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 

August 26, 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 north of the WTC Complex.  Built over an existing Con Edison 

electric power substation, on land 

owned by The Port Authority of 

New York and New Jersey. 

 On September 11, 2001, fires 

burned for almost seven hours, 

from the collapse of 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 2-story lobbies. The north side of the 1st

and 2nd stories was the Con Edison substation. 

 Floors 5 and 6 were 

mechanical spaces with 

transfer elements. 

 Floors 7 through 45 were 

tenant floors. 

 The 46th and 47th floors, 

while mainly tenant floors, 

were structurally reinforced 

to support special loads, 

such as cooling towers. 

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 

East Elevation columns. 

(Core)

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

64 73 66 69 72 75 75 74 73 7654321 7654321

7654321

67 70 `

753

North Elevation South Elevation West Elevation East Elevation

24

22

Ramp Con Edison Substation

421 15 281 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 

Truss 1 

Truss 2 

Truss 3 

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. 
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 slab was 14 in. thick. 

 On Floor 7, had a 8 in. total slab thickness. 

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

Floor 7 

Elevator 

Banks Stairs Stairs

Core 

Area Composite 

Floor 

System

Floors 8 to 45 
13

Roof Layout 

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
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 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

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? 
17

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
18

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 

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 
19

Fire Growth and Spread 

Progression of fires on Floor 12 

• The major fires in WTC 7 were 

modeled using the Fire Dynamics 

Simulator (FDS). 

 There were far fewer photographs and 

videos of WTC 7 than of the towers. 

 The fire simulations for WTC 7 were 

conducted for each floor individually. 

 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 

collapsing towers. 

1:00 pm 

2:00 pm 

3:00 pm 

4:00 pm 

5:00 pm 
20

FDS Simulation of Fires on Floor 12 
21

 Thermal Response to Fire 

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

3:30 p.m. 4:00 p.m. 4:30 p.m. 5:00 p.m. 
22

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 

 Computer 

 64-bit workstation 

 quad-core, 3.0 GHz processor 

 64 GB of random access 

memory (RAM) 

 Analysis Time 

 approximately 6 months 

 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. 
23

LS-DYNA Structural Response Model 

 The LS-DYNA model was used to 

determine the sequence of events 

that led to global collapse. 

 Software: double precision version of 

LS-DYNA 

 Model 

 3,593,049 Nodes 

 3,045,925 Elements 

 Computer 

 High speed Linux computer 

cluster. 

 1.5 terabytes of RAID 5 disk 

storage 

 64-bit 2.6 GHz processors, with 

8 GB to 16 GB of RAM 

 Analysis Time 

 Up to 8 weeks 
24

Summary of Probable Collapse Sequence (1) 

Aerial photograph shot 14 s ± 0.5 s 

after WTC 1 began to collapse. 

 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.
25

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. 
26

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 
27

Summary of Probable Collapse Sequence (2) 

 Photo showing north and east faces. 

 Likely taken between 3:20 - 3:40 p.m. 

 Growth and Spread of Fires. 

 Fires on 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 beginning 

at about 3 p.m. to 4 p.m. 

 The local fires on Floors 19, 

22, 29, and 30 were not 

observed after ~ 1 p.m. 
28

Observed Fires 

 Cropped photograph of north face taken 

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

 Cropped photograph 

showing part of north face 

at 4:39 p.m. ± 120 s. 
29

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. Blue indicates 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. 
30

 Initial Local Failure for Collapse Initiation. 

 Fire-induced thermal expansion of the floor system surrounding 

Column 79 led to the collapse of Floor 13 and 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) 
31

North East Floor System Near Column 79 

Seat 

Bolts 

Column 79 

Girder 
32

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 one-sided framing 

and lack of shear studs. 

Forces from thermal 

expansion failed the 

connection at Column 

79, then pushed the 

girder off the seat. 
33

Physics-Based Visualization of WTC 7 Collapse 

Initiation 

View 

from 

South 
34

Summary Probable Collapse Sequence (4) 

 Vertical Progression of Failure. 

