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Template:Sep11 On September 11, 2001, the two main towers of the World Trade Center complex were each hit by aircraft as part of the September 11, 2001 attacks. The south tower (2 WTC) collapsed at 9:59 am, less than an hour after being hit, and the north tower (1 WTC) followed at 10:28 am, causing massive damage to the rest of the complex and nearby buildings. In all, 2,595 people inside and near the towers were killed, along with the 157 people who were aboard the two airplanes.

The Federal Emergency Management Agency (FEMA) issued a performance study of the buildings in May 2002, declaring the WTC design sound and attributing the collapses wholly to extraordinary factors beyond the control of the builders. In its September 2005 report, the National Institute of Standards and Technology (NIST) concurred with this view, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. It did add, however, that the towers' stairwell design lacked adequate reinforcement.

7 World Trade Center collapsed at 5:20 pm, with no casualties. No conclusive reason for the collapse has ever been established, but NIST has said that the collapse was the result of structural damage sustained during the collapse of Towers 1 and 2, combined with widespread fires in the building.

Design issues

The towers were designed as framed tube structures, with columns grouped around the perimeter and within the core. The perimeter columns supported virtually all lateral loads, such as wind loads, and shared the gravity loads with the core columns. Above the seventh floor there were 59 perimeter columns along each face of the building and there were 47 heavier columns in the core. All of the elevators and stairwells were located in the core. The large, column-free space between the perimeter and core was bridged by pre-fabricated floor trusses. The trusses connected to the perimeter at alternate columns. The towers also incorporated a "hat truss" or "outrigger truss" located between the 107th and 110th floors, which consisted of six trusses along the long axis of core and four along the short axis. This truss system allowed some load redistribution between the perimeter and core columns and supported the transmission tower.

Anticipation of aircraft impact

Like all modern skyscrapers, WTC towers were designed to survive major fires. Fireproofing was also added after a fire in 1975 that spread to six floors before being extinguished. Early tests conducted on steel beams from the WTC show they generally met or were stronger than design requirements.

In a less common move, designers had also considered the consequences of aircraft impact. In 1993, John Skilling, who had been in charge of the structural design of the buildings, said that an aircraft impact would cause a great deal of damage and loss of life, mainly because of the ensuing fires, but the structure would not collapse. After the 2001 attacks, Leslie Robertson, who had participated in the structural design of the towers, said that the towers had in fact been designed to withstand the impact of the largest airliner of the day, the Boeing 707-320, in the event one was lost in fog while looking to land. The modeled aircraft weighed 263,000 lb (119 metric tons) with a flight speed of 180 mph (290 km/h), as in approach and landing. This would have been much slower than the actual impacts of 9/11. Robertson also said that they lacked a good understanding of the effects of such large fires on the structures. These considerations were also reported by FEMA's original building performance study.

The National Institute of Standards and Technology, however, was unable to document the study reported by Robertson and FEMA. Instead it found a reference to a study of the effects of a Boeing 707 hitting the buildings at 600 mph, which would be faster than either of the two planes that hit on 9/11. In line with Skilling's remarks, this study apparently found that the buildings would not collapse in that event. But NIST was unable to find any further details about the study and ultimately suggested that any attempt to compare the performance of the buildings to design expectations would be "speculation".

Impacts of airliners

The towers were struck by hijacked Boeing 767 jet planes, American Airlines Flight 11 and United Airlines Flight 175. 1 WTC was hit at 8:46 am by Flight 11 between the 99th and 93rd floors. 2 WTC was hit at 9:02 am by Flight 175 between the 85th and 77th floor.

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 m³) 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 1 WTC, the north tower; flight 175 hit 2 WTC, the south tower, at about 590 mph (950 km/h). In addition to severing a significant number of load-bearing columns, the resulting explosions in each tower ignited 10,000 gallons (c. 40 m³) of jet fuel and immediately spread the fire to several different floors while consuming paper, furniture, carpeting, computers, books, walls, framing and other items in all the affected floors.

The fires

The lightness and hollowness of the towers allowed the jet fuel to penetrate far inside the towers, igniting many large fires simultaneously over a wide area of the impacted floors. The fuel from the planes probably burned out in less than ten minutes, but the contents of the buildings burned over the next hour or hour and a half. It has been suggested that 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 would then not have become unique failure points. In this scenario, the towers might have stood far longer, perhaps indefinitely. The fires were hot enough to significantly weaken the columns and cause the floor trusses to sag.

