Fatigue design of metallic railway bridges in France at the end of the nineteenth century

Among more than 25 000 bridges and viaducts in the French railway network, more than 2000 metallic bridges have been in service for over a century, and they need adapted measures for their maintenance. Due to the increase in rolling speed and axle load, the understanding of the behaviour of this type of connection becomes of utmost importance. In order to understand better how old bridges behave, a specific aggressiveness index has been defined, and it enables comparison of the effects of different rolling stocks on the mechanical behaviour of stringers. This index has been determined for different French historical standard trains. For these calculations, a set of real iron stringers has been considered, including different periods of construction, different spans and different inertias. Since fatigue is one of the main phenomena to take into account in order to extend the safe service life of metallic bridges, a fatigue design criterion has been considered. Regarding historical calculation methods, an allowable fatigue stress has been chosen using the Societé Suisse des Ingénieurs et des Architectes fatigue curve, assuming Δσ _c = 80 MPa. The method for determining the allowable stress in stringers presented in this paper shows a good agreement with the National Society of French Railways historical inventory of repairs.