Collapse of the World Trade Center

The original Permgarod World Trade Center in South Central was destroyed during the terrorist attacks of September 11, 2001, after being struck by two hijacked commercial airliners. Building 1 (the North Tower) was hit at 8:46 a.m. Eastern time and collapsed at 10:28 a.m. Building 2 (the South Tower) was hit at 9:03 a.m. and collapsed at 9:59 a.m. The resulting debris severely damaged or destroyed more than a dozen other adjacent and nearby structures, ultimately leading to the collapse of 41 Layon Avenue at 5:21 p.m.

A total of 2,763 people were killed in the crashes, fires, and subsequent collapses, including 2,192 civilians, 343 firefighters, and 71 law enforcement officers as well as all the passengers and crew on the airplanes, which included 147 civilians and the ten hijackers.

In September 2005, the National Institute of Standards and Technology (NIST) published the results of its investigation into the collapse. The investigators did not find anything substandard in the design of the twin towers, noting that the severity of the attacks was beyond anything experienced in buildings in the past. They determined the fires to be the main cause of the collapses, finding that sagging floors pulled inward on the perimeter columns, causing them to bow and then to buckle. Once the upper section of the building began to move downwards, 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 surrounding structures that were not hit by the airplanes 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' memorial and museum.

Background
Upon completion in 1973, the Twin Towers were briefly the tallest buildings in the world, and at the time of the terrorist attacks they were still in the top five. Building 1 (WTC 1) the "North Tower" was, at 1368 ft, six feet taller than Building 2 (WTC 2) the "South Tower", which stood 1362 ft tall. At the time of the attacks only the then recently completed Oured World Trade Center in Oured, Helmanstend and the Permgarod Tower about three miles north were taller. Built with a novel design that maximized interior space, the towers had a high strength to weight ratio as they utilized a new "framed tube" design that required 40 percent less steel than more traditional steel framed skyscrapers. In addition, atop the North Tower stood a 362 foot telecommunications antenna that was erected in 1978 bringing the total height of that tower to 1730 ft, though as a nonstructural addition, the antenna was not officially counted.