In the immediate aftermath of the attacks, numerous structural engineers and experts spoke to the media, describing what they thought caused the towers to collapse.
Hassan Astaneh, a structural engineering professor at the University of California at Berkeley, explained that the high temperatures in the fires weakened the steel beams and columns, causing them to become "soft and mushy", and eventually they were unable to structure above. Astaneh also suggested that the fireproofing became dislodged during the initial aircraft impacts. He also explained that once the initial structural failure occurred, then progressive collapse of the entire structure was inevitable.[31] Cesar Pelli, who designed the Petronas Towers in Malaysia and the World Financial Center in New York, remarked that, "no building is prepared for this kind of stress".[32]
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As in the theory which is currently accepted, the fires were taken to be the key to the collapses. Thomas Eagar, an MIT materials professor, had described the fires as "the most misunderstood part of the WTC collapse".[43] This is because the fires were originally said to have "melted" the floors and columns. As Eagar said, "The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel." Jet fuel is essentially kerosene and would have served mainly to ignite very large, but not unusually hot, hydrocarbon fires. This led Eagar, FEMA and others to focus on what appeared to be the weakest point of the structures, namely, the points at which the floors were attached to the building frame. Once these connections failed, the pancake collapse could initiate.[45][46] The NIST report, however, would ultimately vindicate the floor connections; indeed, the collapse mechanism depends on the strength of these connections as the floors pulled the outer walls in.
http://en.wikipedia.org/wiki/Collapse_of_the_World_Trade_Center#Collapse_of_the_South_Tower