https://www.youtube.com/watch?v=C9AoaU7LlTk
If you can watch this video and still insist that Bush and our gov't is telling us everything, and not hiding anything or lying, I really feel sorry for you.
I've never seen someone fail so miserably (and look so uncomfortable doing it) to avoid answering a question.
Based on its comprehensive investigation, NIST concluded that the WTC towers collapsed because: (1) the impact of the planes severed and damaged support columns, dislodged fireproofing insulation coating the steel floor trusses and steel columns, and widely dispersed jet fuel over multiple floors; and (2) the subsequent unusually large number of jet-fuel ignited multi-floor fires (which reached temperatures as high as 1,000 degrees Celsius, or 1,800 degrees Fahrenheit) significantly weakened the floors and columns with dislodged fireproofing to the point where floors sagged and pulled inward on the perimeter columns. This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers. Both photographic and video evidence—as well as accounts from the New York City Police Department aviation unit during a half-hour period prior to collapse—support this sequence for each tower.
NIST estimated the elapsed times for the first exterior panels to strike the ground after the collapse initiated in each of the towers to be approximately 11 seconds for WTC 1 and approximately 9 seconds for WTC 2. These elapsed times were based on: (1) precise timing of the initiation of collapse from video evidence, and (2) ground motion (seismic) signals recorded at Palisades, N.Y., that also were precisely time-calibrated for wave transmission times from lower Manhattan (see NIST NCSTAR 1-5A).
As documented in Section 6.14.4 of NIST NCSTAR 1, these collapse times show that:
“The structure below the level of collapse initiation offered minimal resistance to the falling building mass at and above the impact zone. The potential energy released by the downward movement of the large building mass far exceeded the capacity of the intact structure below to absorb that energy through energy of deformation.
Since the stories below the level of collapse initiation provided little resistance to the tremendous energy released by the falling building mass, the building section above came down essentially in free fall, as seen in videos. As the stories below sequentially failed, the falling mass increased, further increasing the demand on the floors below, which were unable to arrest the moving mass.”
In other words, the momentum (which equals mass times velocity) of the 12 to 28 stories (WTC 1 and WTC 2, respectively) falling on the supporting structure below (which was designed to support only the static weight of the floors above and not any dynamic effects due to the downward momentum) so greatly exceeded the strength capacity of the structure below that it (the structure below) was unable to stop or even to slow the falling mass. The downward momentum felt by each successive lower floor was even larger due to the increasing mass.
From video evidence, significant portions of the cores of both buildings (roughly 60 stories of WTC 1 and 40 stories of WTC 2) are known to have stood 15 to 25 seconds after collapse initiation before they, too, began to collapse. Neither the duration of the seismic records nor video evidence (due to obstruction of view caused by debris clouds) are reliable indicators of the total time it took for each building to collapse completely.
Yes. The basic principles of conservation of momentum and conservation of energy were satisfied in these analyses.
In the case of the aircraft impact analyses, which involved a moving aircraft (velocity) and an initially stationary building, the analysis did, indeed, account for conservation of momentum and energy (kinetic energy, strain energy).
After each tower had finished oscillating from the aircraft impact, the subsequent degradation of the structure involved only minute (essentially zero) velocities. Thus, a static analysis of the structural response and collapse initiation was appropriate. Since the velocities were zero and since momentum is equal to mass times velocity, the momentum terms also equaled zero and therefore dropped out of the governing equations. The analyses accounted for conservation of energy.
NIST developed and reported detailed timelines for various aspects of
the WTC disaster beginning with the impact of the aircraft. These
timelines included the progression of fires through the buildings, the
response of the structure to damage and to fire, egress of occupants
from the towers, and the emergency response. The timelines were
developed based on extensive analysis of the photographic and video
evidence, analysis of computer models, first person interviews, radio
transmissions, and other data documenting the events of Sept. 11, 2001. A
general timeline for each of the towers is reported in NIST NCSTAR 1.
Detailed timelines for specific aspects of the WTC disaster are reported
in NIST NCSTAR 1-2, NIST NCSTAR 1-5, NIST NCSTAR 1-6, NIST NCSTAR 1-7,
and NIST NCSTAR 1-8.
Analyses of the composite floor system under fire exposures determined from fire dynamics simulations and thermal analyses, predicted sagging subsequent to truss web diagonal buckling and failure of some seated connections (see NIST NCSTAR 1-6C). However, the vast majority of the connections remained intact. Further, the shear studs that attached the floor slab to the spandrel and the diagonal steel struts that connected the truss top chord to the intermediate columns also were capable of transferring inward pull forces. Thus, the sagging floors were capable of exerting an inward pull on the exterior columns and spandrel beams.
in the bottom of the tower
Established in 1995,
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