Initial Opinions and Analysis
The first person to theorize that the buildings collapsed due to structural failure due to fire was actually an eye-witness Fox News freelancer named Mark Walsh. At 11:55 am on September 11, Walsh was asked about what he saw from his apartment. "... I was watching with my roommate. It was approximately several minutes after the first plane had hit. I saw this plane come out of nowhere and just ream right into the side of the Twin Tower exploding through to the other side, and then I witnessed both towers collapse, one first, and then the second, mostly due to structural failure because the fire was just too intense." 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. 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."
On September 13, 2001, Zdeněk Bažant, professor of civil engineering and materials science at Northwestern University, circulated 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. Once more than half of the columns on a particular floor buckled, the overhead structure could no longer be supported and complete collapse of the structures occurred. Bažant later published an expanded version of this analysis. Other analyses were conducted by MIT civil engineers Oral Buyukozturk and Franz-Josef Ulm, who also described a collapse mechanism on September 21, 2001. They later contributed to an MIT collection of papers on the WTC collapses edited by Eduardo Kausel called The Towers Lost and Beyond.
Read more about this topic: Collapse Of The World Trade Center
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