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The Twin Towers of the World Trade Center collapsed on September 11, 2001, as a result of being struck by jet airliners hijacked by terrorists affiliated with al-Qaeda during the September 11 attacks. Two of the four hijacked airliners crashed into the Twin Towers, one into the North Tower (1 World Trade Center) and the other into the South Tower (2 World Trade Center). The collapse of the Twin Towers destroyed the rest of the complex, and debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures. The South Tower collapsed at 9:59 am, less than an hour after being hit by the second hijacked airliner, and at 10:28 am the North Tower collapsed. Later that day, 7 World Trade Center collapsed at 5:21 pm from fires that had started when the North Tower collapsed. As a result of the attacks to the towers, a total of 2,763 people died. Of the people who died at ground level and in the towers, 2,192 were civilians, 71 were law enforcement officers, and 343 were firefighters. Aboard the two airplanes, 147 civilians and 10 hijackers also died. Immediately following the attacks, a building performance study (BPS) team of engineering specialists was formed by the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE) and Federal Emergency Management Agency (FEMA). The BPS team issued its report in May 2002, finding that the aircraft impacts caused "extensive structural damage, including localized collapse" and that the resulting fires "further weakened the steel-framed structures, eventually leading to total collapse." They also presented recommendations for more detailed engineering studies of the disaster. The BPS team investigation was later followed by a more detailed investigation conducted by the National Institute of Standards and Technology (NIST), which also consulted outside engineering entities. This investigation was completed in September 2005. The NIST investigators did not find anything substandard in the design of the WTC towers, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. They also emphasized the role of the fires and found that sagging floors pulled inward on the perimeter columns: "This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers."〔(【引用サイトリンク】title=NIST Response to the World Trade Center Disaster )〕 The cleanup of the site involved round-the-clock operations, many contractors and subcontractors, and cost hundreds of millions of dollars. The demolition of the surrounding damaged buildings continued even as new construction proceeded on the World Trade Center's replacement, One World Trade Center, which was structurally completed on May 10, 2013, when the final component of the spire was installed atop the skyscraper. , One World Trade Center, 4 World Trade Center and 7 World Trade Center have been replaced. ==Structural design== The towers were designed as "tube in tube" structures, which provided tenants with open floor plans uninterrupted by columns or walls. Numerous, closely spaced perimeter columns provided much of the strength to the structure, along with gravity load shared with the steel box columns of the core. Above the tenth floor, there were 59 perimeter columns along each face of the building, and there were 47 heavier columns in the core. All of the elevators and stairwells were located in the core, leaving a large column-free space between the perimeter that was bridged by prefabricated floor trusses. The floors consisted of lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors with shear connections to the concrete slab for composite action.〔 The trusses had a span of in the long-span areas and in the short-span area.〔 The trusses connected to the perimeter at alternate columns, and were therefore on centers. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to interior box columns on the interior side. The floors were connected to the perimeter spandrel plates with viscoelastic dampers, which helped reduce the amount of sway felt by building occupants. The towers also incorporated a "hat truss" or "outrigger truss" located between the 107th and 110th floors, which consisted of six trusses along the long axis of core and four along the short axis. This truss system allowed optimized load redistribution of floor diaphragms between the perimeter and core, with improved performance between the different materials of flexible steel and rigid concrete allowing the moment frames to transfer sway into compression on the core, which also mostly supported the transmission tower. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Collapse of the World Trade Center」の詳細全文を読む スポンサード リンク
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