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Encyclopedia > Collapse of the World Trade Center
Ground Zero debris with markup showing building locations.
Ground Zero debris with markup showing building locations.
Sept. 11, 2001 attacks
Timeline
Planning
September 11, 2001
Rest of September
October
Beyond October
Victims
Survivors
Foreign casualties
Hijacked airliners
American Airlines Flight 11
United Airlines Flight 175
American Airlines Flight 77
United Airlines Flight 93
Sites of destruction
World Trade Center
The Pentagon
Shanksville, Pennsylvania
Effects and aftermath
World political effects
World economic effects
Detentions
Airport security
Closings and cancellations
Conspiracy theories
Post 9/11
Audiovisual entertainment
Impact on popular culture
Local health
Response
Global Guardian
Government response
Rescue and recovery effort
Financial assistance
Operation Yellow Ribbon
Memorials and services
Perpetrators
Responsibility
Organizers
Miscellaneous
Communication
WTC collapse
Slogans and terms
Inquiries
U.S. Congressional Inquiry
9/11 Commission Report
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On September 11, 2001, the two main towers of the World Trade Center complex were each hit by aircraft as part of the September 11, 2001 attacks. The south tower (2 WTC) collapsed at 9:59 a.m., less than an hour after being hit, and the north tower (1 WTC) followed at 10:28 a.m., causing massive damage to the rest of the complex and nearby buildings. In all, 2,750 people inside and near the towers were killed, including the 157 passengers and crew aboard the two airplanes.[1] The Federal Emergency Management Agency (FEMA) issued a performance study of the buildings in May 2002, declaring the WTC design sound and attributing the collapses wholly to extraordinary factors beyond the control of the builders.[2] In its September 2005 report, the National Institute of Standards and Technology (NIST) concurred with this view, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. It did add, however, that the towers' stairwell design lacked adequate reinforcement.[3] The collapse of 7 World Trade Center occurred at 5:20 p.m. with no casualties, and was the result of structural damage sustained during the collapse of Towers 1 and 2, combined with widespread fires in the building.[4] Image File history File links Download high resolution version (2992x2242, 2413 KB) A photograph of the of the World Trade Center site (Ground Zero) with an overlay showing the original building locations. ... Image File history File links Download high resolution version (2992x2242, 2413 KB) A photograph of the of the World Trade Center site (Ground Zero) with an overlay showing the original building locations. ... A sequential look at United Flight 175 crashing into the south tower of the World Trade Center The September 11, 2001 attacks (often referred to as 9/11—pronounced nine eleven or nine one one) consisted of a series of coordinated terrorist[1] suicide attacks upon the United States, predominantly... September 11, 2001 attacks timeline Background History 1972: One World Trade Center completed. ... THIS ARTICLE CONTAINS THE PHRASE FUCK THE US GOVERNMENT HIDDEN THROUGHOUT THE ARTICLE...IT REQUIRES IMMEDIATE CLEAN UP // According to the presidents of the United States and the Philippines, the September 11, 2001 attacks originated with Operation Bojinka (a plan that was not executed), which was conceived by Khalid Shaikh... The September 11, 2001 attacks, in addition to being a unique act of aggression, constituted a media event on a scale not seen since the advent of civilian global satellite links, round-the-clock television news organizations and the instant worldwide reaction and debate made possible by the Internet. ... All times, except where otherwise noted, in New York Time (EDT). ... All times, except where otherwise noted, in New York Time (EDT). ... // 2001 November Thursday, November 1, 2001 Afternoon: Deputy Mayor Anthony P. Coles meets with the two firefighter union leaders, Fire Capt. ... According to the 9/11 Commission, between 16,400 and 18,800 civilians were in the World Trade Center complex at the time of the attacks. ... This does not cite its references or sources. ... Flight 11 redirects here. ... United Airlines Flight 175 was a morning flight that regularly flew from Logan International Airport in East Boston, Massachusetts to Los Angeles International Airport (LAX) in Los Angeles, California. ... Security camera image showing American Airlines Flight 77 (highlighted) just before and after impact. ... For other uses of Flight 93 and United 93, see Flight 93. ... For other uses, see World Trade Center (disambiguation). ... This article is about the United States military building. ... Main Street in Shanksville. ... The September 11, 2001 attacks defined the first term of President George W. Bush and led to what he has called the War on Terrorism, or war against terrorism. ... The following worldwide effects arose from the September 11, 2001 attacks: All Canadian