

N

Vertical Displacement of WTC 2 Core fro

Vertical Displacement of WTC 2 Core fro

Floor 77 to Floor 86 at 2,540 s Floor 77 to Floor 86 at 2,540 s

Min value = -13 in Max value = 0.0 

1

MN

MX

X Y

Z

WTC2 Reduced Model At 2540s 

-13.05

-11.6

-10.15

-8.7

-7.25

-5.8

-4.35

-2.9

-1.45

0

MAR 2 2005

10:45:53

NODAL SOLUTIONSTEP=17SUB =523TIME=63.523UZ (AVG)RSYS=0DMX =13.576SMN =-12.982
Summary of Column Loads between Flo

Summary of Column Loads between Flo

and 106 of WTC 2 and 106 of WTC 2 

• Compression is taken as positive

West East North South Core 

(1) Before Impact 8497 8572 7382 7169 17123 (2) After Impact 9170 11272 6488 8432 13380 (3) 600 s 9181 11062 6250 8274 13975 (4) 1,200 s 9279 11121 6310 8350 13683 (5) 1,800 s 9369 11860 6416 8553 12544 (6) 2,400 s 9199 11928 6525 8691 12400 (7) 2,540 s 7092 8026 6551 9173 17900 (8) (2)-(1) 674 2700 -894 1263 -3743 (9) (3)-(2) 11 -211 -238 -157 595 

(10) (4)-(3) 97 59 60 76 -292 (11) (5)-(4) 91 739 106 203 -1139 (12) (6)-(5) -170 68 108 138 -144 (13) (7)-(6) -2107 -3902 27 482 5501 
Summary of Column Loads at Floor 83 

Summary of Column Loads at Floor 83 

WTC 2 

• Compression is taken as positive

West East North South Core 

(1) Before Impact 18065 18114 13567 13284 61828 (2) After Impact 18670 22481 12193 13511 57821 (3) 600 s 18728 22226 11896 13358 58413 (4) 1,200 s 18914 22208 12052 13318 58124 (5) 1,800 s 18876 23681 11770 13365 56967 (6) 2,400 s 18531 23682 11906 13473 56825 (7) 2,540 s 15667 15143 14215 16292 62422 (8) (2)-(1) 604 4368 -1374 227 -4007 (9) (3)-(2) 58 -255 -296 -153 592 

(10) (4)-(3) 186 -18 156 -40 -289 (11) (5)-(4) -38 1473 -282 47 -1157 (12) (6)-(5) -345 1 136 108 -142 (13) (7)-(6) -2864 -8539 2309 2819 5596 
WTC 2 Tilt at 2,540 s (Total Displacemen

WTC 2 Tilt at 2,540 s (Total Displacemen

Contours) Contours)

1

MN

MX

X

Y Z

 WTC-2 Severe Case Temperature Analysis 

3.298

5.332

7.366

9.399

11.433

13.467

15.5

17.534

19.568

21.601

MAR 14 2005

12:38:28

NODAL SOLUTIONSTEP=7SUB =1TIME=.007USUM (AVG)RSYS=0DMX =21.601SMN =3.298SMX =21.601

N

Undeformed 

building edge

X20 Magnification
Variation of Vertical Displacements at Flo

Variation of Vertical Displacements at Flo

2,540 s 2,540 s

~11 in

~10 in

~4.5 in

~2.5 in ~1.2 in

~5

~8.5 in

13 in 12 in 11 in

N

Likely zone the 

tower tilts around
Visual Evidence of Fires in WTC 1 Visual Evidence of Fires in WTC 1

Visual Evidence of Fires in WTC 1 Visual Evidence of Fires in WTC 1

Dark blue line indicates extent of inward bowing observed in visual images.Light blue line is estimated region of inward bowing where visual images were obscured.
1

MN

MX

Y X

Z

WTC1 Severe Temp at 6000s w/5kip pull - South Face (5X) 

-.701059

4.135

8.972

13.808

18.644

23.481

28.317

33.153

37.99

42.8

MAR 30 20

10:23:

NODAL SOLUTIONSTEP=33SUB =1437TIME=150UY (AVG)RSYS=0DMX =42.979SMN =-.701059SMX =42.826

Inward Bowing of the WTC 1 South Face at 600

Inward Bowing of the WTC 1 South Face at 600

Maximum Inward Displacement = 43 in.