Collapse of the two towers

The south tower, 2 WTC, was struck at 9:03 am and collapsed about 56 minutes later, at 9:59 am. The north tower, 1 WTC, was struck at 8:46 am and collapsed at 10:28 am, standing for 102 minutes after impact.

In both cases, the commonly accepted assumption is the damaged portion of the building failed, which allowed the section above the airplane impacts to fall onto the remaining building below. It continued in what was essentially free fall. While it took only about 12 seconds to destroy each building, parts of the cores remained standing for about 15 seconds more. Both buildings collapsed symmetrically and more or less straight down, though there was some tilting of the tops of the towers. As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction.

The collapses were accompanied by loud explosions as the structure gave way. They were also visually explosive, spreading debris in a wide radius around the buildings, damaging other buildings nearby and producing enormous clouds of dust that covered Manhattan for days. These were composed mainly of pulverized gypsum cladding and dry wall, finely ground concrete from the towers' floors, glass particles, asbestos, and lead (from the many computers in the buildings).

Total progressive collapse

Once the collapse was initiated, the enormous weight of the portion of the towers above the impact areas overwhelmed the load bearing capacity of the structures beneath them. This was argued in a paper in the days immediately after the attacks by Zdenek P. Bazant and Yong Zhou. Their analysis of global collapse allowed NIST to concentrate their efforts on the events that brought the structure to the point of global collapse, and NIST did not study the progress of the global collapse at all. NIST did propose an explanation for the ejections of dust from the windows, however. As the floors above the impact point were relatively undamaged (save for fire), the upper portion fell and smashed through the lower floors as a unit. The air that was compressed ahead of the falling section was responsible for the ejections of dust and debris through the windows.

History of investigations

Initial reaction

The collapse of the World Trade Center came as a surprise to engineers. Before 9/11, wrote the New Civil Engineer, "it had been genuinely inconceivable that structures of such magnitude could succumb to this fate." While the initial damage from the airplanes was severe, it was localized to a few floors of each tower. The challenge for engineers was to explain how local damage could result in the complete progressive collapse of three of the biggest buildings in the world. Interviewed by the BBC in October 2001, the British architect Bob Halvorson correctly predicted that there would be "a debate about whether or not the World Trade Center Towers should have collapsed in the way that they did." The autopsy would involve careful analysis of the plans of the WTC, its construction, eye witness testimony, video of the collapses, and examination of the wreckage. Emphasizing the difficulty of the task, Halvorson said that the collapses were "well beyond realistic experience."

Authority

Immediately following the collapses, there was some confusion about who had the authority to carry out an official investigation. In contrast to, for example, aircraft accidents, there were no clear procedures in the case of buildings collapses.

A team was quickly assembled by the ASCE, which eventually handed authority over to FEMA. This investigation was criticized by engineers and lawmakers in the US, however, for its limited funding, authority to conduct an investigation, and access to the WTC site. One major point of contention at the time was that the cleanup of the WTC site was resulting in the destruction of the majority of the buildings' steel components. Indeed, when NIST published its final report it noted "the scarcity of physical evidence" that it had had at its disposal to investigate the collapses. Only a fraction of a percent of the buildings remained for analysis after the cleanup was completed: some 236 individual pieces of steel.

FEMA published its report in May of 2002. While NIST had already announced its intention to investigate the collapses in August of the same year, by September 11, 2002, a year after the disaster, there was growing public pressure for a more thorough investigation. and Congress passed the National Construction Safety Team bill in October 2002. This provided the authority for the NIST investigation, which published its results in September of 2005.

Pancaking floors

The first explanation of the disaster came to be known as the "pancake" collapse theory. It was defended especially by Thomas Eagar and popularized by PBS. On this view, when the connections between the floor trusses and the columns broke, the floors fell down, one on top of the other, quickly exceeding the load that any one floor was designed to carry.

While the early efforts had exaggerated the temperatures of the fires, however, Eagar's theory would prove to underestimate the effect of the fires on the structural steel columns. In a paper published in December of 2001, he had focused on the joints between the floor assemblies and the perimeter columns, which, he argued, would be more vulnerable to the effects of the fires. On this assumption he proposed that "the joints on the most severely burned floors gave way, causing the perimeter wall columns to bow outward and the floors above them to fall".

A similar line was taken by Tim Wilkinson, a civil engineer at the University of Sydney. In an "initial suggestion", written already on September 11, he outlined a range of possible effects related mainly to the effects of the fires.