military bases increased their level of security awareness. ... Major economic effects arose from September 11, 2001 attacks, with initial shock causing global stock markets to drop sharply. ... Soon after the attacks of September 11, 2001, the United States Government began detaining people who fit the profile of the suspected hijackers: mostly male, Arabic or Muslim noncitizens. ... Box-cutter knives were apparently used in the September 11, 2001 attacks, though such knives are not usually considered weapons. ... Many closings and cancellations followed the September 11, 2001 attacks, including major landmarks, buildings, as well as postponement or cancellation of major sporting and other events. ... A conspiracy theories have emerged which contradict the mainstream account of the September 11, 2001 attacks. ... This article talks about the aftermath of the September 11, 2001 attacks. ... The September 11, 2001, attacks had an important impact on the audiovisual entertainment business, not just in terms of television coverage. ... The September 11, 2001 attacks have been the subject of numerous films and other works of art and literature, including: // Hollywoods first reaction to the September 11 attacks was to alter, delay or even cancel films that unintentionally evoked the disaster. ... There has been growing concern over the health effects of the September 11, 2001 attacks in the Financial District of lower Manhattan. ... Global Guardian is an annual training exercise sponsored by the United States Strategic Command in conjunction with Space Command and NORAD. Its main purpose is to test the militarys command and control procedures in the event of nuclear warfare. ... The response of the U.S. government to the September 11, 2001 attacks sparked investigations into the motivations and execution of the attacks, as well as the ongoing War on Terrorism in Iraq. ... A bucket brigade works to clear rubble and debris on September 14, 2001 The area surrounding the World Trade Center became the site of the greatest number of casualties and missing, and physical destruction. ... Charities and relief agencies raised over $657 million in the three weeks following the September 11, 2001 attacks, the vast bulk going to immediate survivors and victims families. ... Operation Yellow Ribbon is the name of the operation that Transport Canada created to handle the diversion of civilian airline flights following the September 11, 2001, attacks. ... The first memorials to the victims of the September 11, 2001 attacks began to take shape online, as hundreds of webmasters posted their own thoughts, links to the Red Cross, and other rescue agencies, photos and eyewitness accounts. ... The United States government identified 19 hijackers as being responsible for the September 11, 2001 attacks, and linked the attacks to Osama bin Laden. ... The September 11, 2001 attacks were carried out by 19 hijackers, with planning and organization of the attacks involving numerous additional members of al-Qaeda. ... Communications problems and successes played an important role in the September 11, 2001 attacks and their aftermath. ... The September 11, 2001 attacks on the United States spawned a number of catchphrases, terms, and slogans, many of which continue to be used a half-decade after the event. ... The Joint Inquiry into Intelligence Community Activities before and after the Terrorist Attacks of September 11, 2001 is the official name of the inquiry conducted by the Senate Select Committee on Intelligence and the House Permanent Select Committee on Intelligence into the activities of the U.S. Intelligence Community in... The cover of the final 9/11 report 9/11 Commission Report, formally titled Final Report of the National Commission on Terrorist Attacks Upon the United States, is the official report of the events leading up to the September 11, 2001 attacks. ... is the 254th day of the year (255th in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... For other uses, see World Trade Center (disambiguation). ... A sequential look at United Flight 175 crashing into the south tower of the World Trade Center The September 11, 2001 attacks (often referred to as 9/11—pronounced nine eleven or nine one one) consisted of a series of coordinated terrorist[1] suicide attacks upon the United States, predominantly... FEMA redirects here. ... NIST logo The National Institute of Standards and Technology (NIST, formerly known as The National Bureau of Standards) is a non-regulatory agency of the United States Department of Commerce’s Technology Administration. ... For other uses of terms redirecting here, see US (disambiguation), USA (disambiguation), and United States (disambiguation) Motto In God We Trust(since 1956) (From Many, One; Latin, traditional) Anthem The Star-Spangled Banner Capital Washington, D.C. Largest city New York City National language English (de facto)1 Demonym American... 7 World Trade Center is a building in New York City located across from the World Trade Center site in Lower Manhattan. ...

