1 

Dr. S. Shyam Sunder 

Director and Lead Investigator 

Building and Fire Research Laboratory 

National Institute of Standards and Technology

U.S. Department of Commerce 

NIST Response to the World Trade Center Disaster 

Federal Building and Fire Safety Investigation 

of 

the World Trade Center Disaster 

November 19, 2008 
2 

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
3 

NIST WTC Investigation Objectives 

• Determine: 

• why and how the WTC Towers collapsed following the initial 

impact of the aircraft, and 

• why and how the 47-story WTC 7 collapsed 

• Determine why the numbers of injuries and fatalities were so low or 

high depending on location, including technical aspects of fire 

protection, occupant behavior, evacuation, and emergency response 

• Determine the procedures and practices that were used in the 

design, construction, operation, and maintenance of the WTC 

buildings 

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

building and fire model codes, standards, and practices that 

warrant revision 
4 

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
5 

World Trade Center 7 

 WTC 7 was a 47 story office building 

located immediately to the north of the 

main WTC Complex.  Built on top of an existing Con Edison 

electric power substation, located on 

land owned by The Port Authority of 

New York and New Jersey. 

 On September 11, 2001, WTC 7 

endured fires for almost seven hours, 

from the time of the collapse of the 

north WTC tower (WTC 1) at 

10:28:22 a.m. until 5:20:52 p.m., 

when WTC 7 collapsed. 

 The collapse of WTC 7 was the first 

known instance of the total 

collapse of a tall building primarily 

due to fires. 
6 

Con Edison Substation 

Used With Permission of Con Edison 
7 

WTC 7 Structural Concept 

 The 47 story building contained approximately 200,000 m2 (2 million ft2) of floor area. 

 Structurally, WTC 7 consisted of four "tiers“: 

 The lowest four floors housed two two-story lobbies, one each on the center of the south side of 

the 1st and 3rd floors. The north side of the 1st and 2nd stories was the Con Edison substation. 

 Floors 5 and 6 were mechanical 

spaces. Between the 5th and 7th

floor slabs were three transfer 

trusses and a series of eight 

cantilever transfer girders. 

 Floors 7 through 45 were tenant 

floors, all structurally similar to each 

other. 

 The 46th and 47th floors, while mainly 

tenant floors, were structurally 

reinforced to support special loads, 

such as the cooling towers and the 

water tanks for fire suppression. 

140 ft. 

120 ft. 66 ft. 

37 ft. 

52 ft. 
8 

WTC 7 Framing 

 The WTC 7 structural 

system was designed to 

distribute the weight of the 

building (gravity loads) and 

resist (lateral) wind loads. 

 The frame included 

columns, floor assemblies, 

spandrel beams, girders, 

and transfer elements. 

 The lateral loads were 

resisted by the exterior 

moment frame. 

 The gravity loads were 

supported roughly equally 

by the 58 exterior columns 

and the 24 interior columns. 

East Elevation (Core)

Con Edison Substation South Elevation (Core) North Elevation (Core)

64 73 66 69 72 75 75 74 73

7

6

5

4

3

2

1

7

6

5

4

3

2

1

7

6

5

4

3

2

1

67 70

` 7

5

3

North Elevation South Elevation West Elevation East Elevation

24

22

Ramp Con Edison Substation

42 1 15 28 1 15 28 42

Con Edison Substation 1
9 

Typical WTC 7 Floor Framing 

From the 7th floor to the 47th floor, WTC 7 was supported by 24 interior columns and 58 exterior 

columns. Columns 58 through 78 formed the building core. Columns 79, 80, and 81 were particularly 

large and supported long floor spans on the east side of the building. 

Column 

Number Floor 

Beams 

Girders 
10

Built-up Columns on Lower Floors 

Built-up 

Column 

Non Builtup Column 

W14  730

Cover plates

Web plates

W14  730

 Plate thickness ranged from 1.5 in to 8 in. 

 Plate thickness t (in.) and steel grade were 

specified as follows: 

 2 < t < 4 ASTM A588 Grade 50

 4 < t < 6 ASTM A572 Grade 42

 t > 6 ASTM A588 Grade 42 
11

Transfer Trusses and Girders 

The layout of the WTC 7 columns did not align with the building foundation and the Con Edison 

columns. Therefore, a set of column transfers were constructed between the 5th and 7th floor slabs. 

Truss 1 

Truss 2 

Truss 3 

78A
12

WTC 7 Floor System 

The floor slabs were reinforced concrete of varying thickness. 

 The concrete on most floors was poured on a 3 in. corrugated metal deck. 

 Floors 2, 3, 4, and 6 had a 6 in. total slab thickness. 

 On Floor 5, the concrete was 14 in. thick. 

 On Floor 7, the south half of the floor had a poured 8 in. slab, and the north half had an 8 in. total 

slab thickness on a 3 in. deep metal deck. 

 On Floors 8 through 47, the concrete was 5.5 in. thick. 

Floors 8 to 45 
13

1 56

5

10

14A

15 24 26 28

30

35

40

53 47 44 42

18

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

21

50

East Penthouse 

West 

Penthouse Screenwall 

Cooling Towers Below

Roof Setback

Roof Layout 
14

Active Fire Protection Systems 

 WTC 7 had the following active fire protection systems: 

 fire alarms 

 smoke and heat detectors 

 manual pull stations 

 smoke control systems 

 automatic sprinklers. 

 The standpipe and automatic sprinkler systems were divided into three zones. Each 

zone had a primary and secondary water supply. 

 The primary water supply for the high zone (Floors 40 through 47) and mid-level 

zone (Floors 21 through 39) was from two water storage tanks on the 46th floor. 

The secondary supply was pumped from the city water main. 

 The primary water supply for the low zone, floors 1 through 20, was a direct 

connection to the city water mains. The secondary supply was from an automatic 

fire pump, which was connected to the city water main as well. 
15

WTC 7 Emergency Power Systems 

Schematic of 5th floor showing location of 

emergency power system components. 
16

WTC 7 Emergency Power Systems 

Base Building System

Salomon Smith Barney 

(SSB) System Mayor's OEM System

Fuel Storage Tank 

Capacities

Two 12,000 gal tanks Two 6,000 gal tanks Single 6,000 gal tank

Tank Locations Below the loading dock Below the loading dock 1

st floor

Locations of Generator(s) Two on 5th floor Nine on the 5th floor Three on the 7th floor

Day Tanks and Locations Single 275 gal tank on the 

5

th floor

Nonea Single 275 gal day tank on the 

7

th floor

Day Tank Pump Locations 

and Capacities

Two, on the 1st floor; 

4.4 gal/min

Two circulating pumps on 

1

st floor, 70 gal/min

Two, on the 1st floor, 

12 gal/min

Ambassador Modification Generator and 50 gal day 

tank on 9th floor; two pumps 

on the 1st floor, 2.4 gal/min

American Express 

Modification

Generator and 275 gal day 

tank on 8th floor; two pumps 

on the 1st floor, 2.8 gal/min

a The NYCBC had a limit of one day tank per floor. Since there was a day tank on the 5th floor for the base generators, the 

SSB system used a pressurized fuel distribution system, in which pumps continuously circulated fuel whenever the generators 

were running. There was enough fuel 35 gal in the valve rig and piping on the 5th floor to start the diesel engines, which, in 

turn, would supply power to operate the circulating pumps. 

b The generator and day tank were removed prior to September 11, 2001. 
17

Some Specific Questions 

 Why did WTC 7 collapse after having withstood fires for 7 hours? 

 What role, if any, did transfer elements (trusses, girders, and cantilever 

overhangs) play in the collapse of WTC 7? 

 What role, if any, did fuel oil systems for emergency power generators in 

WTC 7 play in the collapse? 

 What role, if any, did hypothetical blast events play in the collapse of WTC 7? 

 How well did design, construction, and maintenance practices conform to 

accepted practices? 

 Would WTC 7 have collapsed even if there had been no structural damage 

induced by the collapse of the WTC Towers? 
18

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
19

Analysis of Probable Collapse Sequence 

 Analyzed complete sequence of events: 

 Initial damage due to collapse of WTC 1 

 Growth and spread of fires 

 Thermal response of structural components 

 Thermally-induced initial local failure for collapse initiation 

 Collapse propagation, resulting in global collapse 

 Combined: 

 Physics-based mathematical modeling 

 Analysis of visual evidence (photographs and videos) 

 Analysis of design, construction, and inspection documents 

 Considered possible fuel oil fires, hypothetical blast events, and role of Con Edison 

substation 

NIST developed and used a series of rigorous and comprehensive 

models to determine the probable collapse sequence for WTC 7 

Significantly advanced current state-of-the-art; 

Tested limits of current computational capabilities 
20

Fire Growth and Spread 

Progression of fires on Floor 12 

• The major fires in WTC 7 were modeled using the 

Fire Dynamics Simulator (FDS) in a manner 

similar to those for the WTC towers. 