Eventually, the loss of strength and stiffness of the materials resulting from the fire, combined with the initial impact damage, would have caused a failure of the truss system supporting a floor, or the remaining perimeter columns, or even the internal core, or some combination. Failure of the flooring system would have subsequently allowed the perimeter columns to buckle outwards. Regardless of which of these possibilities actually occurred, it would have resulted in the complete collapse of at least one complete storey at the level of impact.

Among a series of self-published accounts by structural engineers, HERA structural engineer Charles Clifton emphasized that a combination of factors led to the collapse. Clifton stated that the two towers collapsed in markedly different ways, which led some to suggest that there were two modes of failure. The north tower collapsed directly downwards, seemingly "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. Others, like Wilkinson, took these differences to be largely superficial. He argued that the "same mechanism of failure, the combination of impact and subsequent fire damage, is the likely cause of failure of both towers" While NIST did conclude that the collapses varied in their details, they proposed essentially the same "probable collapse sequence" for both towers and rejected "the pancake theory".

The role of the fires

Many identified the fires as the key to the collapses. While NIST would eventually confirm this hypothesis, Thomas Eagar, an MIT materials professor, was nonetheless right to describe the fires as "the most misunderstood part of the WTC collapse". This is because the fires were originally said to have "melted" the floors and columns. As Eagar said, "The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel." Jet fuel is essentially kerosene and would have served mainly to ignite very large, but not unusually hot, hydrocarbon fires. Nontheless, in 2003, three engineers at the University of Edinburgh, published a paper in which they provisionally concluded that the fires alone (without any damage from the airplanes) could have been enough to bring down the WTC buildings, while noting that 'A complete consensus on any detailed explanation of the definitive causes and mechanisms of the collapse of these structures is well nigh impossible given the enormous uncertainties in key data.' In this view, the towers were uniquely vulnerable to the effects of large fires on several floors at the same time.

Even after the conclusions of the NIST study were public, at least one of these engineers, Jose Torero, is pursuing further research into the potentially catastrophic effects of fire on steel framed buildings. Moreover, when the NIST report was published, Barbara Lane, with the UK engineering firm Arup, criticized its conclusion that the structural damage resulting from the airplane impacts was a necessary factor in causing the collapses.

Core failure

There had also been some visual evidence that the north tower's core had collapsed first: in videos, the large antenna on top of the core can be seen starting downward a fraction of a second earlier than the rest of the building. NIST, however, disputed 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."

Other attempts

Another early attempt, which included many of the elements already noted, came from MIT civil engineers Oral Buyukozturk and Franz-Josef Ulm on September 21, 2001.

Some 60 tons or more of jet fuel could have easily caused sustained high temperatures of 1,500ºF and higher. Under these conditions, structural steel loses rigidity and strength. The resulting failure of the 2-3 floor system at the site of impact sent the 30 to 25 floors above free-falling onto the 80 to 85 floor structure below. The enormous energy released by this collapse was too large to be absorbed by the structure below. That impact may have ultimately caused the explosive buckling, floor after floor, of the WTC towers. Similar to a car crash in a wall, the towers crashed into the ground with an almost free-fall velocity.

They would later contribute to an MIT collection of papers on the WTC collapses edited by Eduardo Kausel called The Towers Lost and Beyond, published in May 2002.

According to the NOVA documentary on PBS, "Why the Towers Fell", the core column structure of the south tower was momentarily intact after the floors had collapsed.

Leslie E. Robertson, the lead structural engineer on the team that designed the towers, wrote that "The events of September 11 are not well understood by me . . . and perhaps cannot really be understood by anyone." As NIST would also conclude, however, Robertson conjectured that "the fires raging in the inner reaches of the buildings undermined their strength."

The NIST report

Design of the study

Following pressure from technical experts, industry leaders and families of victims, the Commerce Department's National Institute of Standards and Technology conducted a three year $24 million investigation into the structural failure and progressive collapse of several WTC complex structures. The study included in-house technical expertise and drew upon the knowledge of several outside private institutions for aid to include:

• Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE)
• Society of Fire Protection Engineers (SFPE)
• National Fire Protection Association (NFPA)
• American Institute of Steel Construction (AISC)
• Council on Tall Buildings and Urban Habitat (CTBUH)
• Structural Engineers Association of New York (SEAoNY)

Scope and limits

The scope of the NIST investigation was limited to "the sequence of events from the instant of aircraft impact to the initiation of collapse for each tower." In line with the concerns of most engineers, NIST focused on the airplane impacts and the spread and effects of the fires, modeling these at a very high level of detail. NIST developed several highly detailed structural models for specific sub-systems such as the floor trusses as well as a global model of the towers as a whole which is less detailed. These models are static or quasi-static, including deformation but not the motion of structural elements after rupture as would dynamic models. So, the NIST models are useful for determining how the collapse was triggered, but do not shed light on events after that point. As stated in the report, it "includes little analysis of the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable." (p. xxxvii, fn2) Some engineers have suggested that our understanding of the collapse mechanism could be improved by developing an animated sequence of the collapses based on a global dynamic model, and comparing it with the video evidence of the actual collapses.

