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Collapse of the World Trade Center

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Template:Sep11

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

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

Construction of 1 and 2 WTC

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

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

Impacts of airliners

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

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

Collapse of the two towers

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

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

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

Design criticisms

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


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

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

Seven World Trade Center

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

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

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

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


Conspiracy theories

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

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

Other buildings

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

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

References

Cited references

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

References

External links

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