 There were far fewer photographs and videos of 

WTC 7 than of the towers; thus, details of the 

WTC 7 fires were not as precise as for the towers. 

 The fire simulations for WTC 7 were conducted for 

each floor individually; there were no obvious 

pathways for the flames and heat to pass from 

one floor to another, aside from the debrisdamaged area in the southwest corner of the 

building. 

 Sustained and/or late fires were observed only on 

floors 7 through 9 and 11 through 13. 

 The actual fires on these floors were most likely 

initiated at the time of the incidence of the debris 

from the collapsing towers. 

 A typical single floor simulation took up to two 

days on a Linux cluster with 8 processors. 

1:00 pm 

2:00 pm 

3:00 pm 

4:00 pm 

5:00 pm 
21

FDS Simulation of Fires on Floor 12 
22

 Thermal Response to Fire 

Temperature distribution (oC) on the floor beams of Floor 13. 

Temperature distribution (oC) in the top layer of the floor slabs of Floor 12. 
23

ANSYS Structural Response Model 

 The 16-story ANSYS model was used to 

determine the sequence of events that led to 

fire-induced collapse initiation. 

 Software: double precision version of 

ANSYS 11.0 

 Model 

 93,413 Nodes 

 101,357 Elements 

 21,095 beam elements 

 28,182 rigid beam elements 

 34,461 shell elements 

 7,658 contact elements 

 9,961 break elements 

 Computer 

 64-bit workstation 

 quad-core, 3.0 GHz processor 

 64 GB of random access memory 

(RAM) 

 Analysis Time 

 approximately 6 months 

 Fires were observed on Floors 7 to 9 and 11 to 13. 

 

 The floors and columns in these areas were 

subject to heating and possible thermally induced 

failures. 

 Floor framing between Floors 8 and 14 and 

columns between Floors 7 and 14 were explicitly 

modeled while Floor 7 and below and Floors 15 

and 16 were modeled using superelements. 
24

LS-DYNA Structural Response Model 

 The LS-DYNA model was used to determine the 

sequence of events that led to collapse 

propagation and, ultimately, global collapse. 

 Software: double precision version of LS-DYNA 

 Model 

 3,593,049 Nodes 

 3,045,925 Elements 

 3,006,910 shell elements 

 33,364 discrete spring elements 

 3,190 beam elements 

 2,461 solid elements 

 Computer 

 High speed Linux Beowulf compute 

cluster. 

 Head node - two 64-bit 2.4 GHz 

processors, 4 GB of RAM, 1.5 terabytes 

of RAID 5 disk storage 

 Each compute node - two 64-bit 2.6 GHz 

processors, with 8 GB to 16 GB of RAM 

 Analysis Time 

 Up to 8 weeks 
25

Summary of Probable Collapse Sequence (1) 

 Aerial photograph shot 14 s ± 0.5 s after WTC 1 

began to collapse. 

 Arrows mark arcs formed by debris that seemed to 

be ejected outwards from the debris cloud 

 Collapse of the WTC Towers. 

 Collapse of WTC 2 did not cause any 

structural damage or start any fires in 

WTC 7. 

 Collapse of WTC 1 damaged seven 

exterior columns on the lower floors of 

the south and west faces and initiated 

fires on 10 floors between Floors 7 and 30. 
26

Debris Impact Damage to WTC 7 

 Enlargement of a photograph showing 

part of the west face of WTC 7. 

 Arrows and floor numbers have been 

added for the corresponding rows of 

windows. 
27

Observed Debris Impact Damage to WTC 7 

green (■)- no visible damage 

yellow (■) – window glass broken 

orange (■) – granite and underlying truss 

damage 

red (■) – damage to exterior structural steel 

gray (■) – vertical dark band 

blue (■) – not visible due to smoke, dust, 

and intervening buildings. 

West Face South Face 
28

Summary of Probable Collapse Sequence (2) 

 Cropped photograph showing east edge of the 

north face and oblique view of east face of WTC 7. 

 It was likely taken between 3:20 p.m. and 3:40 p.m. 

 Growth and Spread of Fires. 

 Fires on the lower floors (Floors 7 

to 9 and 11 to 13) grew and spread 

since they were not extinguished 

either by the automatic sprinkler 

system or by FDNY because water 

was not available. 

 Fires were generally concentrated 

on the east and north sides of the 

northeast region beginning at about 

3 p.m. to 4 p.m. 

 The local fires on the upper floors 

(Floors 19, 22, 29, and 30) were not 

observed after approximately 1 p.m. 
29

Observed Fires 

 Cropped photograph of north face of WTC 7 taken 

between 3:11:15 p.m. and 3:16:51 p.m. 

 Cropped photograph showing part 

of north face of WTC 7 at 4:39 p.m. 

± 120 s. 
30

Summary of Fires on North Façade of WTC 7 

 Red represents windows where the glass was broken out and/or fire was observed through 

the window. The remaining windows are colored blue indicating that no direct evidence was 

observed for fires at these locations. 

 Façade map showing where windows were broken and/or fire was observed during the period 

when fire first appeared shortly before 3:00 p.m. until the building collapsed at 5:20:52 p.m. 
31

 Initial Local Failure for Collapse Initiation. 

 Fire-induced thermal expansion of the floor system surrounding Column 79 led to 

collapse of Floor 13 that triggered a cascade of floor failures. 

 This, in turn, led to loss of lateral support to Column 79 over nine stories, resulting in 

the buckling failure of Column 79. 

Summary Probable Collapse Sequence (3) 

Fl 5

No support

Partial support

Fl 9

Full Support

Fl 6

Fl 7

Fl 8

Fl 10

Fl 11

Fl 12

Fl 13

Fl 14

Fl 15

Fl 16

Fl 17

Fl 18

Fl 4

Fl 3

Fl 2

Column 79 

at 

-1.3 (14.7) s

Vertical 

support and 

lateral 

restraint in 

north only

View from southeast

Fl 5

No support

Partial support

Fl 9

Full Support

Fl 6

Fl 7

Fl 8

Fl 10

Fl 11

Fl 12

Fl 13

Fl 14

Fl 15

Fl 16

Fl 17

Fl 18

Fl 4

Fl 3

Fl 2

Column 79 

at 

-1.3 (14.7) s

Vertical 

support and 

lateral 

restraint in 

north only

View from southeast
32

North East Floor System Near Column 79 

Seat 

Bolts 

Column 79 

Girder 
33

Thermal Response of Northeast Floor Framing 

APR 16 

14:

Exterior moment 

frame resisted 

forces from thermal 

expansion. 

Girder was pushed 

laterally by thermal 

expansion of floor 

beams, due to onesided framing and 

lack of shear studs. 

Forces from thermal 

expansion failed the 

connection at 

Column 79, then 

pushed the girder off 

the seat. 
34

Physics-Based Visualization of WTC 7 Collapse 

Initiation 

View from 

South 
35

Summary Probable Collapse Sequence (4) 

 Vertical Progression of Failure. 

 Buckling of Column 79 (the collapse initiation event) triggered a vertical progression of floor 

system failures to the east penthouse and subsequent cascading failure of the adjacent interior 

columns on the east side of the building (i.e., Columns 80 and 81). 

 Vertical progression of floor system failures spread to include the entire east region all the way 

to the top of the building. 

 

View of interior 

columns 
36

Summary of Probable Collapse Sequence (5) 

 Horizontal Progression of Failure. 

 The interior columns buckled in succession from east to west in the lower floors due to 

 loss of lateral support from floor system failures 

 forces exerted by falling debris 

 load redistributed from other buckled columns. 

Buckling of all interior columns at 6.5 s following 

collapse initiation. 

Failure progresses to the third line of interior 

columns from the east at 4.5 s following collapse 

initiation. 
37

Summary Probable Collapse Sequence (6) 

 Global Collapse. 

 The exterior columns buckled at the lower floors (between Floors 7 and 14) due to load 

redistribution to the exterior columns from the downward movement of the building core. 

 The entire building above the buckled-column region then moved downward in a single unit, 

as observed, completing the global collapse sequence. 

