Collapse of the World Trade Center

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The collapse of Two World Trade Center seen from Williamsburg, Brooklyn

The collapse of the World Trade Center occurred after the September 11 attacks. Each of the two towers of the World Trade Center (WTC) in New York City was hit by an airliner that had been hijacked by Al Qaeda operatives. The south tower (Two World Trade Center) collapsed less than an hour after being hit at 9:59 a.m., and the north tower (One World Trade Center) followed at 10:28 a.m.

Inside and near the towers, 2,753 people were killed, including all 157 passengers and crew aboard the two airplanes.[1] The collapse of the twin towers also caused extensive damage to the rest of the complex and nearby buildings. At 5:20 p.m. 7 World Trade Center collapsed as well as a result of damage and fires which had occurred earlier in the day when the north tower collapsed.[2] Debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures.

The Federal Emergency Management Agency (FEMA) completed its performance study of the buildings in May 2002. It declared that the WTC design had been sound and attributed the collapses wholly to extraordinary factors beyond the control of the builders. While calling for further study, FEMA suggested that the collapses were probably initiated by weakening of the floor joists by the fires that resulted from the aircraft impacts. According to FEMA's report – and subsequently contradicted by NIST's findings – the floors detached from the main structure of the building and fell onto each other, initiating a progressive "pancake" collapse.[3]

FEMA's early investigation was revised by a later, more detailed investigation conducted by the National Institute of Standards and Technology (NIST), which also consulted outside engineering entities. This investigation was completed in September 2005. Like FEMA, NIST vindicated the design of the WTC, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. NIST also emphasized the role of the fires, but it did not attribute the collapses to failing floor joists. Instead, NIST found that sagging floors pulled inward on the perimeter columns: "This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers."[4]

The cleanup of the site involved round-the-clock operations, many contractors and subcontractors, and cost hundreds of millions of dollars. The demolition of the surrounding damaged buildings continued even as new construction proceeded on the World Trade Center's replacement, the 1 World Trade Center (Freedom Tower). Of the destroyed buildings, only 7 World Trade Center has been replaced as of 2008.

Contents

Structural design

Main article: Construction of the World Trade Center

Architect Minoru Yamasaki designed the towers as framed tube structures, which provided tenants with open floor plans, uninterrupted by columns or walls. This was accomplished using numerous, closely-spaced perimeter columns to provide much of the strength to the structure, along with gravity load shared 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.[5] All of the elevators and stairwells were located in the core, leaving a large column-free space between the perimeter that was bridged by prefabricated floor trusses.[6]

The floors consisted of 4 inches (10 cm) thick lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors. The trusses had a span of 60 feet (18 m) in the long-span areas and 35 feet (11 m) in the short span area.[6] The trusses connected to the perimeter at alternate columns, and were therefore on 6.8 feet (2.1 m) centers. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to the core columns on the interior side. The floors were connected to the perimeter spandrel plates with viscoelastic dampers, which helped reduce the amount of sway felt by building occupants. The trusses supported a 4 inches (10 cm) lightweight concrete floor slab, with shear connections for composite action.[6]

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.[6]

Safety concerns regarding aircraft impacts

The structural engineers working on the World Trade Center considered the possibility that an aircraft could crash into the building. In July 1945, a B-25 bomber that was lost in the fog had crashed into the 79th floor of the Empire State Building. A year later, another airplane nearly crashed into the 40 Wall Street building, and there was another near-miss at the Empire State Building.[7] During the design of the World Trade Center, Leslie Robertson, one of the chief engineers, considered the scenario of the impact of a jet airliner—a Boeing 707—which might be lost in the fog and flying at relatively low speeds, seeking to land at JFK or Newark Airport.[8][7]

NIST found a three page white paper that mentioned another aircraft impact analysis, involving impact of a jet at 600 miles per hour (970 km/h), but the original documentation of the study was lost when Port Authority offices were destroyed in the collapse of the World Trade Center.[9] In 1993, John Skilling recalled doing the analysis, and remarked, "The building structure would still be there."[10] However, he may have put little thought to how the structure would behave in an intense fire that would result from an aircraft impact, and simply assumed that the World Trade Center's lightweight trusses and columns would perform as well as the heavy masonry and steel structure in the Empire State Building.[11] In its investigation, NIST also found reason to believe that they lacked the ability to properly model the effect of such impacts on the structures, especially the effects of the fires.[12][note 1]

