Collapse of the World Trade Center
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[[Image:World Trade Center Site After 9-11 Attacks With Original Building Locations.jpg|thumb|right|Ground Zero debris with markup showing building locations.]]
{{sep11}}
The '''collapse of the World Trade Center''' was the central event of the [[September 11, 2001 attacks]], during which each of the [[World Trade Center|twin towers]] was hit by a hijacked airliner. The south tower (2 WTC) collapsed at 9:59 a.m., less than an hour after being hit. The north tower (1 WTC) followed at 10:28 a.m.
2,751 people inside and near the towers were killed, including all 157 passengers and crew aboard the two airplanes.<ref>{{cite news |url= http://www.msnbc.msn.com/id/20713270/ |title=Relatives gather at ground zero to mark 9/11 |date=2007-09-09 |publisher=The Associated Press / MSNBC |accessdate= 2007-11-03}}</ref> The collapse of the twin towers also caused extensive damage to the rest of the complex and nearby buildings. As a result, at 5:20 p.m. [[7 World Trade Center]] collapsed as well.<ref name="NIST2">{{cite web | last = | first = | title = PartIIC - WTC 7 Collapse | work = NIST Response to the World Trade Center Disaster| publisher = National Institute of Standards and Technology |date=2005-04-05 | url = http://wtc.nist.gov/pubs/WTC%20Part%20IIC%20-%20WTC%207%20Collapse%20Final.pdf | format = pdf | accessdate = 2006-11-01 }}</ref>
The [[Federal Emergency Management Agency]] (FEMA) completed its performance study of the buildings in May 2002. It declared the WTC design 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 [[joist]]s 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. <ref name="FEMA">{{cite web | last = Hamburger | first = Ronald, et al | title = World Trade Center Building Performance Study | work = | publisher = Federal Emergency Management Agency| date = | url = http://www.fema.gov/pdf/library/fema403_ch2.pdf | format = pdf|accessdate = 2006-07-27 }}</ref>
FEMA's proposed explanation was rejected by a later, more detailed investigation by the [[National Institute of Standards and Technology]] (NIST), which 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 [[United States|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."<ref name="NISTFAQ">
{{cite web
| url = http://wtc.nist.gov/WTC%20Response%20Presentation%2011122002%20text.pdf
| title = NIST Response to the World Trade Center Disaster
| accessdate = 2008-04-23 }}</ref>
The cleanup of the site involved round-the-clock operations, many contractors and sub-contractors, 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 [[Freedom Tower]]. Of the destroyed buildings, only [[7 World Trade Center]] has been replaced [[as of 2008]].
==Design==
{{main|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.<ref>{{cite book |author=National Construction Safety Team |url=http://wtc.nist.gov/NISTNCSTAR1CollapseofTowers.pdf |title=Final Report on the Collapse of the World Trade Center Towers |publisher=NIST |format =PDF |date=September 2005 |chapter=Chapter 1 |pages=p. 6}}</ref> 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.<ref name="NIST-chapter1">{{cite book |author=National Construction Safety Team |url=http://wtc.nist.gov/NISTNCSTAR1CollapseofTowers.pdf |format =PDF |title=Final Report on the Collapse of the World Trade Center Towers |publisher=NIST |date=September 2005}}</ref>
The floors consisted of 4 inch (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 (18.2 m) in the long-span areas and 35 feet (11 m) in the short span area.<ref name="NIST-chapter1"/> The trusses connected to the perimeter at alternate columns, and were therefore on 6 foot 8 inch (2.03 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 [[Viscoelasticity|viscoelastic]] dampers, which helped reduce the amount of sway felt by building occupants. The trusses supported a 4 inch thick (10 cm) lightweight concrete floor slab, with shear connections for composite action.<ref name="NIST-chapter1"/>
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. It was found to play a key role in the collapse sequence.<ref name="NIST-chapter1">{{cite web |url=http://wtc.nist.gov/NISTNCSTAR1CollapseofTowers.pdf |title=Final Report on the Collapse of the World Trade Center (chapter 1) |publisher=National Institute of Standards and Technology (NIST) |date=September 2005}}</ref>
The designers had considered the effects of the [[Construction_of_the_World_Trade_Center#Anticipation_of_aircraft_impact|impact of a passenger jet]], and believed the structures would remain standing in such an event. But NIST 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.<ref name="fact"/> (For more, see [[Collapse_of_the_World_Trade_Center#History_of_investigations|history]].)
==Impacts of airliners==
[[Image:World Trade Center 9-11 Attacks Illustration with Bird's-eye Impact Locations.svg|thumb|200px|left|Impact locations on 1 and 2 WTC]]
Hijackers flew two [[Boeing 767]] jet airliners, [[American Airlines Flight 11]] (a 767-200ER) and [[United Airlines Flight 175]] (a 767-200) into the towers. 1 WTC was hit at 8:46 a.m. by Flight 11 between the 99th and 93rd floors. 2 WTC was hit at 9:03 a.m. by Flight 175 between the 85th and 77th floor.