Contents

Design issues

The towers were designed as framed tube structures, which provided tenants with open floor plans, uninterrupted by columns or walls. This was accomplished using numerous, closely-spaced perimeter columns to provide much of the strength to the structure, along with gravity load shared with the core columns. Above the seventh floor there were 59 perimeter columns along each face of the building and there were 47 heavier columns in the core.[5] All of the elevators and stairwells were located in the core, leaving a large column-free space between the perimeter that was bridged by prefabricated floor trusses.[6] View of the World Trade Centers construction from across the Hudson River The building of the World Trade Center was conceived as an urban renewal project, spearheaded by David Rockefeller, to help revitalize Lower Manhattan. ...


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.[6] 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 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.[6] This article is about the construction material. ... Viscoelasticity, also known as anelasticity, describes materials that exhibit both viscous and elastic characteristics when undergoing plastic deformation. ...


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.[6]


Fire and aircraft potentials

Like all modern skyscrapers, WTC towers were designed to survive major fires, but not necessarily those that involved aviation fuel. Though fireproofing had been incorporated in the original construction, more was added after a fire in 1975 that spread to six floors before being extinguished.[2] Early tests conducted on steel beams from the WTC show they generally met or were stronger than design requirements.[7]


Designers had also considered the consequences of aircraft impact. In 1993, John Skilling, who had been in charge of the structural design of the buildings, said that an aircraft impact would cause a great deal of damage and loss of life, mainly because of the ensuing fires, but the structure would not collapse.[8] After the 2001 attacks, Leslie Robertson, who had participated in the structural design of the towers, said that the towers had in fact been designed to withstand the impact of the largest airliner of the day, the Boeing 707-320, in the event one was lost in fog while looking to land. According to Robertson, the modeled aircraft weighed 263,000 lb (119 metric tons) with a flight speed of 180 mph (290 km/h), as in approach and landing. As FEMA pointed out in its report, this implies a slower and smaller plane than those involved in the actual impacts of 9/11.[2] Robertson also said that they lacked a good understanding of the effects of such large fires on the structures.[9] Pan Am 707 The Boeing 707 is a four engined commercial passenger jet aircraft developed by Boeing in the early 1950s. ...


The National Institute of Standards and Technology, however, was unable to document the study reported by Robertson and FEMA. Instead it found a reference to a study of the effects of a Boeing 707 carrying 23,000 US gallons (87 m³) of fuel hitting the buildings at 600 mph (1,000 km/h), which would not only be faster than either of the two planes that hit on 9/11 but with twice the fuel quantity. In line with Skilling's remarks, this study apparently found that the buildings would not collapse in that event. But NIST was unable to find any further details about the study and stated that without the original calculations which were used to render such conclusions, any attempt to compare the performance of the buildings to design expectations would be "speculation". In examining the collapse of the towers, NIST also stated that, "No building code in the United States has specific design requirements for impact of aircraft, and thus, buildings are not specifically designed to withstand the impact of fuel-laden commercial aircraft."[10] Scenario planning or Scenario thinking is a strategic planning method that some organizations use to make flexible long-term plans. ...


Impacts of airliners

Impact locations for 1 and 2 WTC
Impact locations for 1 and 2 WTC
Impact locations on 1 and 2 WTC
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. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... Image File history File links World_Trade_Center_9-11_Attacks_Illustration_with_Bird's-eye_Impact_Locations. ... Image File history File links World_Trade_Center_9-11_Attacks_Illustration_with_Bird's-eye_Impact_Locations. ... American Airlines Boeing 767-300 at Gatwick Airport, England. ... Flight 11 redirects here. ... United Airlines Flight 175 was a morning flight that regularly flew from Logan International Airport in East Boston, Massachusetts to Los Angeles International Airport (LAX) in Los Angeles, California. ...


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).[11] 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).[6] In addition to severing a number of load-bearing columns, the resulting explosions in each tower ignited 38 m³ (10,000 gallons)[2] 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 including some who were on fire.


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 probably burned out in less than ten minutes, but the contents of the buildings burned over the next hour or hour and a half.[12] 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.[13][14] 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.[15]


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.[16] Deteriorating conditions were also reported by the helicopters of the NYPD aviation unit.[16] For other uses, see 911 (disambiguation). ...

  • 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.[16]

  • 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 NYPD, and the FDNY were experiencing problems with 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.[17][18][19] NYPD redirects here. ... The New York City Fire Department or the Fire Department of New York (FDNY) has the responsibility for protecting the citizens and property of New York Citys five boroughs from fires and fire hazards, providing emergency medical services, technical rescue as well as providing first response to biological, chemical... This article reviews some of the details of radio communications at the September 11, 2001 World Trade Center tragedy. ...


