THE tangle of concrete and steel that used to be the Minneapolis bridge across the Mississippi holds secrets it could take some time to fathom. Five people are known to have died when the bridge buckled during the evening rush hour on 1 August, pitching around 50 vehicles into the water. At the time of going to press, eight others were still missing. The cause of the collapse is unknown and investigators say it may take as long as 18 months to fully uncover what happened.
The bridge carried the Interstate 35 and linked the two halves of Minneapolis. It was built in 1967, which was also the year of one of the worst bridge disasters in US history, when the Silver bridge over the Ohio river linking Point Pleasant with Gallipolis on 15 December, killing 46 people. The questions asked then are exactly the same as those being asked now: what caused the collapse and could it have been prevented?
In the case of last week’s disaster, the answer to the first question will take time coming. The answer to the second is a little clearer: there’s no doubt that we are better equipped than 40 years ago to monitor the state of bridges: modern sensing technology can allow us to keep tabs on the health of a bridge remotely and provide for an effective early-warning system. It is already being used in some places, and now is the time to deploy it as widely as possible.
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Usually engineers inspect bridge structures by eye. After the collapse of the Silver bridge, the US set up a regular system of inspections that classifies the country’s 583,000 major public bridges on a scale from 0 to 9, where 9 represents perfect condition and 0 a state so dangerous that the bridge must be closed pending demolition. The Minneapolis bridge was rated 4 – in “poor condition” but still usable. The trouble with human inspections is that they are subjective and prone to error. In 2000, the Non-destructive Evaluation Validation Center of the Federal Highway Administration (FHA) carried out a study of their effectiveness. In one of its tests, it asked 49 inspectors to look at a steel bridge that had the beginnings of a potentially serious crack. Only a fifth of them found it.
“The FHA asked 49 inspectors to look at a bridge with the beginnings of a serious crack. Only a fifth of them found it”
Technology can make monitoring a lot more reliable. Engineers can use sensors to weigh and count traffic as it passes over a bridge and warn inspectors if the load gets dangerous. Likewise, strain gauges can measure the effects on its infrastructure. This makes it possible to raise the alarm long before a bridge is at risk and even pinpoint structural flaws. There are already systems like this on the Williamsburg bridge in New York City and the Commodore John Barry bridge over the Delaware river between Chester and Bridgeport. More advanced devices are on the way. The FHA has conducted trials on a laser technology that measures to a tenth of a millimetre how much a bridge moves as traffic passes across it. Last month, researchers at the University of Michigan, Ann Arbor, revealed they had developed a “” made from carbon nanotubes that can be sprayed onto a structure to track its stability.
We need such technologies now more than ever. One of the chief causes of bridge fatigue is heavy lorry traffic, which is growing the world over. It is not only the number of trucks that’s increasing, but also their maximum weight. In 1967, the top weight of a truck in the US was 36 tonnes; today it is 40 tonnes. Three and a half times as many articulated goods vehicles pound US roads today as in 1967, and much of this traffic is concentrated on the interstate network, using bridges like the one in Minneapolis. Freight traffic on interstate highways is forecast to double over the next 20 years. In the absence of a major change in transport policy, such as switching traffic from road to rail, things will only get worse.
Technology cannot replace highly trained inspectors, but it can give authorities a far better idea of how infrastructure is bearing up under the strain and allow them to take appropriate action. Widespread use of remote sensing devices would not even be that expensive – in the region of tens of thousands of dollars per bridge. That may sound a lot at first glance, but given the number of people who were on the Minneapolis bridge when it collapsed last week – including a busload of schoolchildren – can the government afford not to take this step?
