2.3 Observations and Findings

The structural damage sustained by each of the two buildings as a result of the terrorist attacks was massive. The fact that the structures were able to sustain this level of damage and remain standing for an extended period of time is remarkable and is the reason that most building occupants were able to evacuate safely. Events of this type, resulting in such substantial damage, are generally not considered in building design, and the ability of these structures to successfully withstand such damage is noteworthy.

Preliminary analyses of the damaged structures, together with the fact the structures remained standing for an extended period of time, suggest that, absent other severe loading events such as a windstorm or earthquake, the buildings could have remained standing in their damaged states until subjected to some significant additional load. However, the structures were subjected to a second, simultaneous severe loading event in the form of the fires caused by the aircraft impacts.

Figure 2-32 The top portion of WTC 2 falls to the east, then south, as viewed from the northeast.

The large quantity of jet fuel carried by each aircraft ignited upon impact into each building. A significant portion of this fuel was consumed immediately in the ensuing fireballs. The remaining fuel is believed either to have flowed down through the buildings or to have burned off within a few minutes of the aircraft impact. The heat produced by this burning jet fuel does not by itself appear to have been sufficient to initiate the structural collapses. However, as the burning jet fuel spread across several floors of the buildings, it ignited much of the buildings' contents, causing simultaneous fires across several floors of both buildings. The heat output from these fires is estimated to have been comparable to the power produced by a large commercial power generating station. Over a period of many minutes, this heat induced additional stresses into the damaged structural frames while simultaneously softening and weakening these frames. This additional loading and the resulting damage were sufficient to induce the collapse of both structures.

Because the aircraft impacts into the two buildings are not believed to have been sufficient to cause collapse without the ensuing fires, the obvious question is whether the fires alone, without the damage from the aircraft impact, would have been sufficient to cause such a collapse. The capabilities of the fire protection systems make it extremely unlikely that such fires would develop without some unusual triggering event like the aircraft impact. For all other cases, the fire protection for the tower buildings provided in-depth protection. The first line of defense was the automatic sprinkler protection. The sprinkler system was intended to respond quickly and automatically to extinguish or confine a fire. The second line of defense consisted of the manual (FDNY/Port Authority Fire Brigade) firefighting capabilities, which were supported by the building standpipe system, emergency fire department use elevators, smoke control system, and other features. Manual suppression by FDNY was the principal fire protection mechanism that controlled a large fire that occurred in the buildings in 1975. Finally, the last line of defense was the structural fire resistance. The fire resistance capabilities would not be called upon unless both the automatic and manual suppression systems just described failed. In the incident of September 11, not only did the aircraft impacts disable the first two lines of defense, they also are believed to have dislodged fireproofing and imposed major additional stresses on the structural system.

Had some other event disabled both the automatic and manual suppression capabilities and a fire of major proportions occurred while the structural framing system and its fireproofing remained intact, the third line of defense, structural fireproofing, would have become critical. The thickness and quality of the fireproofing materials would have been key factors in the rate and extent of temperature rise in the floor trusses and other structural members. In the preparation of this report, there has not been sufficient analysis to predict the temperature and resulting change in strength of the individual structural members in order to approximate the overall response of the structure. Given the redundancy in the framing system and the capability of that system to redistribute load from a weakened member to other parts of the structural system, it is impossible, without extensive modeling and other analysis, to make a credible prediction of how the buildings would have responded to an extremely severe fire in a situation where there was no prior structural damage. Such simulations were not performed within the scope of this study, but should be performed in the future.

Buildings are designed to withstand loading events that are deemed credible hazards and to protect the public safety in the event such credible hazards are experienced. Buildings are not designed to withstand any event that could ever conceivably occur, and any building can collapse if subjected to a sufficiently extreme loading event. Communities adopt building codes to help building designers and regulators determine those loading events that should be considered as credible hazards in the design process. These building codes are developed by the design and regulatory communities themselves, through a voluntary committee consensus process. Prior to September 11, 2001, it was the consensus of these communities that aircraft impact was not a sufficiently credible hazard to warrant routine consideration in the design of buildings and, therefore, the building codes did not require that such events be considered in building design. Nevertheless, the design of WTC 1 and WTC 2 did include at least some consideration of the probable response of the buildings to an aircraft impact, albeit a somewhat smaller and slower moving aircraft than those actually involved in the September 11 events. Building codes do consider fire as a credible hazard and include extensive requirements to control the spread of fire throughout buildings, to delay the onset of fire-induced structural collapse, and to facilitate the safe egress of building occupants in a fire event. For fire-protected steel-frame buildings, like WTC 1 and WTC 2, these code requirements had been deemed effective and, in fact, prior to September 11, there was no record of the fire-induced-collapse of such structures, despite some very large uncontrolled fires.

