Site specific models of traffic action effects for bridge evaluation

Once a structure is in service, site measurements can be made in order to reduce the uncertainty about loads and resistance. In the case of the evaluation of existing road bridges, the consideration of actual traffic and the effects it produces in a given structure enables a more accurate assessment and a better ranking of maintenance needs. The main aim of the study described in this paper was therefore to develop a simple method for the consideration of site specific traffic loads as a function of parameters describing the bridge and traffic, referred to as site characteristics. This method is based on a site specific probabilistic model of traffic action effects derived from the results of computer simulations of traffic effects. The simulation program generates random traffic actions for defined traffic conditions and determines the frequency distribution of maximum static effects. Simulation results have been used to derive empirical relationships between site characteristics and the parameters of a type III extreme value distribution of maximum effects. The paper presents these relationships and explains how this probabilistic model has been used in this study in order to develop a simple method for considering site specific traffic actions. This simple method is based on the application of reduction factors to action effects calculated using the Swiss design traffic load model. The paper is a summary of part of a doctoral thesis [1] and describes the innovative approach which was adopted for calibrating reduction factors using probabilistic methods considering various failure modes of bridge structures.