Deriving a load model for the braking force on road bridges: Comparison between a deterministic and a probabilistic approach

The current provisions for the braking force in the Eurocodes are much more demanding than previously enforced national codes from most European countries. Hence, safety assessments of existing bridges may show a lack of compliance with the new safety requirements of current maintenance codes, making the relevant authorities responsible for their strengthening. The current load model for the braking force was derived from a deterministic evaluation of traffic configurations with characteristics that do not correspond with actual traffic measurements. This procedure differs from models describing vertical traffic loads, where the effects of traffic were analysed within a probabilistic framework in compliance with the partial safety factor design method. This paper investigates the advantages, limitations and hypotheses of a probabilistic model for the braking force when compared with deterministic models. Original results for the characteristic value of the braking force on road bridges are presented, which consider a realistic time-history of bridge crossing vehicles generated by a traffic microsimulation tool with input data from a Swiss Weigh-In-Motion station. The results are compared with the braking force of the Eurocodes, highlighting the role of the probability of braking and the influence of the dynamic characteristics of bridges on the characteristic value of the braking force.