The ability of the two towers to withstand aircraft impacts without immediate collapse was a direct function of their design and construction characteristics, as was the vulnerability of the two towers to collapse a result of the combined effects of the impacts and ensuing fires. Many buildings with other design and construction characteristics would have been more vulnerable to collapse in these events than the two towers, and few may have been less vulnerable. It was not the purpose of this study to assess the code conformance of the building design and construction, or to judge the adequacy of these features. However, during the course of this study, the structural and fire protection features of the buildings were examined. The study did not reveal any specific structural features that would be regarded as substandard, and, in fact, many structural and fire protection features of the design and construction were found to be superior to the minimum code requirements.

Several building design features have been identified as key to the buildings' ability to remain standing as long as they did and to allow the evacuation of most building occupants. These included the following:

• robustness and redundancy of the steel framing system

• adequate egress stairways that were well marked and lighted

• conscientious implementation of emergency exiting training programs for building tenants Similarly, several design features have been identified that may have played a role in allowing the buildings to collapse in the manner that they did and in the inability of victims at and above the impact floors to safely exit. These features should not be regarded either as design deficiencies or as features that should be prohibited in future building codes. Rather, these are features that should be subjected to more detailed evaluation, in order to understand their contribution to the performance of these buildings and how they may perform in other buildings. These include the following:

• the type of steel floor truss system present in these buildings and their structural robustness and redundancy when compared to other structural systems

• use of impact-resistant enclosures around egress paths

• resistance of passive fire protection to blasts and impacts in buildings designed to provide resistance to such hazards

• grouping emergency egress stairways in the central building core, as opposed to dispersing them throughout the structure

During the course of this study, the question of whether building codes should be changed in some way to make future buildings more resistant to such attacks was frequently explored. Depending on the size of the aircraft, it may not be technically feasible to develop design provisions that would enable all structures to be designed and constructed to resist the effects of impacts by rapidly moving aircraft, and the ensuing fires, without collapse. In addition, the cost of constructing such structures might be so large as to make this type of design intent practically infeasible.

Although the attacks on the World Trade Center are a reason to question design philosophies, the BPS Team believes there are insufficient data to determine whether there is a reasonable threat of attacks on specific buildings to recommend inclusion of such requirements in building codes. Some believe the likelihood of such attacks on any specific building is deemed sufficiently low to not be considered at all. However, individual building developers may wish to consider design provisions for improving redundancy and robustness for such unforeseen events, particularly for structures that, by nature of their design or occupancy, may be especially susceptible to such incidents. Although some conceptual changes to the building codes that could make buildings more resistant to fire or impact damage or more conducive to occupant egress were identified in the course of this study, the BPS Team felt that extensive technical, policy, and economic study of these concepts should be performed before any specific code change recommendations are developed. This report specifically recommends such additional studies. Future building code revisions may be considered after the technical details of the collapses and other building responses to damage are better understood.

2.4 Recommendations

The scope of this study was not intended to include in-depth analysis of many issues that should be explored before final conclusions are reached. Additional studies of the performance of WTC 1 and WTC 2 during the events of September 11, 2001, and of related building performance issues should be conducted. These include the following:

• During the course of this study, it was not possible to determine the condition of the interior structure of the two towers, after aircraft impact and before collapse. Detailed modeling of the aircraft impacts into the buildings should be conducted in order to provide understanding of the probable damage state immediately following the impacts.

• Preliminary studies of the growth and heat flux produced by the fires were conducted. Although these studies provided useful insight into the buildings' behavior, they were not of sufficient detail to permit an understanding of the probable distribution of temperatures in the buildings at various stages of the event and the resulting stress state of the structures as the fires progressed. Detailed modeling of the fires should be conducted and combined with structural modeling to develop specific failure modes likely to have occurred.

• The floor framing system for the two towers was complex and substantially more redundant than typical bar joist floor systems. Detailed modeling of these floor systems and their connections should be conducted to understand the effects of localized overloads and failures to determine ultimate failure modes. Other types of common building framing should also be examined for these effects.

• The fire-performance of steel trusses with spray-applied fire protection, and with end restraint conditions similar to those present in the two towers, is not well understood, but is likely critical to the building collapse. Studies of the fire-performance of this structural system should be conducted.

