The collapse of World Trade Center Building 7 (WTC 7) on September 11, 2001, was primarily caused by uncontrolled fires that led to the failure of a critical structural column inside the building. The fires caused steel floor beams and girders to thermally expand, which pushed a key girder off its seat on Floor 13, resulting in the collapse of floors around Column 79. Without lateral support over nine stories, Column 79 buckled, instigating a progressive collapse of the entire structure starting from the inside and spreading outward, eventually causing the whole building to fall. Key points on why WTC 7 collapsed:
- Fires on multiple floors burned uncontrolled during the afternoon, fueled mainly by office contents.
- Thermal expansion of steel components caused structural connections to fail, something exacerbated by the building's design with long-span floors and connections not designed for thermal stresses.
- The critical failure began with interior columns on the building's east side, leading to a rapid sequence of structural failures resulting in complete collapse.
- WTC 7's collapse was the first known instance of a tall steel-framed building collapsing primarily due to fire.
- Investigations ruled out explosives and fuel fires from diesel tanks as causes.
- Lack of water pressure for the fire suppression system meant fires were not controlled.
This explanation is supported by extensive investigation by the National Institute of Standards and Technology (NIST) and other expert analyses, concluding that WTC 7's unique structural features and fire conditions caused the collapse without the use of explosives or other external factors.
