User:Cs32en/911/Sources/NIST/200x/content

Preliminary investigations did not include the mechanics of the actual collapse, concentrating instead on the events leading up to it. However, the final draft report on the collapse of WTC7 by NIST provides a detailed investigation into the collapse timeline, starting with the failure of a critical column, Column 79 (initial failure event). 6 seconds later, the collapse of the East Penthouse on the roof was visible. The collapse of the core columns progressed from east to west for another 6.9 seconds (12.9 seconds total since the initial failure event). At this point, the report says, "all the interior columns had buckled" and "the remaining exterior structure above began to fall vertically as a single unit." To calculate the timeline of the collapse of the rest of the building, NIST focused on the time between the initial collapse of the roofline and the last position that the complete roofline could be observed before portions of it started to become obscured by dust, at the top of Floor 29. NIST calculated the timeline for this observable descent as 5.4 seconds and calculated the theoretical free-fall time for the same portion of the building as 3.9 seconds, and concluded that, "The actual collapse time of the upper 18 floors of the north face of WTC7 (the floors clearly visible in the video evidence) was 40 percent greater than the computed free fall time. This was consistent with physical principles."