Collapse of the World Trade Center

The World Trade Center, in Lower Manhattan, New York City, was destroyed after a series of terrorist attacks on September 11, 2001, killing almost 3,000 people at the site. Two commercial airliners hijacked by al-Qaeda members were deliberately flown into the Twin Towers of the complex, engulfing the struck floors of the towers in large fires that eventually resulted in a total progressive collapse of both skyscrapers, at the time the third and fourth tallest buildings in the world. It was the deadliest and costliest building collapse in history.

The North Tower (WTC 1) was the first building to be hit when American Airlines Flight 11 crashed into it at 8:46 a.m.,^[a] causing it to collapse at 10:28 a.m.^[e] after burning for one hour and 42 minutes.^[f] At 9:03 a.m.,^[g] the South Tower (WTC 2) was struck by United Airlines Flight 175; it collapsed at 9:59 a.m.^[h] after burning for 56 minutes.

The towers' destruction caused major devastation throughout Lower Manhattan, as more than a dozen adjacent and nearby structures were damaged or destroyed by debris from the plane impacts or the collapses. Four of the five remaining World Trade Center structures were immediately crushed or damaged beyond repair as the towers fell, while 7 World Trade Center remained standing for another six hours until fires ignited by raining debris from the North Tower brought it down at 5:21 p.m. the same day.

The hijackings, crashes, fires, and subsequent collapses killed an initial total of 2,760 people. Toxic powder from the destroyed towers was dispersed throughout the city and gave rise to numerous long-term health effects that continue to plague many who were in the towers' vicinity, with at least three additional deaths reported.^[17] The 110-story towers are the tallest freestanding structures ever to be destroyed, and the death toll from the attack on the North Tower represents the deadliest single terrorist act in world history.^[i]

In 2005, the National Institute of Standards and Technology (NIST) published the results of its investigation into the collapse. It found nothing substandard in the towers' design, noting that the severity of the attacks was beyond anything experienced by buildings in the past. The NIST determined the fires to be the main cause of the collapses; the plane crashes and explosions damaged much of the fire insulation in the point of impact, causing temperatures to surge to the point the towers' steel structures were severely weakened. As a result, sagging floors pulled inward on the perimeter columns, causing them to bow and then buckle. Once the upper section of the building began to move downward, a total progressive collapse was unavoidable.

The cleanup of the World Trade Center site involved round-the-clock operations and cost hundreds of millions of dollars. Some of the surrounding structures that had not been hit by the planes still sustained significant damage, requiring them to be torn down. Demolition of the surrounding damaged buildings continued even as new construction proceeded on the Twin Towers' replacement, the new One World Trade Center, which opened in 2014.^[19]

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