Weighing up our bridges
Deciding when to renovate or replace an ageing bridge can be tricky. Risks to safety must be avoided, but acting prematurely may be wasteful. Monitoring techniques developed by an EU-funded project can facilitate such decisions. They are based on an innovative weigh-in-motion system used to check that trucks aren't overloaded.
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To those who understand them, bridges can tell a lot. They can provide information on the vehicles that cross them day in, day out, and on the strain this places on their structure and on our roads and railways.
The Bridgemon project has found a way to decipher this secret language. It has developed advanced techniques to measure the weight of moving trucks or trains, and it has devised an innovative way to monitor the structural health of steel bridges.
The project upgraded an existing weigh-in-motion (WIM) system used for the detection of overloaded trucks, which cause considerable damage to road networks worldwide. The system also generates valuable data that can help to optimise investments into road and rail infrastructure.
Measuring the burden
“Road networks are slowly beginning to feel the pressure of overloaded trucks and unanticipated levels of traffic,” says project coordinator Peter Favai of Cestel, a Slovenian SME specialising in bridge WIM systems. Roads designed decades ago simply weren’t built for today’s massive trucks and sheer numbers of vehicles, he adds, and the excessive wear causes costs and disruption.
Improving the detection of trucks that flout the weight limits would curb the damage, Favai explains. Bridgemon rose to the challenge by upgrading SiWIM, the lead partner’s existing WIM system.
Unlike WIM systems that rely on direct contact with the vehicle, this solution does not involve any alterations to the road surface itself. Instead, the equipment is installed underneath, at point where the road crosses a bridge and is therefore easily accessible from below.
The measurements are taken by sensors that record the micro-deformations of the bridge caused by passing vehicles. This data is used to calculate the gross weight of individual trucks and the number, spacing and load of their axles. As the information is processed instantly, police can stop screened vehicles if their weight exceeds the limit for their category.
Placing the equipment under rather than on a road means that the lane does not have to be blocked during set-up, and that the sensors last a great deal longer. As an added advantage, SiWIM is portable, in contrast to other systems, which tend to involve permanent installations. It can be moved from one site to another as the priorities change.
Detecting signs of fatigue
The partners have also adapted the system for use on rail bridges, to check the weight of trains, and have greatly improved its accuracy, say the partners. The upgraded system provides the basis for another Bridgemon achievement: an innovative approach to monitoring the structural health of steel bridges. By analysing the micro-movements observed in the structure, the partners can predict when the bridge will need to be replaced, Favai explains.
WIM monitoring could also be used to generate crucial statistical data, Favai notes, adding that information on vehicle weight is key to optimised traffic and infrastructure management. “If our goal is to have a good road network, we have to prevent vehicles being overloaded and build better roads,” he says. “This requires knowledge of the real traffic loadings. So we have to weigh the traffic, and not just rely on traffic counters.”
Currently, Favai explains, technical requirements for new roads are based on a potentially outdated tally of passing vehicles, combined with an estimate of their likely weight — which does not necessarily account for the reality of overloading. “So WIM measurement is the future, I think, but we need some time to raise awareness,” he concludes.
That said, Favai and his fellow bridge whisperers don’t seem to have much difficulty getting their point across. The upgraded SiWIM system is meeting with interest from around the globe, he reports.