Buckling of lower exterior columns. 
38

Video Evidence of WTC 7 Collapse 
39

Physics-Based Visualization of WTC 7 Collapse 

View from 

Northwest 
40

Physics-Based Visualization of WTC 7 Collapse 

View from 

South 
41

Collapse Initiation Observations 

Exterior Observations 

 East Penthouse 

Kinks 

 Windows break on 

East Side of North 

Face 

East Penthouse 

Screen Wall 

West Penthouse 

40th Fl

47th Fl

30th Fl

Window 

Breakage 

Column Buckles 

14th Fl

Vertical 

Collapse 

Progression 

7

th Fl

Floors Fail 
42

Global Collapse Observations 

Exterior Observations 

 Screenwall and 

West Penthouse 

Sink 

 North Façade Kinks 

at Edge of Core 

 Windows break on 

West Side of North 

Face 

40th Floor 

Window 

Breakage 

Horizontal 

Progression 

Initiates Failure 

in Columns 

Global Collapse 

Window 

Breakage 

47th Floor 

Kink in North 

Facade
43

Collapse Observations 

P17

57

1

28 15

42

44 53 55

76

77

78

79

80

81

43 45

Cooling Towers 

Screen Wall West 

Penthouse 

Window breakage 

between columns 

54 and 57 

Kink forms in roof of 

East Penthouse 

Kink forms in north face 

of building 

N 

West edge of 

penthouse 

rotates inwards but 

does not move 

downward 

East edge of 

penthouse 

rotates inwards and 

moves downward 

Kink forms in north 

face of East Penthouse 

Floor beams collapse 

Core framing 

Column 

79

East 

Penthouse 

Core 

Window breakage 

between columns 

43 and 45 
44

Vibration Analysis of Video 

No Vibration 

Floors 

Collapse 

East Penthouse Begins 

to Collapse 

Horizontal 

Progression of 

Collapse 

Beginning 

of

Global 

Collapse 

Column 79 Buckles 
45

LS-Dyna Visualization Clock 1012-876543 -210 1 2345678 9Building Vibration 

No Vibration 

Floors Collapse 

Column 79 Buckles 

East Penthouse Collapses 

Comparison of 

Visualization and Video 

Horizontal Progression 

of Collapse 

Beginning of 

Global Collapse 
46

Accuracy of the Probable Collapse Sequence 

• Agreement between observations and simulations is reasonably good, validating probable collapse sequence. 

• Results with and without debris damage bound the observations; results with damage better match the 

observations of the overall global collapse sequence. 

Event Observation Time (s) 

Analysis Time (s) 

 w/Debris Damage 

Analysis Time (s) 

w/o Debris Damage 

Start of cascading failure of floors surrounding 

Column 79  -6 s a -6.6 s -6.6 s 

Buckling of Column 79, quickly followed by 

buckling of Columns 80 and 81 Not observable -1.3 -1.4 

Start of descent of east penthouse 0.0 0.0 0.0 

Descent of east penthouse below roofline (First 

value: observed from the northwest and below; 

second value: observed from the north at the 

roofline)

2.0 2.4, 2.7 2.3, 2.6 

Buckling of columns across core, starting with 

Column 76 Not observable 3.5-6.1 3.2-13.5 

Initial downward motion of the north face roofline 

at the eastern section of the building 6.9 6.3 9.8 

Descent of the east end of the screenwall below 

the roofline (First value: observed from the 

northwest and below, second value: observed 

from the north at the roofline)

8.5 7.3, 7.7 8.7, 9.2 

Descent of the west penthouse below the 

roofline (First value: observed from the 

northwest and below, second value: observed 

from the north at the roofline)

9.3 6.9, 7.3 10.6, 10.9 

a

 Based on vibration analysis 
47

WTC 7 Collapse Visualization w/o Damage 

View from 

Northwest 
48

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
49

Other Possible Hypotheses: Role of Fuel Fires 

 Hypothetical diesel fuel fires on the 7th, 8th, and 9th floors did not contribute to the collapse of 

 WTC7.

 The generators on the 8th and 9th floors were supplied by two 12,000 gal tanks under the building. 

 Nearly all the fuel in these tanks was recovered months after the building collapse. 

 At most, 1000 gal of fuel was unaccounted from these tanks, which was equivalent to only about 

 5 percent of the office combustibles on a single floor. 

 The day tanks that supplied fuel to the emergency generators on these floors did not contain 

enough fuel to be a significant contributor to the combustible load on these floors. 

 It is unlikely that the tanks would have been re-supplied because of multiple safeguards in the fuel 

delivery system. 

 None of the day tanks were located near critical Column 79. 

 Fire did not reach the northeast region of the 7th, 8th, or 9th floors until about 4:00 p.m. 

 The 5th Floor was the only floor with a pressurized fuel line supplying nine of the emergency 

power generators on that floor. Two 6,000 gallon underground tanks supplied these 9 generators, 

which were always kept full for emergencies, and were full on September 11, 2001. 

 Both 6,000 gallon tanks were found to be damaged and empty in the rubble pile several months 

after the collapse. This fuel might have contributed to a fire on Floor 5, a possibility that could not be 

distinguished from the fuel being consumed or dissipated in the burning rubble fire subsequent to the 

collapse of WTC 7. 
50

Role of Fuel Fires (2) 

 NIST analyzed potentially severe pool fires that might have resulted from ignition of spillage of 

the diesel fuel present on the 5th floor or that might have been pumped to that floor: 

 Two types of over-ventilated fires in the vicinity of Column 79 (which was critical because 

observations and analysis indicated clearly that the failure of this column initiated collapse). 

 fuel burning sustained for the  7 h between the collapse of WTC 1 and the collapse of WTC 7. 

 burning rate double that of previous fire, i.e., a high intensity fire with significant duration. 

 Two types of under-ventilated fires in the vicinity of Column 79: 

 air handling system turned off and louvers closed; initial fuel burning rate fit to the air availability. 

 air handling system was turned off, but louvers were open. 

 An additional simulation of an over-ventilated fire near the breach in the south wall. 

 Hypothetical pool fires on the 5th and 6th floors did not contribute to the collapse of WTC 7. 

 These worst-case scenarios could not be sustained long enough, or could not have generated 

sufficient heat, to raise the temperature of a critical column (i.e., Column 79) to the point of 

significant loss of strength or stiffness; or 

 Such fires would have produced large amounts of visible smoke that would have emanated from 

the exhaust louvers; however, no such smoke discharge was observed; or 

 The gas temperatures would have exceeded the boiling point of the coolant for all 9 diesel 

generators in < 3 h, leading to engine failure. Once all 9 generators were down, there would have 

been no power to operate the fuel pumps, and the fires would have burned out well before columns 

in the region of the fire, including Column 79, experienced significant loss of strength. 
51

Role of Fuel Oil Fires (3) 

 The day tank on Floor 5 supplying the remaining two emergency generators on that floor 

did not contain enough fuel for a fire that could threaten Column 79. 

 The contained fuel was only equivalent to a few percent of the combustible furnishings on a tenant 

floor. 

 It is unlikely that the tanks would have been re-supplied because of multiple safeguards in the fuel 

delivery system. 

 None of the day tanks were located near the location of critical Column 79. 

 A diesel fuel spray fire on Floor 5 would have been less damaging than a pool fire. 

 A spray fire would have resulted from a small leak in the fuel supply piping, so the fuel escape rate 

would have been far less than in the over-ventilated pool fire scenarios. 

 Even if the spray had directly hit Column 79, it would have heated only a small area of the steel. 

 Calculations showed that even if the entire column were immersed in a flame as hot as 1400 C, it 

would have taken 6 h to heat the column to the point of significant loss of strength or stiffness. 

 FDNY personnel reported that they did not see any indication of burning liquid fuels before 

the building collapsed. 
52

Other Possible Hypotheses: Role of Blast 

Events 

 Scenarios of a hypothetical blast event that could have occurred in WTC 7 on September 11, 2001, 

were assessed, including blast location, size, and timing. 

 Phase I: Identify hypothetical blast scenarios to initiate structural collapse. 

 A scenario with the minimum amount of required explosive (9 pounds of linear shape charge of 

RDX) was identified, which was appropriate for the columns on the east side of the building. 

 The recommended column preparation for use of this shape charge required at least 30 

minutes (cutting and placing). 

 Additional preparation time was required to clear the column for cutting, such as removing 

walls or other coverings that might have restricted access to bare steel at a column section. 

 

 Phase II: Assess the blast wave propagation inside the building and the corresponding 

response of the WTC 7 windows. 

 2-D SHAMRC blast propagation calculations were performed for the minimum explosive case 

identified in Phase I. 

 Calculations were performed for a lesser charge size of 2 pounds to evaluate threshold 

explosive requirements for breaking windows. 

 There were 4 blast analyses performed in total, consisting of two different charge sizes (2 or 9 

lbs of RDX) and two floor layouts with differing partition layouts in the tenant areas. 
53

Role of Hypothetical Blast Events (2) 

 Phase III: Assess the approximate distance from the building that the blast would have been 

audible in an urban setting. 

 An acoustic analysis was performed to assess the distance from the building that the blast would have 

been audible. 

