Ductile seismic retrofit of steel deck-truss bridges. I: Strategy and modeling

Most deck-truss bridges have been designed and constructed without seismic resistance considerations; as a result, their members, connections, and frequently their substructure are not sized and detailed to provide the ductile response needed during major earthquakes. A ductile seismic retrofit solution proposed here consists of converting the deck slab into a composite slab and replacing the end cross-frames and the lower lateral braced panels adjacent to the supports by special ductile diaphragms, thus creating ductile fuses to protect the remaining superstructure and substructure. This paper illustrates the typical seismic vulnerability of such bridges, presents a model governing transverse seismic response of retrofitted deck-trusses, and a methodology to determine overall stiffness and strength of such ductile panels based on developed seismic performance goals. The results of computer simulations for a retrofitted deck-truss subjected to severe ground excitation validates the proposed methodology and the global modeling. It is concluded that the proposed ductile retrofit solution can significantly enhance seismic performance of deck-trusses.