Bridge WIM Systems

Bridge Weigh-in-Motion (B-WIM) was introduced in the US in the late 1970s by Prof. Fred Moses. An existing bridge is used as a weighing scale to find the axle loads and gross weight of passing trucks. This is done by monitoring strains in a bridge deck (girders or slab) that are induced by vehicles crossing overhead. The most common sensors employed are strain gauges and strain transducers. Significant advances in B-WIM accuracy and effectiveness were achieved in the 1990s as a result of several research projects financed by the European Commission. Today, B-WIM systems are used and further developed in many countries around the world.

Modern B-WIM systems provide identical vehicle-by-vehicle data as the more common pavement WIM systems. Besides, they also measure strain measurements, influence lines, load distribution factors and dynamic amplification of loading, which all improve structural analysis of bridges. While calibration and connectivity to other types of traffic monitoring equipment are identical to those for pavement WIM systems, the instrumentation and maintenance are more straightforward and less intrusive. In most cases, no sensors are needed on the pavement surface and, consequently, traffic is not disrupted during installation and maintenance. Furthermore, the weighing locations cannot be spotted from the vehicles, and drivers of overloaded vehicles will not try to escape them. B-WIM systems can be moved from one site to another and the portability does not affect the accuracy of the results.

Initially, in the US, simply-supported girder bridges were used. Today, the B-WIM concept can be applied to almost any type of bridge, as long as the distance between the two furthest points that affect the measurement – the influence line – is less than about 40 m. In reality, the bridges are selected based on the required level of accuracy. The types of structure used range from short culverts, widely used in Australia, to the most common beam-and-slab and slab bridges, and orthotropic deck bridges, such as the Millau viaduct in France, one of the tallest bridge in the world. In a few cases, even masonry arch bridges have been instrumented.

As the weighing scale consists of at least one bridge span, the results (gross weight and axle loads) are very accurate, provided that the road surface is without potholes or ruts that cause excessive dynamic bouncing or rocking of the vehicle.

If an appropriate bridge is available, the main benefits of using B-WIM are associated with its portability, i.e. when:

  • only a few days or weeks of data are needed from each site,
  • WIM data samples from different road sections are required,
  • locations for pre-selection of potentially overloaded vehicles change regularly,
  • bridge assessment requires information about its true static and dynamic behaviour under the known traffic loading.