 Adjacent buildings and their effect on the sound propagation was not considered in this analysis; 

however, sound from a blast in an urban setting would be reflected and channeled down streets with 

minimum attenuation due to the hard building exteriors. 

 For all four scenarios, significant audible sound was predicted from all building faces. 

 If propagation was unobstructed the sound level from all building perimeter openings at 1 km would be 

approximately 130 to140 dB. 

Peak over pressures for 9 lb shaped charge (left) and 2lb charge (right). 
54

Role of Hypothetical Blast Events (3) 

 Hypothetical blast events did not cause the collapse of WTC 7. NIST concluded 

that blast events did not occur and found no evidence of any blast events. 

 

 The minimum explosive charge (lower bound) required to fail a critical column (i.e., 

Column 79) would have: 

 produced a pressure wave that would have broken windows on the north and east 

faces of the building near Column 79. 

 resulted in a sound level of 130 to 140 decibels (a sound level consistent with a 

gunshot blast, standing next to a jet plane engine, and more than 10 times louder 

than being in front of speakers at a rock concert), at least half a mile away. 

 generated a sound that would be reflected and channeled with minimum attenuation 

down urban streets with hard building exteriors. 

 Although such a blast could be heard a mile away, there were no witness reports of 

such a loud noise, nor was such a noise heard on audio tracks of video tapes that 

recorded the WTC 7. 
55

Role of Hypothetical Blast Events (4) 

 Prior to preparing a column for intentional demolition, walls and/or column enclosures 

and SFRM would have to be removed and replaced without being detected. 

 Preparing the column includes steps such as cutting sections with torches (which produces 

noxious and odorous fumes), and careful placement of charges and an initiation device. 

 A much larger charge would be required to fail the column without prior preparation, including 

cutting; the resulting sound level and extent of window breakage would be much greater. 

 Intentional demolition usually prepares most, if not all, interior columns in a building with 

explosive charges, not just one column. 

 It is unlikely that explosive charges would have been applied to just one column (that being the 

critical column) or that such activity could have taken place without being detected. 

 

 Observations for WTC 7 do not match the typical sequence of events for a controlled 

demolition. 

 Video evidence clearly supports a sequence of failures (initiation, vertical progression, horizontal 

progression, and global collapse) based on a single point of initiation. 

 The observed collapse sequence is inconsistent with a typical controlled demolition where 

charges are placed in multiple locations across the building and detonated in rapid succession. 
56

Role of Con Edison Substation 

 The Con Edison substation played no role in the fires that caused the collapse of WTC 7. 

 Electronic communications into and out of the Con Edison substation show that the substation 

continued to operate until it was intentionally shut down at 4:33 p.m. 

 Additionally, there was no evidence of a fire in the Con Edison substation. 

 

 Even though utility power to the rest of WTC 7 was lost at 9:59 a.m., auxiliary utility power to the Con 

Edison substation from transmission switching station feeders allowed internal operations to be 

maintained. 

 At 4:33 p.m., the utility control center isolated the WTC 7 Con Edison substation by opening the 

138 kV circuit breakers feeding the WTC 7 substation. 

 Fire detector signals from the Con Edison substation were monitored off-site throughout the day. 

 One fire detector within the Con Edison substation gave an alarm when WTC 1 collapsed, and 

stayed in alarm mode until the substation was isolated from incoming feeders at 4:33 p.m. 

 There were no other indications (e.g., no high temperature alarms from the transformers, no 

visible smoke emanating from the Con Edison substation) that a fire occurred within the 

substation during that period of time. 

 Likely causes of the fire alarm were the smoke and dust dispersed in the area of the substation 

from the collapse of WTC 1. 
57

Role of Fire Resistance Ratings (1) 

 Based on the BCNYC, a sprinklered high-rise office building (Class 1-C) would have 

required 1.5 hour fire resistance rating for the floors and an unsprinklered high-rise office 

building (Class 1-B) would have required a 2 hour fire resistance rating for the floors. 

 Thus, the floor system should have had at least a 1.5 hour fire rating. 

 The floor system in WTC 7 was specified to have a 2 h fire rating in bid 

documents consistent with the Class 1-B unsprinklered building. 

 

 The Underwriters Laboratories directory listing for floor beams in composite floors—with 

SFRM applied to the underside of the metal deck—indicates that 0.5 in. of the SFRM 

product used in WTC 7 would provide a 2 hour fire resistance rating. 

 A set of SFRM inspection reports indicated that the target thickness of the SFRM 

on the floor beams was 0.5 inch (for ―2 HR. BEAMS‖) while the applied thickness 

had an average value of 0.534 in. (standard deviation 0.054 in. and coefficient of 

variation of 0.101). The 82 observations typically ranged between 0.425 in. and 0.630 in. 

 

 The SFRM thickness applied to the framing of the composite floor system was 

consistent with a 2 h fire resistance rating.
58

Role of Fire Resistance Ratings (2) 

 Based on thermal calculations, it is unlikely that the collapse of WTC 7 would 

have been prevented had the insulation thickness on the floor beams been 

increased by 50 percent (from 0.5 in. to 0.75 in.). NIST calculations indicated that the 

time to reach the steel temperature of 649 oC would have increased by about 10 to 20 

min. 

 The ASTM E119 test does not capture critical behavior of structural systems, e.g., 

effect of thermal expansion or sagging of floor beams on girders, connections, 

and/or columns. 

 The thermal expansion of the WTC 7 floor beams that initiated the probable 

collapse sequence occurred at temperatures below approximately 400 oC. 

 Thus, to the extent that thermal expansion, rather than loss of structural 

strength, precipitates an unsafe condition, the current fire resistance rating 

system is not conservative. 
59

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
60

Principal Findings for Objective 1: Probable 

Collapse Sequence 

 WTC 7 withstood debris impact damage that resulted in seven exterior columns being severed and 

subsequently withstood conventional fires on several floors for almost seven hours. 

 The collapse of WTC 7 represents the first known instance of the total collapse of a tall building 

primarily due to fires. The collapse could not have been prevented without controlling the fires 

before most of the combustible building contents were consumed. 

 WTC 7 collapsed due to uncontrolled fires with characteristics similar to other tall buildings 

with uncontrolled fires. 

 The fires in WTC 7 were similar to those that have occurred previously in several tall buildings 

(One New York Plaza, 1970, First Interstate Bank, 1988, and One Meridian Plaza, 1991) where 

the automatic sprinklers did not function or were not present. 

 These three buildings, however, did not collapse because of differences between their 

structural designs and that of WTC 7. 

 Fires for the range of combustible contents in WTC 7 (4.0 psf on Floors 7 to 9 and 6.4 psf on 

Floors 11 to 13) persisted in any given location for approximately 20 to 30 minutes. 

 Had a water supply for the automatic sprinkler system been available and had the sprinkler 

system operated as designed, it is likely that fires in WTC 7 would have been controlled and 

the collapse prevented. 
61

Principal Findings for Objective 1: Probable 

Collapse Sequence (2) 

 Observations support a local fire origin on any given floor in WTC 7. 

 In most instances, the fire on any given floor likely initiated near the damaged southwest region, 

though collapse initiation did not occur until nearly seven hours later in the northeast region. 

 Unlike the WTC towers, there was no dispersion of jet fuel in WTC 7 causing simultaneous fire 

initiation over extensive areas of multiple consecutive floors. 

 The probable collapse sequence that caused the global collapse of WTC 7 was initiated by 

the buckling of Column 79, which was unsupported over nine stories, after local fire-induced 

damage led to a cascade of floor failures. 

 The buckling of Column 79 led to a vertical progression of floor failures up to the east penthouse 

and to the buckling of Columns 80 and 81. 

 An east-to-west horizontal progression of interior column buckling followed, due to loss of lateral 

support to adjacent columns, forces exerted by falling debris, and load redistribution from other 

buckled columns. 

 The exterior columns then buckled as the failed building core moved downward, redistributing its 

loads to the exterior columns. 

 Global collapse occurred as the entire building above the buckled region moved downward as a 

single unit. 
62

Principal Findings for Objective 1: Probable 

Collapse Sequence (3) 

 The collapse of WTC 7 was a fire-induced progressive collapse. 

 Progressive collapse—also known as disproportionate collapse—is defined as the spread of 

local damage, from an initiating event, from element to element, eventually resulting in the 

collapse of an entire structure or a disproportionately large part of it (ASCE 7-05). 

 Despite extensive thermal weakening of connections and buckled floor beams, fire-induced 

damage in the floor framing surrounding Column 79 over multiple stories was the 

determining factor causing the buckling of Column 79 and, thereby, initiating progressive 

collapse.

 This is the first known instance where fire-induced local damage (i.e., buckling failure of 

Column 79; one of 82 columns in WTC 7) led to the collapse of an entire tall building. 

o WTC 7 was prone to classic progressive collapse in the absence of fire-induced or debris impact 

damage when a section of Column 79 between Floors 11 and 13 was removed. The collapse 

sequence demonstrated a vertical and horizontal progression of failure upon the removal of the 

Column 79 section, followed by downward movement at the roofline due to buckling of exterior 

columns, which led to the collapse of the entire building. 

 The transfer elements (trusses, girders, and cantilever overhangs) and the 

―strong‖ 5th and 7th floors did not play a significant role in the collapse of WTC 7. 

Likewise, the Con Edison substation did not play a significant role in the collapse 

of WTC 7. 
63

Principal Findings for Objective 1: Probable 

Collapse Sequence (4) 

 There was no evidence of damage to the SFRM that was applied to the steel 

columns, girders, and beams, except in the vicinity of the structural damage 

from the collapse of WTC 1, which was near the west side of the south face of 

the building. This is consistent with observations NIST made in the 130 

Liberty St building that suffered similar debris damage from the collapse of 

WTC 2. 

 Even without the initial structural damage caused by debris impact from 

the collapse of WTC 1, WTC 7 would have collapsed from fires having 

the same characteristics as those experienced on September 11, 2001. 

 Early stage fires in the southwest region did not play a role in the collapse of 

WTC 7. Unlike the northeast region where collapse initiated: 

 there were no columns supporting long span floors in the southwest 

region. 

 the fires in this region were not severe enough since they had not fully 

developed and there were few combustibles in the core area adjoining the 

interior columns. 
64

Principal Findings for Objective 1: Probable 

Collapse Sequence (5) 

 Collapse time of the north face (the floors clearly visible in video evidence) 

was 40 percent greater than the computed free fall time.

• The actual time for the north face to descend 18 stories, based on video 

evidence, was approximately 5.4 s and the computed free fall time was 3.9 s. 

 A detailed analysis of the descent of the north face found 3 stages:

• A slow descent at less than 

gravitational acceleration, 

corresponding to column 

buckling 

• A free fall descent at 

gravitational acceleration over 

approximately 8 stories 

• A decreasing acceleration, as 

the north face encountered 

resistance from the structure 

below. 
65

Principal Findings for Objective 1: Probable 

Collapse Sequence (6) 

 Diesel fuel fires did not play a role in the collapse of WTC 7. The worst-case scenarios associated 

with fires being fed by ruptured fuel lines (or from fuel stored in day tanks on the lower floors): 

 could not have been sustained long enough, or could not have generated sufficient heat, to raise 

the temperature of a critical column (i.e., Column 79) to cause significant loss of strength/stiffness 

 would have produced large amounts of visible smoke that would have emanated from the exhaust 

louvers. No such smoke discharge was observed. 

 Blast events did not play a role in the collapse of WTC 7. 

 Based on visual and audio evidence and use of specialized computer modeling, NIST concluded 

that blast events did not occur, and found no evidence whose explanation required invocation of a 

blast event. 

 Blast from the smallest charge capable of failing a critical column (i.e., Column 79) would have 

resulted in a sound level of 130 dB to 140 dB at a distance of at least half a mile if unobstructed by 

surrounding buildings (such as along Greenwich Street or West Broadway); the sound be reflected 

and channeled with minimum attenuation down urban streets with hard building exteriors. 

 The sound level resulting from such a charge is consistent with a gunshot blast, standing next to a 

jet plane engine, and more than 10 times louder than being in front of speakers at a rock concert. 

 There were no witness reports of such a loud noise, nor was such a noise heard on the audio 

tracks of video recordings of the WTC 7 collapse. 
66

Principal Findings for Objective 2: Evacuation 

and Emergency Response 

 There were no serious injuries or fatalities because the estimated 4,000 occupants of WTC 7 

reacted to the airplane impacts on the two WTC towers and began evacuating before there was 

significant damage to WTC 7. 

 Evacuation drills that had been conducted every six months likely contributed to the speed 

and overall success of the evacuation. 

 Building officials held the occupants in the building lobby until they identified an exit path that was 

safe from the debris falling from WTC 1 across the street.

 Evacuation of the building took just over an hour, about 30 min longer than the estimated minimum time 

if the elevators and stairs had been used to maximum advantage. 

 Occupants were able to use both elevators and stairs, which were as yet not damaged, obstructed, 

or smoke-filled. 

 Some of the additional evacuation time was due to the considerable crowding in the lobby. 

 Occupants arrived in the lobby from both stairwells, from the elevators, and from other WTC 

buildings, and were held in the lobby until a safe exterior exit was identified.

 The decision not to continue evaluating and fighting the fires was made hours before the building 

collapsed, so no emergency responders were in or near the building when the collapse occurred. 

 Evacuation management at every level did not provide timely evacuation instructions to 

building occupants during the event. It was not clear whether specific guidance was delivered to the 

occupants via the public address system. 
67

Principal Findings for Objective 3: Procedures 

and Practices 

 WTC 7 was designed and constructed as a “tenant alteration project” of The Port 

Authority. Its design and construction were based on the 1984 edition of the Tenant 

Construction Review Manual. 

 Although the PANYNJ was not subject to the NYCBC, the 1968 NYCBC, including 

amendments to January 1, 1985, appears to have been used for the design and 

construction provisions of WTC 7, based on citations in the construction documents. 

 The type of building classification used to design and construct the building 

was not clear from the available documents. 

 Based on the height, area, primary occupancy classification, and installation of a fire 

sprinkler and standpipe system, the minimum construction type (permitted by NYCBC) 

was type 1-C (2 h protected) classification. 

 Some documentation, including some building drawings and specifications for bidders 

on the contract for applying SFRM to the structural steel, indicate a type 1-B (3 h 

protected) classification. 

 The design of WTC 7 was generally consistent with the NYCBC. 

 NIST did not conduct an exhaustive review of WTC 7 to determine its compliance with the 

NYCBC. NIST only has documentary or anecdotal evidence indicating compliance. 
68

Principal Findings for Objective 3: Procedures 

and Practices 

 Consistent with the NYCBC, there was no redundancy in the source of water 

supply for the sprinkler system in the lower 20 floors of WTC 7. Since there was 

no gravity-fed overhead tank supplying these floors, the sprinkler system could 

not function when the only source of water, which was from the street mains, was 

not available. 

 The passive fire resistance design of WTC 7 was based on catalogued ASTM E 119 

test data, which is the practice today, and was in the 1980s. Current practice for 

the fire resistance design of structures, based on the use of ASTM E 119 standard 

test method, is deficient since the method was not designed to include key fire 

effects that are critical to structural safety. Specifically, current practice does not 

capture: 

 important thermally-induced interactions between structural subsystems, 

elements, and connections—especially restraint conditions. 

 system-level interactions—especially those due to thermal expansion—since 

columns, girders, and floor subassemblies are tested separately. 

 the performance of connections under both gravity and thermal effects. 

 scale effects in buildings with long span floor systems. 
69

Principal Findings for Objective 3: Procedures 

and Practices (2) 

 Determination of the actual fire performance of the WTC 7 structural system was not the 

responsibility of any of the design professionals associated with the design, construction, or 

maintenance of WTC 7. Current practice does not require design professionals to possess the 

qualifications necessary to ensure adequate passive fire resistance of the structural system. 

 In current practice, architects typically rely on catalogued ASTM E 119 test data to specify the 

required passive fire protection that is needed for the structure to comply with the building code. 

 Architects are not required to explicitly evaluate the fire performance of the structure as a system 

(such as analyzing the effect of the thermal expansion or sagging of floor beams on girders, 

connections, and/or columns). 

 Structural engineers are not required to consider fire as a load condition in structural design. 

 Fire protection engineers may or may not be called upon to assist the architect in specifying the 

required passive fire protection. 

 Thus, none of the key professionals is assigned the responsibility to ensure the adequate fire 

performance of the structural system. 

 

 The design of WTC 7 did not include a specific analysis of how the structure might perform in a 

real fire. There is a critical gap in knowledge about how structures perform in real fires, 

particularly considering: the effects of the fire on the entire structural system; the interactions between 

the subsystems, elements, and connections; and scaling of fire test results to full-scale structures 

(especially for structures with long span floor systems). 
70

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have 

Changed The Outcome 

 Recommendations 
71

Future Factors that Could Have Mitigated 

Structural Collapse 

 More robust connections and framing systems to better resist the effects of thermal 

expansion on the structural system, which is not currently considered in design practice. 

 Structural systems expressly designed to prevent progressive collapse. The current 

model building codes do not require that buildings be designed to resist 

progressive collapse. 

