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However, please note:
From their detailed study NIST concluded that rapidly sequential “pancaking” of the floors below the region of initial impact did not occur as a result of “truss drop”, but that the global collapse of WTC1 and WTC2 was initiated by sagging of the very hot floor joists (truss assembly) that gave rise to inward bowing with subsequent buckling of the perimeter walls (see the next slide).
A possible alternative explanation by D. Bushnell (not an expert on building collapse): The sagging of a hot floor joist (truss assembly) and consequent high joist catenary axial tension that would be capable of causing bending of the columns (such as is shown in the next slide) would give rise to significant shear forces in the 5/8-inch bolts (shown in the previous slide), leading to bolt failure and thus separation of each joist from its column. This separation would mean that there would be columns with greater unsupported lengths, perhaps enough additional multi-floor lengths to cause them to buckle statically. This static buckling would free the upper part of the tower to move downward sort of like a rigid body and slam into the top of the lower, cooler portion of the tower. Perhaps this huge slamming force would be enough to buckle the shorter unsupported single-floor column lengths in the first cooler floor below. That same mode of single-floor column buckling would occur in each successive floor, with the entire building eventually collapsing in a “pancake” mode.
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