Austin Williams reviews what information is currently available on the collapse of one of the towers of the Morandi Bridge (Ponte Morandi) on Tuesday:
The Morandi Bridge in Genoa shortly after the collapse of one of the towers, viewed from Coronata.
Photo by Salvatore Fabbrizio via Wikimedia Commons
The appalling tragedy of the Morandi Bridge, which collapsed on Tuesday, 14 August 2018, is a disaster that is still under investigation. At the moment, there are reports of 40 lives lost, but not much is known about the causes or the wider consequences. I am loath to speculate about either, but there needs to be a wider conversation and not simply kneejerk demands for corporate manslaughter charges against the private bridge-operating company, Autostrade per l’Italia. Heads will roll – deservedly so – but we must look at wider concerns about infrastructure, per se. (I caveat all of this by saying that pre-empting the ongoing investigation may reveal me to be wide of the mark.)
Firstly, the collapse was extraordinary. The bridge – built in the mid-1960s and named after the civil engineer Riccardo Morandi – is architecturally unique, and with this comes unique stresses that may have given rise to the catastrophic failure. A similar collapse of a similar nature and magnitude has not occurred before with a cable-stayed bridge. This bridge had 90m-high concrete towers that held tensioned cables fixed at an angle to the deck (the roadway), meaning that as the cables were being stretched by the loads, the towers were being squashed by the same loads. In engineering terms, the weight of the deck (and the traffic on it) was carried by the combined structure, meaning that the towers were under tremendous compression while the cables were under tension.
Bridge design always makes allowances – contingencies – for localised failure, and therefore even the most efficient, slender structures tend to be over-designed so that the engineers can sleep at night. The bridge has to imagine, for example, wind loads, snow loads, or 250 lorries being on the bridge rather than just 250 cars. But have the owners and managers of this bridge kept up with the reality, that in the 60 years or so since this bridge was constructed, loading conditions have significantly increased owing to increased freight volumes and vehicle sizes? Has this unseen attritional damage finally come home to roost?
In this bridge design, there is only one angled ‘stay’ (made up of four steel cables) encased in a concrete covering. The idea was that the concrete would protect the steel from weathering, corrosion and failure. Maintenance work as recently as 15 or so years ago replaced some of the cable stays and recoated them in concrete. Suggestions that cracks in the concrete exacerbated the ingress of water, which speeded the corrosion of the cables, are possible, but not totally convincing.
Were the cables to snap – possibly requiring more than one to snap due to the engineering contingency discussed above – then the deck would fall and possibly overbalance the entire structure. However, the fact that the towers collapsed in such a horrific fashion – seeming to crumble – may hint that the problem may lie in the foundations, where regular maintenance work was being carried out. We just don’t yet know. As Professor Gordon Masterton, from the University of Edinburgh’s school of engineering, told the Guardian, there needs to be a ‘forensic and thorough’ investigation to get to the root cause.
