The destruction of the towers of the World Trade Center in New York

The finite element model of the WTC floor, which allows to evaluate the structural system of the building

Dimensions of the structure in terms of 63.4 × 63.4 m, stiffness cores - 26.8 × 42.1 m. The WTC towers were designed as a “pipe in pipe” (trunk structural scheme), in order to create continuous floors space not separated by walls or columns . This was achieved due to the fact that the outer wall of the tower actually consisted of many columns installed nearby, which absorbed the main vertical load, while the wind load fell mainly on the power columns located in the center of the tower (stiffness core). Starting from the tenth floor, each tower wall was made up of 59 columns, 49 power columns were installed in the center of the tower ^[5] All elevators and stairwells passed in the core of rigidity, leaving a large free space between the core and the perimeter of the tower for offices ^[6] .

The floor structure was 10 cm of lightweight concrete laid on fixed formwork from profiled flooring . Decking was laid on secondary (auxiliary) trusses , based on the main trusses, transferring the load to the central and extreme columns. The main trusses were 11 and 18 meters long (depending on the span), laid in increments of 2.1 m, and were attached on the outside to the lintels connecting the extreme columns at the level of each floor, and from the inside to the central columns. The floor was fastened through elastic dampers , designed to reduce the impact of building vibrations on people working in it ^[6] .

Each tower also had an additional reinforcement structure for the upper part, including upper trusses and cantilever trusses located between 107th and 110th floors, which consisted of six trusses along the long axis (core) and four along the short axis, which served to increase the total the stability of the building and the redistribution of load from the antenna spire, which was installed only on one of the towers. NIST found that this design played a key role in the sequence of events leading to the complete destruction of the towers ^[6]

Opportunities to counter fire and aircraft

Like all modern high-rise buildings, the WTC towers were designed and built in anticipation of an ordinary fire. Many fire elements were laid at the design and construction stage, others were added after the 1975 fire, which spanned six floors, before it was localized and extinguished ^[3] . Tests made before the disaster showed that the steel structures of the towers meet or even exceed existing fire resistance requirements ^[7] .

Structural engineers who designed the World Trade Center took into account the possibility that an airplane could collide with the building. In July 1945, the B-25 Mitchell bomber lost its orientation in the fog and crashed into the 79th floor of the Empire State Building ^[8] . A year later the twin-engine Beechcraft C-45 collided with a skyscraper on Wall Street 40 ^[9] , and the other plane was close to yet another collision with the Empire State Building ^[8] .

NIST stated that “American building standards do not contain requirements for the sustainability of buildings when an airplane enters them. ... and thus, buildings are not designed to withstand the impact of a fully refueled commercial airliner. ” ^[10] , however, the WTC designers and architects discussed this problem and recognized its importance. , one of the World Trade Center's chief engineers, recalled that he was considering the scenario of a jet landing on a Boeing 707 airliner, which had lost its orientation in the fog and was flying at a relatively low speed in search of John F. Kennedy Airport or Newark Liberty Airport ^{[8] [11] [12]} . John Skilling, another WTC engineer, said in 1993 that his subordinates conducted an analysis that showed that the biggest problem in the event of a Boeing 707 airliner colliding with one of the WTC towers would be getting a large mass of fuel inside the building, resulting in a “terrible fire” "And many casualties, but the building itself will remain standing ^{[12] [13]} . FEMA wrote that the WTC buildings were built taking into account a collision with a Boeing 707 airliner weighing 119 tons and flying at a low speed of about 290 km / h, which is much less in weight and speed than those planes that were used in the attacks September 11 ^{[12] [14]} .

NIST found a three-page report in the archives summarizing a study simulating a hit in a Boeing 707 or Douglas DC-8 building at a speed of 950 km / h. The study showed that the building should not be destroyed as a result of such a hit ^[12] . But, as NIST experts noted, "the 1964 study did not contain a simulation of the effect of the fire caused by the spraying of aviation fuel in a building" ^[15] . NIST also noted that in the absence of initial calculations used to model the situation ^[16] , further commenting on this topic will be mainly “speculation” ^[17] . Another document found by NIST was the calculation of the building’s oscillation period in the event of a plane getting into the 80th floor of the WTC tower, but it made no assumptions about the fate of the building after the collision ^[12] . In the property risk assessment report prepared for Silverstain Properties , the case of an aircraft entering the WTC towers was considered unlikely, but possible. The authors of the report referred to the construction engineers of the WTC, according to which the towers should have survived in the event of a collision of a large passenger aircraft, but burning fuel flowing out of the aircraft to ground level would damage the cladding of the building ^{[12] [16]} . Some of the documents related to the analysis of the scenario of an aircraft entering a tower were lost as a result of the destruction of WTC 1 and WTC 7, which contained documents from the Port Authority of New York and New Jersey and Silverstain Properties ^[16] .

