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In the September 11, 2001 attacks, a hijacked airliner crashed into each of the two main towers (1 WTC and 2 WTC) of the World Trade Center complex. This led to the total destruction of both towers, which collapsed at 9:59 am (2 WTC) and 10:28 am (1 WTC), causing massive damage to the rest of the complex and nearby buildings. Having been evacuated seven hours earlier, 7 WTC collapsed at 5:20 pm, with no casualties. In all, 2,595 people inside and near the towers were killed, along with the 157 people who were aboard the flights.

The combined effects the airplane impacts and the heat of the fires undermined the structural integrity of the two towers. The dislodging of fireproofing by the impacts proved to be a decisive factor, exposing the steel to intense heat. This gradually weakened them and ultimately made them unable to support the enormous loads above the impacted floors. The towers collapsed abruptly when the perimeter walls buckled because sagging floors had pulled them inwards. Once the perimeter columns buckled and collapse was initiated, total structural failure was inevitable.

The collapse of 7 WTC has not yet been adequately explained, but investigations are in progress.

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. The National Institute of Standards and Technology (NIST) concurred with this view in their Final Report on the Collapse of the World Trade Center Towers, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in US cities in the past. It did add, however, that the towers' stairwell design lacked adequate reinforcement.

Construction of 1 and 2 WTC

Structural Details

Construction of the towers began in 1968 and was completed in 1970 (WTC 1) and 1972 (WTC 2). WTC 1 and WTC 2 were nearly identical structures. Both had 110 stories, although WTC 1 was 1,368 feet (417.0 meters) tall and WTC 2 was 1,362 feet (415.1 meters) tall. WTC 1 also supported a 360 foot tall (110 meter) antenna. The core in WTC 1 was oriented with the long axis east to west, while that of WTC 2 was oriented north to south.

The towers were designed as framed tube structures. The columns were grouped around the perimeter and within the core. The perimeter columns supported virtually all lateral loads such as wind loads and shared the gravity loads with the core columns. All columns were founded on bedrock, which is unusually shallow in Manhattan.

Above the seventh floor there were 280 perimeter columns, evenly spaced around the towers on 3 foot 4 inch centers (1.02 meters). The perimeter columns had a square cross section, 14 inches on a side (0.36 meters), and were constructed of welded steel plate. The thickness of the plates and grade of steel were varied over the height of the tower, with the steel strength and plate thickness decreasing with height. The columns were connected with deep spandrel plates, which were typically 52 inches (1.3 meter) deep. The spandrel plates were located at each floor, and served to transmit shear flow between columns thus allowing them to work together in resisting lateral loads.

The perimeter was assembled from prefabricated modules consisting of three columns and three spandrel plates. The spandrel plates were welded to the columns at the fabrication shop. These modules extended for two full floors and half of two more floors. Adjacent modules were bolted together, with the splices occurring at mid-span of the columns and spandrels. The joints between modules were staggered vertically, so the column splices between adjacent modulus were not at the same floor.

The core of each tower was a rectangular area 87 by 133 feet (27 by 41 meters) and contained 48 steel columns running from the bedrock to the tops of the tower. The columns tapered with height, and consisted of welded box-sections at lower floors and rolled wide-flange sections at upper floors. All of the elevators and stairwells were located in the core.

The large, column-free space between the perimeter and core was bridged by pre-fabricated floor trusses. The trusses had a span of a span of 60 feet (18.2 m) in the long-span areas and 36 feet (11.0 m) in the short span area. The trusses connected to the perimeter at alternate columns, and were therefore on 6 foot 8 inch (2.03 meter) centers. The top chord 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. Approximately 10,000 of the trusses were also connected to the perimeter columns at their bottom chord through viscoelastic dampers to reduce wind-induced sway for occupant comfort. The trusses supported a 4 inch (0.10 meter) thick lightweight concrete floor slab, with shear connections for composite action.

The towers also incorporated a "hat truss" or "outrigger truss" located between the 106th and 110th floors. The truss system 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.

Commentary

The WTC towers were innovative in many ways, and were significantly different from earlier generations of skyscrapers such as the Empire State Building. One of the most innovative features is the tube structural system, which allowed large, open floor spaces uninterrupted by columns. This helped maximize the rentable space, and allowed tenants more flexibility in configuring their office space. The towers also made extensive use of pre-fabricated modules such as perimeter sections and floor trusses and extensively used light-weight materials.

