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] ] looks up at the remains of the World Trade Center, two days after its collapse]] ] ] looks up at the remains of the World Trade Center, two days after its collapse]]
Following the ] that sent one hijacked airliner into each of the two main towers of the ] complex, 1 WTC, 2 WTC and 7 WTC collapsed. Both main towers lost structural integrity and fell that morning, killing 2,595 people within and nearby, as well as 157 people who were aboard the flights. Other nearby buildings, including 7 WTC, were damaged by the debris. 7 WTC was evacuated, and collapsed seven hours later at 5:20pm, without casualties. Following the ] that sent one hijacked airliner into each of the two main towers of the ] complex, 1 WTC, 2 WTC and 7 WTC collapsed. Both main towers lost structural integrity and fell that morning, killing 2,595 people within and nearby, as well as 157 people who were aboard the flights. Other nearby buildings, including 7 WTC, were damaged by the debris. 7 WTC was evacuated, and collapsed seven hours later at 5:20pm, without casualties.

A titled "World Trade Center Building Performance Study"<ref>Federal Emergency Management Agency, .pdf, ''World Trade Center Building Performance Study'', URL accessed ], ]</ref> issued by the ] (FEMA) in May 2002, pronounced the WTC design fundamentally safe and attributed the collapse wholly to extraordinary factors beyond the control of the builders.

The ] (NIST) created a computer model of each building to determine whether any unusual structural features of the Twin Towers might have been at fault. In 2005, NIST issued a series of reports<ref>National Institutes of Standards and Technology, , (4/25/2006), URL accessed ], ]</ref> documenting emergency response efforts and events leading up to the collapse. NIST concluded "the buildings would likely not have collapsed under the combined effects of aircraft impact and the subsequent jet-fuel ignited multi-floor fires, if the fireproofing had not been dislodged or had been only minimally dislodged by aircraft impact." NIST also found the Towers' stairwell design lacked adequate reinforcement.

The FEMA and NIST reports have not resolved all disagreements among engineers. ''New Civil Engineer'' published an article titled ''Row erupts over why twin towers collapsed'', in which one party claims "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." Fireproofing was added after a fire in 1975<ref>Federal Emergency Management Agency, .pdf, ''World Trade Center Building Performance Study'', URL accessed ], ]</ref> that spread to six floors before being extinguished.


==Construction of 1 and 2 WTC== ==Construction of 1 and 2 WTC==
], ]]] ], ]]]
Construction of the towers began in 1968 and was completed in 1972 and 1973.
Construction of the towers began in 1968 and was completed in 1972 and 1973. During the period, implementation of an innovative elevator system halved the number of elevator shafts. The express elevators took people to "sky lobbies" on the 44th and 78th floors, where they could board local elevators. Also unique was the grouping of columns into the core and perimeter of the building, a structural system called a "tube".

To meet the challenges of wind load, gravity load and related architectural stresses, the WTC's structural engineers took a then-unusual approach in its construction: instead of employing a traditional grid-like plan with beams evenly spaced throughout a floor, the WTCs columns were grouped in the building's core and perimeter. The core of each tower was a rectangular area 87 by 133 feet (27 by 41 meter) and consisted of steel box columns running from the bedrock to the tops of the tower. The columns tapered to the top, where they transitioned to lightweight H-beams, but the exact dimensions are unknown as the blueprints are under the jurisdiction of the Port Authority and are not public domain. Each tower had 240 steel perimeter columns (from 2.5 inches thick at the bottom tapering to .25 inch at the top ) placed 14 inches (36 cm) around the perimeter. This signature feature of columns grouped in the core and perimeter allowed large tracts of uninterrupted floorspace, a significant marketing feature for the towers.


