Response to cbrer (Reply #27)
Mon Feb 20, 2012, 08:19 PM
William Seger (7,077 posts)
28. I meant "shown" as in "demonstrated conclusively"
Has Chandler demonstrated either in the paper or the video that the acceleration was really constant? And why does it matter?
> And again, I believe I have a rudimentary understanding of the issue.
Apparently not, judging by this:
> It seems that his evidence of constant acceleration means that there was nothing there to slow them down when they collided.
No, he claims there was a "constant" downward acceleration of 0.64% of gravity, which means there was something slowing it down, but he claims not as much as should be expected.
> Is a constant acceleration AKA "free fall"?
No, free fall on Earth is a constant 32 feet per second per second, whereas Chandler is claiming a "constant" acceleration a little over 20 feet per second per second for the collapse.
Imagine a frozen pond with a brick sitting on the surface. When frozen, the water easily supports the brick. Now imagine that the ice suddenly melted, allowing the brick to fall to the bottom of the pond. It would fall smoothly through the water at a constant acceleration, but it would be something less than the acceleration it would have if falling through a vacuum because it has to push water out of the way -- agreed?
Chandler's "constant acceleration" claim amounts to saying that the tower collapses showed something like that kind of behavior, and he claims that means some "extra energy" must have come from somewhere to reduce the load capacity of the structure to only 36% of the upper block's weight.
The aforementioned university professor who writes structural mechanics textbooks and peer-reviewed journal papers (Bazant, in case you were wondering) claims that in order to halt a collapse after it begins, the structure below must somehow absorb the gravitational energy released in that initial fall, and he provides calculations to show that even with conservative estimates, there was much more energy released than the structure could absorb.
Right from the outset, the professor's claim sounds reasonable to me, while Chandler's sounds very dubious -- he appears to be ignoring any dynamic behavior -- but is there any empirical way to tell who is right? Well, Chandler would seem to predict that a progressive vertical failure is impossible without something weakening the structure, but the video is a demonstration of a European demolition technique called Verinage. It uses a system of cables and jacks to cause pairs of load-bearing walls to collapse, simply dropping the top of a building onto the bottom and letting gravity do the rest, with no "extra energy" (e.g. explosives) required. It's impossible not to notice the similarity to the tower collapses.
A Verinage demolition doesn't fall with a constant acceleration, however. It's an initial free fall followed by a collision in which velocity is lost but the impulse force causes a structural failure at that level, so the accumulating mass falls freely again to a collision with the next floor. Very careful measurements would show that the acceleration of the collapse varies because of that alternating sequence of collision jolts and free falls. But since the velocity is always increasing, if you only took position measurements with low resolution or at widely spaced points (or both), you wouldn't see those variations: It would look like a fairly constant acceleration and furthermore it would be in the ballpark of 0.64% g, more or less.
That's why it's critical that Chandler doesn't really demonstrate that the acceleration was constant. All he does is plot some points on a graph, draw a line through them, and then claim that the straight line means a constant acceleration. No, it does not; not if you don't know what's happening between the points. Furthermore, for that matter, Chandler doesn't justify that any impulse jolts could be measured at the roof line, even with better measurements, given that the tower collapses involved a tilted fall rather than the straight-down fall of a Verinage demolition. (That tilt would mean that collisions were happening in different places at different times, so instead of a single massive jolt that could easily be observed at one point in time, there would be much smaller jolts distributed in time.) In a previous post, I described a couple of failure modes that would result in local failure followed by local free fall, and I noted that what we really see is the average result of thousands of such events. I claim that what we see in the towers is simply that the average acceleration was 0.64% of gravity, although it was not really constant.
But that isn't the only problem with Chandler's hypothesis: He wants to conclude that his analysis means there must have been a controlled demolition but he fails to describe any demolition technique involving explosives that would produce a constant, like-a-brick-through-water smooth acceleration of 0.64% g. I don't believe that's possible, but feel free to give it a try. So not only does Chandler assume constant acceleration without justification, he also assumes an unjustified reason for it.
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I meant "shown" as in "demonstrated conclusively"
|William Seger||Feb 2012||#28|
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