Traffic load model based on weigh in motion measurements

The assessment of the structural safety of existing bridges and viaducts becomes increasingly important in many countries. Within the actions applied to the bridges, the traffic load is, in general, the most significant variable action to be considered when the ultimate limit states are under investigation. Over the last years a considerable increase in traffic loads and intensities has been observed. In this paper design traffic loads for bridges are derived based on an extensive measurement program of vehicle characteristics. Traffic load measurements were carried out in several highways using the Weigh in Motion (WIM) system; for every vehicle, variables recorded by the system include vehicle speed, number of axles, inter axle separation, and axle and vehicle weight. Part of the study is the composition of a Non Parametric Continuous Bayesian Belief Net in order to be able to obtain insight in the several measured parameters. This model has been quantified with data from eight WIM locations in the Netherlands. It consists of 705 nodes representing univariate margins and more than 2,300 arcs. Design loads should be derived such that they provide a sufficient safety level of the bridge; this corresponds to a bridge design using traffic loads with a return period in the order of magnitude of tens of thousands of years. Extrapolation of the Weigh in Motion data is therefore carried out through several statistical methods. For the lane loads a probabilistic traffic flow model was constructed including axle and vehicle loads, traffic intensities, lorry distances and speeds for free and congested traffic. Results are distributed loads on single and multiple lanes for different bridge spans. The value can be used in bridge design in combination with dynamic amplification factors and factors taking into account the uncertainties in the model and the future trends