 Better thermal insulation (i.e., reduced conductivity and/or increased thickness) to limit 

heating of structural steel and to minimize both thermal expansion and weakening effects. 

Insulation has been used to protect steel strength, but it could be used to maintain 

a lower temperature in the steel framing to limit thermal expansion. 

 Automatic fire sprinkler systems with independent and reliable sources for the primary and 

secondary water supply. 

 Improved compartmentation in tenant areas to limit the spread of fires. 

 Thermally resistant window assemblies which limit breakage, reduce air supply, and retard 

fire growth. 
72

Human Performance Factors 

 Human performance factors contributed to the outcome of no loss of life at WTC 7. 

 Reduced number of people in WTC 7 at the times of airplane impact on the 

towers. 

 Participation of the building occupants in recent fire drills. 

 Shortness of delay in starting to evacuate. 

 Evacuation assistance provided by emergency responders to evacuees. 

 Decision not to continue reconnaissance of the building and not to fight the fires 

within. 

 Other human performance factors did not play a life safety role in WTC 7 on 

September 11, 2001, but could have been important had the fires been more 

widespread, the building damage more severe, or the building occupancy at full 

capacity. 

 Accuracy and reliability of communications among emergency responders and 

building occupants. 

 Efficiency of management of large-scale emergency incidents. 
73

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
74

WTC 7 Recommendations 

 Based on its findings, NIST identified one new recommendation and reiterated 12 

recommendations from its Investigation of the WTC towers. 

 The urgency of prior recommendations is significantly reinforced by their pertinence to 

the collapse of a tall building with a structural system design that is in widespread use. 

 A few of the prior recommendations have been modified to reflect the findings of this 

Investigation. 

• The 13 recommendations for improvements to codes, standards, and practices fall 

into 7 of the 8 major groups that were identified for the recommendations from 

the investigation of the WTC towers: 

 Increased Structural Integrity 

 Enhanced Fire Resistance of Structures 

 New Methods for Designing Structures to Resist Fires 

 Improved Active Fire Protection 

 Improved Building Evacuation (not applicable for WTC 7) 

 Improved Emergency Response 

 Improved Procedures and Practices 

 Education and Training Programs 

Structural 

Design 

Life 

Safety 

Systems 

Practices & 

Development 
75

Comparison of WTC 7 and WTC Towers 

 WTC 7 was unlike the WTC towers in many respects: 

 It was a more typical tall building in the design of its structural system. 

 It was not struck by an aircraft. 

 The fires in WTC 7 were quite different from those in the towers. Since WTC 7 

was not doused with thousands of gallons of jet fuel, large areas of any floor 

were not ignited simultaneously. 

 Instead, the fires in WTC 7 were similar to those that have occurred in several 

tall buildings where the automatic sprinklers did not function or were not present. 

 

 These other buildings did not collapse due to differences in structural design, 

while WTC 7 succumbed to its fires. 
76

Key Premise 

 The partial or total collapse of a building due to fires is an infrequent event. 

 This is particularly true for buildings with a reliably operating active fire 

protection system such as an automatic fire sprinkler system. 

 A properly designed and operating automatic sprinkler system will contain fires 

while they are small and, in most instances, prevent them from growing and 

spreading to threaten structural integrity. 

 The intent of current practice, based on prescriptive standards and codes, is to 

achieve life safety, not collapse prevention. 

 Buildings, however, should not collapse in infrequent (worst-case) fires 

that may occur when active fire protection systems are rendered 

ineffective (e.g., when sprinklers do not exist, are not functional, or are 

overwhelmed by the fire). 

 Fire scenarios for structural design based on single compartment or single floor 

fires are not appropriate representations of infrequent fire events. Such events 

occurred in several tall buildings resulting in unexpected substantial losses. 
77

Historic Tall Building Fires 

First Interstate Bank 

Los Angeles, 1988 

Los Angeles Times Photo by Boris Yaro 

One Meridian Plaza 

Philadelphia, 1991 

Michael Wirtz/Philadelphia Inquirer 
78

Characteristics of Infrequent (Uncontrolled) Fire 

Events for Structural Design Based on Historical Data 

 Ordinary combustibles and combustible load levels. 

 Local fire origin on any given floor. 

 No widespread use of accelerants. 

 Consecutive fire spread from combustible to combustible. 

 Fire-induced window breakage providing ventilation for continued fire 

spread and accelerated fire growth. 

 Concurrent fires on multiple floors.

 Active fire protection systems rendered ineffective (e.g., sprinklers do not 

exist, are not functional, or are overwhelmed by fire). 

 The fires in WTC 7 had all of these characteristics. 
79

Recommendations for Group 1: Increased 

Structural Integrity 

Recommendation A (NIST NCSTAR 1 Recommendation 1). NIST recommends 

that: 

 progressive collapse be prevented in buildings through the development 

and nationwide adoption of consensus standards and code provisions, 

along with the tools and guidelines needed for their use in practice; and 

 a standard methodology be developed—supported by analytical design 

tools and practical design guidance—to reliably predict the potential for 

complex failures in structural systems subjected to multiple hazards. 

Relevance to WTC 7: Had contemporaneous standards and practices been available 

to expressly design WTC 7 for prevention of fire-induced progressive collapse, it would 

have been sufficiently robust to withstand local failure due to the fires without suffering 

total collapse. 
80

Recommendations for Group 2: Enhanced Fire 

Endurance of Structures 

Recommendation B (New), NIST recommends that buildings be explicitly evaluated 

to ensure the adequate performance of the structural system under maximum 

credible (infrequent) design fires with any active fire protection system rendered 

ineffective. Of particular concern are the effects of thermal expansion in buildings 

with one or more of the following features: 

 long-span floor systems which experience significant thermal expansion and sagging 

effects, 

 connection designs (especially shear connections) that cannot accommodate thermal 

effects, 

 floor framing that induces asymmetric thermally-induced (i.e., net lateral) forces on girders, 

 shear studs that could fail due to differential thermal expansion in composite floor 

systems, and 

 lack of shear studs on girders. 

Careful consideration should also be given to the possibility of other design features that may 

adversely affect the performance of the structural system under fire conditions. 

Relevance to WTC 7: The effects of restraint of free thermal expansion on the steel 

framing systems, especially for the long spans on the east side of WTC 7, were not 

considered in the structural design and led to the initiation of the building collapse. 
81

Details of Recommendation B (New) 

 Typical floor span length in tall office buildings is in the range of 40 ft to 50 ft; 

this range is considered to represent long-span floor systems. 

 Thermal effects (e.g., thermal expansion) that may be significant in long-span 

buildings may also be present in buildings with shorter span lengths, 

depending on the design of the structural system. 

 Building owners, operators, and designers are strongly urged to act 

upon this recommendation. Engineers should be able to design costeffective fixes to address any areas of concern identified by the evaluations 

using existing, emerging, or anticipated future capabilities. 

 Industry should partner with the research community to fill critical gaps in 

knowledge about how structures perform in real fires, particularly 

considering: 

 the effects of fire on the entire structural system; 

 the interactions between subsystems, elements, and connections; 

 scaling of fire test results to full-scale structures, especially for structures 

with long span floor systems. 
82

Recommendations for Group 2: Enhanced Fire 

Endurance of Structures (2) 

Recommendation C (NIST NCSTAR 1 Recommendation 4). NIST recommends evaluating, and 

where needed improving, the technical basis for determining appropriate construction 

classification and fire rating requirements (especially for tall buildings)—and making related code 

changes now as much as possible—by explicitly considering factors including:

 timely access by emergency responders and full evacuation of occupants, or the time required for 

burnout without partial collapse;

 the extent to which redundancy in active fire protection (sprinkler and standpipe, fire alarm, and smoke 

management) systems should be credited for occupant life safety;

 the need for redundancy in fire protection systems that are critical to structural integrity; 

 the ability of the structure and local floor systems to withstand a maximum credible fire scenario without 

collapse, recognizing that sprinklers could be compromised, not operational, or non-existent;

 compartmentation requirements (e.g., 12,000 ft2) to protect the structure, including fire rated doors and 

automatic enclosures, and limiting air supply (e.g., thermally resistant window assemblies) to retard fire 

spread in buildings with large, open floor plans;

 the effect of spaces containing unusually large fuel concentrations for the expected occupancy of the 

building; and

 the extent to which fire control systems, including suppression by automatic or manual means, should be 

credited as part of the prevention of fire spread. 