The publication of the NIST report did not end all disagreements about the collapses. The trade journal, the New Civil Engineer reported that some still believe that "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." In August of 2006, NIST posted a webpage addressing frequently-asked questions.

7 World Trade Center

The WTC complex comprised seven buildings, three of which underwent total progressive collapse on September 11, 2001. At 5:20 pm, 7 WTC, a 47-story steel-frame skyscraper across the street from the rest of the complex, became the third building to collapse.

FEMA's provisional study was inconclusive and the collapse of 7 WTC was not included the final report of the NIST investigation into the collapse of the World Trade Center when it was published in September of 2005. With the exception of a letter to the Journal of Metallurgy, which suggested that some of the structural steel had been exposed to temperatures sufficient to melt it, no studies of the collapse of 7 WTC have been published in scientific journals.

The final report from NIST regarding the collapse of 7 WTC was due in July 2005, but its release date has been postponed. NIST released a progress report in June of 2004 outlining its working hypothesis. On this hypothesis a local failure in a critical column, caused by damage from either fire or falling debris from the collapses of the two towers, progressed first vertically and then horizontally to result in "a disproportionate collapse of the entire structure". In answer to the question of whether "a controlled demolition hypothesis is being considered to explain the collapse", NIST says that it "would like to determine the magnitude of hypothetical blast scenarios that could have led to the structural failure of one or more critical elements." At this point NIST is not committed to any one hypothesis, and new hypotheses may yet emerge.

Remarks by Osama bin Laden

Although its authenticity was questioned, a videotape of Osama bin Laden that was verified by the Pentagon as indicating that Bin Laden, who is a trained Civil Engineer, had not believed that the buildings would collapse completely, but would collapse only above the levels where the planes struck:

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.

Other buildings

The entire WTC complex was destroyed on September 11, 2001, and many of the surrounding buildings were also either damaged or destroyed as the towers fell. 5 WTC suffered a large fire and a partial collapse of its steel structure.

Other buildings destroyed include St. Nicholas Greek Orthodox Church, Marriott World Trade Center (Marriott Hotel 3 WTC), South Plaza (4 WTC), and U.S. Customs (6 WTC). The World Financial Center buildings, 90 West Street, and 130 Cedar Street suffered fires. The Deutsche Bank Building, Verizon, and World Financial Center 3 suffered impact damage from the towers' collapse, as did 90 West Street. One Liberty Plaza survived structurally intact but sustained surface damage including shattered windows. 30 West Broadway was damaged by the collapse of 7 WTC. The Deutsche Bank Building, known through images of it being covered in a large black 'shroud' after September 11 to cover the building's damage, is currently being deconstructed because of water, mold, and other severe damage caused by the neighboring towers' collapse.

Aftermath

Site Cleanup

The cleanup of the WTC site was coordinated by the City of New York Department of Design and Construction (DDC). On September 22, a preliminary cleanup plan was delivered by Controlled Demolition Inc. (CDI) of Phoenix. Mark Loizeaux, president of CDI, emphasised the importance of protecting the slurry wall (or "bathtub") which kept the Hudson river from flooding the WTC's basement.

The cleanup of the WTC site was a massive operation. It involved round-the-clock operations, many contractors and sub-contractors, and cost billions of dollars. By early November, with a third of the debris removed, officials began to reduce the amount of firefighters and police officers assigned to recovering the remains of victims in order to prioritize the removal of debris. This occasioned confrontations with firefighters.

The debris smouldering fires

The colossal pile of debris left on the site burned for more than five months. The smouldering fires resisted intensive attempts by the fire fightering crew to extinguish them until most of the rubble was removed. The effects of the smouldering products on the health of the emergency workers were patent but the details are still a matter of debate.

Air quality and the EPA's response

On September 18, the Environmental Protection Agency issued a statement assuring the public that the air in Manhattan was "safe to breathe". In a report published in 2003, however, the EPA's inspector general found that the agency did not at that time have sufficient data to make such a statement. Also, it found that the White House had influenced the EPA to remove cautionary statements and include assuring ones, in part motivated by the desire to re-open Wall Street. In fact, the collapse of the World Trade Center resulted in serious reductions in air quality and is likely the cause of many respiratory illnesses among first responders, residents and office workers in lower Manhattan.

