Conceptually, at least, a single low energy event can cascade through a stable structure and destroy it, if that event is enough to destabilize a part of the structure and cause another structural element to be then stressed and fail and then another one to be overstressed and fail and so on.
Yes, but it is usually unlikely to happen naturally in a way that goes through the path of most resistance.
The avalanche/domino examples are not really going through that path, the collapse progression just happens to go through the path of least resistance for each element. The dominoes, after all, prefer falling sideways. The fact that the collapse is going sideways is to their liking.
Tall buildings, on the other hand, are expressly designed to NOT like falling down.
ROOSD shows one way that that can be made to change, but the impossible condition required to initiate it is the key to understanding what it means.
In the case of a structure which is storing gravitational energy such as a building a progression of failures through the structure driven by the release of gravitational energy is possible if the safety factors do not permit the structure to remain standing with some of its structure gone.
That's what happens in those verinage examples. But again, these events are rigged to weaken the crucial elements beforehand. I think everybody is satisfied that that was not the case for WTC.
The underlying structure still had enormous redundancy.
The destruction or the twin towers was enabled by the weakest link being over loaded and this weakest link happens to occur on the connection of the floor system to the columns which support them.
I would contend the opposite in this case, but it is something that is very amenable to testing.
How these floor elements behave in situ under collapse conditions is a big issue.
Even so, if it is the wall connections failing you still have to explain the shattering of the concrete and exterior columns: most did not come off as large chunks but were dismembered element by element. The high energy ejections argue against this interpretation.
There was no need to beef up the floor connections lower in the towers only beef up the columns which carried the cumulative loads of everything above bearing down on them.
That is an excellent point, but it has problems in explaining the destruction of the facade and the crushing of the concrete.
If the wall connections were strong enough while these two things happened, why then did they suddenly fail afterwards. Why if they provide the resistance to the pancaking sufficient in size to allow the concrete to be crushed and perimeter to be shattered do we not see a judderry-stuttering collapse?
If the concrete was so light-weight as to become powder at the merest provocation, how is it supplying the mass needed to overcome the wall connections, surely the steel could not be made to behave coherently so quickly.
The truth movement sees the blasts occurring all around the facade as the collapse begins and explosions all around the building, exploding the floors yet no panels are seen blown off only ejected through the windows.
I would have to disagree with you there.
Personally it doesn't matter to me where any explosives were placed if they were placed, but I believe Achimspok would want to have a word with you about that particular assertion.
If enough columns were taken out by the plane strike perhaps the others would be overloaded and begin buckle and then fail and cause the floors to crash and being a progressive collapse.
I think that all parties agree that could not have been the case. Anyway, the initial tilt before the failure suggests a fulcrum at the far perimeter.
The 3 floors they supported would have no support on the facade side and be cantilevered from the core. Their core side connections would fail and they would drop. Three floors would drop down to the one below. But this would simply strip the floors from the core and the core would remain standing as the floors collapsed down and the facade either fell away or remained as a hollowed tube.
Much of the core did it fact remain standing for a while in one of the towers.
But this only really makes sense if you take a 1-dimensional slice. In 2-dimensions you can see the thing becomes asymmetrical very quickly. Once one side of the building fails the material will aggregate towards that side.
The symmetry-asymmetry problem is one of the biggest problems in my mind, because it forces you to accept Bazant's solid block to get the collapse to go all the way to the bottom all around the core.
The facade which was supporting the hat truss is not being hung from it and the hat truss is acting like a boom crane boom and the facades the counter weight and the object lifted with the core being the central truss tower of the crane.
That is why NIST stipulates core failure as the initiation event, Femr can tell us if that matches observation.
Again, the problem is going to be accommodating the tilt into this picture though.
Something managed to get the columns up there at the crash zone out of alignment and plunging downward. What was it?
That is the problem, they were never out of alignment as the ROOSD scenario requires. The tilt means that even though that scenario is the most likely with the step-out initiation, the step-out initiation is completely irreconcilable with observation.
To the extent that ROOSD tells you what would be required for a natural collapse to have occurred it also reductio ad absurdum
tells you that it could not have occurred.
To be fair ROOSD doesn't claim to be the only possible natural collapse scenario, but I have yet to see anyone propose anything better.
I am of the opinion though that, ironically, it is Bazant himself who manages to rule out any natural collapse scenario positively.