Some Misunderstandings Related to WTC Collapse Analysis

[Lauren Johnson]

This is a technical paper refuting the Progressive Column Failure theory of North Tower Collapse from the International Journal of Protective Structures. A couple of key paragraphs:

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....

As one can readily see, this mode of damage is a distinct process, whereby each floor
becomes crushed, one after the other. An attempt has been made to smear the process out into
a continuum event and then use differential equations to obtain a solution [2]. Yet,
justification of such an approach must be performed, by showing that little is lost in
translation. As no such justification exists, the whole approach in [2] seems questionable....

[Summary]A number of simple, transparent calculations of the North Tower collapse were presented in
[5] and the conclusion was that assuming even a modest resistance of columns during their
destruction would cause an unacceptably long collapse time. It is only when perfectly
frangible columns were adopted that the fall time was as low as 15.3 s. This removes the PCF
mode, as defined here, as a viable hypothesis of collapse.

Yet, the PCF achieved significant popularity, as based on [1] and [2], while the next work
[12] did not contribute anything new to the core of the subject. These papers, purporting to
explain the collapse, suffered from three fatal errors, as detailed above. Also, the whole
methodology was not justified. Some incredibly short fall times were quoted by the authors,
while all solutions were of a black-box type. The presentations in these papers are not a valid
description of what happened. The reasons for a smooth motion history and promptness of
collapse of the North Tower remain yet to be determined.

http://911blogger.com/news/2013-07-09/so...e-analysis