No shortage of commentary existed after the attacks on the World Trade Center, which resulted in the collapse of the two-towered structure. There was surprise among technologists and non-technologists alike as to the degree of structural failure brought on by the event.

Never has such a large building collapsed with such a devastating loss of life. Who could have credibly advanced a scenario of a direct impact from a jumbo jet and the ensuing petroleum-fueled fire? A review of the design concept and construction of the building helps explain the failure’s origin and sequence.

Figure 1 is a simplified drawing of one of the tower structures. A central core housed elevators and supported the weight of the floors. Around the outside of the building were closely spaced (39-inch centers) perimeter columns that were designed to resist lateral wind loads and support the outer edge of each floor.