The terrorists sent two Boeing 767 airliners, American Airlines flight 11 (767-200ER) and 175 United Airlines flight (767-200) to the towers. The North Tower (1 WTC) was hit at 8:46 a.m. flight 11, between 93 and 99 floors. Flight 175 crashed into the south tower (2 WTC) at 9:03, between floors 77 and 85.

The Boeing 767-200 airliner has a length of 48.5 m, a wingspan of 48 m, carries on board from 62 tons (-200) to 91 tons (-200ER) of aviation fuel ^[18] . Aircraft crashed into towers at a very high speed. Flight 11 flew at a speed of approximately 700 km / h at the time it crashed into the north tower; Flight 175 crashed into the south at a speed of about 870 km / h ^[6] . In addition to the fact that the hits led to serious destruction of the supporting columns, they caused an explosion of approximately 38 tons of aviation fuel in each tower ^[3] , which led to the almost instantaneous spread of a strong fire on several floors, where there was office furniture , paper , carpets , books and other combustible materials. The shock wave from reaching the north tower spread down to the first floor, at least one shaft of a high-speed elevator , broke glass on the first floor, which caused several people to be injured.

The light construction of the towers and the absence of solid walls and ceilings led to the fact that aviation fuel spread in a fairly large volume of buildings, leading to numerous fires on several floors close to the zones of aircraft entry. Aviation fuel itself burned out for several minutes, but combustible materials in the building itself maintained intense fires for another hour or a half ^[19] . It is possible that if more traditional constructions appeared in the way of the planes, then the fires would not be so centralized and intense - the wreckage of aircraft and aviation fuel could remain mainly in the peripheral area of ​​the building, and not penetrate directly to its central part. In this case, the towers would probably stand, or in any case, would stand much longer ^{[20] [21]} .

Situation development

The calls of people locked on the upper floors of the towers to the service 9-1-1 give some idea of ​​the further development of the situation. At 9:37 a man called on the 105th floor of the south tower called the service and said that the floors under it “in the region of 90 floors” collapsed ^[22] . The deterioration of the situation was also reported from helicopters of the fire department, which were in the air in the vicinity of burning towers ^[22] .

• 9:52 - a helicopter of the fire department reports on the radio that “large pieces of the building may fall from the upper floors of the south tower. We see large parts of the building in limbo. ”
• 9:59 - from a helicopter they report that the south tower is falling.

Also, helicopters reported on the development of the situation with the north tower of the WTC ^[22] .

• 10:20 a.m. - A fire department helicopter reports that the upper floors of the north tower may be unstable.
• 10:21 - it is reported that the southeast corner of the tower has squinted, and that the tower is beginning to slope southward.
• 10:27 - it is reported that the roof of the north tower may collapse at any moment.
• 10:28 - the fire department receives a message that the north tower collapsed.

Overloading dispatchers and poor communications led to the fact that the fire and police departments of New York experienced great problems in ensuring timely interaction, both with their units and with each other. As a result, the fire brigades in the towers did not receive an evacuation order and 343 firefighters died in the collapse of the buildings ^{[23] [24] [25]} .

At 9:59, the South Tower collapsed, 56 minutes after the strike. The north tower stood until 10:28, 102 minutes after the plane hit it. The collapsed towers created a huge cloud of dust that covered a significant part of Manhattan . In both cases, a similar process occurred, the upper damaged part of the building collapsed on the lower floors. Both towers fell almost vertically, although there was a significant deviation from the vertical of the upper part of the south tower. It was also observed that debris and dust flew from the windows of the building below the rapidly advancing collapse zone.

Tower Mechanism

The NIST investigation showed that due to the fact that the planes hit the towers in an unequal manner, the process of destruction of the north and south towers was also slightly different, although in general it was similar. After the aircraft hit, the internal power columns were seriously damaged, although the external columns were relatively weak. This caused a serious redistribution of the load between them. A significant role in this redistribution was played by the upper power structure of the towers ^[4] , the weight of which now accounted mainly for peripheral columns, not internal ones.

