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Following the September 11, 2001 attacks, that sent one hijacked airliner into each of the main towers of the World Trade Center complex, 1 WTC, 2 WTC and 7 WTC collapsed. Both main towers lost structural integrity and fell that morning, killing 2,595 people within and nearby, as well as 157 people who were aboard the flights. Other nearby buildings, including 7 WTC, were damaged by the debris. 7 WTC collapsed seven hours later at 5:20pm, but without casualties as it had already been evacuated.

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. Radical design decisions made by the WTC team were compared to more time-tested skyscraper designs. 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.

The National Institute of Standards and Technology (NIST) has analyzed the events leading up to the initiation of the collapse to determine whether any unusual structural features of the Twin Towers may have been wholly or partially at fault. In 2005, NIST issued a series of reports documenting events leading up to the initiation of 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 resolved all disagreements among engineers. New Civil Engineer published an article titled Row erupts over why twin towers collapsed, in which one party claims "the towers would have collapsed after a major fire on three floors at once, even with fireproofing in place and without any damage from plane impact." Fireproofing was added due to a minor electrical fire in 1975 (pg 12) that affected six floors before being put out. Other articles have been titled WTC investigators resist call for collapse visualisation and Huge investigation but questions remain .

Construction of 1 and 2 WTC

Construction of the towers began in 1968 and were completed in 1972 and 1973. During the period, implementation of an innovative elevator system halved the number of elevator shafts. The express elevators took people to "sky lobbies" on the 44th and 78th floors, where they could board local elevators. Also unique was its grouping of columns into the core and perimeter of the building, a structural system called a "tube".

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 employing a traditional grid-like plan with beams evenly spaced throughout a floor, the WTCs columns were grouped in the building's core and perimeter. The core of each tower was a rectangular area 87 by 133 feet and consisted of steel box columns running from the bedrock to the tops of the tower. The columns tapered to the top, where they transitioned to lightweight H-beams, but the exact dimensions are unknown as the blueprints are under the jurisdiction of the Port Authority and are not public domain. Each tower had 240 steel perimeter columns (from 2.5 inches thick at the bottom tapering to .25 inch at the top) placed 14 inches around the perimeter. This signature feature of grouping columns in the core and perimeter allowed large tracks of floorspace that were uninterrupted by columns, a significant marketing feature for the towers.

Impacts of airliners

After the airliners hit, it appeared to most ground observers that the buildings had been severely but not fatally damaged. However, intense heat from the burning jet fuel and combustibles near the cores of the towers was weakening the central steel columns, the longspan floor trusses and cross trusses, and the joints connecting the floorplates to the external columns. The strength of the steel dropped markedly with exposure to fire, the steel losing half its strength at a temperature of 650C (1,202F).

A combination of factors including the impact from aircraft flying at high speed was the cause of internal structural damage. The resultant fires from the aviation fuel spread widely through the impact zone and ultimately led to the collapse of the Twin Towers. The destabilization, debris, and associated fires resulting from the collapse of the Towers also damaged many of the buildings in the immediate vicinity. The lightness and hollowness of the towers were prime factors allowing the jet fuel (and resulting fires) to penetrate so far inside the Towers. 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— 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 might 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 as the economic cruising speeds of a Boeing 767 is around 850 km/h. 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 in each tower ignited ten thousand gallons of jet fuel and immediately spread the fire to several different floors simultaneously while consuming paper, furniture, carpeting, computers, books, walls, 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 after impact. The south tower, 2 WTC, was struck at 9:02:54 am and collapsed 56 minutes and 10 seconds later, at 9:59:04 am. A combination of three factors may have 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 (as seen in this video of the building's collapse) 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.

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The two towers collapsed in markedly different ways which may indicate that there were 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, then pulverized into dust in mid-air, and the tower then continued straight down.

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 would have 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. Witnesses from within the tower stated they heard "something like a heavy freight train approaching". It could be said these statements support this theory. 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. However later analysis by NIST dispute this claim stating "that observations from a single vantage point can be misleading and may result in incorrect interpretation. When records from east and west vantage points were viewed, it was apparent that the building section above the impact area tilted to the south as the building collapsed."

In the south tower, it is assumed to be fire that warped and severed 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 their breaking point as the joints, floorplates and columns weakened. The momentum of the collapsing structure crushed everything below it.

Design criticisms

The collapse of the towers set off intense debates within the structural engineering and architectural professions. 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 minority takes exception to that view.

Their criticisms of the WTC design feature six 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 below the top floor is catastrophically damaged, 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 less likely 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.
6. According to the National Commission on Terrorist Attacks on the United States, each tower's core consisted only of a hollow steel shaft.

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 while looking to land. The modeled aircraft was a 707 weighing 263,000 lb (119,000 kg) with a flight speed of 180 mph (290 km/h), as would be used in approach and landing situations (, 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).

In its final report, the 9/11 Commission 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 that indicates 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 fail as they did or that the buildings would collapse so quickly after impact. It is a common belief that bin Laden, who is a civil engineer , had not believed that the buildings would collapse completely, but rather would collapse only above the levels where the planes respectively struck:

Although a small group question its authenticity, a videotape of bin Laden verified by the Pentagon states

We calculated in advance the number of casualties from the enemy, who would be killed based on the position of the tower. We calculated that the floors that would be hit would be three or four floors. I was the most optimistic of them all. (...Inaudible...) Due to my experience in this field, I was thinking that the fire from the gas in the plane would melt the iron structure of the building and collapse the area where the plane hit and all the floors above it only. This is all that we had hoped for. (Transcript of Osama bin Laden videotape)

A federal technical building and fire safety investigation of the collapses of the Twin Towers and 7 WTC was conducted by the United States Department of Commerce's National Institute of Standards and Technology (NIST). The goals of this investigation were to investigate the building construction, the materials used, and the technical conditions that contributed to the outcome of the WTC disaster. The investigation will serve as the basis for:

• improvements in the way buildings are designed, constructed, maintained, and used;
• improved tools and guidance for industry and safety officials;
• revisions to building and fire codes, standards, and practices; and
• improved public safety.

