As I understand it, Dave's hypothesis starts with the upper section falling at FFA for 12 feet.
negligible: too small to be considered
This does not happen instantaneously. FFA requires instantaneous failure of the remaining ~120 exterior columns and 38 core columns.
Good points!
I was going to bring up the freefall problem with Dave Thomas' analysis myself but wanted to be sure he admitted that is what he assumed first. Which he does above where he mentions that he thinks he is being misunderstood.
Dave should know that some of us certainly do understand that his analysis assumes freefall punctuated by brief collisions and that he is trying to show how that could mimic an average continuous acceleration of 2/3rds g. The issue is that his analysis cannot possibly be legitimately describing the collapses of WTC 1 for several reasons:
1. Collapsing/buckling columns still provide a fair amount of resistance. If anyone doubts this they can look at figures 5d and e in Dr. Bazant's paper, where he shows a classic graph of resistance during buckling
http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/405.pdf. The resistance during the buckling never falls below 25% of the original intact column resistance. In the case of WTC 1, where the columns were at least 3 times stronger than needed to support the static load, that means the resistance is almost capable of supporting the static load. In other words, the minimum resistance is 25% of a strength which could support 3 times the static load, or 75% of the minimum support needed for the static load. The fall through this resistance can only occur at about 0.25 g, so the minimum resistance was not negligible and does not support anything close to near freefall acceleration as Dave is claiming.
2. The velocity drops which Dave assumes in his calculations are far too small to provide the energy needed to continue collapse. Dave seems to think I am violating the conservation of momentum here, but in reality his argument is circular. To determine the velocity loss Dave is taking the kinetic energy difference between freefall acceleration and 2/3rds g. If there wasn't much energy loss required by the upper section for his calculations to determine the velocity drop that doesn't negate the reality of what the columns required to cause their failure. It actually points to other problems and shows he is not considering the full picture.
3. The Verinage demolitions show what a natural collapse should look like in terms of the deceleration of the upper section at impact, since a load amplification is necessary for the insufficient static load mass of the upper section to overcome the reserve strength below. The same measurement techniques used for WTC 1's descent, where they show no deceleration, show significant deceleration of the roofline in every single Verinage demolition. This proves that the measurement technique and frame rate are not the problem and that the effects of any impulse capable of causing continuing collapse are observable. There is obviously no deceleration in the fall of the upper section of WTC 1 and it proves there was no dynamic load.
