World Trade Center
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Location: New York, New York, USA
Completion Date: 1972 (Tower One), 1973 (Tower Two)
Cost: $400 million
Height: 1,368 feet (Tower One), 1,362 feet (Tower Two)
Facing Materials: Aluminum, steel
Engineer(s): Skilling, Helle, Christiansen & Robertson
Constructed by the Port Authority of New York and New Jersey in the early 1970s, the World Trade Center towers were, for their time, the best known examples of tube buildings. Tube buildings are strengthened by closely spaced columns and beams in the outer walls. The closely spaced columns and beams in each tower form a steel tube that, together with an internal core, withstand the tremendous wind loads that affect buildings this tall.
Aside from withstanding enormous wind loads, the World Trade Center towers were also constructed to withstand settlement loads. Because the towers were built on six acres of landfill, the foundation of each tower had to extend more than 70 feet below ground level to rest on solid bedrock.
The two towers were unable to survive the effects of a direct hit by two hijacked commercial jetliners during terrorist attacks on the morning of September 11, 2001. Although they were in fact designed to withstand being struck by an airplane, the resultant fires weakened the infrastructure of the building, collapsing the upper floors and creating too much load for the lower floors to bear. Shortly after the attack, both towers collapsed.
You can explore a comprehensive site on why the Twin Towers fell, including interviews with an engineer, a survivor, an interactive explanation of how metal behaves when heated, and much more, at http://www.pbs.org/wgbh/nova/wtc/.
At the time of their completion in 1973, the World Trade Center towers were the two tallest buildings in the world. Two years later, the Sears Tower in Chicago seized the coveted title.
Here's how this skyscraper stacked up against the biggest skyscrapers in the world.
(height, in feet)