the falling of the world trade centers

he twin towers of the World Trade Center collapsed on September 11, 2001, as a result of the September 11 attacks,[1] in which terrorists affiliated with al-Qaeda hijacked four commercial passenger jet airliners, crashing two of them into the World Trade Center, one into the North Tower (1 World Trade Center) and the other into the South Tower (2 World Trade Center).[2] 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 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.[3] As a result of the attacks to the towers, 2,752 people died, including all 157 passengers (including the hijackers) and crew aboard the two airplanes.[4] 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.[5] 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."[6] 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. as of 2013 4 World Trade Center and 7 World Trade Center have been replaced. Contents [hide] 1 Structural design 1.1 Safety concerns regarding aircraft impacts 1.2 Fireproofing 2 September 11, 2001 2.1 Aircraft impact 2.2 Fires 2.3 Collapse of the South Tower 2.4 Collapse of the North Tower 2.5 Collapse of 7 World Trade Center 3 Mechanics of Twin Towers' collapse 3.1 Collapse initiation 3.2 Total progressive collapse 4 Initial opinions and analysis 5 Investigations 5.1 FEMA building performance study 5.2 NIST report 5.3 7 World Trade Center 5.4 Other investigations 6 Aftermath 6.1 Other buildings 6.2 Cleanup 6.3 Health effects 7 See also 8 Notes 8.1 Footnotes 8.2 Citations 9 References 10 External links Structural design[edit] Main article: Construction of the World Trade Center 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.[7] The floors consisted of 4-inch-thick (10 cm) 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.[7] The trusses had a span of 60 feet (18 m) in the long-span areas and 35 feet (11 m) in the short-span area.[7] The trusses connected to the perimeter at alternate columns, and were therefore on 6.8-foot (2.1 m) 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. Safety concerns regarding aircraft impacts[edit] The structural engineers working on the World Trade Center considered the possibility that an aircraft could crash into the building. In July 1945, a B-25 bomber that was lost in the fog had crashed into the 79th floor of the Empire State Building. A year later, another airplane nearly crashed into the 40 Wall Street building, and there was another near-miss at the Empire State Building.[8] Leslie Robertson, one of the chief engineers working on the design of the World Trade Center, has since claimed to have personally considered the scenario of the impact of a jet airliner"a Boeing 707"which might be lost in the fog and flying at relatively low speeds, seeking to land at JFK Airport or Newark Airport. However in an interview with the BBC, Robertson states, "with the 707, the fuel load was not considered in the design, I don't know how it could have been considered." Robertson has provided no documentation for his opinion.[8][9] NIST found a three-page white paper that mentioned another aircraft-impact analysis, involving impact of a Boeing 707 at 600 miles per hour (970 km/h), but the original documentation of the study, which was part of the building's 1,200-page structural analysis, was lost when the Port Authority offices were destroyed in the collapse of the 1 WTC; the copy was lost in 7 WTC.[10] In 1993, John Skilling, lead structural engineer for the WTC, recalled doing the analysis, and remarked, "Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a horrendous fire. A lot of people would be killed," he said. "The building structure would still be there."[11] In its report, NIST stated that the technical ability to perform a rigorous simulation of aircraft impact and ensuing fires is a recent development, and that the technical capability for such analysis would have been quite limited in the 1960s.[12][note 1] Fireproofing[edit] In April 1970, the New York City Department of Air Resources ordered contractors building the World Trade Center to stop the spraying of asbestos as an insulating material.[13] Fireproofing was incorporated in the original construction and more was added after a fire in 1975 that spread to six floors before being extinguished. After the 1993 bombing, inspections found fireproofing to be deficient. The Port Authority was in the process of replacing it, but replacement had been completed on only 18