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 on August 21 following an extensive, three-year scientific and technical building and fire safety investigation. This was the first known instance of fire causing the total collapse of a tall building, the agency stated as it released for public comment its WTC investigation report and 13 recommendations for improving building and fire safety.
"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," said 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."
"Video and photographic evidence combined with detailed computer simulations show that neither explosives nor fuel oil fires played a role in the collapse of WTC 7," Sunder said. The NIST investigation team also determined that other elements of the building's construction—namely trusses, girders and cantilever overhangs that were used to transfer loads from the building superstructure to the columns of the electric substation (over which WTC 7 was constructed) and foundation below—did not play a significant role in the collapse.
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.
Citing its one new recommendation (the other 12 are reiterated from the previously completed investigation of the World Trade Center towers, WTC 1 and 2), the NIST investigation team said that "while the partial or total collapse of a tall building due to fires is a rare event, we strongly urge building owners, operators and designers to evaluate buildings to ensure the adequate fire performance of the structural system. Of particular concern are the effects of thermal expansion in buildings with one or more of the following features: long-span floor systems, connections not designed for thermal effects, asymmetric floor framing and/or composite floor systems." Engineers, the team said, should be able to design cost-effective fixes to address any areas of concern identified by such evaluations.
NIST welcomes comments on the draft report and recommendations received by noon Eastern Daylight Time on Sept.15, 2008. Instructions for submitting comments—as well as links to the complete text of the draft WTC 7 investigation report, archived videos of the Aug. 21 press briefing and an Aug. 26 technical briefing, a video describing the WTC 7 investigation findings, and other materials—may be accessed at http://wtc.nist.gov.