Engineers have said that the towers collapsed because the steel beams melted in the intense heat of the explosions. With both jets carrying full fuel tanks on impact, the temperature of the fire has been estimated at up to 500°C.
The federal investigators confirmed that the beams exceeded the requirement to bear 16,300 kg per square inch and had in fact helped save thousands of lives by staying upright for more than an hour after the explosions.
However, with support columns damaged by the planes' impact, the weakened beams were unable to hold up the weight of the floors.
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