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Old 12th April 2012, 06:05 AM   #4041
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Originally Posted by Christopher7 View Post
This is utter claptrap. If you insist that every data point is accurate then WTC 7 was moving up and down according to femr2's graph.
Not WTC7, but the analyzed corner probably was moving as the internal collapse progressed. And the points are somewhat accurate within a margin of error. There is some noise to consider. The estimation of the error due to noise is one of the main points where femr2 failed, though.

ETA: But don't miss the fact that the graphs (for clarity, I mean the ones below) show acceleration, not velocity. If acceleration goes above zero, as it does, it just means that the fall velocity decreases, not that the building moves up.

Originally Posted by Christopher7 View Post
The 0.1% error is the line representing the average, not the individual data points. It is negligible - too small to be considered.
I'm not considering only the average. I'm considering the whole acceleration profile, something that NIST only did very roughly.

Originally Posted by Christopher7 View Post
It is possible that the core could momentarily pull the north face down at slightly more than FFA at the very beginning of the FFA but the data does not reflect that.

Once the core and north wall have equalized their common FFA, neither can speed up or slow down the other because there are no stresses within the system. The entire upper part of the building [minus the part that had already collapsed] was falling at FFA.
It's interesting that you admit that, even if you get some details wrong.

The whole block did NOT need to fall at g for over-g to be reached by the fašade at a couple of points. Remember that the core collapsed progressively, so one part was falling in advance to another part and could have pulled the fašade earlier, then the rest might have pulled it again. That's consistent with the observations.

I think that you will at least understand that if the core is pulling the perimeter down, then the extra load induced by that pull would overload the bottom columns and therefore cause them to buckle all at the same time. That explains what we saw. You have not explained it satisfactorily at any moment.

There was a period of over-g acceleration. That is a fact you can't deny. No matter what data you use, the over-g is there, and as you note, it's there at the beginning of the fall.

Here's tfk's graph again:

Here's NIST's as annotated by tfk (it shows velocity rather than acceleration but it illustrates over-g too):

Here is one of the smoothed graphs of the acceleration that femr2 got:

Here's another:

They all use data extracted from the evidence (the video), just with different obtention techniques and different smoothing (and axes scaling and labeling).

Originally Posted by Christopher7 View Post
There you go again comparing a heavy steel H beam and moment frame to a stick. You have no knowledge of steel structures and no reference to back up that absurd statement. Look at the NIST model! The frame is NOT snapping like a stick, it is folding and bending but hanging together.


Again you are talking thru your hat. You have no idea how much resistance buckling columns will provide. Provide a source or stop making that claim.
I happen to have a reference.
The high-strength steel has a much lower ductility, which must have caused fractures with a drop of axial force to zero very early during buckling, [...]
Ba×ant, Zdeněk P.; Le, Jia-Liang; Greening, Frank R.; Benson, David B. (October 2008). What Did and Did Not Cause Collapse of WTC Twin Towers in New York? Journal of Engineering Mechanics 134 (10) p. 896.

Bolding mine. Change "zero" to "negligible" for a more accurate statement though (Again, remember the crane...)

Last edited by pgimeno; 12th April 2012 at 06:12 AM.
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