==Impacts of airliners== ==Impacts of airliners==
] ]
]'']] ]'']]
The towers were each struck by hijacked ] jet planes, ] and ].
The towers were each struck by hijacked ] jet planes, ] and ]. A typical Boeing 767 is 180 feet (55 m) long and has a wingspan of 156 feet (48 m), with a capacity of up to 24,000 US gallons (91,000 L) of jet fuel. The planes hit the towers at very high speeds. Flight 11 was traveling roughly 490 mph (790 km/h) when it crashed into the north tower, Flight 175 hit the south tower at about 590 mph (950 km/h). The resulting explosions in each tower ignited ten thousand gallons<ref>Federal Emergency Management Agency, .pdf, ''World Trade Center Building Performance Study'', URL accessed ], ]</ref> (c. 40,000 L) of jet fuel and immediately spread the fire to several different floors simultaneously while consuming paper, furniture, carpeting, computers, books, walls, framing and other items in all the affected floors.

The jet fuel probably burned out in less than 10 minutes; what did burn over the next hour, or hour and a half, was much of the contents of the buildings, according to the lead investigator of the NIST investigation.<ref>Fire/Rescue News, , (February 7, 2004), URL accessed ], ]</ref>

However, intense heat from the burning ] and combustibles near the cores of the towers was weakening the central steel columns, the longspan floor ]es and cross trusses, and the joints connecting the floorplates to the external columns. The strength of the steel dropped markedly with exposure to fire, losing half its strength at a temperature of 1,202F (650C).<ref>Eagar, Thomas W., Christopher Musso, , ''Journal of Metals'', JOM, Volume 53, Issue 12 pp.8-11, URL accessed ], ]</ref>

]
Stuctural Engineer Charles Clifton says that a combination of factors including the impact from aircraft flying at high speed caused internal structural damage.<ref>Clifton, G. Charles, .pdf, URL accessed ], ]</ref> The lightness and hollowness of the towers allowed the jet fuel (and resulting fires) to penetrate far inside the Towers. This lightness and hollowness were functions, primarily, of the absence of building-wide rows of columns (and attendant walls), the absence of ] elements or heavy steel in the facades, and the use of ] ] rather than reinforced concrete to encase stairways and elevator shafts. The collapse certainly would not have occurred without the fires, but the fires may not have been as centrally positioned nor as intense had traditionally heavy high-rise construction been standing in the way of the aircraft&#8212; debris and fuel would likely have remained mostly outside the buildings and/or concentrated in more peripheral areas away from the building cores, which themselves would not have been unique failure points. In this scenario, the towers might have stood far longer, perhaps indefinitely.<ref>Gross, John L., Therese P. McAllister,, ''Federal Building and Fire Safety Investigation of the World Trade Center Disaster'', NIST NCSTAR 1-6, URL accessed ], ]</ref><ref>Wilkinson, Tim, , World trade Center - Some Engineering Aspects, (April 3, 2006), URL accessed ], ]</ref> Fires resulting from the aviation fuel spread widely through the impact zone and ultimately led to the collapse of the Twin Towers. As the Towers collapsed, the destabilization, debris, and associated fires damaged many of the buildings in the immediate vicinity.
<br clear all>


==Collapse of the two towers== ==Collapse of the two towers==
The north tower, 1 WTC, was struck at 8:46:26 am and collapsed at 10:28:31 am, standing for 102 minutes and 5 seconds after impact. The south tower, 2 WTC, was struck at 9:02:54 am and collapsed about 56 minutes later, around 10:00 am. According to the 9-11 commission report, "at 9:58:59, the South Tower collapsed in ten seconds, .... The building collapsed into itself, causing a ferocious windstorm and creating a massive debris cloud." The north tower, 1 WTC, was struck at 8:46:26 am and collapsed at 10:28:31 am, standing for 102 minutes and 5 seconds after impact. The south tower, 2 WTC, was struck at 9:02:54 am and collapsed about 56 minutes later, around 10:00 am. Eagar went on to state that "the joints on the most severely burned floors gave way, causing the perimeter wall columns to bow outward and the floors above them to fall".<ref>Eagar, Thomas W, Christopher Musso, , (December 2001), URL accessed ], ]</ref>


