Mechanics of the WTC collapse

Two theories of the WTC collapse are examined. The first one is the theory of progressive failure, and the other one is the theory of fracture waves. The collapse in the regime of progressive failure is shown to occur at an acceleration, which is several times less than the gravitational acceleration and, hence, this theory contradicts to the observed free fall. Evidently, the WTC towers were disintegrated at the very beginning of each collapse. To explain this fact an alternative model based on the theory of failure waves is proposed.