The use of express elevators also decreased the amount of space lost to elevator shafts. The express elevators took people to "sky lobbies" on the 44th and 78th floors, where they could board local elevators.

Some of these innovative features have been criticized as contributing factors to the collapse. These criticisms include:

1. 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.
2. 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.
3. The WTC used lightweight materials exclusively especially in the facade. Had the facade contained even minimal masonry elements and/or traditional heavy steel outermost column rows, it would have been 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.
4. The use of gypsum cladding instead of reinforced concrete to shield stairwells. Almost all skyscrapers, including those built since the WTC, shield stairwells in reinforced concrete. On September 11, it was the collapse of all stairways above the impact level that consigned all people above the impact zone in 1 WTC to death. 2 WTC had two of its three stairwells taken out above the impact area by the plane: some people above the impact zone survived by using 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.

Fireproofing was added after a fire in 1975 that spread to six floors before being extinguished. Early tests conducted on steel beams from the WTC show they generally met or were stronger than design requirements, ruling them out as a contributing cause of the collapse of the towers.

Impacts of airliners

The towers were struck by hijacked Boeing 767 jet planes, American Airlines Flight 11 and United Airlines Flight 175. 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 1 WTC, the north tower; flight 175 hit 2 WTC, the south tower, at about 590 mph (950 km/h). In addition to severing a significant amount of load-bearing columns, the resulting explosions in each tower ignited 10,000 gallons (c. 40,000 l) 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 buildings has in fact been designed to withstand the impact of the largest impact of the day, the Boeing 707-320, lost in fog while looking to land. The modeled aircraft weighed 263,000 lb (119,000 kg) with a flight speed of 180 mph (290 km/h), as in approach and landing. The 767s that hit the towers had a kinetic energy more than seven times greater than the modeled impact. Nonetheless, the impacts alone did not cause the towers to collapse.

The fires

While the towers were designed to survive aircraft impact, and in fact did survive such impacts, little was known about the fires that might result from them. It was ultimately the fires that brought the buildings down. The lightness and hollowness of the towers 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 airplanes probably burned out in less than 10 minutes; but the contents of the buildings burned over the next hour or hour and a half.

It has been suggested that 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. Debris and fuel would likely have remained mostly outside the buildings and/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.

The strength of steel drops markedly with heat, losing half its strength at a temperature of 1,202°F (650°C). The heat from the fires quickly began to weaken the central steel columns, the longspan floor trusses, and cross trusses.

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 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.

Physical features of the collapses

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 NIST report would later estimate 12 seconds. It should also be noted that the buildings collapsed symmetrically but not "into themselves", spreading debris in a wide radius around them. One of the most characteristic features of the collapses were the enormous dustclouds that covered Manhattan for days. These were composed mainly of pulverized gypsum cladding and dry wall, finely ground concrete from the towers' floors, glass particles, lead (from the many computers in the buildings), and some radioactive material (from the fire detectors).

The collapse mechanism

In the most comprensive of study to date, published in September of 2005, NIST developed computer models of the tops of the towers, which indicated that heat from the fires made floor assemblies sag in the middle of the spans. The collapses of the two towers were found to differ in some respects, but in both cases, the same sequence of events apply.

After the initial aircraft impact had severed exterior columns and damaged core columns, the weight that these columns had supported was redistributed to other columns. NIST found that the hat trusses at the top of buildings played a significant role in this redistribution of the loads in the structure. The impacts also dislodged some of the fireproofing from the steel, increasing its exposure to the heat of the fires.

In the 56 and 102 minutes before the collapse of, respectively, 2 WTC and 1 WTC, the fires, and events associated with them, weakened the core, until it was unable to carry loads. The NIST report provides a useful image of the situation.

At this point, the core of WTC 1 could be imagined to be in three sections. There was a bottom section below the impact floors that could be thought of as a strong, rigid box, structurally undamaged and at almost normal temperature. There was a top section above the impact and fire floors that was also a heavy, rigid box. In the middle was the third section, 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 WTC 2. 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 am, 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. At 10:28 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 had their fire proofing partially upgraded.

Thinking the unthinkable: early attemps to understand the collapses

On September 13, 2001, the cover of the New Civil Engineer in the UK consisted of a picture WTC 1 during its collapse with a single word written across it: "unthinkable". "Just hours earlier, it had been genuinely inconceivable that structures of such magnitude could succumb to this fate." In subsequent effort to make sense of the facts, a number of different collapse mechanisms were proposed, culminating in the NIST report of September 2005. Interviewed by the BBC in October 2001, the British architect Bob Holvorson 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 carefully 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, Holvorson noted that the collapses were "well beyond realistic experience."

