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http://en.wikipedia.org/wiki/Collaps...d_Trade_Center
In the immediate aftermath of the attacks, numerous structural engineers and experts spoke to the media, describing what they thought caused the towers to collapse. Hassan Astaneh, a structural engineering professor at the University of California at
Berkeley, explained that the high temperatures in the fires weakened the steel beams and columns, causing them to become "soft and mushy", and eventually they were unable to support the structure above. Astaneh also suggested that the fireproofing became dislodged during the initial aircraft impacts. He also explained that, once the initial structural failure occurred, progressive collapse of the entire structure was inevitable.[26] Cesar Pelli, who designed the Petronas Towers in Malaysia and the World Financial Center in New York, remarked, "no building is prepared for this kind of stress".[27]
On September 13, 2001, Zdeněk Bažant, a professor of civil engineering and materials science at Northwestern University, published a draft paper with results of a simple analysis of the World Trade Center collapse. Bažant suggested that heat from the fires was a key factor, causing steel columns in both the core and the perimeter to weaken and experience deformation, before losing their carrying capacity and buckling. Then, once more than half of columns on a particular floor buckled, the above structure could no longer be supported and complete collapse of the structures occurred. Bažant's paper was later expanded and published in the Journal of Engineering Mechanics, in January 2002.[28] Other analyses were conducted by MIT civil engineers Oral Buyukozturk and Franz-Josef Ulm, who also described a collapse mechanism on September 21, 2001.[29] They would later contribute to an MIT collection of papers on the WTC collapses edited by Eduardo Kausel called The Towers Lost and Beyond, published in May 2002.[30]
7 World Trade Center
7 World Trade Center on fire after the collapse of the Twin Towers on 9/11
Main article: 7 World Trade Center
FEMA's provisional study was inconclusive regarding the collapse of 7 World Trade Center.[48] The investigation of the collapse of 7 WTC was done separately, and subsequent to the investigation of the Twin Towers' collapse. In June 2004, NIST released a progress report outlining its working hypothesis for 7 WTC, which was that a local failure in a critical column, caused by damage from either fire or falling debris from the collapses of the two towers, progressed first vertically and then horizontally to result in "a disproportionate collapse of the entire structure".[49][50]
On August 21, 2008, NIST released a draft of its final report on the collapse of 7 World Trade Center. In the report, NIST explains that fire was the main reason for the collapse, along with lack of water to fight the fire. The fire continued to burn throughout the afternoon on the lower floors, and at 5:20 pm. a critical column on the 13th floor buckled and triggered a progressive collapse of the entire structure.[51]
Other investigations
In 2003, three engineers at the University of Edinburgh published a paper in which they provisionally concluded that the fires alone (without any damage from the airplanes) could have been enough to bring down the WTC buildings. In their view, the towers were uniquely vulnerable to the effects of large fires on several floors at the same time.[52] When the NIST report was published, Barbara Lane, with the UK engineering firm Arup, criticized its conclusion that the structural damage resulting from the airplane impacts was a necessary factor in causing the collapses.[53] Jose L Torero from the BRE Centre for Fire Safety Engineering at the University of Edinburgh is pursuing further research into the potentially catastrophic effects of fire on real-scale buildings.[54][55][56