Triple-chord trussed submerged floating tunnels: hybrid construction concept, feasibility and design.

Submerged floating tunnels (SFT) offer a promising solution for deep-water crossings and intercontinental transportation. However, current SFT designs struggle to meet the high structural performance demands associated with the harsh service environments while remaining economically viable, thus limiting their implementation in practice. Here, we propose a conceptual SFT design using a triple-chord trussed concrete-filled double-skin tubular (CFDST) hybrid structure, featuring CFDST chords and hollow steel tubular braces. This design is highly adaptable and allows the steel tubes and sandwiched concrete to work synergistically, achieving efficiency in withstanding multiple loading conditions including lateral flow, internal fire, fatigue and impact loading. We further develop a multi-scale structural analysis methodology that integrates three-dimensional solid finite element (3-D FE) and simplified fibre modelling for the efficient evaluation of global deformations, fire performance and joint behaviour. The results demonstrate that the proposed design leads to considerably enhanced resistance against lateral flow loading, vibrations and internal fire, and is more adaptable and cost-effective than existing solutions.