The collapse of the World Trade Center was caused by the September 11, 2001 attacks that sent one hijacked airliner into each of the main towers of the World Trade Center complex, 1 WTC and 2 WTC. Both towers lost structural integrity and fell that morning, killing almost 3,000 people within and nearby. Other nearby buildings, including 7 WTC, were destroyed or damaged by the debris.

Structural engineers and architects in the United States and elsewhere have extensively analyzed the collapse, sometimes contentiously, to determine whether the unusual structural features of the Twin Towers may have been wholly or partially at fault.

In the first few months after the attacks, most representatives from these professions who gave statements to media outlets lauded the "performance" of the Twin Towers, suggesting that loss of life could have been far worse if design and construction of the buildings had been of lesser quality. As time went on, dissenting voices were heard. Radical design decisions made by the WTC team were compared to more time-tested skyscraper design. A report entitled "World Trade Center Building Performance Study" issued by the Federal Emergency Management Agency (FEMA) in May 2002, pronounced the WTC design fundamentally safe and attributed the collapse wholly to extraordinary factors beyond the control of the builders.

In 2005, the National Institute of Standards and Technology (NIST) issued a series of reports documenting the collapse and emergency response efforts. NIST concluded "the buildings would likely not have collapsed under the combined effects of aircraft impact and the subsequent jet-fuel ignited multi-floor fires, if the fireproofing had not been dislodged or had been only minimally dislodged by aircraft impact." NIST also found the Towers' stairwell design lacked adequate reinforcement.

The FEMA and NIST reports have not completely quieted debate among architecture and construction professionals.

One and Two World Trade Center

Template:Sep11 To meet the challenges of wind load, gravity load and related architectural stresses, the WTC's structural engineers took a then-unusual approach in its construction. Instead of the typical high-rise infrastructure, in which the walls held building-wide rows of heavy columns, each tower was essentially a hollow steel tube staked to the earth by a relatively narrow array of columns running up the building core. As secondary supports, each tower had 240 thin steel columns sheathing the facade, a signature feature allowing the number of internal columns to be very small for such huge structures. The result was super-tall, super-wide office buildings with uniquely large expanses of column-free floorspace.

After the airliners hit, it appeared to most ground observers that the buildings had been severely but not fatally damaged. But intense heat from the burning jet fuel and combustibles deposited near the cores of the towers by the two aircraft was weakening the central steel columns, the longspan floor trusses and the joins connecting the floorplates to the external columns. The strength of steel drops markedly with prolonged exposure to fire, becoming more elastic as the temperature rises.

Thus it could be said that the towers burned down, essentially, or were destroyed by fire, and that any steel of any building would have degraded in the same way. This is something of a tautological argument, however, because the lightness and hollowness of the towers were prime factors allowing the jet fuel (and resulting fires) to penetrate so far inside in the first place. This lightness and hollowness were functions, primarily, of the absence of building-wide rows of columns (and attendant walls), the absence of masonry elements or heavy steel in the facades, and the use of gypsum cladding rather than reinforced concrete to encase stairways and elevator shafts. Debates between engineers have looped along this circular cause-and-effect chain: collapse certainly would not have occurred without the fires, but the fires may not have been as centrally positioned nor as intense had traditionally heavy high-rise construction been standing in the way of the aircraft. With that traditionally heavy construction, debris and fuel would likely have remained mostly outside the buildings and/or concentrated in more peripheral areas away from the building cores, which themselves would not have been unique failure points. In this scenario, the towers may have stood far longer, perhaps indefinitely.

The towers were each struck by hijacked Boeing 767 jet planes, American Airlines Flight 11 and United Airlines Flight 175. A typical Boeing 767 is 180 feet (55 m) long and has a wingspan of 156 feet (48 m), with a capacity of up to 24,000 US gallons (91,000 L) of jet fuel. The planes hit the towers at very high speeds: Flight 11 was traveling roughly 490 mph (790 km/h) when it crashed into the north tower, Flight 175 hit the south tower at about 590 mph (950 km/h). The resulting explosions ignited thousands of gallons of the jet fuel and immediately spread the fire to several different floors simultaneously in each tower, consuming paper, furniture, carpeting, computers, books, walls and framing, and other items in all the affected floors.

Collapse of the two towers

The north tower, 1 WTC, was struck at 8:46:26 am and collapsed at 10:28:31 am, standing for 102 minutes and 5 seconds. The south tower, 2 WTC, was struck at 9:02:54 am and collapsed at 9:59:04 am, 56 minutes and 10 seconds later. A combination of three factors allowed the north tower to remain standing longer: the region of impact was higher (so the gravity load on the most damaged area was lighter); the speed of the airplane was lower (so there was less impact damage); and the affected floors had had their fire proofing partially upgraded. Also, the hottest part of the fire in the south tower burned near a corner of the building and apparently led to a sudden bursting of bolts in that section, while the failures in the north tower core involved slower creep and softening effects. [1]

The two towers collapsed in markedly different ways, indicating that there were in fact two modes of failure. The north tower collapsed directly downwards, "pancaking" in on itself, while the south tower fell at an angle during which the top 20 or so stories of the building remained intact for the first few seconds of the collapse.

