The towers collapsed because the airplanes' impacts damaged key structural elements and removed fireproofing, allowing fires fueled by building contents to weaken the steel columns and floor trusses. Over time, the steel weakened from high temperatures, causing core columns to buckle and floors to sag. This led to the perimeter walls being pulled inward and buckling, triggering a progressive collapse where the floors above the impact zones fell onto the floors below, initiating the total collapse of both towers. This process was influenced by the combination of impact damage and prolonged heating, which made the steel soft and unable to support the loads above it, resulting in a gravity-driven failure of the structures.
Specifically, the South Tower was hit lower and off-center, causing more unbalanced damage and faster collapse, while the North Tower was hit more centrally and stood longer. Investigations, including those by NIST, found no evidence that explosives or controlled demolitions caused the collapse, attributing it instead to structural damage from the impacts and intense fires.
