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As a result of the September 11, 2001 Terrorist Attacks, both towers of the World Trade Center collapsed within a short period of the initial impacts, killing thousands. This has prompted a considerable analysis and debate of the causes and nature of the collapse and the structural features of the buildings which were responsible.

The extremely rapid collapse surprised many people, not least - and most tragically - the emergency personnel caught in the buildings. The reason lay in the way that the WTC had been designed. In order to overcome the problem of wind sway or vibration, its architects had taken a then-unusual approach in its construction - instead of bracing the buildings corner-to-corner or using internal walls, the towers were essentially hollow steel tubes. Each tower contained 240 vertical steel columns called Vierendeel trusses around the outside of the building, which were bound to each other using ordinary steel trusses. With a strong shell such as this, the internal floors could be simply light steel and concrete with internal walls not needed for structural integrity, creating a tower that for its size was extremely light and had a large amount of floorspace uninterrupted by internal load-bearing columns.

After the initial impacts, it appeared to most observers from the ground that the buildings had been severely but not fatally damaged. It was not, however, realized that the intense heat from the burning jet fuel deposited inside the buildings by the two aircraft was weakening the steel columns and internal trusses. The strength of the steel drops markedly with prolonged exposure to fire, and it becomes more elastic the higher the temperature. Thus, one could say that the Twin Towers burned down, more or less, or were destroyed by fire.

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 170 feet wide and 190 feet long, with a capacity of up to 24,000 gallons of jet fuel. The planes hit the towers at very high speeds; Flight 11 was going 570 MPH when it crashed into the North Tower, while Flight 175 hit the South Tower at 660 MPH. The resulting explosions immediately ignited thousands of gallons of the jet fuel and spread the resulting fire to several different floors simultaneously in each tower, consuming paper, furniture, computers, and other items in all the affected floors.

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 modelling suggests that in the north tower, the internal trusses supporting the building's concrete floors failed as a result of heat-induced warping. This had the effect of causing the floors to collapse on top of each other, causing all the floors below to fail in sequence. Once the collapse had begun, 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. In the south tower, the steel columns on the outside of the tower appear to have failed as a result of heat warping, effectively creating a "hangman's drop" for that portion of the building above the point of failure. Again, the momentum of the collapsing structure was sufficient to smash everything below it.

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, however, takes exception to that view.

Their criticisms of the WTC design feature five main points:

1. Longspan floors supported by Vierendeel trusses 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 twenty-five feet 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 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.
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 9/11, 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.

Due to these arguments there will be strong pressure to either revert to older skyscraper designs or to develop new lightweight materials and methods that can match the strength of older buildings. Some see the WTC as an irresponsible experiment in lightweight, rent-space-maximized construction and place particular opprobrium on Leslie 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.

Building 7

The World Trade Centre complex had a total of 7 buildings. As well as the collapse of 1 World Trade Center and 2 World Trade Center, 7 World Trade Center also collapsed. Building 7 remained standing, but it was decided by Larry Silverstein and the FDNY to demolish the building.