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 IIC – WTC 7 Collapse

April 5, 2005
NIST WTC Investigation Objectives NIST WTC Investigation Objectives

• Determine:

• why and how the WTC Towers collapsed following the 

impact of the aircraft, and 

• why and how the 47-story WTC 7 collapsed

• Determine why the numbers of injuries and fatalities were 

high depending on location, including technical aspects of 

protection, occupant behavior, evacuation, and emergenc

• Determine the procedures and practices that were used in 

design, construction, operation, and maintenance of the Wbuildings

• Identify, as specifically as possible, areas in current n

building and fire model codes, standards, and practic

warrant revision
WTC Investigation P

WTC Investigation P

Analysis 

of Steel

Structural 

Collapse

Evacuation

Baseline 

Performance& 

Impact 

Damage

Analysis of 

Codes and 

Practices

Fire 

Re

Active Fire 

Suppression

Thermal and 

Tenability 

Environment

Public InputsDocumentsVideo/ Photographic RecordsOral History DataEmergency Response RecordsRecovered Structural SteelBPAT RecommendationsGovernment, Industry, Professional, Academic InputsPublic Inputs
Schedule for Completion of Investigatio

Schedule for Completion of Investigatio

• Major focus is on writing the Investigation reports; technical work is nearly com• The time required to write and review the comprehensive set of draft docume

constitute final WTC investigation findings and recommendations is driving the 

the report.

• The NIST reports include the overall investigation report for the WTC towers, 

separate project reports, and 34 supporting technical reports, totaling some 1This enormous task has taken NIST longer to accomplish than originally antic

• NIST is committed to putting accuracy, quality, and completeness ahead of sc

whatever time is required to do the job right.

• NIST plans to release a draft of the final report for public comment in June 20

comment period of about 6 weeks after release of the draft reports; NIST plan

Investigation report in September 2005.

• WTC 7 report will be issued as a supplement to the main report: draft planned 

2005; final for December 2005.

• Decoupling of WTC 7 report necessary to accommodate overlapping staffing 

on WTC towers.

• This change affects mainly the collapse analysis; other WTC 7 work will be re

other Investigation reports.

• WTC Conference: Putting Recommendations into Practice, September 2
WTC Investigation Reports WTC Investigation Reports

WTC Towers 

Investigation Report

Building and 

Fire Codes

Baseline 

Performance & 

Aircraft Impact

Steel

Analysis

Active Fire 

Protection

Thermal 

Reconstruction

Occupant 

Behavior & 

Egress

Structural 

Response & 

Collapse

1A Design andConstruction1B StructuralCode Comparison1C StructuralModifications1D Fire Protection &Life Safety Provisions1E Fire CodeComparison 1F Comparisonof NYC Codes1G Fire Protection& Life Safety Amend.1H Post-Const.Modifications WTC1&21I Post-Const.Modifications WTC 71J WTC 7 FuelSystem 

8 

reports 

totaling 

about 

1,500 

pages

34 

reports 

totaling 

about 

8,000 

pages

43 reports for WTC Towers; 4 reports for WTC 7

3A Contemp.Steel Specs.3B SteelInventory3C SteelDamage3D MechanicalSteel3E PhysicalProperties

2A BaselinePerformance2B AircraftImpact

4A FireHistory4B FireSprinklers4C FireAlarms4D SmokeManagement

5A VisualEvidence5B SteelExperiments5C SingleWorkstation5D CeilingTile5E MultipleWorkstation 5F FireSimulation5G FireStructure Interface

6A Passive FireProtection6B FireResistance Test6C ComponentAnalysis6D GlobalAnalysis

7A PublishedAccounts7B SurveyInstruments
Working Collapse Hypothesis for WTC 7 Working Collapse Hypothesis for WTC 7

If it remains viable upon further analysis, the working collapse hypothesis fosuggests that it was a classic progressive collapse, including: 

An Initiating Event 

 An initial local failure at the lower floors (below Floor 13) of the building due to fire 

induced structural damage of a critical column (the initiating event), which support

floor bay with an area of about 2,000 ft2

A Vertical Progression at the East Side of the Building

 Vertical progression of the initial local failure up to the east penthouse, as large flo

unable to redistribute the loads, bringing down the interior structure below the eas

A Subsequent Horizontal Progression from the East to the West Side 

 Horizontal progression of the failure across the lower floors (in the region of Floor

were much thicker than the rest of the floors), triggered by damage due to the ver

Disproportionate Global Collapse

 Events resulted in a disproportionate collapse of the entire structure

NIST has seen no evidence that the collap

was caused by bombs, missiles, or controlled 
Building Description Building Description

Major Features Relative to the Working Collapse Hypo

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

Perimeter Moment Frame

Braced Core a(Ground to 7th 

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

7th

1st

22nd

47 Stories

Regular

Irregular

Braced Frame East & West F(Ground to 5t
WTC 7 and Con Edison Substation WTC 7 and Con Edison Substation

