Why did the World Trade Center collapse? Science, Engineering, and Speculation: Fid-Bau Portal

Why did the World Trade Center collapse? Science, Engineering, and Speculation

Eagar, T.W. / Musso, C.

There have been numerous reports detailing the cause of the WTC (World Trade Center) tower collapse on September 11, 2001. Most have provided qualitative explanations; however, simple quantitative analyses show that some common conclusions are incorrect. In order to separate the fact from the fiction, starting with some design properties authors have attempted to quantify various details of the collapse, and to present some guidelines for improvements in future structures. The main catastrophic events include the airplane impact with damage to the columns, the ensuing fire with loss of steel strength and distorsion, and the collapse, which generally occurred inward without significant tipping (a 500000 t structure has too much inertia to fall in any direction other than nearly straight down). While the aircraft impact undoubtely destroyed several columns in the WTC perimeter wall, the number of columns lost on the initial impact was not large and the loads were shifted to remaining columns in this highly redundant structure. Of equal or even greater significance during this initial impact was the explosion when 90000 L of jet fuel ignited. The ensuing fire (maximum flame temperature of about 1000 deg C) was clearly the principal cause of the collapse. The failure of the steel was due to two factors, the loss of strength due to the temperature of the fire, and the loss of structural integrity due to distorsion of the steel from the non-uniform temperatures in the fire. Like nearly every large building, the WTC towers had redundant design that allows for loss of one primary structural member, such as a column. However, when multiple members fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down.