Immediately after 9/11, experts such as Bazant and Zhou explained exactly why the towers collapsed. What occured is known as "total progressive collapse", and has occured many times before. British structural engineer Chris Wise commented that "It was the fire that killed the buildings. "<ref>BBC News, , (September 13, 2001), URL accessed ], ]</ref> Similarly, John Knapton, a professor of ] at the ] said "The 35 tonnes of aviation fuel will have melted the steel..."<ref>BBC News,, (October 17, 2001), URL accessed ], ]</ref>

These early assessments that steel in the buildings had melted and caused the collapse, were further enhanced by ] materials professor Thomas Eagar, who said "The temperature of the fire at the WTC was not unusual" It didn't need to be unusual. Eagar's theory, published in December 2001, focuses on the joints between the floor assemblies and the perimeter columns. Eagar went on to state that "the joints on the most severely burned floors gave way, causing the perimeter wall columns to bow outward and the floors above them to fall".<ref>Eagar, Thomas W, Christopher Musso, , (December 2001), URL accessed ], ]</ref>

In 2005, the NIST developed computer models of the tops of the towers. The computer models indicated that heat from the fires made floor assemblies sag in the middle of the spans. Quoting the NIST report, "The primary role of the floors in the collapse of the towers was to provide inward pull forces that induced inward bowing of perimeter columns". These perimeter columns buckled and failed, which shifted tremendous weight bearing load onto the core columns. This was too much for the core columns to handle, so they also buckled and failed.

This is similar to what occured in 1992 at the ] in Houston Texas, among many other examples of progressive collapse.

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 airplane was lower (so there was less impact damage); and the affected floors had had their fire proofing partially upgraded. For the south tower, intense fires led to a sudden bursting of ]s in one section, while the failures in the north tower core involved slower ] and softening effects.<ref>Wilkinson, Tim, , World trade Center - Some Engineering Aspects, (April 3, 2006), URL accessed ], ]</ref>
] ]
] ]

The two towers collapsed in markedly different ways which may indicate that there were two modes of failure. 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, then pulverized into dust in mid-air, and the tower then continued straight down. In spite of these differences, an Australian structural engineer believes that the "same mechanism of failure, the combination of impact and subsequent fire damage, is the likely cause of failure of both towers"<ref>Wilkinson, Tim, , World trade Center - Some Engineering Aspects, (April 3, 2006), URL accessed ], ]</ref>

Subsequent modeling suggests that in the north tower the internal trusses supporting the building's concrete floors failed as a result of heat-induced warping.<ref>Eagar, Thomas, , ''NOVA'', (May 2002), URL accessed ], ]</ref> This would have placed additional stress on the bunched core columns which themselves were losing integrity from both impact damage and heat. When the core columns gave out on one of the impact floors, this floor collapsed into the floor below. Once the collapse started, it was unstoppable; the huge mass of the falling structure had sufficient momentum to act as a battering ram, smashing through all the intact floors below. Witnesses from within the tower stated they heard "something like a heavy freight train approaching". It could be said these statements support this theory. There is some visual evidence that it was the core that collapsed first. It can be seen in videos that the large antenna, which was built on top of the core, starts downward a fraction of a second earlier than the rest of the building.<ref>Bazant, Zdenek P., Yong Zhou, , ''Theoretical and Applied Mechanics'', (September 22, 2001), URL accessed ], ]</ref> However later analysis by NIST dispute this claim stating "that observations from a single vantage point can be misleading and may result in incorrect interpretation. When records from east and west vantage points were viewed, it was apparent that the building section above the impact area tilted to the south as the building collapsed."<ref>Gross, John L., Therese P. McAllister,, ''Federal Building and Fire Safety Investigation of the World Trade Center Disaster'', NIST NCSTAR 1-6, URL accessed ], ]</ref>


