Assessment and reduction of seismic risk on existing bridges

Indispensable to the functioning of our society, land transport networks are vulnerable to certain major natural hazards, such as earthquakes. Feedback from past earthquakes shows that, along road routes, bridges are both specific points of weakness and strategic nodes, for both emergency access to the disaster zone and the medium-term resumption of socio-economic activity.

Scientific advances and the evolution of earthquake engineering design regulations have raised questions about the level of resistance of existing structures to this hazard. In France, the first paraseismic rules, known as "PS69", applied from 1975 onwards, provided for simplified consideration of seismic actions, leading to structures that were generally undersized compared with more recent standards. Since 1996, application of the PS92 seismic regulations has, in theory, guaranteed adequate earthquake protection for structures located in the country's most seismic zones, but most bridges currently in service were designed earlier. It is therefore legitimate to question their ability to withstand the stresses generated by an earthquake, at least at the regulatory level. What's more, changes in national seismic zoning, imposed by Decree no. 2010-1255 of October 22, 2010 on the delimitation of seismic zones in France, have extended the number of structures likely to be affected by this hazard. This observation also raises the question of the structural requalification of recent constructions in zones where seismicity was at a lower level.

This specific problem, which is not covered for bridges by either the new national legislation or by the Eurocode 8-2 standard, raises a number of decisional and technical questions. Which structures should be diagnosed first? At what risk level should existing bridges be reinforced? What level of performance should be aimed for when reinforcing them? What diagnostic methods and strengthening techniques are available?