Accurate measurements of gross vehicle weight through bridge weigh-in-motion: a case study

In a bridge design code, the design live loads should be based on actual traffic loads. In the past, the information about the actual truck loads on highway bridges was obtained from truck surveys, in which the trucks are stopped for measurement and weighed on ‘static’ weighing scales. During the past few decades, however, the information about truck loads is collected either by pavement weigh-in-motion or bridge weigh-in-motion (BWIM) systems. This paper provides details of three BWIM methods, which were calibrated with extensive testing on a slab-on-girder bridge in Winnipeg, Canada. The first two methods are based on the assumption that truck loads could be represented by an equivalent uniformly distributed load. The first method uses the asymmetry in the shape of the bending moment diagram to calculate the gross vehicle weight (GVW), while the second method compares the girder response at two instrumented transverse sections. The third method calculates GVW using strain signal area as proposed by Ojio and Yamada in 2002. The results show that the first method was inconsistent in its accuracy; the second method’s accuracy reduced with the length of the vehicle. The third method proved consistent and accurate to within 5 % for the test trucks provided that the vehicle speeds could be estimated accurately. Accurate calculations of the truck speeds were achieved by using the time delay in the peak responses of the straps that confine the externally restrained deck slab of the bridge.