The World Trade Center, in Lower Manhattan, New York City, was destroyed after a series of terrorist attacks on September 11, 2001, killing nearly 3,000 people at the site. Two commercial airliners hijacked by al-Qaeda members were deliberately flown into the complex's Twin Towers, at the time among the five tallest buildings in the world. The plane impacts inflicted significant structural damage to the skyscrapers and started large fires in the towers, eventually resulting in both undergoing a total progressive collapse.
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., causing it to collapse at 10:28 a.m. after burning for one hour and 42 minutes. At 9:03 a.m., the South Tower (WTC 2) was struck by United Airlines Flight 175; it collapsed at 9:59 a.m. 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. Raining debris from the North Tower cut a large gash in the south side of 7 World Trade Center and ignited fires that burned out of control for six hours. It remained standing until it collapsed at 5:21 p.m. that day.
The hijackings, crashes, fires, and subsequent collapses initially killed 2,760 people. Toxic dust 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. 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 human history.
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.
Background
When they opened in 1973, the Twin Towers were the tallest buildings in the world. They had surpassed the previous record-holder, the 1931 Empire State Building in Midtown Manhattan. At the time of the attacks, only the Willis Tower (known then as the Sears Tower) in Chicago and the Petronas Towers in Kuala Lumpur, Malaysia, had held the title of tallest buildings in the world after them. Built with a novel "framed tube" design that maximized interior space, the towers had a high strength-to-weight ratio requiring 40 percent less steel than more traditional steel-framed skyscrapers. In addition, atop WTC 1 stood a 362 ft (110 m) telecommunications antenna erected in 1978, bringing that tower's total height to 1,730 ft (530 m), though as a nonstructural addition, the antenna was not officially counted.
Structural design
The towers were designed as framed tube structures, which provided tenants with open floor plans uninterrupted by columns or walls. The buildings were square and 207 ft (63 m) on each side but had chamfered 6 ft 11 in (2.11 m) corners, making each building's exterior roughly 210 ft (64 m) wide. One World Trade Center (WTC 1), the "North Tower", was, at 1,368 ft (417 m), six feet taller than Two World Trade Center (WTC 2), the "South Tower", which was 1,362 ft (415 m) tall. Numerous closely spaced perimeter columns provided much of the structural strength, 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 spaced 3 ft 4 in (1.02 m) on center. While the towers were square, the interior cores were rectangular and supported by 47 columns that ran the full height of each tower. All the elevators and stairwells were in the core, leaving a large column-free space between it and the perimeter that was bridged by prefabricated floor trusses. As the core was rectangular, this created a long and short span distance to the perimeter columns.
The floors consisted of 4 in-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. The trusses had a span of 60 ft (18 m) in the long-span areas and 35 ft (11 m) in the short-span area. The trusses connected to the perimeter at alternate columns, and were therefore on 6.8 ft (2.1 m) centers. The top chords of the trusses were bolted to seats welded to the spandrels on the perimeter side and a channel welded to interior box columns on the core 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 had a "hat truss" or "outrigger truss" between the 107th and 110th floors, consisting of six trusses along the long axis of core and four along the short axis. This 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. These trusses were installed in each building to support future transmission towers, but only the North Tower was ultimately fitted with one.
Evaluations for aircraft impact
Though fire studies and even an analysis of the impacts of low-speed jet aircraft impacts had been undertaken before the towers' completion, the full scope of those studies no longer exists. Nevertheless, since fire had never before caused a skyscraper to collapse and aircraft impacts had been considered in their design, their destruction initially surprised some in the engineering community.
The structural engineers working on the World Trade Center considered the possibility that aircraft could crash into the building. In July 1945, a B-25 bomber that was lost in fog had crashed into the 79th floor of the Empire State Building. A year later, a C-45F Expeditor crashed into the 40 Wall Street building. Once again, fog was believed to have been the contributing factor in the collision. Leslie Robertson, one of the chief engineers working on the design of the World Trade Center, said that he considered the scenario of the impact of a Boeing 707, which might be lost in the fog and flying at relatively low speeds while seeking to land at either JFK or Newark Airports. In an interview with the BBC two months after the towers collapsed, Robertson said: "with the 707, the fuel load was not considered in the design. I don't know how it could have been considered." He also said that the main difference between the design studies and the event that caused the towers to collapse was the velocity of the impact, which greatly increased the absorbed energy, and was never considered during the construction process.
