The Great Thermite Debate...

[eomer]

121. My answer about intervening to change assumptions, and other similar things.

The Investigation Team then defined three cases for each building by combining the middle, less severe, and more severe values of the influential variables. Upon a preliminary examination of the middle cases, it became clear that the towers would likely remain standing. The less severe cases were discarded after the aircraft impact results were compared to observed events. The middle cases (which became Case A for WTC 1 and Case C for WTC 2) were discarded after the structural response analysis of major subsystems were compared to observed events. The more severe case (which became Case B for WTC 1 and Case D for WTC 2) was used for the global analysis of each tower.

Complete sets of simulations were then performed for Cases B and D. To the extent that the simulations deviated from the photographic evidence or eyewitness reports, the investigators adjusted the input, but only within the range of physical reality. Thus, for instance, the observed window breakage was an input to the fire simulations and the pulling forces on the perimeter columns by the sagging floors were adjusted within the range of values derived from the subsystem computations.

The results were a simulation of the structural deterioration of each tower from the time of aircraft impact to the time at which the building became unstable, i.e., was poised for collapse. Cases B and D accomplished this in a manner that was consistent with the principal observables and the governing physics.

There are a couple of points about this account that I want to discuss. The first is the one I said I would come back with, which is my statement that they intervened in the middle of the model and set assumptions as needed to make it match the observations. The second paragraph above is their statement that they did that. So they did not just use simplifying assumptions. They customized key assumptions, apparently down at the level of individual structures, to be whatever they needed them to be in order to make the model resemble the observations. So the model resembling the observations does not validate the model. In fact the opposite is true. The model did not resemble the observations just based on simplifying assumptions and physics. It had to be customized down at the level of individual components to make it resemble the observations, which says that the original (un-rigged) model failed to validate against observations.

Another point is their use of the phrase "within the range of physical reality". This is putting it backwards and I think putting it backwards on purpose in order to mislead. Contrary to their claim, there is no range in the physical reality of this event. The range is in the uncertainty of their models and analyses. And since the range of uncertainty spans from no to yes on the question of collapse (according to them), what their whole exercise demonstrates is the opposite of what they imply. They imply that they've demonstrated that the towers would collapse from the effects of impact and fires. In fact what they've demonstrated is that they don't know whether the towers would collapse from just those effects.

A third point is about the construction and selection of the most severe case. Apparently to construct this case they took the most severe results of each of the subsystems, combined them together, and this forms the input for the most severe case of the global analysis. Thinking of this in terms of probability, it seems they chose an answer somewhere in the tail of a distribution for each assumption and they repeated this over significant number of different assumptions. If each of the individual assumptions was on the edge of plausibility (most severe) then combining them this way would normally push the combination beyond plausibility, again from a probability point of view. In their account above you can see one revealing instance of this: they say that they discarded the less severe cases of all the subsystems because the aircraft impact results didn't match observations. How does the result for the aircraft impact subsystem justify discarding the less severe case for their heat transfer subsystem, for example, and for all the other subsystems?

So they discarded the less severe and middle cases. They also discarded all the possible combinations of the severe cases for some subsystems with the less severe or middle cases for other subsystems. Only when they combined all the marginally plausible cases for all the subsystems could they get the result they wanted.

These things taken together show that they had to push their analyses out toward (and beyond in my opinion) the limits of plausibility. And, even more than that, even if someone believes that they didn't go beyond the limits of plausibility, there is still the unavoidable fact that at best they didn't demonstrate that the towers would have collapsed; at best they demonstrated that they don't know.


Edit: remove an inadvertent smilie.