Seismic behaviour of a suspension bridge with dissipative foundations

This paper evaluates, with the aid of dynamic numerical analyses, the seismic performance of a long-span suspension bridge that was provided with a dissipative foundation system, consisting of a gravel layer interposed between the foundation caisson and a group of foundation piles. This system was conceived to permit a controlled development of relative horizontal displacements between the caisson and the piles during strong motion, providing an intrinsic limitation to the maximum seismic forces transmitted to the tower structure. In the paper, the performance of this innovative foundation layout is evaluated through a series of numerical analyses, in which the soil-structure interaction is studied using a direct approach, taking into account explicitly the tower’s subsoil down to the bedrock and developing a simplified description of the main structural elements of the bridge. The results of the numerical analyses showed that the effect of the dissipative layer built into the tower foundations is limited to frequencies higher than those associated to the fundamental tower modes. Moreover, the strength of the dissipative layer is activated only for strong earthquakes, while the horizontal displacements of the towers are controlled not only by the resistance of the dissipative gravel layer, but notably by the strength of the foundation soils, that provide a first important contribution to the attenuation of the motion amplitudes over a large frequency range.