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 weigh-scales to find the weights of passing trucks and their axles. 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. Great advances in B-WIM accuracy and effectiveness were achieved as a result of a number of 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 to the more common pavement WIM systems, plus some additional information that can help with the structural assessment of bridges: strain measurements, influence lines, load distribution factors and dynamic amplification of loading. While calibration and connectivity to other types of traffic monitoring equipment are identical to those for pavement WIM systems, the instrumentation and maintenance are simpler 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 thus drivers of overloaded vehicles will not try to escape them. B-WIM systems are completely portable and the portability does not affect the accuracy of the results.
Initially, in the US, simply supported girder bridges were used. Over the years the B-WIM concept was extended to almost any type of bridge, as long as the length of the influence line, i.e. the distance between the two furthest points that affect the measurement, is less than about 30 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, the tallest bridge in the world. In a few cases even masonry arch bridges have been used.
As the weighing scale consists of the entire bridge or at least one bridge span, the results (gross weight and axle loads) are very accurate, provided that the road surface is smooth, without potholes or ruts that may cause excessive dynamic bouncing or rocking of the vehicle.
If an appropriate bridge is available, then the main benefits of using B-WIM are associated with its portability, i.e. when:
- WIM data from several different road sections are required,
- only a few days or weeks of data are needed from each site,
- locations for pre-selection of potentially overloaded vehicles are varied on a regular basis or
- bridge assessment requires realistic information about traffic loads and true bridge behaviour of the structure under the traffic loading.