SFTs under seismic loading: conceptual design and optimization tools

Submerged Floating Tunnels (SFTs), also named Archimedes Bridges, are a way of crossing waterbodies that has recently attracted the interest of Researchers. However, a number of issues are still open to conceptualization and optimization, as those pertaining to the general arrangement of the SFTs and of the approaches to the shores in case the construction site is in an earthquakes prone area. This paper describes the development of a semi-analytical model for supporting the design of the approaches and the optimization of the mooring systems. The main criteria are that only the transverse response to seismic excitation is considered, and this is assumed as perfectly correlated along the tunnel length. Hydrodynamic effects due to seaquake are modeled as an equivalent (linearized) damping. The anchoring system is treated as a distributed transverse stiffness which, contrary to the works in the literature, will be considered variable along the tunnel length. The general solution to the dynamic equilibrium problem of the SFT is sought through an application of the Rayleigh-Ritz approach, which yields a classical algebraic eigenvalue problem. Standard solution techniques are adopted to evaluate the natural frequencies and eigenvectors of the SFT and a modal analysis approach is adopted to study the forced vibration problem in case of earthquake excitation. The proposed semi-analytical procedure has been implemented inside a Matlab script, and the results highlight the role of two basic design parameters such as the ratio of the tunnel to the anchoring system stiffness and the degree of variability of the lateral stiffness of the anchoring system.