Fireproofing

Fireproofing was incorporated in the original construction and more was added after a fire in 1975 that spread to six floors before being extinguished. After the 1993 bombing, inspections found fireproofing to be deficient. The Port Authority was in the process of replacing it, but replacement had been completed on only 18 floors in 1 WTC, including all the floors affected by the aircraft impact and fires,[13] and on 13 floors in 2 WTC, although only three of these floors (77, 78, and 85) were directly affected by the aircraft impact.[14] Although replacement fireproofing was specified at 1.5 inches (3.8 cm) in thickness, NIST found the average thickness to be 2.5 inches (6.4 cm).[15] NIST concluded that "the existing condition of the fireproofing prior to aircraft impact and the fireproofing thickness on the WTC floor system did not play a significant role".

September 11, 2001

Impact locations on 1 and 2 WTC

Aircraft impact

On September 11, 2001, hijackers associated with al-Qaeda took control of two early morning Los Angeles-bound flights—both Boeing 767 jetliners—soon after take off from Boston's Logan International Airport. In its final moments, American Airlines Flight 11 flew south over Manhattan and crashed at roughly 440 miles per hour (710 km/h) into the northern facade of the World Trade Center's North Tower at 8:46 a.m., impacting between the 93rd and 99th floors. Seventeen minutes later, United Airlines Flight 175 approached from the southwest, over New York Harbor, and crashed into the South Tower's southern facade between the 77th and 85th floors at 540 miles per hour (870 km/h).[6] In addition to severing numerous load-bearing columns on the perimeter and inflicting other structural damage, the resulting explosions in each tower ignited 10,000 US gallons (38,000 l) of jet fuel along with office contents.[3] The force of the explosion from the initial impact in 1 WTC traveled through at least one express elevator shaft all the way down to the lobby floor, blowing out all of the windows and leaving a number of people injured.

Fires

The light construction and hollow nature of the structures 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 burned at most for a few minutes, but the contents of the buildings burned over the next hour or hour and a half.[16] It has been suggested that the fires might 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 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.[17][18] The fires were hot enough to weaken the columns and cause floors to sag, pulling perimeter columns inward and reducing their ability to support the mass of the building above.[19]

In the North Tower, jet fuel ran down at least two elevator shafts to the basement, and two or more elevators plummeted to the lower levels. Fire continued to burn in the shafts, which may have helped weaken the core.

Collapse of the South Tower

As the fires continued to burn, occupants trapped in the upper floors of the South Tower provided information about conditions via 9-1-1. At 9:37 a.m., an occupant on the 105th floor of the South Tower reported that floors beneath him "in the 90-something floor" had collapsed.[20] The aviation unit also relayed information about deteriorating conditions of the buildings to police commanders, who issued orders for its personnel to evacuate the towers.[21] At 9:52 a.m., the NYPD aviation unit reported over the radio that "large pieces may be falling from the top of WTC 2. Large pieces are hanging up there".[20] With the warnings, the NYPD issued orders for its personnel to evacuate. During the emergency response, there was minimal communication between the NYPD and the FDNY, and overwhelmed 9-1-1 dispatchers did not pass along information to FDNY commanders on-scene. At 9:59 a.m., the south tower collapsed, 56 minutes after being struck. Only 14 people escaped from the impact zone of the South Tower after it was hit, and only four people from the floors above it. They escaped via Stairwell A, the only stairwell which had been left intact after the impact.

Collapse of the North Tower

After the South Tower collapsed, NYPD helicopters relayed information about the deteriorating conditions of the North Tower. At 10:20 a.m., the NYPD aviation unit reported that "the top of the tower might be leaning," and a minute later reported that the North Tower, "is buckling on the southwest corner and leaning to the south". At 10:27 a.m., the aviation unit reports that "the roof is going to come down very shortly."[20] The north tower collapsed at 10:28 a.m., after burning for 102 minutes.

After the South Tower collapsed, FDNY commanders issued orders for firefighters in the North Tower to evacuate. Due to radio communications problems, firefighters inside the towers did not hear the evacuation order from their supervisors on the scene, and most were unaware that the other tower had collapsed.[22] 343 firefighters died in the Twin Towers, as a result of the collapse of the buildings.[23][21][24] No one was able to escape from above the impact zone in the North Tower after it was hit, as all stairwells and elevator shafts on those floors were destroyed.