A Boeing 767-200 is 48.5 m (160 ft) long and has a wingspan of 48 m (156 ft), with a capacity of up to 62.2 (-200) or 91 (-200ER) m³ of jet fuel (16,700 or 24,000 US gallons).<ref name="Jane's">{{cite web | last =Jane's All the World's Aircraft | year = 2001 | url = http://www.janes.com/aerospace/civil/news/jawa/boeing_767.shtml | title = Boeing 767 | publisher = Jane's | accessdate = 2007-08-19}}</ref> The planes hit the towers at very high speeds. Flight 11 was traveling roughly 700 km/h (440 mph) when it crashed into the 1 WTC, the north tower; flight 175 hit 2 WTC, the south tower, at about 870 km/h (540 mph).<ref name="NIST-chapter1"/> In addition to severing a number of load-bearing columns, the resulting explosions in each tower ignited 38 m³ (10,000 gallons)<ref name="FEMA"/> 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 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.
==The 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.<ref>{{cite web | last = Field | first = Andy | year = 2004 | url = http://cms.firehouse.com/content/article/article.jsp?sectionId=46&id=25807 | title = A Look Inside a Radical New Theory of the WTC Collapse | work = | publisher = Fire/Rescue News | accessdate = 2006-07-28}}</ref> 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.<ref name="Gross">{{cite web | last = Gross | first = John L., Therese P. McAllister | year = 2004 | url = http://wtc.nist.gov/NISTNCSTAR1-6.pdf | format = pdf |title = Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers | work = Federal Building and Fire Safety Investigation of the World Trade Center Disaster NIST NCSTAR 1-6 | publisher = National Institute of Standards and Technology | accessdate = 2006-07-28}}</ref><ref name="Wilkinson">{{cite web | last = Wilkinson | first = Tim | year = 2006 | url = http://www.civil.usyd.edu.au/wtc.shtml | format = |title = World Trade Center - Some Engineering Aspects | work = | publisher = | accessdate = 2006-07-28}}</ref> 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.<ref>{{cite book |author=National Construction Safety Team |url=http://wtc.nist.gov/NISTNCSTAR1CollapseofTowers.pdf |title=Final Report on the Collapse of the World Trade Center Towers |publisher=NIST |chapter=Executive Summary |date=September 2005}}</ref>
===Deteriorating conditions===
Calls from occupants trapped in the upper floors relayed information via [[9-1-1]] about conditions. 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.<ref name="nist-ncstar1-8-p37">{{cite web |url=http://wtc.nist.gov/NISTNCSTAR1-8.pdf |title=NIST NCSTAR 1-8 - The Emergency Response |author=Lawson, J. Randall, Robert L. Vettori |publisher=National Institute of Standards and Technology |date=September 2005 |pages=p. 37}}</ref> Deteriorating conditions were also reported by the helicopters of the NYPD aviation unit.<ref name="nist-ncstar1-8-p37">{{cite web |url=http://wtc.nist.gov/NISTNCSTAR1-8.pdf |title=NIST NCSTAR 1-8 - The Emergency Response |author=Lawson, J. Randall, Robert L. Vettori |publisher=National Institute of Standards and Technology |date=September 2005 |pages=p. 37}}</ref>
* 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"
* 9:59 a.m. - they report that the South Tower is coming down.
NYPD helicopters report deteriorating conditions of the North Tower.<ref name="nist-ncstar1-8-p37"/>
* 10:20 a.m. - the NYPD aviation unit reports that the top of the tower might be leaning.
* 10:21 a.m. - they report that the North Tower is buckling on the southwest corner and leaning to the south.
* 10:27 a.m. - the aviation unit reports that the roof is going to come down very shortly.
* 10:28 a.m. - the NYPD reports that the tower is collapsing.
With dispatchers overwhelmed, there was minimal communication with the [[New York City Police Department|NYPD]], and the [[New York City Fire Department|FDNY]] were experiencing problems with [[September 11, 2001 radio communications|faulty radios]]. Firefighters inside the towers did not hear the evacuation order from their supervisors on the scene. 343 firefighters died in the Twin Towers, as a result of the collapse of the buildings.<ref name="mckinsey-ems">{{cite web |url=http://www.nyc.gov/html/fdny/pdf/mck_report/ems_response.pdf |title=McKinsey Report - Emergency Medical Service response |publisher=FDNY / McKinsey & Company |date=2002-08-09 |accessdate=2007-07-12}}</ref><ref name="mckinsey-nypd">{{cite web |url=http://www.nyc.gov/html/nypd/pdf/nypdemergency.pdf |title=McKinsey Report - NYPD |date=2002-08-19 |accessdate=2007-07-10}}</ref><ref>"NY Firefighters attack Giuliani," BBC News, July 12, 2007 http://news.bbc.co.uk/2/hi/americas/6294198.stm</ref>
==Collapse of the twin towers==
[[Image:World trade center 2.jpg|thumb|250px|As seen from [[Williamsburg, Brooklyn]]]]
At 9:59 a.m., the south tower collapsed, 56 minutes after being struck. The north tower, struck at 8:46 a.m., collapsed at 10:28 a.m., 102 minutes after impact. The collapses produced enormous clouds of dust that covered Manhattan for days. 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.
