I guess that the beginning of this threat bears an error. Dr. G gave time and distance for the first 5 seconds of the Sandwell East Tower demolition. He wrote that the tower didn't accelerate faster than 5m/s².
The given times and distances appear like the onset of the collapse at 1.4 seconds, acceleration about 8m/s² for the next 1.5 seconds and nearly a constant velocity after that.
For a short time the acceleration could be higher than that e.g. if the downward movement indeed started at about 1.5 seconds then the first 0.5 seconds need to accelerate at free fall.
I haven't seen a video of the Sandwell demolition but probably they took just one floor or less out. The upper part would hit the lower part and immediately start to decelerate.
The astonishing WTC7 collapse had a relatively long period at or near free fall. Like peterene pointed out somewhere these pattern fits perfectly the loss of the lobby height before the upper part hypothetically hit the ground and decelerated.
The major difference between a seismic collapse and the WTC7 collapse can be described this way:
A) In a seismic collapse the sway of the building is caused by the ground shaking. The building sways back and forth on all columns at ground level. The displacement of the upper block is parallel to its corners. At a certain amplitude of the sway the columns will start to kink or bend and the upper block starts to "sink" sideways until these ground level columns break. -> Collapse.
b) For any unknown reason WTC7 started to sway some seconds prior to the east penthouse collapse. First it sways to the west and away from column 79. Imo the columns 79 - 81 can be excluded for a cause. Shortly after that the east penthouse collapsed and fell near free fall acceleration through the "core" of WTC7 - means about no resistance inside the building! Still the building sways back and forth but now at a higher amplitude. Finally the onset of the global collapse did not occur in the moment of the highest amplitude but about in the moment of the buildings upright "zero position". The whole building - while swaying "sinus-like" back to the west - immediately changed its direction towards east where the penthouse was collapsed already.
Btw, all known videos are showing the WTC7 from the north. Column 79 supported the north wall of the east penthouse while columns 80 and 81 supported the south wall of that penthouse. Nevertheless, in no video is any visible sign of the south wall still in place while the north wall just started to fall. Imo columns 81 and 80 were already gone just some seconds before the east penthouse collapsed. There is a visible downwards suction in the smoke above the penthouse shortly before the east penthouse started to collapse. This suction might indicate the collapse of the roof and probably the north side of the penthouse.
The collapse time given by NIST is wrong. According to the seismic data the collapse started about 15 seconds earlier. Comparing the seismic data of LDEO (PAL) with the sway of the building some remarkable events becoming visible: The sway started exactly in the moment of a seismic spike. But there are TWO seismic spikes. The second spike occurred shortly prior to the east penthouse collapse.
All further ground shaking can be explained by the falling debris of the penthouse hitting the ground and - after a short pause - the debris of the global collapse. The magnitudes of both collapses (penthouse, rest) are comparable probably because the penthouse fell from a high elevation seemingly without deceleration. The outer shell fell "just" for some meters and started to decelerate itself. While doing so just the lowest floor collapsed one by one. Therefore the transmitted energy would not exceed the necessary energy to crush just one floor. Most of that energy will be consumed by the crushing of that actual floor. Just a fraction of that energy will be transformed into seismic energy. Therefore the appearance of both seismic events looks pretty similar even if the first was caused by the small penthouse and the second by the rest of the building.
Finally we never heard some good and unedited sound recording of the collapse. But that sound exits somewhere and sure, NIST listened to it because they used the well known West St video for a lot of analysis e.g. the motion detection of the sway. According to the sway-analysis NIST has a longer version of that video then the public ever had seen. They included this shot in their own WTC7-promotion-video but without sound. Nevertheless there is a version of that shot aired on Sep. 12 with a completely different sound than e.g. in the Naudet video. The "most likely" original sounds recorded in a distance of 0.4 miles has two loud "Bangs" at very low frequencies. (It sounds pretty much like described by a witness even if NIST claims that there was no "Bang" at all and of course no witness to those sounds.) That sound recording started after the first seismic spike and prior to the second seismic spike (after subtracting the time that sound had to travel trough air).
According to Won-Young from LDEO (PAL) the 1993 bombing of the WTC caused no measurable seismic activity. I can only guess about the reasons for it. Maybe because of a very short pressure pulse that wasn't directed against the foundation but against the inner structure. However, the seismic spikes prior to the WTC7 collapse probably must be interpreted as falling debris rather than as "explosion". If so then the timing of the thunder sound matches an hypothetical explosion that caused a lot of falling debris which is accountable for the seismic activity.
According to the building sway and seismic spikes there was another similar event but still without any known authentic sound recording.
Imo the first event took out column 80 and probably some symmetrical support of the west side -> the building started to sway west. Some seconds later column 79 was taken out and caused the penthouse collapse. Probably further support of the west side was gone simultaneously (the building still sway in a sinus-pattern around the upright "zero-position". Simultaneously with the penthouse hitting the ground either the debris smashed further support* or further support was taken out and caused the global collapse.
*the NIST theory of the "penthouse smashing transfer trusses" lacks of one explanation. In front of the penthouse all lower floors must hit these trusses. If these lower debris wasn't able to destroy these trusses then we should expect some kind of debris pillow for the penthouse. There is no usable explanation why the global collapse should wait for the penthouse hitting the ground.
Anyone any idea?
Imho WTC7 is not comparable to a seismic collapse and it didn't fall too fast. But it fell too fast for being initiated by a buckling core column 79 and some tiny toon chain reaction that totally negated all support from below in a perfect manner.
Most disturbing is the fact that the NIST Interim Report on WTC7 concluded that an unsupported column length of 4 floors wouldn't cause the column to buckle at a steel temperature 250°C. In some oxygen guided simulation (including additional fire sources on the north side etc.) NIST reached some simulation of a burning inferno for hours on several floors one above the other and surprisingly in the area around column 79 (core area without that much fuel) and finally an unsupported column length of 5 floors. I try to imagine how surprised the NIST engineers looked at the screen to see that happen. But while that unsupported column 79 buckled at less than 250°C we still have the problem of extremely hot "hot spots" in the rubble and close the column 81.
Falling Penthouse:
http://www.youtube.com/watch?v=JjQyXLRpULUSway:
http://www.youtube.com/watch?v=v5rAK2CeP0YSound, witness accounts and seismic spikes:
http://www.youtube.com/watch?v=7sI2gP7_xdA
and I'll consider a new thread.)