why did the twin towers collapse

The Twin Towers collapsed primarily due to the combination of the aircraft impacts and the subsequent fires. The planes damaged the structural steel and removed fireproofing in the impact zones, allowing fires fueled by jet fuel and building contents to weaken the steel structure significantly. The steel weakened by the intense heat buckled and failed, causing the floors above the impact zones to fall onto the floors below in a chain reaction progressive collapse, bringing down each tower almost straight down. The design allowed the towers to withstand the impact of the planes initially, but not the prolonged and intense fires that followed, which caused the core and perimeter columns to lose their strength and fail under the load. This explanation is supported by detailed investigations, including those by NIST and engineering analyses, which concluded that fire-induced weakening of steel after the plane impacts was the key cause of the collapses.