Reply #152: Energy and column buckling...

[Carefulplease]

What's your basis for saying the columns were disconnected?

If the idea is that the falling floors sheared away the connections, that is a process that would require energy -- and thus would slow the fall.

That the columns mostly failed at the splices in an observation that NIST makes. You can find this in this report:

http://wtc.nist.gov/NISTNCSTAR1-3.pdf

As for the energy required there seems to have been plenty. Bazant and Zhou, and also F. R. Greening have provided anayses that confirm this. The energy spend in destroying the structure below has an effect on the timing of the collapse that is much less significant than momentum transfer (the effects of the mere inertia of the floors below). These authors even assume that the energy goes into buckling intact and fully braced columns. If you want to question this you have to come up with some specific argument and calculation like Gordon Ross attempted to do.

Are you saying the columns were disconnected because that seems like the only possibility? But is it really a possibility? You say the columns "yielded" once the falling of the floors had removed their lateral support. That's a lot of assuming. As can be seen from construction photos, the columns were braced against one another. They did not depend on the floors for lateral support.

The only braces that there are are floor truss assemblies in (most of) the tenant areas, and beam frames in the core. All of these are parts of the floors. When the floors from the upper block hit and remove the floors from the lower section there are no bracing beams that can remain.

And how did they yield? You say they "fail at the splices." Wherever they failed, they would need a mechanism and a cause. Was it in buckling that they failed at the splices, for lack of lateral support? Remember that these are columns that were designed to support lots of floors. Now that the floors are gone along with, you say, their lateral support, their load is lightened vastly -- they have only themselves to support. So what would cause them to buckle? Further, if they did simply buckle, long pieces of them would remain visible in videos of the collapses. None did.

What causes them to buckle is the combination of both loss of lateral support and the top section (containing such things as 50 heavy elevator motors and the massive hat truss) coming down crashing on them. The top sections of WTC1 and WTC2 got progressively destroyed in the process and in the end tall vertical unbraced sections of both cores remained standing for a while (several seconds) until off-center deflection was sufficient to rupture the spices and/or buckle then near the base and bring them down under their own weight.

It looks like the only possibility left is that the columns failed first and it was the columns' failure that brought the buildings down. After all, it was the columns that had been holding the buildings up.

That is correct. The columns failed first in the impact and fire region. After collapse initiation floor failure would have taken the lead because the columns above mostly hit the floors below.