Stopping bridge collapses | The Economist

ON AUGUST 2nd a century-old bridge carrying the road from Mumbai to Goa over the Savitri river collapsed (see picture), killing at least 20 people.

The probable cause was that the river, swollen by monsoon rains, had scoured away the foundations of the bridge’s piers. Such erosion-induced collapses are not peculiar to India. In 2009 the Malahide viaduct, north of Dublin, failed similarly just after a train had crossed it. This was despite its having been inspected and pronounced safe a few days earlier. In America, meanwhile, foundation-scouring is reckoned to be the leading reason for bridge failure. Half of the 500 collapses that happened there between 1989 and 2000 were caused by it.

If detected early enough, foundation-scouring is easy to fix. Dumping rubble, known as riprap, into the water around a bridge’s piers stabilises the riverbed they are sunk into. But until now such detection has involved the deployment of teams of divers, which is expensive. Hence a search for technology which can substitute for the men and women in the wetsuits.

Stopping bridge collapses

Ken Loh of the University of California, San Diego, thinks he has an answer. He has created flexible rods that, when inserted into a riverbed, monitor erosion quite simply. The exposed portion of a rod undulates in the water. Piezoelectric polymers in the rod convert this motion into electricity, with the frequency of the undulation (and therefore of the electric current) indicating the length of the rod’s exposed part. As the bed erodes, this portion gets longer and the frequency drops. That tells the riprap tippers when to get busy.

Genda Chen of Missouri University of Science and Technology has a more unusual proposal: to throw magnetic “rocks” (artificial boulders with magnets embedded inside them) into the river. These rocks roll around in the riverbed until they settle in dips in the sediment, which are generally places where erosion is at its greatest. Sensors fitted to a bridge’s piers then estimate the amount of scouring, and where it is, from the strength and direction of the magnetic field they detect.Some researchers, like Luke Prendergast of Delft University of Technology in the Netherlands, think installing sensors below the waterline like this is too expensive—and is also unreliable. He worries that heavy storms will wash them away when they are needed most. He has focused instead on monitoring the part of the bridge above the water, using accelerometers of the sort found in most smartphones. All bridges vibrate, as traffic bumps over them or winds rattle their decks. If their foundations begin to erode, the pattern of these vibrations will change, much as the pitch of a tuning fork varies with its length. Accelerometers, Dr Prendergast suggests, could monitor such changes and forewarn of problems.

Accelerometers are not the only way to measure vibrations, though. David Mascareñas of Los Alamos National Laboratory videos them. He then uses a computer algorithm to analyse the resulting footage and determine a structure’s properties, even if the vibrations recorded have an amplitude of less than a millimetre.

Whether methods that study vibrations in these ways can detect problems early enough to prevent collapses remains to be seen. Branko Glisic of Princeton University, by contrast, thinks the best approach is to detect threatening cracks directly. He has created special sensor sheets, designed to be pasted onto the sides of a bridge. Wires within a sheet elongate if a crack opens underneath them. That changes their resistance. The arrangement of the wires means such changes in resistance give away precisely where the crack is.

If methods such as these can be made to work in practice, then it will, more often, be possible to send the rip rappers in at the appropriate moment to save a bridge that is otherwise sound. And, for those bridges that are not, timely warning will be provided that a crossing needs to be closed before someone is killed traversing it.

Source: Stopping bridge collapses | The Economist


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