COL301

COL359

FL93FL95FL97FL99

Looking from the outside of the building

Inward displacement is shown as positive displacement
Visual Evidence of Fires in WTC 2 Visual Evidence of Fires in WTC 2
Visual Evidence of Fires in WTC 2 Visual Evidence of Fires in WTC 2

Dark blue line indicates extent of inward bowing observed in visual images.Light blue line is estimated region of inward bowing where visual images were obscured.
Visual Evidence of Fires in WTC 2 Visual Evidence of Fires in WTC 2

Dark blue line indicates extent of inward bowing observed in visual images.Light blue line is estimated region of inward bowing where visual images were obscured.
Inward Bowing of the WTC 2 East Face at 2,540 

Inward Bowing of the WTC 2 East Face at 2,540 

1

MN

MX

Y

X

Z

WTC2 Reduced Model At 2540s Temp UnCn - East Face 

-4.143

3.179

10.5

17.821

25.142

32.463

39.785

47.106

54.427

61.

FEB 27 2

13:45

NODAL SOLUTIONSTEP=17SUB =523TIME=63.523UY (AVG)RSYS=0DMX =61.911SMN =-4.143SMX =61.748

Maximum inward displacement = 60 in.
• In general, the affected floor systems in WTC 1 had upgraded or thi

fireproofing (1.5 in. specified); affected floors in WTC 2 had the origfireproofing (0.5 in. specified).

• Structural response is sensitive to variability in fireproofing thi

along the length of components; it is possible to determine a thequivalent uniform thickness that should be greater than the spthickness. The thermally equivalent thicknesses were used in t

• In the analysis under the original condition, the fireproofing thi

the bridging trusses was considered to be one-half the thickne

primary trusses from interpretation of photographic evidence. For 

upgraded condition, the fireproofing thickness on the bridging considered to be equal to the thickness for the primary trusses

Fireproofing Conditions Fireproofing Conditions

Upgraded

Original

Status

2.5 (0.24) 2.2 1.50.75 (0.4) 0.6 0.5

Specified Thermally Equivalent As-Applied Avg. (COV*)

* Coefficient of Variation
Analysis of Fireproofing Damage Analysis of Fireproofing Damage

• NIST developed and used a rigorous technical approach to eval

role of the post-impact condition of the fireproofing in the collapWTC towers.

• The technical approach was based on a comprehensive aircraft im

analysis that predicted in detail (1) the damage to structural comand building partitions and furnishings (the partitions and moduworkstations were modeled explicitly in the impact region), (2) tthe debris field that was generated by aircraft impact, and (3) the 

of jet fuel.

• NIST determined the most influential parameters that governed the r

aircraft impact damage analysis based on formal statistically-based mNIST then conducted analyses for two sets of values for the moinfluential parameters for each WTC tower to estimate the range caused by aircraft impact.

• NIST determined conservative estimates for the extent of dislod

fireproofing by considering fireproofing damage only to structucomponents in the direct path of debris. 
Analysis of Fireproofing Damage (2) Analysis of Fireproofing Damage (2)

• Consistent with a conservative approach, NIST estimates ignored the p

fireproofing on structural components in a much larger region that was direct path of the debris was dislodged by shock or strong vibrations. 

• The WTC towers shook vigorously during the 0.5-0.7 seconds of aircraft 

Video analysis showed that WTC 2 vibrated for over 4 minutes after air

with amplitudes in excess of 20 inches at the roof top.

• Considerable photographic evidence shows fireproofing dislodged from 

columns not directly impacted by debris.

• First-person interviews of building occupants indicate that building vibra

aircraft impact were strong enough to dislodge ceiling tiles and collapse throughout the height of both WTC towers and to cause nearly all elevatfunctioning.

• Difficult to establish robust criteria to generate a coherent pattern of vibr

dislodging.

• The variation of influential parameters used in determining the probable 

sequence included some variation in the extent of dislodged fireproofing
Examples of Types of Core Column Firep

SFRM – Sprayed fire resistive materialWallboard – Gypsum wallboard enclosure

SFRM Only SFRM + 

Wallboard

Wallboa

Only
Types of Fireproofing on Core Columns 

Affected Floors

All 11% 46% 43%

98 11% 45% 45%

97 11% 45% 45%

96 11% 45% 45%

95 9% 49% 43%

94 13% 49% 38%

% 

Wallboard 

only

%

SFRM +

Wallboard

%

SFRM

only

Floor

All 22% 55% 

83 15% 57% 82 23% 55% 81 17% 51% 80 17% 51% 79 19% 68% 78 38% 49% 

%

SFRM 

Wallboa

%

SFRM

only

Floor

WTC 1 WTC 2

SFRM – Sprayed fire resistive materialWallboard – Gypsum wallboard enclosure

Information from original design drawings
Application of SFRM to External Column

• By design, uniform thickness• As applied, region between flange tips filled

(for example, see column 246 at right)

