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Key Findings of NIST's June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster

The findings address four objectives. These are:

  • To determine (a) why and how the WTC 1 and WTC 2 collapsed following the initial impact of the aircraft, and (b) why and how the 47-story WTC 7 collapsed;
  • To determine why the loss of life and injuries were so low or so high depending on location, including technical aspects of fire protection, occupant behavior, evacuation and emergency response;
  • To determine the procedures and practices which were used in the design, construction, operation and maintenance of the WTC buildings; and
  • To identify, as specifically as possible, areas in national building and fire codes, standards and practices that warrant revision.

The following must be considered when reviewing the interim findings:

  • Buildings are not specifically designed to withstand the impact of fuel-laden commercial airliners. While documents from the Port Authority of New York and New Jersey (PA NY NJ) indicate that the impact of a Boeing 707 flying at 600 miles per hour, possibly crashing into the 80th floor, was analyzed during the design of the WTC towers in February/March 1964, the effect of subsequent fires was not considered. Building codes do not require building designs to consider aircraft impact.
  • Buildings are not designed for fire protection and evacuation under the magnitude and scale of conditions similar to those caused by the terrorist attacks of Sept. 11, 2001.
  • The load conditions induced by aircraft impact and the extensive fires on Sept. 11, 2001, which triggered the collapse of the WTC towers, fall outside the norm of design loads considered in building codes.
  • Prior evacuation and emergency response experience in major events did not include the total collapse of tall buildings such as the WTC towers and WTC 7 that were occupied and in everyday use; instead, that experience suggested that major tall building fires result in burnout conditions, not global building collapse.
  • The PA NY NJ was created as an interstate entity, under a clause of the U.S. Constitution permitting compacts between states, and is not bound by the authority of any local, state or federal jurisdiction, including local building and fire codes. The PA NY NJ's long-standing stated policy is to meet, and where appropriate, exceed the requirements of local building and fire codes.

Collapse of the WTC Towers – Working Hypothesis

NIST is interested in determining how and why WTC 1 stood nearly twice as long as WTC 2 before collapsing (103 minutes versus 56 minutes), even though they were hit by virtually identical aircraft.In addition, NIST is interested in determining what factors related to normal building and fire safety considerations not unique to the terrorist attacks of Sept. 11, 2001, if any, could have delayed or prevented the collapse of the WTC towers.

The NIST investigation team has formulated the following chronological sequence of major events leading to the eventual collapse of the towers:

  • Aircraft impact damaged the perimeter columns, causing redistribution of column loads to adjacent perimeter columns and to the core columns via the hat truss (the steel structure that supported the antenna atop the towers and was connected to the core and perimeter columns).
  • After breaching the building's exterior, the aircraft continued to penetrate into the buildings, damaging core columns with redistribution of column loads to other intact core and perimeter columns via the hat truss and floor systems.
  • The subsequent fires, influenced by the post-impact condition of the fireproofing, weakened columns and floor systems (including those that had been damaged by aircraft impact), triggered additional local failures that ultimately led to column instability.
  • Final column instability resulted when redistributing loads could not be accommodated any further.

Among the factors relevant to the condition and collapse of the WTC towers—and currently under analysis—were:

  • The innovative structural system at the time they were built, incorporating many new and unusual features, including:
    • a composite floor system, using open-web bar joist elements, and
    • the use of wind tunnel testing to estimate lateral wind loads in the design;
  • The relative roles of the aircraft impacts and subsequent fires;
  • The post-impact condition of the fireproofing on the floor systems; and
  • The qualities and properties of the structural steel used.

