World Trade Center 7 Demolition Acceleration
Lead investigator for NIST, Shyam Sunder, stated (before freefall was admitted in the final WTC7 report):
"a free fall time would be an object that has no structural components below it . . . there was structural resistance that was provided in this particular case. And you had a sequence of structural failures that had to take place. Everything was not instantaneous."
From the August 26 2008 NIST Technical Briefing :
Question to Dr. Sunder:
Q: Any number of measurements using a variety of methods, indicate the northwest corner of WTC7 fell with an acceleration within a few percent of the acceleration of gravity. Yet, the report contradicts this claiming 40% slower than the freefall based on a single data point.
A: Well, um, the, first of all, um, gravity is the loading function that applies to the structure, uh, a, um, applies to applies to every body every all bodies on a, on a, on on this particular on this planet, not just um, um, in in ground zero. Um, the a, the analysis showed there's a difference in time between the freefall time, a freefall time would be an object that has no ah structural components below it, uh and if you look at the analysis of the video, it shows that the time it takes for the 17, uh for the a the roof line on the video to collapse down 17 floors that you can actually see in the video, below which you cant see anything in the video is about uh 3.9 seconds. What the analysis shows, and um the structural analysis shows the collapse analysis shows, the same time that it took for the structural model to come down from the roof line all the way for those 17 floors to disappear is um 5.4 seconds it's a about 1.5 seconds or roughly 40% more time for that freefall to happen, and that is not at all unusual because there was structural resistance that was provided in this particular case, and you had a sequence of structural failures that had to take place, and where everything was not instantaneous.
WTC7: NIST Finally Admits Freefall: https://www.youtube.com/watch?v=Rkp-4sm5Ypc&list=PL206C1F5EDFC83824
NIST tracked the downward displacement of a point near the center of the roofline, fitting the data using a smooth function. (The time at which motion of the roofline was first perceived was taken as time zero.) The fitted displacement function was then differentiated to estimate the downward velocity as a function of time, shown as a solid curve in Figure 3-15. Velocity data points (solid circles) were also determined from the displacement data using a central difference approximation. The slope of the velocity curve is approximately constant between about 1.75 s and 4.0 s, and a good straight line fit to the points in this range (open-circles in Figure 3-15) allowed estimation of a constant downward acceleration during this time interval. This acceleration was 32.2 ft/s2 (9.81 m/s2), equivalent to the acceleration of gravity g.
In Stage 1, the descent was slow and the acceleration was less than that of gravity. This stage corresponds to the initial buckling of the exterior columns in the lower stories of the north face. By 1.75 s, the north face had descended approximately 2.2 m (7 ft).
In Stage 2, the north face descended at gravitational acceleration, as the buckled columns provided negligible support to the upper portion of the north face. This free fall drop continued for approximately 8 stories or 32.0 m (105 ft), the distance traveled between times t = 1.75 s and t = 4.0 s.
In Stage 3, the acceleration decreased somewhat as the upper portion of the north face encountered increased resistance from the collapsed structure and the debris pile below. Between 4.0 s and 5.4 s, the north face corner fell an additional 39.6 m (130 ft).
The three stages of collapse progression described above are consistent@@color(black):*@@ with the results of the global collapse analyses discussed in Chapter 12 of NIST NCSTAR 1-9. 1A.pdf NCSTAR1A_p87
 (see comment 22)
WTC7 in Freefall - No Longer Controversial: http://www.youtube.com/watch?v=gC44L0-2zL8
Re: Freefall Acceleration of WTC 7: http://www.youtube.com/watch?v=RveHSPbtTmY
FAQ On Measurements In The Videos Above: http://www.911speakout.org/WTC7-Measurement-FAQ.html
[NCSTAR 1-A pg 45 [pdf pg 87]
"The slope of the velocity curve is approximately constant between about 1.75 s and 4.0 s, and a good straight line fit to the points in this range (open-circles in Figure 3-15) allowed estimation of a constant downward acceleration during this time interval. This acceleration was 32.2 ft/s2 (9.81 m/s2), equivalent to the acceleration of gravity g." [within 0.1%]