Cable Supported Bridge (CSB), such as suspension and cable stayed ones, are widely used for waterway crossings and the only bridge typology able to surpass long spans. However, the effectiveness of their cable system greatly reduces and its cost enormously rises as the crossing length increases. Submerged Floating Tunnels (SFT) represent an innovative technical solution in the field of waterway crossings, being tubular structures, submerged in the water at a fixed depth, kept in position through anchorage groups made up of cables or tethers linked to the seabed. The anchorage system of SFTs features several striking advantages as it is stable and, above all, provides a supporting condition to the tunnel which is unnoticeably affected by the crossing length but it can be less effective and more costly when large water depths are encountered. Even though Cable Supported Bridges and Submerged Floating Tunnels appear to be well distinct structural solutions, several similarities can be noticed between the two: the bridge deck/tunnel is supported through vertical or sloped cables, the load conditions are analogous, the gravity loads on the cable supported bridges being replaced by the upward residual buoyancy in the SFTs and the role of wind being played by currents and waves actions; the tunnel structure can be realized with a multi cell crosssection having a fluid-dynamic shape, as most of the modern cable supported bridges feature. Therefore some of the knowledge and ideas developed in the cable supported bridges field can be conveniently transferred to the SFTs one.
As a matter of fact, the cable system configurations in use for cable supported bridges can be combined with the concept of a tunnel floating underwater, giving rise to a new typology of submerged floating tunnel, the Cable Supported Immersed Inversed Bridge (CSIB). This solution seems to be convenient in intermediate to deep waters, featuring several advantages with respect to traditional Cable Supported Bridges under both the economic and the environmental impact points of view. In case of deep waters and crossing length belonging to the range of feasibility of CSBs, this new solution features some important advantages also with respect to the “classic” SFT. The aims of this paper are to give a description of the differences and points in common existing between conventional Cable Supported Bridges and SFTs and to illustrate the main features and the potential applications of this alternative submerged floating tunnel solution.