Tony Szamboti said:
The deceleration of the upper section is indicative of the load it is applying.
This sounds stupid.
Shouldn't the deceleration depend on the resistance provided by the lower part?
Seymour,
It ain't you.
The cornerstone to Tony's (and Heiwa's. and Chandler's) rap is to be intentionally confusing with his terminology. That ain't an accident. (And it is precisely what pisses me off about Tony. This amounts to willful "techno-bullying" by these bozos of the non-tech crowd. And I hate bullies.)
Just as soon as anyone is simple, clear & consistent with their terminology, all of the mystery evaporates. This is exactly why Tony will never be clear. Will not be consistent. And will not define his terms.
Tony constantly intermingles the concepts of average accelerations with instantaneous ones, average forces with instantaneous ones, time-average stresses with instantaneous stresses, and spatially average stresses with local stresses.
In each of these cases, the dynamic, short term, instantaneous values can be much, much greater than the time-averaged or spatially-averaged values. And in fracture mechanics, average values don't mean squat. Only the instantaneous & local values of stress determine whether or not something breaks.
In this particular case, the use of the word "deceleration" is deceptive. The correct use of the word "deceleration" means that a velocity is getting smaller.
In this particular case, he is talking about the downward velocity of the upper block.
Is Tony talking about "average velocity" or "instantaneous velocity"? He is never clear. But the answer is clear.
His data is so coarse (taking one position data every 150 msec), he can only speak about really gross "average" velocity.
And that velocity is not decreasing. From the moment the collapse begins, it only increases from zero, then levels out to a steady state (i.e., "terminal velocity"). At no time does this average velocity decrease. Ergo, there is NO deceleration of the upper block. There is only acceleration. (Acceleration in the downward direction. But this is NOT, by any definition of the term, "deceleration".)
For the short period of time (about 5 - 10 seconds, perhaps 30 - 50 stories or so) that the block is accelerating downwards, the average resisting force is less than the weight of the upper block. And BECAUSE the average resisting force is less than the weight, the block keeps accelerating. And it accelerates UNTIL the average resisting force equals the weight of the upper block.
Once this "force balance" situation has been reached (i.e., average resisting force = weight of the upper block), then the block accelerates no more, but maintains a constant downward velocity, i.e., "terminal velocity".
Notice that the block starts out with a low velocity and high downward acceleration. As the velocity increases, the average resisting forces increase, and the resultant downward acceleration of the block continuously decreases.
Once the block gets moving, if you want to slow the block down, then you must exert a force on it greater than its weight.
If you want to bring it to a halt in a short period of time (i.e., a short distance), then you've got to exert a force on it much greater than its weight. The shorter the distance in which you want to stop it, the higher the force that you've got to exert on it.
Once it gets moving, if you want to stop it in a distance that allows the stopping columns to survive (i.e., perhaps a couple of inches or so) and the descent to arrest, then the forces you've got to exert on the upper block are enormous. The columns of the lower block cannot generate those forces. And the columns of the upper block cannot withstand them either.
This is precisely what Bazant proved.
The reality is that there is no practical way to get those forces into the columns. And applying these levels of force to any other part of the structure (i.e., the wafer thin concrete floors or the delicate trusses) simply fractures them effortlessly.
And the collapse continues. Just like we saw.
Is this explanation (of the acceleration & forces) clear?
Tom