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Last Updated: Sep 19th, 2011 - 19:28:10

 


Why Did the World Trade Center Towers Collapse?
By U.S. Department of State's Bureau of International Information Programs
Aug 25, 2011, 7:18pm




The U.S. National Institute of Standards and Technology (NIST) conducted an extremely thorough, three-year investigation into what caused the WTC twin towers to collapse. The U.S. National Institute of Standards and Technology (NIST) conducted an extremely thorough, three-year investigation into what caused the WTC twin towers to collapse.

Its conclusion is that the twin towers collapsed because the impact of the planes severed and damaged support columns and dislodged fireproofing insulation coating the steel floor trusses and steel columns, which meant that the fire, which reached 1000 degrees Celsius, weakened the floors and columns to the point where they bowed and buckled, causing the towers to collapse.

Here is part of their report on the collapse of the World Trade Center

NIST’s Draft Summary Report stated (pp. 171-172):

The two aircraft hit the towers at high speed and did considerable damage to principal structural components: core columns, perimeter columns, and floors. However, the towers withstood the impacts and would have remained standing were it not for the dislodged insulation and the subsequent multifloor fires. …

In WTC 1, the fires weakened the core columns and caused the floors on the south side of the building to sag. The floors pulled the heated south perimeter columns inward, reducing their capacity to support the building above. Their neighboring columns quickly became overloaded as the south wall buckled. The top section of the building titled to the south and began its descent. …

In WTC 2, the core was damaged severely at the southeast corner …. The steady burning fires on the east side of the building caused the floors there to sag. The floors pulled the heated east perimeter columns inward, reducing their capacity to support the building above. Their neighboring columns quickly became overloaded as the east wall buckled. The top section of the building tilted to the east and to the south and began its descent. …

The WTC towers would likely not have collapsed under the combined effects of aircraft impact and the extensive, multifloor fires if the thermal insulation had not been widely dislodged or had been only minimally dislodged by aircraft impact.

Source: U.S. Department of State's Bureau of International Information Programs

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