The World Trade Center: Collapse of the Twin Towers

Terrorist Attacks on New York's World Trade Center Towers

Dateline: September 12, 2001 This article summarizes news reports and commentary at the time of the terrorist attacks on the World Trade Center in New York City. For complete information about the World Trade Center, reconstruction plans, memorials, and other resources related to the September 11 terrorist attacks, visit our World Trade Center Resource Page. Built in the 1970s, World Trade Center Twin Towers in New York City were designed to withstand normal fires and hurricane-force winds. According to some reports, engineers believed that even the impact of a Boeing 707 would not bring down the towers. But the two planes which struck the Twin Towers on September 11, 2001 were much larger than the Boeing jets of the 1970s. Experts say that no engineer could have prepared for the terrorist attack which reduced the World Trade Center to rubble. Indeed, the two towers showed remarkable strength, standing for about an hour after the air strikes. Their ultimate collapse was caused primarily by the jet fuel fire, engineers told reporters for the New York Times. Temperatures soaring from 1,000 to 2,000 degrees caused the steel columns around the tower facades to buckle. With their supports weakened, the concrete-slab floors plunged. The South Tower of the World Trade Center was most heavily impacted on floors 87 through 93. The North Tower was impacted at floors 96 through 103. Because the jets hit the towers near the top, the buildings themselves became their own means of destruction as the weight of many floors crushed downward. Later, falling debris and more fires led to the collapse of the smaller 7 World Trade Center building adjacent to the towers. Designed by Minoru Yamasaki and Associates, the World Trade Center towers utilized tube construction, which was a popular innovation for skyscrapers of the 1970s. Older structures like the Empire State Building use heavy internal supports and thick masonry. In 1945, when a ten-ton B-25 bomber left a 20-foot hole in the landmark skyscraper, the Empire State Building remained standing. But the heavy masonry building would surely not have survived an impact from the larger passenger jets which struck the World Trade Center towers. The Twin Towers were constructed of lightweight steel and glass supported by exterior columns. Each concrete-slab floor was supported by steel trusses with special plates designed to lessen the effect of high winds. Stairs and elevators were supported by columns at the core of the towers. However, the primary support for the towers came from their external sheathing. The demolition of the World Trade Center complex has altered New York's skyline forever. By all accounts, the buildings were miracles of design. Could they have been made stronger or safer? Tell us what you think. Why the World Trade Center Towers Fell Engineers who studied the Twin Towers after the September 11 attacks explain why the buildings stood as long as they did, and why they eventually collapsed. Read more about the construction of the Twin Towers See pictures of the proposed plans for reconstruction on the World Trade Center site.