===The collapse mechanism===
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.<ref name="NIST">
{{cite web
| title = NIST's Responsibilities Under the National Construction Safety Team Act
| url = http://www.nist.gov/public_affairs/factsheet/constructionact.htm
| accessdate - 2008-04-23 }}</ref>
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 [[deformation|plastic deformation]] and [[creep (deformation)|creep]] from the weight of higher floors. The NIST report provides a useful model of the situation.
{{cquote|''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, partially 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.<ref name="NIST"/>
===Total progressive collapse===
[[Image:September 17 2001 Ground Zero 02.jpg|250px|thumb|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]]
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 cores of the buildings began to fall 15 to 25 seconds after the initial start of the collapse. These times are approximate because dust obscured the view.<ref name="fact"/><ref name="BazantWhy">{{cite paper |last=Bažant |first=Zdeněk P. |authorlink=Zdeněk Bažant |coauthor=Jia-Liang Le, Frank R. Greening and David B. Benson |title=Collapse of World Trade Center Towers: What Did and Did Not Cause It? |version=2007-06-22 |publisher=Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois 60208, USA |date=2007-05-27 |url=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 |id=Structural Engineering Report No. 07-05/C605c (page 12)|accessdate=2007-09-17 }}</ref>
The NIST report analyzes the failure mechanism in detail. 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,<ref>{{cite journal|
last=Bažant|
first=Zdeněk P.|
coauthors=Yong Zhou|
title=Why Did the World Trade Center Collapse?—Simple Analysis|date=2002-01-01|
month=January|
journal=Journal of Engineering Mechanics|
volume=128|
issue=1|
pages=pp. 2–6|
doi=0.1061/(ASCE)0733-9399(2002)128:1(2)|
url=http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/405.pdf|
accessdate=2007-08-23|
doi_brokendate=2008-06-21
}}</ref> crushing it and adding to the kinetic energy. This scenario repeated with each successive story, crushing the entire tower at near free-fall speed.<ref name="Bazant07"/>
==Collapse of 7 World Trade Center==
[[Image:Wtc7onfire.jpg|thumb|7 World Trade Center on fire after the collapse of the Twin Towers on 9/11]]
{{main|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 (or [[theodolite]]) was used to measure the extent of a visible bulge.<ref>{{cite web|author=Hayden, Peter| title= WTC: This Is Their Story| publisher=Firehouse Magazine|date=April 2002| url=http://www.firehouse.com/terrorist/911/magazine/gz/hayden.html}} </ref>
FEMA's provisional study was inconclusive<ref>{{cite web |last = |first = | year = |url = http://www.fema.gov/pdf/library/fema403_ch8.pdf | format = pdf | title = Observations, findings and Recommendations | work = World Trade Center Building Performance Study, (Chapter 8.2.5.1) | publisher = Federal Emergency Management Agency |accessdate = 2006-07-28}}</ref> and the collapse of 7 WTC was not included in the final report of the NIST investigation into the collapse of the World Trade Center when it was published in September 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,<ref>{{cite web | last = Barnett | first = J.R. | coauthors = R.R. Biederman, R.D. Sisson Jr. | year = 2001 | url = http://www.tms.org/pubs/journals/JOM/0112/Biederman/Biederman-0112.html | title = An Initial Microstructural Analysis of A36 Steel from WTC Building 7 | work = Feature: Letter | publisher = The Journal of Materials | accessdate = 2006-05-12}}
</ref> no studies of the collapse of 7 WTC have been published in scientific journals.