Collapse of the twin towers

A view of New York City after the first of two complete structural collapses of the World Trade Center towers

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. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...


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.[3]


The impacts also dislodged some of the fireproofing from the steel, increasing its exposure to the heat of the fires. In the 102 minutes before the collapse of 1 WTC, the fires reached temperatures that, although well below the melting point, were high enough to weaken the core columns so that they underwent plastic deformation and creep from the weight of higher floors. The NIST report provides a useful model of the situation. In engineering mechanics, deformation is a change in shape due to an applied force. ... Creep is the term used to describe the tendency of a material to move or to deform permanently to relieve stresses. ...

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.[3]


Total progressive collapse

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
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

Enormous advancing dust clouds obscured the collapses, making it impossible to calculate the collapse times through visual evidence, but analysis of seismic data from the nearby Lamont-Doherty Earth Observatory at Columbia University shows that the north tower collapsed in 12.73 seconds, or 57.7% longer than free fall, and that the duration of the south tower collapse was 10.53 seconds, or 42% longer than free fall.[20] Image File history File linksMetadata Download high-resolution version (3000x2400, 2890 KB) Ground Zero, New York City, N.Y. (Sept. ... Image File history File linksMetadata Download high-resolution version (3000x2400, 2890 KB) Ground Zero, New York City, N.Y. (Sept. ... Lamont-Doherty Earth Observatory is a world-class research institution specializing in earth sciences and part of Columbia University. ... Alma Mater Columbia University is a private university in the United States and a member of the Ivy League. ...


While the NIST report analyzes the initial failure mechanism in detail, it does not address the subsequent total collapse of the WTC towers. 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,[21] crushing it and adding to the kinetic energy. This scenario repeated with each successive story, crushing the entire tower at an ever-increasing pace. While it is the most widely held view among engineers,[22] it has been been criticized for ignoring the resistance of the underlying structure, which may have slowed a progressive collapse much more dramatically and even prevented it altogether.[23]


Collapse of 7 World Trade Center

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

The WTC complex comprised seven buildings, three of which completely collapsed on September 11, 2001. 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 WTC 7 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.[24] Image File history File linksMetadata Size of this preview: 511 × 600 pixel Image in higher resolution (1167 × 1370 pixel, file size: 485 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): 7 World Trade... Image File history File linksMetadata Size of this preview: 511 × 600 pixel Image in higher resolution (1167 × 1370 pixel, file size: 485 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): 7 World Trade... 7 World Trade Center is a building in New York City located across from the World Trade Center site in Lower Manhattan. ... 7 World Trade Center is a building in New York City located across from the World Trade Center site in Lower Manhattan. ... An optical theodolite, manufactured in the Soviet Union in 1958 and used for topographic surveying. ...


FEMA's provisional study was inconclusive[25] 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 of 2005. With the exception of a letter to the Journal of Metallurgy, which suggested that some of the structural steel had been exposed to temperatures sufficient to melt it,[26] no studies of the collapse of 7 WTC have been published in scientific journals.


As of August, 2006, NIST anticipated that it would release a draft report on the collapse of 7 WTC in early 2007.[27] It released a progress report in June of 2004 outlining its working hypothesis. On this hypothesis a local failure in a critical column, caused by damage from either fire or falling debris from the collapses of the two towers, progressed first vertically and then horizontally to result in "a disproportionate collapse of the entire structure".[28][29] In answer to the question of whether "a controlled demolition hypothesis is being considered to explain the collapse", NIST says that it "would like to determine the magnitude of hypothetical blast scenarios that could have led to the structural failure of one or more critical elements."