• Observation of the debris generated by the collapse of the towers and of damaged adjacent structures suggests that spray-applied fire proofing may be vulnerable to mechanical damage from blasts and impacts. This vulnerability is not well understood. Tests of these materials should be conducted to understand how well they withstand such mechanical damage and to determine whether it is appropriate and feasible to improve their resistance to such damage.

• In the past, tall buildings have occasionally been damaged, typically by earthquakes, and experienced collapse within the damaged zones. Those structures were able to arrest collapse before they progressed to a state of total collapse. The two WTC towers were able to arrest collapse from the impact damage, but not from the resulting fires when combined with the impact effects of the aircraft attacks. Studies should be conducted to determine, given the great size and weight of the two towers, whether there are feasible design and construction features that would permit such buildings to arrest or limit a collapse, once it began.

2.5 References

BBC News. 2001. «We Ran for Our Lives.» Account of Mike Shillaker. September 13.

Cauchon, D. 2001a. «For Many on Sept. 11, Survival Was No Accident,» USA Today.com. December 19. Cauchon, D. 2001b. «Four Survived by Ignoring Words of Advice,» USA Today.com. December 19. Computers and Structures, Inc. (CSI). 2000. SAP-2000. Berkeley, CA. Dateline NBC. 2001. «The Miracle of Ladder Company 6.» September 28. Hearst, D. 2001. «Attack on America: Survivors: Suddenly they started to yell out, `get out now': Bravery and fear mingled with disbelief,» Guardian Home Pages, page 15. Account of Simon Oliver. September 13.

Labriola, J. 2001. Personal account. Channel 4 News, «Inside the World Trade Center,» broadcast. September 13.

Masetti, A. 2001. Personal account received by email. December 21. Mayblum, A. 2001. Personal account. www.worldtradecenternews.org/survivorstory.html, World Trade Center Miracles section. September 18.

New York Board of Fire Underwriters. 1975. One World Trade Center Fire, February 13, 1975. Nicholson, W. J.; Rohl, A. N.; Wesiman, I.; and Seltkoff, I.J. 1980. Environmental Asbestos Concentrations in the United States, page 823. Environmental Sciences Laboratory, Mount Zion Hospital, New York, NY.

Scripps, H. 2001. «I walked out... I made it out alive,» Boston Herald.com. Account of John Walsh. September 14.

Shark, G., and McIntyre, S. December 5, 2001. ABS. Personal account. Smith, D. 2002. Report from Ground Zero. Viking Penguin, New York. p. 29. Zalosh, R. G. 1995. «Explosion Protection,» SFPE Handbook of Fire Protection Engineering, 2nd edition. Quincy, MA.

Isn't it comforting that a supposedly scientific article about the collapse of the WTC (apart from articles on the 1975 WTC fire) only quotes survival stories from media sources?

The author of the comments (italic) is unknown. This is one of many pages, mostly concerning 9/11, which appeared anonymously during 2002-2003 at nerdcities.com/guardian/. The website disappeared in April 2003, but is preserved at http://thewebfairy.com/nerdcities/.

Это коммюнике было взято с сервера Министерства обороны США. С ним можно ознакомиться на сайте архивов Йельского университета:

• http://www.yale.edu/lawweb/avalon/sept_11

Part of Pentagon Collapse after Terrorists Crash Plane into Building — депеша Associated Press от 11 сентября 2001 г.

The Day the World Changed in The Christian Science Monitor от 17 сентября 2001 г.;

• http://www.csmonitor.com/

Inside the Pentagon Minutes Before Raid Рика Пирсона (Rick Pearson), in Chicago Tribune от 12 сентября 2001 г.;

• http://www.chicagotribune.com/

DodNews Briefing от 12 сентября 2001 г., 15 часов 25 минут.

Special Edition, Christian Science Monitor от 17 сентября 2001 г.; можно скачать с сайта:

• http://www.csmonitor.com/pdf/csm20010917.pdf

Доступно на: http://www.senate.gov/~armed_services

Доступно на: http://www.peterson.af.mil/norad/presrelNORADTimelines.htm

См. также: Military Alerted Before Attacks Брадли Грэма (Bradley Graham) в The Washington Post от 15 сентября 2001 г.;

• http://www.washingtonpost.com

US Jets Were Just Eight Minutes Away from Shooting Down Hijacked Plane Андрю Гам-бела (Andrew Gumbel) в The Independent от 20 сентября 2001 г.;

• http://news.independent.co.uk

Ср. Public Report of the White House Security Review (10 мая 1995 г.);

• http://www. fas. org/irp/agency/ustreas/usss/tlpubrpt. html

Официальный сайт базы Эндрюс:

• http:/www. dcmilitary. com/baseguides/airforce/andrews

В Newsday от 23 сентября 2001 г.

