Submerged Floating Tunnels under Seismic Motion: Vibration Mitigation Issues

In the work here described, a 3D finite element model of a SFT (Submerged Floating Tunnel) has been developed within the ANSYS numerical code. The model, which takes geometrical and mechanical nonlinearities and soil-structure interaction into account, has been subjected to multiple-support seismic excitation. It is found, from transient dynamic analyses, that in the anchoring elements near to the tunnel ends the elastic limit of the material is largely exceeded under design seismic actions. By introducing inelastic material behavior in the bars, the effect of energy dissipation due to hysteresis on the global response is estimated. Attention has been devoted as well to the control of longitudinal oscillations of the tunnel; in most design proposals the tunnel itself acts as a continuous element, since no intermediate structural joints are allowed due to tightness requirements. Consequently, at the tunnel ends, elastic-plastic spring devices can be applied, trying to reduce the relative displacement between the structure and the shore though limiting the axial stress in the tunnel. In the paper the strain-stress relationships in the bars, the displacements and stress time histories at the different sections along the tunnels and the displacement-force curve of the onlinear elastic-plastic spring device are given, allowing for a comparison between the elastic and inelastic behavior.