In the south tower, it is assumed to be fire that warped<ref>NOVA, , ''The Structure of Metal'', (May 2002), URL accessed ], ]</ref> and severed the single-bolt connections between the floorplates and the initially-intact external columns surrounding the impact hole, effectively creating a "hangman's drop" for that portion of the building above the point of failure. Eventually, the gravity load on these bolts increased beyond their breaking point as the joints, floorplates and columns weakened. The momentum of the collapsing structure crushed everything below it.<ref>Wilkinson, Tim, , World trade Center - Some Engineering Aspects, (April 3, 2006), URL accessed ], ]</ref> In the south tower, it is assumed to be fire that warped<ref>NOVA, , ''The Structure of Metal'', (May 2002), URL accessed ], ]</ref> and severed the single-bolt connections between the floorplates and the initially-intact external columns surrounding the impact hole, effectively creating a "hangman's drop" for that portion of the building above the point of failure. Eventually, the gravity load on these bolts increased beyond their breaking point as the joints, floorplates and columns weakened. The momentum of the collapsing structure crushed everything below it.<ref>Wilkinson, Tim, , World trade Center - Some Engineering Aspects, (April 3, 2006), URL accessed ], ]</ref>
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==Design criticisms== ==Design criticisms==


The WTC design was intended to maximize rentable floor space, and minimize the intrusion of columns into that floor space. To accomplish this goal, the structural columns were grouped along the perimeter and within the core of the towers. The space between the perimeter and core columns was bridged by long floor trusses, with a span of 60 feet (18.2 m) in the long-span areas and 36 feet (11.0 m) in the short span areas.<ref>National Institute of Standards and Technology, "Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Component, Connection and Subsystem Structural Analysis", Page 3, September 2005</ref> This is in contrast to earlier generations of skyscrapers, such as the Empire State Building, in which the columns are arranged in a grid pattern with floor spans of no more than about 30 feet (9 m).<ref>Robertson, Leslie E. , The Bridge, Volume 32, Number 1 - Spring 2002</ref> Criticisms of this aspect of the WTC design feature the following points: This is in contrast to earlier generations of skyscrapers, such as the Empire State Building, in which the columns are arranged in a grid pattern with floor spans of no more than about 30 feet (9 m).<ref>Robertson, Leslie E. , The Bridge, Volume 32, Number 1 - Spring 2002</ref> Criticisms of this aspect of the WTC design feature the following points:


#The large column-free area between the perimeter and core may have allowed the aircraft and the fuel they carried to penetrate deeper into the structure than they would have in a building with a more traditional grid-type column arrangement. This would spread the fire more rapidly through the building, and made damage to the stairwells and the fire proofing of the interior columns more likely.
#A structure with the columns grouped along the perimeter and within the interior core may be inherently less redundant and robust than one with the columns arranged in a grid pattern.<ref> Dunn, Vincent , URL accessed May 11, 2006</ref>


There are additional criticisms regarding the use of lightweight materials in the WTC towers: There are additional criticisms regarding the use of lightweight materials in the WTC towers:
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# The World Trade Center exclusively used lightweight materials<ref>Eagar, Thomas W., Christopher Musso, , ''Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, JOM, vol. 53/12 pp. 8-11, (2001), URL accessed ], ]</ref> especially in the facade. Had the WTC facade contained even minimal masonry elements and/or traditional heavy steel outermost column rows, it is less likely the aircraft would have cleanly penetrated to the core of each tower&mdash; a significant portion of debris and jet fuel would have remained outside, a much different scenario. # The World Trade Center exclusively used lightweight materials<ref>Eagar, Thomas W., Christopher Musso, , ''Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, JOM, vol. 53/12 pp. 8-11, (2001), URL accessed ], ]</ref> especially in the facade. Had the WTC facade contained even minimal masonry elements and/or traditional heavy steel outermost column rows, it is less likely the aircraft would have cleanly penetrated to the core of each tower&mdash; a significant portion of debris and jet fuel would have remained outside, a much different scenario.
# Single-bolt connections binding the longspan floorplates with the load-bearing external columns were extremely lightweight for their assigned task. One study group from the ] has concluded the proximal cause of the south tower collapse was failure of these bolts in the southeast corner of the building.<ref>Eagar, Thomas W., Christopher Musso, , ''Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, JOM, vol. 53/12 pp. 8-11, (2001), URL accessed ], ]</ref> Double-bolts should have been used. # Single-bolt connections binding the longspan floorplates with the load-bearing external columns were extremely lightweight for their assigned task. One study group from the ] has concluded the proximal cause of the south tower collapse was failure of these bolts in the southeast corner of the building.<ref>Eagar, Thomas W., Christopher Musso, , ''Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, JOM, vol. 53/12 pp. 8-11, (2001), URL accessed ], ]</ref> Double-bolts should have been used.
# The use of ] cladding instead of reinforced concrete to shield stairwells. Almost all skyscrapers, including those built since the WTC, shield stairwells in reinforced concrete. On September 11th, it was the collapse of all stairways above the impact level that consigned all people above the impact zone in Tower One to death. Tower Two had two of its three stairwells taken out above the impact area by the plane. Some people above the impact zone survived, as they used the third stairwell. Computer models have shown that most of the stairwells in both towers would likely have remained usable until the general collapse had they been shielded in concrete.