Collapse of 7 World Trade Center

The WTC complex comprised seven buildings, three of which completely collapsed on the day of the attacks. At 5:20 p.m., 7 World Trade Center, a 47-story steel-frame skyscraper across the street from the rest of the complex, became the third building to collapse. Unlike the Twin Towers, the collapse of 7 WTC had been anticipated for several hours and the building had been evacuated. A transit was used to measure the extent of a visible bulge.[25]

Initial opinions and analysis

Impact locations for 1 and 2 WTC

In the immediate aftermath of the attacks, numerous structural engineers and experts spoke to the media, describing what they thought caused the towers to collapse. Hassan Astaneh, a structural engineering professor at the University of California at Berkeley, explained that the high temperatures in the fires weakened the steel beams and columns, causing them to become "soft and mushy", and eventually they were unable to support the structure above. Astaneh also suggested that the fireproofing became dislodged during the initial aircraft impacts. He also explained that, once the initial structural failure occurred, progressive collapse of the entire structure was inevitable.[26] Cesar Pelli, who designed the Petronas Towers in Malaysia and the World Financial Center in New York, remarked, "no building is prepared for this kind of stress".[27]

On September 13, 2001, Zdeněk Bažant, a professor of civil engineering and materials science at Northwestern University, published a draft paper with results of a simple analysis of the World Trade Center collapse. Bažant suggested that heat from the fires was a key factor, causing steel columns in both the core and the perimeter to weaken and experience deformation, before losing their carrying capacity and buckling. Then, once more than half of columns on a particular floor buckled, the above structure could no longer be supported and complete collapse of the structures occurred. Bažant's paper was later expanded and published in the Journal of Engineering Mechanics, in January 2002.[28] Other analyses were conducted by MIT civil engineers Oral Buyukozturk and Franz-Josef Ulm, who also described a collapse mechanism on September 21, 2001.[29] 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.[30]

Investigations

Immediately following the collapses, there was some confusion about who had the authority to carry out an official investigation. While there are clear procedures for the investigation of aircraft accidents, no agency had been appointed in advance to investigate building collapses.[31] A team was quickly assembled by the Structural Engineers Institute of the American Society of Civil Engineers. It also involved the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association, and the Society of Fire Protection Engineers.[32] ASCE ultimately invited FEMA to join the investigation, which was completed under the auspices of the latter.[32]

The investigation was criticized by some engineers and lawmakers in the U.S. It had little funding, no authority to demand evidence, and limited 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.[33] 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, although 95% of structural beams and plates and 50% of the reinforcement bars were recovered.[34]

FEMA published its report in May 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.[35] Congress passed the National Construction Safety Team bill in October 2002, giving NIST the authority to conduct an investigation of the World Trade Center collapses.[36]

FEMA Building Performance Study

FEMA developed an early explanation of the collapses, which had come to be known as the "pancake" theory. It was defended by Thomas Eagar and popularized by PBS.[37] According to this explanation, 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.[38] A number of self-published accounts by structural engineers suggested that a combination of factors led to the collapse, but most suggested a version of pancake collapse.[39][18]

As in the theory which is currently accepted, the fires were taken to be the key to the collapses. Thomas Eagar, an MIT materials professor, had described the fires as "the most misunderstood part of the WTC collapse".[38] 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. This led Eagar, FEMA and others to focus on what appeared to be the weakest point of the structures, namely, the points at which the floors were attached to the building frame. Once these connections failed, the pancake collapse could initiate.[40][41] The NIST report, however, would ultimately vindicate the floor connections; indeed, the collapse mechanism depends on the strength of these connections as the floors pulled the outer walls in.

NIST report

The outer shell of the south tower (tower 2) of the WTC is still standing at right. The 22 story Marriott Hotel in the foreground was crushed when the adjacent tower collapsed.

After the FEMA report had been published, and 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.[42] The study included in-house technical expertise, along with assistance from several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers, Society of Fire Protection Engineers, National Fire Protection Association, American Institute of Steel Construction, Simpson Gumpertz & Heger Inc., Council on Tall Buildings and Urban Habitat, and the Structural Engineers Association of New York.

The scope of the NIST investigation was focused on identifying "the sequence of events" that triggered the collapse, and did not include detailed analysis of the collapse mechanism itself (after the point at which events made the collapse inevitable).[43][44] In line with the concerns of most engineers, NIST focused on the airplane impacts and the spread and effects of the fires, modeling these using the computer code FDS. 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.