• Missing SFRM from outer flange indicated by

shadowing and exposed red paint
Damaged SFRM - North Face of WTC 2

Removed from flanges (red)Intact (green)

Removed from outer web (white)

Covered by weatherproofing coating 
Damaged SFRM - East Face of WTC2
Damaged SFRM - North Face of WTC 1

Left Side of Impact Hole
Damaged SFRM - North Face of WTC 1:

Missing from Trusses
Analysis of Fireproofing Damage (3) Analysis of Fireproofing Damage (3)

• In the case of core columns, the analysis required the debris imp

strong enough to fail building partitions immediately in front of th

• If the wall partitions remained intact in the core area after interaction 

debris field, then the fireproofing on core columns behind these parassumed to remain intact. 

• If wall partitions were damaged or destroyed by the debris field, the

on core columns behind these partitions was assumed to be dislodgfloor height.

• For aircraft impact damaged core columns, fireproofing was assume

dislodged only if the columns were subject to direct debris impact thwall partitions.

• The representative strength of building partitions was assumed 

psi based on a survey of data for partition walls and modular w

while the representative laboratory cohesive and adhesive strefireproofing measured by NIST was generally less than 12 psi (

• Core columns had spray-on fireproofing, gypsum wallboard enclosu

combination.
Analysis of Fireproofing Damage (4) Analysis of Fireproofing Damage (4)

• In the case of perimeter columns, the analysis requ

debris impact to be strong enough to damage or deroom furnishings (modular office workstations) adjacthe columns.

• If the room furnishings remained intact after interaction with 

field, then the fireproofing on the inside face of the perimetebehind these furnishings was assumed to remain intact.

• If the room furnishings were damaged or destroyed after int

with the debris field, then the fireproofing on the inside face perimeter columns in the same vicinity was assumed to be over that floor height. 

• The other three faces of the perimeter columns were protec

windows and/or aluminum cladding and were assumed to hfireproofing damage.
Analysis of Fireproofing Damage (5) Analysis of Fireproofing Damage (5)

• In the case of floor trusses, the analysis requir

debris impact to be strong enough to damage destroy room furnishings (modular office workin the same area of the affected floor.

• If the debris field momentum was not strong enough 

damage the furnishings, then the debris field was aconsidered not to extend high enough or be strong 

dislodge the fireproofing.

• If the debris field momentum was strong enough to 

the furnishings, then the debris field was also considextend high enough or be strong enough to dislodge fireproofing.
Analysis of Fireproofing Damage (6) Analysis of Fireproofing Damage (6)

• The thermal analysis of the WTC towers was conducted with rigor 

properly taking into account the estimated post-impact condition fireproofing on each structural component.

• For floor trusses without dislodged fireproofing, a thermally equivalent unifor

was considered to appropriately account for the average application thicknes

variability (floor trusses had original fireproofing thickness in the affected floo

and upgraded fireproofing thickness in the affected floors of WTC 1).

• For columns without dislodged fireproofing, the specified thickness as determ

available documents was used. No information was available on the in-plac

of the fireproofing on the perimeter columns, and little information was availacolumns.

• For structural components with significantly dislodged fireproofing, the firepro

considered to be missing on each such component. Separate analysis showsignificant regions of missing fireproofing in a component is essentially equiv

being no fireproofing.

• For structural components with partially dislodged fireproofing, the fireproofin

considered to be missing on specifically identified faces for each such compon inner face of perimeter columns that bowed inwards minutes prior to collaWTC tower).
Estimation of Floor Effects in Global Mo

Estimation of Floor Effects in Global Mo

• Floor Subsystem Models

• Analyzed structural response of single truss-with-slab-section model 

effects; estimated magnitude of inward pull forces from exterior conn

• Analyzed structural response of full floor model to thermal effects; es

regions where floor sags and extent of disconnections from columns

• Wall Subsystem Model

• Analyzed structural response of wall model to thermal effects (South 

1; East face of WTC 2).

• Imposed floor disconnections due to aircraft impact damage; made a

to match observations (e.g., floor 83 of WTC 2).

