Partial factors for fatigue loads in the Eurocode system for road bridge design
In the Eurocode system, for fatigue design of bridges, the recommended partial factor for fatigue traffic loads is set to 1. In this paper, the adequacy of this approach is investigated by performing a reliability analysis on two types of welded joint in a main girder of a steel motorway bridge. For this purpose, a weigh in motion measurement dataset belonging to a main Dutch motorway has been compared with the fatigue load model 4 of Eurocode EN 1991 2 with respect to the stress spectrum and the fatigue damage of two structural steel details. Several structural schemes have been considered to study the effect of the shape and length of the influence line. The distributions of the stochastic variables such as dynamic amplification, accuracy of the structural model, and future traffic trends have been estimated or taken from literature. Partial factors for fatigue loads have then been calibrated in such a way that the target reliability is obtained. The influence of each stochastic variable on partial factors has been studied by derivation of the sensitivity factors. The results show that a considerably higher fatigue partial factor is required for fatigue loads on road bridges than the value of 1 currently recommended in EN 1991 2. 1. The factor gMf depends on the choice of fatigue assessment method as well as the consequences of failure. For the safe life fatigue assessment method and a structure with high consequence of failure, gMf is recommended as 1.35. In this paper, the adequacy of these partial factors is investigated in a probabilistic approach where all influential random variables are attempted to be modelled as close to reality as possible. For this purpose, several structural schemes are defined and for each one of them, two structural details are designed to meet the Eurocode's safety requirements. Subsequently, the reliability of each designed case at the end of service life (100 years) is evaluated by the probabilistic approach and is compared with the safety requirement of the Eurocode (EN1990, 2002) Partial factors are calibrated in such a way that the deterministic design with partial factors arrives at the target reliability level.