jomper said:
Why wouldn't you expect the building to collapse asymmetrically towards the most fire weakened or structurally damaged area as it begins to move down?
Because it conserves its motion. If one face fails first, then the tower will rock. This converts some downward motion into a rotational motion of the whole falling tower top. That rotational motion is conserved within every separate structural element, and thus will alter all the positions of, and all the force relationships of, all future impact events.
These impact events would not usually have ever occurred within the structure and would be individually spaced over time, allowing inertia forces to be transmitted from the surrounding structure to accumulate for long enough to damage the weaker of the two impactors sufficiently to allow the downward velocity vector of the descending tower top to increase. The proportion of G overridden being around 30%, this is a measure of the kinetic energy loss during collapse. More precisely it is proportional to the area between the graph of the actual fall and the graph of pure free fall.
The kinetic energy that was not overridden became the heat found at the wreckage base, less the energy lost in deforming that steel. That heat was so tightly insulated that even hydrogen didn't burn down there, but at the surface it did.
Can someone --
other than Jazzy -- attempt to explain what this means, please? I don't have difficulties with technical arguments as a rule but I'm finding this one a bit of a puzzler.
First we have this.
Because it conserves its motion. If one face fails first, then the tower will rock. This converts some downward motion into a rotational motion of the whole falling tower top.
No problem here -- it's really what I'm asking: why does the buckling of the exterior of the building (which of course occurs below the line that any video evidence shows and cannot be seen or be verified in any way) not cause some asymmetrical rotation or cause the roofline to deviate from the horizontal as it descends?
The most damaged face would be expected to move first -- a shift of 3% from the perpendicular in one corner at the moment of initiation does not explain the symmetry of the descent. Even in Mick's can experiment it is obvious that the buckling of the walls of the can -- as he stands on it and kinks it in the side, creating asymmetrical damage -- stops the top of the can from moving downwards with the kind of symmetry observed in the video evidence of the collapse of WTC7.
So we have
some downward motion [converted] into a rotational motion of the whole falling tower top.
Where does it go?
It seems
That rotational motion is conserved within every separate structural element, and thus will alter all the positions of, and all the force relationships of, all future impact events.
So the rotational movement is conserved within the structure. Of course, the structure at this point is hypothesised to be only a shell, so the rotational movement would not be conserved by the interior supports of the building, and while the shell may be moment resisting it has not been shown that this means it must descend symmetrically once it has been subject to asymmetrical damage.
In fact we would expect the rotational movement caused by collapse due to asymmetrical damage to do as Jazzy says, and
alter all the positions of, and all the force relationships of, all future impact events.
So this doesn't answer the question. And what are these impact events? They are in the past, aren't they? There are no further impact events once the internal structure has collapsed that might cause the exterior to start falling, with or without symmetry; if these "impact events" refer to fire and structural damage, they are certainly chaotic and asymmetrical.
Now comes the next part.
These impact events would not usually have ever occurred within the structure and would be individually spaced over time
Well, yes. We wouldn't expect impact events to occur within a structure in the usual course of events. Again, what are these impact events Jazzy is talking about? Anyway, even though they're not to be expected, they would normally be
individually spaced over time allowing inertia forces to be transmitted from the surrounding structure to accumulate for long enough to damage the weaker of the two impactors
So it's two "impactors", is it? It's really starting to look like whatever these impactors are should have been defined properly at the start if this is to be a cogent argument.
Anyhow, these impactors can be damaged, whatever that means, and one's weaker than the other, and they are subject to inertia forces -- which one assumes come from the rigidity of the structure, but that's merely an assumption from a context that is becoming increasingly opaque.
So anyway: the "weaker" of the two impactors -- which have not been differentiated, still less defined or explained -- are "damaged"
sufficiently to allow the downward velocity vector of the descending tower top to increase
which simply seems to mean that the building is falling at an increasing rate, but certainly seems to have no bearing on why it would fall symmetrically.
So then
The proportion of G overridden being around 30%, this is a measure of the kinetic energy loss during collapse.
we're talking about gravity, it would seem -- and although where this figure of 30% comes from is not explained it has apparently been measured by something somewhere.
Fortunately Jazzy defines what he means in the next sentence.
More precisely it is proportional to the area between the graph of the actual fall and the graph of pure free fall.
Has anyone mentioned a graph or a graph-based comparison on free-fall rates, still less posted actual graphs, on this thread? I don't think so. But since the question is about how the building fell so symmetrically, rather than why or how it fell at the speed it fell, how does a graph-based comparison of descent speeds help explain the issue?
Certainly Jazzy's final sentence
The kinetic energy that was not overridden became the heat found at the wreckage base, less the energy lost in deforming that steel. That heat was so tightly insulated that even hydrogen didn't burn down there, but at the surface it did.
...does absolutely nothing to answer the question of why the collapse had to be symmetrical.
As I said, I don't really think further elucidation from Jazzy is going to help here. He may know what he means but he's made his post and given his answer.
Does anyone else understand what Jazzy means and can they demonstrate their understanding?
Pete Tar: you "liked" this post when Jazzy originally made it, and I'm sure you wouldn't "like" something you don't understand. So perhaps you could make up for the nasty names you've called me on this thread by doing me the courtesy of re-phrasing this explanation to aid the general comprehension of all of us.