• Imposed floor disconnections from full floor thermal model.• Imposed inward pull forces estimated from single truss with slab sec• Compared results with observed inward bowing.• Estimated magnitude and extent of inward pull forces required to ma

inward bowing.
Sagging in Detailed Truss Model Sagging in Detailed Truss Model

• After web members 

buckled, the truss pulled exterior columns in.

• The tension force ranged 

from 9 kip to 14 kip per column in this model at the exterior column.

MN

MX

-42.11

-37.357

-32.603

-27.849

-23.095

-18.342

-13.588

-8.834

-4.081

.673211

Vertical displacement contour

Buckling of we

-100

-80-60-40-20

0

2040

0 100 200 300 400 500 

Temperature of truss (°

Total Horizontal Reaction Force 

(kip)

web member buckling

interior seat bolt shear-off
Floor Sagging and Inward Pull Forces Floor Sagging and Inward Pull Forces

650 oC925 oC500 oCMaximum TemperatureTmax

25 to 27 in.18 to 24 in.16 to 17 in.Floor Sag

• 2 story perimeter 

column 

• fixed at end 

supports

• 6 story perimeter 

column

• pinned at end 

supports

• 13 story single 

perimeter column

• column area 

doubled to account for adjacent column participation

• pinned at end 

supports

Perimeter Column Model

• 1 floor 9 to 14 kip• fire on floor 

below

• creep 

included

NIST3

• 5 floors 16 to 27 ki• fire on 4 floors

Duthinh2

• 12 floors 16 to 20 ki• fire on 3 floors

Usmani, et al1

Tensile Forces

Floor Model

1. Fire Safety Journal, v 38, n 6, October, 2003, p 501-533; Temperature profile assumed for floors with Tmax 

column and a linear reduction toward the core

2. NIST Special Publication 1000-5, June 2004 Progress Report on the Federal Building and Fire Safety Inves

World Trade Center Disaster, v 5, App. M; Temperature profile developed from NIST Fire Dynamics Simila

3. A uniform temperature profile was assumed
WTC 1 - Vertical Displacement of Floors 95 to 9

Vertical Displacement of Floors 95 to 9

6,000 s 6,000 s 

1

MN

MX

X Y

Z

WTC1 FL98 - Maximum Damage Case Temperature at 6000 sec 

-49.045

-43.113

-37.181

-31.249

-25.318

-19.386

-13.454

-7.522

-1.59

4.341

OCT 29 2004

09:46:42

NODAL SOLUTIONSTEP=15SUB =18TIME=6000UZ (AVG)RSYS=0DMX =49.136SMN =-49.045SMX =4.341

1

MN

MX

X Y

Z

WTC1 FL97 - Maximum Damage Case Temperature at 6000 sec 

-37.368

-32.731

-28.095

-23.459

-18.823

-14.186

-9.55

-4.914

-.277601

4.359

OCT 25 2004

00:20:47

NODAL SOLUTIONSTEP=12SUB =8TIME=6000UZ (AVG)RSYS=0DMX =37.426SMN =-37.368SMX =4.359

1

MN

MX

X Y

Z

WTC1 FL96 - Maximum Damage Case Temperature at 6000 sec 

-22.431

-19.594

-16.756

-13.918

-11.08

-8.243

-5.405

-2.567

.27036

3.108

SEP 27 2004

09:20:52

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

Floor 98, max sag = 49 in. Floor 97, max sag = 37 in.

Floor 96, max sag = 22 in.

