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The Cause Of The Fall Of The World Trade Center

On September 11, 2001, 19 members of the terrorist faction Al-Qaeda hijacked four planes. Two of these flights, the American Airlines Flight 11 and the United Airlines Flight 175, were deliberately crashed into the Twin Towers of the World Trade Center.

Located in New York, the Twin Towers were built between 1966 and 1970. Each skyscraper was 110 stories tall, although the construction plans were originally intended for a height of only 80 stories. Built on a tube structural system, the Twin Towers were capable of facing heavy winds and lateral loads. Due to the height regime, the structure had been planned and crafted so as to remain in position in the event of a high-velocity impact generated by a Boeing 707, a lighter and faster version of the commercial airliners involved in the collision. Besides the tube frame, the towers were reinforced with heavy load steel columns distributed along the perimeter in a tight array, thus eliminating the need for interior columns. Adjacent trusses distributed the potential shear stress between the heavy duty exterior columns, allowing them to function in a unitary manner.

To allow the intense traffic expected by the architects, the high-speed elevator system was built around sky lounges. Such lounges were circulation nodes, meant to allow the passengers to exchange a large elevator, serving the entire building, with a slower elevator for the top floors. Furthermore, to prevent the elevator shafts from buckling and collapsing under the pneumatic pressure of the high-speed cabins, the constructors endowed the shafts with lightweight steel cores. While the cores maintained the necessary lightness of the structure, they were not fireproofed by being dressed in heavy masonry. In fact, most of the tubes, trusses, and beams of the towers were not sufficiently insulated and fireproofed.

When the American Airlines jet crashed into the fa├žade of the north tower at 8:46 AM (National Transportation Safety Board 2002), leaving a gash in the vicinity of the 80th floor and killing hundreds, the structure remained upright for one hour and forty minutes. The south building burned for almost fifty minutes before collapsing. Not only did the towers have insufficient fireproofing, they also had poor fire-suppression systems, which might have been damaged in the crash. Indeed, MIT experts pointed out that the kerosene fires were maintained by the 20,000 gallons of fuel carried by each jet. Vulnerable to temperatures above 800 degrees, the steel was exposed to the jet fuel fires burning at 1,500 degrees. Consequently, the aluminum fuselage melted and triggered a chemical reaction that maintained the temperature of the molten alloy at 1,800 degrees. This creates circumstantial evidence showing that the building was designed to withstand the initial impact, but not the high-temperature kerosene fires that proved fatal for the towers.

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