Failure Knowledge Database / 100 Selected Cases

The World Trade Center Collapse
September 11, 2001, New York, U.S.A.
Masayuki Nakao
Institute of Engineering Innovation, School of Engineering, the University of Tokyo

Two passenger planes taken over by terrorists crashed one after the other into two
skyscrapers of the World Trade Center (WTC 1, WTC 2) located in the southernmost tipof Manhattan, New York City. Both towers were engulfed in flames and collapsed inless than 2 hours.
Victims of the incident totaled up to 2,752 people (last announcement of New York
City Police Authorities on October 30, 2003). The damage total is said to be 83 billion to95 billion dollars.

Picture 1. The flared WTC by the crash of a passenger plane [1]1. Event
Two passenger planes taken over by terrorists crashed one after the other into two
skyscrapers of the World Trade Center (WTC 1, WTC 2) located in the southernmost tipof Manhattan, New York City. Both towers were engulfed in flames, and collapsed inless than 2 hours afterwards. Thousands of people were killed.
2. Course
At 8:45am, Tuesday, September 11, 2001, American Airlines flight 11 (Boeing 767)
from Boston to Los Angeles with 81 passengers and 11 crew plunged into the upperfloors of the World Trade Center 1 (the north side) located in the financial district of the1Failure Knowledge Database / 100 Selected Cases

southern part of Manhattan, New York. Dark smoke spewed into the sky from a largehole created in the upper floors, and the wreckage of the plane poured onto streets ofthe financial district.
At 9:03 am, United Airlines flight 175 (Boeing 767) from Boston to Los Angeles with56 passengers and 9 crew plunged into the middle floors of WTC 2 (the south side).
In WTC 1, the people who could not bear terrible high temperature and smoke began
to jump off the building. People who jumped off numbered about 200.
WTC 2 collapsed at about 10:05am.
At about 10:28am, WTC 1 collapsed.
WTC 3, 4 and 5 then collapsed one after another.
At about 17:20, WTC 7 collapsed. While only WTC 6 did not collapse, it was greatlydamaged.
Victims of the incident summed to 2,752 people (last announcement of New York
City Police Authorities on October 30, 2003).
The damage total is said to be 83 billion to 95 billion dollars.
3. Cause
The direct cause was hijacks by international terrorists (said to be Al Qaeda) whoflew the planes into the buildings. Source say that President Bush was aware, before
September 11, that Osama Bin Laden and his group (Al Qaeda) might hijack American
planes, however, the threat had been more of a general one pointing to a moreconventional airline hijacking, and that the Administration had no prior knowledge
that terrorists would use commercial jetliners as weapons to attack the World TradeCenter. Acting on the information received, the Administration notified the appropriate
agencies that hijackings in the traditional sense were possible. The warning was nevermade public (according to the White House press secretary Ari Fleischer at the press
conference on May 15, 2002). The airport security at the time failed to detect the knives
that the hijackers used to threat crews and passengers.
First, we will examine the cause of how these high-rise buildings collapsed; themechanism that enlarged the size of disaster.
In 1945 before the World Trade Center was built, a B25 bomber, which was going toland at the Newark Airport, crashed into the Empire State Building in New York.
Dense fog was blamed for this accident. So the WTC design was intended not tocollapse even if an airplane collided with the buildings.
Figure 1 was a typical floor plan of the WTC. The vertical plane is constructed with aperimeter of 990mm spaced steel columns, with an internal lift, and the horizontal
plane consists of trussed steel and concrete slabs of about 100mm thickness.

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Failure Knowledge Database / 100 Selected Cases

Figure 1. WTC floor plan [1]Despite the design intension, the building actually collapsed. The following areestimated factors.
(i)Damage/collapse of many columns from the impact of passenger planes (about200t) crashing in the structure.

(ii)

Falling of fireproof coatings on the structural material due to the impact.

(iii)

Material (especially horizontal truss steel) strength dropped due to the fire fed
with a large volume of jet fuel of 60,000 pounds (27t).

(iv)

Structural fragility of the pin joints of beams/columns.

For these factors, it was estimated that the floors that could not bear the weight of
the upper floors first dropped, and the lower floors, which could not resist this impact
collapsed in a chain reaction. Picture 2 is the collapsing building, and Picture 3 is thestate after the building collapsed.

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Failure Knowledge Database / 100 Selected Cases

Picture 2. The collapsing WTC[1]Picture 3. The state after the WTC collapsed [1]4. Immediate Action
The following actions were taken after the incident.
At 9:18am of September 11, the Newark Airport of New York was closed.
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At 9:21am, tunnels and bridges in New York City (Manhattan Island) were closed.
At 9:25am, President Bush held a press interview.
At 9:40am, the Federal Aviation Administration (FAA) decided to shutdown all thedomestic airlines.
At 10:15am, all international flights arriving in the U.S.A. were diverted to Canada.
5.

CountermeasureThe

directcauseoftheincident

wasterrorism.