James Quintiere, professor of fire protection engineering at the University of Maryland, called the spoliation of the steel "a gross error" that NIST should have openly criticized.[45] He also noted that the report lacked a timeline and physical evidence to support its conclusions.[46] Some engineers have suggested that 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. In October 2005, the New Civil Engineer reported criticism of NIST's computer modeling. Colin Bailey at the University of Manchester and Rober Plank at the University of Sheffield called on NIST to produce computer visualizations of the collapses in order to correlate the collapse models with observed events.[47]

7 World Trade Center

7 World Trade Center on fire after the collapse of the Twin Towers on 9/11
Main article: 7 World Trade Center

FEMA's provisional study was inconclusive regarding the collapse of 7 World Trade Center.[48] The investigation of the collapse of 7 WTC was done separately, and subsequent to the investigation of the Twin Towers' collapse. In June 2004, NIST released a progress report outlining its working hypothesis for 7 WTC, which was that 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".[49][50]

On August 21, 2008, NIST released a draft of its final report on the collapse of 7 World Trade Center. In the report, NIST explains that fire was the main reason for the collapse, along with lack of water to fight the fire. The fire continued to burn throughout the afternoon on the lower floors, and at 5:20 pm. a critical column on the 13th floor buckled and triggered a progressive collapse of the entire structure.[51]

Other investigations

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. In their view, the towers were uniquely vulnerable to the effects of large fires on several floors at the same time.[52] 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.[53] Jose L Torero from the BRE Centre for Fire Safety Engineering at the University of Edinburgh is pursuing further research into the potentially catastrophic effects of fire on real-scale buildings.[54][55][56]

Collapse mechanism

Ground Zero debris with markup showing building locations
Portions of the outer shells of the South Tower at right and the North Tower at center left, as well as damage to all the other buildings at the WTC site are shown

In both cases, the commonly accepted process is that the damaged portion of the buildings failed, which allowed the section above the airplane impacts to fall onto the remaining structure below. Both buildings collapsed symmetrically and more or less straight down, though there was some tilting of the tops of the towers and a significant amount of fallout to the sides. As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction.

Owing to differences in the initial impacts, the collapses of the two towers were found to differ in some respects, but in both cases, the same sequence of events applies. After the impacts had severed exterior columns and damaged core columns, the loads on these columns were redistributed. The hat trusses at the top of each building played a significant role in this redistribution of the loads in the structure.

The impacts also dislodged some of the fireproofing from the steel, increasing its exposure to the heat of the fires. In the 102 minutes before the collapse of 1 WTC, the fires reached temperatures that – although well below the melting point – were high enough to weaken the core columns so that they underwent plastic deformation and creep from the weight of higher floors. The NIST report provides a model of the situation.

“ At this point, the core of WTC 1 could be imagined to be in three sections. There was a bottom section below the impact floors that could be thought of as a strong, rigid box, structurally undamaged and at almost normal temperature. There was a top section above the impact and fire floors that was also a heavy, rigid box. In the middle was the third section, damaged by the aircraft and weakened by heat from the fires. The core of the top section tried to move downward, but was held up by the hat truss. The hat truss, in turn redistributed the load to the perimeter columns. (p. 29) ”

The situation was similar in 2 WTC. In both towers, perimeter columns and floors were also weakened by the heat of the fires, causing the floors to sag and exerting an inward force on exterior walls of the building.

At 9:59 a.m., 56 minutes after impact, the sagging floors finally caused the eastern face of 2 WTC to buckle, transferring its loads back to the failing core through the hat truss and initiating the collapse; the section above the impact area then tilted in the direction of the failed wall. At 10:28 a.m., 102 minutes after the impact, the south wall of 1 WTC buckled, with similar consequences. After collapse ensued, the total collapse of the towers was inevitable due to the enormous weight of the towers above the impact areas.

A combination of three factors 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 plane was lower (so there was less impact damage), and the affected floors had received partially upgraded fire proofing.

Total progressive collapse

Analysis of video footage capturing the initial collapse and analysis of seismic data from Palisades, New York shows that the first fragments of the outer walls of the collapsed north tower struck the ground 9 seconds after the collapse started, and parts of the south tower after 11 seconds. The lower portions of both buildings cores (60 stories of WTC 1 and 40 stories of WTC 2) remained standing for up to 25 seconds after the start of the initial collapse before they too collapsed. These times are approximate because dust obscured the view.[12][57]

An early analysis explains that the kinetic energy of the upper portion of the building falling onto the story below exceeded by an order of magnitude the amount of energy that the lower story could absorb,[28] crushing it and adding to the kinetic energy. This scenario repeated with each successive story, crushing the entire tower at near free-fall speed.[58]

