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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 involved. Other buildings were also destroyed or damaged by the debris from the towers.

1 and 2 World Trade Center

Despite the obviously grave damage done by the speeding jetliners that struck both towers, the collapse surprised many people, including the emergency personnel caught in the buildings. There is growing speculation that these catastrophic failures were due as much to the way the WTC was designed as to the unprecedented force of the impacts.

In order to overcome the problem of wind load and other common architectural stresses, the WTC's structural engineers took a then-unusual approach in its construction— instead of bracing the towers corner-to-corner with building-wide rows of columns within internal walls, the towers were essentially hollow steel tubes supported primarily by a cross-sectionally narrow array of columns running up the center shafts. Each tower contained 240 vertical steel columns called Vierendeel trusses around the outside of the building, bound to each other using ordinary steel trusses. With a shell such as this the floors could be simply light steel and concrete, creating a tower that for its size was extremely light with a broad expanse of floorspace uninterrupted by spaced rows of internal columns and their attendant walls.

After the impacts it appeared to most observers from the ground that the buildings had been severely but not fatally damaged. It was not realized that the intense heat from the burning jet fuel and combustibles deposited near the cores of the buildings by the two aircraft was weakening the central steel columns, longspan floor trusses and the joins connecting the floorplates to the Vierendeel trusses. The strength of steel drops markedly with prolonged exposure to fire, becoming more elastic the higher the temperature. Thus it could be said that the towers burned down, more or less, 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 had much to do with the jet fuel reaching so far inside in the first place.

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 156 feet wide (48 m) and 180 feet (55 m) long, 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 going roughly 490 mph (790 km/h) when it crashed into the north tower, while Flight 175 hit the south tower at about 590 mph (950 km/h). 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 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. 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 Vierendeel trusses surrounding the impact hole, 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 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 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.

These arguments will bring 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 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 (no one imagined intentional strikes). Mr. Robertson and others involved in design and construction of the WTC have repeatedly 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. Yet a comparison of the 707-320 with the Boeing 767-200's that struck the towers shows surprisingly small differences between them and, factoring in the 707's higher typical cruise speed, a case can be made that the design team actually modeled an aircraft with greater kinetic energy than those which struck:

The Boeing 747, with an empty weight more than twice that of the 767, was in the final design phase when the WTC was being planned and its dimensions were widely known. The first order for a 747 was made in April, 1966. Facts like these are prompting a rethinking of statements made in the immediate aftermath of September 11th, which generally characterized the performance of the towers under the circumstances as "admirable".

7 World Trade Center

The World Trade Center 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, as seen live on television. FEMA's report on the disaster states that the tower appears to have collapsed due to the fire on the middle floors.

As part of the electrical backup system, there were tanks containing thousands of gallons of fuel oil on several floors and pumps to distribute it. These are seen as a possible cause of the fires that caused the collapse. Structural members required to transfer building weight off of the pre-existing electrical sub-station that the 7 WTC building was built over may have failed in the fire leading to the internal mechanism of collapse.

Other buildings

Numerous other buildings in the World Trade Center and surrounding it were damaged or destroyed as the Towers fell. 5 World Trade Center suffered a large fire and a partial collapse of its steel structure. This is also an unprecedented event.

Other buildings destroyed include St. Nicholas Greek Orthodox Church, Marriot Hotel (3 WTC), South Plaza (4 WTC), U.S. Customs (6 WTC), and the Winter Garden at the World Financial Center. World Financial Center buildings 4, 5, 6, and 7, 90 West Street, and 130 Cedar Street suffered fires. The Bankers Trust Building, Verizon, and World Financial Center 3 suffered impact damage from the Towers collapse. 30 West Broadway was damaged by the collapse of 7 WTC.

External links

• World Trade Center Building Performance Study (May 2002) - a study by FEMA and the ASCE.
• Video of the collapse of building 7

• New York City history
• World Trade Center