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NIST Federal Building and Fire Safety Investigation of the World Trade Center Disaster Answers to FAQ's-Supplement (12/14/2007)

National Institute of Standards and Technology (NIST) Federal Building and Fire Safety Investigation of the World Trade Center Disaster (12/14/2007, ARCHIVE, incorporated into 9/19/2011 update)

Answers to Frequently Asked Questions - Supplement (December 14, 2007)

Since the release of the Final Report on the Collapse of the World Trade Center Towers in October 2005, NIST has received many questions from interested readers curious about NIST's findings and the technical basis for them. The complexity of the investigation and the length of the final report (including all of the supporting volumes) have made understanding of the investigation a challenge for many interested readers. In response, NIST has prepared simplified answers to the most frequently asked questions.  

1.  Was there enough gravitational energy present in the World Trade Center Towers to cause the collapse of the intact floors below the impact floors? Why was the collapse of WTC 1 and 2 not arrested by the intact structure below the floors where columns first began to buckle?

Yes, there was more than enough gravitational load to cause the collapse of the floors below the level of collapse initiation in both WTC Towers. The vertical capacity of the connections supporting an intact floor below the level of collapse was adequate to carry the load of 11 additional floors if the load was applied gradually and 6 additional floors if the load was applied suddenly (as was the case). Since the number of floors above the approximate floor of collapse initiation exceeded six in each WTC Tower (12 and 29 floors, respectively), the floors below the level of collapse initiation were unable to resist the suddenly applied gravitational load from the upper floors of the buildings. Details of this finding are provided below:

Consider a typical floor immediately below the level of collapse initiation and conservatively assume that the floor is still supported on all columns (i.e., the columns below the intact floor did not buckle or peel-off due to the failure of the columns above). Consider further the truss seat connections between the primary floor trusses and the exterior wall columns or core columns. The individual connection capacities ranged from 94,000 lb to 395,000 lb, with a total vertical load capacity for the connections on a typical floor of 29,000,000 lb (See Section 5.2.4 of NIST NCSTAR 1-6C). The total floor area outside the core was approximately 31,000 ft2, and the average load on a floor under service conditions on September 11, 2001 was 80 lb/ft2.  Thus, the total vertical load on a floor outside the core can be estimated by multiplying the floor area (31,000 ft2) by the gravitational load (80 lb/ft2), which yields 2,500,000 lb (this is a conservative load estimate since it ignores the weight contribution of the heavier mechanical floors at the top of each WTC Tower). By dividing the total vertical connection capacity (29,000,000 lb) of a floor by the total vertical load applied to the connections (2,500,000 lb), the number of floors that can be supported by an intact floor is calculated to be a total of 12 floors or 11 additional floors.

This simplified and conservative analysis indicates that the floor connections could have carried only a maximum of about 11 additional floors if the load from these floors were applied statically. Even this number is (conservatively) high, since the load from above the collapsing floor is being applied suddenly. Since the dynamic amplification factor for a suddenly applied load is 2, an intact floor below the level of collapse initiation could not have supported more than six floors. Since the number of floors above the level where the collapse initiated, exceeded 6 for both towers (12 for WTC 1 and 29 for WTC 2), neither tower could have arrested the progression of collapse once collapse initiated. In reality, the highest intact floor was about three (WTC 2) to six (WTC 1) floors below the level of collapse initiation. Thus, more than the 12 to 29 floors reported above actually loaded the intact floor suddenly. 

2.  Were the basic principles of conservation of momentum and energy satisfied in NIST's analysis of the structural response of the towers to the aircraft impact and the fires? 

Yes. The basic principles of conservation of momentum and conservation of energy were satisfied in these analyses.   

In the case of the aircraft impact analyses, which involved a moving aircraft (velocity) and an initially stationary building, the analysis did, indeed, account for conservation of momentum and energy (kinetic energy, strain energy).

After each tower had finished oscillating from the aircraft impact, the subsequent degradation of the structure involved only minute (essentially zero) velocities. Thus, a static analysis of the structural response and collapse initiation was appropriate. Since the velocities were zero and since momentum is equal to mass times velocity, the momentum terms also equaled zero and therefore dropped out of the governing equations. The analyses accounted for conservation of energy.

3.  How does NIST explain the absence of a timeline for the WTC Towers?

NIST developed and reported detailed timelines for various aspects of the WTC disaster beginning with the impact of the aircraft. These timelines included the progression of fires through the buildings, the response of the structure to damage and to fire, egress of occupants from the towers, and the emergency response. The timelines were developed based on extensive analysis of the photographic and video evidence, analysis of computer models, first person interviews, radio transmissions, and other data documenting the events of September 11, 2001. A general timeline for each of the towers is reported in NIST NCSTAR 1. Detailed timelines for specific aspects of the WTC disaster are reported in NIST NCSTAR 1-2, 1-5, 1-6, 1-7, and 1-8.

4.  Why was physical evidence not collected immediately following the collapse of the WTC Towers?

The complete collapse of the WTC Towers destroyed virtually all physical evidence except the major pieces of steel and mechanical equipment. In the initial days and weeks following the WTC disaster, the emphasis was on rescue and later on recovery, necessitating the removal of steel and disturbing the collapse site. FEMA, which had launched its Building Performance Study in early October 2001, sent a team of experts to review the steel at the WTC site and the salvage yards.  These experts, including one from NIST, identified pieces of steel of potential interest to a follow-on investigation. Beginning in February 2002, NIST, on its own initiative, began identifying additional steel pieces of potential interest at the salvage yards and transporting them to NIST to preserve and secure the evidence in anticipation of launching its own investigation, which it did in August 2002. NIST did not receive the legal authority to collect and preserve physical evidence from a disaster/failure site until the National Construction Safety Team Act became law in October 2002. NIST NCSTAR 1-3 fully documents the steel recovered from the site.

5.  How did NIST derive the temperatures in the WTC Towers and how valid are they?   

Using all the visual and physical evidence available, NIST conducted simulations of the fires in each of the towers from the time of airplane impact to the collapses. The computational model used to simulate the fires was the Fire Dynamics Simulator. This model had been validated in numerous experiments and fire recreations prior to the World Trade Center Investigation. Additional large-scale experiments conducted during the Investigation (NIST NCSTAR 1-5) provided further assurance of the validity of the model output. This output was in the form of maps of the air temperatures on each of the floors over the duration of the fires (shown in NIST NCSTAR 1-5F). 

In a following set of computations using the Fire Structure Interface, the evolving temperatures of the concrete and steel structural components of the towers were calculated by exposing them to the mapped air temperatures (shown in NIST NCSTAR 1-5G). 

Both sets of computations are based on the fundamental laws of combustion, heat transfer, and air flow. The methods have been documented extensively and have been successfully subjected to technical peer review and published in professional journals.

6.  At least one private sector source has asked (1) Whether the Commerce Department's legal structure impeded NIST's ability to obtain information and therefore prevented NIST from finding the facts; and (2) Why NIST did not use its subpoena authority?

No. The Commerce Department's legal structure facilitated NIST's ability to obtain valuable information and evidence for the investigation.

NIST is required to comply with laws regarding the treatment of human subjects, the Paperwork Reduction Act, copyright, and other applicable laws. NIST is also required to comply with the provisions of the National Construction Safety Team Act. Therefore, NIST staff followed all the prescribed procedures when seeking to acquire relevant documents, interview building occupants and first responders, and acquire visual evidence. NIST has stated publicly that although documentary information was lost in the collapse of the WTC Towers, the information obtained from other sources was sufficient to conduct its investigation.

Under the National Construction Safety Team Act, NIST was granted subpoena authority. NIST's experience during the investigation was that it was able to obtain all essential documentary and visual evidence without the need to invoke subpoena authority. The existence of subpoena authority was helpful to NIST in getting access to data.

7.  Why did NIST not conduct large-scale/small-scale tests to evaluate the response of the WTC Towers structures to the aircraft impact and the fires in the buildings?

For studying the impact on a 110 story building by an actual Boeing 767 aircraft, a full-scale test was not feasible.  For a test to capture the response of the towers as a system, it would have been necessary to construct a test assembly that included the core columns, exterior columns, floors and hat truss. Even to replicate experimentally the response of the floors near and above the impact zones would have required test assemblies of about 20 stories for WTC 1 and 30 stories for WTC 2. No facility exists to conduct such a test, either with fire or in the absence of fire; and, indeed, such tests are not conducted in current engineering practice. 

Therefore, NIST relied on high-fidelity finite element modeling of the aircraft impact event and subsequent fires. The analyses were calibrated against the observed structural response of the towers upon impact (videos, photographs, and physical evidence) and the evolution of the ensuing fires.  

NIST did not conduct reduced scale system-level tests because there are no generally accepted scaling laws that apply to fire propagation, temperature evolution, and structural response.

Furthermore, fire test facilities with the capability to apply arbitrary fire exposures (in contrast to the standard time-temperature exposure) and arbitrary loads to structural components did not exist in the U.S. at the time of the investigation. Even had such a facility been existent, each large-scale structural fire test would have evaluated only a single set of conditions, e.g., structural system, fire exposure, amount of fireproofing, etc. Even a modest parametric series of such tests would have been prohibitively expensive.

NIST did conduct full-scale fire tests of single and multiple workstations. These tests were of sufficient size to properly capture the combustion physics. These tests established burning histories, mass burning rates, and heat release rates. The results were used to validate the fire dynamics calculations for fire growth and spread. See NIST NCSTAR 1-5E.  NIST also conducted full-scale fire tests exposing insulated and bare structural elements to real fires to validate the fire and thermal modeling approaches. See NIST NCSTAR 1-5B.  

8.  Why did NIST conduct ASTM E119 testing of floor systems that were not representative of the condition of the fireproofing on September 11, 2001? Why did NIST ignore the results of these tests, which showed that the floor system did not collapse, in its analysis of the thermal-structural response of the towers?

NIST's review of available documents related to the design and construction of the WTC Towers indicated that the fire performance of the composite floor system was an issue of concern to the building owners and designers from the original design and throughout the service life of the buildings (NIST NCSTAR 1-6A). NIST found no evidence to determine the technical basis for the selection of fireproofing material for the WTC floor trusses and of the fireproofing thickness to achieve a 2 hour rating. Further, NIST found no evidence that fire resistance tests of the WTC Towers' floor system were ever conducted.

Therefore, NIST conducted a series of four Standard Fire Tests (ASTM E 119) for the following purposes, as stated clearly in NIST NCSTAR 1-6B:

•  to establish the baseline performance of the floor system of the WTC Towers as they were originally built,

• to differentiate the factors (thermal restraint, fireproofing thickness, and scale of test) that most influenced the collapse of the WTC Towers as they may relate to normal building and fire safety considerations and those unique to the terrorist attacks of September 11, 2001, and

•  to study the procedures and practices used to accept an innovative (at the time) structural and fireproofing system.

The thickness of fireproofing material used in this series of four tests ranged from ½ in. to ¾ in. This range of thickness was, indeed, consistent with the thickness of fireproofing as originally applied to the floor steel in the WTC Towers.

NIST concluded from its aircraft impact analyses (NIST NCSTAR 1-2) that the fireproofing was dislodged as a result of the aircraft debris and dispersed fuel traveling, initially, over 500 miles per hour (700 ft/s) (NIST NCSTAR 1-6D). Since the test assemblies for all four Standard Fire Tests were protected with sprayed fire-resistant material (SFRM), conclusions could not be drawn for the response of the WTC Towers to the fires on September 11, 2001, because the aircraft impact resulted in there having been unprotected steel in the fire-affected region. 

The fire-affected floors in WTC 2 had the originally applied fireproofing, which was specified to be ½ in. and averaged approximately ¾ in. in thickness. The fire-affected floors of WTC 1 had upgraded fireproofing on the order of 2½ in. thickness.  However, the fireproofing thickness did not matter, since much of the fireproofing was dislodged as a result of the aircraft debris and dispersed fuel.

9.  NIST conducted a single workstation burn and a multiple workstation burn as a part of its investigation. Why did NIST only provide temperature data for one of these tests? Was the ventilation used in these tests representative of the ventilation that was present in the WTC Towers on September 11, 2001?

