Why Did the World Trade Center Collapse ?

Each tower of the World Trade Center was designed to withstand, as a whole, the horizontal impact of a large commercial aircraft. Why, then, did total collapse occur? Zden ek P. Ba zant, Walter P. Murphy Professor of Civil Engineering and Materials Science at Northwestern University, and Yong Zhou, a graduate research assistant at Northwestern, suggest that the collapse was the dynamic consequence of the prolonged heating of the steel columns to very high temperatures. Such heating would have caused creep buckling of the columns of the framed tube along the perimeter of the structure that transmitted the vertical load to the ground. They offer the following as a likely scenario of the failure.