Aftermath

Other buildings

Portions of the outer shell of the North Tower lean against the remains of 6 WTC which suffered massive damage when the North Tower collapsed. The remains of 7 WTC are at upper right

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, which was 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.[59]

Cleanup

September 15, 2001

The cleanup was a massive operation coordinated by the City of New York Department of Design and Construction. On September 22, a preliminary cleanup plan was delivered by Controlled Demolition, Inc. (CDI) of Phoenix, Maryland. Mark Loizeaux, president of CDI, emphasized the importance of protecting the slurry wall (or "the bathtub") which kept the Hudson River from flooding the WTC's basement.[60] It involved round-the-clock operations, many contractors and subcontractors, and cost hundreds of millions of dollars.[61] The large pile of debris left on the site burned for three months, resisting attempts to extinguish the blaze until the majority of the rubble was finally removed from the site.[62][63] By early November, with a third of the debris removed, officials began to reduce the number of firefighters and police officers assigned to recovering the remains of victims, in order to prioritize the removal of debris. This caused confrontations with firefighters.[64] In 2007, the demolition of the surrounding damaged buildings was still ongoing as new construction proceeded on the World Trade Center's replacement, the Freedom Tower.

Health effects

Main articles: Health effects arising from the September 11 attacks and EPA 9/11 pollution controversy

The collapse of the World Trade Center produced enormous clouds of dust that covered Manhattan for days. On September 18, 2001, the Environmental Protection Agency issued a statement assuring the public that the air in Manhattan was "safe to breathe".[65] 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 reopen 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.[66]

Controlled demolition conspiracy theories

Main articles: World Trade Center controlled demolition conspiracy theories and 9/11 conspiracy theories

According to a 2006 poll, 16 percent of American adults believed that the World Trade Center may have been destroyed by controlled demolition rather than resulting from the plane impacts.[67] This idea has been rejected by NIST, which concluded that there were no explosives or controlled demolition involved in the collapses of the WTC towers.[12] Controlled demolition is also dismissed in the engineering literature[58] and is pursued mainly as part of larger conspiracy theories about the events of 9/11.[68]

Notes

  1. ^ The three-page white paper titled Salient points with regard to the structural design of The World Trade Center towers described an analysis of a Boeing 707 weighing 336,000 lb (152 metric tons) and carrying 23,000 US gallons (87 m³) of fuel impacting the 80th floor of the buildings at 600 miles per hour (970 km/h). It is unclear whether the effect of jet fuel and aircraft contents was a consideration in the original building design but this study is in line with remarks made by John Skilling following the 1993 WTC bombing. Without original documentation for either study, NIST said any further comments would amount to speculation.