Some see the WTC as an irresponsible experiment in lightweight, rent-space-maximized construction and place particular opprobrium on ], its Chief Structural Engineer. Others see it as a landmark in structural engineering simply in need of refinement due to unforeseen, and probably unforeseeable, variables.

One of those variables was the size and kinetic energy of aircraft that might accidentally strike the WTC. Mr. Robertson and others involved in design and construction of the WTC have stated that back in the 1960s they could not have planned for the jetliners of 2001. Specifically, they modeled the effects of a hit by the largest aircraft of the day, the Boeing 707-320, and presumably calibrated their design to withstand it. <br><br>
{| cellpadding="2" cellspacing="0" border="true" align="center"
!Parameter
!colspan="2"|]
!colspan="2"|]
|-
|fuel capacity
|23,000 US gal
|87,000 L
|23,980 US gal
|90,780 L
|-
|max takeoff weight
|333,600 lb
|151,300 kg
|387,000 lb
|175,500 kg
|-
|empty weight
|146,400 lb
|66,400 kg
|164,800 lb
|74,800 kg
|-
|wingspan
|145.75 ft
|44 m
|156.08 ft
|48 m
|-
|wing area
|3010 ft&sup2;
|280 m&sup2;
|3050 ft&sup2;
|283 m&sup2;
|-
|length
|152.92 ft
|47 m
|159.17 ft
|49 m
|-
|cruise speed
|557 mph
|896 km/h
|530 mph
|853 km/h
|}<br>

According to the Federal Emergency Management Agency, WTC towers 1 and 2 were designed to withstand the impact of a 707 lost in fog while looking to land. The modeled aircraft was a 707 weighing 263,000 lb (119,000 kg) with a flight speed of 180 mph (290 km/h), as would be used in approach and landing situations.<ref>Federal Emergency Management Agency, .pdf, ''World Trade Center Building Performance Study'', Pg. 17, URL accessed ], ]</ref> The 767s that actually hit the towers had a kinetic energy more than seven times greater than the specifically modeled 707 impact. (The ], with an empty weight more than twice that of the 767, was in the final design phase when WTC drafting began and the first 747s were constructed simultaneously with the WTC towers; however the known attributes of the 747 were apparently not modeled in designing the towers).

Although an article in ''The Guardian''<ref>Larson, Mark, , ''Guardian Unlimited'', URL accessed ], ]</ref>
questioned its authenticity, a ] of ] verified by the Pentagon indicated that Bin Laden had not believed that the buildings would collapse completely, but rather would collapse only above the levels where the planes respectively struck:
<blockquote>We calculated in advance the number of casualties from the enemy, who would be killed based on the position of the tower. We calculated that the floors that would be hit would be three or four floors. I was the most optimistic of them all. (...Inaudible...) Due to my experience in this field, I was thinking that the fire from the gas in the plane would melt the iron structure of the building and collapse the area where the plane hit and all the floors above it only. This is all that we had hoped for.<ref>NPR, , URL accessed ], ]</ref></blockquote>