On June 29, 2007, NIST issued a press release stating that it expected to release its draft report on the collapse of 7 WTC for public comment by the end of 2007.[30]


Some authors have cited the decision to locate the Office of Emergency Management headquarters on the 23rd floor of 7 WTC as a possible contributing factor to the collapse of the building,[31] noting especially the placement of large diesel tanks, against warnings from the Fire Department, to provide backup power for the facility.[32]


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."[33] 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.[22] 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."[34]


Authority

Immediately following the collapses, there was some confusion about who had the authority to carry out an official investigation. In contrast to, for example, aircraft accidents, there were no clear procedures in the case of building collapses.[35]


A team was quickly assembled by the Structural Engineers Institute (SEI) of the American Society of Civil Engineers, with the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association, and the Society of Fire Protection Engineers also involved.[36] ASCE also invited FEMA to join the investigation, which later became a joint ASCE-FEMA effort.[36] This investigation was criticized by engineers and lawmakers in the US, however, for its limited funding, authority to conduct an investigation, and access to the WTC site. One major point of contention at the time was that the cleanup of the WTC site was resulting in the destruction of the majority of the buildings' steel components.[37] 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.[3] “ASCE” redirects here. ... AISC means: American Indian Student Commission American Institute of Steel Construction [1] Category: ... The National Fire Protection Association (established 1896) is an independent, voluntary-membership, nonprofit (tax-exempt) organization. ... The Society of Fire Protection Engineers (SFPE) is a professional society for fire protection engineering established in 1950. ... New FEMA seal The Federal Emergency Management Agency or FEMA is an agency of the United States government dedicated to swift response in the event of disasters, both natural and man-made. ...


FEMA published its report in May of 2002. While NIST had already announced its intention to investigate the collapses in August of the same year, by September 11, 2002, a year after the disaster, there was growing public pressure for a more thorough investigation.[38] and Congress passed the National Construction Safety Team bill in October 2002. This provided the authority for the NIST investigation, which published its results in September of 2005.[39] is the 254th day of the year (255th in leap years) in the Gregorian calendar. ... Also see: 2002 (number). ...


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 especially by Thomas Eagar and popularized by PBS.[40] On this view, when the connections between the floor trusses and the columns broke, the floors fell down, one on top of the other, quickly exceeding the load that any one floor was designed to carry.[41] 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.[42][14]

A woman stands in the gash in 1 WTC.

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".[41] 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.[43][44] 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. Image File history File links Edna_Cintron_standing_in_WTC1. ... Image File history File links Edna_Cintron_standing_in_WTC1. ... Mapúa Institute of Technology (MIT, MapúaTech or simply Mapúa) is a private, non-sectarian, Filipino tertiary institute located in Intramuros, Manila. ... Jet fuel is a type of aviation fuel designed for use in jet-engined aircraft. ...


Early column failure theories

However, 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 that was very close to the current consensus already on September 21, 2001. is the 264th day of the year (265th in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ...

Some 60 tons or more of jet fuel could have easily caused sustained high temperatures of 1,500 °F [820 °C] and higher. Under these conditions, structural steel loses rigidity and strength. The resulting failure of the 2-3 floor system at the site of impact sent the 20 to 25 floors above free-falling onto the 80 to 85 floor structure below. The enormous energy released by this collapse was too large to be absorbed by the structure below. That impact may have ultimately caused the explosive buckling, floor after floor, of the WTC towers. Similar to a car crash in a wall, the towers crashed into the ground with an almost free-fall velocity.[45]

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.[46]


The NIST report

Design of the study

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.
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.[47] The study included in-house technical expertise and drew upon the knowledge of several outside private institutions for aid to include: Image File history File links Download high-resolution version (2076x1088, 377 KB) [edit] Summary Photo of World Trade Center 3 with remains of WTC1 (left background) and WTC2 (right foreground) visible. ... Image File history File links Download high-resolution version (2076x1088, 377 KB) [edit] Summary Photo of World Trade Center 3 with remains of WTC1 (left background) and WTC2 (right foreground) visible. ... NIST logo The National Institute of Standards and Technology (NIST, formerly known as The National Bureau of Standards) is a non-regulatory agency of the United States Department of Commerce’s Technology Administration. ...

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

Note: This page soon to be renamed National Fire Protection Association. ... AISC means: American Indian Student Commission American Institute of Steel Construction [1] Category: ... The Council on Tall buildings and Urban Habitat (CTBUH) was founded at Lehigh University in 1969. ...