As documented in NIST NCSTAR 1-5C, a series of tests of single workstations was conducted to gain an appreciation for the open burning behavior and the general effect of jet fuel. The principal quantity measured was the rate of heat release. This quantity, combined with the ventilation, heat losses to walls, etc., determines the temperatures that would be reached if the workstation were burned in an actual fire. NIST NCSTAR 1-5C contains the heat release rate curves from all of the single workstation fires. The series of multiple workstation fire tests, conducted in a replication of part of a WTC floor, is fully documented in NIST NCSTAR 1-5E. In these tests, the window openings were close to the same size and layout as in the fire floors in the towers. There was no glass in the windows, replicating the broken windows seen in the photographs of the vicinity of the tower fires. The report includes, for all of the tests, plots of the heat release rate and temperature histories in multiple locations.  

10. Why didn't NIST fully model the collapse initiation and propagation of WTC Towers?

The first objective of the NIST Investigation included determining why and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft (NIST NCSTAR 1). Determining the sequence of events leading up to collapse initiation was critical to fulfilling this objective. Once the collapse had begun, the propagation of the collapse was readily explained without the same complexity of modeling, as shown in the response to question #1 above. 

11. Why didn't NIST consider the "base" and "less severe" cases throughout its analysis of the WTC Towers? What was the technical basis for selecting only the "more severe" case for its analyses? 

All three cases, (the base case, less severe case, and more severe case) are reasonable and realistic representations, each within the range of uncertainty, of the conditions in the WTC Towers on September 11, 2001. Of the three, the more severe case resulted in the closest agreement with the visual and physical evidence. (Refer to NIST NCSTAR 1-2, Section 7.1 and NIST NCSTAR 1-6, Section 9.2.4.)

12. What was the source of the material properties that were used in NIST's thermal/structural analyses of the WTC Towers? Were these properties obtained from physical testing of steel recovered from the WTC Towers?

NIST conducted extensive property measurement tests on the recovered WTC steel, which included all the many grades of structural steel used in the WTC Towers. To account for natural variation in properties from different batches of manufactured steel, NIST augmented the experimental data with published data for steels of the same construction era. These data included room temperature, high-strain rate, and high-temperature mechanical properties, along with physical properties. 

13. NIST states that the fires in WTC 1 were generally ventilation limited. If this was the case, wouldn't the fires have burned out in about 2 minutes? Why do NIST's models show the fires burning longer?

Nearly all fires are limited either by the burning rate of combustible fuel (fuel-limited fires) or by the availability of air (ventilation-limited fires). Many fires that are ventilation limited do continue to burn, with the burning rate determined by the chemistry of the combustion and the rate at which the oxygen arrives. This was generally the case for the WTC Tower fires. Of course, if the rate of air inflow were too slow (e.g., due to very few broken windows), the limited combustion would not have generated sufficient heat to continue pyrolyzing fuel, and the fire would have gone out. This was not the case on the fire floors in the WTC Towers.

The Fire Dynamics Simulator, used to reconstruct the fires in the WTC towers, included the burning characteristics of the building combustibles and the ventilation through the broken windows and the damaged building façade. The simulation showed that there were ample perforations in the building facade to maintain the ventilation-limited combustion until the fuel supply was depleted.

14 The collapse sequence for WTC 1 proposed by NIST includes, aircraft impact, core weakening, floor sagging and disconnection, inward bowing of the south wall, and collapse initiation. If the floors are disconnecting from the south wall, how were the floors able to exert forces on the exterior walls to cause the inward bowing?

Analyses of the composite floor system under fire exposures determined from fire dynamics simulations and thermal analyses, predicted sagging subsequent to truss web diagonal buckling and failure of some seated connections (see NIST NCSTAR 1-6C).  However, the vast majority of the connections remained intact. Further, the shear studs that attached the floor slab to the spandrel, and the diagonal steel struts that connected the truss top chord to the intermediate columns were also capable of transferring inward pull forces. Thus, the sagging floors were capable of exerting an inward pull on the exterior columns and spandrel beams.

Created May 25, 2010, Updated June 2, 2021