Posted: Sep 14, 2021 11:19 pm
Well, to be blunt, I actually don't know a damn thing about all this. Never did pay much attention to this subject in the past.
Anyway, does this document you just gave me not answer your questions concerning time? That the momentum of the collapsing floors was so great that it completely overwhelmed the floor below the impact floors, rendering its effect on the collapsing structure's velocity minimal at best, resulting in the structure collapsing at essentially free fall speed?
Where did you get 18 seconds, anyway? The document you just cited claims that one tower fell in roughly 11 seconds, and the other in roughly 9 seconds.
Is that why you want to know more about the concrete used? To check their math?
Anyway, does this document you just gave me not answer your questions concerning time? That the momentum of the collapsing floors was so great that it completely overwhelmed the floor below the impact floors, rendering its effect on the collapsing structure's velocity minimal at best, resulting in the structure collapsing at essentially free fall speed?
Where did you get 18 seconds, anyway? The document you just cited claims that one tower fell in roughly 11 seconds, and the other in roughly 9 seconds.
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.
Is that why you want to know more about the concrete used? To check their math?