Insteadof

discussing

countermeasures against terrorist attacks, this report describes some of the responses
and actions taken after the incident.
The Emergency Operation Center (EOC) of the Office of Emergency Management(OEM) was located on the 23rd floor of WTC 7. A total of 13 million dollars had built
this OEM in 1999 after the lessons of the WTC bombing in1993. This office, however,
could not be used after the collapse of WTC 1 and WTC 2.
Mayor Giuliani of New York City entered the temporary Emergency CommandCenter (ECC) set at a block north of the WTC, but escaped for his safety on foot after
the first collapse of the WTC 2.
At about 11:00am, the ECC was re-located in the fire station at Houston Street,
which was about 2km north from the incident site.
At 11:02am, Mayor Giuliani gave an evacuation order to the district south of Canal
Street and closed the district south of 14th street to the public except residents and theECC transferred to the police academy on 20th street.
The EOC was transferred to a building owned by the city at pier 94 facing theHudson River on September 17, operated until November 23 when it closed.
Subsequently, OEM carried on the functions of EOC.
6. Summary
In the incident of September 11, 2001, four passenger planes in total were hijacked
in the United States. Besides those which went into the WTC, one plane plunged intothe Pentagon (the Department of Defense) in Arlington, Virginia adjacent toWashington DC, and another crashed in the Stoney Creek area in Pennsylvania.
Therefore it is called "9.11 (nine eleven) series of terrorist attacks".
The scene of the burning WTC from the first airplane crash went out live on nationalTV, then the scene of the second plane plunging into the building took place live on air.
It was like a war movie for those watching the news.
The building collapse after the plane crash was totally unexpected and since there
were firefighters on the rescue and others that were in the middle of evacuation, thecollapse added more to the count victims.

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7. Knowledge
Unexpected events occur sometimes. However, there are often signs. Proactive
actions and measures are important when such signs appear.
The Office of Emergency Management (OEM) should not be built in an important
institution (such as a possible target of an attack).
A disaster countermeasure office should not be built in the area where damage has
occurred or the damage is severe.
Military OEMs are commonly built on the bedrock about 40m underground.
However, it is still a possible target of terrorism. The best strategy is not to specify its
location on a map or by a physical address.
8. Background
The attacks led to the so-called the War on Terror, or the war against terrorism, andthe first military action on Afghanistan was initiated by the U.S.
Afghanistan is a country at the crossroads of Asia, geographically located between
Central Asia and the Indian subcontinent. Through the ages, the region known as
Afghanistan today has been invaded by a host of peoples who sought expansion ofterritory. The ones still fresh in our minds would be the British intervention during theperiod of the Great Game (between British India and the outlying regions of TsaristRussia) in the 19th century, and the Soviet occupation during the Cold War era that led
to the Soviet war in Afghanistan in late 20th century.
Following the removal of the Soviet forces in 1989 and the downfall of theSoviet-backed Democratic Republic of Afghanistan in 1992, Afghanistan was throwninto civil war between competing warlords. The Taliban, with its radical ideology andthe 1994 legend (called the Taliban legend) and backed by Pakistan, Saudi Arabia andother strategic allies, developed as a politico-religious force, and eventually seized thecapital city Kabul. It captured the seat of government and established the Islamic
Emirate of Afghanistan in 1996. The Taliban were able to capture 90% of the country
including Bamiyan seized in 1998, aside from the Afghan Northern Alliancestrongholds primarily found in the northeast in the Panjsher valley.
In response to the 1998 U.S. embassy bombings in the East African capital cities, UN
Security Council sanctions, Security Council Resolution 1267 (October 1999) and 1333
(December 2000), were imposed against Taliban to pressure it to cease the provision ofsanctuary and training for international terrorists and their organizations as well as toturn over Osama bin Laden, the leader of Al Qaeda, who allegedly directed 1998 U.S.
embassy bombings.
A London-based Arab newspaper, Al-Arab, reported that international terrorist
Osama bin Laden had warned three weeks before the 9/11 attacks that he wasplanning an attack “against American interests” and threatened that it would be a bigone. The World Trade Center situated near the south end of Manhattan in the6Failure Knowledge Database / 100 Selected Cases

downtown financial district provided almost four percent of Manhattan’s entire officeinventory, and its Towers were undeniable icons of New York City.
There had been another incident at the World Trade Center prior to the 9/11 attacks.
In February 1993, terrorists drove a truck packed with explosives into the basement
parking garage, killing 6 and injuring 1,000 people.
<References>[1]

Sydney University, the World Trade Center - Some Engineering Aspects:
http://www.civil.usyd.edu.au/latest/wtc.php[2]Disaster Prevention Research Institute Kyoto University, Research Center for
Disaster Reduction Systems, The investigation study of the damage expansion
process of the American World Trade Center and victims correspondence
http://infoshako.sk.tsukuba.ac.jp/~toshiw3/Labo/murao/wtc/blocker/wtcpdf2/21.pdf[3]Architectural Institute of Japan Fire Safety Committee, The collapse of NewYork World Trade Center by the passenger plane collisionhttp://www.aij.or.jp/jpn/databox/2001/010919-2.pdf

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