Morandi bridge a week on: the key thing, I believe, is a lack of structural redundancy. For the bridge to collapse as it did, something needed to give way – and it was unlikely to be a member in compression, so that rules the struts out. We know the tower remained standing after the roadway had gone, and then first one side of the giant pylon twisted and crashed, and then the next side did the same – in the opposite direction. That implies torsion – and where else is it going to get torsion, except from one of the giant cable-stays collapsing before the other, and twisting the tall thin pylons around their axis. They weren’t made to twist – they didn’t stand a chance.
So I’ve put together a small animation – something sets the whole catastrophe in motion – and the consensus so far seems to be that it is likely to be the corroded steel and concrete stays. Apparently the concrete used to encase the steel was only poured at 10 MpA – predictably therefore, the concrete was crumbling and the stays were suffering from rust and perhaps other damage. Perhaps they were hit by lightening. Perhaps a truck went out of control in the wet and spun out and hit the cable, but whatever caused the crack, one of those mighty cable stays was severed – and then instantly the entire structure went from a situation dynamically counter-balanced at rest, and turned it into a horrible twisting, flailing mess in which the rest of the structure could not survive. There was no structural redundancy once one cable had gone….
The central “drop-in” span would have disappeared quite quickly as the main deck started to subside, dropping out and down onto the tracks 45m below. The remaining load then transfers from the deck onto the one remaining cable stay – and there’s no way that single cable could hold up the bridge one-handed, so to speak. But it tries – so it resists the falling deck, and pulls hard against gravity – and so induces fierce torsion into the tall inverted V pylon, causing cracking and splintering. Now the other side goes too, and the central heavily strutted structure caves into itself centrally. It rips off the other side – paradoxically, the rapid separation leaves a stable structure on either side. The pylon collapses and just the two side spans now remain standing.
Something like that anyway.
The authorities will likely conclude something like this in a few months or a year or two, but it seems fairly obvious on the sequence of events – just not the crucial question “why?”. Morandi used this same design of bridge in a number of other bridges too, with the giant pair of cable stays – one in Venezuela (General Rafael Urdaneta Bridge, which partially collapsed in 1964), one in Libya (the huge Wadi el Kuf bridge, now closed due to fears of collapse?) and others in Canada, Ecuador, Columbia, and of course, several more in Italy. He was a great bridge builder of some structurally proud exciting bridges, but after this calamity, sadly, his name will probably always be associated with disaster.