Reply #112: You want sources to refute a water tester?

[William Seger]

And why would you think that I'd be afraid to put the American Institute of Steel Construction (AISC) up against your water tester? Their Manual of Steel Construction is used as the basis for most of the steel building codes in the country. You can Google for "AISC column safety factor" and verify that they recommend factors of safety ranging from 1.67 up to 1.92, depending on the column's slenderness ratio and restraint conditions:

http://www.google.com/search?hl=en&lr=&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=8Lr&q=aisc+column+safety+factor&btnG=Search

In a preliminary report, NIST reported that the original design criteria included a factor of safety of 1.67 for the core columns. But after NIST did their computer model, we no longer have to guess about that: That model effectively reverse-engineered the "as built" core columns. They found that the actual factors of safety for individual columns ranged from about 1.6 up to about 2.8. (The numerical average would be 2.1 if we gave each column equal weight, but in fact it was generally the smaller columns that had the larger numbers -- probably because of rounding up to standard steel sizes -- so the total effective factor of safety for the entire core was definitely less than 2.) The results are in NIST NCSTAR 6D on page 232 (but note that their diagram shows "demand-to-capacity" ratios, which is the inverse of the factor of safety -- divide those numbers into 1 to get the FoS):

http://wtc.nist.gov/NISTNCSTAR1-6D.pdf

So, yes indeed, I'll take AISC Standards and computer analysis of the actual columns over your water tester's hyperbole multiplied by your misremembered or misunderstood lecture added to your misunderstanding Eagar's comment to mean that the core was designed to carry the whole tower -- any day.

> > The core was not designed to carry any wind loading at all.

> The core was an essential part of the system. Without the core, the
hat truss could not do its job.

The wind design defeats your scenario. Buckled perimeter columns (assuming
there were some) can not carry compressive loads, you're right. But the
wind design provides for the forces to be taken by the tensile restraint,
mediated by the hat trusses, of the perimeter columns on the other side.

Sheesh, you're getting as bad as dailykoff. No, that's total gibberish; the hat trusses had nothing to do with the wind loading; they were for redistributing vertical gravity loads. The floor diaphragms and the spandrel plates made the perimeter walls a rigid tube, which was all that was necessary to redistribute lateral wind loads.

And what do you mean by "buckled perimeter columns (assuming there were some)." After all the direct, unambiguous proof you've been shown in the form of videos and pictures, I suppose I should be glad you're now willing to "assume there were some," but still... that's simply annoying as hell.

> Those perimeter columns were rated at 2000% of the live load, which was equal
to the dead load. That's 20X. Again, this comes from Ryan.

Well then, again, it appears that Ryan is full of shit. Perhaps, like you, he got carried away with multiplying exaggerations by his own misunderstandings. You're changing the subject away from the core, but because the perimeter columns were also used to handle the wind loads, they had about 5x (500%) the capacity they would have needed to carry just the gravity loads alone, which is likely where your original 5x number came from before you misapplied it to the core columns. But that number is irrelevant since we know from the videos that perimeter columns buckled inward at the beginning of the collapse -- hard to do with explosives! -- and we also know that as the collapse progressed, the perimeter column sections were literally ripped away from the building well before they could reach their compressive limits. Which takes us right back to an important principle that I can assure you (from working with them for 5 years) that structural engineers understand very well, whether or not certain water testers and other CTers do: ALL of the theoretical load-carrying capacities of structural members assume that structural integrity is maintained. Destroy the structural integrity, and the maximum limits under ideal conditions are totally meaningless. And that, my friend, is the real story of the towers' complete collapse in a nutshell, and that's why structural engineers aren't impressed with CTer's hand-waving pseudo-analysis.