Why did the World Trade Center towers collapse?

Press Release, September 13, 2001

by Zdenek P. Bazant

McCormick School Professor and Walter P. Murphy Professor
Department of Civil and Environmental Engineering
Northwestern University

The towers of the World Trade Center were designed to withstand as a whole the horizontal impact of a large commercial aircraft. So why did a total collapse occur? The reason is the dynamic consequence of the prolonged heating of the steel columns to very high temperature. The heating caused creep buckling of the columns of the framed tube along the perimeter of the structure, which transmits the vertical load to the ground. The likely scenario of failure may be explained as follows...

Note: After September 13, 2001 this press release was circulated around the world by email and over the web, and there soon appeared similar versions of the same basic analysis. For example, E. Kausel of MIT, in spite of his attempts to acknowledge the priority of this article, has been quoted several times (MIT Tech Talk 9/26/01, Scientific American 10/9/01) regarding the predictions of the "two simple models" above.

Further Reading

My father is an expert on structural stability and collapse. Although he has analyzed the collapse of many buildings in the past, he could never have imagined the gruesome cause and mode of failure of the World Trade Center towers in New York City in the terrorist attacks of September 11, 2001. Following the initial impacts by hijacked passenger airplanes, thousands of victims within the towers and hundreds of brave rescue workers who rushed to their aid were surprised, and senselessly murdered, by the sudden and complete collapse of both 110 story towers. The brief article above, written by my father for a press release shortly after the disaster, gives a probable physical explanation for the collapse of these apparently very well designed buildings. Of course, there is no acceptable explanation for the intent of the terrorists, and their sponsors, to kill so many innocent and peaceful men, women, and children. -- Martin Z. Bazant, September 13, 2001.

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