The long-anticipated report was partially released in draft for public comment on April 6, 2005. In its over 10,000 pages the conclusion reached was that the fireproofing on the steel infrastructure was blown off by the initial impact of the planes into the towers. If this had not occurred the WTC would have likely remained standing. A further finding of the report was that the staircases were not adequately reinforced to provide emergency escape for people above the impact zone.

According to the Executive Summary of NIST's final report on the Collapses of the World Trade Center Towers, one of its goals was to "Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft and why and how WTC 7 collapsed" (p xli). However, the report elsewhere says that its "probable collapse sequence" "does not actually include the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable." (p xliii)

Seven World Trade Center

The World Trade Center complex had a total of 7 buildings. The third building to collapse in the September 11, 2001 attacks was 7 World Trade Center which fell at 5:20pm, as seen live on television. WTC 7 was a 47 story steel-frame skyscraper that stood across the street from the rest of the WTC complex.

Firefighters appeared to have abandoned the building and let the fires burn. It remains unknown what criteria they used to determine that it should be abandoned or who decided to abandon it. Since little could be seen from the outside, and no one was able to observe what happened within the building, the cause of the collapse is disputed.

As part of the electrical backup system, there may have been up to a total of 160,000 litres (42,000 gallons) of diesel fuel stored in five tanks within the building on several floors, as well as pumps to distribute it. It has been claimed that the diesel fuel and emergency generators spilled and ignited inside building 7 although the FEMA report states that all tanks remained fully intact. Another speculation is that the building's unusual architecture may have contributed to its collapse. Theoretically, cantilevers and structural members, required to transfer building weight off of the pre-existing Con Ed electrical substation that the 7 WTC building was built over, may have failed in the fire leading to the internal mechanism of collapse.

FEMA's report on the disaster states that "Although the total diesel fuel on the premises contained massive potential energy, the best hypothesis has only a low probability of occurrence. Further research, investigation, and analyses are needed to resolve this issue" (Chapter 8.2.5.1) . NIST continued this work and released a progress report in June of 2004 in which they outlined the working hypothesis of the collapse of 7 WTC :

• An initial local failure at the lower floors (below Floor 13) of the building due to fire and/or debris induced structural damage of a critical column (the initiating event), which supported a large span floor bay with an area of about 2,000 square feet (190 m²).
• Vertical progression of the initial local failure up to the east penthouse, as large floor bays were unable to redistribute the loads, bringing down the interior structure below the east penthouse.
• Horizontal progression of the failure across the lower floors (in the region of Floors 5 and 7, that were much thicker than the rest of the floors), triggered by damage due to the vertical failure, resulting in the disproportionate collapse of the entire structure.

The final report from the NIST regarding the collapse of 7 WTC was due in July of 2005, but is still ongoing.

In "WTC part IIC - WTC7 Collapse Final", released in April 2005, NIST concludes about the fuel "This finding allows for the possibility, though not conclusively, that the fuel may have contributed to a fire on Floor 5."

Controlled demolition speculations

A small number of people has challenged the findings of U.S. Government engineers and independent researchers, arguing that controlled demolition may have occurred. For more information, please see: 9/11 conspiracy theories.

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.

Other buildings destroyed include St. Nicholas Greek Orthodox Church, Marriott 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, as did 90 West Street. 30 West Broadway was damaged by the collapse of 7 WTC. The Deutsche Bank building, though left standing, is currently being demolished because of water and mold damage, and severe damage caused by the neighboring towers' collapse.

Notes

1. Therese McAllister (ed.) (2002): World Trade Center Building Performance Study, FEMA/ASCE report 403, May 2002. Retrieved from www.fema.gov
2. National Institute of Standards and Technology - Ongoing investigation into the WTC collapse.
3. 9/11 Commission Hearing

References

• NIST and the World Trade Center
• World Trade Center - Some Engineering Aspects
• World Trade Center Building Performance Study
• Why Did the World Trade Center Collapse? Science, Engineering, and Speculation
• Journal of Engineering Mechanics ASCE, in press 9/13/01, Expanded 9/22/01, Appendices 9/28/01 Why Did the World Trade Center Collapse?—Simple Analysis
• Collapse of the World Trade Centre Towers G. Charles Clifton, HERA Structural Engineer
• NOVA online, The Collapse: An Engineer's Perspective Dr. Thomas Eagar, professor of materials engineering and engineering systems at MIT
• A.S. Usmani, Y.C. Chung, J.L. Torero; "How did the World Trade Center Collapse: A New Theory", Fire Safety Journal, 38, 6, 501-591, 2003
• NOVA online The structure of metal, discussing the molecular bending of metal
• Jeremy Baker: Silverstein, FDNY Razed WTC 7. Public Broadcasting Service documentary. Available as part of America Rebuilds. PBS home video. ISBN 0-7806-4006-3

External links

• Bill Biggart's Final Exposures contains a photo of the WTC Marriott severely damaged by the collapse of 2 WTC immediately before the collapse of 1 WTC in which the photographer, Bill Biggart, was killed.

• World Trade Center
• September 11, 2001 attacks