Whereas NIST did attempt to analyze and model the collapse
of WTC 7, it did not do so in the case of the Twin
Towers. In NIST’s own words, “The focus of the investigation
was on the sequence of events from the instant of
aircraft impact to the initiation of collapse for each tower….this
sequence is referred to as the ‘probable collapse
sequence,’ although it includes little analysis of the structural
behaviour of the tower after the conditions for collapse
initiation were reached and collapse became inevitable.”[5]
Thus, the definitive report on the collapse of the Twin
Towers contains no analysis of why the lower sections
failed to arrest or even slow the descent of the upper
sections—which NIST acknowledges “came down essentially
in free fall” [5-6]—nor does it explain the various
other phenomena observed during the collapses. When a
group of petitioners filed a formal Request for Correction
asking NIST to perform such analysis, NIST replied that
it was “unable to provide a full explanation of the total
collapse” because “the computer models [were] not able
to converge on a solution.”
However, NIST did do one thing in an attempt to substantiate
its assertion that the lower floors would not be
able to arrest or slow the descent of the upper sections in
a gravity-driven collapse. On page 323 of NCSTAR 1-6,
NIST cited a paper by civil engineering professor Zdeněk
Bažant and his graduate student, Yong Zhou, that was
published in January 2002 [7] which, according to NIST,
“addressed the question of why a total collapse occurred”
(as if that question were naturally outside the scope of
its own investigation). In their paper, Bažant and Zhou
claimed there would have been a powerful jolt when the
falling upper section impacted the lower section, causing
an amplified load sufficient to initiate buckling in the
columns. They also claimed that the gravitational energy
would have been 8.4 times the energy dissipation capacity
of the columns during buckling.
