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Old 10th September 2006, 07:57 PM   #4236
R.Mackey
Philosopher
 
Join Date: Apr 2006
Posts: 7,854
Responding to Abby's Blog -- WTC Collapse

For those of you who are completely lost, some time ago I looked at Gordon Ross's paper, the one that claims after the upper block of a WTC tower fell, it would have actually been supported and carried aloft by the remaining structure, rather than leading to a progressive collapse. My original critique is found here.

Shortly afterward, Dr. Greening, who had previously written an excellent whitepaper demonstrating quite the opposite, took Ross to task, much better than I could have done. His response was published in the "journal" put out by Steven Jones's gang -- along with a rebuttal, by Ross. Always know the hazards of an uneven playing field. In any case, I also reviewed Ross's response, in this post.

Recently, a rather brash newcomer has arrived on scene, calling himself "TruthSeeker1234," accusing me (and NIST, and lots of other folks) of misdeeds in this thread, while providing nothing that resembles evidence of his own. He was directed to my critiques and promised to respond.

Apparently he has now done so, but not here -- posting instead in Abby's blog. The text is shown below in entirety.

Originally Posted by TruthSeeker1234, we think
I regard the first point of critique, that of one-dimensionality, as a little unfair, considering it does call for nothing less than a complete 3-dimensional FEA, which NIST, with its multimillion dollar budget, should have provided, not Ross. NIST instead decided not to inquire beyond what they deemed conditions of collapse initiation.

Momentum was examined in order to determine post-impact velocities and correlating kinetic energies. It's a necessary step, so I wonder why it's supposedly suspicious? The author rightfully says that plastic deformation demands could be calculated regardless, but fails to mention that the entire point was to calculate energy expended by accelerating the correlating mass in the first place. Afterwards, he suddenly brings up this very energy by complaining that Ross didn't regard it any further even though it should have deformed/twisted the support columns. Which is incorrect, as it didn't exceed elastic limits.

Next thing, he's complaining about the arbitrary safety factor of 4. Considering building codes often demand safety factors up to 6 and that the WTC, not exactly being a barn in Alabama, was probably engineered on the safer side, I fail to see the problem. He also claims that "there are more detailed descriptions of the WTC design available, so there's no need to guess."

Oh really? WTC blueprints remain secret up to this day. If the author can somehow access them, there would be several thousands of people happy to have a look.

His last paragraph of critique is simply outrageous! All of the balance factors Ross provides are justified:

Upper Stories Plastic & Elastic Strain: Of course the upper stories will consume some energy by deformation! Does the author somehow expect them to wait until all available KE was transferred into the lower stories? He then claims that the available KE must be doubled because the process traverses more height than one floor (when finally the deformations occur), but neglects to mention that this is accounted for under "Potential Energy Additional Downward Movement". Unbelievable...

So, in my humble opinion, the guy's been trying to poke holes into Ross' calculations, but succeeds only if you don't really pay attention. Please do, there's quite something at stake here.
I can only assume it was put there, instead of here, to delay my finding and responding to his complaints. However, I shall do so now.

Originally Posted by TruthSeeker1234, we think
I regard the first point of critique, that of one-dimensionality, as a little unfair, considering it does call for nothing less than a complete 3-dimensional FEA, which NIST, with its multimillion dollar budget, should have provided, not Ross. NIST instead decided not to inquire beyond what they deemed conditions of collapse initiation.
Wrong. My objection is that Ross, doing essentially a back-of-envelope calculation, should have accounted for a range of possible column bending energies. This takes no FEA whatsoever to compute. Ross assumed that each column would be pinned at top and bottom, i.e. one-dimensional strain, which leads to the highest possible column breaking energy. He should have also considered the case where the column top was more-or-less free to deflect, which leads to a lower breaking energy.

This consideration turns out to be closer to the truth. As the NIST FAQ recently released explains:
Originally Posted by NIST
Instead, the NIST investigation showed conclusively that the failure of the inwardly bowed perimeter columns initiated collapse and that the occurrence of this inward bowing required the sagging floors to remain connected to the columns and pull the columns inwards. Thus, the floors did not fail progressively to cause a pancaking phenomenon.
Inward bowing is consistent with the lower breaking energy I am referring to.

The person complaining, whomever he may be, is obviously not well-versed in this subject. Far from requiring a "complete 3-dimensional FEA," all that is required is to substitute a different energy value into a very simple equation. This energy value can be looked up, or computed with first-year strength of materials knowledge.

Let's look at the next complaint:
Originally Posted by TruthSeeker1234, we think
Momentum was examined in order to determine post-impact velocities and correlating kinetic energies. It's a necessary step, so I wonder why it's supposedly suspicious? The author rightfully says that plastic deformation demands could be calculated regardless, but fails to mention that the entire point was to calculate energy expended by accelerating the correlating mass in the first place. Afterwards, he suddenly brings up this very energy by complaining that Ross didn't regard it any further even though it should have deformed/twisted the support columns. Which is incorrect, as it didn't exceed elastic limits.
Again, the complainer misunderstands my comments. This is somewhat tricky to explain. Indeed, the full intricacy of the calculation escaped Gordon Ross.

Ross's key argument is as follows: When the upper block hits lower floors, the upper block and a portion of the lower block match speeds, as dictated by Conservation of Momentum. But it's not just the lower contact floor, it's several floors below that rebounding, carried along by linear strain in the columns. Once the velocities of all the floors involved are computed, you know the kinetic energy of each block, and whatever energy remains goes into deforming the structure. If this energy is less than the energy required to break the columns, then the columns don't break and the building stays up.