Airplanes that got into the buildings peeled off the refractory coating from a significant part of the steel structures, which led to the fact that they were under the direct influence of fire. In the 102 minutes preceding the fall of the north tower, the temperature of the fire, although it was much lower than the melting point of the metal , reached a sufficient value to cause weakening of the power columns in the center of the building, which began to deform and bend under the weight of the upper floors. The NIST report describes this situation as follows:

You can imagine the central power frame of the north tower in the form of three sections. The lower section (below the destruction zone) was a rigid, stable, undamaged structure with a temperature close to normal. The upper section, above the destruction zone, was also a rigid box, which also has a large weight. The middle section located between them was damaged by the blow and explosion of the aircraft, and was also weakened by the fire. The upper part of the power frame tended to fall below, but it was supported by the upper truss, based on peripheral columns. As a result, this design created a large load on the perimeter of the building.

Original text

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.

- NIST Report, p. 29

A similar situation was in the south tower (internal power columns were seriously damaged). The peripheral columns and floor structures of both towers were weakened by fire, which caused the floor to subside on damaged floors and created a significant load on the peripheral columns directed inside the building.

At 9:59 a.m., 56 minutes after the impact, the settling floor caused serious bending inside the outer columns on the eastern side of the south tower, the upper power structure transferred this bending force to the central columns, which caused their destruction and the collapse of the building, the upper part of the tower deviated towards the damaged wall (there is no explanation why that part of the south tower, which tilted east, and began a free fall was not found on the whole earth). At 10:28, the south wall of the north tower warped, which caused approximately the same sequence of events. As a result of the collapse of the upper floors, the complete destruction of the towers became inevitable, due to the enormous weight of that part of the buildings that were above the damage zone.

The reason that the north tower stood longer than the south one was a combination of the following three factors: the zone where the plane hit the north tower was higher (and the weight of the upper part of the building, respectively, less), the speed of the plane that hit the tower was lower, in addition, the plane hit floors, fire protection of which was previously partially improved ^[4] .

Theory of Complete Progressive Collapse

A huge cloud of dust covered the collapsing towers, which made it impossible to establish the exact duration of the destruction based on visual evidence.

Since the NIST report deals mainly with the mechanisms of the initial collapse, it does not address the issue of the subsequent complete collapse of both WTC towers. Initial analyzes explained the collapse in that the kinetic energy of the falling upper floors was much greater than what the floor floors could withstand, which also collapsed, adding kinetic energy to the falling building. ^[26] This scenario was repeated at an increasing rate until the towers were completely destroyed. Despite the fact that this is the most widely held point of view among construction engineers ^[27] , it was criticized for not taking into account the resistance of lower structures, which was supposed to slow down the collapse of towers, or even stop it. ^[28]

The WTC complex consisted of seven buildings, three of which completely collapsed on the same day when the terrorist attacks occurred. At 17:20, the 47-story building 7 WTC collapsed, located across the street from the main towers, it became the third completely destroyed WTC building. Unlike the twin towers, the collapse of 7 WTC was predicted in a few hours, and the building was completely evacuated. To observe the burning and collapsing building, geodetic instruments were used ^[29] . The initial FEMA study was inconclusive ^[30] , and the collapse of 7 WTC was not included in the final NIST report published in September 2005 . With the exception of the letter published by the Metallurgical Journal , in which it was assumed that the steel frame of the building could melt from the fire ^[31] , no other studies of this issue were published in scientific journals. The fall of 7 WTC was investigated separately from the fall of 1 WTC and 2 WTC, and in June 2004, NIST issued a working report, which contained several hypotheses of what happened. One of the hypotheses was the destruction of one of the critical support columns of the building, caused by a fire or large debris from falling towers, which led to a “disproportionate collapse of the entire structure” ^{[32] [33]} .