NIST released a progress report in June 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".<ref>{{cite web |last = |first = | year = 2004 |url = http://www.nist.gov/public_affairs/factsheet/wtc_keyfindings.htm | format = | title = Key Findings of NIST’s June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster | work = Fact sheets from NIST | publisher = National Institute of Standards and Technology |accessdate = 2006-07-28}}</ref><ref>{{cite web |last = |first = | year = 2004 |url = http://wtc.nist.gov/progress_report_june04/appendixl.pdf | format = pdf | title = Interim Report on WTC 7 | work = Appendix L | publisher = National Institute of Standards and Technology |accessdate = 2006-07-28}}</ref> Though it is not considering a [[controlled demolition hypothesis for the collapse of the World Trade Center|controlled demolition hypothesis]], NIST is developing hypothetical blast scenarios that could have caused the buildings structure to fail. It anticipates the release of a draft report of 7 World Trade Center in 2008.<ref>{{cite conference |url=http://wtc.nist.gov/media/NCSTAC_December18(Sunder).pdf |title=Opening Remarks and Overview of WTC 7 Investigation |conference=NCST Advisory Committee Meeting |author=Sunder, S. Shyam |booktitle=Proceedings |publisher=NIST |date=2007-12-18}}</ref>
==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."<ref>{{cite web | last = Oliver | first = Anthony |date=2005-06-30 | url = http://cruachan.televisual.co.uk/asset/GetArticle.exe?DB=e2&DATABASE=e2&LABEL=emap2&RECORD=191265&SEARCH=1| title = Lasting lessons of WTC | work = | publisher = New Civil Engineer | accessdate = 2006-07-28}}</ref> 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.<ref name="Bazant07">{{cite journal|last=Bažant|first=Zdeněk P.|coauthors=Mathieu Verdure|title=Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions|year=2007|month=March|journal=J. Engrg. Mech.|volume=133|issue=3|pages=pp. 308–319|doi=10.1061/(ASCE)0733-9399(2007)133:3(308)|url=http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/466.pdf|accessdate=2007-08-22}}</ref>
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."<ref>{{cite web | last = Whitehouse | first = David | year = 2001 | url = http://news.bbc.co.uk/1/hi/sci/tech/1579092.stm| title = WTC collapse forces skyscraper rethink | work = | publisher = BBC News | accessdate = 2006-07-28}}</ref>
===Authority===
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.<ref>Snell, Jack. "The Proposed National Construction Safety Team Act." NIST Building and Fire Research Laboratory. 2002.[http://www.fire.nist.gov/bfrlpubs/build02/PDF/b02044.pdf]</ref> 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]].<ref name="enr-20010924">{{cite news |title=Experts Debate Future of the Skyscraper in Wake of Disaster |publisher=Engineering News-Record |date=2001-09-24}}</ref> ASCE ultimately invited [[FEMA]] to join the investigation, which was completed under the auspices of the latter.<ref name="enr-20010924"/>
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.<ref>Glanz, James and Eric Lipton. "Nation Challenged: The Towers; Experts Urging Broader Inquiry In Towers' Fall". ''New York Times'' [[December 25]], [[2001]]</ref> 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.<ref name="NIST"/>
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.<ref>Dwyer, Jim. "Investigating 9/11: An Unimaginable Calamity, Still Largely Unexamined". ''New York Times''. [[September 11]], [[2002]] [http://www.skyscrapersafety.org/html/article_09112002.html]</ref> 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 2005.<ref name="NIST"/>
===Design analysis===
[[Image:World Trade Center 9-11 Attacks Illustration with Vertical Impact Locations.svg|thumb|230px|right|Impact locations for 1 and 2 WTC]]
One question that investigators tried to answer was whether the buildings had been designed to survive events like those on 9/11. The WTC towers were, of course, designed to survive foreseeable fires<ref>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 World Trade Center bombing|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, (NCSTAR 1-6, p lxxi) and on 13 floors in 2 WTC, although only three of these floors (77,78 & 85) were directly affected by the aircraft impact (NCSTAR 1-6, p lxvii-lxix). Although replacement fireproofing was specified at 1.5 inches in thickness, NIST found the average thickness to be 2.5 inches (NCSTAR 1-6A, p xl). 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".</ref> and, while "no building code in the United States has specific design requirements for impact of aircraft",<ref>{{cite web| last =Lew| first =H.S.| authorlink =| coauthors =Richard W. Bukowski and Nicholas J. Carino| title =Design, Construction and Maintenance of Structural and Life Safety | work =NIST NCSTAR 1-1 Pages 70-71| publisher =National Institutes of Standards and Technology|date=2006| url =http://wtc.nist.gov/NISTNCSTAR1-1.pdf| format =pdf| accessdate = 2007-10-15}}</ref> the scenario was considered by the structural engineers. NIST, however, found it difficult to document how the buildings were designed to anticipate aircraft impact.<ref>[http://wtc.nist.gov/pubs/MediaUpdate%20_FINAL_ProgressReport051303.pdf NIST Progress Report] May 2003 pdf</ref><ref>[http://query.nytimes.com/gst/fullpage.html?res=9F02E2DD1F3FF93BA3575AC0A9649C8B63&sec=&spon=&pagewanted=1 The Height of Ambition] [[New York Times]] [[September 8]] [[2002]]</ref>
Two accounts of the designers' anticipation of aircraft impact have been suggested in the course of the investigations. FEMA described the modeled aircraft as weighing 263,000 lb (119 metric tons) with a flight speed of 180 mph (290 km/h), both of which were exceeded in the actual impacts of 9/11.<ref name="FEMA"/> This description follows Leslie Robertson's remarks in the days immediately following the collapses.<ref name="Robertson">{{cite web | last =Robertson | first = Leslie E. | year = 2002 | url = http://www.nae.edu/nae/bridgecom.nsf/weblinks/CGOZ-58NLCB?OpenDocument | title = Reflections on the World Trade Center | work = The Bridge Volume 32, Number 1 | publisher = National Academy of Engineering | accessdate = 2006-07-28}}</ref> Robertson, who had participated in the structural design of the towers, recalled he "addressed the question of an airplane collision, if only to satisfy his engineer's curiosity". NIST was unable to document this reported study; Robertson could not find a copy and it was not an official study done by the WTC engineers. Journalists investigating the issue were also unable to find anyone to verify Robertson's recollection.<ref>Glanz, James and Eric Lipton. "The Height of Ambition" in the ''New York Times Magazine'', September 8, 2002.</ref>
NIST did find a three page [[white paper]] from 1964 summarizing an extensive WTC structural study that included the [[Scenario planning|effects]] 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 {{convert|600|mi/h|km/h|-2|abbr=on}}. While this suggests planes that were faster than those that crashed into the towers on 9/11, the study found that the buildings would not collapse on such a scenario.<ref>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.</ref> This study is more in line with remarks made by John Skilling's after the 1993 bombing.<ref name="Nader">Nalder, Eric. "Twin Towers Engineered to Withstand Jet Collision". ''The Seattle Times''. Saturday, February 27, 1993.[http://archives.seattletimes.nwsource.com/cgi-bin/texis.cgi/web/vortex/display?slug=1687698&date=19930227]</ref> When asked about this study by ''[[The New York Times Magazine]]'', Robertson insisted that he had no knowledge of this study and that it had not actually been carried out.
