Dynamic response of Submerged Floating Tunnels: An enhanced semi-analytical approach

The dynamic behavior of Submerged Floating Tunnels (SFTs) is often investigated by resorting to continuous models, where the tunnel is described as a beam resting on a continuous elastic foundation that translates the effect of the mooring system. These models are invaluable tools for conceptual design and optimization of SFTs, but their application to real-world problems requires addressing two main issues that have been typically neglected in the literature: (1) the effects of the spatial variability of the mooring stiffness; (2) the evaluation of the internal forces (bending moments and shear forces) along the tunnel length, whenever in presence of dynamic external loads, such as the ones induced by a seismic event. This paper specifically addresses the aforementioned issues and presents a continuous SFT model explicitly accounting for the spatial variability of the mooring stiffness. A procedure to reconstruct the tunnel internal forces is also developed, by considering the SFT as a beam subject to support settlements and a suitable distribution of inertia forces. Spatial variations of the mooring stiffness are shown to be responsible for mode localization phenomena. The proposed model is then used to investigate the seismic response of a proposal design for the Messina Strait crossing.