References

  1. ^ "Relatives gather at ground zero to mark 9/11". The Associated Press / MSNBC. 2007-09-09. http://www.msnbc.msn.com/id/20713270/. Retrieved on 2007-11-03. 
  2. ^ "PartIIC - WTC 7 Collapse" (pdf). NIST Response to the World Trade Center Disaster. National Institute of Standards and Technology. 2005-04-05. http://wtc.nist.gov/pubs/WTC%20Part%20IIC%20-%20WTC%207%20Collapse%20Final.pdf. Retrieved on 2006-11-01. 
  3. ^ a b Hamburger, Ronald, et al. "World Trade Center Building Performance Study" (pdf). Federal Emergency Management Agency. http://www.fema.gov/pdf/library/fema403_ch2.pdf. Retrieved on 2006-07-27. 
  4. ^ "NIST Response to the World Trade Center Disaster" (PDF). http://wtc.nist.gov/WTC%20Response%20Presentation%2011122002%20text.pdf. Retrieved on 2008-04-23. 
  5. ^ National Construction Safety Team (September 2005). "Chapter 1" (PDF). Final Report on the Collapse of the World Trade Center Towers. NIST. pp. 6. http://wtc.nist.gov/NISTNCSTAR1CollapseofTowers.pdf. 
  6. ^ a b c d e "Final Report on the Collapse of the World Trade Center (chapter 1)" (PDF). National Institute of Standards and Technology (NIST). September 2005. http://wtc.nist.gov/NISTNCSTAR1CollapseofTowers.pdf. 
  7. ^ a b Glanz, James and Eric Lipton (2002-09-08). "The Height of Ambition". The New York Times. http://query.nytimes.com/gst/fullpage.html?res=9F02E2DD1F3FF93BA3575AC0A9649C8B63&sec=&spon=&pagewanted=10. 
  8. ^ Robertson, Leslie E. (2002). "Reflections on the World Trade Center". The Bridge Volume 32, Number 1. National Academy of Engineering. http://www.nae.edu/nae/bridgecom.nsf/weblinks/CGOZ-58NLCB?OpenDocument. Retrieved on 2006-07-28. 
  9. ^ Sadek, Fahim. Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center Towers(NCSTAR 1-2 appendix A). NIST 2005. pp. 305-307.
  10. ^ Nalder, Eric (1993-02-27). "Twin Towers Engineered to Withstand Jet Collision". The Seattle Times. http://archives.seattletimes.nwsource.com/cgi-bin/texis.cgi/web/vortex/display?slug=1687698&date=19930227. 
  11. ^ Glanz, James and Eric Lipton (2003). City in the Sky: The Rise and Fall of the World Trade Center. Times Books. pp. 138–139. 
  12. ^ a b c "Answers to Frequently Asked Questions". NIST & The World Trade Center. National Institute of Standards and Technology. http://wtc.nist.gov/pubs/factsheets/faqs_8_2006.htm. Retrieved on 2006-09-17. 
  13. ^ NCSTAR 1-6, p lxxi
  14. ^ NCSTAR 1-6, p lxvii-lxix
  15. ^ NCSTAR 1-6A, p xl
  16. ^ Field, Andy (2004). "A Look Inside a Radical New Theory of the WTC Collapse". Fire/Rescue News. http://cms.firehouse.com/content/article/article.jsp?sectionId=46&id=25807. Retrieved on 2006-07-28. 
  17. ^ Gross, John L., Therese P. McAllister (2004). "Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers" (pdf). Federal Building and Fire Safety Investigation of the World Trade Center Disaster NIST NCSTAR 1-6. National Institute of Standards and Technology. http://wtc.nist.gov/NISTNCSTAR1-6.pdf. Retrieved on 2006-07-28. 
  18. ^ a b Wilkinson, Tim (2006). "World Trade Center - Some Engineering Aspects". http://www.civil.usyd.edu.au/wtc.shtml. Retrieved on 2006-07-28. 
  19. ^ National Construction Safety Team (September 2005). "Executive Summary" (PDF). Final Report on the Collapse of the World Trade Center Towers. NIST. http://wtc.nist.gov/NISTNCSTAR1CollapseofTowers.pdf. 
  20. ^ a b c Lawson, J. Randall, Robert L. Vettori (September 2005). "NIST NCSTAR 1-8 - The Emergency Response" (PDF). National Institute of Standards and Technology. p. 37. http://wtc.nist.gov/NISTNCSTAR1-8.pdf. 
  21. ^ a b "McKinsey Report - NYPD". August 19, 2002. http://www.nyc.gov/html/fdny/html/mck_report/toc.html. Retrieved on 2007-07-10. 
  22. ^ Dwyer, Jim, Kevin Flynn and Ford Fessenden (2002-07-07). "9/11 Exposed Deadly Flaws In Rescue Plan". The New York Times. http://query.nytimes.com/gst/fullpage.html?res=9E05E7DA1E31F934A35754C0A9649C8B63&sec=&spon=&pagewanted=all. 
  23. ^ "McKinsey Report - Emergency Medical Service response" (PDF). FDNY / McKinsey & Company. 2002-08-09. http://www.nyc.gov/html/fdny/pdf/mck_report/ems_response.pdf. Retrieved on 2007-07-12. 
