In the September 11 attacks , two commercial airliners were deliberately crashed into the Twin Towers of the World Trade Center in New York City . The impact and resulting fires caused both towers to collapse within less than two hours . The impacts severed exterior columns and damaged core columns , redistributing the loads that these columns had carried . This redistribution of loads was greatly influenced by the hat trusses at the top of each building . The impacts dislodged some of the fireproofing from the steel , increasing its exposure to the heat of the fires . Temperatures became high enough to weaken the core columns to the point of creep and plastic deformation under the weight of higher floors . The heat of the fires also weakened the perimeter columns and floors , causing the floors to sag and exerting an inward force on exterior walls of the building . WTC Building 7 also collapsed later that day ; the 47 story skyscraper collapsed within seconds due to a combination of a large fire inside the building and heavy structural damage from the collapse of the north tower
September 11 attacks
September 11 attacks
Structural integrity and failure