Pedestrian Bridge

Con-Edison Substation

WTC 7 3rd floor

North Face of Building Above

N

Promenade

Below 7th floor, braced frame constructed over Con Ed substa
Transfer System Between Floors 5 and 7 Transfer System Between Floors 5 and 

Truss#1

Truss#2

Cantilever

Girders

Truss#3

Girders

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

7th Floor Plan

Built-up Column Locations

Built-up Column

Non Builtup Column

Built-up Column w

1⁄2

Built-up Column wi

5/16-
Roof Layout Roof Layout

57 56 55 45 53 52 51 50 49 48 47 46 45 4

58 61 64 67 70 73

59

62 65 68 71 74

76

79

77 80

60 63 66 69 72 75 78 81

EAST 

PENTHOUSE WEST SCREEN WALL PENTHOUSE

COOLING TOWERS BELOW 

N 

WTC 7 was modified:

• Penthouse structures added to the roof• East half of floors 41 and 43 were removed and replaced• HVAC and communication systems were added from floor 28 • Fuel tanks and backup generators were added
Building Conditions Building Conditions

Data for building conditions has been expanded and enhanc

through photographs, video, and interview records:

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

After WTC 2 collapsed: 

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

After WTC 1 collapsed: 

 Heavy debris on Vesey Street and WTC 7 Promenad No heavy debris observed in lobby area, white dust c

 SW Corner Damage – floors 8 to 18 South face damage between two exterior columns - ro

down 5 to 10 floors, extent not known

 South Face Damage –

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

• large debris hole near center around 14th floor

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

•

8th / 9th floor from inside, visible south wall gone with mo

damage to west, 2 elevator cars dislodged into elevator 
WTC 7 

Roof After WTC 1 Collapse Collapse

Minor Da

Parape
WTC 7 

SW Corner After WTC 1 Collapse Collapse

SW 

Sta
Vesey Street by Pedestrian Bridge Street by Pedestrian Bridge 

After WTC 1 Collapse After WTC 1 Collapse
Vesey Street by Promenade Street by Promenade 

After WTC 1 Collapse After WTC 1 Collapse
Estimated Extent of Debris Damage Estimated Extent of Debris Damage

58 61 64 67 70 73

59

62 65 68 71 74

76 79

80

60

N 63 66 69 72 75 81

ApproximaLocation oin East Penthouse 

TRUSS #1

TRUSS #3

78

77

TRUSS #2

78A

Cooling Towers 

Screenwall West Penthous

e

Approximate Region of Impact Damage by Large WTC1 Debris

Roof and Upper Level Debris Damage

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

-2:30 pm

2:30 pm

General

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

6 from stairwell and lob

 Fire reported at west wall of floor 

7 around 12:15 pm

 In east stairwell, smoke was observed near floors 19-20; sig

observed on floor 23

Looking from southwest corner to the south face

 Fire in SW corner near floors 10 or 11 Fire on floors 6, 7, 8, 21, 30 Multiple fires observed on floors numbered 20’s and 30’s Heavy black smoke coming out of south face gash; no fire o

Looking from southeast corner to the south face

 Fire on floor 12;1 area above covered with smoke

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

1 fires reported on floor 14, but photographs showed east face fire
Observed Fires Observed Fires

Fires in WTC 7—which began soon after WTC 1 collapsed—were observed on Flooand 11 near the middle about half an hour before collapse; Floor 12 was burned out 

Fires were also seen on Floors 12, 13, 22, 29, and 30 at various times during the day

East Face Fires on Floors 11-12 n
Observed Fires Observed Fires

North Face Fires on Floors 7 and 12 

near 3 PM
First 

Observation Observation of Collapse of Collapse 

East Penthouse Kink 

at Center of Roofline
Observed Sequence of Failures in WTC 

Observed Sequence of Failures in WTC 
Failure Sequence Timeline Failure Sequence Timeline

Global collapse occurs as windows fail between floo

0.3 8.2 around column 55 

0.1 7.9 West penthouse submerged

Façade kink formed along column 46-47

0.5 7.7 East end of roof starts to move

0.2 7.2 West end of roof starts to move 

Movement of entire north face of WTC7 (visible abov

3.0 7.0 North side of west penthouse moves

1.8 4.0 Windows break along column 46 at floors 37 and 40

0.5 2.2 East penthouse completely submerged

0.4 1.7 3 windows break at floors 41 to 44

East penthouse submerged from view (now inside bu

0.3 1.2 4 windows fail at floor 40

2 windows at floor 40 fail between columns 44 - 45 

0.9 0.9 East penthouse kinks between columns 44 and 45 0.0 0.0 Movement of east penthouse roofline

Observation from CNN Net Dub 7 47.avi

Total Time 

(sec)

Time Interval 

(sec)
Initiating Event & Vertical Progression Initiating Event & Vertical Progression