Relevance to WTC 7: The floor systems in WTC 7 failed at shorter fire exposure times than the specified fire rating (two 

hours) and at lower temperatures because thermal effects within the structural system, especially thermal expansion, were 

not considered in setting the fire rating requirements in the construction classification, which are determined using the 

ASTM E 119 or equivalent testing standard. Such evaluation is not required under current codes and standards. 
83

Recommendations for Group 2: Enhanced Fire 

Endurance of Structures (3) 

Recommendation D (NIST NCSTAR 1 Recommendation 5). NIST recommends that 

the technical basis for the century-old standard for fire resistance testing of 

components, assemblies, and systems be improved through a national effort. 

Necessary guidance also should be developed for extrapolating the results of 

tested assemblies to prototypical building systems. A key step in fulfilling this 

recommendation is to establish a capability for studying and testing the 

components, assemblies, and systems under realistic fire and load conditions. 

Relevance to WTC 7: The floor systems failed in WTC 7 at shorter fire exposure times 

than the specified fire rating (two hours) and at temperatures lower than the endpoint 

criteria (593 C) because thermal effects within the structural system, especially thermal 

expansion, were not considered in setting the endpoint criteria when using the ASTM E 

119 or equivalent testing standard. 

 The structural breakdowns that led to the initiating event and the eventual collapse of 

WTC 7 occurred at temperatures that were hundreds of degrees below the criteria 

that determine structural fire resistance ratings. 

 The United States currently does not have the capability for studying and 

testing these important fire-induced phenomena critical to structural integrity. 
84

Recommendations for Group 2: Enhanced Fire 

Endurance of Structures (4) 

Recommendation E (NIST NCSTAR 1 Recommendation 7). NIST recommends the 

adoption and use of the ―structural frame‖ approach to fire resistance ratings. 

 The definition of the primary structural frame should be expanded to include 

bracing members that are essential to the vertical stability of the primary 

structural frame under gravity loading (e.g., girders, diagonal bracing, 

composite floor systems that provide lateral bracing to the girders) whether or 

not the bracing members carry gravity loads. 

Relevance to WTC 7: Thermally-induced breakdown of the floor system in WTC 7 was 

a determining step in causing collapse initiation and progression. Therefore, the floor 

system should be considered as an integral part of the primary structural frame. 
85

Recommendations for Group 3: New Methods 

for Fire Resistant Design of Structures 

Recommendation F (NCSTAR Recommendation 8). NIST recommends that the fire 

resistance of structures be enhanced by requiring a performance objective that 

uncontrolled building fires result in burnout without partial or global (total) collapse. 

Current methods for determining the fire resistance rating of structural assemblies do not 

explicitly specify a performance objective.

Relevance to WTC 7: Large, uncontrolled fires led to failure of a critical column and 

consequently the complete collapse of WTC 7. In the region of the collapse initiation (i.e., 

on the east side of Floor 13), the fire had not consumed the combustible building contents, 

yet collapse occurred. 
86

Recommendations for Group 3: New Methods 

for Fire Resistant Design of Structures (2) 

Recommendation G (NIST NCSTAR 1 Recommendation 9). NIST recommends the 

development of: 

 performance-based standards and code provisions, as an alternative to 

current prescriptive design methods, to enable the design and retrofit of 

structures to resist real building fire conditions, including their ability to 

achieve the performance objective of burnout without structural or local floor 

collapse: and 

 the tools, guidelines, and test methods necessary to evaluate the fire 

performance of the structure as a whole system. Standards development 

organizations, including the American Institute of Steel Construction, have already 

begun developing performance-based provisions to consider the effects of fire in 

structural design. 

Relevance to WTC 7: A performance-based assessment of the effects of fire on WTC 7, 

had it considered all of the relevant thermal effects (e.g., thermal expansion effects that 

occur at lower temperatures), would have identified the vulnerability of the building to fireinduced collapse and allowed alternative designs for the structural system. 
87

Recommendations for Group 4: Improved 

Active Fire Protection 

Recommendation H (NIST NCSTAR 1 Recommendation 12). NIST recommends 

that the performance and possibly the redundancy of active fire protection 

systems (sprinklers, standpipes/hoses, fire alarms, and smoke management 

systems) in buildings be enhanced to accommodate the greater risks associated 

with increasing building height and population, increased use of open spaces, 

high-risk building activities, fire department response limits, transient fuel loads, 

and higher threat profile. 

Relevance to WTC 7: No water was available for the automatic suppression system on 

the lower 20 stories of WTC 7 once water from street-level mains was disrupted. This 

lack of reliability in the source of the primary and secondary water supply allowed the 

growth and spread of fires that ultimately resulted in collapse of the building. 
88

Recommendations for Group 6: Improved 

Emergency Response 

Recommendation I (NIST NCSTAR 1 Recommendation 24). NIST recommends the 

establishment and implementation of codes and protocols for ensuring effective and 

uninterrupted operation of the command and control system for large-scale building 

emergencies.

Relevance to WTC 7: 

 The New York City Office of Emergency Management (OEM) was located in WTC 7 

and was evacuated early in the day before key fire ground decisions had to be made. 

The location of OEM in WTC 7 contributed to the loss of robust interagency command 

and control on September 11, 2001. 

 Due to the collapse of the WTC towers and the loss of responders and fire control 

resources, there was an evolving site leadership during the morning and afternoon. 

Key decisions (e.g., decisions not to fight the fires in WTC 7 and to turn off the power 

to the Con Edison substation) were reasonable and would not have changed the 

outcome on September 11, 2001, but were not made promptly. Under different 

circumstances (e.g., if WTC 7 had collapsed sooner and fire fighters were still 

evaluating the building condition), the outcome could have been very different. 
89

Recommendations for Group 7: Improved 

Procedures and Practices 

Recommendation J (NIST NCSTAR 1 Recommendation 27). NIST recommends that 

building codes incorporate a provision that requires building owners to retain 

documents, including supporting calculations and test data, related to building 

design, construction, maintenance and modifications over the entire life of the 

building. Means should be developed for offsite storage and maintenance of the 

documents. In addition, NIST recommends that relevant building information be 

made available in suitably designed hard copy or electronic format for use by 

emergency responders. Such information should be easily accessible by 

responders during emergencies. 

Relevance to WTC 7: The efforts required in locating and acquiring drawings, 

specifications, tenant layouts, material certifications, and, especially, shop fabrication 

drawings, significantly lengthened the investigation into the collapse of WTC 7. 
90

Recommendations for Group 7: Improved 

Procedures and Practices (2) 

Recommendation K (NIST NCSTAR 1 Recommendation 28). NIST recommends that 

the role of the ―Design Professional in Responsible Charge‖ be clarified to ensure 

that: 

 all appropriate design professionals (including, e.g., the fire protection 

engineer) are part of the design team providing the standard of care when 

designing buildings employing innovative or unusual fire safety systems, and 

 all appropriate design professionals (including, e.g., the structural engineer 

and the fire protection engineer) are part of the design team providing the 

standard of care when designing the structure to resist fires, in buildings that 

employ innovative or unusual structural and fire safety systems. 

Relevance to WTC 7: Following typical practice, none of the design professionals in 

charge of the WTC 7 project (i.e., architect, structural engineer, and fire protection 

engineer) was assigned the responsibility to explicitly evaluate the fire performance of the 

structural system. Holistic consideration of thermal and structural factors during the 

design or review stage could have identified the potential for the failure and might have 

prevented the collapse of the building. 
91

Recommendations for Group 8: Education and 

Training 

Recommendation L (NIST NCSTAR 1 Recommendation 29). NIST recommends that 

continuing education curricula be developed and programs be implemented for : 

 training fire protection engineers and architects in structural engineering 

principles and design, 

 training structural engineers, architects, fire protection engineers, and code 

enforcement officials in modern fire protection principles and technologies, 

including fire-resistance design of structures, and 

 training building regulatory and fire service personnel to upgrade their 

understanding and skills to conduct the review, inspection, and approval 

tasks for which they are responsible. 

The outcome would further the integration of the disciplines in effective fire-safe design of 

buildings. 

 

Relevance to WTC 7: Discerning the fire-structure interactions that led to the collapse of 

WTC 7 required research professionals with expertise in both disciplines. Assuring the 

safety of future buildings will require that participants in the design and review processes 

possess a combined knowledge of fire science, materials science, heat transfer, and 

structural engineering and design. 
92

Recommendations for Group 8: Education and 

Training 

Recommendation M (NIST NCSTAR 1 Recommendation 30). NIST recommends 

that academic, professional short-course, and web-based training materials in 

the use of computational fire dynamics and thermostructural analysis tools be 

developed and delivered to strengthen the base of available technical 

capabilities and human resources. 

Relevance to WTC 7: NIST stretched the state-of-the-art in the computational tools 

needed to reconstruct a fire-induced building collapse. This enabled identification of 

the critical processes that led to that collapse. Making these expanded tools and 

derivative, validated, and simplified modeling approaches usable by practitioners could 

prevent future disasters. 
93

Implementing the WTC Recommendations 

 NIST believes these recommendations are realistic, 

appropriate, and achievable within a reasonable period of 

time. 

