Timber-concrete composite (TCC) bridges have the potential to achieve significant levels of structural efficiency through the synergistic use of Engineering Wood Products (EWPs) and reinforced concrete. Although TCC beam type bridges are currently employed for short-span crossings, this bridge configuration is unsuitable for longer spans. Therefore, this thesis focuses on the application of under-deck tendons to TCC bridges, making the under-deck cable-stayed TCC bridges competitive for the medium- and long-span road bridge solutions. Currently, there is no available design guidance covering under-deck cable-stayed TCC bridges. This is a reflection of the lack of knowledge on the matter. Moreover, the dynamic responses of this newly-proposed bridges are expected to be a significant design driver given timber's high strength-to-weight ratio. To this end, 2D MATLAB and 3D Abaqus Models are developed and validated in this thesis against analytical estimations and experimental results. Based on these models, the static, dynamic, stability, and fatigue behaviour of the newly-proposed cable-stayed TCC bridges with short, medium and long spans are systematically explored, and a comprehensive set of design recommendations are offered.

A suite of 2D MATLAB models are validated against analytical predictions. These models are employed in the design of TCC bridges with and without under-deck tendons. It is demonstrated that the tendons significantly change the critical limit states governing the design of TCC bridges, and remarkably increase their deck slenderness and structural efficiency of the cable-staying system. In addition, a systematic analysis on the influence of key design parameters is conducted to explore the best practical configurations.

Subsequently, the dynamic response of under-deck cable-stayed TCC bridges is studied via 3D finite element models. The dynamic investigation focuses on modal frequencies, dynamic amplification factors and acceleration distribution. The most critical dynamic load cases and corresponding vibration characteristics are examined for under-deck TCC bridges with short, medium and long spans. In addition, appropriate strategies for reducing the unfavourable accelerations, without the need of implementing supplemental damping devices, are explored by carrying out parametric studies on some key design variables. Finally, the practical implications of the findings as well as modelling and design recommendations are given, together with suggestions for further works. The under-deck cable-stayed TCC bridges under study can be systematically designed following current European design philosophy as expressed in the relevant Eurocode provisions and related design recommendations. It is expected that a synergistic integration of post-tensioning steel and engineered timber elements will lead to a new generation of infrastructure with inherent low-carbon characteristics and optimal whole-life performance, especially for medium and long span structures.