Reason for collapse
The collapse of WTC 7 was attributed to fires that burned for nearly seven hours after the collapse of WTC 1 (North Tower). Debris from WTC 1 ignited fires on at least 10 floors of WTC 7. These fires, primarily fueled by office contents, were uncontrolled because the building's sprinkler system was rendered inoperable due to damaged water mains.
The heat from the fires caused the steel floor beams and girders to thermally expand. This expansion led to a chain of events that resulted in the failure of a critical support column (Column 79) on the 13th floor, triggering a progressive collapse that brought the entire building down.
Investigation and findings
The National Institute of Standards and Technology (NIST) conducted an extensive investigation into the collapse of WTC 7. Their 2008 report concluded that the fires were the primary cause of the collapse. While debris from the North Tower did damage some exterior columns, it did not initiate the collapse. NIST also considered and ruled out the possibility of explosives being involved in the collapse.
The collapse of WTC 7 is significant because it was the first known instance of a tall building collapsing primarily due to uncontrolled fires. This event has led to revisions in building codes and practices, including improvements in fireproofing, sprinkler systems, and structural integrity requirements,