Why did the World Trade Center collapse? Science, engineering, and speculation

Abstract

Following the collapse of the World Trade Center (WTC) towers on September 11, 2001 came the wide speculation that the buildings were structurally deficient, that the steel columns melted, or that the fire suppression equipment failed to operate. To separate the fact from the fiction, an attempt is made to quantify various details of the collapse. The major events are detailed: (i) the airplane impact with damage to the columns; (ii) the ensuing fire with loss of steel strength and distortion; (iii) the collapse, which generally occurred inward without significant tipping.