 NIST strongly urges that immediate and serious 

consideration be given to these recommendations by the 

building and fire safety communities. 

 Implementation of these recommendations will achieve 

appropriate improvements in the way buildings are 

designed, constructed, maintained, and used—with the goal 

of making buildings safer in future emergencies. 
94

Implementing the WTC Recommendations (2) 

• NIST has assigned top priority to work vigorously with the building and fire safety 

communities to assure that there is a complete understanding of the recommendations and 

to provide needed technical assistance. 

• NIST held a conference September 13-15, 2005 attended by over 200 people, including 

all major standards and codes developers. 

• NIST awarded a contract to the National Institute of Building Sciences—a Congressionally 

authorized non-profit, non-governmental organization—to turn appropriate 

recommendations into code language suitable for submission of code change proposals 

to the two national model code developers. NIBS has drawn upon broad, representative 

group of building code experts to carry out this task. 

• Key U.S. standards and codes development organizations are seriously considering 

the WTC recommendations and the first comprehensive set of model building code 

changes have been adopted in the International Building Code. 

• The WTC recommendations already are having a significant impact on the 

construction of iconic high-rise buildings worldwide, including the Freedom Tower. 

• NIST has implemented a web-based system so that the public can track progress on 

implementing the recommendations. 
95

Changes to Building Codes and Standards 

• First comprehensive set of eight model building code changes based on 

recommendations from NIST’s WTC investigation adopted by the International 

Building Code in the 2007 supplement to the 2006 IBC.

• A set of 23 comprehensive and far reaching model building code changes based on 

NIST’s WTC recommendations were approved by the ICC for the 2009 edition of the 

IBC and the IFC. 

• NIST’s WTC Recommendations have spurred actions to develop new provisions/ 

guidelines within other standards, codes, and industry organizations, such as: 

• National Fire Protection Association 

• American Society of Mechanical Engineers 

• ASTM International 

• American Society of Civil Engineers 

• Council on Tall Buildings and Urban Habitat 
96

 Web site http://wtc.nist.gov 

Email to wtc@nist.gov 

Facsimile to (301) 975-6122

Regular mail: 

WTC Technical Information Repository, Stop 8610, 

100 Bureau Drive, Gaithersburg, MD 20899-8610.
97

Critical Analysis Inter-Dependencies 
98

Early Fire Development (10:28 am to 2:30 pm) 

 Ignition of local fires on several floors of WTC 7 was likely caused by flaming debris, impact-induced 

electrical failures, etc., on the southwest corner due to collapse of WTC 1. Each ignition was likely a 

single workstation component or office furnishing item; growth over the full cluster of workstations or 

office took several minutes. 

 Floors 7 through 9: 

 Initial fire spread by flame contact with an adjacent workstation. 

 Once a cluster was burning, radiative ignition of a nearby workstation would have occurred (across 

an aisle); by the time this second cluster was fully involved, the prior cluster would have burned out. 

 The path of the fires likely jumped from workstation to workstation, meandering toward the windows, 

toward the core, or parallel to the façade. 

 Eventually the upper gas layer over enough of the huge open space would likely be hot enough for 

radiatively enhanced ignition of multiple workstations, i.e., non-linear growth. 

 The fires on Floors 7 and 8 didn't enhance much by 2:30 p.m. 

 Floors 11 through 13: 

 The fire would likely grow within an office, reaching flashover in several minutes. 

 After about 15 minutes, the ceiling tile system would likely fail and the hot gases would create a 

local hot upper layer. 

 Adjacent offices would likely ignite. Offices across a corridor would likely ignite more slowly. 

 Fires on the 7th and 8th floors couldn't spread to the east along the south face because of barriers to 

spread or lack of combustibles. The interior layout of the 9th floor was not well documented; fires were 

not observed until about 4:00 pm. The office-to-office spread along the south of Floors 11 through 13 

was slow due to presence of walls. 
99

LS-DYNA Model Loading Sequence 

1. Gravity loads were applied smoothly over 

4.5 s using a sinusoidal curve to 

eliminate dynamic response. 

2. Debris impact damage from the collapse 

of WTC 1 was then applied 

instantaneously by removing elements 

from the model. The structure was then 

allowed to damp residual vibrations over 

2 s. 

3. Elevated temperatures from the ANSYS 

thermal analysis were next applied 

smoothly over 2 s using a sinusoidal 

curve to eliminate dynamic response. 

4. Fire-induced damage (buckled beams 

and connection capacities that were 

reduced) obtained from the 16-story 

ANSYS structural analysis was finally 

applied instantaneously and the 

computation model proceeded to initiate 

and propagate collapse, up to global 

collapse. 

 The response of the building with fire-induced damage from the 

16-story ANSYS analysis at 4.0 h (both with and without debris 

impact damage) was consistent with the timing of key events in 

the recorded videos of the collapse. 

 The response of the building to fire-induced damage from the 

16-story ANSYS analysis at 3.5 h (damage in the south and 

southeast regions of WTC 7 on multiple floors) did not result in 

collapse of the building. 

 The LS-DYNA analysis approach minimized any spurious 

dynamic effects associated with loading sequence. 
100

Estimated Debris Impact Damage by Floor 

Floor 8 Floor 9 

Floors 10 and 11 Floors 12 to 14 
101

Details of Collapse Initiation (1) 

 The simple shear connection between Column 79 and the girder that spanned the distance to 

the north face (to Column 44) failed on Floor 13. 

 The connection failed due to shearing of erection bolts, caused by lateral thermal expansion 

of floor beams supporting the northeast floor system. 

 Further thermal expansion of the floor beams pushed the girder off its seat. 

 The displaced girder and other local fire-induced damage led to the failure of the floor 

system surrounding Column 79 on Floor 13. 

 The collapse of Floor 13 onto the floors below—some of which were already weakened by 

fires—triggered a cascade of floor failures in the northeast region. 

 This, in turn, led to loss of lateral support to Column 79 in the east-west direction over nine 

stories (between Floors 5 and 14). 

 The increase in unsupported length led to the buckling failure of Column 79, which was the 

collapse initiation event. 
102

Details of Collapse Initiation (2) 

 The failure of the girder-to-column connections was caused primarily by the thermal expansion 

of the large span-length northeast floor beams. 

 Additional factors that contributed to the failure were the absence of girder shear studs that 

would have provided lateral restraint and the one-sided lateral support to the girder provided by 

the northeast corner floor beams. 

 

 In addition to the complete failure of the connection to Column 79 for the girder spanning to 

Column 44 on Floor 13 with the most severe fire condition, the same connection on Floors 8 to 

12 were partially damaged due to the failure of some or all of the 4 bolts. 

 Complete failure of the connection required girder walk off in addition to the failure of all four 

bolts. 

 The temperatures in the columns in WTC 7 did not exceed 300 °C and, therefore, the columns 

did not buckle due to direct fire-induced thermal expansion or weakening. 

 The interior columns were not thermally restrained and, therefore, did not develop increased 

loads due to thermal expansion which could have led to buckling. 
103

Details of Horizontal Progression 

 The first north-south line of core columns (i.e., Columns 76 to 78) buckled 

since they had lost lateral support, were impacted by falling debris, and had 

loads redistributed to them from adjacent buckled columns. 

 Truss 2, which collapsed due to accumulated weight and impact loads from 

falling debris, precipitated the buckling of Columns 77 and 78 a few moments 

earlier than Column 76. The failure of Truss 2 caused the failure of the 

transfer girder linking Columns 77 and 78A which supported Column 78. 

 Truss 2 was not critical to the horizontal progression of failure. 
104

WTC 7 Collapse Initiation w/o Debris Damage 

View from 

South 
105

WTC 7 Collapse Visualization w/o Damage 

View from 

South 
106

Principal Findings for Objective 3: Procedures 

and Practices (3) 

 The calculated stairwell capacity was insufficient to meet the requirements of the NYCBC in 

effect during the design and construction period, if the building were occupied at the calculated 

maximum level ( 14 000 people). 

 The capacity was sufficient for the normal occupancy of the building ( 8 000 workers plus 

visitors), estimated by NIST, and was more than sufficient for the occupancy on September 

11, 2001 ( 4 000 people), also estimated by NIST. 

 The stairwell capacity met the requirement of the (subsequent) 2000 edition of the IBC, but 

not the 2003 edition of NFPA 5000. 

 The separation of the stairwell doors met the requirement of the 1968 NYCBC. 

 On some floors, the separation of the stairwell doors was below the remoteness 

requirements in the 2000 IBC and the 2003 NFPA 5000.