Thanks for the comments - we are closer to understanding each other.I agree that the collapse can be divided into those three stages, and that during "transition", there is significant "mutual destruction" of upper and lower structure - initially perhaps near enough to being "at the same rate" (and this is even a result of B&V for the first story!).
BUT the upper parts were initially something like 15 and nearly 30 floors - and as your "Transition" phase is "the very much chaotic process for the first few stories of dropping", I think you agree that by the time Transition has turned into "Progression", there is still a major portion of upper part left undismantled, simply because "the first few" stories is surely less than 15.
Two points from this first part:
(a) We agree three stages - which is an important basis for this and related thread topics. Not distinguishing the stages causes much confusion. Not only for truthers. << THAT is the main reason I raise the issue in this thread - it is relevant to some of the arguments.
(b) My apology for "first few". I have always had in mind the 15 or near 30 stories and the mutual destruction of Top Block and upper levels of lower towers. (Which may not have been exactly equal numbers of floors of Top Block and lower tower) If you read my posts the destruction of all the Top Block is usually implicit but not stated explicitly - my apology for the confusion. The reason for that boundary - which I explained as "fuzzy" - is that before that point the Top Block remnants are still partially acting as integral whilst after that point the top block remnants still attached are acting as debris...not "structure".
We should take a rain check on this. It has little if any direct relevance to the thread topic. BUT B&V's CD/CU is 1D and homogenous. Their goal was a generic "Bazant's Method" for progressive collapse that was NOT 1D homogenous. Stated alternatively - a model that would apply to WTC style tube frame constructions. And that is MY interpretation - I doubt that either B or V ever saw it or expressed it so simply. >> A derail for another tine/place.During Progression, ROOSD is driven by the debris layer at least as much as by the weight of the remaining upper part - and the debris layer has a large velocity relative to the lower part, but a low velocity relative to the upper part; and that is why in the process, the Crush Down goes at a higher rate than Crush Up.
And that is why B & V were NOT entirely wrong, not even in the case of WTC. Reality sure is messier than their simple and idealized (essentially 1D and homogenous) model, but it points in the direction of something that surely played a role.