Chandler’s Downward Acceleration predicts that no structure can undergo top-down accelerating collapse.
When I realised that I knew there had to be something wrong with it; it must be possible to contrive a structure that can. Then I realised the problem; his model is one dimensional. The diagram is two dimensional, but only one dimension appears in the maths; the vertical. Nothing can go past anything else on the way down; the material above is forced to go through the material beneath by this limitation of the model.
Chandler’s model can be applied in three dimensions, but there must be no variation in the structure in the two vertical dimensions. It applies to a block, and indeed Chandler refers to the sections above and below the damaged zones as blocks, but they are not, they are mostly empty apart from air.
So I immediately wondered what would happen if the vertical frame members got displaced laterally ie. they got out of line, and that starting point eventually led me to understanding the way the collapses proceeded.