It is unclear whether the effect of jet fuel and aircraft contents was a consideration in the original building design. "One view," writes NIST, "suggests that an analysis was done indicating the biggest problem would be the fact that all the fuel would dump into the building and there would be a horrendous fire. Another view suggests that the fuel load, and the fire damage that it would cause, may not have been considered."<ref>[http://wtc.nist.gov/pubs/WTC_Part%20I_Introduction.pdf NIST Response to the World Trade Center Disaster] [[NIST]] pdf</ref><ref>{{cite web |url=http://wtc.nist.gov/progress_report_june04/appendixq.pdf |title=NIST’s Working Hypothesis for Collapse of the WTC Towers (Appendix Q) |date=June 2004 |publisher=NIST |accessdate=2007-12-21}}</ref> Without the original calculations, which were used to render its conclusions,<ref>[http://wtc.nist.gov/pubs/MediaUpdate%20_FINAL_ProgressReport051303.pdf NIST was informed by the Port Authority that the documents cited were destroyed in the collapse of 1 WTC and the WTC owners documents held in 7 WTC were also lost.]</ref> NIST said, any further comment would amount to speculation.<ref>{{cite web| last =Lew| first =H.S.| authorlink =| coauthors =Richard W. Bukowski and Nicholas J. Carino| title =Design, Construction and Maintenance of Structural and Life Safety | work =NIST NCSTAR 1-1 Page 71| publisher =National Institutes of Standards and Technology|date=2006| url =http://wtc.nist.gov/NISTNCSTAR1-1.pdf| format =pdf| accessdate = 2007-10-15}}</ref>
===FEMA's pancake collapse theory===
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.<ref>{{cite web | last = Eagar | first = Thomas | year = 2002 | url = http://www.pbs.org/wgbh/nova/wtc/collapse.html | title = The Collapse: An Engineer's Perspective | work = | publisher = NOVA | accessdate = 2006-07-28}}</ref> 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.<ref name="Eagar">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.</ref> 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.<ref name="Clifton">{{cite web | last = Clifton | first = G. Charles | year = 2002 | url = http://www.hera.org.nz/PDF%20Files/World%20Trade%20Centre.pdf#search='collapse%20of%20the%20world%20trade%20center' | format = pdf | title = Collapse of the World Trade Centre Towers | work = | publisher = | accessdate = 2006-07-28}}</ref><ref name="Wilkinson"/>
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".<ref name="Eagar"/> 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.<ref>{{cite web | last = | first = | year = 2001 | url = http://news.bbc.co.uk/1/hi/world/americas/1540044.stm | title = How the World Trade Center fell | work = | publisher = BBC News | accessdate = 2006-07-28}}</ref><ref>{{cite web | last = | first = | year = 2001 | url = http://news.bbc.co.uk/1/hi/england/1604348.stm | title = Twin towers' steel under scrutiny | work = | publisher = BBC News|accessdate = 2006-07-28}}</ref> 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.
===Early column failure theories===
NIST's column failure theory had already been articulated, not least by Bažant and Zhou. MIT civil engineers Oral Buyukozturk and Franz-Josef Ulm, probably following Bažant's early proposal, also described a collapse mechanism on [[September 21]], [[2001]].<ref>{{cite web |last = Buyukozturk |first = Oral, Franz-Josef Ulm |year = 2001 |url = http://web.mit.edu/newsoffice/2001/skyscrapers.html |title = How safe are our skyscrapers?: The World Trade Center collapse |work = | publisher = Massachusetts Institute of Technology | accessdate = 2006-06-26}}</ref>
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.<ref>{{cite web |last = Kausel |first = Eduardo |year = 2002 |url = http://web.mit.edu/civenv/wtc/ |title = The Towers Lost and Beyond |work = |publisher = Massachusetts Institute of Technology |accessdate = 2006-06-26}}</ref>
===The NIST report===
====Design of the study====
[[Image:WTC1.jpg|right|250px|thumb|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.<ref>{{cite web |last = Newman |first = Michael E. |year = 2002 |url = http://www.nist.gov/public_affairs/releases/n02-14.htm |title = Commerce's NIST Details Federal Investigation of World Trade Center Collapse |work = |publisher = National Institute of Standards and Technology |accessdate = 2006-07-28}}</ref> 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]]
:*[[Society of Fire Protection Engineers]]
:*[[National Fire Protection Association]]
:*[[American Institute of Steel Construction]]
:*[[Council on Tall Buildings and Urban Habitat]]
:*[[Structural Engineers Association of New York]]
====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" and "includes little analysis of the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable."<ref>NIST final report (2005). NCSTAR 1, p. xxxvii.</ref> 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 [[statics|static]] or quasi-static, including [[deformation]] but not the motion of structural elements after rupture as would [[Dynamics (mechanics)|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.
===Ongoing 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.<ref>{{cite web | last = Usmani | first = A.S., Y.C. Chung, J.L. Torero | year = 2003 | url = http://hdl.handle.net/1842/1216 | format = pdf | title = How did the World Trade Center Collapse: A New Theory | work = | publisher = Fire Safety Journal, 38, 6 | accessdate = 2007-05-07}}</ref> 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.<ref>{{cite web | last = | first = | year = 2005 | url = http://cruachan.televisual.co.uk/asset/GetArticle.exe?DB=e2&DATABASE=e2&LABEL=emap2&RECORD=194310&SEARCH=1 | format = | title = Row erupts over why twin towers collapsed | work = | publisher = New Civil Engineer | accessdate = 2006-07-28}}</ref> [[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 [[Dalmarnock tests|fire on real-scale buildings]].<ref>{{cite web | last = BRE Centre for Fire Safety Engineering, University of Edinburgh | year = 2006 | url = http://www.civ.ed.ac.uk/research/fire/dalmarnock.html | format = | title = Dalmarnock Full-Scale Experiments 25 & 26 July 2006 | work = | publisher = BRE Centre for Fire Safety Engineering, University of Edinburgh | accessdate = 2008-04-23}}</ref><ref>{{cite web | last = Christian | first = Nicholas | year = 2006 | url = http://news.scotsman.com/scitech.cfm?id=1068572006 | format = | title = Glasgow tower block to shed light on 9/11 fire | work = | publisher = Scotsman | accessdate = 2006-07-28}}</ref><ref>{{cite web | last = | first = | year = 2007 | url = http://www.bbc.co.uk/sn/tvradio/programmes/horizon/broadband/tx/firegrid | format = | title = Skyscraper Fire Fighters | work = | publisher = BBC Horizon | accessdate = 2007-07-31}}</ref>
===Criticism===
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.<ref>{{cite web |url=http://commdocs.house.gov/committees/science/hsy24133.000/hsy24133_0.HTM#259 |title=THE INVESTIGATION OF THE WORLD TRADE CENTER COLLAPSE: FINDINGS, RECOMMENDATIONS, AND NEXT STEPS |accessdate=2007-04-01 |author=Committee on Science |date=2005-10-26 |publisher=commdocs.house.gov |pages=259 |quote= }}</ref><!--and said that the "spoilation of fire scene evidence can border on a crime."<ref>Quintere, James (2005-07-27). "[http://www.nistreview.org/NIST-NCSTAR1-QUINTIERE.pdf Comments on the NIST NCSTAR 1 Draft]".</ref>--> He also noted that the report lacked a timeline and physical evidence to support its conclusions.<ref>{{cite web|author=Quintiere, James|title="2004 REPORT TO CONGRESS OF THE NATIONAL CONSTRUCTION SAFETY TEAM ADVISORY COMMITTEE"|date=December 2004|url=http://www.nist.gov/public_affairs/ncst/Final%20NCSTAC%202004%20Report%20to%20Congress.pdf|publisher=NIST|page=8}}</ref>
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. In October 2005, the ''[[New Civil Engineer]]'' reported criticism of NIST's computer modelling. 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.<ref>Parker, Dave. "WTC investigators resist call for collapse visualisation." ''New Civil Engineer'' October 6, 2005.</ref>
==Remarks by Osama bin Laden==
Although its translation is contested,<ref>[http://web.archive.org/web/20021218105636/www.wdr.de/tv/monitor/beitraege.phtml?id=379 "Bin-Laden-Video: Falschübersetzung als Beweismittel?] WDR, Das Erste, MONITOR Nr. 485 am [[December 20]], [[2001]]</ref><ref>{{cite web |last = Larson |first = Mark | year = 2001 |url = http://www.guardian.co.uk/afghanistan/story/0,,619181,00.html | format = | title = Unwinding the Bin Laden tape | work = | publisher = Guardian Unlimited |accessdate = 2006-07-28}}</ref> a [[Videos of Osama bin Laden#December 13, 2001|videotape]] of [[Osama bin Laden]] verified by the Pentagon indicates that Bin Laden did not believe that the buildings would collapse completely, but only those levels above where the planes struck:
{{cquote|''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.''<ref>{{cite web |last = |first = | year = 2001 |url = http://www.npr.org/news/specials/response/investigation/011213.binladen.transcript.html | format = | title = Transcript of Bin Laden videotape | work = | publisher = National Public Radio |accessdate = 2006-07-28}}</ref>}}
==Other buildings==
[[Image:September 17 2001.jpg|right|250px|thumb|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]]
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 World Trade Center|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), [[4 World Trade Center|South Plaza]] (4 WTC), and [[6 World Trade Center|U.S. Customs]] (6 WTC). The World Financial Center buildings, [[90 West Street, New York|90 West Street]], and 130 Cedar Street suffered fires. The [[Deutsche Bank Building]], Verizon, and [[Three World Financial Center|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.<ref>{{cite web |last = | first = | year = 2005 |url = http://wcbstv.com/topstories/topstories_story_302000152.html | format = | title = Bone Fragments Found Near WTC Said Human | work = | publisher = Associated Press |accessdate = 2006-09-11}}</ref>
==Aftermath==
===Site cleanup===
[[Image:ground-zero-2001-11-18.jpg|right|thumb|140px|November 18, 2001]]
[[Image:WTC-Fireman requests 10 more colleages.jpg|left|thumb|200px|September 15, 2001]]
The colossal 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.<ref>http://english.people.com.cn/200112/20/eng20011220_87119.shtml</ref><ref>[http://www.cbsnews.com/stories/2001/12/19/archive/main321907.shtml CBS News | WTC Fires All But Defeated | December 19, 2001 23:22:25<!-- Bot generated title -->]</ref> The cleanup was a massive operation coordinated by the City of New York Department of Design and Construction (DDC).[[Image:Ground Zero WTC.JPG|left|thumb|200px|Ground zero of the World Trade Center in 2008]] On [[September 22]], a preliminary cleanup plan was delivered by Controlled Demolition Inc. (CDI) of Phoenix. Mark Loizeaux, president of CDI, emphasized the importance of protecting the slurry wall (or "[[The Bathtub|the bathtub]]") which kept the Hudson river from flooding the WTC's basement.<ref>Post, Nadine M. and Debra K. Rubin. "Debris Mountain Starts to Shrink." Engineering News Record, 10/1/01. [http://www.construction.com/NewsCenter/Headlines/ENR/20011001b.asp]</ref> It involved round-the-clock operations, many contractors and sub-contractors, and cost hundreds of millions of dollars.<ref>{{cite news |first=Sara |last=Kugler |title=Officials Wanted More Searching at WTC |url=http://www.washingtonpost.com/wp-dyn/content/article/2006/10/23/AR2006102300062_pf.html|publisher=Washington Post |date=2006-10-23 |accessdate=2007-10-29 }}</ref> 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.<ref>Rubin, Debra K. and Janice L. Tuchman. "WTC Agency Begins Ramping Up Operations." Engineering News Record, 11/01/01. [http://www.construction.com/NewsCenter/Headlines/ENR/20011108c.asp]</ref> In 2007, the demolition of the surrounding damaged buildings was still ongoing, even as new construction proceeded on the World Trade Center's replacement, the [[Freedom Tower]].
===Air quality and the EPA's response===
{{main|EPA 9/11 pollution controversy}}
On [[September 18]], [[2001]], the Environmental Protection Agency issued a statement assuring the public that the air in Manhattan was "safe to breathe".<ref>{{cite web | url=http://www.epa.gov/wtc/stories/headline_091801.htm | title=EPA Response to September 11 | accessdate=2007-08-27}}</ref> 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.<ref>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.[http://www.epa.gov/oig/reports/2003/wtc/wtc_report_press_statement.htm]</ref>
==Controlled demolition conspiracy theories==
{{main article|Controlled demolition hypothesis for the collapse of the World Trade Center|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, not by the effects of the airplanes.<ref>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.[http://newspolls.org/story.php?story_id=55]</ref> 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.<ref name="fact">{{cite web | last = | first = | year = | url = http://wtc.nist.gov/pubs/factsheets/faqs_8_2006.htm | title = Answers to Frequently Asked Questions | work = NIST & The World Trade Center| publisher = National Institute of Standards and Technology | accessdate = 2006-09-17}}</ref> Controlled demolition is also dismissed in the engineering literature<ref name="Bazant07"/> and is pursued mainly as part of larger conspiracy theories about the events of 9/11.<ref>{{cite web |last = James B.
|first = Meigs |year = 2006 |url = http://www.popularmechanics.com/science/research/4199607.html |title = The Conspiracy Industry |work = Popular Mechanics |publisher = Hearst Communications, Inc. |accessdate = 2006-12-11}}</ref>
==See also==
*[[Health effects arising from the September 11, 2001 attacks]]
*[[Plans to rebuild the World Trade Center]]
==References==
===Cited references===
{{reflist|2}}
{{refbegin}}
*{{cite book
| last = Dwyer
| first = Jim
| coauthor = Kevin Flynn
| year = 2004
| title = 102 Minutes: The Untold Story of the Fight to Survive Inside the Twin Towers
| publisher = Times Books
| id = ISBN 0805076824
}}
*{{cite web
| last = National Institute of Standards and Technology
| first = Technology Administration
| year = 2006
| url = http://wtc.nist.gov/
| title = NIST and the World Trade Center
| work = NIST building and fire safety investigation
| publisher = US Department of Commerce
| accessdate = 2006-05-02
}}
*{{cite web
| last = Wilkinson
| first = Tim
| year = 2006
| url = http://www.civil.usyd.edu.au/wtc.shtml
| title = World Trade Center - Some Engineering Aspects
| publisher = School of Civil Engineering, The University of Sydney
| accessdate = 2006-05-02
}}
*{{cite web
| last = McAllister
| first = Therese
| coauthors = [[W. Gene Corley]], et al.
| year = 2002
| url = http://www.fema.gov/pdf/library/fema403_cover-toc.pdf
| title = World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations (FEMA 403)
| format = pdf
| publisher = Federal Emergency Management Agency
| accessdate = 2006-05-02
}}
*{{cite web
| last = Eagar
| first = Thomas W.
| coauthors = Christopher Musso
| year = 2001
| url = http://www.tms.org/pubs/journals/JOM/0112/Eagar/Eagar-0112.html
| title = Why Did the World Trade Center Collapse? Science, Engineering, and Speculation
| work = JOM, 53 (12)
| publisher = The Minerals, Metals & Materials Society
| accessdate = 2006-05-02
}}
*{{cite web
| last = Bažant
| first = Zdeněk P.
| coauthors = Yong Zhou
| year = 2001
| url = http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/405.pdf#search=%22Bazant%2C%20Zdenek%20P.%3B%20Yong%20Zhou%20(2001)%22
| format = pdf
| title = Why Did the World Trade Center Collapse? - Simple Analysis
| work = Journal of Engineering Mechanics ASCE, 9/13/01
| publisher = Northwestern University
| accessdate = 2006-09-11
}}
*{{cite web
| last = Clifton
| first = G. Charles
| year = 2001
| url = http://www.caddigest.com/subjects/wtc/select/clifton/p1.htm
| title = Collapse of the World Trade Centre Towers
| work = CAD Digest
| publisher = TenLinks, Inc.
| accessdate = 2006-05-02
}}
*{{cite web
| last = Edgar
| first = Dr. Thomas
| coauthors = NOVA
| year = 2002
| url = http://www.pbs.org/wgbh/nova/wtc/collapse.html
| title = The Collapse: An Engineer's Perspective
| work = Why the Towers Fell
| publisher = WGBH Educational Foundation
| accessdate = 2006-05-02
}}
*{{cite web
| last = Usmani
| first = A.S.
| coauthors = Y. C. Chung, J. L. Torero
| year = 2003
| url = http://www.era.lib.ed.ac.uk/handle/1842/1216
| title = How did the WTC towers collapse: a new theory
| format = pdf
| work = Fire Safety Journal, Volume 38, Issue 6
| publisher = Elsevier Ltd.
| accessdate = 2007-05-07
}}
*{{cite web
| last = NOVA online
| year = 2002
| url = http://www.pbs.org/wgbh/nova/wtc/metl_bend.html
| title = The structure of metal
| work = Why the Towers Fell
| publisher = WGBH Educational Foundation
| accessdate = 2006-05-02
}}
*{{cite web
| last = Kean
| first = Thomas H.
| year = 2004
| url = http://www.9-11commission.gov/archive/hearing11/9-11Commission_Hearing_2004-05-18.htm
| title = Eleventh Public Hearing
| work = Hearings
| publisher = National Commission on Terrorist Attacks Upon the United States
| accessdate = 2006-05-02
}}
*{{cite visual
| crew = Seth Kramer, Daniel Miller
|date=2002
| title = America Rebuilds: A Year at Ground Zero
| medium = Television series
| location = United States
| distributor = PBS home video
}}
*{{cite web
| year = 2006
| url = http://wtc.nist.gov/pubs/factsheets/faqs_8_2006.htm
| title = Answers to Frequently Asked Questions (August 30, 2006)
| work = NIST Federal Building and Fire Safety Investigation of the World Trade Center Disaster
| publisher = National Institute of Standards and Technology
| accessdate = 2008-04-21
}}
{{refend}}
==External links==
{{commonscat|9/11}}
* [http://wtc.nist.gov/ NIST and the World Trade Center] The National Institute of Standards and Technology's page on the collapse of the WTC. Contains most recent developments in investigations and FAQs.
* [http://www.civil.usyd.edu.au/wtc.shtml World Trade Center - Some Engineering Aspects] Early suggestion by University of Sydney engineering instructor about how the towers might have collapsed.
* [http://digitaljournalist.org/issue0111/biggart_intro.htm 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 was killed.
* [http://video.google.com/videoplay?docid=-2991254740145858863&q=9%2F11+duration=short Video showing fire in corner of 2 WTC]
* [http://podcasts.zaxis.com/pac/world-trade-center Forensic animations used in court]
* [http://news.bbc.co.uk/2/hi/americas/7485331.stm New light on 7 WTC collapse]
{{WTC navigation}}
[[Category:World Trade Center]]
[[Category:September 11, 2001 attacks]]
[[Category:Building fires in New York City]]
[[Category:Collapsed buildings in the United States]]
[[Category:Environmental disasters in the United States]]
[[fr:Effondrement des tours du World Trade Center]]