The collapse of the World Trade Center occurred after the September 11 attacks. Each of the two towers of the World Trade Center (WTC) in New York City was hit by an airliner that had been hijacked by Al Qaeda operatives. The south tower (Two World Trade Center) collapsed at 9:59 a.m., less than an hour after being hit, and the north tower (One World Trade Center) followed at 10:28 a.m.
Inside and near the towers, 2,750 people were killed, including all 157 passengers and crew aboard the two airplanes. The collapse of the twin towers also caused extensive damage to the rest of the complex and nearby buildings. At 5:21 p.m., 7 World Trade Center collapsed as well, as a result of damage and fires which had occurred earlier in the day when the north tower collapsed. Debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures.
The Federal Emergency Management Agency (FEMA) completed its performance study of the buildings in May 2002. It declared that the WTC design had been sound and attributed the collapses wholly to extraordinary factors beyond the control of the builders. While calling for further study, FEMA suggested that the collapses were probably initiated by weakening of the floor joists by the fires that resulted from the aircraft impacts. According to FEMA's report – and subsequently contradicted by the findings of the National Institute of Standards and Technology (NIST) – the floors detached from the main structure of the building and fell onto each other, initiating a progressive "pancake" collapse.
FEMA's early investigation was revised by a later, more detailed investigation conducted by NIST, which also consulted outside engineering entities. This investigation was completed in September 2005. Like FEMA, NIST vindicated the design of the WTC, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. NIST also emphasized the role of the fires, but it did not attribute the collapses to failing floor joists. Instead, NIST 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."
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, 1 World Trade Center (Freedom Tower). Of the destroyed buildings, only 7 World Trade Center has been replaced as of 2008.
Structural design
Main article: Construction of the World Trade CenterArchitect Minoru Yamasaki designed the towers as framed tube structures, which provided tenants with open floor plans, uninterrupted by columns or walls. This was accomplished using numerous, closely-spaced perimeter columns to provide much of the strength to the structure, along with gravity load shared with the core columns and concrete. Above the seventh 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 4 inches (10 cm) thick lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors. 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. The trusses connected to the perimeter at alternate columns, and were therefore on 6.8 feet (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 the core 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 trusses supported a 4 inches (10 cm) lightweight concrete floor slab, with shear connections for composite action.
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 some load redistribution between the perimeter and core columns and supported the transmission tower.
Safety concerns regarding aircraft impacts
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. During the design of the World Trade Center, Leslie Robertson, one of the chief engineers, 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, but Robertson provided no documentation for this assertion.
NIST found a three-page white paper that mentioned another aircraft-impact analysis, involving impact of a jet at 600 miles per hour (970 km/h), but the original documentation of the study was lost when Port Authority offices were destroyed in the collapse of the World Trade Center. 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." In its investigation, NIST found reason to believe that they lacked the ability to properly model the effect of such impacts on the structures, especially the effects of the fires, though NIST offers no evidence for this belief.
Fireproofing
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 floors in 1 WTC, including all the floors affected by the aircraft impact and fires, and on 13 floors in 2 WTC, although only three of these floors (77, 78, and 85) were directly affected by the aircraft impact. Although replacement fireproofing was specified at 1.5 inches (3.8 cm) in thickness, NIST found the average thickness to be 2.5 inches (6.4 cm). NIST concluded that "the existing condition of the fireproofing prior to aircraft impact and the fireproofing thickness on the WTC floor system did not play a significant role".
September 11, 2001
Aircraft impact
On September 11, 2001, hijackers associated with al-Qaeda took control of two early morning Los Angeles-bound flights—both Boeing 767 jetliners—soon after take off from Boston's Logan International Airport. In its final moments, American Airlines Flight 11 flew south over Manhattan and crashed at roughly 440 miles per hour (710 km/h) into the northern facade of the World Trade Center's North Tower at 8:46 a.m., impacting between the 93rd and 99th floors. Seventeen minutes later, United Airlines Flight 175 approached from the southwest, over New York Harbor, and crashed into the South Tower's southern facade between the 77th and 85th floors at 540 miles per hour (870 km/h). In addition to severing numerous load-bearing columns on the perimeter and inflicting other structural damage, the resulting explosions in each tower ignited 10,000 US gallons (38,000 l) of jet fuel along with office contents. Jet fuel from the impact traveled down at least one elevator shaft and exploded on the 77th and 22nd floors, as well as on the west side lobby.
Fires
The light construction and hollow nature of the structures allowed the jet fuel to penetrate far inside the towers, igniting many large fires simultaneously over a wide area of the impacted floors. The fuel from the planes burned at most for a few minutes, but the contents of the buildings burned over the next hour or hour and a half. It has been suggested that the fires might not have been as centrally positioned, nor as intense, had traditionally heavy high-rise construction been standing in the way of the aircraft. Debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the
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