Why did it take nearly two hours for the twin towers to collapse after the planes hit?
The towers apparently withstood the initial impact of the crashes. “The subsequent fires caused the collapse,” said Jon Magnuson, head of the company that designed the buildings’ structure. “What happens in a fire is steel at 1,500, 1,600 degrees Fahrenheit, steel loses its strength. Then it just collapses.” He said nobody could design a building and account for the possibility of heat generated by burning jet fuel. The impact of the plane could have damaged the sprinkler system, allowing the fire to burn uncontrolled. The combination of the plane crashes and the heat from the fires on the steel columns probably began the collapse. A structural engineer said that after the upper floors gave way, each then overloaded the floor beneath it, continuously to the ground. There is a theory that there could have been detonations from the bottom of the buildings, but experts said that from watching replays on television, the buildings seemed to collapse from the top down.