Scope and limits

The scope of the NIST investigation was limited to "the sequence of events from the instant of aircraft impact to the initiation of collapse for each tower." In line with the concerns of most engineers, NIST focused on the airplane impacts and the spread and effects of the fires, modeling these at a very high level of detail. NIST developed several highly detailed structural models for specific sub-systems such as the floor trusses as well as a global model of the towers as a whole which is less detailed. These models are static or quasi-static, including deformation but not the motion of structural elements after rupture as would dynamic models. So, the NIST models are useful for determining how the collapse was triggered, but do not shed light on events after that point. As stated in the report, it "includes little analysis of the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable." (p. xxxvii, fn2) Some engineers have suggested that our understanding of the collapse mechanism could be improved by developing an animated sequence of the collapses based on a global dynamic model, and comparing it with the video evidence of the actual collapses.[48] Statics is the branch of physics concerned with physical systems in static equilibrium, that is, in a state where the relative positions of subsystems do not vary over time, or where components and structures are at rest under the action of external forces of equilibrium. ... In engineering mechanics, deformation is a change in shape due to an applied force. ... In physics, dynamics is the branch of classical mechanics that is concerned with the effects of forces on the motion of objects. ...


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.[49] 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.[50] Recently, Jose L Torero, from the BRE Centre for Fire Safety Engineering at the University of Edinburgh, is pursuing further research into the potentially catastrophic effects of fire on real-scale buildings.[51][52][53] The University of Edinburgh (Scottish Gaelic: ), founded in 1582,[4] is a renowned centre for teaching and research in Edinburgh, Scotland. ... Arup is a professional services firm providing engineering, design, planning, project management and consulting services for all aspects of the built environment. ... Professor José Luis Torero (born in Lima, Peru) is a professor in Fire Safety Engineering at the University of Edinburgh. ... The BRE Centre for Fire Safety Engineering is the research group at The Institute of Infrastructure and Environment, University of Edinburgh conduction research in fire, structures and environment. ... The Dalmarnock tests were a series of fire experiments conducted in a real high-rise. ...


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.[54] He also noted that the report lacks a timeline and physical evidence to support its conclusions.[55]


In October of 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 Sheffiled called on NIST to produce computer visualizations of the collapses in order to correlate the collapse models with observed events.[56] New Civil Engineer is the weekly magazine of the Institution of Civil Engineers, the UK chartered body that oversees the practice of civil engineering in the UK. New Civil Engineer was a co-founder of the British Construction Industry Awards. ...


Remarks by Osama bin Laden

Although the translation is contested,[57][58] a videotape of Osama bin Laden was released that was verified by the Pentagon as indicating that Bin Laden had not believed that the buildings would collapse completely, but would collapse only above the levels where the planes struck: There have been several videos released by Osama bin Laden. ... Osama bin Muhammad bin Awad bin Laden (Arabic: ‎; born March 10, 1957[1]), most often mentioned as Osama bin Laden or Usama bin Laden, is a Saudi Arabian militant Islamist and is widely believed to be one of the founders of the organization called al-Qaeda. ...

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.[59]

Other buildings

Portions of the outer shell of the North Tower lean against the remains of 6 WTC which suffered massive damage when the North Tower collapsed. The remains of 7 WTC are at upper right
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 WTC suffered a large fire and a partial collapse of its steel structure. Image File history File linksMetadata Download high-resolution version (3000x2400, 3036 KB) Ground Zero, New York City, N.Y. (Sept. ... Image File history File linksMetadata Download high-resolution version (3000x2400, 3036 KB) Ground Zero, New York City, N.Y. (Sept. ... is the 254th day of the year (255th in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... 5 WTC, upper right corner. ...


Other buildings destroyed include St. Nicholas Greek Orthodox Church, Marriott World Trade Center (Marriott Hotel 3 WTC), South Plaza (4 WTC), and U.S. Customs (6 WTC). The World Financial Center buildings, 90 West Street, and 130 Cedar Street suffered fires. The Deutsche Bank Building, Verizon, and World Financial Center 3 suffered impact damage from the towers' collapse, as did 90 West Street. One Liberty Plaza survived structurally intact but sustained surface damage including shattered windows. 30 West Broadway was damaged by the collapse of 7 WTC. The Deutsche Bank Building, known through images of it being covered in a large black 'shroud' after September 11 to cover the building's damage, is currently being deconstructed because of water,[60] mold, and other severe damage caused by the neighboring towers' collapse. On August 18, 2007, a 7-alarm blaze was ignited on the 17th floor due to careless smoking. The fire left two firefighters dead from smoke inhalation and carbon monoxide poisoning. St. ... Marriott with Twin Towers, July 2001 The Marriott World Trade Center Hotel was a 22-story[1] steel-framed building with 825 rooms. ... World Trade Center 4 was a 9-story low-rise office building in the southeast corner of the site, in Lower Manhattan, New York City. ... 6 World Trade Center was the U.S. Customs House: a 537,693 square foot (49,953 m²), 7 story low rise building, in Lower Manhattan, New York City. ... 90 West Street or West Street Building is a building in Lower Manhattan designed by architect Cass Gilbert for the West Street Improvement Corporation. ... For the current Deutsche Bank building, see 60 Wall Street View from the southeast in 1997. ... Three World Financial Center Three World Financial Center is one of the largest skyscrapers in New York City, located on West Street between Liberty and Vesey Streets in Lower Manhattan. ... One Liberty Plaza is a skyscraper in Manhattan, New York, which currently resides at the location of the former Singer Building (in 1968, the tallest building to be demolished)). The building is 743 feet (225m) high and 54 floors. ... is the 230th day of the year (231st in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ...


