This paper examines the elastic loading and plastic shortening phases of the columns of WTC 1 after impact of the upper 16 storeys of the building upon the lower storeys and its effect on the momentum transfer after the collision. An energy balance is derived showing that there is an energy deficit before completion of the plastic shortening phase that would not allow the collapse to continue under the constraints of this paper. Previous analysis of the momentum transfer in the collapse of the towers has viewed them as being floors suspended in space and have examined the momentum transfer as a series of elastic or inelastic collisions, which are independent of each other. This type of analysis takes the momentum transfer out of the context given by the other effects of the collisions. This is becaus
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