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This schematic is from:
http://www.debunking911.com/collapse.htm
Comments by David Bushnell (4 paragraphs written May 2015. David Bushnell is not an expert on the collapse of buildings!):
Immediately after the initial global collapse shown in this schematic (which reproduces the rigid-body tilting of the top many stories of the building photographed in the previous slide), general collapse occurred almost as rapidly as free fall. There seemed to be no further tilting. However, it was impossible to see any details of the vertical collapse because the building where current collapse was occuring and above was enveloped in a dense cloud of dust.
From a brief search of the literature there seem to be essentially two alternative major theories concerning which local failure in the neighborhood of the initial impact precipitated global collapse of the entire building:
1. The truss assembly that supported the most critical (hottest?) floor became separated from its supporting walls (perimeter wall and/or core), that floor then crashed to the floor below causing loss of edge support of the truss assembly of that lower floor, followed by loss of edge support of successive floor truss assemblies floor by floor, accompanied by intermittant Euler buckling of the outer wall, now laterally unsupported over several floors, as global collapse accelerated downward. (See the next slide for a diagram of the floor support by perimeter wall and core and see the following two slides for diagrams of a typical floor truss assembly.)
2. Sagging hot truss assemblies over several floors in the neighborhood of the initial impact pulled the perimeter wall inward, leading to buckling over several floors of the perimeter wall allowing the undeformed upper stories of the building to crash into the undeformed lower stories of the building, thus initiating rapid downward global failure.
Writing by one research team:
Text from:
https://911inacademia.files.wordpress.com/2011/01/preview-some-misunderstandings-of-wtc-collapse-analysis.pdf
“Some Misunderstandings Related to WTC
Collapse Analysis”, by Gregory Szuladzinski, Anthony Szamboti, and Richard Johns, International Journal of Protective Structures, Vol. 4, No. 2, 2013.
ABSTRACT: This article elaborates on variables associated with the collapse of the North Tower [WTC1] of the World Trade Center. The previously published quantifications of inertia, column capacity, and the assumptions related to the beginning of downward motion, are examined and corrected. The reasons for false conclusions reached in several previous analyses are presented.
INTRODUCTION: This presentation is not so much about how the WTC towers failed, but about how they could not fail. The objective is to eliminate erroneous concepts supported by false assumptions and by the use of incorrect values for velocity, mass, and column resistance. The only complete hypothesis of the global collapse mechanism of the Towers is a successive flattening of stories associated with compressive column failure and referred to as a Progressive Column Failure mode or PCF in brief. (In the past this mode was often referred to as pancaking, but this term is not used here to avoid ambiguities). It is explained here why PCF could not be the mode of the ultimate destruction.
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