Yes, can you explain this in terms of debris driving a collapse?:
(followed by broken link, I used the link in post 342 instead)
I'm taking your questions from post 342 also.
Keep in mind that you're asking me to speculate. I'm happy to oblige but in the absence of any scientific investigation of the detailed dynamics of the collapse (due to computer modeling systems being inadequate to the task) that's all I can do.
(1) linearity of the collapse fronts.
Debris (especially concrete floor slabs from the floors above) landing on floors is causing floors to pancake. (You know, of course, that NIST ruled out the pancaking theory of collapse
initiation, which is not the same thing as declaring that pancaking never occurred during the collapse.)
One process helping the progression of floor failure to remain roughly horizontal is the resistance of the structure. This resistance slows the collapse to slower than free-fall speed.
Now, consider the collapse from the point of view of a single undamaged floor. When does that floor provide the most resistance to further collapse? The answer is, while it's stationary and its connections to the columns are still completely intact (for however brief a period that might be, once the debris starts impacting it). As it starts to tear free and starts to accelerate, its resistance decreases. That means that the first debris to reach the floor encounters more resistance than debris that hits it later (we're talking small fractions of a second difference here). So the leading debris is slowed down more. This would tend to keep the falling debris on an even front.
(2) several floor lag between the adjoining faces of the collapse.
No such lag is evident in the video you linked. In each frame, lines aligned with the linear collapse fronts of the two visible sides intersect in the corner.
However, the collapse front at the corner itself does lag behind. This suggests that the standing corner structure is stronger, which makes sense (the meeting of two walls at a right angle is stronger than a span of flat wall). The corner floor trusses also had a stronger two-way arrangement. Corner collapse might well have lagged behind due to the increased resistance there, with corners shearing off as the main portion of the floors fell.
Also, you showed some images and videos on another thread that did appear to show the collapse front running several floors apart on
opposite sides of the building. I wouldn't be surprised if floors on opposite sides of the long axis of the core did pancake separately, one side ahead of the other. The floor trusses between the short walls of the core and the walls ran parallel to the long axis of the core, so the only thing preventing half a floor from separating from the other half along that axis would have been the relatively weak (and few) bridging trusses, some deck support angles, four inches of concrete (brittle under shear), and the thin floor pan. The increased resistance of having to part the floors in two would have tended to keep the two sides even, but that might not have been enough to compensate for a head start of one side's collapse due to the initial tilt of the top section.
Until the technology exists to model the whole process under a whole array of starting conditions within the range of uncertainty of the known starting conditions, and observe the range of behaviors and probe the causes of each phenomenon of interest, such answers will be only speculative. That's why the published peer-reviewed scientific papers so far have focused on the overall characteristics of the collapse that don't depend on knowing what pieces broke in what order, such as the calculations that show ample potential energy available to destroy the structure in the time observed.
Respectfully,
Myriad