Aftermath

Site cleanup

The cleanup of the WTC site was coordinated by the City of New York Department of Design and Construction (DDC). On September 22, a preliminary cleanup plan was delivered by Controlled Demolition Inc. (CDI) of Phoenix. Mark Loizeaux, president of CDI, emphasized the importance of protecting the slurry wall (or "bathtub") which kept the Hudson river from flooding the WTC's basement.[61] is the 265th day of the year (266th in leap years) in the Gregorian calendar. ...


The cleanup of the WTC site was a massive operation. It involved round-the-clock operations, many contractors and sub-contractors, and cost hundreds of millions of dollars.[62] 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 occasioned confrontations with firefighters.[63]


Demolition of surrounding damaged buildings was continuing in 2007 even as new construction proceeded on the World Trade Center's replacement, the Freedom Tower. For the building in Miami, Florida of the same name, see Freedom Tower (Miami). ...


The debris smoldering fires

The colossal pile of debris left on the site burned for three months, resisting attempts to extinguish the blaze until finally the majority of the rubble had actually been removed from the site.[64][65]


Air quality and the EPA's response

On September 18, 2001 the Environmental Protection Agency issued a statement assuring the public that the air in Manhattan was "safe to breathe".[66] 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.[67] The EPA 9/11 pollution controversy was a result of a report released by the Office of the Inspector General of the Environmental Protection Agency in August of 2003 which claimed that the White House put pressure on the EPA to delete cautionary information about the air quality in New... is the 261st day of the year (262nd in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ...


Controlled demolition conspiracy theories

According to a 2006 poll, 16 percent of polled American adults speculate that the World Trade Center was destroyed by controlled demolition, not by the effects of the airplanes.[68] The theory was rejected by NIST, which concluded that there were no explosives or controlled demolition involved in the collapses of the WTC towers,[27] and the only prominent, peer-reviewed engineering publication to consider it,[22] and is pursued mainly as part of larger conspiracy theories about the events of 9/11.[69] The South Tower destruction viewed from across the Hudson River According to the controlled demolition hypothesis, the World Trade Center was not destroyed by the planes that crashed into it as part of the September 11th attacks, nor by the fires that followed, but by explosives or other devices planted... A conspiracy theories have emerged which contradict the mainstream account of the September 11, 2001 attacks. ...


See also

There has been growing concern over the health effects of the September 11, 2001 attacks in the Financial District of lower Manhattan. ... Proposal for Rebuilt Twin Towers, from Triroc. ...

References

Cited references

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  68. ^ 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.[8]
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Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 288th day of the year (289th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 231st day of the year (232nd in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 193rd day of the year (194th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 191st day of the year (192nd in leap years) in the Gregorian calendar. ... Zdeněk Pavel Bažant (b. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 260th day of the year (261st in leap years) in the Gregorian calendar. ... 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A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 280th day of the year (281st in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 132nd day of the year (133rd in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 260th day of the year (261st in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 195th day of the year (196th in leap years) in the Gregorian calendar. ... Wayne Barrett is a writer for the Village Voice. ... Dan Collins (born 26 February 1987 in Syracuse, New York) is a major junior ice hockey forward. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 298th day of the year (299th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... is the 359th day of the year (360th in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... is the 254th day of the year (255th in leap years) in the Gregorian calendar. ... Also see: 2002 (number). ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 177th day of the year (178th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 177th day of the year (178th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 127th day of the year (128th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 212th day of the year (213th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 91st day of the year (92nd in leap years) in the Gregorian calendar. ... is the 354th day of the year (355th in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 254th day of the year (255th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 296th day of the year (297th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 302nd day of the year (303rd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 239th day of the year (240th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 345th day of the year (346th in leap years) in the Gregorian calendar. ...