On November 20, 2008, NIST published the final report on the fall of 7 WTC ^[34] . In the NIST report, the main cause of the destruction was called the fire along with the lack of water for fighting fire in firefighters and in the automatic fire extinguishing system. NIST restores the sequence of events as follows: at 10:28, the debris from the falling 1 WTC caused damage in the nearby 7 WTC. Fire also occurred, probably from burning debris from 1 WTC. Firefighters arrived immediately at 7 WTC, but at 11:30 they found that there was no water in the fire hydrants to fight the fire - water came from the city's water supply system, which was destroyed by the fall of the 1 WTC and 2 WTC towers. The Fire Department of New York , fearing for the life of firefighters in the event of the destruction of 7 WTC, at 14:30 recalled the firefighters and stopped the struggle to preserve the building. Fire was observed on 10 floors from the 7th to the 30th, and on floors 7–9 and 11–13 the fire went out of control ^{[N08 1] [N08 2]} . Thermal expansion of beams heated to approximately 400 ° C around column 79 in the eastern part of the building in the 13-14 floor area led to the failure of fire-weakened ceilings (steel does not lose its strength characteristics at a temperature of 400 degrees. Steel begins to weaken at temperatures above 800 degrees), adjacent to the column 79, from the 13th to the 5th floor. The destruction of the ceilings deprived the column 79 of horizontal support, and it began to bend, which was the direct cause of the complete destruction of the building after a few seconds ^{[N08 2] [N08 3] [35] [36]} . The bending of the column 79 led to the transfer of load to the columns 80 and 81, which also began to bend, as a result of which all the ceilings connected with these columns were destroyed to the top of the building. Falling floors destroyed Farm 2, which led to the fall of columns 77, 78, and 76. As a result of the increased load transferred from the bent columns, falling from the top of the floor debris and the lack of horizontal support from the side of the destroyed floors, all internal columns from east to west began to bend sequentially. Following this, in the area of ​​floors 7-14, the external columns began to bend, to which the load transferred from the descending inner columns and the center, and all floors above the bent columns began to descend as a whole, which completed the final destruction of the building at 17:20 ^{[N08 2] [35]} .

Some authors criticized the decision of the city authorities to place the Office of Emergency Management headquarters on the 23rd floor of the 7th WTC . It was thought that this could be a significant factor in the destruction of the building. ^[37] Particularly noted was the placement in the building of large tanks with diesel fuel , which were supposed to be used to power emergency power generators ^[38] . NIST concluded that diesel did not play a role in the destruction of the building, but the rapid evacuation of the Emergency Office was one of the reasons for the poor interaction of various services and the loss of control over the situation ^{[N08 4]} . The main reason for the destruction of the building was a fire, damage from the debris of the falling 1 WTC accelerated the fall of the building, but NIST calculations showed that 7 WTC would also have collapsed from an uncontrolled fire ^{[36] [N08 5]} .

First reaction

The destruction of the WTC towers was a surprise to construction engineers. “Until September 11,” the New Civil Engineer magazine wrote, “we could not sincerely assume that a structure of such a magnitude could befall such a fate” ^[39] . Despite the fact that the damage from the air strikes was very serious, they affected only a few floors of each building. It was a challenge for engineers to find out how such local injuries caused a complete progressive collapse of three, one of the largest buildings in the world ^[27] . In an interview with the BBC in October 2001 , English architect Bob Halvorson quite successfully predicted that there would be numerous “debates about whether the World Trade Center could collapse in the way he did.” Architectural and design should go into full analysis. WTC plans, witness testimonies, videos of the destruction, debris research data, and so on. Emphasizing the complexity of this task, Halvorson said that the destruction of the WTC towers is "far beyond the ordinary experience" ^[40] .

Research authority

Immediately after the disaster, a situation of uncertainty arose over who has sufficient authority to conduct an official investigation. In contrast to the practice that exists in the investigation of air crashes , clear procedures for investigating the collapse of buildings simply did not exist ^[41] .

Shortly after the disaster, a working group was created on the basis of the Structural Engineers Institute (SEI), the ), and specialists were also involved. Institute, American Institute of Steel Construction , American Concrete Institute , National Fire Protection Association and Society of Fire Protection Engineers, Society of Fire Protection Engineers ) ^[42] . ASCE also invited the Federal Emergency Management Agency (FEMA), which later received the joint management of ASCE-FEMA ^[42], to join the group. This investigation was subsequently criticized by American engineers and lawyers , however, the authority of the above organizations was sufficient to conduct an investigation and to provide access to the scene of the disaster for the group experts. One of the most pressing moments of the investigation was that clearing the crash site actually led to the destruction of the remaining components of the buildings ^[43] . Indeed, when NIST published its final report, it noted a “lack of physical evidence”, which became one of the main problems of the investigation. Only a fraction of a percent of the remains of the building remained available for investigation after the completion of the clearing work; in total, 236 individual steel fragments were collected ^[4] .

FEMA published its report in May 2002 . Despite the fact that NIST had already declared its involvement in the investigation in August of that year, in October 2002, under increasing pressure from the public to demand a more detailed investigation, Congress passed a bill to create a new group under the management of NIST, which published its report in September 2005 ^{[44] [45]} .

