pgimeno wrote:I don't know if it was the same as this one:
It wasn't, but that one's nice too.
In any case, this one has two interesting features:
- An over-g mechanism similar to the most likely one in WTC7.
I thought I'd let this lie fallow for a while, in the hopes you wouldn't notice my response. I really have no interest in reviving the pissing contest we had some time back in another thread. This case is the same as the last... whatever mechanism for over-g was present in WTC7, if any
was present, this was probably not it.
It is established by direct observation that the period of high acceleration in the early descent of WTC7 was accompanied by insignificant global tilt. The mechanism of over-g in this video is the same as any possible over-g condition in the tipping brick wall and the tipping entertainment center we were discussing in the other thread. The over-g condition results from kinematic constraint and is apparent in only a portion of the total mass - that which is effectively leveraged, away from the pivot through the center of mass.
Since it's quite obvious that global tilt was not present in WTC7, the only means by which this type of mechanism could be responsible is if there is local tilt; that is, distortion in building geometry which results in planar sections undergoing the same sort of rotation about a pivot. All things considered, there is only one way for that to happen and that is rotation of the outer about the inner (i.e. toroidal) where the inner provides the pivot.
There are several problems with this, the most severe being the interior portion is not fixed relative to ground! It is instead presumed to be in collapse progression as well, with some lead time. A moment's reflection on this situation - that the alleged pivot about which the perimeter could swing down at over g - is not stopped but instead already descending with greater velocity, suggests an entirely different and mutually exclusive effect: the already descending interior is dragging the exterior portions down with it, even mildy 'slingshotting' it downward (with some elastic response coming from god knows where).
The latter mechanism is by far the most typically proposed mechanism for over g from those who either believe it did or can occur in the case of building seven. The mechanism you cite, however, is used to demonstrate over-g is possible in the general case. I don't think anyone else is proposing this mechanism is present in WTC7's collapse, that was the point I was trying to make to you when you posted a tipping wall video at JREF and I called you on it here. Subsequent to calling you on it, you made some pretense of how you weren't trying to say it applied to WTC7, just that you had some (invalid) point to make about one of Major_Tom's comments.
Well, here you are explicitly claiming it is the likely mechanism for over-g in B7, so I guess we can dispense with the pretense of that other thread. My point remains the same: no, this is not the likely mechanism of over-g, if there was over-g. I can get a very good third degree fit to about 90% of g over that period. With all due respect to femr2 (and much more is due than you've probably ever given), over-g is not proven. I think femr2 makes an excellent case, and I wouldn't fault anyone for going with what appears to be the most solid empirical evidence he's provided.
However, those who DO go with this evidence, and that's everyone from femr2 to his biggest detractor (tfk) cite instead drag-down from the interior as the most likely cause. I have to agree, it is the most likely cause if indeed there's any effect which needs explanation. But there are problems when examining it from the other perspective, namely first principles. Kinematically, the perimeter MUST draw in for over-g to occur from simple inelastic drag-down when the interior remains coupled. Additionally, the core-floor and perimeter-floor connections must not only survive rotation and ductile elongation well in excess of anything which seems reasonable, the connections and the floor assemblies must remain sufficiently intact to mediate tension to accomplish this pull down.
I'm not saying the above can't happen, WTF do I really know? I'm saying it's highly speculative and seems unlikely to me, given the existing literature on small angle failures of all types of connections, welds and fasteners. This is why I've suggested the notion of a debris train providing sustained impulse against the floor assemblies (near
the walls), which permits things to break apart as they are wont to do in a collapsing piece of shit like that building apparently was, yet accounts for the appearance of a continuous force applied over a substantial period of time. A series of discrete impulses which do result in disintegration but result in acceleration in total. The differential speed between the moving debris and the impulsively entrained perimeter would be the cause, and would still pretty much require negligible resistance below. No hard coupling like the drag down - more like the action near stall speed in a torque converter within an automatic transmission, or a clutch while partially engaged.
Or, one could take the less speculative route which is that sub-g acceleration for all descent is within error bands of measurement, so maybe there's no over-g to explain.