The objective of this study was to present a new approach for determining the truck weight limit of simply supported steel girder bridges. This approach is based on the fatigue reliability of bridge girders under the action of random traffic flows considering the presence of multiple trucks. A simply supported steel girder bridge designed according to the current codes was used as the bridge model to illustrate the presented approach. Based on the collected traffic data from Wisconsin, random traffic flows were generated using the Monte Carlo method and were used as the traffic loading. Numerical simulations were carried out to investigate the effects of three important parameters, including the fraction of traffic in the fast lane (FTFL), violation rate (VR), and truck weight limit (TWL), on the average fatigue damage accumulation (AFDA) induced by each truck in the random traffic flow. Based on the Miner's cumulative damage model and the S-N curve, the effects of these parameters on the fatigue reliability index of bridge girders were also analyzed. The truck weight limit that can ensure the target fatigue reliability of the bridge girders after 75years of service was determined based on the fatigue reliability analysis of the bridge girder.
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