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NIST Report Determines Fire Caused WTC 7 Collapse, Not Explosives

The fall of the 47-story World Trade Center building 7 (WTC 7) in New York City late in the afternoon of Sept. 11, 2001, was primarily due to fires, the National Institute of Standards and Technology (NIST) announced following an extensive, three-year scientific and technical building and fire safety investigation.



“Our 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,” says NIST WTC Lead Investigator Shyam Sunder. “Heating of floor beams and girders caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down.”


According to the report, 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.


The investigators also reported that if the city water main had not been cut by the collapse of World Trade Center towers 1 and 2 (WTC 1 and WTC 2), operating sprinklers in WTC 7 would likely have prevented its collapse. “Nevertheless, we recommend that building standards and codes be strengthened beyond their current intent to achieve life safety by preventing structural collapse even during severe fires like this one, when sprinklers do not function, do not exist or are overwhelmed by fire,” ” Sunder says.

Sunder identified several existing, emerging or even anticipated capabilities that could have helped prevent WTC 7’s collapse. He cautioned that the degree to which these capabilities improve performance remains to be evaluated. Possible options for developing cost-effective fixes include:

    •    More robust connections and framing systems to better resist effects of thermal expansion on the structural system.
    •    Structural systems expressly designed to prevent progressive collapse, which is the spread of local damage from a single initiating event, from           element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it. 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.
    •    Improved compartmentation in tenant areas to limit the spread of fires.
    •    Thermally resistant window assemblies to limit breakage, reduce air supply and retard fire growth.

The 12 recommendations reiterated from the WTC towers investigation address several areas, including specific improvements to building standards, codes and practices; changes to, or the establishment of, evacuation and emergency response procedures; and research and other appropriate actions needed to help prevent future building failures.



posted on 9/3/2008

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