This revision includes reference to "concrete..... core walls" as well as other information about calculated amounts of high explosives, perhaps the reason the .pdf is no longer avaialbe at its original source.
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Collapse of World Trade Center Towers:
What Did and Did Not Cause It?
Zdenek P. Bazant 6/21/07
1, Hon.M. ASCE, Jia-Liang Le
2, Frank R. Greening
3, and David B. Benson
4, Abstract: Previous analysis of progressive collapse showed that gravity alone suffices to explain the overall collapse of the World Trade Center towers. However, it has not been checked whether the allegations of controlled demolition by planted explosives have any scientific merit. The present analysis proves that they do not. The video record available for the first few seconds of collapse agrees with the motion history calculated from the differential equation of progressive collapse but disproves the free fall hypothesis (on which the aforementioned allegations rest). Although, due to absence of experimental crushing data for the lightweight concrete used, the theory of comminution cannot predict the size range of pulverized concrete particles, it is shown that the observed size range (0.01 mm – 0.1 mm) is fully consistent with this theory and is achievable by collapse driven gravity alone, and that only about 7% of the total gravitational energy converted to kinetic energy of impacts would have sufficed to pulverize all the concrete slabs and core walls (while at least 158 tons of TNT per tower, installed into many small holes drilled into each concrete floor slab and core wall, would have been needed to produce the same degree of pulverization). The exit speed of air ejected from the building by the crushing front of gravitational collapse must have attained, near the ground, 465 mph (208 m/s) on the average, and fluctuations must have reached the speed of sound. This explains loud booms and wide spreading of pulverized concrete and glass fragments, and shows that the lower margin of dust cloud could not have coincided with the crushing front. The resisting upward forces due to pulverization and air ejection, neglected in previous studies, are found to be negligible during the first few seconds of collapse but not insignificant near the end of crush-down (these forces extended the crush-down duration by about 4%; they augmented, by about 25%, the resisting force due to column buckling at the end of crush-down, and doubled that force at the beginning of crush-up). The calculated crush down duration is found to match a logical interpretation of seismic record, while the free fall duration is found to be in conflict.
http://72.14.205.104/search q=cache:H5djFQBfSzsJ:www.civil.northwestern.edu/people/bazant/PDFs/Papers/00%2520WTC%2520Collapse%2520-%2520What%2520did%2520%26%2520Did%2520Not%2520Cause%2520It%2520-%2520Revised%25206-22-07.pdf+Bazant&hl=en&ct=clnk&cd=2&gl=us&client=firefox-a
This is the latest revision of the orginal analysis provided to NIST by Bazant et al, the first one was included in the NIST report. The above url was reconstructed after the ,pdf was no longer available on the server.
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