Following are key points related to each of the four relevant factors:

Innovative Structural System

  • The fire protection of a truss-supported floor system by directly applying spray-on fireproofing was innovative and not consistent with prevailing practice at the time of construction.
  • The  fireproofing thickness (specified to meet a 2-hour fire endurance  rating)  was 1/2 inch at construction and was upgraded on some floors to 1-1/2 inches prior to Sept. 11, 2001.
  • Unrelated to the WTC buildings, a model code evaluation system service recommended in June 2001 a minimum thickness of 2 inches for a similar floor system to achieve the 2-hour fire rating.
  • The three-to-four-fold difference (between 1/2 inch and 2 inches) in specifying the fireproofing thickness to meet the required   fire rating is extraordinarily large and confirms the lack of technical basis in selecting a thickness.
  • While the building designers recognized the benefits of conducting a full-scale fire endurance test to determine the required fireproofing thickness, no such tests were conducted on the floor system used in the WTC towers (NIST will be conducting this test later this summer).
  • If a "structural frame" approach (considering that the floor truss was connected to the interior and perimeter columns, essentially forming a single structural unit) had been used, the needed fire rating would likely have been 3 hours, as it was for the perimeter columns alone.
  • NIST computer simulations indicate that flames in a given location lasted about 20 minutes before spreading to adjacent, yet unburned combustibles, and that this spread was generally continuous because of the even distribution of combustibles throughout the floors and the lack of interior partitions.
  • The results of two sets of wind tunnel tests on the WTC towers conducted by independent laboratories in 2002 and provided to NIST show large differences – as much as 40 percent – in resultant forces on the structures. Additionally, the wind loads estimated from these tests are about 20-60 percent higher than those apparently used in the original design of the WTC towers.
  • Wind load capability is a key factor in determining the overall strength of a tall building and important in determining its ability to withstand not only winds but also its reserve capacity to withstand unanticipated events such as a major fire or impact damage.
  • NIST is conducting an independent analysis to establish the baseline performance of the WTC towers under the original design wind loads and will compare those wind load estimates with the then-prevailing code requirements.

Relative Roles of Aircraft Impact and Fires

  • The two WTC towers withstood the initial impact of virtually identical aircraft (Boeing 767 200ER) during the terrorist attacks of Sept. 11, 2001. The robustness of the perimeter structural system and the large dimensional size of the WTC towers helped the buildings withstand the aircraft impact.
  • Following impact, the WTC towers displayed and withstood vibrational forces that were as much as half the levels (in extreme wind conditions) for which the buildings were designed.
  • Preliminary aircraft impact damage analysis indicates that the impact of a fuel filled wing section resulted in extensive damage to the exterior wall panel, including complete failure of the perimeter columns.
  • Fires  played a major role in further reducing the structural capacity  of the  buildings, initiating collapse. While aircraft impact damage did not, by itself, initiate building collapse, it contributed greatly to the subsequent fires by:
    • compromising the sprinkler and water supply systems;
    • dispersing jet fuel and igniting building contents over large areas;
    • creating large accumulations of combustible matter containing aircraft and building contents;
    • increasing the air supply into the damaged buildings that permitted significantly higher energy release rates than would normally be seen in ventilation-limited building fires, allowing the fires to spread rapidly within and between floors; and;
    • damaging ceilings that enabled "unabated" heat transport over the floor to ceiling partition walls and to structural components
  • The  jet fuel, which ignited the fires, was mostly consumed within  the first  few minutes after impact. The fires that burned  for almost the entire time that the buildings remained standing  were due mainly to burning building contents and, to a lesser extent,  aircraft contents, not jet fuel.
  • The  typical WTC office workstation furnishings were able to sustain intense fires for at least an hour on a given WTC floor.

Role of Fireproofing Conditions

  • Most of the floor systems in WTC 1 impacted by the aircraft crash and fires had upgraded or thicker (1-1/2 inches) fireproofing while most of the affected floors in WTC 2 had the original (1/2-inch) thickness.
  • The response of a structural component to fire is sensitive to variability in fireproofing thickness along its length.
  • As applied—both in the original spraying and in later upgrades—the fireproofing was found to be thermally equivalent to uniform thicknesses that were greater than the specified minimums required by the building owner.
  • It was found that the acceleration of a structural component would have to be about 100-150 times the acceleration due to gravity to dislodge 1-inch-thick fireproofing similar to that used in the WTC towers. NIST is currently conducting analytical studies to estimate the magnitude of accelerations of the structural components due to aircraft impact. This will help identify those regions where fireproofing may have been dislodged.


