Why Did the World Trade Center Collapse?—Simple Analysis

This paper presents a simplified approximate analysis of the overall collapse of the towers of World Trade Center in New York on September 11, 2001. The analysis shows that if prolonged heating caused the majority of columns of a single floor to lose their load carrying capacity, the whole tower was doomed. DOI: 10.1061/~ASCE!0733-9399~2002!128:1~2! CE Database keywords: New York; New York City; Disasters; Buildings; High-rise; Collapse; Analysis; Terrorism. RAPID COMMUNICATION Introduction and Failure Scenario The 110-story towers of the World Trade Center were designed to withstand as a whole the forces caused by a horizontal impact of a large commercial aircraft ~Appendix I!. So why did a total collapse occur? The cause was the dynamic consequence of the prolonged heating of the steel columns to very high temperature. The heating lowered the yield strength and caused viscoplastic ~creep! buckling of the columns of the framed tube along the perimeter of the tower and of the columns in the building core. The likely scenario of failure is approximately as follows. In stage 1 ~Fig. 1!, the conflagration, caused by the aircraft fuel spilled into the structure, causes the steel of the columns to be exposed to sustained temperatures apparently exceeding 800°C. The heating is probably accelerated by a loss of the protective thermal insulation of steel during the initial blast. At such temperatures, structural steel suffers a decrease of yield strength and exhibits significant viscoplastic deformation ~i.e., creep—an increase of deformation under sustained load!. This leads to creep buckling of columns ~Bažant and Cedolin 1991, Sec. 9!, which consequently lose their load carrying capacity ~stage 2!. Once more than half of the columns in the critical floor that is heated most suffer buckling ~stage 3!, the weight of the upper part of the The original version with Eqs. ~1! and ~2! was originally submitted to ASCE on September 13, 2001, and an expanded version with Eq. ~3! was submitted to ASCE on September 22, 2001. The appendices were added between September 28 and October 5, 2001; The preliminary report ~Bažant and Zhou 2001! on which this article is based was posted on September 14, 2001, at ^http://www.civil.northwestern.edu/news&, ^http:// www3.tam.uiuc.edu/news/200109wtc&, and ^http://math.mit.edu/ bazant&. Walter P. Murphy Professor of Civil Engineering and Materials Science, Northwestern Univ., Evanston, IL 60208. E-mail: [email protected] Graduate Research Assistant, Northwestern Univ., Evanston,