N

N N

1

MN

MX

X Y

Z

WTC1 FL95 - Maximum Damage Case Temperature at 6000 sec 

-15.681

-13.544

-11.407

-9.27

-7.134

-4.997

-2.86

-.723545

1.413

3.55

OCT 20 2004

12:00:48

NODAL SOLUTIONSTEP=11SUB =8TIME=6000UZ (AVG)RSYS=0DMX =15.742SMN =-15.681SMX =3.55

N

Floor 95, max sag = 15 in.
WTC 2 - Vertical Displacements of Floor 

1

MN

MX

X Y

Z

WTC2 FL82 - Maximum Damage Case Temperature at 2400 sec 

-47.415

-41.674

-35.933

-30.192

-24.451

-18.711

-12.97

-7.229

NODAL SOLUTIONSTEP=5SUB =8TIME=2400UZ (AVG)RSYS=0DMX =47.561SMN =-47.415SMX =4.253

1

MN

MX

X Y

Z

WTC2 FL82 - Maximum Damage Case Temperature at 1200 sec 

-42.452

-37.358

-32.264

-27.171

-22.077

-16.983

-11.889

-6.795

-1.702

3.392

OCT 18 2004

13:42:00

NODAL SOLUTIONSTEP=3SUB =27TIME=1200UZ (AVG)RSYS=0DMX =42.53SMN =-42.452SMX =3.392

1,200 s max sag = 42 in. 2,400 s max sag = 47 in

1

MN

MX

X Y

Z

WTC2 FL82 - Maximum Damage Case Temperature at 3000 sec 

-48.708

-42.823

-36.938

-31.054

-25.169

-19.284

-13.4

-7.515

-1.63

4.254

OCT 18 2004

13:46:03

NODAL SOLUTIONSTEP=6SUB =16TIME=3000UZ (AVG)RSYS=0DMX =48.876SMN =-48.708SMX =4.254

3,000 s max sag = 48 in.
1

X Y

Z

 WTC 1 Severe at 5400 s 

MAR 18 2005

14:13:43

ELEMENTSUROTF

Pull-in Forces on the WTC 1 South Wall in Forces on the WTC 1 South Wall

1

X Y

Z

 WTC 1 Severe at 6,000 s 

ELEMENTSUROTF

Between 4,800 s and 5,400 s Between 5,400 s and 6

FL95FL99FL97 

5 kip pull 
Pull-in Forces on the WTC 2 East Wall in Forces on the WTC 2 East Wall

359

301

83

78

1.0 kip pull

329

359

83

78

1.0 kip pull 

329

359

301

83

78

1.5 kip pull 3.0 kip pull

329

359

301

83

78

1.5 kip pull 3.0 kip pull

329

Time = 10 mins Time = 

Time = 40 mins
Estimation of Floor Effects in Global Mo

Estimation of Floor Effects in Global Mo

• Global Model with Creep

• Analyzed structural response of global model with thermal effects.• Imposed floor disconnections and inward pull forces estimated from wa

(South face of WTC 1; East face of WTC 2).

• Compared results with observed inward bowing.• Adjusted magnitude of inward bowing forces to match observations.

Time Interval (s) Inwar

1,800 – 2,540 1

0 – 1,800 

WTC 2

(East Face of Floors 

Time Interval (s) Inward Pull (kip)

4,800 – 6,000

5

0 – 4,800 

0

WTC 1 

(South Face of Floors 95 to 99)

* Pull forces applied to each of t
WTC 1 Floor Disconnection and Pull WTC 1 Floor Disconnection and Pull

-In 

Locations for the South Face Locations for the South Face

South

9293949596979899

100

360 350 340 330 320 310 

Colum ID

Floor Number

Disconnection Inward Pull Connected Impact Damag
WTC 2 Floor Disconnection and Pull WTC 2 Floor Disconnection and Pull

-In 

Locations for the East Face Locations for the East Face

7778798081828384

360 350 340 330 320 310 300

Colum ID

Floor Number

Disconnection Inward Pull Connected Impact Damage

7778798081828384

360 350 340 330 320 

Colum ID

Floor Number

Disconnection Inward Pull Connected 

1200 s 2400 s
Estimation of Floor Effects in Global Mo

Estimation of Floor Effects in Global Mo

• Magnitude of Inward Pull Forces

• Represent small fraction (0.3 to 2.5%) of column axial loads (average column 

about 200 kip for WTC 1 and 300 kip for WTC 2).

• Consistent with magnitudes estimated from single truss-with-slab section mod

studies by Usmani (U. Edinburgh) and Duthinh (NIST) with simplified subsyst

single trusses over multiple floors found similar magnitudes of pull-in forces.

• Extent of Inward Pull Forces

• Greater than estimates obtained from thermal analysis of full floor model.

• Fireproofing dislodged from floor trusses over a larger region (estimates 

conservative).

• Additional floor sag over large region due to effects of thermally-induced 

spalling/delamination. 

• Additional factors that may influence Magnitude and Extent of Inward Pull Forces:

• Boundary conditions on full floor model were too rigid (far end of top and 

at floor supports were fixed)

• Full floor model included large deflections, nonlinear buckling, and temp

dependent plasticity but did not include creep.
Findings on Probable Collapse Sequenc

Findings on Probable Collapse Sequenc

• Inward bowing and primary tilting direction at collapse initiation in WTC 1 

• Not observed on face where aircraft impact occurred.• Observed on face parallel to the longer dimension of the building core.

