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The collapse of the World Trade Center towers was initiated by the impact of the upper falling part onto the underlying intact story. At the moment of impact, the velocity of the upper part must have decreased. The fact that no velocity decrease can be discerned in the videos of the early motion of the tower top has been recently exploited to claim that the collapse explanation generally accepted within the structural mechanics community was invalid. This claim is here shown to be groundless. Calculations show that the velocity drop is far too small to be perceptible in amateur video records and is much smaller than the inevitable error of such video records. [ABSTRACT FROM AUTHOR]

In this article, the authors present their views in response to a comment on their article related to the claims against the scientific basis for gravity-driven progressive collapse occurred in the towers of the World Trade Center (WTC) in New York City. As per them, the claims include violation of momentum conservation principles and sustained column heating. They state that structural mechanics without standard simplifying hypothesis made structural analysis feasible.