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Pooh, thanks for your reply.
As best I remember, I got Nikko to do the momentum calculations with his spreadsheet model; it’s probably around comment page 100. But Nikko had started wrong, by letting the topmost floor assembly fall onto the one beneath, and he got a collapse that stopped. In fact the damaged zone was over ten floors down. I pointed this out and Nikko corrected his model such that the top ten floors dropped, and he got a collapse that accelerated.
But Nikko also had an arbitrary ejection of mass each time the falling material entrained another floor. I think he chose 5% or 10%, which is way too much, see this comment above. It took me a while to persuade him, but he eventually reduced the ejection to nothing, and thereby got the limiting case. It was well within the collapse as estimated from the video record, and accorded quite well with my manual guestimate, which might be around page 93 or 95 – basically a falling mass of ten units hitting one stationary unit slows the collapse by only one eleventh, and thus a twelfth at the next floor down, then a thirteenth, and so on.
The spare energy calculations come later.