No detailed technical investigation of the Polcevera Viaduct – also known as the Morandi bridge – can begin until the search and recovery operation is complete. Investigators will be hunting for clues to how such a vital structure on a major motorway could collapse so suddenly, sending at least 39 road users tumbling to their deaths 45m below. Already a number of theories are circulating – but until a thorough investigation is completed, it is only possible to speculate.
Investigators are likely to focus first on the collapsed tower’s foundations. Reports are still confused, but it appears that some form of strengthening works had either been done recently or was still in progress. This might have left the foundations unprotected against fast flowing flood water, which could have eroded the ground below the foundations. Should this be ruled out, attention will surely turn towards the steel cables that stitch the bridge deck together and those massive cables that swoop down from the top of the towers, the cable stays.
This is an unusual bridge, one of the first wave of cable-stayed crossings, a new type that would eventually replace the suspension bridges traditionally used for long spans. In the 1960s its designers were really pushing the envelope. Working without any assistance from powerful computers, and designing for much lower traffic levels, they created an elegant structure that would have been difficult to construct and a nightmare to maintain.
Nobody would design a bridge this way today. Modern cable stay bridges use fans of multiple cables, as on the Second Severn Crossing. These are easier to build and much simpler to maintain. If one of the stay cables has problems, it is possible to remove and replace it with the minimum of fuss.
Not so on the Morandi Bridge. With just a pair of cable stays running down each side of the tower the bridge has no redundancy. Should just one of these cables fail, total collapse would be inevitable. The hazy video of the moment of failure appears to show the tower collapsing in two phases, which might suggest that one cable stay did fail, causing an out of balance loading on the tower which led to progressive collapse.
Failure could occur at the top or bottom anchorages, probably more likely than within the massive cable itself. The cable is made up of bundles of fine steel wires, apparently protected against corrosion by a concrete surround. This has proven to be an ineffective solution. Corrosion could not only be severe after many years – it would also be very hard to detect.
Giant hollow precast concrete “Lego” blocks are locked into place by tensioned internal steel cables to form the bridge deck. Protective techniques at the time usually involved sheathing the tendons in plastic ducting, which was then pumped full of a mixture of cement, sand and water. Again, this has proved to be a flawed solution.
Recently more than £100M had to be spent strengthening the Hammersmith Flyover in west London, which used the same deck construction technique. It was found that de-icing salts were making their way through leaking waterproofing and dripping onto the cable ducts below, causing serious corrosion of the tendons.
It is reported that the Morandi Bridge had undergone a number of strengthening procedures over the decades. No shame in that: many bridges of a similar age have had to be strengthened to cope with levels of traffic their designers never dreamed of. The Forth Road Bridge is a typical example. Maintenance is another matter.
Local politics might still determine exactly who carries the blame for this tragedy. At the very least the national government might launch a thorough investigation of major crossings on Italian motorways. The results might well be alarming.
A version of this article first appeared in the Telegraph, 16 August 2018. Like what you’ve read? To receive New Civil Engineer’s daily and weekly newsletters click here.