• Observed on face associated with less stiff (longer period) direction of the build

• Occurred on face associated with long span direction of floor system; truss cho

capacity higher in long span direction.

• Inward bowing on South face and Southward tilting for WTC 1:

• Extent of fires similar on all faces; somewhat more extensive on East and Wes

in extent on North and South faces.

• Although North face had extensive impact damage, inward bowing occurred o

building tilted to South at collapse initiation.

• Thermal weakening of perimeter columns with floor sagging (which induced in

occurred on South side) caused inward bowing on South face and tilting in tha

• Inward bowing on East face and primary tilting towards East for WTC 2:

• Fires more extensive on East face; less extensive on North face and South fac

significant on East side of both faces; no observed fires on West face.

• Although South face had extensive impact damage, inward bowing occurred o

building tilted more to the East and less to the South at collapse initiation.

• Thermal weakening of perimeter columns with floor sagging caused inward bo

face and primary tilting in that direction (with additional Southward tilting due to 

impact damage).
Findings on Probable Collapse Sequenc

Findings on Probable Collapse Sequenc

• The time it took for each WTC tower to collapse was due primarily 

• Asymmetric structural damage resulting from aircraft impact in WTC 2 c

WTC 1; and greater damage (severed or heavy damage) to core columthan in WTC 1; higher aircraft speed/impact energy and impact location span resistance and off center position) caused greater WTC 2 core da

• Time it took for the fires, in combination with aircraft impact damage, to 

core.

• Time it took for fires to traverse from their initial location to the critical si

towers, and:

• time it took for heat to weaken and sag floor system, resulting in inward pull on 

• time it took for heat to weaken perimeter columns observed to be bowing inwa

collapse of each tower (traverse time of fires to South face of WTC 1 was muc

East face of WTC 2 where fires already existed).

• Time it took for heat to weaken and buckle those perimeter columns tha

simultaneously subject to inward pull forces and, to a lesser extent in W

additional vertical loads redistributed from core.
Findings on Probable Collapse Sequenc

Findings on Probable Collapse Sequenc

• The time to destructive heating was determined by the fires, whose 

intensity was determined by the large mass and wide distribution of the nature and (rather low) loading of combustibles, the sparseness of initsurviving building partitions, and the ease with which windows weallowing oxygen to feed the fires.

• Separate analyses showed that heating of structural members was 

sensitive to effect of dislodged fireproofing due to debris from aircthan to episodic regions of missing fireproofing or thinness of firepfire-affected region. As-built fireproofing conditions elsewhere did role in fire-induced collapse. 

• Debris field generated by aircraft impact removed significant fireproofing 

board enclosures, as well as some of the walls. Structural components 

thermally weakened were generally determined by impact of the debris f

fireproofing not been dislodged by debris field, temperature rise of 

components would likely have been insufficient to induce global co
Findings on Probable Collapse Sequenc

Findings on Probable Collapse Sequenc

• Role of the Building Core

• Core weakened significantly due to aircraft impact damage and thermal 

(thermal effects dominated WTC 1; aircraft impact damage dominated W

• Loads redistributed to perimeter faces; additional axial loads on perimet

not significant (only about 20-25 percent on average)

• Role of the Building Floors

• Primary role was to provide inward pull forces that induced inward bowi

on exterior face (South face of WTC 1; East face of WTC 2).

• Sagging floors continued to support floor loads despite extensive fires a

fireproofing; there would be no inward pull forces if floors had failed/disc

• Role of Exterior Frame-Tube

• Column instability over an extended region of the exterior face ultimatel

system failure (loads could not be redistributed via hat truss to already wbuilding core; load transfer via spandrels propagated column instability faces) causing initiation of building collapse.