FEMA Pancake Stack Theory

In its early investigations, FEMA developed a theory to explain the collapse of the WTC towers, called the Pancake Theory . This theory was defended by Thomas Iga and was widely covered by PBS ^[46] . According to this theory, the connection between the floor-supporting farms and the columns of the building was destroyed, as a result of which the floor fell a floor below, creating loads for its construction that were not designed for it ^[47] . Some individual publications suggested other sets of factors that caused the collapse of the towers, but in general the majority accepted this theory ^{[21] [48]} .

The main key factor in this theory remained fires. Thomas Iga, professor of materials science at MIT , described fires as “the most obscure part of the collapse of the WTC towers” ^[47] . Despite the fact that it was initially believed that the fires “melted” the steel structures, Iga stated that “the temperature of the fires in the towers of the WTC was unusually high, but still definitely not enough to cause the steel to melt or seriously soften.” The ignition of aviation kerosene usually leads to extensive fires, but these fires do not have a very high temperature. This led Iga, FEMA and other researchers to believe that there was a weak point, and this point was called the fastening of the floor to the supporting structures of the building. Because of the fire, these mounts weakened and, when they collapsed under the weight of the floor, a collapse began ^{[49] [50]} . On the other hand, the NIST report fully and unequivocally states that these mounts were not destroyed. Moreover, it was their strength that was one of the main causes of the collapse, since through them the force that bent the columns inward was transmitted to the peripheral columns.

At temperatures above 400-500 ° C, a sharp decrease in the tensile strength and yield strength (3-4 times) occurs, at 600 ° C they are close to zero and the bearing capacity of the steel is exhausted ^[51] .

NIST Report

Research Organization

As a result of increasing pressure from experts, leaders in the construction industry and family members of the victims following the publication of the FEMA report, the National Institute of Standards and Technology ( NIST ) of the Department of Commerce conducted a three-year investigation into the causes of the destruction and collapse of the WTC towers, which cost $ 24 million ^[52] . The study included a series of experiments, in addition, leading experts from many third-party organizations were involved in it:

• 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 )

Field of study and its limitations

The NIST research area was limited to studying the issue of “the sequence of events from the moment an aircraft hit to the beginning of the collapse of each tower”, and also included “a small analysis of the structural behavior of the tower structure after the conditions of its destruction were reached and collapse became inevitable” ^{[ 53]} . Like many other engineers involved in this matter, NIST experts focused on getting planes into towers, simulating the effects of hits such as structural damage, fire spread, etc. with a very high degree of detail. NIST has developed several very highly detailed models of various building components, such as floor-supporting lintels, in addition, the entire buildings were modeled, but with a lower level of detail. These models were static, or quasistatic, included deformation modeling, but did not include modeling the movement of structural elements after they were disconnected from each other. Thus, NIST models are useful for elucidating the reasons for the beginning of the collapse of towers, but they do not allow modeling the collapse itself.

Parallel Investigations

In 2003, three engineers at the University of Edinburgh published a report that suggested that the mere action of fires, even without taking into account the destructive effects of aircraft strikes, was enough to completely destroy the WTC towers. По их мнению, конструкция башен делала их исключительно уязвимыми для обширных пожаров, охватывающих сразу несколько этажей ^[54] Когда вышел отчёт NIST, Барбара Лейн и английская инженерная компания Arup критиковали содержащееся в нём заключение, что разрушения, вызванные попаданием самолётов, стали необходимым фактором для начала разрушения зданий ^{[55] [56] [57] [58]} .

Criticism

Джеймс Квинтере, профессор по противопожарной защите Университета Мэриленда , указывал, что отчёту NIST недостаёт чёткого изложения последовательности событий и физических свидетельств происшедшего ^[59] .

11 сентября 2001 года был разрушен весь комплекс зданий Всемирного торгового центра и маленькая православная греческая церковь Св. Николая , стоявшая на Либерти-стрит напротив южной башни ВТЦ. Кроме того, в той или иной степени пострадали многие окружающие комплекс здания.

Расчистка места катастрофы

Огромная гора обломков на месте ВТЦ продолжала гореть и тлеть ещё в течение трёх месяцев, попытки справиться с огнём не имели успеха до тех пор, пока не было вывезено значительное количество обломков и мусора ^{[60] [61]} . Расчистка стала весьма масштабной операцией, координацией которой занимался Строительный Департамент (DDC). Предварительный план расчистки был подготовлен ещё 22 сентября, компанией Controlled Demolition Inc. (CDI) из Феникса. Марк Лозо, президент CDI, особенно подчёркивал важность защиты глиняной стены (или «ванны»), которая предохраняла фундамент ВТЦ от затопления водами Гудзона ^[62] . Расчистка велась круглосуточно, с привлечением большого количества подрядчиков, и обошлась в сотни миллионов долларов ^[63] . В начале ноября, после того как была убрана примерно треть мусора, правительство города стало сокращать участие в расчистке полицейских и пожарных, занимавшихся поиском останков погибших, и перенесло приоритеты на вывоз мусора. Это вызвало возражения пожарных ^[64] .

