What Did and Did Not Cause Collapse
of WTC Twin Towers in New York
Zdenˇek P. Baˇzant1, Hon.M. ASCE, Jia-Liang Le2, Frank R. Greening3, and David B. Benson4
Abstract: Previous analysis of progressive collapse showed that gravity alone suffices to explain
the overall collapse of the World Trade Center (WTC) towers. However, it remains to be checked
whether the recent allegations of controlled demolition have any scientific merit. The present analysis
proves that they do not. The video record available for the first few seconds of collapse is shown to
agree with the motion history calculated from the differential equation of progressive collapse but,
despite uncertain values of some parameters, it is totally out of range of the free fall hypothesis, on
which these allegations rest. It is shown that the observed size range (0.01 mmó0.1 mm) of the
dust particles of pulverized concrete is consistent with the theory of comminution caused by impact,
and that less than 10% of the total gravitational energy, converted to kinetic energy, sufficed to
produce this dust (whereas more than 150 tons of TNT per tower would have to be installed, into
many small holes drilled into concrete, to produce the same pulverization). The air ejected from the
building by gravitational collapse must have attained, near the ground, the speed of almost 500 mph
(or 223 m/s, or 803 km/h) on the average, and fluctuations must have reached the speed of sound.
This explains the loud booms and wide spreading of pulverized concrete and other fragments, and
shows that the lower margin of the dust cloud could not have coincided with the crushing front. The
resisting upward forces due to pulverization and to ejection of air, dust and solid fragments, neglected
in previous studies, are found to be indeed negligible during the first few seconds of collapse but not
insignificant near the end of crush-down. The calculated crush-down duration is found to match a
logical interpretation of seismic record, while the free fall duration grossly disagrees with this record.
To structural engineers, the collapse of the World Trade Center (WTC) towers on 9/11/2001
came as the greatest surprise since the collapse of Tacoma Narrows Bridge in 1940. Immediately
after the aircraft impact, the structural frame behaved as expected, but not after the fire.
To explain the collapse, it was proposed (on September 13, 2001; Baˇzant 2001; Baˇzant and
Zhou 2002) that viscoplastic buckling of heated and overloaded columns caused the top part of
tower to fall through the height of at least one story, and then shown that the kinetic energy of
the impact on the lower part must have exceeded the energy absorption capacity of the lower
part by an order of magnitude. A meticulous investigation of unprecedented scope and detail,
conducted by S. Shyam Sunderís team at the National Institute of Standards and Technology
(NIST 2005), supports this explanation. Although NIST did not analyze the overall process of
dynamic progressive collapse below the fire zone, it verified a sequence of effects that triggered
the collapse: (1) scraping of much of steel insulation by flying objects during aircraft impact
(without which the towers would not have collapsed, as concluded by NIST); (2) cutting of many
columns, and damage with large deflections of others during aircraft impact; (3) subsequent load
redistributions among columns; (4) sagging of heated floor trusses and their catenary action,
evidenced by multistory inward bowing of perimeter columns; and (5) viscoplastic buckling of
heated, damaged and overloaded columns.
Universally though has the foregoing explanation of collapse been accepted by the communities
of structural engineers and structural mechanics researchers, some outside critics have
nevertheless exploited various unexplained observations to disseminate allegations of controlled
demolition. The objective of this paper, based on the report by Baˇzant et al. (2007), is to examine
whether those allegations might be scientifically justifiable, and to show that the concept
of gravity-driven collapse does not conflict with any observations.
1McCormick Institute Professor and W.P. Murphy Professor of Civil Engineering and Materials
Science, Northwestern University, 2145 Sheridan Road, CEE/A135, Evanston, Illinois 60208; zbazant@
2Graduate Research Assistant, Northwestern University.
3Engineering Consultant, Hamilton, Ontario L8S 3X7.
4Professor Emeritus, http://www.civil.northwestern.edu/people...0Cause%20It.pdf
You certainly can find much more which says that NIST`s data gives sufficent evidence to rule out explosive demolition. The funny stuff about Stevie`s claims along with those who claimed to have published papers is that all of it goes straight back to conspiracy websites, not scientific journals. Your link to Bentham goes to the same letter I posted a link for. As any yard ape can tell, there is nothing enclosed within this letter which can be peer reviewed. It`s not a paper.