• Column instability induced by thermal weakening of columns, inward pu

sagging floors, and to a much lesser degree, additional axial loads from 
Findings on Probable Collapse Sequenc

Findings on Probable Collapse Sequenc

• Performance of WTC floor system with intact fireproofing:

• WTC 1 did not collapse during the major 1975 fire which engulfed about 9,

Southeast quadrant of the 11th floor and spread mostly via utility closets to 10 

time, office spaces in the WTC towers were unsprinklered. The fire caused mito the floor system, and at no time was the load carrying capacity of the flcompromised. The fire “did not damage a single primary, fireproofed element. 

chord members (not needed for structural integrity), some bridging members (u

floor tremor and the like) and some deck support angles (used only as construc

were buckled in the fire—all were unfireproofed steel.” (SCHR Letter Report 197

• The load carrying capacity of the floor system was not compromised by th

temperatures in any of the four fire resistance (ASTM E 119) tests conducte

2004 up until the time they were stopped which was approximately 2 hours. Th

were about twice those on September 11, 2001. The high temperature cond

were at least as severe and lasting as the WTC fires (although the top of the slaheated)

• A detailed thermal-structural analysis, which did not include slab delamin

effects, showed that a full collapse of the WTC floor system would not occnumber of failed trusses or connections.

• The horizontal and vertical capacity of the floor connections to the perimeter an

exceeded the demand under design load conditions.
Role of Fire Resistance Tests Role of Fire Resistance Tests

• The fire resistance tests cannot be used to determine the actual per

the floor systems in the collapse of the WTC towers, nor can the teswhether or not the actual floor systems as built met code requireme

the PANYNJ could have taken the highly unusual step of reclassifyistructure to Class 1-C, with a 1-1/2 hour required rating for floors an

rating for columns, when installation of the sprinkler system was coprior to September 11, 2001.

• The fire resistance tests provided valuable insights into the behavior of th

for use in analyzing the thermal response and collapse of the WTC towe

• The occurrence and spread of jet-fuel due to the terrorist attacks on Sept

2001 ignited multi-floor fires in the WTC towers. These fires were significfrom the fires to which floor systems in standard U.S. fire rating tests are Consider, for example:

• Combustible fuel load of the hijacked jets.• Extent and number of floors involved in fires.• Rate of fire spread across and between floors.• Ventilation conditions in the fire-affected floors.

• The probable collapse sequence for the WTC towers are based on the b

thermally weakened structural components that had extensive damage to gypsum board fire protection induced by the debris field generated by air
Factors that Enhanced Building Perform

Factors that Enhanced Building Perform

on September 11, 2001 on September 11, 2001

• The unusually dense spacing of perimeter columns, coupled with deep s

that was an inherent part of both the architectural and structural design 

walls, resulted in a robust building that was able to redistribute loads froperimeter columns to adjacent intact columns.

• The wind loads used for the WTC towers, which governed the design of 

frame-tube system, significantly exceeded the prescriptive requirements 

York City building code and selected other building codes of the era (Ch

York State), including the relevant national model building code (BOCA)

• The robustness of the perimeter frame-tube system and the large dimen

the WTC towers helped the buildings withstand the aircraft impact. 

• The composite floor system with open-web bar joist elements, framed to 

way flat plate action, enabled the floors to redistribute loads without collaplaces of aircraft impact damage to other locations, avoiding larger scale 

upon impact.
Factors that Enhanced Building Perform

Factors that Enhanced Building Perform

on September 11, 2001 (2) on September 11, 2001 (2)

• The hat truss resisted the significant weakening of the core, due to airc

damage and subsequent thermal effects, by redistributing loads from th

core columns to adjacent intact columns and, ultimately, by redistributin

the perimeter walls from the thermally weakened core columns that losto support the buildings’ weight.

• As a result of the above factors, the buildings would likely not have coll

under the combined effects of aircraft impact and the subsequent jet-fumulti-floor fires, if the fireproofing had not been dislodged or had been o

minimally dislodged by aircraft impact. The existing condition of the fire

prior to aircraft impact and the fireproofing thickness on the WTC floor snot play a significant role in initiating collapse on September 11, 2001.
Future Technologies and Practices that Future Technologies and Practices that 

Potentially Could Have Improved Perfor

Potentially Could Have Improved Perfor

on September 11, 2001 (Requires Analys

on September 11, 2001 (Requires Analy

• Fireproofing not dislodged or only minimally dislodged by aircraf• Perimeter columns and floor framing with greater mass to enhan

and buckling performance. 

• Other passive and active fire protection features (e.g., compartm

retard spread of building fires; thermally-resistant window assemair supply and retard the spread of fires; fire-protected and struchardened elevators for firefighter access with continuous, redunsupply for standpipes).