  24. ^ "NY Firefighters attack Giuliani," BBC News, July 12, 2007 http://news.bbc.co.uk/2/hi/americas/6294198.stm
  25. ^ Hayden, Peter (April 2002). "WTC: This Is Their Story". Firehouse Magazine. http://www.firehouse.com/terrorist/911/magazine/gz/hayden.html. 
  26. ^ Perlman, David (2001-09-12). "Jets hit towers in most vulnerable spots". San Francisco Chronicle. http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2001/09/12/MN101866.DTL&hw=STRUCTURAL&sn=025&sc=505. 
  27. ^ Gugliotta, Guy (2001-09-12). "'Magnitude Beyond Anything We'd Seen Before'; Towers Built to Last But Unprepared For Such an Attack". The Washington Post. http://www.washingtonpost.com/wp-dyn/content/article/2001/09/12/AR2005033107985.html. 
  28. ^ a b Bažant, Zdeněk P.; Yong Zhou (2002-01-01). "Why Did the World Trade Center Collapse?—Simple Analysis". J. Engrg. Mech. 128 (1): pp. 2–6. doi:0.1061/(ASCE)0733-9399(2002)128:1(2). http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/405.pdf. Retrieved on 2007-08-23. 
  29. ^ Buyukozturk, Oral, Franz-Josef Ulm (2001). "How safe are our skyscrapers?: The World Trade Center collapse". Massachusetts Institute of Technology. http://web.mit.edu/newsoffice/2001/skyscrapers.html. Retrieved on 2006-06-26. 
  30. ^ Kausel, Eduardo (2002). "The Towers Lost and Beyond". Massachusetts Institute of Technology. http://web.mit.edu/civenv/wtc/. Retrieved on 2006-06-26. 
  31. ^ Snell, Jack. "The Proposed National Construction Safety Team Act." NIST Building and Fire Research Laboratory. 2002.[1]
  32. ^ a b "Experts Debate Future of the Skyscraper in Wake of Disaster". Engineering News-Record. 2001-09-24. 
  33. ^ Glanz, James and Eric Lipton. "Nation Challenged: The Towers; Experts Urging Broader Inquiry In Towers' Fall". New York Times December 25, 2001
  34. ^ Sylvie Boulanger and Sylvain Boulanger "Steel and sustainability 2: Recovery strategies" Canadian Institute of Construction. March 17, 2004.[2]
  35. ^ Dwyer, Jim. "Investigating 9/11: An Unimaginable Calamity, Still Largely Unexamined". New York Times. September 11, 2002 [3]
  36. ^ "NIST's Responsibilities Under the National Construction Safety Team Act". http://www.nist.gov/public_affairs/factsheet/constructionact.htm. Retrieved on 2008-04-23. 
  37. ^ Eagar, Thomas (2002). "The Collapse: An Engineer's Perspective". NOVA. http://www.pbs.org/wgbh/nova/wtc/collapse.html. Retrieved on 2006-07-28. 
  38. ^ a b Eagar, Thomas W.; Christopher Musso (2001). Why Did the World Trade Center Collapse? Science, Engineering, and Speculation. JOM, 53 (12). The Minerals, Metals & Materials Society. Retrieved on 2006-05-02.
  39. ^ Clifton, G. Charles (2002). "Collapse of the World Trade Centre Towers" (pdf). http://www.hera.org.nz/PDF%20Files/World%20Trade%20Centre.pdf#search='collapse%20of%20the%20world%20trade%20center'. Retrieved on 2006-07-28. 
  40. ^ "How the World Trade Center fell". BBC News. 2001. http://news.bbc.co.uk/1/hi/world/americas/1540044.stm. Retrieved on 2006-07-28. 
  41. ^ "Twin towers' steel under scrutiny". BBC News. 2001. http://news.bbc.co.uk/1/hi/england/1604348.stm. Retrieved on 2006-07-28. 
  42. ^ Newman, Michael E. (2002). "Commerce's NIST Details Federal Investigation of World Trade Center Collapse". National Institute of Standards and Technology. http://www.nist.gov/public_affairs/releases/n02-14.htm. Retrieved on 2006-07-28. 
  43. ^ Bazant (2007), p. 2
  44. ^ NIST final report (2005). NCSTAR 1, p. xxxvii.
  45. ^ Committee on Science (2005-10-26). "THE INVESTIGATION OF THE WORLD TRADE CENTER COLLAPSE: FINDINGS, RECOMMENDATIONS, AND NEXT STEPS". commdocs.house.gov. 259. http://commdocs.house.gov/committees/science/hsy24133.000/hsy24133_0.HTM#259. Retrieved on 2007-04-01. 
  46. ^ Quintiere, James (December 2004). ""2004 REPORT TO CONGRESS OF THE NATIONAL CONSTRUCTION SAFETY TEAM ADVISORY COMMITTEE"" (PDF). NIST. 8. http://www.nist.gov/public_affairs/ncst/Final%20NCSTAC%202004%20Report%20to%20Congress.pdf. 
  47. ^ Parker, Dave. "WTC investigators resist call for collapse visualisation." New Civil Engineer October 6, 2005.