Fire: "the most misunderstood part of the collapses"

Many identified the fires as the key to the collapses. While NIST would eventually confirm this hypothesis, Thomas Eagar, an MIT materials professor, was nonetheless right to describe the fires as "the most misunderstood part of the WTC collapse". This is because the fires were originally said to have "melted" the floors and columns. As Eagar pointed out, "The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel." Jet fuel is is essentially kerosene and would have served mainly to ignite very large, but not unusually hot, hydrocarbon fires.

The controlled demolition hypothesis

On the day of the attack, Van Romero, an explosives expert in New Mexico, told the Albuquerque Journal that the collapses of the towers looked "too methodical" to be triggered by the aircraft impacts and fires. He said they looked like they had been triggered by "explosive devices inside the buildings". He later clarified his position, saying that he did not believe that any such explosives had in fact been involved; he was merely noting that that is what it looked like. Indeed, NIST would find "no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to September 11, 2001." This clarification was added to Part III and the executive summary of the final report in "consideration of public comments" that had been made to an earlier draft. Indeed, the controlled demolition hypothesis had not been proposed in mainstream engineering scholarship prior to the publication of the report, and has not been suggested in such scholarship since. New York magazine, however, has described the rapid collapse of the World Trade Center as the "grassy knoll" of 9/11 conspiracy theories.

Eagar and NOVA: "pancaking" floors

The most influential of the early theories of the collapse sequence was the so-called "pancake" collapse theory, was proposed by Thomas Eagar and popularized by PBS. 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.

While the early efforts had exaggerated the temperatures of the fires, however, Eagar's theory would prove to underestimate the effect of the fires on the structural steel columns. In a paper published in December of 2001, he had focused on the joints between the floor assemblies and the perimeter columns, which, he argued, would be more vulnerable to the effects of the fires. On this assumption he proposed 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".

Right idea, wrong direction: a suggestion from Sydney

A similar line was taken by Tim Wilkinson, a civil engineer at the University of Sydney. In an "initial suggestion", written already on September 11, he outlined a range of possible effects related mainly to the effects of the fires.

Eventually, the loss of strength and stiffness of the materials resulting from the fire, combined with the initial impact damage, would have caused a failure of the truss system supporting a floor, or the remaining perimeter columns, or even the internal core, or some combination. Failure of the flooring system would have subsequently allowed the perimeter columns to buckle outwards. Regardless of which of these possibilities actually occurred, it would have resulted in the complete collapse of at least one complete storey at the level of impact.

It will be noted that this hypothesis, while identifying all the elements that would eventually make up NIST's explanation, suggests exactly the opposite effect.

Combinations of factors

Among a series of self-published accounts by structural engineers, Charles Clifton emphasized that a combination of factors led to the collapse (needs expanding).

One or several collapse mechanisms?

The two towers collapsed in markedly different ways, which led some to suggest that there were two modes of failure. The north tower collapsed directly downwards, seemingly "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 continued straight down. Others, like Wilkinson, took these difference to be largely superficial. He argued that the "same mechanism of failure, the combination of impact and subsequent fire damage, is the likely cause of failure of both towers" While NIST did conclude that the collapses varied in their details, they proposed essentially the same "probable collapse sequence" for both towers.

Questions about the core

There had also been some visual evidence that the core had collapsed first: in videos the large antenna on top of the core can be seen starting downward a fraction of a second earlier than the rest of the building. NIST, however, disputed 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."

MIT: The Towers Lost and Beyond

Another early attempt, which included many of the elements already noted, came from MIT civil engineers Oral Buyukozturk and Franz-Josef Ulm on September 21, 2001.

Some 60 tons or more of jet fuel could have easily caused sustained high temperatures of 1,500 F and higher. Under these conditions, structural steel looses rigidity and strength. The resulting failure of the 2-3 floor system at the site of impact sent the 30 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.

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.

The designer speaks

A paper by Leslie E. Robertson, the lead structural engineer on the team that designed the towers, should also be noted. "The events of September 11," he said, "are not well understood by me . . . and perhaps cannot really be understood by anyone." As NIST would also conclude, however, Robertson conjectured that "the fires raging in the inner reaches of the buildings undermined their strength."

Were the fires enough?