В соответствии с информацией, полученной от конструктора;

• http://www.boeing.%20com/commercial/757-200/product.html

Виртуальное посещение Пентагона на сайте:

• http://www. defenselink. mil/pubs/pentagon

Фотография принадлежит: DoD, Tech. Sgt. Cedric H. Rudisill

• www.defenselink.mil/photos/Sep2001/010914-F-8006R-002.html

Inside the Ring, хроника Билла Герца (Bill Gertz) в Washington Times от 21 сентября 2001 г.;

• http://www. washtimes. com

Пресс-конференция под председательством Виктории Кларк, заместителя министра обороны, Пентагон, 12 сентября 2001 г.;

• www.defenselink.mil/news/Sep2001/t09122001^912asd.html

Nation s Capital Under State of Emergency, CNN от 12 сентября 2001 г.;

• http://www. cnn. com (исходно статья находилась на: http://www.cnn.com/2001/US/09/1Idc.terrorism/index.html)

Полностью текст доступен на сервере канцелярии египетского президента:

• http://www.presidency.gov.eg/html/14-Sept2001 _press_2.html

За исключением самолета рейса 93, взорвавшегося над Пенсильванией. Пассажиры сообщали, что у пиратов имеется коробка, в которой, по их словам, упрятана бомба.

Пистолеты из синтетических материалов не обнаруживаются детекторами в аэропортах.

См. пример на сайте: • http://www. glock. com

Global Hawks, the DoD's Newest Unmaned Air Vehicl, Министерство обороны, февраль 1997 г.;

• http://www.defenselink.mil/photos/Febl997/970220-D-OOOG-001.html

Selling Out the Investigation Билла Мэннинга (Bill Manning) в Fire Engineeringза январь 2002 г.

См. также: WTCInvestigation? A Call to Action (петиция, опубликованная в том же номере журнала). Например, показания пожарного Луи Каккиоли (Louie Cacchioli) из 47-й бригады;

• http://people.aol.eom/people/special/0,11859,174592-3,00.html

Explosives Planted in Towers, N. M: Tech. Expert Says Оливера Уттебрука (Olivier Uyttebrouck) в Albuquerque Journal от 11 сентября 2001 г. Опровержение в Fire, Not Extra Explosives, Doomed Buildings, Expert Says Джона Флека (John Fleck) в Albuquerque Journal от 21 сентября 2001 г.;

• http://www. abqjournal. com

Secret С. I. A. Site in New York was Destroyed on Sept. 11 Джеймса Райзена (James Risen), The New York Times, 4 ноября 2001 г.;

• http://www. nytimes. com

Мы ознакомились с несколькими свидетельскими показаниями, в соответствии с которыми база ЦРУ находилась на 9-м и 10-м уровнях Башни 7. Она скрывалась за вывеской «New York Electronic Crime Task Force», относящейся к Секретной службе. Те же источники предоставили нам фотографии Башни 7, сделанные после обрушения Южной башни, на которых ясно виден пожар на 9-м уровне;

• http://members.aol.com/erichuf/eh_wtcl6. jpg.

У нас не было возможности проверить подлинность фотографий и достоверность информации.

«Окончательный итог количества жертв Всемирного торгового центра — 2843 погибших», сообщение Associated Press от 9 февраля 2002 г. Эту цифру предоставили городские власти Нью-Йорка. Само же агентство печати предположило более низкую цифру — 2799 погибших.

На'aretz от 26 сентября 2001 г.;

• http://www. haaretzdaily. com/.

Сообщение, подхваченное Даниелем Сибергом (Daniel Sieberg) на Си-эн-эн: FBI Probing «Threating» Message, Firm Says 28 сентября;

• http://www. cnn. com.

См. также: Instant Message To Israel Warned of WTC Attack Брайана Макуильям-ca (Brian McWillliams) в Newsbytes от 27 сентября и Odigo Clarifies Attack Messages того же автора в номере следующего дня;

• http://www. newsbytes. com.

Эта информация была передана в искаженном виде комментатором Al-Jezeera, который попытался ее использовать с целью внушения того, что теракты были осуществлены Моссадом и что он заранее предупредил евреев-служащих во ВТЦ. Телеканал Катара тут же уволил журналиста-интригана.

