Investigation of the World Trade Center Disaster

Investigation of the World Trade Center Disaster


On August 21, 2002, NIST announced its building and fire safety investigation of the World Trade Center (WTC) disaster. The WTC Investigation was conducted under the authority of the National Construction Safety Team (NCST) Act, which was signed into law on October 1, 2002.




The specific objectives of the investigation were to: 1) determine why and how the towers, WTC 1 and WTC 2, collapsed following the initial impacts of the aircraft and why and how the WTC 7 building collapsed; 2) determine why the injuries and fatalities were so high or low depending on location, including all technical aspects of fire protection, occupant behavior, evacuation, and emergency response; 3) determine what procedures and practices were used in the design, construction, operation, and maintenance of WTC 1, 2, and 7; and 4) identify, as specifically as possible, areas in current building and fire codes, standards, and practices that warrant revision.

NIST completed the study of the World Trade Center towers and released the final report in October 2005. The final report entitled, “Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers” (NCSTAR 1) and the 42 companion reports are available on the NIST WTC web site:

The results of this extensive research led to the conclusion that the tragic consequences of the September 11, 2001, attacks were directly attributable to the combination of initial structural damage and the resulting multi-floor fires resulting from the impact of large, jet-fuel laden commercial airliners into the WTC towers. Buildings for use by the general population are not designed to withstand attacks of such severity; building regulations do not require building designs to consider aircraft impact. In U.S. cities, there has been no other experience with a disaster of such magnitude, nor has there been any in which the total collapse of a high-rise building occurred so rapidly and with little warning.

NIST also completed the investigation of WTC 7, the third building that collapsed on September 11, 2001. The study found that the fires in WTC 7, which were uncontrolled but otherwise similar to fires experienced in other tall buildings, caused an extraordinary event. Heating of floor beams and girders caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down. A key factor leading to the eventual collapse of WTC 7 was thermal expansion of long-span floor systems at temperatures hundreds of degrees below those typically considered in current practice for fire resistance ratings. WTC 7 used a structural system design in widespread use.

As a result of its investigation of the WTC towers, NIST compiled a list of 30 recommendations to improve the safety of tall buildings, occupants, and emergency responders based on its investigation of the procedures and practices that were used for the WTC towers. The recommendations call for action by specific entities regarding standards, codes and regulations, their adoption and enforcement, professional practices, education, and training; and research and development. Additionally, as a result of the investigation of WTC 7, NIST has issued one additional recommendation and reiterated 12 of the recommendations from the WTC towers investigation.

Responding to the recommendations, the International Code Council (ICC) has adopted 23 code changes that were incorporated in the 2009 edition of the International Building Code and the International Fire Code. In addition, the National Fire Protection Association (NFPA) approved fifteen changes that were incorporated into the 2009 editions of the NFPA 5000 Building Code, the NFPA 1 Fire Code, and the NFPA 101 Life Safety Code. These far-reaching building and fire code changes will lead to future buildings—especially tall structures—that are increasingly resistant to fire, more easily evacuated in emergencies, and safer overall for occupants and emergency responders. NIST is continuing to work with the codes and standards bodies and the technical community toward implementing additional changes to codes and standards based on the recommendations of the WTC investigation.

Start Date:

October 1, 2009

Lead Organizational Unit:


S. Shyam Sunder, Director
Engineering Laboratory
Graphic showing the buckling of WTC 7 Column 79 (circled area), the local failure identified as the initiating event in the building’s progressive collapse.
Graphic showing the buckling of WTC 7 Column 79 (circled area), the local failure identified as the initiating event in the building’s progressive collapse

Filed Under: Anatomy of BuildingsFire Protection EngineeringScience & Technology


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