По состоянию на 2007 год продолжался снос некоторых окружающих ВТЦ зданий, на фоне строительства замены ВТЦ, мемориального комплекса и «Башни Свободы» ( Freedom Tower ).

Существует версия, что башни ВТЦ могли быть разрушены в результате спланированного контролируемого сноса, а не в результате попадания самолётов. Эта теория была отвергнута NIST, который пришёл к заключению, что в процессе обрушения башен не было участия взрывчатых веществ ^[65] . NIST заявил, что он не выполнял тесты для поиска остатков взрывоопасных веществ в любом роде в обломках ввиду того, что в этом не было необходимости:

12. Did the NIST investigation look for evidence of the WTC towers being brought down by controlled demolition? Was the steel tested for explosives or thermite residues? The combination of thermite and sulfur (called thermate) «slices through steel like a hot knife through butter.»

NIST did not test for the residue of these compounds in the steel.

The responses to questions number 2, 4, 5 and 11 demonstrate why NIST concluded that there were no explosives or controlled demolition involved in the collapses of the WTC towers.

— http://www.nist.gov/public_affairs/factsheet/wtc_faqs_082006.cfm

В отчёте 2008 года NIST также проанализировал гипотезу взрыва в башне 7 WTC и пришёл к выводу, что взрыв не мог вызвать наблюдаемых последствий. В частности, наименьшее количество взрывчатки, которое смогло бы разрушить колонну 79, вызвало бы шум в 130—140 децибел на расстоянии 1 км от 7 WTC, но такой шум не был замечен ни звукозаписывающей аппаратурой, ни свидетелями ^{[N08 6] [36]} . Данная теория стала заметной частью большинства всех «теорий заговора», возникших в результате событий 11 сентября ^[66] .

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• NIST and the World Trade Center (neopr.) . NIST building and fire safety investigation . US Department of Commerce. Date of treatment May 2, 2006. Archived March 4, 2012.
  • NIST NCSTAR 1A: Final Report on the Collapse of World Trade Center Building 7 21-23, 4446. NIST (November 2008). Date of treatment February 23, 2010. Archived February 23, 2010.
• Therese McAllister, W. Gene Corley et al. World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations (FEMA 403) (neopr.) . Federal Emergency Management Agency (2002). Date of treatment May 2, 2006. Archived March 4, 2012.
• Thomas W. Eagar, Christopher Musso. Why did the World Trade Center Collapse? Science, Engineering, and Speculation (Neopr.) . JOM, 53 (12) . The Minerals, Metals & Materials Society (2001). Date of treatment May 2, 2006. Archived March 4, 2012.
• G. Charles Clifton. Collapse of the World Trade Center Towers (Neopr.) (Link not available) . CAD Digest . TenLinks, Inc. (2001). Date of treatment May 2, 2006. Archived March 4, 2012.
• AS Usmani, YC Chung, JL Torero. How did the WTC towers collapse: a new theory (unopened) (pdf). Fire Safety Journal, Volume 38, Issue 6 . Elsevier Ltd. (2003). Date of treatment May 7, 2007. Archived March 4, 2012.
• The structure of metal (neopr.) . Why the Towers Fell . WGBH Educational Foundation (2002). Date of treatment May 2, 2006. Archived June 13, 2002.
• Thomas H. Kean. Eleventh Public Hearing (Neopr.) . Hears . National Commission on Terrorist Attacks Upon the United States (2004). Date of treatment May 2, 2006. Archived March 4, 2012.
• Evgeny Dodolev , Marina Lesko . The riddles of September 11, or 27 classified pages // Spark : Journal. - 2003. - No. 52 . Archived on October 9, 2007.
• World Trade Center - Some Engineering Aspects One of the first studies of the destruction of the WTC of Sydney University.
• Bill Biggart's Final Exposures contains photographs of the seriously damaged WTC Marriott Hotel, taken after the destruction of the WTC’s south tower and before the north’s destruction, during which the photographer himself died.
• Video showing fire in corner of 2 WTC
• Forensic animations used in court