References

  • Dwyer, Jim (2004). 102 Minutes: The Untold Story of the Fight to Survive Inside the Twin Towers. Times Books. ISBN 0805076824. 
  • National Institute of Standards and Technology, Technology Administration (2006). NIST and the World Trade Center. NIST building and fire safety investigation. US Department of Commerce. Retrieved on 2006-05-02.
  • Wilkinson, Tim (2006). World Trade Center - Some Engineering Aspects. School of Civil Engineering, The University of Sydney. Retrieved on 2006-05-02.
  • McAllister, Therese; W. Gene Corley, et al. (2002). World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations (FEMA 403) (pdf). Federal Emergency Management Agency. Retrieved on 2006-05-02.
  • 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.
  • Bažant, Zdeněk P.; Yong Zhou (2001). Why Did the World Trade Center Collapse? - Simple Analysis (pdf). Journal of Engineering Mechanics ASCE, 9/13/01. Northwestern University. Retrieved on 2006-09-11.
  • Clifton, G. Charles (2001). Collapse of the World Trade Centre Towers. CAD Digest. TenLinks, Inc.. Retrieved on 2006-05-02.
  • Edgar, Dr. Thomas; NOVA (2002). The Collapse: An Engineer's Perspective. Why the Towers Fell. WGBH Educational Foundation. Retrieved on 2006-05-02.
  • Usmani, A.S.; Y. C. Chung, J. L. Torero (2003). How did the WTC towers collapse: a new theory (pdf). Fire Safety Journal, Volume 38, Issue 6. Elsevier Ltd.. Retrieved on 2007-05-07.
  • NOVA online (2002). The structure of metal. Why the Towers Fell. WGBH Educational Foundation. Retrieved on 2006-05-02.
  • Kean, Thomas H. (2004). Eleventh Public Hearing. Hearings. National Commission on Terrorist Attacks Upon the United States. Retrieved on 2006-05-02.
  • Seth Kramer, Daniel Miller. (2002) America Rebuilds: A Year at Ground Zero [Television series]. United States: PBS home video.

Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... May 2 is the 122nd day of the year in the Gregorian calendar (123rd in leap years). ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... May 2 is the 122nd day of the year in the Gregorian calendar (123rd in leap years). ... Dr. W. Gene Corley, P.E. is a structural engineer and preeminent expert on building collapse investigations and building codes. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... May 2 is the 122nd day of the year in the Gregorian calendar (123rd in leap years). ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... May 2 is the 122nd day of the year in the Gregorian calendar (123rd in leap years). ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 254th day of the year (255th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... May 2 is the 122nd day of the year in the Gregorian calendar (123rd in leap years). ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... May 2 is the 122nd day of the year in the Gregorian calendar (123rd in leap years). ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 127th day of the year (128th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... May 2 is the 122nd day of the year in the Gregorian calendar (123rd in leap years). ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... May 2 is the 122nd day of the year in the Gregorian calendar (123rd in leap years). ...

External links

Wikimedia Commons has media related to:
9/11
  • Bill Biggart's Final Exposures contains a photo of the WTC Marriott severely damaged by the collapse of 2 WTC immediately before the collapse of 1 WTC in which the photographer, Bill Biggart, was killed.
  • Video showing fire in corner of 2 WTC
  • Forensic animations used in court
  • World Trade Center - Some Engineering Aspects University of Sydney engineering instructor Tim Wilkinson writes an article explaining how the towers collapsed.
  • Victims of the World Trade Center Collapse

  Results from FactBites:
 
Definition of Collapse of the World Trade Center (2148 words)
As a result of the September 11, 2001 Attacks, both towers of the World Trade Center collapsed within a short period of the initial impacts, killing thousands.
The north tower collapsed directly downwards, "pancaking" in on itself, while the south tower fell at an angle during which the top 20 or so stories of the building remained intact for the first few seconds of the collapse.
A federal technical building and fire safety investigation of the collapses of the Twin Towers and 7 WTC is being conducted by the United States Department of Commerce's National Institute of Standards and Technology (NIST).
  More results at FactBites »

 
 

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