Initiating Event

 First exterior sign of failure was at the east penthouse 

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

Vertical Progression

 Columns 79, 80, and 81 supported large tributary are

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

-Structural Response 

Structural Response 

Critical Components Critical Components

Column 79 at Floors 5-7

Possible Modes of Column 

 Squashing (Yielding) o Cover Plate Weld Failu Failure of Column Spli

Thermal Structural Analysis

 Uniform and Gradient 

Temperatures

 Damaged and Intact F Temperature Depende

Properties

 Thermal Softening Axial Thermal Expans Bowing from Thermal 

Temperature Distribution

P

W14 x 730

Cover plate

0.5-in Welds

Web plate

W14 x 730

0.313-in 

Welds

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

NIST continues to evaluate the factors that could have caused colu

80, or 81 to fail

Possible contributing factors include:

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

damage

 Unusual fuel loads (fuel lines, high density of building conte

Analysis to date indicates:

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

fires and/or damaged fireproofing to initiate thermally-relate
Vertical Progression: Floor Response to 

Vertical Progression: Floor Response to 

of Column Support of Column Support

Col 79

Beam Hinges 

Æ

Å Slab Force

Col 80

Beam Hinges 

Æ

Å Slab Force

Col 81

Beam Hinges 

Æ

Å Slab Force
Initiating Event Initiating Event

& Vertical Progression & Vertical Progression

Exterior Observations:

 East Penthouse Kink  Window Breakage

East Penthouse Screenwall 

Substation

Roof40th Fl

30th Fl

13th Fl

Vertical CollapProgression

5th Fl7th Fl

Failed Column
Horizontal Progression & Global Collaps

Horizontal Progression & Global Collaps

Horizontal Progression

 A vertical failure would pile debris on the east side of the bu

damaging or severing transfer girders and trusses between and 7.

 This secondary damage has been postulated to cause a ho

progression of failure in the core columns at or near floors 5 

Global Collapse

 The global collapse occurred with few external signs and is 

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

N

6

5 68 71

74 77

80

Truss #2

78A

Truss #1

78

79

Truss 

#3

Braced 

Frames

Cantilever Transfers

81
Horizontal Progression Horizontal Progression 

– Truss #1 Truss #1

79 76 73 70 67 

5th

7th

9th

Column Fails 

in Bending

Truss #1

Splices Fail

in Bending
Horizontal Progression Horizontal Progression 

– Truss #2 Truss #2

80 77 74 71 68 65

5th

7th9th

Truss #2

S

Column Fails in Bending
Horizontal Horizontal 

Collapse Collapse Progression Progression

Exterior Observations:

 East Penthouse Sinks  Further Window 

Breakage

 No Movement for 5 sec

East Penthouse 

Sinks into Building

Substation

Roof

40th Fl

30th Fl

13th Fl

10th Fl

5th Fl

7th Fl

Sky Visible
Global Collapse Global Collapse

Exterior Observations:

 Center and West 

Penthouses Sink 

 North Façade Kink at 

Column 76

 Window Breakage in 

Front of Truss #3

Roof

40th Fl

30th Fl

13th Fl

10th Fl

5th Fl

7th Fl

Global 

Collapse

Remainder of 

Building Sinks
Fuel System for Emergency Power in W

Fuel System for Emergency Power in W

• NIST has reviewed and documented the fuel system for emergency pow• Floor 5—which did not have any exterior windows and contained th

pressurized fuel distribution system on the south, west and north fareas—is considered a possible fire initiation location, subject to fuand/or analysis that improve knowledge of fire conditions in this area.

• The two 6,000 gallon tanks supplying the 5th floor generators through a 

piping system were always kept full for emergencies and were full that d

• Both tanks were found to be damaged by debris and empty several mon

collapse. Some fuel contamination was found in the gravel below the tasand below the slab on which the tanks were mounted, but no contaminfound in the organic marine silt/clay layer underneath.

• This finding allows for the possibility, though not conclusively, that the fu

contributed to a fire on Floor 5.
Observations on Fifth Floor Observations on Fifth Floor

• Critical columns (79, 80, 81) carrying large loads from about 2,000 ft

2 of floor 

present on the 5th floor.

• The 5th floor was the only floor with a pressurized fuel line supplying the eme

generators.

• Two 6,000 gallon fuel tanks supplying a pressurized line possibly contributed 

were found to be damaged by debris and empty several months after collapse.

• In a 1997 facility condition survey, fireproofing was observed to be prominently mis

framing above main lobby; possible repair not confirmed.

• A majority of the 5th floor was not protected by sprinkler systems, with the exceptio

space to east and office area to north side of building; no evidence of sprinklers in 

5th floor (also on floors 7, 8, and 9) which housed OEM generators and day tanks. 

generator room may have been sprinklered, conflicting data.

• Two of the three sprinkler risers which were located next to stairs (#1 and #2) on th

the building transferred towards center on the 5th floor along with stairs.

• Sprinkler systems on floors 1 through 20 were supplied directly from the city distrib

through an automatic pump located on the 1st floor; water supply could be interrupt

power to fire pump or significant damage to underground city main in vicinity of bu
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
Visual Observations for WTC 7 Visual Observations for WTC 7

 The first exterior sign of structural 

failure in WTC 7 was the sinking of the east penthouse roof structure into the building.

 Witnesses reported structural 

damage to WTC 7 on its south face and southwest corner from WTC 1 debris.