 Buckling of Column 79 triggered a vertical progression of floor system 

failures to the east penthouse, and subsequent cascading failure of 

Columns 80 and 81 on the east side of the building, that spread to include 

the entire east region all the way to the top of the building. 

 

View of interior 

columns 
35

Summary of Probable Collapse Sequence (5) 

 Horizontal Progression of Failure. 

 Interior columns buckled from east to west in the lower floors due to 

(1) loss of lateral support from floor system failures, (2) forces exerted 

by falling debris, and (3) load redistributed from other buckled columns. 

Buckling of all interior columns at 6.5 s 

following collapse initiation. 

Buckling of third line of interior columns 

at 4.5 s following collapse initiation. 
36

Summary Probable Collapse Sequence (6) 

 Global Collapse. 

 The exterior columns buckled between Floors 7 and 14 due to load 

redistribution to these columns as the building core moved downward. 

 The entire building above the buckled-column region then moved 

downward in a single unit, as observed. 

Buckling of lower exterior columns. 
37

Video Evidence of WTC 7 Collapse 
38

Physics-Based Visualization of WTC 7 Collapse 

View 

from 

Northwest 
39

Physics-Based Visualization of WTC 7 Collapse 

View 

from 

South 
40

Collapse Initiation Observations 

Exterior Observations 

 East Penthouse 

Kinks 

 Windows break on 

East Side of North 

Face 

Vertical Collapse 

Progression 

East Penthouse 

Screen Wall 

West Penthouse 

40th Fl

47th Fl

30th Fl

Window 

Breakage 

Column Buckles 

14th Fl

7

th Fl

Floors 

Fail 
41

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 

47th Floor 

Kink in North 

Facade 
42

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 

Windows broke 

between 

Columns 54 

and 57 

Kink formed 

in roof of 

East 

Penthouse 

Kink formed 

in north face 

N 

East and west 

edges of penthouse 

rotated inward 

Kink formed 

in East 

Penthouse 

Floor beams 

collapsed 

Core 

framing 

Column 79 

East 

Penthouse 

Core 

Windows 

broke 

between 

Columns 

43 and 45 
43

Vibration Analysis of Video 

No 

Vibration 

Floors 

Collapsed 

East Penthouse 

Begins to Collapse 

Horizontal 

Progression of 

Collapse 

Beginning 

of

Global 

Collapse 

Column 79 

Buckled 
44

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 
45

Accuracy of the Probable Collapse Sequence 

Event Observation 

Time (s) 

Analysis 

w/Debris

Damage (s) 

Analysis 

w/o Debris 

Damage (s) 

Cascading failure of floors 

surrounding Column 79  -6 s -6.6 s -6.6 s 

Buckling of Column 79, followed 

by Columns 80 and 81

Not 

observable -1.3 -1.4 

East penthouse starts to fall 0.0 0.0 0.0 

East penthouse falls below roofline 2.0 2.4, 2.7 2.3, 2.6 

Buckling of columns across core Not 

observable 3.5-6.1 3.2-13.5 

Initial downward motion of the 

north face roofline 6.9 6.3 9.8 

East end of the screenwall falls 

below the roofline 8.5 7.3, 7.7 8.7, 9.2 

West penthouse falls below the 

roofline 9.3 6.9, 7.3 10.6, 10.9 

Agreement between observations and simulations is good; results with debris 

damage better match observations of overall global collapse sequence. 
46

WTC 7 Collapse Visualization w/o Damage 

View from 

Northwest 
47

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
48

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 gallon 

tanks. 

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

 At most, 1000 gal of fuel was unaccounted from these tanks, which was 

equivalent to about 5 percent of the office combustibles on a single floor. 

 The day tanks did not contain enough fuel to be a significant contributor. 

 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. 
49

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. 

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

 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 heat 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 smoke from the exhaust 

louvers; 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. 
50

Role of Fuel Oil Fires (3) 

 The day tank on Floor 5 supplying 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 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 a 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.
51

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 was identified. 

 The recommended column preparation required at least 30 minutes 

(cutting and placing). 

 Additional preparation time was required to prepare the column for cutting 

and placing charges. 

 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 charge of 9 pounds. 

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

evaluate threshold explosive requirements for breaking windows. 