Analysis of Recovered WTC Steel

  • The collection of 236 pieces of steel in NIST's possession is adequate for analyzing the quality and properties of the steel for the investigation, emphasizing regions of impact and fire damage. Pieces of all specified grades of steel (for the exterior panels, core columns and steel trusses in the floor systems) were acquired.
  • Analysis of the recovered steel indicates that each of the structural components of the WTC towers had the grade specified in the design drawings.
  • Metallography and mechanical property tests indicate that the strength and quality of the steel was as specified, typical of the era and likely met all qualifying test requirements.
  • The room-temperature strength of the steel used in the towers met the relevant standards and, in many instances, exceeded the requirements by 5-10 percent.
  • Analysis is ongoing of the performance of the steel components under impact and fire conditions up to the starting point of the total building collapse.

Collapse of WTC 7—Working Hypothesis

NIST is interested in determining why and how the 47-story WTC7 building, a more typical tall building, collapsed even though it was not directly hit by an aircraft.

The NIST investigation team has formulated the following chronological sequency of major events leading to the eventual collapse of WTC7:

  • An initial local failure at the lower floors (below Floor 13) of the building due to fire and/or debris induced structural damage of a critical column (the initiating event), which supported a large span floor bay with an area of about 2,000 square feet.
  • Vertical progression of the initial local failure up to the east penthouse, as large floor bays were unable to redistribute the loads, bringing down the interior structure below the east penthouse.
  • Horizontal progression of the failure across the lower floors (in the region of Floors 5 and 7, that were much thicker than the rest of the floors), triggered by damage due to the vertical failure, resulting in the disproportionate collapse of the entire structure.

The working hypothesis is consistent with all evidence currently held by NIST, including photographs and videos, eyewitness accounts and emergency communication records.

Based on a review of the fuel system for emergency power in WTC 7, Floor 5 – which did not have any exterior windows and contained the only pressurized fuel distribution system on the south, west and north floor areas – is considered a possible fire initiation location, subject to further data and/or analysis that improve knowledge of fire conditions in this area.

Evacuation and Emergency Response

NIST is interested in determining what factors related to normal building and fire safety considerations, if any, could have saved additional WTC occupant lives on Sept. 11, 2001, or could have minimized the loss of life among the first responders.


Based on information and data gathered during the first-person interviews of WTC surviving occupants, the following was learned:

  • It is estimated that 17,400 occupants (± 1,200) were present in the WTC towers on the morning of Sept. 11, 2001. The initial population of each tower was similar.
  • About 6 percent of the surviving occupants reported a pre-existing limitation to their mobility. These limitations included obesity, heart condition, needing assistance to walk, pregnancy, asthma, being elderly, chronic condition, recent surgery or injury, and other.
  • About 7 percent of the surviving occupants reported having special knowledge about the building. These included fire safety staff, floor wardens, searchers, building maintenance and security staff.
  • Two-thirds of surviving occupants reported having participated in a fire drill in the 12 months prior to Sept. 11, 2001, while 17 percent reported that they received no training during that same period. Of those participating in fire drills, 93 percent were instructed about the location of the nearest stairwell. Overall, slightly over half of the survivors, however, had never used a stairwell at the WTC prior to Sept. 11, 2001.
  • Approximately 87 percent of the WTC tower occupants, including more than 99 percent of those below the floors of impact, were able to evacuate successfully.
  • Rough estimates indicate that about 20 percent or more of those who were in the WTC towers and lost their lives may have been alive in the buildings just prior to their collapse.
  • Overall, about 7,900 survivors evacuated WTC 2 in 73 minutes (i.e. from the instant the WTC 1 was struck by aircraft until WTC 2 collapsed) while about 7,500 survivors evacuated WTC 1 in 103 minutes. Thus, the overall evacuation rate in WTC 2 (108 survivors per minute) was about 50 percent faster than that in WTC 1 (73 survivors per minute). Functioning elevators allowed many survivors to evacuate WTC 2 prior to aircraft impact. Most of the elevators in WTC 1 were not functioning, and survivors could only use the stairways.
  • After the first airplane struck WTC 1 and before the second airplane struck WTC 2, the survivors in WTC 2 were twice as likely as those in WTC 1 to have already exited the building (41 percent versus 21 percent). The rate of evacuation completion in WTC 2 was twice the rate in WTC 1 during that same period.
  • Soon after the airplane struck WTC 2 until about 20 minutes before each building collapsed, the survivors in WTC 2 and WTC 1 had exited at about the same rate (the prior evacuation rate of WTC 1).
  • During the last 20 minutes before each building collapsed, the evacuation rate in both buildings had slowed to about one-fifth the immediately prior evacuation rate. This suggests that for those seeking and able to reach and use undamaged exits and stairways, the egress capacity (number and width of exits and stairways) was adequate to accommodate survivors.

Based on use of existing egress models and actual evacuation time on Sept. 11, 2001, it is estimated that a full capacity evacuation of each WTC tower with 25,000 people – three times the number present on Sept. 11, 2001 – would have required about 4 hours.

To achieve a significantly faster total evacuation at full capacity would have required increases in egress capacity (number and width of exits and stairways).


Emergency Communication Systems

The analysis of the emergency responder communication tapes from Sept. 11, 2001, indicates that:

  • After the first aircraft struck WTC 1, there was an approximate factor of 5 peak increase in traffic level over the normal level of emergency responder radio communications, followed by an approximate factor of 3 steady increase in the level of subsequent traffic.
  • A surge in communications traffic volume made it more difficult to handle the flow of communications and delivery of information. Roughly a third to a half of the radio messages transmitted during these radio traffic surge conditions were not complete messages or understandable.
  • The Fire Department of New York's (FDNY) citywide high-rise Channel 7 (PAPD Channel 30) radio repeater at the WTC site was operating.
  • New York Police Department (NYPD) aviation unit personnel reported critical information about the impending collapse of the WTC towers several minutes prior to their collapse. No evidence has been found to suggest that the information was further communicated to all emergency responders at the scene.

Command and Control

Based on face-to-face interviews, NIST has determined that first responders – including key incident commanders – did not have adequate information (voice, video and data) on, nor an overall perspective of, the conditions in the WTC buildings and what was happening elsewhere at the WTC site. Interagency information sharing was inadequate.


Active Fire Protection Systems

Investigation of the design, capabilities and performance of the active fire protection systems in the WTC towers and WTC 7 indicates that:

  • The smoke management systems in the WTC towers were not activated during the fires on Sept. 11, 2001, likely due to damage inflicted by the aircraft impacts.
  • HVAC (heating, ventilation, and air-conditioning) ductwork was a major path for vertical smoke spread in the buildings.
  • Computer modeling shows that stair pressurization systems would have provided minimal resistance to the passage of smoke in WTC 1 and WTC 2 had they been installed on Sept. 11, 2001.
  • The fire alarm system in WTC 7 sent only one signal (at 10:00:52 a.m. shortly after the collapse of WTC 2) to the monitoring company indicating a fire condition. The signal did not contain any specific information about the location of the fire within the building. Since the system was placed on TEST for a period of 8 h beginning at 6:47:03 a.m. on September 11, 2001, alarm signals would not have been shown on the operator's display; instead, they would have to be recorded into the history file.
  • The resistance to failure of the fire alarm system communications paths between the fire command station and occupied WTC tower floors could have been enhanced if fiber optic communications cable had been used instead of copper lines.
  • Although the fire sprinkler system was damaged by aircraft impact, the water supply riser system lacked redundancy and there   existed the potential for single point failure of the water supply connection on each floor.