Dust Cleanup

Concerns remain about the adequacy of the cleanup of the dust that covered Manhattan for days after the collapses, and which settled in offices and apartments. Plans to clean these interior spaces are still being developed.

Controlled demolition conspiracy theory

According to a 2006 poll, 16 percent of polled American adults speculate that the World Trade Center was destroyed by controlled demolition, not by the effects of the airplanes. The hypothesis has been explicitly rejected by both NIST and the engineering community, and is pursued mainly as part of larger conspiracy theories about the events of 9/11.

See also

• Health effects of September 11, 2001 attacks
• 9/11 Truth Movement

References

Cited references

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13. Due to the dust produced by the collapse, the exact times are difficult to determine. See NIST's answers to frequently asked questions, August 2006
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35. Buyukozturk, Oral, Franz-Josef Ulm (2001). "How safe are our skyscrapers?: The World Trade Center collapse". Massachusetts Institute of Technology. Retrieved 2006-06-26.{{cite web}}: CS1 maint: multiple names: authors list (link)
36. Kausel, Eduardo (2002). "The Towers Lost and Beyond". Massachusetts Institute of Technology. Retrieved 2006-06-26.
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42. "Key Findings of NIST's June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster". Fact sheets from NIST. National Institute of Standards and Technology. 2004. Retrieved 2006-07-28.
43. "Interim Report on WTC 7" (pdf). Appendix L. National Institute of Standards and Technology. 2004. Retrieved 2006-07-28.
44. Larson, Mark (2001). "Unwinding the Bin Laden tape". Guardian Unlimited. Retrieved 2006-07-28.
45. "Transcript of Bin Laden videotape". National Public Radio. 2001. Retrieved 2006-07-28.
46. "Bone Fragments Found Near WTC Said Human". Associated Press. 2005. Retrieved 2006-09-11.
47. Post, Nadine M. and Debra K. Rubin. "Debris Mountain Starts to Shrink." Engineering News Record, 10/1/01.
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49. United States Environmental Protection Agency Office of Inspector General. "EPA's Response to the World Trade Center Collapse." Report No. 2003-P-00012. August 21, 2003.
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References

• National Institute of Standards and Technology, Technology Administration (2006). "NIST and the World Trade Center". NIST building and fire safety investigation. US Department of Commerce. Retrieved 2006-05-02.
• Wilkinson, Tim (2006). "World Trade Center - Some Engineering Aspects". School of Civil Engineering, The University of Sydney. Retrieved 2006-05-02.
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• Eagar, Thomas W. (2001). "Why Did the World Trade Center Collapse? Science, Engineering, and Speculation". JOM, 53 (12). The Minerals, Metals & Materials Society. Retrieved 2006-05-02. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Bazant, Zdenek P. (2001). "Why Did the World Trade Center Collapse? - Simple Analysis" (pdf). Journal of Engineering Mechanics ASCE, 9/13/01. Northwestern University. Retrieved 2006-09-11. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Clifton, G. Charles (2001). "Collapse of the World Trade Centre Towers". CAD Digest. TenLinks, Inc. Retrieved 2006-05-02.
• Edgar, Dr. Thomas (2002). "The Collapse: An Engineer's Perspective". Why the Towers Fell. WGBH Educational Foundation. Retrieved 2006-05-02. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Usmani, A.S. (2003). "How did the WTC towers collapse: a new theory" (pdf). Fire Safety Journal, Volume 38, Issue 6. Elsevier Ltd. Retrieved 2006-05-02. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• NOVA online (2002). "The structure of metal". Why the Towers Fell. WGBH Educational Foundation. Retrieved 2006-05-02.
• Kean, Thomas H. (2004). "Eleventh Public Hearing". Hearings. National Commission on Terrorist Attacks Upon the United States. Retrieved 2006-05-02.
• America Rebuilds: A Year at Ground Zero (Television series). United States. 2002. {{cite AV media}}: Unknown parameter |crew= ignored (help); Unknown parameter |distributor= ignored (|publisher= suggested) (help)

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.
• Video showing fire in corner of WTC2
• Forensic animations used in court
• World Trade Center - Some Engineering Aspects University of Sydney engineering teacher Tim Wilkinson writes an article explaining how the towers collasped.

• World Trade Center
• September 11, 2001 attacks
• Building fires in the United States
• Collapsed buildings
• History of New York