NIST: Fire caused the collapse of WTC 7
Oct 1, 2008
“Uncontrolled building fires—similar to fires experienced in other tall buildings—caused an extraordinary event, the collapse of World Trade Center 7 (WTC 7),” S. Shyam Sunder, lead investigator for the National Institute of Standards and Technology (NIST), reported at a news conference in August. This conclusion was reached, Sunder said, “by reconstructing the building, beam by beam, column by column, connection by connection into a computer model.” The investigation results are included in a draft of the report and recommendations available online at
http://wtc.nist.gov; NIST was accepting comments on the document up to September 15, 2008. The study took three years to complete.
Among the findings contained in the report are the following:
The collapse was not caused by explosives or fuel oil fires.
Fires, which undermined floor beams and a critical structural column, led to the eventual collapse. The fires on Floors 7 through 9 and 11 through 13 were particularly severe.
Thermal expansion of long-span floor systems in the east side of the building was a critical factor. The building’s exterior columns were more closely spaced than the interior ones. Thermal expansion of the floor beams damaged connections between the steel beams and concrete slab of the composite floor system. Some beams buckled; others pushed the girders, causing some to buckle. A few girders lost their connections to columns, triggering floor failures.
WTC 7 is the first modern high-rise to collapse primarily as a result of a fire. Debris from the falling WTC Tower 1 damaged structural columns and ignited fire on at least 10 floors. The main fuels for the fire were office paper and furnishings.
The water supply for the sprinkler system had been cut off on six of the lower floors, which caused these floors to burn with intensity. The upper floors had a backup water supply.
The collapse started on the 13th floor when a girder disconnected from a critical column (79), which supported a long open floor span. Once the floor gave way, the floors below it, down to the fifth floor, also compressed. The buckling of Column 79 (and then Columns 80 and 81) occurred when lateral support for nine stories was lost. As the building started to fall, a kink occurred at the top of the building as the east penthouse failed.
Sunder noted that thermal expansion effects currently “are not explicitly considered in design practice for fire resistance ratings. No professional is assigned the explicit responsibility for ensuring the adequate fire safety performance of a building’s structural system.” He added that the important lesson for engineers and architects to consider for other skyscrapers is how the heat from fires can weaken structural elements, potentially causing a “so-called progressive collapse.” Owners of tall buildings with a similar floor design, he said, “should immediately consider whether to install reinforcements.” He added, “Perhaps codes should be changed to address the weakness.”