The sudden fall of World Trade Center 7 (WTC 7), a massive 47-story steel-framed building two blocks from the Twin Towers, has been a troublesome issue for the official narrative. The imminent demise of the building was predicted repeatedly during the afternoon of September 11. Police and firefighters cleared a radius of several blocks, and people, including several reporters, were told that this was “the building that would come down next”  — even though no steel structure had ever globally collapsed due to fire prior to 9/11. The event was witnessed by hundreds of people directly and many cameras.
At about 5:20 in the afternoon, the East Penthouse collapsed into the building, accompanied by window breakage on multiple floors.  A few seconds later, the West Penthouse began to collapse into the building, but before it disappeared the entire building underwent a sudden transition to free fall, which lasted for over two seconds. 
In its initial report on the building collapses, the National Institute of Standards and Technology (NIST) discussed only the Twin Towers, omitting WTC 7. A separate report on this building was repeatedly delayed because, in the words of project director Shyam Sunder, “We’ve had trouble getting a handle on building No. 7.”  NIST’s Final Report on the Collapse of World Trade Center Building 7 was released in November 2008 (when the Bush administration was ready to leave office). NIST offered a computer-generated graphical simulation to finally explain the collapse.
The Official Account
WTC 7 collapsed because of fire alone.  Here are the central features of the collapse:
- Intense heating on the 12th floor caused an overhead beam to lengthen, due to thermal expansion, and to push a 13th-floor girder off of the seat that had connected it to interior column #79.
- This failure propagated for several floors, leaving column 79 unsupported, thereby causing it to buckle. Nearby columns were unable to absorb the transfer of load. This inability initiated a progressive collapse, which led to catastrophic failure of the entire building.
- This collapse of the building, which appeared to be sudden although in reality it was progressive, is shown to be plausible by computer simulations.
- The mechanism behind the collapse is supported by graphical output, which is included in the NIST Final Report  and explained by animations posted on the NIST website. 
The Best Evidence
- A building undergoing progressive collapse would come down in a sequential manner. Sections would be expected to fail as they lost support. However, from measurements of the collapse time, it could not have been progressive or sequential:
- From the time of the collapse of the East Penthouse to the onset of global collapse, the building appeared, from all external signs, to retain its overall integrity. The transition from total support to freefall was sudden.  The building fell with a horizontal roofline, implying that catastrophic failure across the entire width of the building (100 meters east to west) occurred virtually simultaneously within a fraction of a second.
- The building buckled horizontally near the middle about 1.5 seconds prior to the onset of freefall, but this was not accompanied by downward motion. The building retained its full height until the onset of global collapse. The initial downward motion, measured at the northwest corner of the building, was a sudden transition to freefall.
- In addition to the fact that the collapse of a steel-framed building entering into freefall in the absence of explosives to remove the steel supports is inherently implausible, the graphical output from NIST’s computer simulations does not match the actual observations at all. The two cases presented in NIST’s Final Report represent two, very different, scenarios:
- In one of these, damage caused by debris from the North Tower collapse was a contributing factor.
- In the other one, there is no mention of debris-caused damage.
The scenario that included debris-caused damage to the south face of WTC 7 somewhat resembled the observed fall, but NIST concluded that debris-caused damage was not a significant factor in the collapse.
It is quite apparent, from even casual inspection, that the simulation does not account for observations:
- The simulated building shows marked deformations that would be easily detectable from exterior views.  These were not observed.The actual building did not even undergo window breakage during the interval leading up to the freefall collapse, whereas window breakage was quite conspicuous during the much smaller local collapse under the East Penthouse. If, as NIST reported, the interior collapse of the majority of the building had been actually occurring, one would expect window breakage to be at least as evident as what was observed in the smaller collapse event.When the simulated building starts to collapse, it does not enter freefall, whereas freefall is the most notable aspect of the actual collapse and the feature most in need of explanation.The animation depicting the collapse of the simulated building is cut short. It does not cover the entire period of observed freefall.
- In statements following the release of NIST’s Final Report, project director Shyam Sunder still struggled to come to terms with the reality of the observed period of freefall. His claim, presumably speaking for NIST, was that we are not seeing the actual collapse, but only the north facade of the building. According to this explanation, the interior collapsed first (in progressive collapse, as described by their model), leaving the facade to fall at freefall as a separate event.This explanation does not pass scrutiny, however, for four reasons:
- There is video footage showing both the north and west faces of the building. Both of these faces, along with the corners connecting to the south and east faces of the building, appear to remain stable and intact until the sudden onset of free fall.
- The West Penthouse, and therefore the interior structure that supported it, did not fail until about a second before the onset of freefall. Therefore the interior of the building got no more than about half a story head start. 
- Even though the smaller collapse of the East Penthouse brought about a brief period of window breakage, no further windows were broken until the onset of freefall, so it is not believable that total internal collapse was occurring.
- The roiling clouds of debris that raced down the street, often likened to pyroclastic flow, did not occur until the visible collapse of the building. If the interior of the building had collapsed earlier, we would expect the flow of debris to have started earlier.
We can conclude that the computer simulations do not, in fact, correlate to the key features of the building collapse. NIST’s attempt to “decouple” developments in the unseen interior of the building from what happened to its easily observable exterior is, therefore, contrary to evidence. NIST’s position appears to be no more than an attempt to evade legitimate questions.
This conclusion is reinforced by NIST’s refusal to release its computer models, combined with the fact that progressive collapse resulting in freefall could never be replicated experimentally — for the simple reason that a progressive collapse involving freefall is physically impossible.
References for WTC 7 Point 4
- David Chandler, “WTC 7: Sound Evidence for Explosions,” YouTube: DavidChandler911, July 5, 2010.
- “9/11: WTC 7 east penthouse collapse (partial video),” YouTube: Nathan Flach.
- David Chandler, “WTC 7 in Freefall No Longer Controversial.” NIST measured 2.25 seconds of absolute freefall with a gradual transition into freefall (NIST NCSTAR 1A (archive copy), November 2008, p. 46, pdf-p.@nbsp;88), but NIST’s measurements were taken near the midpoint of the roofline using video from a street-level camera. The horizontal folding of the roofline at that point was erroneously interpreted as downward motion, giving the appearance of a gradual transition into freefall.
- Statement by Shyam Sunder on March 20, 2006, in answer to reporters’ questions. Reported in: Mark Jacobson, “The Ground Zero Grassy Knoll,” New York Magazine, March 20, 2006; also in “Profile: Shyam Sunder” at History Commons.
- NIST NCSTAR 1-9, Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7, Volume 1, Chapter 8; NIST NCSTAR 1A (archive copy), November 2008, Executive Summary, p. xxxvi (pdf: 39).
- NIST NCSTAR 1-9, Vol. 2, Chapter 12, pp. 588-97 (pdf: 654 ff). A picture of the distorted collapsing model is emphasized on the cover of NIST NCSTAR 1A (archive copy). NIST has not released the actual simulation data.
- The videos showing the models have been removed from the NIST website but they have been widely downloaded and reposted. They are available, for example, here: “NIST WTC7 Models.”
- David Chandler, “WTC7 in Freefall: No Longer Controversial,” February 12, 2010.
- The deformations may also be seen at this official source: NIST NCSTAR 1A, “Final Report on the Collapse of World Trade Center Building 7” (archive copy), November 2008, 42.
- The collapse of the west penthouse is plainly visible. It begins to collapse, but gets only about half way to the main roofline when the whole building collapses. After that the penthouse and main building move together with the penthouse continuing to be visible during the freefall part of the collapse.