• Steels with improved high-temperature properties (e.g., yield str

stiffness) and creep behavior.________________________There is far greater knowledge of how fires influence structures in 2005 case in the 1960s. The analysis tools available to calculate the respons

to fires are also far better now than they were when the WTC towers we
Limitations and Uncertainties in Determ

Limitations and Uncertainties in Determ

Probable Collapse Sequences Probable Collapse Sequences

• NIST recognized the inherent limitations and uncertainties in the 

performed for determining the probable collapse sequences:

• As-built condition of the WTC towers, as well as occupancy and use.• Estimated aircraft impact damage to WTC towers (structure, partitions, debris, 

fuel dispersion, material failure criteria, model size limitations and uncertainties

from exterior of buildings

• Estimated growth and spread of building fires (fuel load from building and aircr

ventilation within the core, compartmentation, fire dynamics model size limitatiuncertainties), especially interior building fires

• Estimated response of the aircraft-impact damaged WTC tower structures to th

fireproofing damage, material and structural failure criteria, thermal and structu

limitations and uncertainties)

• NIST validated the probable collapse sequences with availab

evidence, carefully considering the sensitivity of its results tuncertainties.
Web site Web site http:// http://wtc.nist.gov wtc.nist.gov

Email to Email to wtc@nist.gov wtc@nist.gov

Facsimile to (301) 975 Facsimile to (301) 975-6122

Regular mail: Regular mail:

WTC Technical Information Repository, Stop 8

Technical Information Repository, Stop 8

100 Bureau Drive, Gaithersburg, MD 20899 Bureau Drive, Gaithersburg, MD 20899-86
Background Slides Background Slides
WTC 1 Damage for Base Case WTC 1 Damage for Base Case 

Composite Summary for Floors 94 to 98 Composite Summary for Floors 94 to 98

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Base Case Damage WTC 1 Base Case Damage

Floor 94 

Floor 96 Floor 97 

Floors 93 to 98 Floor 93

Cumulative Damage

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 93 Damage for Base Case WTC 1 Floor 93 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 94 Damage for Base Case WTC 1 Floor 94 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 95 Damage for Base Case WTC 1 Floor 95 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 96 Damage for Base Case WTC 1 Floor 96 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 97 Damage for Base Case WTC 1 Floor 97 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 98 Damage for Base Case WTC 1 Floor 98 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 93 Damage for Severe Case WTC 1 Floor 93 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 94 Damage for Severe Case WTC 1 Floor 94 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 95 Damage for Severe Case WTC 1 Floor 95 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 96 Damage for Severe Case WTC 1 Floor 96 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 97 Damage for Severe Case WTC 1 Floor 97 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Floor 98 Damage for Severe Case WTC 1 Floor 98 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Damage for Base Case WTC 2 Damage for Base Case

Composite Summary for Floors 78 to 83 Composite Summary for Floors 78 to 83

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Base Case Damage WTC 2 Base Case Damage

Floor 79 

Floor 81 Floor 82 

Floors 78-83 Floor 78

Cumulative Damage

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 77 Damage for Base Case WTC 2 Floor 77 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 78 Damage for Base Case WTC 2 Floor 78 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 79 Damage for Base Case WTC 2 Floor 79 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 80 Damage for Base Case WTC 2 Floor 80 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 81 Damage for Base Case WTC 2 Floor 81 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 82 Damage for Base Case WTC 2 Floor 82 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 83 Damage for Base Case WTC 2 Floor 83 Damage for Base Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 77 Damage for Severe Case WTC 2 Floor 77 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 78 Damage for Severe Case WTC 2 Floor 78 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 79 Damage for Severe Case WTC 2 Floor 79 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 80 Damage for Severe Case WTC 2 Floor 80 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 81 Damage for Severe Case WTC 2 Floor 81 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 82 Damage for Severe Case WTC 2 Floor 82 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 2 Floor 83 Damage for Severe Case WTC 2 Floor 83 Damage for Severe Case

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
Visual Evidence of Fires in WTC 1: 8:48 

Visual Evidence of Fires in WTC 1: 8:48 
Visual Evidence of Fires in WTC 1: 9:04 

Visual Evidence of Fires in WTC 1: 9:04 
Visual Evidence of Fires in WTC 1: 9:20 

Visual Evidence of Fires in WTC 1: 9:20 
Visual Evidence of Fires in WTC 1: 9:38 

Visual Evidence of Fires in WTC 1: 9:38 
Visual Evidence of Fires in WTC 1: 10:04 

Visual Evidence of Fires in WTC 1: 10:04 
Visual Evidence of Fires in WTC 1: 10:22 

Visual Evidence of Fires in WTC 1: 10:22