Analysis of the Thermal Exposure in the Impact Areas of the World Trade Center Terrorist Attacks

The collapse of each tower was caused by extensive damage resulting from the aircraft impact to each tower and subsequent weakening of the steel at the impact zone by heating of unprotected steel by fire. While the fire was massive in spatial extent, the thermal environment created by the fire was significantly less severe than normally occurs in fully developed compartment fires. The spray applied fireproofing was removed from structural members directly exposed to the impact debris field. The heating of the unprotected columns resulted in the loss of structural capacity and ultimately resulted in the collapse of the towers. The collapse mechanisms for the two towers were different as a result of the differing impact dynamics rather than differences in fire dynamics.