It was shown that if progressive failure would be the only regime of the collapses of the New York towers, the time of collapses of both twin towers would be equal to the free fall time, namely 9.4 s, only if the critical floor, one and the same for both towers, would be significantly lower than the floors hit by terrorists and subjected to fire. This conclusion follows also from the present hybrid theory taking into account both fracture waves and progressive failure. The hybrid theory was shown to explain the observations not understood earlier, namely (i) free fall time of all collapses, almost independent of the position of the critical floor, (ii) visual observations of the towers during the collapses that the lower parts of towers remained intact until failure front reached them, and (iii) visual observations of dust clouds, the markers of fracture waves. It proves that all collapses started on the floors located significantly lower than the floors hit by terrorists and subjected to fire. The latter conclusion follows also from the equations of pure progressive collapse given in [1] and [6].
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