You have an excellent grasp of engineering reality, and space-time mechanics. Your questions make this obvious.
The answer to your question is not so simple.
It probably won’t hurt to read my latest three or four posts on the live thread, but I’ll try to sum up here.
Structural failures can progress rapidly, if design loads are exceeded rapidly and by orders of magnitude. On the other hand, structural failures can proceed slowly if the loads imposed are only at or near design limits.
The trusses used in this bridge were designed with both cantilever and simple truss load distribution mechanisms. Simply put, the trusses acted as balanced cantilevers near the mainspan piers, and as simple trusses near midspan on the approaches and center span.
When the mainspan failed, the counter-balancing weight for the northern cantilever assemblies was removed. Had this been a full and true cantilever design, the northern sidespan would probably have failed much more quickly.
But because it was not a full and true cantilever design, the unbalancing forces were comparatively small compared to the case of a true cantilever.
With the counterbalancing weight of the mainspan removed, the sidespan went into (terminal, but we didn’t know that for sure then) instability. Because the counterbalance weight was not the only thing holding up the sidespan, the forces were not in excess of design limits by orders of magnitude. Design limits were exceeded, but only by relatively small increments.
Instead of catastrophic failure, the structure began an accelerating series of load redistributions. In essence, the simple truss portion of the design scheme began trying to take the additional loads imposed by the loss of the counterweight.
Because the simple truss mechanism was still structurally viable, even with the counterweight removed, it was able to successfully redistribute greater than design loads for an appreciable time after the mainspan failure sequence terminated.
Individual truss member components, instead of tearing, shearing off, or buckling instantly, slowly elongated (tension members) or distorted (compression members) and a whole series of load re-distributions took place, back and forth, and members changed shape and began to shed loads they could no longer accomodate.
Eventually point loads accumulated on individual members that they could not, by design (and in implementation during the throes of failure), shed, or bear any longer and these individual members then underwent catastrophic failure.
Due to the non-redundant design of the bridge, it probably didn’t take many single point failures before big changes began to take place, and at this point, full failure was in progress. That would be when the video would begin to show visible results.
An important part of the failure sequence is missing from the video. There is no audio signal to go with it.
Between mainspan failure termination and the onset of catastrophic sidespan collapse, I’m reasonably confident that any human observers witnessing the event were fully aware of what was taking place. That steel would have been emitting a cacaphony of noise, screeches and groans, popping sounds, and in the late stages, the reports of tearing steel. The structure was dying and in spite of its best efforts to redistribute loads to survive, the loss of the counterbalancing weight of the mainspan proved to be a mortal injury.
We can’t hear that on the silent video, but anyone there would have. It’s probably one of the saddest (and most terrifying) sounds they ever heard.
Has any thought been given to the possibility of a train derailment at that point, with that impact causing the bridge collapse?
Marked. Good comments, as with the others here.