A federal technical building and fire safety investigation of the collapses of the Twin Towers and 7 WTC was conducted by the ] ]'s ] (NIST). The goals of this investigation were to investigate the building construction, the materials used, and the technical conditions that contributed to the outcome of the WTC disaster. The investigation<ref>National Institutes of Standards and Technology, , (4/25/2006), URL accessed ], ]</ref> will serve as the basis for:

* improvements in the way buildings are designed, constructed, maintained, and used;
* improved tools and guidance for industry and safety officials;
* revisions to building and fire codes, standards, and practices; and
* improved public safety.

The long-anticipated report was partially released in draft for public comment on ], ]. In its over 10,000 pages the conclusion reached was that the fireproofing on the steel infrastructure was blown off by the initial impact of the planes into the towers. If this had not occurred the WTC would have likely remained standing. A further finding of the report was that the staircases were not adequately reinforced to provide emergency escape for people above the impact zone.

According to the Executive Summary of NIST's final report on the Collapses of the World Trade Center Towers, one of its goals was to "Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft and why and how WTC 7 collapsed" (p xli). However, the report elsewhere says that its "probable collapse sequence" "does not actually include the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable." (p xliii)

Early tests conducted on steel beams from the World Trade Center show they generally met or were stronger than design requirements, ruling them out as a contributing cause of the collapse of the towers.<ref>{{cite web | last = Barrett | first = Devlin | year = 2003 | url = http://www.boston.com/news/nation/articles/2003/08/28/steel_type_in_wtc_met_standards_group_says?mode=PF | title = Steel type in WTC met standards, group says | work = The Boston Globe | publisher = Associated Press | accessdate = 2006-05-02}}</ref>


==Seven World Trade Center== ==Seven World Trade Center==
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The World Trade Center complex had a total of 7 buildings. The third building to collapse in the ] was ] which fell at 5:20pm, as seen live on television. WTC 7 was a 47 story steel-frame skyscraper that stood across the street from the rest of the WTC complex. The World Trade Center complex had a total of 7 buildings. The third building to collapse in the ] was ] which fell at 5:20pm, as seen live on television. WTC 7 was a 47 story steel-frame skyscraper that stood across the street from the rest of the WTC complex.

<blockquote>The 2 million-square-foot building, 7 World Trade Center, had suffered mightily from the fire, and had been wounded by beams falling off the towers. But experts said no building like it, a modern, steel-reinforced high-rise, had ever collapsed because of an uncontrolled fire. They have been trying to figure out exactly what occurred, and whether they should be worried about other buildings like it around the country. &ndash; </blockquote>

As part of the electrical backup system, there may have been up to a total of 160,000 litres (42,000 gallons) of ] stored in five tanks within the building on several floors, as well as pumps to distribute it. It has been claimed that the diesel fuel and emergency generators spilled and ignited inside building 7.<!--- although the FEMA report states that all tanks remained fully intact.<ref>Federal Emergency Management Agency, .pdf, ''World Trade Center Building Performance Study'', Pg. 17, URL accessed ], ]</ref> This is report for WTC1 & WTC2 & does not say this - nor does report for WTC7--> Another speculation is that the building's unusual architecture may have contributed to its collapse. Theoretically, cantilevers and structural members, required to transfer building weight off of the pre-existing ] electrical substation that the 7 WTC building was built over, may have failed in the fire leading to the internal mechanism of collapse.


