Diagram of the composite WTC floor system [from National Institute of Standards and Technology (NIST)]

This and the next four slides were created by the National Institute of Standards and Technology (NIST).

The schematic shown here is from:
http://www.nist.gov/el/disasterstudies/wtc/faqs_wtctowers.cfm
“NIST’s findings do not support the ‘pancake theory’ of collapse, which is premised on a progressive failure of the floor systems in the WTC towers (the composite floor system—that connected the core columns and the perimeter columns—consisted of a grid of steel ‘trusses’ integrated with a concrete slab; see diagram). Instead, the NIST investigation showed conclusively that the failure of the inwardly bowed perimeter columns [see the schematic 5 slides hence] initiated collapse and that the occurrence of this inward bowing required the sagging floors to remain connected to the columns and pull the columns inwards. Thus, the floors did not fail progressively to cause a pancaking phenomenon.”

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