 Two partition layouts, based on the tenant area layouts, were analyzed. 
52

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. 

 For all four scenarios, windows would have been broken and significant 

sound levels were predicted from all building faces. 

 The sound level at 1 km (1/2 mile) would have been approximately 130 

to140 dB. 

Peak over pressures for 9 lb shaped charge (left) and 2lb charge (right). 
53

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. 
54

Role of Hypothetical Blast Events (4) 

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

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

 Preparing the column includes cutting sections with torches, careful 

placement of charges, and an initiation device. 

 A larger charge would be required to fail the column without prior preparation; 

the resulting sound level and window breakage would be much greater. 

 Demolition usually prepares many interior columns with explosive charges. 

 It is unlikely 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 demolition where 

charges are placed across the building and detonated in rapid succession. 
55

Role of Con Edison Substation 

 The Con Edison substation played no role in the fires that caused the 

collapse of WTC 7. 

 Electronic communications 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. 

 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. 

 There were no other indications that a fire occurred within the substation 

during that period of time. 

 Likely causes of the alarm were smoke and dust from the collapse of WTC 1. 
56

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 indicates that 0.5 in. of the SFRM product 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. 

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

was consistent with a 2 h fire resistance rating.
57

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. 
58

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
59

Principal Findings for Objective 1: Probable 

Collapse Sequence 

 WTC 7 withstood debris impact damage that severed seven exterior columns 

and subsequent 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. 

 WTC 7 collapsed due to uncontrolled fires with characteristics similar 

to previous fires in tall buildings. 

 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. 

 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. 
60

Principal Findings for Objective 1: Probable 

Collapse Sequence (2) 

 Observations support a single point of fire ignition on any given floor. 

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

damaged southwest region. 

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

causing simultaneous fire initiation over extensive areas of multiple floors. 

 The probable collapse sequence 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. 

 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. 
61

Principal Findings for Objective 1: Probable 

Collapse Sequence (3) 

 The collapse of WTC 7 was a 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.

 The transfer elements (trusses, girders, and cantilever overhangs) 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. 
62

Principal Findings for Objective 1: Probable 

Collapse Sequence (3) 

 Prior to the collapse, there was no 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. 

 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 adjacent core area. 

 Collapse time of the north face of the upper 18 floors (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 fall 18 floors was ~ 5.4 s 

 The computed time for free fall (i.e., with no air friction) was 3.9 s.
63

Principal Findings for Objective 1: Probable 

Collapse Sequence (4) 

 Diesel fuel fires did not play a role in the collapse of WTC 7. The worstcase scenarios for fires being fed by ruptured fuel lines or day tanks: 

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

cause significant loss of strength/stiffness in a critical column 

 would have produced large amounts of smoke from the exhaust louvers. No 

such smoke discharge was observed. 

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

 Based on visual and audio evidence and computer modeling, NIST 

concluded that blast events did not occur, and found no evidence of a blast. 

 Blast from the smallest charge capable of failing a critical column would have 

resulted in a sound level of 130 dB to 140 dB at approximately half a mile. 

 This sound level is consistent with a gunshot blast, standing next to a jet 

engine, and 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. 
64

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 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 evacuation took just over an hour, about 30 min longer than the 

estimated minimum time for the elevators and stairs. 

 Occupants were able to use both elevators and stairs. 

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

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

WTC buildings, and were held in the lobby until a safe 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. 
65

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 followed the requirements of 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, indicate a type 1-B (3 h protected) classification. 

 The design of WTC 7 was generally consistent with the NYCBC. 
66

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. 

 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: 

 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. 
67

Principal Findings for Objective 3: Procedures 

and Practices (2) 

 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 to comply with the building code. 

 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.

 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). 
68

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have 

Changed The Outcome 

 Recommendations 
69

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. 

 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. 
70

Human Performance Factors 

 There were factors that 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. 

 There were also factors that 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. 
71

Outline 

 Investigation Objectives 

 WTC 7 Building Design 

 Probable Collapse Sequence 

 Other Possible Hypotheses 

 Principal Findings 

 Factors that Could Have Changed 

The Outcome 

 Recommendations 
72

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. 

