Dave Rogers
Bandaged ice that stampedes inexpensively through
I hope you don't mind someone else addressing this one first, but the answers to these questions seem fairly straightforward.
The answer to (1) is that something like it must have. We know that the collapse started at a high level, and that the material in the floors below collapse initiation was fragmented. Therefore, whatever the mechanism of collapse, there was a continual accumulation of material as progressively lower floors were added to the falling mass. This isn't the same as what would be normally referred to as "snowballing", because that would imply adhesion of material to the falling mass; however, that's not necessary for the material to accumulate in this instance because it's all moving in the same direction due to gravity.
There are two completely separate things that you might mean by 'the "pancake" explanation', so it would help if you could clarify which you mean. Do you mean:
(a) The pancake initiation theory proposed by FEMA and rejected by NIST, in which the suggested cause of collapse initiation was separation of floor trusses from the support columns, resulting in vertical progression of failure as each floor fell on the one below it and detached it, ultimately removing so much lateral support from the columns that they could no longer resist buckling and so collapsed;
or
(b) Pancake collapse, in which, once the collapse had been initiated, the falling mass progressively destroyed each lower floor as it fell?
(a) was, as we know, rejected by NIST as a possible cause based on the observation of inward bowing of perimeter columns, indicating that the trusses must still have been connected in order to exert a lateral tensile force on the columns. (b) is, however, undisputed as a feature of the collapse following initiation, and indeed is an inevitable feature of top-down collapse, more or less by definition.
If you mean (a), then the answer is yes, it's completely unrelated. If (b), then no, it's more or less the same thing.
Very simply, every time the collapse progressed downwards by the height of another floor, it added the material in that height to the falling mass. If the amount of debris ejected in that interval was less than the amount of material added, then there was an accumulation. It's extremely difficult to estimate how much debris was ejected, but note that much of it was in dust clouds, which can be extremely diffuse and yet appear very large.
Another point to note is that collapse propagation doesn't, strictly speaking, require much accumulation of mass. Once the first impact has taken place, if collapse continues, then the kinetic energy available in the second will be greater even if no mass has been added, because the falling block has accelerated. In order for collapse to be arrested at a lower level, the falling mass would have to become smaller, at a faster rate than its downward velocity increased.
Dave
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