A Force Upon the Plain: the American Militia Movement and the Politics of Hate Кеннета Стерна (Kenneth Stern), изд-bo «Саймон энд Шустер» (Simon & Schuster ed.), 1996 г. См. также: первую часть The Secret Life of Bill Clinton: the Unreported Stories Лмброза Эванс-Притчарда (Ambrose Evans-Pritchard), изд-во Regnery Publishing, 1997.

Remarks by the President in Town Hall Meeting, Orange County Convention Center, Orlando, Florida:

• http://www. whitehouse. gov/ news/releases/2001 /12/print/20011204-17. html

The United States Government Interagency Domestic Terrorism Concept of Operation Plan (CONPLAN), который возможно скачать с сайта:

• http://www.fbi.gov/publications/conplan/ conplan.pdf

Полный текст беседы доступен на сайте:

• http://stacks.msnbc.com/news/629714.asp

См.: The Options Николаса Леманна (Nicolas Lemann), в The New Yorker от 25 сентября 2001 г.;

• http://www.newyorker.com

См., например: White House Said Targeted Сандры Собье-рай (Sandra Sobieraj) в Washington Post от 12 сентября 2001 г.;

• http://www.washingtonpost. com;

и Inside the Bunker in The New York Times от 13 сентября 2001 г.;

• http://www.nytimes.com

Digital Moles in White House? Terrorists had top-secret Presidential codes в World Net Daily от 20 сентября 2001 г.;

• http://www.worldnetdaily.com

Bin Laden vs Magic Carpet — Secret US Promis Sofhvare Майкла Рупперта (Michael С. Ruppert) в From the Wilderness от 20 ноября 2001 г.;

• http://www. copvcia. com

В это управление входят руководящие подразделения полнительной власти, подчиненные непосредственно президенту: Управление Белого дома (White House Office), Административно-бюджетное управление (Office of Management and Budget), Совет национальной безопасности (National Security Council) и т. д. (Прим. ред.}

Брифинг министра юстиции Джона Ашкрофта (John Ashcroft) 12 сентября 2001 г.;

• http://www.usdoj.gov/ag/speeches/2001/0913pressconference.htm

Пресс-конференция министра юстиции Джона Ашкрофта и директора ФБР Роберта Мюллера III (Robert Mueller III) 14 сентября 2001 г.;

• http://www.usdoj.gov/ag/agcrisisremarks9_14.htm

Fighting the Forces of Invisibility Салмана Рушди в Washington Post от 2 октября 2001 г.;

• http://www.washingtonpost.com

Брифинг министра юстиции Джона Ашкрофта и директора ФБР Роберта Мюллера III (Robert Mueller III) 28 сентября 2001 г.;

• http://www.usdoj.gov/ag/agcrisisremarks9_28.htm

Несколько европейских газет ошибочно сообщили, что ФБР обнаружила этот документ в руинах Пентагона.

Любопытно, что выдающийся журналист Боб Вудвард (Bob Woodward) отмечает эту странность в тот же день, но не делает из нее никаких выводов. Ср.: In Hijache's Bags, a Call to Planning, Prayer and Death в Washington Post от 28 сентября 2001 г.;

• http://www. washingtonpost. com

Saudi Minister Asserts That Bin Laden Is a «Tool» of Al Qaeda, Not a Mastermind Дугласа Джела (Douglas Jehl) в Washington Post от 10 декабря 2001 г.;

• http://www. washingtonpost. com

Эти коммюнике были распространены агентством Ассошиэйтед Пресс.

Black Tuesday: The World Largest Insider Trading Scam? Дона Радлоэра (Don Radlauer), International Policy Institute for Counterterrorism, Израиль, 9 сентября 2001 г.;

• http://www.ict.org.il/articles/articledet.cfm?articleid=386

Официальный сайт IOSCO: • http://www.iosco.org/iosco.html

Suspicious Profits Sit Uncollected Airlines Investors Seem to Be Lying Low Кристиана Бертельсена (Christian Bermelsen) и Скотта Винокура (Scott Winokur) в San Francisco Chronicle от 29 сентября 2001 г.;

• http://www.sfagate.com

«Разоблачение» Дени Робера и Эрнеста Бакера, изд-во Les Arenes, 2001;

• http://www.arenes.fr/livres/pagelivrel.php? numero_livre:=4&nurn_page= 1

ФБР не проявило большего интереса и в отношении владельцев многозначительных доменных имен в Интернете: по сведениям регистрирующей компании «Верисайн» (VeriSign), неопознанный оператор купил в 2000 году сроком на 1 год 17 доменных имен в Интернете.