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By way of an introduction:

This is the first known instance where fire-induced local damage (i.e., buckling failure of Column 79; one of 82 columns in WTC 7) led to the collapse of an entire tall building. P.62.

A detailed analysis of the descent of the north face found 3 stages: • A slow descent at less than gravitational acceleration, corresponding to column buckling • A free fall descent at gravitational acceleration over approximately 8 stories • A decreasing acceleration, as the north face encountered resistance from the structure below. P.64. [Norm’s emphasis]

Source of the foregoing two quotes: NIST Response to the World Trade Center Disaster: Federal Building and Fire Safety Investigation of the World Trade Center Disaster — November 19, 2008 (Here)

[This is a reposting of this video which was taken down from the original site, originally posted in August 2008 just after the release of the final draft of the NIST WTC7 Report, prior to the final version which was altered to admit a 2.25 second period of freefall.]
This video tracks the motion of the NW corner of Building 7 of the World Trade Center on 9/11 2001. The building was in freefall for a period of ~2.5 seconds. This means it was falling through itself for over 100 feet with zero resistance, an impossibility in any natural scenario. This period of freefall is solid evidence that explosives had to be used to bring the building down. In the final draft for public comment (August 2008) NIST denied that WTC7 fell at freefall. In the final report in Nov 2008 they reversed themselves and admitted freefall, but denied its obvious significance.
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[The WTC7 series has elicited a number of questions from people unclear on the details of how I did the measurements, compared to how NIST did them and how the representatives of NIST described their measurements. I have therefore created a WTC7 Measurement FAQ page:http://www.911speakout.org/WTC7-Measu… . I will also use this FAQ as a place of reference for other questions that arise as well.]
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My new DVD: “9/11 Analysis” is now out. Go to http://www.911speakout.org .

Pertinent to David Chandler’s videos, a long quote from David Ray Griffin’s piece, Left-Leaning Despisers of the 9/11 Truth Movement: Do You Really Believe in Miracles? :

[. . .]

  1. WTC 7’s Descent in Absolute Free Fall

Even if some readers question whether the two previously discussed features of the collapse of WTC 7, when understood within the framework of NIST’s fire theory, imply miracles, there can be no doubt about a third feature: the now-accepted (albeit generally unpublicized) fact that WTC 7 came down in absolute free fall for over two seconds.

Although members of the 9/11 Truth Movement had long been pointing out that this building descended at the same rate as a free-falling object, or at least virtually so, NIST had long denied this. As late as August 2008, when NIST issued its report on WTC 7 in the form of a Draft for Public Comment, it claimed that the time it took for the upper floors – the only floors that are visible on the videos – to come down “was approximately 40 percent longer than the computed free fall time and was consistent with physical principles.”52

As this statement implied, any assertion that the building did come down in free fall, assuming a non-engineered collapse, would not be consistent with physical principles – meaning basic laws of Newtonian physics. Explaining why not during a “WTC 7 Technical Briefing” on August 26, 2008, NIST’s Shyam Sunder said:

“[A] free fall time would be [the fall time of] an object that has no structural components below it. . . . [T]he . . . time that it took . . . for those 17 floors to disappear [was roughly 40 percent longer than free fall]. 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. Everything was not instantaneous.”53

In saying this, Sunder was presupposing NIST’s theory that the building was brought down by fire, which, if it could have produced a collapse of any type, could have produced only a progressive collapse.

In response, high-school physics teacher David Chandler, who was allowed to submit a question at this briefing, challenged Sunder’s denial of free fall, stating that Sunder’s “40 percent longer” claim contradicted “a publicly visible, easily measurable quantity.”54 Chandler then placed a video on the Internet showing that, by measuring this publicly visible quantity, anyone understanding elementary physics could see that “for about two and a half seconds. . . , the acceleration of the building is indistinguishable from freefall.”55 (This is, of course, free fall through the air, not through a vacuum.)

In its final report on WTC 7, which came out in November 2008, NIST – rather amazingly – admitted free fall. Dividing the building’s descent into three stages, NIST described the second phase as “a freefall descent over approximately eight stories at gravitational acceleration for approximately 2.25 s[econds].”56 NIST thereby accepted Chandler’s case – except for maintaining that the building was in absolute free fall for only 2.25, not 2.5, seconds (a trivial difference). NIST thereby affirmed a miracle, meaning a violation of one or more laws of physics.

