Why the building (WTC7) fell

The National Institute of Standards and Technology (NIST), Gaithersburg, Mass., conducted an extensive three-year scientific and technical investigation of the Sept. 11, 2001, collapse of the 47-story World Trade Center building 7 (WTC 7) in NYC. This video describes the results of this study, which concluded that fires on multiple floors 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.