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The image is from:
https://www.era.lib.ed.ac.uk/handle/1842/1216
(Edinburgh Research Archive, 2003)
A.S. Usmani, Y.C.Chung and J.L.Torero.
“HOW DID THE WTC TOWERS COLLAPSE: A NEW THEORY”, date and publisher not given in the “pdf” file. The latest citation is dated Devember 2002.
Text from:
http://en.wikipedia.org/wiki/Collapse_of_the_World_Trade_Center
“They [NIST] also emphasized the role of the fires and found that sagging floors pulled inward on the perimeter columns: ‘This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers.’”
Text 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 [5 slides previous]). Instead, the NIST investigation showed conclusively that the failure of the inwardly bowed perimeter columns 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.”
Comments by David Bushnell (not an expert on building collapse or the effect of temperature on material properties or the failure of fasteners):
Various theories have been offered as to what local thermal/stress effects in the region of initial impact initiated the global collapse of the entire building. Certainly the strength and stiffness of the various parts of each floor joist (truss) were significantly reduced by the high temperature in the region of initial impact. Perhaps the weight of one of the sagging floors in the hot region gave rise to enough tension in the upper horizontal member of each truss to cause enough stress in the edge fasteners to fail these fasteners and therefore allow the edge of the floor to slip off its seat where the floor joist meets the exterior wall and/or where the floor joist meets the core. That floor would then crash down on the floor below, causing failure of the edge fasteners of the truss joists on that floor. Then perhaps the structural columns of the exterior wall buckled over a now laterally unsupported height greater than the distance between adjacent floors. Once this happened the weight of the entire building above the site of initial failure would rapidly drive the global collapse of the building floor by floor. According to NIST, the speed of global collapse was approximately 2/3 of the free-fall velocity.
Text from:
http://www.globalresearch.ca/mechanisms-of-destruction-and-collapse-of-the-world-trade-center-wtc-buildings-on-911/5333553
Mechanisms of Destruction and Collapse of the World Trade Center (WTC) Buildings on 9/11, “An Analysis of Peer Reviewed Technical Literature 2001 - 2012”, by Timothy E. Eastman and Jonathan H. Cole
Journal of 9/11 Studies
Eastman and Cole write:
“What is most striking about our results is the fact that there is serious disagreement as to how the WTC structures fell on September 11, 2001. While precise sequences of every building component failure cannot be determined, the overall basic mechanism of destruction (i.e. some type of fire-induced natural gravitational collapse (PC), or some type of planned demolition CD) is clearly in dispute. There is no consensus. At this point, almost 12 years later, there should not be any significant disagreement about such a fundamental issue as to how three buildings were destroyed so completely given the magnitude of the event, the implications of the event, and repercussions for existing and future structural design.”
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