An article in ''the Journal of Metallurgy'' discussed microstructural changes that resulted in the erosion of a piece of a steel beam collected from WTC7: An article in ''the Journal of Metallurgy'' discussed microstructural changes that resulted in the erosion of a piece of a steel beam collected from WTC7:
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</ref></blockquote> </ref></blockquote>


]'s report on the disaster states that "Although the total diesel fuel on the premises contained massive potential energy, the best hypothesis has only a low probability of occurrence. Further research, investigation, and analyses are needed to resolve this issue".<ref>Federal Emergency Management Agency, .pdf, ''World Trade Center Building Performance Study'', (Chapter 8.2.5.1), URL accessed ], ]</ref> ] continued this work and released a progress report in June of 2004 in which they outlined the working hypothesis of the collapse of 7 WTC:<ref>National Institute of Standards and Technology, , ''Fact sheets from NIST'', (June 18, 2004), URL accessed ], ]</ref><ref>National Institute of Standards and Technology, .pdf, URL accessed ], ]</ref>
:* An initial local failure at the lower floors (below Floor 13) of the building due to fire and/or debris induced structural damage of a critical column (the initiating event), which supported a large span floor bay with an area of about 2,000 square feet (190 m&sup2;).
:* Vertical progression of the initial local failure up to the east penthouse, as large floor bays were unable to redistribute the loads, bringing down the interior structure below the east penthouse.
:* Collapse of the interior structure first, pulling the outer structure down and inward
:* Horizontal progression of the failure across the lower floors (in the region of Floors 5 and 7, that were much thicker than the rest of the floors), triggered by damage due to the vertical failure, resulting in the disproportionate collapse of the entire structure.

The final report from the ] regarding the collapse of 7 WTC was due in July of 2005, but is still ongoing.<ref>Sunder, S. Shyam, .pdf, ''NIST Response to the World Trade Center Disaster'', (October 19, 2004), URL accessed ], ]</ref><ref>National Institutes of Science and Technology, , URL accessed ], ]</ref>

In "WTC part IIC - WTC7 Collapse Final", released in April 2005, NIST concludes about the fuel "This finding allows for the possibility, though not conclusively, that the fuel may have contributed to a fire on Floor 5."<ref>Sunder, S. Shyam, .pdf, ''NIST Response to the World Trade Center Disaster'', (October 19, 2004), URL accessed ], ]</ref>


==Conspiracy theories== ==Conspiracy theories==

Revision as of 15:16, 21 June 2006

Template:Sep11

File:WTC-remnant.jpg
A New York City firefighter looks up at the remains of the World Trade Center, two days after its collapse

Following the September 11, 2001 attacks that sent one hijacked airliner into each of the two main towers of the World Trade Center complex, 1 WTC, 2 WTC and 7 WTC collapsed. Both main towers lost structural integrity and fell that morning, killing 2,595 people within and nearby, as well as 157 people who were aboard the flights. Other nearby buildings, including 7 WTC, were damaged by the debris. 7 WTC was evacuated, and collapsed seven hours later at 5:20pm, without casualties.

Construction of 1 and 2 WTC

Schematic of 1 WTC with impact damage. Note narrow central core into which all internal columns are bunched. Adapted from NIST report "Baseline Structural Performance and Aircraft Impact Damage Analysis", October 19, 2004

Construction of the towers began in 1968 and was completed in 1972 and 1973.

Impacts of airliners

Impact locations for towers 1 and 2.
File:Edna Cintron standing in WTC1.jpg
A woman stands in the gash in 1 WTC. FEMA

The towers were each struck by hijacked Boeing 767 jet planes, American Airlines Flight 11 and United Airlines Flight 175.

Collapse of the two towers

The north tower, 1 WTC, was struck at 8:46:26 am and collapsed at 10:28:31 am, standing for 102 minutes and 5 seconds after impact. The south tower, 2 WTC, was struck at 9:02:54 am and collapsed about 56 minutes later, around 10:00 am. Eagar went on to state that "the joints on the most severely burned floors gave way, causing the perimeter wall columns to bow outward and the floors above them to fall".

Ground Zero debris with markup showing building locations.
File:Cut metal WTC.jpg
Rubble at ground zero, severed box column in background

In the south tower, it is assumed to be fire that warped and severed the single-bolt connections between the floorplates and the initially-intact external columns surrounding the impact hole, effectively creating a "hangman's drop" for that portion of the building above the point of failure. Eventually, the gravity load on these bolts increased beyond their breaking point as the joints, floorplates and columns weakened. The momentum of the collapsing structure crushed everything below it.