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

practices fall into 7 of the 8 major groups : 

 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 
73

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. 
74

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. 
75

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 
76

Characteristics of Infrequent (Worst-Case) 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. 
77

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 WTC 7 been expressly designed 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. 
78

Recommendations for Group 2: Enhanced Fire 

Endurance of Structures 

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

evaluated to ensure adequate performance of the structural system under 

worst-case design fires with active fire protection systems rendered 

ineffective. Of particular concern are buildings with one or more of the 

following features: 

 long-span floor systems, connection designs that cannot 

accommodate thermal effects, floor framing that induces asymmetric 

thermally-induced 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 thermal expansion on steel 

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

considered in the structural design and led to the initiation of global collapse. 
79

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. 
80

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. 

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. 
81

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 lower 

temperatures 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 United States currently does not have the capability for studying 

and testing fire-induced phenomena critical to structural integrity. 
82

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. 
83

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 consumed virtually all of 

the combustible building contents, yet collapse was not prevented. 
84

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. 

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 fire-induced collapse and allowed alternative 

designs for the structural system. 
85

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. 
86

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) in WTC 7 was 

evacuated before key decisions were made. The location of OEM in WTC 

7 contributed to the loss of robust interagency command and control. 

 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 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 in the building), the outcome could have 

been very different. 
87

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. 
88

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 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 

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 might have 

prevented the collapse of the building. 
89

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 of: 

 fire protection engineers and architects in structural engineering 

principles and design, 

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

enforcement officials in modern fire protection principles and 

technologies, including fire-resistance design of structures, and 

 building regulatory and fire service personnel to upgrade their skills 

to conduct the tasks for which they are responsible. 

 

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. 
90

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. 
91

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. 
92

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. 

• 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. 
93

Changes to Building Codes and Standards 

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

recommendations from NIST’s WTC investigation were adopted by the 

International Building Code in 2007. 

• Second set of eight model building code changes based on NIST’s WTC 

recommendations were approved by technical committees and are 

awaiting approval, along with potential appeals on several other code 

changes, at the Final Action Hearing for the 2009 edition of the 

International Building Code 

• 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, 

and the Council on Tall Buildings and Urban Habitat 
94

Process for Submitting Public Comments 

• NIST welcomes comments from the public on the draft investigation reports 

for the WTC 7 building. 

 The public is welcome to comment on any of the three draft reports issued by 

NIST, totaling about 1,000 pages. 

 NIST especially encourages public comment on the approximately 75-

page draft summary report, which contains the principal findings and 

recommendations for changes to codes, standards, and practices. 

 NIST will consider all comments received from the public on the three draft 

reports before they are issued in final form. 
95

Process for Submitting Public Comments (2) 

 Comments must be as specific as possible. 

 A short reason must be provided for any suggested change. 

 Suggested language must be provided for the requested revision. 

 All comments shall include the following information: 

Name: 

Affiliation: 

Contact: Phone number or e-mail address where you can be contacted 

in case of questions.

Report Number: (e.g., NCSTAR 1A, NCSTAR 1-9, or NCSTAR 1-9A)

Page Number: 

Paragraph/Sentence: (e.g., paragraph 2/sentences 2-4)

Comment: 

Reason for Comment: 

Suggestion for Revision: 
96

Process for Submitting Public Comments (3) 

 NIST will accept public comments on any of the draft reports 

until 12 Noon EDT, Monday, September 15, 2008. 

 Comments may be submitted: 

 through a link on the WTC Investigation web site: 

http://wtc.nist.gov 

 Via e-mail to wtc@nist.gov 

 By fax to 301-869-6275

 By mail to: 

WTC Technical Information Repository 

Attention: Mr. Stephen Cauffman 

National Institute of Standards and Technology 

100 Bureau Drive Stop 8611 

Gaithersburg, MD 20899-8611
97

Next Steps 

 Following the public comment period, NIST will revise the 

WTC 7 report as appropriate and release the report in final 

form. 

 NIST will continue to vigorously promote implementation of 

the recommendations from its investigation of the WTC 

towers and WTC 7 and provide necessary technical support 

to standards and code development organizations. 
98

 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.