During its investigation into the collapse, the National Institute of Standards and Technology (NIST) obtained a three-page white paper that stated the buildings would survive an aircraft-impact of a Boeing 707 or DC 8 flying at 600 mph (970 km/h). In 1993, John Skilling, lead structural engineer for the WTC, said in an interview conducted after the 1993 World Trade Center bombing: "Our analysis indicated the biggest problem would be the fact that all the fuel would dump into the building. There would be a horrendous fire. A lot of people would be killed. The building structure would still be there." 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. In its final report on the collapses, the NIST stated that it could find no documentation examining the impact of a high-speed jet or of a large-scale fire fueled by aviation fuel.
Fireproofing
Until the mid-1970s, the use of asbestos for fireproofing was widespread in the construction industry. But 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 and vermiculite plaster was used instead.
After the 1993 bombing, inspections found fireproofing to be deficient. Before the collapses, the towers' owner, the Port Authority of New York and New Jersey, was in the process of adding fireproofing, but had completed only 18 floors in WTC 1, including all the floors affected by the aircraft impact and fires, and 13 floors in WTC 2, although none were directly affected by the aircraft impact.
NIST concluded that the aircraft impact sheared off a significant portion of the fireproofing, contributing to the buildings' collapse. In WTC 1, the impact stripped the insulation off most core columns (43 of 47) on more than one floor, as well as floor trusses over a space of 60,000 sq ft (5,600 m2). In WTC 2, the impact knocked off insulation from 39 of the 47 columns on multiple floors, and also from floor trusses spanning an area of 80,000 sq ft (7,400 m2).
After the collapses, Leslie E. Robertson said: "To the best of our knowledge, little was known about the effects of a fire from such an aircraft, and no designs were prepared for that circumstance. Indeed, at that time, no fireproofing systems were available to control the effects of such fires."
Effects of the airplane impacts
During the September 11 attacks, four teams of al-Qaeda terrorists hijacked four different jetliners. Two of these jetliners, American Airlines Flight 11 and United Airlines Flight 175, both Boeing 767s, were hijacked after takeoff from Boston's Logan International Airport. In its final moments, American Airlines Flight 11 flew south over Manhattan and crashed at roughly 440 mph (710 km/h) into the northern facade of the North Tower (WTC 1) at 8:46 a.m., hitting the 93rd through 99th floors. Seventeen minutes later, United Airlines Flight 175 flew northeast, over New York Harbor, and crashed into the southern facade of the South Tower (WTC 2) at 9:03 a.m., striking between the 77th through 85th floors at 540 mph (870 km/h).
The impacts caused failures in the exterior columns in the regions hit by the fuselage, engines, and fuel-filled wing sections (34 columns in the North Tower and 26 in the South Tower). Damage also extended to the tips of the wings and tailfin. In addition, several core columns were either severed or heavily damaged, especially in the path of the fuselage.
About one third of the jet fuel was consumed in the initial impact and resulting fireball. Some fuel from the impact traveled down at least one elevator shaft and exploded on the 78th floor of the North Tower, as well as in the main lobby. The towers' light construction and hollowness allowed the burning fuel to penetrate far inside them, igniting many large fires simultaneously over a wide area of the impacted floors. The fuel burned for at most a few minutes, but the buildings' contents burned over the next hour or hour and a half.
As Flight 175 struck the South Tower, the shockwave shattered glass on the east face of the North Tower adjacent to the fireball, which aggravated the fires already burning in the North Tower and released plumes of smoke from the newly opened windows. It is unknown whether Flight 11's impact did the same to windows on the South Tower. In any case, the major debris from Flight 11 flew past the South Tower, while the more significant pieces of wreckage from Flight 175 similarly missed the already burning North Tower. In both instances, some of these parts landed on other nearby buildings, resulting in further destruction.
The fires in each building had different attributes, as was evident in the responses and behavior of people trapped in each. Countless windows in the North Tower were smashed by occupants seeking relief from the hellish conditions inside. While some windows were broken in the South Tower, it was relatively uncommon by comparison. Victims were only occasionally spotted inside open windows, and no crowds were present outside of the tower, as in the Impending Death photograph of the North Tower burning.
Numerous people fell or jumped to their deaths from the burning towers. Three were spotted from an east-facing window on the south side of the 79th floor of the South Tower, while the 100–200 people who fell or jumped from the four faces of the North Tower had no other means of escape from the heat, smoke and fire consuming its top 18 stories.
Such differences imply that conditions did not deteriorate as rapidly, or become as intolerable, in the South Tower as in the North. The damage to the North Tower by Flight 11's centered impact severed all escape routes above the 91st floor and left the stranded workers in an inferno from which jumping was their only means of escape; Flight 175 struck the South Tower through the southeast corner of the skyscraper's southern facade and left the northwesternmost stairwell undamaged from top to bottom. The intact stairway meant people in the South Tower were not completely trapped, which may have influenced their decision to jump.