In 2003, three engineers at the University of Edinburgh, published a paper suggesting that the fires alone would have been enough to bring down the WTC buildings. On this view, the towers were uniquely vulnerable to the effects of large fires on several floors at the same time. Even after the conclusions of the NIST study were public, at least one of these engineers, Jose Torero, is pursuing further research into the potentially catastrophic effects of fire on steel framed buildings. Moreover, when the NIST report was published, Barbara Lane, with the UK engineering firm Arup, critized its conclusion that the structural damage resulting from the airplane impacts was a necessary factor in causing the collapses.

National Institute for Standards and Technology (The NIST Report)

Design of the study

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. The study included in-house technical expertise and drew upon the knowledge of several outside private institutions for aid to include:

• Structural Engineering Institute of the American Society of Civil Engineers (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 (SEAoNY)

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.

The last word?

The publication of the NIST report did not end all disagreements about the collapses. The trade journal, the New Civil Engineer reported that some still believe that "the towers would have collapsed after a major fire on three floors at once, even with fireproofing in place and without any damage from plane impact."

Seven World Trade Center

The WTC complex had seven buildings. The third building to collapse was 7 WTC, which fell at 5:20 pm, as seen live on television. 7 WTC was a 47-story steel-frame skyscraper across the street from the rest of the complex.

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.

As part of the electrical backup system, there may have been up to 160,000 l (42,000 gallons) of diesel fuel 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. 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 Con Ed electrical substation that 7 WTC 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 7 WTC:

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.

The FEMA 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". NIST continued this work and released a progress report in June 2004 in which they outlined the working hypothesis of the collapse of 7 WTC:

• An initial local failure at the floors below floor 13 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 m²).
• 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 it.
• Collapse of the interior structure first, pulling the outer structure down and inward.
• Horizontal progression of the failure across the floors in the region of floors 5 and 7, 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 NIST regarding the collapse of 7 WTC was due in July 2005, but is still ongoing.

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."

Remarks by Osama bin Laden

Although its autenticity was questioned, a videotape of Osama bin Laden that was verified by the Pentagon indicated that Bin Laden had not believed that the buildings would collapse completely, but would collapse only above the levels where the planes struck:

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.

Other buildings

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

Other buildings destroyed include St. Nicholas Greek Orthodox Church, Marriott Hotel (3 WTC), South Plaza (4 WTC), and U.S. Customs (6 WTC). 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.

Aftermath

Effects on NYC

New Building Codes

The investigation 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.

References

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References

• 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 2006-05-02.
• Wilkinson, Tim (2006). "World Trade Center - Some Engineering Aspects". School of Civil Engineering, The University of Sydney. Retrieved 2006-05-02.
• McAllister, Therese (2002). "World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations (FEMA 403)" (pdf). Federal Emergency Management Agency. Retrieved 2006-05-02. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Eagar, Thomas W. (2001). "Why Did the World Trade Center Collapse? Science, Engineering, and Speculation". JOM, 53 (12). The Minerals, Metals & Materials Society. Retrieved 2006-05-02. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Bazant1, Zdenek P. (2001). "Why Did the World Trade Center Collapse? - Simple Analysis". Journal of Engineering Mechanics ASCE, 9/13/01. University of Illinois at Urbana-Champaign, College of Engineering. Retrieved 2006-05-02. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: numeric names: authors list (link)
• Clifton, G. Charles (2001). "Collapse of the World Trade Centre Towers". CAD Digest. TenLinks, Inc. Retrieved 2006-05-02.
• Edgar, Dr. Thomas (2002). "The Collapse: An Engineer's Perspective". Why the Towers Fell. WGBH Educational Foundation. Retrieved 2006-05-02. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Usmani, A.S. (2003). "How did the WTC towers collapse: a new theory" (pdf). Fire Safety Journal, Volume 38, Issue 6. Elsevier Ltd. Retrieved 2006-05-02. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• NOVA online (2002). "The structure of metal". Why the Towers Fell. WGBH Educational Foundation. Retrieved 2006-05-02.
• Kean, Thomas H. (2004). "Eleventh Public Hearing". Hearings. National Commission on Terrorist Attacks Upon the United States. Retrieved 2006-05-02.
• America Rebuilds: A Year at Ground Zero (Television series). United States. 2002. {{cite AV media}}: Unknown parameter |crew= ignored (help); Unknown parameter |distributor= ignored (|publisher= suggested) (help)

External links

• 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 WTC2

• World Trade Center
• September 11, 2001 attacks