why did building 7 fall

The collapse of World Trade Center Building 7 (WTC 7) was primarily caused by uncontrolled fires on multiple floors. According to the extensive investigation by the National Institute of Standards and Technology (NIST), the heat from these fires caused the steel floor beams and girders to thermally expand, which led to the failure of a critical support column (Column 79). This failure initiated a progressive fire-induced collapse of the entire building. The fires burned for several hours, and the building lacked sufficient water pressure for fire suppression. The collapse sequence started with floor beams losing connection to this key column, causing floors to collapse in succession, which then caused the column to buckle and triggered a chain reaction of structural failures across the core and perimeter columns until the entire building fell.