Collapse of the World Trade Center

From Academic Kids

A New York City firefighter looks up at what remains of the World Trade Center, two days after its collapse
A New York City firefighter looks up at what remains of the World Trade Center, two days after its collapse

The Collapse of the World Trade Center came as a result of the September 11, 2001 Attacks. Within a short period of time after impacts by hijacked airliners, both main towers of the World Trade Center complex collapsed, killing thousands. Analyses of the causes and nature of the collapse, and to what extent the unusual structural features of the towers may or may not be faulted, have been ongoing. Other buildings were also destroyed or damaged by the debris from the towers.

In the first few months after the attacks, most representatives from the architectural and structural engineering 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 examined and compared to more time-tested skyscraper materials and methods. 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 to extraordinary factors beyond the control of the towers' builders. The report has not resulted in a quieting of the debate among professionals.


One and Two World Trade Center

Missing image
Schematic of 1 WTC with impact damage. Note narrow central shaft into which all internal columns are bunched. Adapted from NIST report "Baseline Structural Performance and Aircraft Impact Damage Analysis", October 19, 2004

To meet the challenges of wind load, gravity load and other common architectural stresses, the WTC's structural engineers took a then unusual approach in its construction— instead of the typical high-rise infrastructure utilizing building-wide rows of heavy columns within walls, each tower was essentially a hollow steel tube staked to the earth by a cross-sectionally 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 that allowed the number of internal columns to be very small for such huge structures. The result was super-tall, super-wide office buildings with maximized expanses of column-free floorspace.

After the aircraft impacts on September 11, 2001, it appeared to most observers from the ground that the buildings had been severely but not fatally damaged. They did not realize that 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, longspan floor trusses and the joins connecting the floorplates to the external columns. As is well known, 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, basically, 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 (and resulting fire) reaching 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 shield stairways and elevator shafts .

Missing image
Impact locations for towers 1 and 2.
Missing image
Impact locations for towers 1 and 2.

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, human beings, and other items in all the affected floors. The fires reached sustained intense temperatures rarely observed in building conflagrations, in places exceeding 760 C (1400 F).

The north tower, 1 WTC, was impacted 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 impacted at 9:02:54 am and collapsed at 9:59:04 am, standing for 56 minutes and 10 seconds. The fact that the north tower stood much longer than the south is attributed mainly to three facts: 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); 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 warping and softening effects.

Collapse of the two towers

Missing image
Ground Zero debris with markup showing building locations.

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 joins, 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 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 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. 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:

Parameter Boeing 707-320 Boeing 767-200
fuel capacity 23,000 US gal 87,000 L 23,980 US gal 90,780 L
max takeoff weight 333,600 lb 151,300 kg 387,000 lb 175,500 kg
empty weight 146,400 lb 66,400 kg 164,800 lb 74,800 kg
wingspan 145.75 ft 44 m 156.08 ft 48 m
wing area 3010 ft² 280 m² 3050 ft² 283 m²
length 152.92 ft 47 m 159.17 ft 49 m
cruise speed 557 mph 896 km/h 530 mph 853 km/h

According to the Federal Emergency Management Agency, however, 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 lbs with a flight speed of 180 mph, as would be used in approach and landing situations ([1] (, page 17). The 767s that actually hit the towers had a kinetic energy more than seven times the modeled 707 impact.

The 9/11 Commission, in its final report, did not address the question of whether 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.

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 [2] ( 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 released 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.

Seven 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 suggests the collapse was due to the fire on the middle floors, although this "hypothesis has only a low probability of occurrence and further research, investigation, and analyses are needed"[3] ( NIST continued this work and released a progress report in June of 2004 in which they outlined the working hypothesis of the collapse of WTC 7 [4] ([5] (

  • 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.
  • 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.

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 source of fuel for 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.

A final report from the NIST regarding the collapse of 7 WTC is due in July of 2005 [6] (

Conspiracy theory: Was 7 WTC demolished?

In a PBS documentary originally aired in Sept. of 2002 entitled "America Rebuilds", Larry Silverstein, the real estate developer who owned the leases on WTC buildings 1, 2 and 7, possibly indicates that Seven World Trade Center was deliberately demolished, which has generated concern that One and Two World Trade Center might also have been deliberately demolished.

In the documentary, Silverstein is quoted as saying:

"I remember getting a call from the, er, fire department commander, telling me that they were not sure they were gonna be able to contain the fire, and I said, 'We've had such terrible loss of life, maybe the smartest thing to do is pull it.' And they made that decision to pull and we watched the building collapse."

Mr Silverstein's comments came after FEMA and the Society of Civil Engineers conducted an investigation into the collapse of 7 WTC. The study was released in May of 2002. The study specifically concluded:

"Loss of structural integrity was likely a result of weakening caused by fires on the 5th to 7th floors. The specifics of the fires in 7 WTC and how they caused the building to collapse remain unknown at this time. 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."[7] (

It must be noted that the demolition of a building requires extensive planning and placement of a large number of explosive charges, a process that takes several days. Furthermore, Silverstein's comments are subject to interpretation. Opponents of the above interpretation state that Silverstein's "pull it" comment refers to pulling fire teams off of 7 WTC, to prevent further loss of life. Lastly, New York City fire department personnel did not have the proper qualifications for such demolition, nor did they have reasonable access given the significant damage and ongoing large fires in the structure to conduct such a controlled demolition.

In November of 2004, responding to such claims Silverstein's spokesman Howard J. Rubenstein stated "It is unfortunate that this group is peddling grossly inaccurate conspiracy theories." FEMA [the Federal Emergency Management Agency] conducted a detailed study and concluded that the collapse was caused by fires ignited by falling debris." [8] (

Another frequent claim in the theory that 7 WTC was intentionally demolished was the nature of the collapse of the building, in that it collapsed mostly within its own footprint. However, NIST has stated the collapse sequence within their working hypothesis "is consistent with all evidence currently held by NIST, including photographs and videos, eyewitness accounts and emergency communication records." [9] ( Thus, according to the NIST the building could indeed fall within its own footprint without there being a controlled demolition.

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.

See also


  • 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
  • Therese McAllister (ed.) (2002): 'World Trade Center Building Performance Study, FEMA/ASCE report 403, May 2002. Retrieved from (

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