Reply #26: Of course NIST is gessing...

[Carefulplease]

Or rather, they are not even trying to guess what the total time of the collapse is. What they say is this:

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 NCSTAR 1-5A).

http://wtc.nist.gov/pubs/factsheets/faqs_8_2006.htm

The collapse isn't finished when those panels hit the ground. Also, both their data on the start (from videos) and the end times (from seismic data) have a 1 second uncertainty.

They also say:

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.

So, it's anybodies guess what the total time of the collapse was. Those who attempt to guess it from the analysis of video data (such as Hoffman) get something around 15 seconds. This is consistent with the acceleration that others have measured in the first few seconds of the videos when specific features of the towers can be tracked before dust obscures the view.

NIST also say:

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.”

This is rather lousy. The structural resistance was indeed little. However the claim does not take into account the more significant effect of momentum transfer from the falling mass to the floors below.