Procedures and Practices

NIST seeks to determine the building and fire safety procedures and practices that were used over the life of the WTC buildings and how well those procedures and practices conformed to accepted national building and fire safety practices, standards and codes.


Applicable Building Codes

The Port Authority of New York and New Jersey (PA NY NJ) adopted the provisions of the proposed 1968 edition of the New York City Building Code, more than three years before it went into effect.The 1968 edition allowed the PA NY NJ to take economic advantage of less restrictive provisions compared with the 1938 edition that was in effect when design began for the WTC towers in 1962. The1968 code:

  • Eliminated a "fire tower" (a smoke-free stairwell) as a required means of egress;
  • Reduced the number of required stairwells from 6 to 3, and the size of doors leading to the stairs from 44 inches to 36 inches;
  • Reduced the fire rating of the shaft walls in the building core from 3 hours to 2 hours;
  • Changed partition loads from 20 pounds per square foot to one based on weight of partitions per unit length (that reduced such loads for many buildings including the WTC buildings); and
  • Permitted a 1-hour reduction in fire rating for all structural components (columns from 4 hours to 3 hours and floor framing members from 3 hours to 2 hours).

The New York City Department of Buildings reviewed the WTC tower drawings in 1968 and provided comments to the PA NY NJ concerning the plans in relation to the 1938 NYC building code. The architect-of-record submitted responses to those comments to the PA NY NJ, noting how the drawings conformed to the 1968 NYC building code.


Standards, Codes and Regulations

NIST has reviewed past and current standards, codes and regulations relevant to assessing the procedures and practices used in the design, construction, operation and maintenance of the WTC buildings. Based on that review, the following issues merit further consideration:

  • Code provisions with detailed procedures to analyze and evaluate data from fire resistance tests of other building components and assemblies to qualify an untested building element.
  • Code provisions that require the conduct of a fire resistance test if adequate data do not exist from other building components and assemblies to qualify an untested building element.
  • Regulations that would adopt code provisions using the "structural frame" approach to fire resistance ratings that requires structural members – other than columns – that are essential to the stability of the building as a whole to be fire protected to the same rating as columns.
  • Code provisions that ensure that structural connections are provided the same degree of fire protection as the more restrictive protection of the connected elements.
  • Code provisions and standards to establish whether the minimum mechanical and durability related properties of spray-applied fire resistive materials (SFRM) are sufficient to ensure acceptable in-service performance in buildings. While minimum bond strength requirements exist, there are no requirements for such materials to withstand typical shock, impact, vibration or abrasion effects over the life of a building.
  • Rigorous field application and inspection provisions and regulatory requirements to assure that the as-built condition of the passive fire protection, such as SFRM, conforms to conditions found in fire resistance tests of building components and assemblies.
  • Rigorous provisions and regulatory requirements for in-service inspections of passive fire protection during the life of the building.
  • Early installation of sprinklers in existing buildings, not as an option in lieu of compartmentation (office space separated by internal walls).
  • Standards and code provisions that provide minimum structural integrity to protect the means of egress (stairwells and elevator shafts) in the building core which are critical to life safety.
  • Standards and code provisions to install fire-protected elevators and permit their use for routine emergency access by first responders or as a secondary method (after stairwells) for emergency evacuation of building occupants.
  • Explicit standards and code provisions for structural integrity that mitigate progressive collapse.
  • Standards and code provisions for conducting wind tunnel tests and for the methods used in practice to estimate design wind loads from test results.
  • Regulatory requirements for retention of documents related to the design, construction, operation, maintenance and modifications of buildings, including retention off-site.

