Traffic Loads on Bridges

The results of a study are described which aimed at the derivation of a traffic load model for the design of road bridges. Attention is given to the Ultimate Limit State, the Serviceability Limit State and the Fatigue Limit State under static and dynamic conditions. A set of measurements done in 1978 in Rheden, The Netherlands, served as the basis for the vehicle model. Starting from the measurements of 16,000 vehicles, statistical models for the weight of 16 types of lorry configurations were derived. An additional set of measurements done in 1985 at Moerdijk, The Netherlands, was used to derive a statistical model for the axle loads, once the vehicle type and the total vehicle load is given. Based on various sources, a probabilistic traffic flow model was constructed, comprising traffic intensities, lorry distances and lorry speeds for three types of traffic: free, congested and full stop, each of which governs the traffic flow during some part of the day. Given both models, design values with predefined probabilities were derived for the axle, tandem, tridem and vehicle loads and for distributed loads on single and multiple lanes of various influence lengths. The axle and vehicle loads were derived by direct calculation. The distributed lane loads were derived using a simplified numerical traffic model. For short periods (1, 10 and 30 days), this simplified model was verified by Monte Carlo simulations on the full model. The values obtained in the way described above can not be used directly in design. The vehicle data is very limited and comes from one place and one time. The traffic flow model contains many simplifications and assumptions. These effects have been taken into account by an extensive trend and uncertainty analysis.