The former belief of mechanical engineers that progressive collapse is a free fall has been just recently refuted in the paper "Mechanics of the WTC collapse" by Cherepanov published in Int. J. Fracture (2006) 141: 287-289. The present paper uses the model of progressive collapse advanced in that paper but analyzed only using an additional assumption of zero resistance. The effect of resistance slows down the progressive collapse. Applying this analysis of dynamics of progressive collapse to the New York events on September 11, 2001 shows that the collapses of twin towers of the World Trade Center started from a floor located significantły lower than the floors hit by terrorists and subjected to fire.
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