Fire Safety and Egress Design Methods

Performance-based methods that explicitly define the design objectives and specific design-basis fire hazards or evacuation events are better suited to risk analysis than traditional prescriptive methods of deriving code provisions and standards. Historical fire loss data suggest that prescriptive methods have considerable built-inconservatism to adequately protect building occupants. Performance-based methods enable appropriate protection to be provided where it is needed.

The increasing use of performance-based methods, as an alternative to prescriptive design, in fire safety and egress design, raises the following issues that merit further consideration:

  • Considering fire as a design condition in structural design, including evaluation of the fire performance of the structure as a whole system. This is already done with other hazards such as wind and earthquakes.
  • Detailing procedures to select appropriate design-basis fire scenarios for performance-based design of the sprinkler system, compartmentation and passive protection of the structure.
  • Validating and verifying tools for use in performance-based design practice to analyze the dynamics of building fires and their effects on the structural system that would allow engineers to evaluate structural performance under alternative fire scenarios and fire protection strategies.
  • Developing the technical basis to establish whether the construction classification and fire rating requirements are risk-consistent. Specifically, it is not apparent how the current height and area tables in building codes consider the technical basis for the progressively increasing risk to an occupant on the upper floors of tall buildings that are much greater than 200 ft in height. The maximum fire rating in current codes applies to any building more than about 12 stories in height.
  • While sprinklers improve safety in most common building fires and prevent them from becoming large fires, the technical basis is not available to establish the sprinkler trade-off in current codes which allows a lower fire rating to be used for structural components in spinklered buildings.
  • Designing egress systems to achieve a target performance (e.g., evacuation rate or time) for a given occupant population by adequately considering travel distance, remoteness requirements, and human factors (such as occupant size, stairwell environmental conditions, visibility and congestion).

Building Practices

The PA NY NJ entered into agreements with the NYC Department of Buildings (DOB) in the 1990s with regard to conformance of its buildings to the NYC Building Code. However, the PA NY NJ did not yield jurisdictional authority for regulatory and enforcement oversight to the DOB. The PA NY NJ was created as an interstate entity and is not bound by the authority of any local, state or federal jurisdiction.

The architect is responsible for specifying the fire protection in current building practice. The structural engineer is not required to evaluate and certify that the passive fire protection is adequate to protect the structural system. In accordance with established practice, the structural engineer was not responsible for the passive fire protection in the design of the WTC tower structures.

In addition, there is no requirement to involve a fire protection engineer in the design and evaluation of a building's fire protection system. In some cases, architects retain fire protection engineers to assist with the fire protection design for a building.

There are few academic degree programs or continuing education programs that qualify engineers (or architects) to evaluate the fire performance of structures. The current state-of-practice is not sufficiently advanced for engineers to routinely analyze the performance of a whole structural system under a prescribed design-basis fire scenario.

Approach to Recommendations

NIST does not set building codes and standards but provides technical support to the private sector and other government agencies in the development of U.S. building and fire practices, standards and codes. NIST recommendations are given serious consideration by private sector organizations that develop national standards and model codes—which provide minimum requirements for public welfare and safety.

The NIST building and fire safety investigation of the WTC disaster has not yet formulated recommendations. However, in doing so, NIST consider the following:

  • Findings from the first three independent investigation objectives related to building performance, evacuation and emergency response, and procedures and practices.
  • Whether findings relate to the unique circumstances surrounding the terrorist attacks of Sept. 11, 2001, or to normal building and fire safety considerations, including evacuation and emergency response.
  • What technical solutions are needed, if any, to address potential risks to buildings, occupants, and first responders, considering both identifiable hazards and the consequences of those hazards.
  • Whether the risk is in all buildings or limited to certain building types (such as height and area or structural system), buildings that contain specific design features, iconic/signature buildings or buildings that house critical functions.
Created June 21, 2010, Updated March 15, 2018