A semi-analytical model for the design and optimization of SFTs under seismic loading

Submerged Floating Tunnels (SFTs) are a promising way of crossing water bodies that lends itself prominently to long crossings due to economic reasons. Due to the length of such structures conceptualization and optimization issues, such as those pertaining to the arrangement of the SFT's mooring system and of the approaches to the shores, cannot easily be solved within numerical approaches such as is the FEM, due to computational costs and practicality, and the help of faster semi-analytical methods for supporting the design has to be sought. More so when the leading excitation source for the dynamic response of the SFT are earthquakes. Indeed, the optimization of the stiffness of the mooring system can have an important effect on the value of the internal forces near the shores, where they are larger. Recently, a semi-analytical procedure of this type has been proposed by the authors on the base that the SFT and its mooring system can be treated as a beam on elastic foundation with non-constant modulus of subgrade reaction, the vertical and transverse response to seismic excitation are decoupled and the seismic excitation is assumed as perfectly correlated along the tunnel length, while hydrodynamic effects due to the ensuing seaquake are modelled as an equivalent (linearized) damping. A classical algebraic eigenvalue problem and standard solution techniques are used 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. A comparison between the results from the proposed semi-analytical procedure, implemented inside a Matlab script, and the results from a FE model of a possible crossing of the Messina strait will serve to highlight the performances and precision of the proposed approach.