Subsequent modeling suggests that in the north tower the internal trusses supporting the building's concrete floors failed as a result of heat-induced warping. This placed additional stress on the bunched core columns, which themselves were losing integrity from both impact damage and heat. When the core columns gave out on one of the impact floors, this floor collapsed into the floor below. Once the collapse started, it was unstoppable; the huge mass of the falling structure had sufficient momentum to act as a battering ram, smashing through all the intact floors below. This theory is supported by witnesses from within the tower stating they heard "something like a heavy freight train approaching". There is some visual evidence that it was the core that collapsed first. It can be seen in videos that the large antenna, which was built on top of the core, starts downward a fraction of a second earlier than the rest of the building.

In the south tower, heat warping weakened the single-bolt connections between the floorplates and the initially intact external columns surrounding the impact hole, effectively creating a "hangman's drop" for that portion of the building above the point of failure. Eventually, the gravity load on these bolts increased beyond the breaking point as the joints, floorplates and columns weakened. Again, the momentum of the collapsing structure was sufficient to smash everything below it.

Design criticisms

The collapse of the towers set off intense debates within the structural engineering and architectural professions, with no clear end in sight. The largest camp appears to be those who feel the towers did well under the circumstances by standing long enough for the majority of occupants to escape. A large and apparently growing minority takes exception to that view.

Their criticisms of the WTC design feature five main points:

1. Longspan floors supported by external columns are inherently weaker than the traditional box frame column/girder arrangement with internal walls.
2. The bunching of all internal columns in a relatively narrow center shaft in a building is an "all your eggs in one basket" configuration-- if that region on any floor is catastrophically damaged (as it certainly was by the fire in the north tower), the entire building is doomed. This stands in stark contrast to earlier generations of skyscrapers which utilize full skeletons of stepped columns, usually one row approximately every 25 feet (7.6 m) from the center to the perimeter.
3. The World Trade Center exclusively used lightweight materials, especially in the facade. Had the WTC facade contained even minimal masonry elements and/or traditional heavy steel outermost column rows, it is unlikely the aircraft would have cleanly penetrated to the core of each tower— a significant portion of debris and jet fuel would have remained outside, a much different scenario.
4. Single-bolt connections binding the longspan floorplates with the load-bearing external columns were extremely lightweight for their assigned task. One study group from the Massachusetts Institute of Technology has concluded the proximal cause of the south tower collapse was failure of these bolts in the southeast corner of the building. Double-bolts should have been used.
5. The use of gypsum cladding instead of reinforced concrete to shield stairwells. Almost all skyscrapers, including those built since the WTC, shield stairwells in reinforced concrete. On September 11th, it was the collapse of all stairways above the impact level that consigned all people above the impact zone in Tower One to death. Tower Two had two of its three stairwells taken out above the impact area by the plane. Some people above the impact zone survived, as they used the third stairwell. Computer models have shown that most of the stairwells in both towers would likely have remained usable until the general collapse had they been shielded in concrete.

Some see the WTC as an irresponsible experiment in lightweight, rent-space-maximized construction and place particular opprobrium on Leslie E. Robertson, its Chief Structural Engineer. Others see it as a landmark in structural engineering simply in need of refinement due to unforeseen, and probably unforeseeable, variables.

One of those variables was the size and kinetic energy of aircraft that might accidentally strike the WTC. Mr. Robertson and others involved in design and construction of the WTC have stated that back in the 1960s they could not have planned for the jetliners of 2001. Specifically, they modeled the effects of a hit by the largest aircraft of the day, the Boeing 707-320, and presumably calibrated their design to withstand it.

According to the Federal Emergency Management Agency, WTC towers 1 and 2 were designed to withstand the impact of a 707 lost in fog, looking to land. The modeled aircraft was a 707 weighing 263,000 lb (119,000 kg) with a flight speed of only 180 mph (290 km/h), as would be used in approach and landing situations ([2], page 17). The 767s that actually hit the towers had a kinetic energy more than seven times greater than the specifically modeled 707 impact. (The Boeing 747, with an empty weight more than twice that of the 767, was in the final design phase when WTC drafting began and the first 747s were constructed simultaneously with the WTC towers; however the known attributes of the 747 were apparently not modeled in designing the towers).

Although the two major government reports largely avoided faulting the WTC design, the construction industry has already made changes that show an implicit acknowledgement of the critics' arguments. For example, the plans for the main tower that is to replace 1 and 2 WTC have been revised a number of times to include heavier materials and more traditional column/girder internals. Additionally, extensive retrofittings of `60s/`70s era skyscrapers that share the WTC's main features, such as Chicago's Sears Tower and John Hancock Center, are underway.

The 9/11 Commission, in its final report, did not address the question of whether Osama bin Laden and the other masterminds of the plot planned on, or even suspected, that the attacks would result in the collapse of the towers. No information has been released by the U.S. government indicating whether interrogators of Khalid Sheikh Mohammed have addressed this issue. Given the complexity of the variables, it seems improbable that the al Qaeda leadership could have anticipated that the steel cores and superstructures would melt as they did, and that the buildings would collapse so quickly after impact. Bin Laden, trained in construction and engineering, had not believed that the buildings would collapse completely, but rather would collapse only above the levels where the planes respectively struck: