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The Twin Towers of the ] ] on September 11, 2001, as a result of being struck by two ]s ] by 10 ] affiliated with ], during the ].<ref name="cbc-2004">{{cite news |title=Bin Laden claims responsibility for 9/11 |publisher=CBC News |date=October 29, 2004 |url=http://www.cbc.ca/world/story/2004/10/29/binladen_message041029.html |accessdate=January 11, 2009 |quote=al-Qaeda leader Osama bin Laden appeared in a new message aired on an Arabic TV station Friday night, for the first time claiming direct responsibility for the 2001 attacks against the United States.}}</ref> Two of the four hijacked airliners crashed into the Twin Towers, one into the North Tower (]) and the other into the South Tower (]).<ref name="SecCounc">{{cite web |title=Security Council Condemns, 'In Strongest Terms', Terrorist Attacks on the United States |publisher=United Nations |date=September 12, 2001 |url=http://www.un.org/News/Press/docs/2001/SC7143.doc.htm |accessdate=September 11, 2006 |quote=The Security Council today, following what it called yesterday’s "horrifying terrorist attacks" in New York, Washington, D.C., and Pennsylvania, unequivocally condemned those acts, and expressed its deepest sympathy and condolences to the victims and their families and to the people and Government of the United States.}}</ref> The collapse of the Twin Towers destroyed the rest of the complex, and debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures. The South Tower collapsed at 9:59&nbsp;am, less than an hour after being hit by the second hijacked airliner, and at 10:28&nbsp;am the North Tower collapsed. Later that day, ] collapsed at 5:21&nbsp;pm from fires that had started when the North Tower collapsed.<ref name="NIST2">{{cite web |title=PartIIC – WTC 7 Collapse |work=NIST Response to the World Trade Center Disaster |publisher=National Institute of Standards and Technology |date=April 5, 2005 |url=http://wtc.nist.gov/pubs/WTC%20Part%20IIC%20-%20WTC%207%20Collapse%20Final.pdf |format=PDF |accessdate=2006-11-01}}</ref> As a result of the attacks to the towers, ]. Of the people who died in the towers, 2,192 were ]s, 343 were ]s, and 71 ]s. Aboard the two airplanes, 147 civilians and 10 hijackers also died.<ref>{{cite news |url=http://www.nytimes.com/2009/09/12/nyregion/12groundzero.html |title=9/11's Litany of Loss, Joined by Another Name |date=September 11, 2009 |work=The New York Times |accessdate=August 29, 2010 |first=Lisa W. |last=Foderaro}}</ref>

Immediately following the attacks, a building performance study (BPS) team of engineering specialists was formed by the Structural Engineering Institute of the ] (SEI/ASCE) and ] (FEMA). The BPS team issued its report in May 2002, finding that the aircraft impacts caused "extensive structural damage, including localized collapse" and that the resulting fires "further weakened the steel-framed structures, eventually leading to total collapse". They also presented recommendations for more detailed engineering studies of the disaster.<ref>{{Citation |title=Executive Summary| editor-first = T. | editor-last = McAllister |work=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=ASCE/FEMA |first1=G. |last1=Corley |first2=R. |last2=Hamburger |first3=T. |last3=McAllister |url=http://www.fema.gov/pdf/library/fema403_execsum.pdf}}</ref>

The BPS team investigation was later followed by a more detailed investigation conducted by the ] (NIST), which also consulted outside engineering entities. This investigation was completed in September 2005. The NIST investigators did not find anything substandard in the design of the WTC towers, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. They also emphasized the role of the fires and 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>{{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 |format=PDF}}</ref>

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, ], which was structurally completed on May 10, 2013, when the final component of the spire was installed atop the skyscraper. {{As of|2014}}, One World Trade Center, ] and ] have been replaced.

==Structural design==
{{see also|Construction of the World Trade Center}}
The towers were designed as "tube in tube" structures, which provided tenants with open floor plans uninterrupted by columns or walls. Numerous, closely spaced perimeter columns provided much of the strength to the structure, along with gravity load shared with the steel box columns of the core. Above the tenth floor, there were 59 perimeter columns along each face of the building, and there were 47 heavier columns in the core. All of the elevators and stairwells were located in the core, leaving a large column-free space between the perimeter that was bridged by prefabricated floor trusses.<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=] (NIST) |date=September 2005 |format=PDF}}</ref>

The floors consisted of {{convert|4|in|cm|adj=mid|-thick}} lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors with shear connections to the concrete slab for composite action.<ref name="NIST-chapter1"/> The trusses had a span of {{convert|60|ft|m}} in the long-span areas and {{convert|35|ft|m}} in the short-span area.<ref name="NIST-chapter1"/> The trusses connected to the perimeter at alternate columns, and were therefore on {{convert|6.8|ft|m|adj=on}} centers. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to interior box columns on the interior side. The floors were connected to the perimeter spandrel plates with ] dampers, which helped reduce the amount of sway felt by building occupants.

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 optimized load redistribution of floor diaphragms between the perimeter and core, with improved performance between the different materials of flexible steel and rigid concrete allowing the moment frames to transfer sway into compression on the core, which also mostly supported the transmission tower.

===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 ] that was lost in the fog had ] of the ]. A year later, a ]<ref>{{cite web|url=http://www.aerospaceweb.org/question/history/q0311.shtml|title=Aerospaceweb.org – Ask Us – B-25 Empire State Building Collision|work=aerospaceweb.org|accessdate=9 July 2015}}</ref> crashed into the ] building,<ref name="NYT">"Pilot Lost in Fog", ''New York Times,'' May 21, 1946, p. 1</ref> and there was a near-hit at the Empire State Building.<ref name="height">{{cite news |url=http://www.nytimes.com/2002/09/08/magazine/the-height-of-ambition.html?pagewanted=all |title=The Height of Ambition |author1=Glanz, James |author2=Eric Lipton |date=September 8, 2002 |work=The New York Times}}</ref> ], one of the chief engineers working on the design of the World Trade Center, has since said he personally considered the scenario of the impact of a ] or another jet airliner, which might be lost in the fog and flying at relatively low speeds while seeking to land at ] or ]. However, in an interview with the BBC, Robertson states, "with the 707, the fuel load was not considered in the design, I don't know how it could have been considered."<ref name="height"/><ref name="Robertson">{{cite web |last=Robertson |first=Leslie E. |year=2002 |url=http://www.nae.edu/Publications/TheBridge/Archives/V32-1EngineeringandHomelandSecurity/ReflectionsontheWorldTradeCenter.aspx |title=Reflections on the World Trade Center |work=The Bridge Volume 32, Number 1 |publisher=National Academy of Engineering |accessdate=2006-07-28}}</ref>

NIST found a three-page ] that mentioned another aircraft-impact analysis, involving impact of a Boeing 707 at {{convert|600|mph|km/h}}, but the original documentation of the study, which was part of the building's 1,200-page structural analysis, was lost when the Port Authority offices were destroyed in the collapse of the 1 WTC; the copy was lost in 7 WTC.<ref name="Sadek">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> In 1993, John Skilling, lead structural engineer for the WTC, recalled doing the analysis, and remarked, "Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a horrendous fire. A lot of people would be killed", he said. "The building structure would still be there."<ref name="Nalder">{{cite news |author=Nalder, Eric |title=Twin Towers Engineered to Withstand Jet Collision |work=The Seattle Times |date=February 27, 1993 |url=http://archives.seattletimes.nwsource.com/cgi-bin/texis.cgi/web/vortex/display?slug=1687698&date=19930227}}</ref> In its report, NIST stated that the technical ability to perform a rigorous simulation of aircraft impact and ensuing fires is a recent development, and that the technical capability for such analysis would have been quite limited in the 1960s.<ref name="NISTFAQ"/><ref group="note">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 {{convert|336,000|lb|t}} and carrying {{convert|23000|USgal|m3}} of fuel striking the 80th floor of the buildings at {{convert|600|mi/h|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 made by John Skilling following the 1993 WTC bombing. Without original documentation for either study, NIST said any further comments would amount to speculation.—NIST 2005. pp. 305–307</ref>

===Fireproofing===
In April 1970, the New York City Department of Air Resources ordered contractors building the World Trade Center to stop the spraying of ] as an insulating material.<ref>New York Times: April 28, 1970, p. 83</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 ], inspections found fireproofing to be deficient. The Port Authority was in the process of replacing it, but replacement had been completed on only 18&nbsp;floors in 1&nbsp;WTC, including all the floors affected by the aircraft impact and fires,<ref>NCSTAR 1–6, p lxxi</ref> and on 13&nbsp;floors in 2&nbsp;WTC, although only three of these floors (77, 78, and 85) were directly affected by the aircraft impact.<ref>NCSTAR 1–6, p lxvii–lxix</ref>{{refn|group="note"|Despite reports that both towers had asbestos fireproofing to their 64th floors<ref></ref> and that the fireproofing was being replaced due to its asbestos content, in fact the builders had been informed of a proposed ban on using asbestos/vermiculite fireproofing during construction and had ceased using it. By this time, only the fireproofing of the lower 40 floors of the north tower had been completed, and more than half of this was later replaced before the building was completed.<ref></ref> NIST concluded that the thickness of the fireproofing was {{convert|0.75|in|cm}} in the North Tower and {{convert|1.5|in|cm}} in the South Tower. NIST noted that upon reviewing the building records, it could not determine how this thickness was arrived at. NIST further concluded that the aircraft impact removed a significant portion of the fireproofing, contributing to the buildings' collapse, though many engineers, including one of NIST's top advisers, strongly disputed that claim, instead saying the fireproofing was simply not thick enough.
<!-- You need to provide a quote for the inspection done after the 1993 bombing that found the fireproofing to be deficient -->}}

==September 11, 2001==
]

===Aircraft impacts===
On September 11, 2001, hijackers associated with ] took control of two early morning Los Angeles-bound flights—both ] jetliners—soon after takeoff from Boston's ]. In its final moments, ] flew south over ] and crashed at roughly {{convert|440|mph|km/h}} into the northern facade of the North Tower at 8:46&nbsp;am, impacting between the 93rd and 99th floors. Seventeen minutes later, ] approached from the southwest, over ], and crashed into the South Tower's southern facade at 9:03&nbsp;am between the 77th and 85th floors at {{convert|540|mph|km/h}}.<ref name="NIST-chapter1"/> In addition to severing numerous load-bearing columns on the perimeter and inflicting other structural damage, the resulting explosions in each tower ignited {{convert|10000|USgal|L}} of jet fuel along with office contents.<ref group="note">According to NIST estimates, Flight 11 was carrying around {{convert|10000|USgal|L}} when it hit the North Tower. Up to {{convert|1500|USgal|L}} was instantly consumed in the initial fireball and a similar amount was consumed in the fireball outside the building. Approximately {{convert|7000|USgal|L}} burnt inside the office spaces igniting combustibles.<br/>Flight 175 was carrying around {{convert|9100|USgal|L}} when it hit the South Tower. Up to {{convert|1500|USgal|L}} was instantly consumed in the initial fireball and up to {{convert|2275|USgal|L}} was consumed in the fireball outside the building. More than {{convert|5325|USgal|L}} was burnt in the office spaces.<br />NIST estimated that each floor of both buildings contained around four pounds per square foot (60 tons per floor) of combustibles.</ref><ref name="FEMA403_CH1">{{Citation| editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=ASCE/FEMA |chapter=Chapter 1 - Introduction |first1=T. |last1=McAllister |first2=J. |last2=Barnett |first3=J. |last3=Gross |first4=R. |last4=Hamburger |first5=J. |last5=Magnusson |url=http://www.fema.gov/pdf/library/fema403_ch1.pdf}}</ref> Jet fuel from the impact traveled down at least one elevator shaft and exploded on the 78th floor of the North Tower, as well as in the main lobby.<ref>NCSTAR 1-5A, p 80</ref>

===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 |publisher=Fire/Rescue News |accessdate=2006-07-28}}</ref> It has been suggested{{by whom|date=March 2011}} 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/NCSTAR1/PDF/NCSTAR%201-6.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=2010-11-24}}</ref><ref name="Wilkinson">{{cite web |last=Wilkinson |first=Tim |year=2006 |url=http://www.civil.usyd.edu.au/wtc.shtml |title=World Trade Center – Some Engineering Aspects |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 |format=PDF}}</ref>

===Emergency response and evacuation===
{{See also|Casualties of the September 11 attacks}}

===Collapse of the South Tower===
{{Main|World Trade Center (1973–2001)}}
As the fires continued to burn, occupants trapped in the upper floors of the South Tower provided information about conditions to ] dispatchers. At 9:37&nbsp;am, 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://www.nist.gov/customcf/get_pdf.cfm?pub_id=101049 |title=NIST NCSTAR 1–8 – The Emergency Response Operations |author=Lawson, J. Randall, Robert L. Vettori |publisher=National Institute of Standards and Technology |date=September 2005 |page=37 |format=PDF}}</ref> The ] aviation unit also relayed information about the deteriorating condition of the buildings to police commanders.<ref name="mckinsey-toc">{{cite web |url=http://www.nyc.gov/html/fdny/html/mck_report/toc.html |title=McKinsey Report – Increasing FDNY's Preparedness |date=August 19, 2002 |accessdate=2016-01-16}}</ref> Only 14 people escaped from above the impact zone of the South Tower after it was hit (including ], who saw the plane coming at him), and only four from the floors above it. They escaped via Stairwell A, the only stairwell which had been left intact after the impact. Numerous police hotline operators who received calls from individuals inside the South Tower were not well informed of the situation as it rapidly unfolded. Many operators told callers not to descend the tower on their own, even though it is now believed that Stairwell A was most likely passable at and above the point of impact.<ref>{{Cite book |last=National Commission on Terrorist Attacks |title=The 9/11 Commission Report |publisher=W. W. Norton & Company |date=July 22, 2004 |edition=first |url=http://govinfo.library.unt.edu/911/report/911Report_Ch9.pdf |pages=294 |isbn=0-393-32671-3}}</ref> At 9:52&nbsp;am, 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".<ref name="nist-ncstar1-8-p37" /> With the warnings, the NYPD issued orders for its officers 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&nbsp;am, the South Tower collapsed, 56 minutes after being struck.

===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&nbsp;am, 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 10:28&nbsp;am, the aviation unit reported that "the roof is going to come down very shortly".<ref name="nist-ncstar1-8-p37"/> The North Tower collapsed at 10:28&nbsp;am, after burning for 102 minutes.

After the South Tower collapsed, FDNY commanders issued orders for firefighters in the North Tower to evacuate. Due to ], 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.<ref>{{cite news |url=http://query.nytimes.com/gst/fullpage.html?res=9E05E7DA1E31F934A35754C0A9649C8B63&sec=&spon=&pagewanted=all |title=9/11 Exposed Deadly Flaws in Rescue Plan |author=Dwyer, Jim, Kevin Flynn and Ford Fessenden |date=July 7, 2002 |work=The New York Times}}</ref> 343 firefighters died in the Twin Towers, as a result of the collapse of the buildings.<ref name="mckinsey-nypd">{{cite web |url=http://www.nyc.gov/html/nypd/pdf/nypdemergency.pdf |title=McKinsey Report – Improving NYPD Emergency Preparedness and Response |date=August 19, 2002 |accessdate=2007-07-10 |format=PDF |archiveurl=https://web.archive.org/web/20070615060709/http://www.nyc.gov/html/nypd/pdf/nypdemergency.pdf |archivedate=2007-06-15}}</ref><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=August 9, 2002 |accessdate=2007-07-12 |format=PDF}}</ref><ref>"NY Firefighters attack Giuliani", BBC News, July 12, 2007 </ref> No one was able to escape the North Tower from the impact zone or above, as all stairwells and elevator shafts on those floors were destroyed or blocked.<ref>, National Museum of American History, Smithsonian Institution, 2002</ref>
After the collapse, light dust reached as far as the ], located {{convert|2.93|mi|km}} away.

===Collapse of 7 World Trade Center===
]
<!--Do not add ] here. It is copyrighted and the fair use claim does not extend to use in this article. Also, the file is 1.5 MB and adds 5 to 15 seconds to the page load time. -->
{{Main|7 World Trade Center}}
As the North Tower collapsed, heavy debris hit 7&nbsp;World Trade Center, causing damage to the south face of the building<ref name="NIST-june2004">{{cite web |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=August 20, 2007}}</ref> and starting fires that continued to burn throughout the afternoon.<ref name="ncstar1-a">{{cite book |url=http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201A.pdf |format=PDF |publisher=NIST |title=NIST NCSTAR1-A: Final Report on the Collapse of World Trade Center Building 7 |date=November 2008 |accessdate=July 11, 2011}}</ref> Structural damage occurred to the southwest corner between Floors 7 and 17 and on the south face between Floor 44 and the roof; other possible structural damage includes a large vertical gash near the center of the south face between Floors 24 and 41.<ref name="ncstar1-a">{{cite book |url=http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201A.pdf |format=PDF |publisher=NIST |title=NIST NCSTAR1-A: Final Report on the Collapse of World Trade Center Building 7 |date=November 2008 |accessdate=July 11, 2011}}</ref> The building was equipped with a ], but had many single-point vulnerabilities for failure: the sprinkler system required manual initiation of the electrical fire pumps, rather than being a fully automatic system; the floor-level controls had a single connection to the sprinkler water riser; and the sprinkler system required some power for the ] to deliver water. Also, water pressure was low, with little or no water to feed sprinklers.<ref>{{cite web |url=http://wtc.nist.gov/media/P4InvestigationofActiveFireProtection.pdf |format=PDF |title=Active Fire Protection Systems Issues |author=Grosshandler, William |publisher=NIST |accessdate=September 11, 2007}}</ref><ref>{{cite web |url=http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201-4.pdf |format=PDF |title=Active Fire Protection Systems |author=Evans, David D |publisher=NIST |date=September 2005 |accessdate=August 26, 2010}}</ref>

Some firefighters entered 7&nbsp;World Trade Center to search the building. They attempted to extinguish small pockets of fire, but low water pressure hindered their efforts.<ref>{{cite web |url=http://graphics8.nytimes.com/packages/pdf/nyregion/20050812_WTC_GRAPHIC/9110313.PDF |format=PDF |title=Oral Histories From Sept.&nbsp;11 – Interview with Captain Anthony Varriale |work=The New York Times |date=December 12, 2001 |accessdate=August 22, 2007}}</ref> Fires burned into the afternoon on the 11th and 12th floors of 7&nbsp;World Trade Center, the flames visible on the east side of the building.<ref>{{cite video |people=Spak, Steve |date=September 11, 2001 |url=http://video.google.com/videoplay?docid=6790722824543352916&hl=en |title=WTC 9-11-01 Day of Disaster |medium=Video |location=New York City |publisher=Spak, Steve}}</ref><ref>{{cite journal |author=Scheuerman, Arthur |url=http://wtc.nist.gov/media/ScheuermanStatementDec2006.pdf |format=PDF |title=The Collapse of Building 7 |date=December 8, 2006 |publisher=NIST |accessdate=June 29, 2007}}</ref> During the afternoon, fire was also seen on floors 6–10, 13–14, 19–22, and 29–30.<ref name="NIST-june2004"/> In particular, the fires on floors 7 through 9 and 11 through 13 continued to burn out of control during the afternoon.<ref name="nist-questions">{{cite web |url=http://www.nist.gov/public_affairs/factsheet/wtc_qa_082108.cfm |title=Questions and Answers about the NIST WTC 7 Investigation |publisher=NIST |accessdate=August 26, 2010}}</ref> At approximately 2:00&nbsp;pm, firefighters noticed a bulge in the southwest corner of 7&nbsp;World Trade Center between the 10th and 13th floors, a sign that the building was unstable and might cave to one side or "collapse".<ref name="hayden">{{cite web |url=http://www.firehouse.com/article/10567885/deputy-chief-peter-hayden?page=4 |title=WTC: This Is Their Story, Interview with Chief Peter Hayden |publisher=Firehouse.com |date=September 9, 2002 |accessdate=March 3, 2011}}</ref> During the afternoon, firefighters also heard creaking sounds coming from the building and issued uncertain reports about damage in the basement.<ref>{{cite web |url=http://www.firehouse.com/article/10568001/captain-chris-boyle?page=2 |title=WTC: This Is Their Story, Interview with Captain Chris Boyle |publisher=Firehouse.com |date=August 2002 |accessdate=March 3, 2011}}</ref> Around 3:30&nbsp;pm FDNY Chief ] decided to halt rescue operations, surface removal, and searches along the surface of the debris near 7&nbsp;World Trade Center and evacuate the area due to concerns for the safety of personnel.<ref>{{cite news |url=http://www.nytimes.com/packages/html/nyregion/20050812_WTC_GRAPHIC/Nigro_Daniel.txt |title=Oral Histories From Sept.&nbsp;11 – Interview with Chief Daniel Nigro |work=The New York Times |date=October 24, 2001 |accessdate=June 28, 2007}}</ref> At 5:20:33&nbsp;pm EDT on September 11, 2001, 7&nbsp;World Trade Center started to collapse, with the crumble of the east mechanical penthouse, while at 5:21:10&nbsp;pm EDT the entire building collapsed completely.<ref name="autogenerated3">{{cite book |author=Gilsanz, Ramon, Edward M. DePaola, Christopher Marrion, and Harold "Bud" Nelson |url=http://www.fema.gov/pdf/library/fema403_ch5.pdf |format=PDF |publisher=FEMA |title=World Trade Center Building Performance Study |chapter=WTC7 (Chapter 5) |date=May 2002 |accessdate=February 17, 2008}}</ref><ref>{{cite video |url=https://archive.org/details/cbs200109111651-1733 |people=CBS News |date=September 11, 2001 |title=CBS Sept.&nbsp;11, 2001 4:51&nbsp;pm&nbsp;– 5:33&nbsp;pm (September 11, 2001) |publisher=], CBS 9, Washington, D.C |medium=Television}} – View of the collapse captured by CBS</ref> There were no ] associated with the collapse.

] after 7 WTC's collapse]]

When 7&nbsp;World Trade Center collapsed, debris caused substantial damage and contamination to the ]'s ] building, located adjacent at 30&nbsp;West Broadway, to the extent that the building was not salvageable. In August 2007, Fiterman Hall was scheduled for dismantling.<ref>{{cite web |url=http://www.lowermanhattan.info/construction/project_updates/fiterman_hall_39764.aspx |title=Fiterman Hall&nbsp;— Project Updates |publisher=Lower Manhattan Construction Command Center/LMDC |accessdate=August 23, 2007}}</ref> A revised plan called for demolition in 2009 and completion of the new Fiterman Hall in 2012, at a cost of $325&nbsp;million.<ref> BMCC News, November 17, 2008</ref><ref>Agovino T Crain's New York Business November 13, 2008</ref> The building was finally demolished in November 2009 and construction of its replacement began on December 1, 2009.<ref>{{cite web |url=http://www.lowermanhattan.info/construction/project_updates/fiterman_hall_39764.aspx |work=LowerManhattan.info |title=Lower Manhattan: Fiterman Hall |accessdate=January 18, 2010 |publisher=Lower Manhattan Construction Command Center}}</ref> The adjacent ], an ] building constructed in 1926, had extensive damage to its east facade from the collapse of 7&nbsp;World Trade Center, though it was successfully restored at a cost of US$1.4&nbsp;billion.<ref name="nyconstruction">{{cite web |url=http://newyork.construction.com/projects/TopProjects04/Verizon.asp |title=Verizon Building Restoration |publisher=New York Construction (McGraw Hill) |accessdate=June 28, 2007}}</ref>

7 World Trade Center housed ] (SEC) files relating to numerous Wall Street investigations, as well as other federal investigative files. All the files for approximately 3,000 to 4,000 SEC cases were destroyed. Although some were backed up elsewhere, others were not, especially those classified as confidential.{{citation needed|date=September 2012}} Files relating to Citigroup's connection to the ] scandal were lost.<ref name="street">{{cite news |title=Citigroup Facing Subpoena in IPO Probe |url=http://www.thestreet.com/markets/matthewgoldstein/10036925.html |work=The Street |accessdate=July 9, 2008}}</ref> The Equal Employment Opportunity Commission estimated over 10,000 cases would be affected.<ref name="NYLEq">{{cite news |title=Federal Agencies: Re-Creating Lost Files |date=September 14, 2001 |url=http://www.nylawyer.com/login.php?source=/news/01/09/091401b |work=New York Lawyer |accessdate=July 9, 2008}}</ref> Investigative files in the ]'s largest field office, of more than 200 employees, were also lost in the collapse of WTC 7. "All the evidence that we stored at 7 World Trade, in all our cases, went down with the building", said one agent.<ref name="TTV">{{cite news |title=Ground Zero for the Secret Service |date=July 23, 2002 |url=http://www.g4tv.com/techtvvault/features/27904/Ground_Zero_for_the_Secret_Service.html |accessdate=July 9, 2008}}</ref>

===Mechanics of Twin Towers' collapse===
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. In both cases, the section of the building that had been damaged by the airplanes failed, which allowed the floors above the impact zone to fall onto the undamaged structure below. As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction, caused by the sudden rush of air from the upper levels. The first fragments of the outer walls of the collapsed North Tower struck the ground 11 seconds after the collapse started, and parts of the South Tower after 9 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.<ref name="NISTFAQ"/>

While the buildings were designed to support enormous ], they provided little resistance to the moving mass of the sections above the floors where the collapses initiated. Structural systems respond very differently to static and dynamic loads, and since the motion of the falling portion began as a free fall through the height of at least one story (roughly three meters or 10&nbsp;feet), the structure beneath them was unable to stop the collapses once they began. Indeed, a fall of only half a meter (about 20&nbsp;inches) would have been enough to release the necessary energy to begin an unstoppable collapse.<ref name="BazantWhy">{{Cite journal |last1=Bažant |first1=Z. K. P. |authorlink1=Zdeněk Bažant |last2=Le |first2=J. L. |last3=Greening |first3=F. R. |last4=Benson |first4=D. B. |title=What Did and Did Not Cause Collapse of World Trade Center Twin Towers in New York? |doi=10.1061/(ASCE)0733-9399(2008)134:10(892) |journal=Journal of Engineering Mechanics |volume=134 |issue=10 |pages=892 |year=2008 |url=http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/00%20WTC%20Collapse%20-%20What%20Did%20%26%20Did%20Not%20Cause%20It.pdf |publisher=]}}</ref>

====Collapse initiation====
After the planes struck the buildings, but before the buildings collapsed, the cores of both towers consisted of three distinct sections. Above and below the impact floors, the cores consisted of what were essentially two rigid boxes; the steel in these sections was undamaged and had undergone no significant heating. The section between them, however, had sustained significant damage and, though they were not hot enough to melt it, the fires were weakening the structural steel. As a result, the core columns were slowly being crushed, sustaining ] and ] deformation from the weight of floors above. As the top section tried to move downward, however, the hat truss redistributed the load to the perimeter columns. Meanwhile, the perimeter columns and floors were also being weakened by the heat of the fires, and as the floors began to sag they pulled the exterior walls inwards. In the case of 2 WTC, this caused the eastern face to buckle, transferring its loads back to the failing core through the hat truss and initiating the collapse. In the case of 1 WTC, the south wall later buckled in the same way, and with similar consequences.<ref>NIST report, p. 29</ref>

====Total progressive collapse====
The collapse of the World Trade Center has been called "the most infamous paradigm" of ],<ref name="Bazant07">{{Cite journal |last=Bažant |first=Zdeněk P. |author2=Mathieu Verdure |title=Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions |date=March 2007 |journal=J. Engrg. Mech. |volume=133 |issue=3 |pages=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 |format=PDF}}</ref> also called "Pancaking".<ref>{{cite web |title=Pancacking, Explained|url=http://izmirlian.net/wordpress/?page_id=184}}</ref> Once the collapse initiated, the mass of failing floors overwhelmed the floors below, causing a progressive series of floor failures which accelerated as the sequence progressed. Soon, large portions of the perimeter columns and possibly the cores were left without any lateral support, causing them to fall laterally towards the outside pushed by the increasing pile of rubble. The result was the walls peeling off and separating away from the buildings by a large distance (about 500 feet in some cases), hitting other neighboring buildings. Some connections broke as the bolts snapped, leaving many panels randomly scattered.<ref>{{cite web |last=Hamburger |first=Ronald |title=World Trade Center Building Performance Study |publisher=Federal Emergency Management Agency |pages=2–27 & 2–35 |url=http://www.fema.gov/pdf/library/fema403_ch2.pdf |format=PDF |accessdate=2006-07-27 |display-authors=etal}}</ref> Significant parts of the naked cores (about 60 stories for the North Tower and 40 for the South Tower) remained standing for a few seconds before they also collapsed.<ref name="NISTFAQ"/>

==Initial opinions and analysis==
]
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. ], a structural engineering professor at the ], 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, ] of the entire structure was inevitable.<ref>{{cite news |url=http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2001/09/12/MN101866.DTL&hw=STRUCTURAL&sn=025&sc=505 |title=Jets hit towers in most vulnerable spots |author=Perlman, David |work=San Francisco Chronicle |date=September 12, 2001}}</ref> ], who designed the ] in Malaysia and the ] in New York, remarked, "no building is prepared for this kind of stress."<ref name="gugliotta">{{cite news |url=http://www.washingtonpost.com/wp-dyn/content/article/2001/09/12/AR2005033107985.html |title='Magnitude Beyond Anything We'd Seen Before'; Towers Built to Last But Unprepared For Such an Attack |author=Gugliotta, Guy |work=The Washington Post |date=September 12, 2001}}</ref>

On September 13, 2001, ], professor of civil engineering and materials science at ], circulated 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. Once more than half of the columns on a particular floor buckled, the overhead structure could no longer be supported and complete collapse of the structures occurred. Bažant later published an expanded version of this analysis.<ref name="Bazant02">{{Cite journal |first=Zdeněk P. |last=Bažant |author2=Yong Zhou |date=January 2002 |title=Why Did the World Trade Center Collapse?—Simple Analysis |journal=Journal of Engineering Mechanics |volume=128 |issue=1 |pages=2–6 |doi=10.1061/(ASCE)0733-9399(2002)128:1(2) |url=http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/405.pdf |accessdate=June 18, 2009}}</ref> Other analyses were conducted by MIT civil engineers Oral Buyukozturk and Franz-Josef Ulm, who 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 |publisher=Massachusetts Institute of Technology |accessdate=2006-06-26}}</ref> They later contributed to an MIT collection of papers on the WTC collapses edited by ] called ''The Towers Lost and Beyond''.<ref>{{cite web |last=Kausel |first=Eduardo |year=2002 |url=http://web.mit.edu/civenv/wtc/ |title=The Towers Lost and Beyond |publisher=Massachusetts Institute of Technology |accessdate=2006-06-26}}</ref>

==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.<ref>Snell, Jack. "The Proposed National Construction Safety Team Act." NIST Building and Fire Research Laboratory. 2002. </ref> A team was quickly assembled by the Structural Engineers Institute of the ], led by ], Senior Vice President of ]. It also involved the ], the ], the ], and the ].<ref name="enr-20010924">{{cite news |title=Experts Debate Future of the Skyscraper in Wake of Disaster |publisher=Engineering News-Record |date=September 24, 2001}}</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". ''The 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, although 95% of structural beams and plates and 50% of the reinforcement bars were recovered.<ref>Sylvie Boulanger and Sylvain Boulanger "Steel and sustainability 2: Recovery strategies" Canadian Institute of Construction. March 17, 2004. </ref>

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". ''The New York Times''. September 11, 2002. </ref> 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.<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 |archiveurl=https://web.archive.org/web/20100419072759/http://www.nist.gov/public_affairs/factsheet/constructionact.htm |archivedate=April 19, 2010}}</ref>

===FEMA building performance study===
FEMA suggested that fires in conjunction with damage resulting from the aircraft impacts were the key to the collapse of the towers. Thomas Eagar, Professor of Materials Engineering and Engineering Systems at ], described the fires as "the most misunderstood part of the WTC collapse". This is because the fires were originally said to have "melted" the floors and columns.<ref>{{harvnb|Eagar|Musso|2001|p=8}}: "The fire is the most misunderstood part of the WTC collapse. Even today, the media report (and many scientists believe) that the steel melted."</ref> ] is essentially kerosene and would have served mainly to ignite very large, but not unusually hot, hydrocarbon fires.<ref>{{harvnb|Eagar|Musso|2001|p=10}}: "The maximum flame temperature increase for burning hydrocarbons (jet fuel) in air is, thus, about 1,000 °C—hardly sufficient to melt steel at 1,500 °C."</ref> As Eagar said, "The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel."<ref>{{harvnb|Eagar|Musso|2001|p=9}}: "The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel."</ref> 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.<ref>{{harvnb|Eagar|Musso|2001|p=11}}: "It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. Many structural engineers believe that the weak points—the limiting factors on design allowables—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure."</ref>
{{quotation|The large quantity of jet fuel carried by each aircraft ignited upon impact into each building. A significant portion of this fuel was consumed immediately in the ensuing fireballs. The remaining fuel is believed either to have flowed down through the buildings or to have burned off within a few minutes of the aircraft impact. The heat produced by this burning jet fuel does not by itself appear to have been sufficient to initiate the structural collapses. However, as the burning jet fuel spread across several floors of the buildings, it ignited much of the buildings’ contents, causing simultaneous fires across several floors of both buildings. The heat output from these fires is estimated to have been comparable to the power produced by a large commercial power generating station. Over a period of many minutes, this heat induced additional stresses into the damaged structural frames while simultaneously softening and weakening these frames. This additional loading and the resulting damage were sufficient to induce the collapse of both structures.{{sfn|FEMA(403) Executive Summary|p = 2}}}}

===NIST report===
] in the foreground was crushed when the south 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 ] conducted a three-year, $16 million investigation into the ] and ] 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 |publisher=National Institute of Standards and Technology |accessdate=2006-07-28}}</ref> The study included in-house technical expertise, along with assistance from several outside private institutions, including the Structural Engineering Institute of the ], ], ], ], ], ], and the ].

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).<ref>{{cite web |title=Reports of the Federal Building and Fire Investigation of the World Trade Center Disaster |publisher=NIST |url=http://wtc.nist.gov/NCSTAR1/ |accessdate=May 17, 2011}}</ref><ref>Bazant (2007), p. 2</ref><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 using the software program ]. 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 ] or quasi-static, including deformation but not the motion of structural elements after rupture as would ] 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.<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=October 26, 2005 |publisher=commdocs.house.gov |page=259}}</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 |format=PDF}}</ref> 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.<ref>{{Cite journal |last=Parker |first=Dave |title=WTC investigators resist call for collapse visualisation |magazine=] |date=October 6, 2005 |publisher=EMAP Publishing |location=England & Wales}}</ref> The NIST report for WTC 7 concluded that no blast sounds were heard on audio and video footage, or were reported by witnesses.<ref>{{cite web|url=http://www.nist.gov/el/disasterstudies/wtc/faqs_wtc7.cfm|title=Questions and Answers about the NIST WTC 7 Investigation|year=2012|accessdate=January 30, 2016|publisher=NIST}}</ref>

===7 World Trade Center===
]

In May 2002, FEMA issued a report on the collapse based on a preliminary investigation conducted jointly with the Structural Engineering Institute of the American Society of Civil Engineers under leadership of Dr. ], P.E. FEMA made preliminary findings that the collapse was not primarily caused by actual impact damage from the collapse of 1&nbsp;WTC and 2&nbsp;WTC but by fires on multiple stories ignited by debris from the other two towers that continued unabated due to lack of water for sprinklers or manual firefighting. The report did not reach conclusions about the cause of the collapse and called for further investigation.{{citation needed|date=November 2011}}

In response to FEMA's concerns, the National Institute of Standards and Technology (NIST) was authorized to lead an investigation into the structural failure and collapse of the World Trade Center twin towers and 7&nbsp;World Trade Center.<ref>{{cite web |url=http://www.nist.gov/public_affairs/factsheet/safety.cfm |title=NIST’s World Trade Center Investigation |publisher=NIST |accessdate=August 26, 2010}}</ref> The investigation, led by Dr S. Shyam Sunder, drew not only upon in-house technical expertise, but also upon the knowledge of several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE), the ] (SFPE), the ] (NFPA), the American Institute of Steel Construction (AISC), the ] (CTBUH), and the ] (SEAoNY).<ref name="NIST-WTCreport">{{cite web |url=http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201.pdf |format=PDF |title=Final Report on the Collapse of the World Trade Center Towers |publisher=National Institute of Standards and Technology (NIST) |date=September 2005 |accessdate=August 26, 2010}}</ref>

The bulk of the investigation of 7 World Trade Center was delayed until after reports were completed on the collapse of the World Trade Center twin towers.<ref name="nist-questions"/> In the meantime, NIST provided a preliminary report about 7 World Trade Center in June 2004, and thereafter released occasional updates on the investigation.<ref name="NIST-june2004"/> According to NIST, the investigation of 7&nbsp;World Trade Center was delayed for a number of reasons, including that NIST staff who had been working on 7&nbsp;World Trade Center were assigned full-time from June 2004 to September 2005 to work on the investigation of the collapse of the twin towers.<ref name="NISTFAQ">{{cite web |url=http://www.nist.gov/public_affairs/factsheet/wtc_faqs_082006.cfm |title=Answers to Frequently Asked Questions |publisher=National Institute of Standards and Technology |date=August 2006 |accessdate=2012-05-25}}</ref> In June 2007, Shyam Sunder explained, "We are proceeding as quickly as possible while rigorously testing and evaluating a wide range of scenarios to reach the most definitive conclusion possible. The 7&nbsp;WTC investigation is in some respects just as challenging, if not more so, than the study of the towers. However, the current study does benefit greatly from the significant technological advances achieved and lessons learned from our work on the towers."<ref name="NIST7STAT1">{{cite press release |url=http://www.nist.gov/public_affairs/releases/wtc_062907.cfm |title=NIST Status Update on World Trade Center 7 Investigation |last=Newman |first=Michael |date=June 29, 2007 |publisher=National Institute of Standards and Technology |accessdate=August 26, 2010}}</ref>

] captured footage of the south face of 7&nbsp;World Trade Center, including a glimpse of a gash, extending approximately 10 stories.]]

In November 2008, NIST released its final report on the causes of the collapse of 7&nbsp;World Trade Center.<ref name="ncstar1-a"/> This followed their August 21, 2008 draft report which included a period for public comments.<ref name="nist-questions"/> In its investigation, NIST utilized ] to model events leading up to collapse initiation and ] models to simulate the global response to the initiating events.<ref>{{cite web |url=http://wtc.nist.gov/media/WTC7_Approach_Summary12Dec06.pdf |format=PDF |title=WTC 7 Technical Approach and Status Summary |author=McAllister, Therese |publisher=NIST |date=December 12, 2006 |accessdate=February 17, 2008}}</ref> NIST determined that diesel fuel did not play an important role, nor did the structural damage from the collapse of the twin towers, nor did the transfer elements (trusses, girders, and cantilever overhangs). But the lack of water to fight the fire was an important factor. The fires burned out of control during the afternoon, causing floor beams near Column 79 to expand and push a key girder off its seat, triggering the floors to fail around column 79 on Floors 8 to 14. With a loss of lateral support across nine floors, Column 79 soon buckled – pulling the East penthouse and nearby columns down with it. With the buckling of these critical columns, the collapse then progressed east-to-west across the core, ultimately overloading the perimeter support, which buckled between Floors 7 and 17, causing the entire building above to fall downward as a single unit. From collapse timing measurements taken from a video of the north face of the building, NIST observed that the building's exterior facade fell at free fall acceleration through a distance of approximately 8 stories (32 meters, or 105 feet), noting "the collapse time was approximately 40 percent longer than that of free fall for the first 18 stories of descent."<ref>{{cite web |url=http://www.nist.gov/customcf/get_pdf.cfm?pub_id=861610 |title=NIST NCSTAR 1A: Final Report on the Collapse of World Trade Center Building 7 |publisher=National Institute of Standards and Technology |date=November 2008 |pages=44–46 |format=PDF}}</ref> The fires, fueled by office contents, along with the lack of water, were the key reasons for the collapse.<ref name="ncstar1-a"/>

The collapse of the old 7 World Trade Center is remarkable because it was the first known instance of a tall building collapsing primarily as a result of uncontrolled fires.<ref name="nist-questions" /> Based on its investigation, NIST reiterated several recommendations it had made in its earlier report on the collapse of the twin towers, and urged immediate action on a further recommendation: that fire resistance should be evaluated under the assumption that sprinklers are unavailable; and that the effects of thermal expansion on floor support systems be considered. Recognizing that current building codes are drawn to prevent loss of life rather than building collapse, the main point of NIST's recommendations is that buildings should not collapse from fire even if sprinklers are unavailable.<ref name="ncstar1-a"/>

===Other investigations===
In 2003, Asif Usmani, Professor of Structural Engineering at ], published a paper with two colleagues. They provisionally concluded the fires alone, without any damage from the airplanes, could have been enough to bring down the buildings. In their view, the towers were uniquely vulnerable to the effects of large fires on several floors at the same time.<ref>{{cite journal |last=Usmani |first=A.S. |author2=Y. C. Chung |author3=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 |journal=Fire Safety Journal |volume=38 |issue=6 |pages=501–533 |publisher=Elsevier Ltd. |accessdate=2007-05-07 |doi=10.1016/S0379-7112(03)00069-9}}</ref> When the NIST report was published, Barbara Lane, with the UK engineering firm ], criticized its conclusion that the loss of fire proofing was a necessary factor in causing the collapses; "We have carried out computer simulations which show that the towers would have collapsed after a major fire on three floors at once, even with fireproofing in place and without any damage from plane impact."<ref>{{cite web |year=2005 |url=http://cruachan.televisual.co.uk/asset/GetArticle.exe?DB=e2&DATABASE=e2&LABEL=emap2&RECORD=194310&SEARCH=1 |title=Row erupts over why twin towers collapsed |publisher=New Civil Engineer |accessdate=2006-07-28}}</ref> ], formerly of the ] at the University of Edinburgh, pursued further research into the potentially catastrophic effects of ].<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 |title=Dalmarnock Full-Scale Experiments 25 & 26 July 2006 |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 |title=Glasgow tower block to shed light on 9/11 fire |work=The Scotsman |accessdate=2006-07-28}}</ref><ref>{{cite web |year=2007 |url=http://www.bbc.co.uk/sn/tvradio/programmes/horizon/broadband/tx/firegrid |title=Skyscraper Fire Fighters |work=BBC ] |accessdate=2007-07-31}}</ref>

==Aftermath==

===Other buildings===
]

Many of the surrounding buildings were also either damaged or destroyed as the towers fell. ] suffered a large fire and a partial collapse of its steel structure. Other buildings destroyed include ], ] (Marriott Hotel 3 WTC), ] (4 WTC), and ] (6 WTC). The World Financial Center buildings, ], and 130 Cedar Street suffered fires. The ], the ], and ] suffered impact damage from the towers' collapse, as did ]. ] 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, was deconstructed because of water, mold, and other severe damage caused by the neighboring towers' collapse.<ref>{{cite news |first=Nicholas |last=Varchaver |title=The tombstone at Ground Zero |date=March 20, 2008 |url=http://money.cnn.com/2008/03/19/news/companies/ground_zero.fortune/?postversion=2008032004 |publisher=CNN |accessdate=2011-04-11}}</ref><ref>{{cite web |url=http://www.lowermanhattan.info/construction/project_updates/130_liberty_street__77170.aspx#activities |title=130 Liberty Street: Project Updates |accessdate=2011-04-11 |date=March 25, 2011 |publisher=Lower Manhattan Construction Command Center}}</ref>

===Lost artworks===
Many works of art were destroyed in the collapse.
*''Ideogram'' (1967), stainless steel sculpture by ]
*''Cloud Fortress'' (1975), a large, black granite piece by Japanese artist ], destroyed in the 9/11 rescue and recovery efforts.
*'']'', a 20' x 35' tapestry by ] that hung in the South Tower Lobby.
*''Sky Gate, New York'' (1977–78), large wooden sculpture by ]
*A memorial fountain for the victims of the ] by ]
*'']'' (1971), a 25' red steel sculpture by ]. Approximately 30% of the sculpture was recovered.
*Some 300 sculptures and drawings by ], part of the ] collection.
*''Needle Tower'' (1968) by ].
*''Recollection Pond'', a tapestry by ].
*''Path Mural'' by ].
*''Commuter Landscape'', a large mural by ].
*''Fan Dancing with the Birds'', a mural by ].
*''The Entablature Series'' by ]
* Approximately 40,000 negatives of photographs by ] documenting the presidency of ].
* '']'', an abstract sculpture by ], survived the collapse but was seriously damaged, and now serves as a memorial.
Many other works of art and valuable artifacts, found in safe deposit boxes located throughout the towers, were also destroyed. Two other sculptures were damaged, but not destroyed by the attacks. These are '']'' by ] and '']'' by ], located down the street from the World Trade Center. They were repaired and still stand today.

===Cleanup===
] calls for 10 more rescue workers to make their way into the rubble of the World Trade Center.]]
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 ] (CDI) of ].<ref>Post, Nadine M. and Debra K. Rubin. "Debris Mountain Starts to Shrink." Engineering News Record, 10/1/01. </ref> It involved round-the-clock operations, many contractors and subcontractors, 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 |work=The Washington Post |date=October 23, 2006 |accessdate=2007-10-29}}</ref> The large pile of debris left on the site burned for three months, despite efforts to extinguish the blaze until the majority of the rubble was finally removed from the site.<ref>{{cite web |url=http://english.people.com.cn/200112/20/eng20011220_87119.shtml |title=WTC Fire Extinguished |work=People's Daily Online |date=December 20, 2001 |accessdate=2013-02-19}}</ref><ref>{{cite news |url=http://www.cbsnews.com/stories/2001/12/19/archive/main321907.shtml |publisher=CBS News |title=CBS News – WTC Fires All But Defeated – December 19, 2001 23:22:25 |date=December 19, 2001}}</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>{{cite web |url=http://www.construction.com/NewsCenter/Headlines/ENR/20011108c.asp |title=WTC Cleanup Agency Begins Ramping Up Operations |accessdate=2009-10-20 |last=Rubin |first=Debra K. |author2=Janice L. Tuchman |date=November 8, 2001 |work=Engineering News Record |publisher=McGraw-Hill |archiveurl=https://web.archive.org/web/20071012104219/http://construction.com/NewsCenter/Headlines/ENR/20011108c.asp |archivedate=October 12, 2007}}</ref> In 2007, the demolition of the surrounding damaged buildings was still ongoing as new construction proceeded on the World Trade Center's replacement, ].

===Health effects===
]
{{Main|Health effects arising from the September 11 attacks|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 ] (EPA) 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. 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. Asbestosis is such an illness, and ] would have been present in the dust.<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. </ref> In 2011 significant long term medical and psychological effects were found among first responders including elevated levels of ], ], ] and ].<ref>{{cite web |url=http://www.sciencedaily.com/releases/2011/09/110901214424.htm |title=First Long-term Study of WTC Workers Shows Widespread Health Problems 10 Years After Sept. 11 |work=ScienceDaily}}</ref>

The thousands of tons of toxic debris resulting from the collapse of the Twin Towers contained more than 2,500 contaminants, including known carcinogens.<ref>{{Cite news |first=Anita |last=Gates |title=Buildings Rise from Rubble while Health Crumbles |work=The New York Times |date=September 11, 2006 |url=http://www.nytimes.com/2006/09/11/arts/television/11dust.html?ref=nyregionspecial3 |accessdate=2011-09-04}}</ref><ref>{{Cite news |url=http://www.nytimes.com/imagepages/2006/09/05/nyregion/20060905_HEALTH_GRAPHIC.html |title=What was Found in the Dust |work=The New York Times |date=September 5, 2006 |accessdate=2011-09-04}}</ref> Subsequent ] among rescue and recovery workers are said to be linked to exposure to these carcinogens.<ref name=autogenerated2>{{Cite news |title=New York: 9/11 toxins caused death |publisher=CNN |date=May 24, 2007 |url=http://www.cnn.com/2007/US/05/24/wtc.dust/index.html |accessdate=2011-09-04 |archiveurl=https://web.archive.org/web/20070618154824/http://www.cnn.com/2007/US/05/24/wtc.dust/index.html |archivedate=June 18, 2007}}</ref><ref>{{Cite news |url=http://www.nytimes.com/2006/05/13/nyregion/13symptoms.html |title=Tracing Lung Ailments That Rose With 9/11 Dust |last=DePalma |first=Anthony |date=May 13, 2006 |work=The New York Times |accessdate=2011-09-04}}</ref> The Bush administration ordered the ] (EPA) to issue reassuring statements regarding air quality in the aftermath of the attacks, citing national security; however, the EPA did not determine that air quality had returned to pre-September 11 levels until June 2002.<ref>{{Cite news |last=Heilprin |first=John |title=White House edited EPA's 9/11 reports |work=Seattle Post-Intelligencer |date=June 23, 2003 |url=http://www.seattlepi.com/default/article/White-House-edited-EPA-s-9-11-reports-1122465.php?rand=26878.51 |accessdate=2011-09-04}}</ref>

Health effects also extended to residents, students, and office workers of Lower Manhattan and nearby ].<ref>{{cite web |title=Updated Ground Zero Report Examines Failure of Government to Protect Citizens |publisher=Sierra Club |year=2006 |url=http://www.sierraclub.org/groundzero/ |accessdate=2011-09-04}}</ref> Several deaths have been linked to the toxic dust, and the victims' names will be included in the World Trade Center memorial.<ref>{{Cite news |last=Smith |first=Stephen |url=http://www.cbsnews.com/stories/2008/04/28/national/main4049362.shtml |title=9/11 "Wall of Heroes" To Include Sick Cops |publisher=CBS News |date=April 28, 2008 |accessdate=2011-09-04}}</ref> Approximately 18,000 people have been estimated to have developed illnesses as a result of the toxic dust.<ref>{{cite news |url=http://www.bbc.co.uk/news/world-us-canada-14738140 |title=Toxic dust legacy of 9/11 plagues thousands of people |publisher=BBC News |first=David |last=Shukman |date=September 1, 2011 |accessdate=September 11, 2011}}</ref> There is also scientific speculation that exposure to various toxic products in the air may have negative effects on fetal development. A notable children's environmental health center is currently analyzing the children whose mothers were pregnant during the WTC collapse, and were living or working nearby.<ref>{{cite web |title=CCCEH Study of the Effects of 9/11 on Pregnant Women and Newborns |work=World Trade Center Pregnancy Study |publisher=Columbia University |year=2006 |url=http://www.familiesofseptember11.org/docs/CCCEH%20Study%20Intro.pdf |accessdate=2011-09-04 |format=PDF}}</ref> A study of rescue workers released in April 2010 found that all those studied had impaired lung functions, and that 30–40% were reporting little or no improvement in persistent symptoms that started within the first year of the attack.<ref>{{cite news |last=Grady |first=Denise |title=Lung Function of 9/11 Rescuers Fell, Study Finds |url=http://www.nytimes.com/2010/04/08/nyregion/08lung.html |accessdate=2011-09-04 |newspaper=The New York Times |date=April 7, 2010}}</ref>

Years after the attacks, legal disputes over the costs of illnesses related to the attacks were still in the court system. On October 17, 2006, a federal judge rejected New York City's refusal to pay for health costs for rescue workers, allowing for the possibility of numerous suits against the city.<ref>{{Cite news |first=Anthony |last=DePalma |title=Many Ground Zero Workers Gain Chance at Lawsuits |work=The New York Times |date=October 18, 2006 |url=http://www.nytimes.com/2006/10/18/nyregion/nyregionspecial3/18toxic.html?ref=nyregion |accessdate=2011-09-04}}</ref> Government officials have been faulted for urging the public to return to lower Manhattan in the weeks shortly after the attacks. Christine Todd Whitman, administrator of the EPA in the aftermath of the attacks, was heavily criticized by a U.S. District Judge for incorrectly saying that the area was environmentally safe.<ref>{{Cite news |first=Larry |last=Neumeister |url=http://www.sfgate.com/cgi-bin/article.cgi?file=/n/a/2006/02/02/national/a142556S81.DTL |title=Judge Slams Ex-EPA Chief Over Sept. 11 |agency=Associated Press |work=San Francisco Chronicle |date=February 2, 2006 |accessdate=2011-09-04 |archiveurl=https://web.archive.org/web/20080524084609/http://www.sfgate.com/cgi-bin/article.cgi?file=/n/a/2006/02/02/national/a142556S81.DTL |archivedate=May 24, 2008}}</ref> Mayor Giuliani was criticized for urging financial industry personnel to return quickly to the greater ] area.<ref>{{Cite news |last=Smith |first=Ben |title=Rudy's black cloud. WTC health risks may hurt Prez bid |url=http://www.nydailynews.com/archives/news/2006/09/18/2006-09-18_rudy_s_black_cloud__wtc_heal.html |publisher=Daily News |location=New York |date=September 18, 2006 |accessdate=2011-09-04}}</ref> Some Americans, alarmed at the prospect of flying, instead traveled by car. This resulted in an estimated 1,595 additional highway deaths in the ensuing year.<ref>{{cite book |last=Gardner |first=Daniel |title=The Science of Fear: Why We Fear the Things We Shouldn't—and Put Ourselves in Greater Danger |year=2008 |publisher=Dutton Adult |isbn=0-525-95062-1 |page=3 |url=http://www.amazon.com/Science-Fear-Shouldnt-Ourselves-Greater/dp/0525950621#reader_0525950621}}</ref>

==See also==
{{Portal|New York City|September 11 attacks}}
*]
*]

==References==

===Explanatory notes===
{{Reflist|group=note}}

===Citations===
{{reflist|30em}}

===Bibliography===
{{Refbegin|30em}}
*{{Cite book |last=Dwyer |first=Jim |author2=Kevin Flynn |year=2004 |title=] |publisher=] |isbn=0-8050-7682-4 |oclc=156884550}}
*{{Citation |last1=Corley |first1=G. |last2=Hamburger |first2=R. |last3=McAllister |first3=T. |year=2002 |editor=FEMA Report 403 <!-- not really the editor, but helps readability (supplies "in FEMA Report 403") --> |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations and Recommendations |url=http://www.fema.gov/library/viewRecord.do?fromSearch=fromsearch&id=1728 |publisher=] |chapter=Executive Summary| chapter-url = http://www.fema.gov/library/file?type=publishedFile&file=403_execsum.pdf&fileid=fa4606e0-50b9-11e0-be57-001cc4568fb6 |accessdate=2012-11-25 |ref={{harvid|FEMA(403) Executive Summary}} }}
*{{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=] |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 |author2=] |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|display-authors=etal}}
*{{Citation |last1=Eagar |first1=T. W. |last2=Musso |first2=C. |doi=10.1007/s11837-001-0003-1 |title=Why did the world trade center collapse? Science, engineering, and speculation |url=http://eagar.mit.edu/EagarPapers/Eagar185.pdf |journal=JOM |volume=53 |issue=12 |pages=8 |year=2001 |accessdate=2012-11-29 |bibcode=2001JOM....53l...8E}}
*{{cite journal |last=Bažant |first=Zdeněk P. |author2=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 |journal=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 |author2=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 journal |last=Usmani |first=A.S. |author2=Y. C. Chung |author3=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 |journal=Fire Safety Journal |volume=38 |issue=6 |pages=501–533 |publisher=Elsevier Ltd. |accessdate=2007-05-07 |doi=10.1016/S0379-7112(03)00069-9}}
*{{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 video |people=Seth Kramer, Daniel Miller |date=2002 |title=America Rebuilds: A Year at Ground Zero |medium=Television series |location=United States |publisher=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}}

===Further reading===
{{refbegin|30em}}
*{{Citation |chapter=Executive Summary | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |first1=G. |last1=Corley |first2=R. |last2=Hamburger |first3=T. |last3=McAllister |url=http://www.fema.gov/media-library-data/20130726-1512-20490-7075/403_execsum.pdf}}
*{{Citation | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |chapter=Chapter 1. Introduction |first1=T. |last1=McAllister |first2=J. |last2=Barnett |first3=J. |last3=Gross |first4=R. |last4=Hamburger |first5=J. |last5=Magnusson |url=http://www.fema.gov/media-library-data/20130726-1512-20490-1698/fema403_ch1.pdf |displayauthors=3}}
*{{Citation |chapter=Chapter 2. WTC 1 and WTC 2 | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |first1=R. |last1=Hamburger |first2=W. |last2=Baker |first3=J. |last3=Barnett |first4=C. |last4=Marrion |first5=J. |last5=Milke |first6=H. |last6=Nelson |url=http://www.fema.gov/media-library-data/20130726-1512-20490-1698/fema403_ch2.pdf |displayauthors=3}}
*{{Citation |chapter=Chapter 3. WTC 3 | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |first1=W. |last1=Baker |url=http://www.fema.gov/media-library-data/20130726-1512-20490-1698/fema403_ch3.pdf}}
*{{Citation |chapter=Chapter 4. WTC 4, 5, and 6 | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |first1=J. |last1=Barnett |first2=R. |last2=Gewain |first3=R. |last3=Gilsanz |first4=H. |last4=Nelson |url=http://www.fema.gov/media-library-data/20130726-1512-20490-1698/fema403_ch4.pdf |displayauthors=3}}
*{{Citation |chapter=Chapter 5. WTC 7 | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |first1=R. |last1=Gilsanz |first2=E. M. |last2=DePaola |first3=C. |last3=Marrion |first4=H. |last4=Nelson |url=http://www.fema.gov/media-library-data/20130726-1512-20490-1698/fema403_ch5.pdf |displayauthors=3}}
*{{Citation |chapter=Chapter 6. Bankers Trust Building | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |first1=R. |last1=Smilowitz |first2=A. |last2=Hapij |first3=J. |last3=Smilow |url=http://www.fema.gov/media-library-data/20130726-1512-20490-1698/fema403_ch6.pdf}}
*{{Citation |chapter=Chapter 7. Peripheral Buildings | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |first1=T. |last1=McAllister |first2=D. |last2=Biggs |first3=E. M. |last3=DePaola |first4=D. |last4=Eschenasy |first5=R. |last5=Gilsanz |url=http://www.fema.gov/media-library-data/20130726-1512-20490-1698/fema403_ch7.pdf |displayauthors=3}}
*{{Citation | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |chapter=Appendix A. Overview of Fire Protection in Buildings |first1=J. |last1=Milke |first2=V. |last2=Kodur |first3=C. |last3=Marrion |url=http://www.fema.gov/media-library-data/20130726-1511-20490-8496/403_apa.pdf}}
*{{Citation | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |chapter=Appendix B. Structural Steel and Steel Connections |first1=J. |last1=Fisher |first2=N. |last2=Iwankiw |url=http://www.fema.gov/media-library-data/20130726-1512-20490-8107/403_apb.pdf}}
*{{Citation | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |chapter=Appendix C. Limited Metallurgical Examination |first1=J. |last1=Barnett |first2=R. R. |last2=Biederman |first3=R. D. |last3=Sisson, Jr. |url=http://www.fema.gov/media-library-data/20130726-1512-20490-8452/403_apc.pdf}}
*{{Citation | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |chapter=Appendix D. WTC Steel Data Collection |first1=R. |last1=Gilsanz |first2=A. |last2=Massa |url=http://www.fema.gov/media-library-data/20130726-1512-20490-6875/403_apd.pdf}}
*{{Citation | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |chapter=Appendix E. Aircraft Information |url=http://www.fema.gov/media-library-data/20130726-1512-20490-6060/403_ape.pdf}}
*{{Citation | editor-first = T. | editor-last = McAllister |title=World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations |year=2002 |publisher=]/] |chapter=Appendix F. Structural Engineers Emergency Response Plan |first1=E. M. |last1=DePaola |url=http://www.fema.gov/media-library-data/20130726-1512-20490-3191/403_apf.pdf}}
*{{Cite journal |last1=Eagar |first1=T. W. |last2=Musso |first2=C. |doi=10.1007/s11837-001-0003-1 |title=Why did the world trade center collapse? Science, engineering, and speculation |journal=JOM |volume=53 |issue=12 |pages=8 |year=2001 |publisher=Springer |url=http://eagar.mit.edu/EagarPapers/Eagar185.pdf |bibcode=2001JOM....53l...8E}}
*{{Citation |last1=Banovic |first1=S. W. |last2=Foecke |first2=T. |last3=Luecke |first3=W. E. |last4=McColskey |first4=J. D. |last5=McCowan |first5=C. N. |last6=Siewert |first6=T. A. |last7=Gayle |first7=F. W. |year=2007 |journal=JOM |volume=59 |issue=11 |title=The role of metallurgy in the NIST investigation of the World Trade Center towers collapse |doi=10.1007/s11837-007-0136-y |publisher=Springer |pages=22–30 |url=http://www.tms.org/pubs/journals/JOM/0711/banovic-0711.html |bibcode=2007JOM....59k..22B |displayauthors=3}}
*{{cite journal|last1=Hammett|first1=Kingsley|title=WHY THE WORLD TRADE CENTER COLLAPSED: An Architect's Perspective|journal=Designer/Builder|date=November 2001|url=https://archive.org/details/WhyTheWorldTradeCentreCollapsed|accessdate=24 October 2015}}
{{refend}}

==External links==
{{Commons and category|Collapse of the World Trade Center|September 11 attacks}}
* The National Institute of Standards and Technology's page on the collapse of the WTC. Contains most recent developments in investigations and FAQs.
* (NIST)
* Early suggestion by University of Sydney engineering instructor about how the towers might have collapsed.
* 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.
*
* ] (Principal Investigator)

{{September 11 attacks}}
{{World Trade Center}}

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Revision as of 20:11, 23 November 2016

Circles is UnderArmourKid