Why this would be a miracle was explained by Chandler, who said: “Free fall can only be achieved if there is zero resistance to the motion.”57 In other words, the upper portion of Building 7 could have come down in free fall only if something had suddenly removed all the steel and concrete in the lower part of the building, which would have otherwise provided resistance (to make a considerable understatement). If everything had not been removed and the upper floors had come down in free fall anyway, even if for only a fraction of a second, this would have been a miracle – meaning a violation of physical principles. Explaining one of the physical principles involved, Chandler said:

“Anything at an elevated height has gravitational potential energy. If it falls, and none of the energy is used for other things along the way, all of that energy is converted into kinetic energy – the energy of motion, and we call it ‘free fall.’ If any of the energy is used for other purposes, there will be less kinetic energy, so the fall will be slower. In the case of a falling building, the only way it can go into free fall is if an external force removes the supporting structure. None of the gravitational potential energy of the building is available for this purpose, or it would slow the fall of the building.”58

That was what Sunder himself had explained, on NIST’s behalf, the previous August, saying that a free-falling object would be one “that has no structural components below it” to offer resistance. But NIST then in November, while still under Sunder’s leadership and still defending its fire theory of WTC 7’s collapse, agreed that, as an empirical fact, free fall happened. For a period of 2.25 seconds, NIST admitted, the descent of WTC 7 was characterized by “gravitational acceleration (free fall).”59

Besides pointing out that the free fall descent of WTC 7 implied that the building had been professionally demolished, Chandler observed that this conclusion is reinforced by two features of the collapse mentioned above:

“[P]articularly striking is the suddenness of onset of free fall. Acceleration doesn’t build up gradually. . . . The building went from full support to zero support, instantly. . . . One moment, the building is holding; the next moment it lets go and is in complete free fall. . . . The onset of free fall was not only sudden; it extended across the whole width of the building. . . . The fact that the roof stayed level shows the building was in free fall across the entire width. The collapse we see cannot be due to a column failure, or a few column failures, or a sequence of column failures. All 24 interior columns and 58 perimeter columns had to have been removed . . . simultaneously, within a small fraction of a second.”60

[…]

Notes

[…]

52 NIST NCSTAR 1-9, Draft for Public Comment, Vol. 2 (http://wtc.nist.gov/media/NIST_NCSTAR_1-9_vol2_for_public_comment.pdf), 596.

53 “WTC 7 Technical Briefing,” NIST, August 26, 2008. Although NIST originally had a video and a transcript of this briefing at its Internet website, it recently removed both of them. However, Nate Flach has made the video available at Vimeo (http://vimeo.com/11941571), and the transcript, under the title “NIST Technical Briefing on Its Final Draft Report on WTC 7 for Public Comment,” is available at David Chandler’s website (http://911speakout.org/NIST_Tech_Briefing_Transcript.pdf).

54 Ibid.

55 David Chandler, “WTC7 in Freefall – No Longer Controversial,” September 4, 2008 (http://www.youtube.com/watch?v=rVCDpL4Ax7I) , at 2:45.

56 NIST NCSTAR 1-9, Vol. 2: 607.

57 Chandler, “WTC7 in Freefall – No Longer Controversial,” at 3:27.

58 Chandler, “WTC7: NIST Finally Admits Freefall (Part III),” January 2, 2009 (http://www.youtube.com/watch?v=v3mudruFzNw) , at 1:19.

59 “Questions and Answers about the NIST WTC 7 Investigation,” NIST, August 21, 2008, updated April 21, 2009. Whereas the original version of this document denied free fall, the updated version affirms it. Although both versions have been removed from NIST’s website, Jim Hoffman’s website has both the 2008 version (http://911research.wtc7.net/mirrors/nist/wtc_qa_082108.html ) and the 2009 version (http://911research.wtc7.net/mirrors/nist/wtc_qa_042109.html).

60 Chandler, “WTC7: NIST Finally Admits Freefall (Part III),” at 2:20, 3:15.