  48. ^ "Observations, findings and Recommendations" (pdf). World Trade Center Building Performance Study, (Chapter 8.2.5.1). Federal Emergency Management Agency. http://www.fema.gov/pdf/library/fema403_ch8.pdf. Retrieved on 2006-07-28. 
  49. ^ "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. http://www.nist.gov/public_affairs/factsheet/wtc_keyfindings.htm. Retrieved on 2006-07-28. 
  50. ^ "Interim Report on WTC 7" (pdf). Appendix L. National Institute of Standards and Technology. 2004. http://wtc.nist.gov/progress_report_june04/appendixl.pdf. Retrieved on 2006-07-28. 
  51. ^ "Final Report on the Collapse of World Trade Center Building 7 - Draft for Public Comment" (PDF). NIST. August 2008. xxxii. http://wtc.nist.gov/media/NIST_NCSTAR_1A_for_public_comment.pdf. 
  52. ^ Usmani, A.S., Y.C. Chung, J.L. Torero (2003). "How did the World Trade Center Collapse: A New Theory" (pdf). Fire Safety Journal, 38, 6. http://hdl.handle.net/1842/1216. Retrieved on 2007-05-07. 
  53. ^ "Row erupts over why twin towers collapsed". New Civil Engineer. 2005. http://cruachan.televisual.co.uk/asset/GetArticle.exe?DB=e2&DATABASE=e2&LABEL=emap2&RECORD=194310&SEARCH=1. Retrieved on 2006-07-28. 
  54. ^ BRE Centre for Fire Safety Engineering, University of Edinburgh (2006). "Dalmarnock Full-Scale Experiments 25 & 26 July 2006". BRE Centre for Fire Safety Engineering, University of Edinburgh. http://www.civ.ed.ac.uk/research/fire/dalmarnock.html. Retrieved on 2008-04-23. 
  55. ^ Christian, Nicholas (2006). "Glasgow tower block to shed light on 9/11 fire". Scotsman. http://news.scotsman.com/scitech.cfm?id=1068572006. Retrieved on 2006-07-28. 
  56. ^ "Skyscraper Fire Fighters". BBC Horizon. 2007. http://www.bbc.co.uk/sn/tvradio/programmes/horizon/broadband/tx/firegrid. Retrieved on 2007-07-31. 
  57. ^ Bažant, Zdeněk P.; Jia-Liang Le, Frank R. Greening and David B. Benson (2007-05-27) (PDF). Collapse of World Trade Center Towers: What Did and Did Not Cause It?. 2007-06-22. Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois 60208, USA. Structural Engineering Report No. 07-05/C605c (page 12). http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/00%20WTC%20Collapse%20-%20What%20did%20%26%20Did%20Not%20Cause%20It%20-%20Revised%206-22-07.pdf. Retrieved on 2007-09-17. 
  58. ^ a b Bažant, Zdeněk P.; Mathieu Verdure (March 2007). "Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions" (PDF). J. Engrg. Mech. 133 (3): pp. 308–319. doi:10.1061/(ASCE)0733-9399(2007)133:3(308). http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/466.pdf. Retrieved on 2007-08-22. 
  59. ^ "Bone Fragments Found Near WTC Said Human". Associated Press. 2005. http://wcbstv.com/topstories/topstories_story_302000152.html. Retrieved on 2006-09-11. 
  60. ^ Post, Nadine M. and Debra K. Rubin. "Debris Mountain Starts to Shrink." Engineering News Record, 10/1/01. [4]
  61. ^ Kugler, Sara (2006-10-23). "Officials Wanted More Searching at WTC". Washington Post. http://www.washingtonpost.com/wp-dyn/content/article/2006/10/23/AR2006102300062_pf.html. Retrieved on 2007-10-29. 
  62. ^ http://english.people.com.cn/200112/20/eng20011220_87119.shtml
  63. ^ CBS News | WTC Fires All But Defeated | December 19, 2001 23:22:25
  64. ^ Rubin, Debra K. and Janice L. Tuchman. "WTC Agency Begins Ramping Up Operations." Engineering News Record, 11/01/01. [5]
  65. ^ "EPA Response to September 11". http://www.epa.gov/wtc/stories/headline_091801.htm. Retrieved on 2007-08-27. 
  66. ^ 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.[6]
  67. ^ Hargrove, Thomas and Guido H. Stempel III.More than 40 percent of the New York City and NY State population indicate they believe the WT Centers were destroyed by controlled demolition in turn indicating government complicity according to several Zogby polls. "Anti-government anger spurs 9/11 conspiracy belief", Scripps Howard News Service, August 2, 2006.[7]
  68. ^ James B., Meigs (2006). "The Conspiracy Industry". Popular Mechanics. Hearst Communications, Inc.. http://www.popularmechanics.com/science/research/4199607.html. Retrieved on 2006-12-11. 

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