Dynamic soil-structure interaction for a long-span suspension bridge with dissipative foundations

This paper deals with the seismic behaviour of the Izmit Bay sus-pension bridge. The towers of this bridge were provided with a dissi-pative foundation system intended to limit the seismic loads transmit-ted to the superstructure. The bridge is in close proximity to the North Anatolian fault, responsible of a high seismic hazard characterised by important near-source effects. This paper describes an innovative ap-proach for predicting the behaviour of the bridge accounting for the soil-structure interaction effects and for the behaviour of the above dissipative system. The approach entailed the use of three-dimensional numerical analyses, carried out in the time domain using nonlinear constitutive assumptions for the soil. The numerical model included the entire sequence of the soil deposits extending down to the bedrock, and a simplified structural model of the towers, that was calibrated to reproduce the main aspects of the dynamic behaviour of the super-structure. The results of the analyses show that the seismic perform-ance of the dissipative foundations is strongly affected by the ampli-tude of the vertical component of the seismic motion. The dissipative foundation is fairly efficient in limiting the seismic forces transmitted into the superstructure; however, it appears that the horizontal perma-nent displacements of the tower foundations derive mostly from per-manent strains occurring within the subsoil, rather than in the dissipa-tive device, as originally intended in the design of the bridge.