Design criticisms

This is in  contrast to earlier generations of skyscrapers, such as the Empire State Building, in which the columns are arranged in a grid pattern with floor spans of no more than about 30 feet (9 m).  Criticisms of this aspect of the WTC design feature the following points:


There are additional criticisms regarding the use of lightweight materials in the WTC towers:

  1. The World Trade Center exclusively used lightweight materials especially in the facade. Had the WTC facade contained even minimal masonry elements and/or traditional heavy steel outermost column rows, it is less likely the aircraft would have cleanly penetrated to the core of each tower— a significant portion of debris and jet fuel would have remained outside, a much different scenario.
  2. Single-bolt connections binding the longspan floorplates with the load-bearing external columns were extremely lightweight for their assigned task. One study group from the Massachusetts Institute of Technology has concluded the proximal cause of the south tower collapse was failure of these bolts in the southeast corner of the building. Double-bolts should have been used.

Seven World Trade Center

File:WTC7-collapse-animation.gif
Video of the collapse of WTC7

The World Trade Center complex had a total of 7 buildings. The third building to collapse in the September 11, 2001 attacks was 7 World Trade Center which fell at 5:20pm, as seen live on television. WTC 7 was a 47 story steel-frame skyscraper that stood across the street from the rest of the WTC complex.

An article in the Journal of Metallurgy discussed microstructural changes that resulted in the erosion of a piece of a steel beam collected from WTC7:

Rapid deterioration of the steel was a result of heating with oxidation in combination with intergranular melting due to the presence of sulfur. The formation of the eutectic mixture of iron oxide and iron sulfide lowers the temperature at which liquid can form in this steel. This strongly suggests that the temperatures in this region of the steel beam approached ~1000°C by a process similar to making a 'blacksmith's weld' in a hand forge.


Conspiracy theories

Main article: 9/11 conspiracy theories § World Trade Center towers

Some conspiracists disagree with the findings of U.S. Government engineers and independent researchers, arguing that controlled demolition better explains the full range of observations and available data.

Other buildings

Numerous other buildings in the World Trade Center and surrounding it were damaged or destroyed as the Towers fell. 5 World Trade Center suffered a large fire and a partial collapse of its steel structure.

Other buildings destroyed include St. Nicholas Greek Orthodox Church, Marriott Hotel (3 WTC), South Plaza (4 WTC), U.S. Customs (6 WTC), and the Winter Garden at the World Financial Center. World Financial Center buildings 4, 5, 6, and 7, 90 West Street, and 130 Cedar Street suffered fires. The Bankers Trust Building, Verizon, and World Financial Center 3 suffered impact damage from the Towers collapse, as did 90 West Street. 30 West Broadway was damaged by the collapse of 7 WTC. The Deutsche Bank Building, though left standing, is currently being demolished because of water and mold damage, and severe damage caused by the neighboring towers' collapse.

References

Cited references

  1. Eagar, Thomas W, Christopher Musso, Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, (December 2001), URL accessed May 25, 2006
  2. NOVA, Bend, The Structure of Metal, (May 2002), URL accessed May 1, 2006
  3. Wilkinson, Tim, Why did it collapse?, World trade Center - Some Engineering Aspects, (April 3, 2006), URL accessed May 1, 2006
  4. Robertson, Leslie E. "Reflections on the World Trade Center", The Bridge, Volume 32, Number 1 - Spring 2002
  5. Eagar, Thomas W., Christopher Musso, The Design, Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, JOM, vol. 53/12 pp. 8-11, (2001), URL accessed May 1, 2006
  6. Eagar, Thomas W., Christopher Musso, The Design, Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, JOM, vol. 53/12 pp. 8-11, (2001), URL accessed May 1, 2006
  7. Barnett, J.R. (2001). "An Initial Microstructural Analysis of A36 Steel from WTC Building 7". Feature: Letter. The Journal of Materials. Retrieved 2006-05-12. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

References

External links

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