Ross's mistakes are these: First, the kinetic energy that floors below the contact floor receive is transmitted through the support columns. This is not a harmless energy sink. Ross would have tens of lower floors suddenly accelerated to several meters per second. Far from harmlessly dissipating the energy, it adds inward stress to the columns -- just like the NIST discussed above.

Second, Ross stops his calculation at this point -- with many floors moving downward at up to several meters per second. Buildings that stay up have their floors moving at ZERO meters per second. He never bothers to show how this new kinetic energy is dissipated.

That's why I find this suspicious.

In Greening's critique of Ross (see excerpt in my second critique, linked above), Greening is satisfied to castigate Ross for claiming that 24 floors rebounded absorbing energy, when the video evidence shows that only four reacted as Ross claims. Greening further cites the video as showing that this energy, far from dissipating harmlessly, causes visible damage lower in the tower, further weakening the structure. Greening thus supports my suspicions.

This is a subtle argument, clearly too subtle for the complainer with his dubious background. Nonetheless, he should have asked for clarification.

Let's tackle complaint number three:

Originally Posted by TruthSeeker1234, we think
Next thing, he's complaining about the arbitrary safety factor of 4. Considering building codes often demand safety factors up to 6 and that the WTC, not exactly being a barn in Alabama, was probably engineered on the safer side, I fail to see the problem. He also claims that "there are more detailed descriptions of the WTC design available, so there's no need to guess."
I was complaining about the arbitrariness of Ross's assumptions. Ross does not cite it. Neither does the complainer. This appears to be an argument to incredulity.

Dr. Greening also found Ross's assertion doubtful. As he said in his rebuttal, found in the "Journal" of 9/11 Studies:
Originally Posted by Dr. Greening
On pages 6 and 7 of Ross’ article we see an undefined “safety factor”, arbitrarily set at 4, used to calculate the elastic strain energy of the lower and upper storeys. In looking for any justification for the use of a safety factor of 4 for the WTC we read in Reference /6/: “The factor of safety is typically not greater than 2 in building structural designs.” (Note added July 19th, 2006: S. Sunder at a NIST Progress Report on the WTC Building Performance, presented Oct 19th, 2004, stated that the safety factor for the yielding and buckling of core columns is 1.67.)
I therefore bluntly reject this complaint about my analysis.

Let us next consider the fourth complaint:

Originally Posted by TruthSeeker1234, we think
His last paragraph of critique is simply outrageous! All of the balance factors Ross provides are justified:

Upper Stories Plastic & Elastic Strain: Of course the upper stories will consume some energy by deformation! Does the author somehow expect them to wait until all available KE was transferred into the lower stories? He then claims that the available KE must be doubled because the process traverses more height than one floor (when finally the deformations occur), but neglects to mention that this is accounted for under "Potential Energy Additional Downward Movement". Unbelievable...
Again, the complainer has missed my argument, though this one is simple. In Ross's energy balance equation, he has budgeted the gravitational collapse energy of a single floor. However, in the liability columns, he requires that two floors are destroyed -- the highest floor of the lower block, and the lowest floor of the upper block.

This is not bookkept under "Potential Energy Additional Downward Movement" as the complainer states. That item in Ross's paper refers to the gravitational energy gained when the lower floors compressed, not when a floor failed. I give credit to Ross for accounting for this item, since it is a subtle point, one that I might have missed. However, Ross's assumption that both floors would have to be destroyed before collapse could continue assumes that the upper and lower block would fail symmetrically, which is the absolute best case performance of the structure. I guess it's possible, but it is disingenuous to claim as a "conservative" estimate, as Ross does.

The complainer, alas, understands neither my critique, nor Ross's paper itself.

At last we have reached the finale:

Originally Posted by TruthSeeker1234, we think
So, in my humble opinion, the guy's been trying to poke holes into Ross' calculations, but succeeds only if you don't really pay attention. Please do, there's quite something at stake here.
This is notable because the complainer absolutely ignores both of my strongest criticisms of Ross. They are as follows, reprinted from my original analysis:
Originally Posted by Myself, in the JREF forum
Missed that Ross also double-counted the concrete-crunching energy. In fact, neither floor would have to be pulverized before collapse could initiate -- the floors are not holding up the structure. They can be crushed later.
Originally Posted by ibid
A final point that Ross has not addressed is that the floor that the upper stories fell upon was not in blueprint condition! It was immediately below the raging fire that collapsed the impact floor, suffered deformation from proximity to the impact floor, and was heated enough to weaken its yield strength. Again, even if we take Ross's numbers as correct, but add another floor's worth of gravitational energy, we still get collapse initiation.
In summary, I reject this complaint in its entirety:
  1. It was not posted where I would be likely to respond to it, despite claims that it would be
  2. The complainer demonstrates a thorough misunderstanding of structural mechanics
  3. The complainer demonstrates unfamiliarity with Ross's papers
  4. The complainer's appeal to incredulity regarding safety factor has been rebuffed, by experts, in a scholarly conference
  5. The complainer fails to address or even acknowledge my two strongest objections

Unless further debate is forthcoming from the complainer, I consider this matter closed.

Thank you all for reading. I apologize for its length. I confess a personal weakness for thorough discussion.
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