September 11, 2001 is a date that few people in America will forget. It is still hard to believe that the World Trade Centers, buildings that represented America's strength and power, are now visible only as ruins in the Manhattan skyline. Soon after the buildings collapsed, a preliminary study of the structural foundations was conducted by a group of Boston-based civil and structural engineers.
The panel concluded that what eventually led to the towers collapsing were the raging fires as gallons of airplane fuel ignited. The supports could no longer stand the heat and crashed on top of each other. The group agreed that no "existing or foreseeable economically viable skyscraper structure" would not have been toppled by the airplane crashes.
However, two main theories as to how the towers may have remained standing longer, were developed by Professor Jerome Connor of MIT, and Professor Zdenek Bazant, a professor at Northwestern University. Connor suggested that it was the weaknesses in how horizontal and vertical beams were tied together. "The floor trusses sat on beams and were tied down so the core was locked to the exterior. If you lose the connection between them, however, you lose the ability to carry the floor loads and allow the floors to slide back and forth under stress. If a damaged floor system were to fall, down the floors would go."Bazant's theory was explained by MIT professor Eduardo Kausel who said: "I believe that the intense heat softened or melted the structural elements so that they became like chewing gum, and that was enough to trigger the collapse. The floor trusses are likely to have been the first to sag and fail. As soon as the upper floors became unsupported, debris from the failed floor systems rained down onto the floors below, which eventually gave way, starting an unstoppable sequence."