| Summary: | Under-deck cable-stayed bridges (UDCSBs) are an innovative typology of cable-stayed bridges, in which the stay cables are located underneath the deck. UDCSBs with prestressed concrete decks present an efficient structural behaviour due to the enhanced axial response. Hence, UDCSBs lead to a more sustainable construction compared to bridges without cable-staying systems. However, UDCSBs with steel-concrete decks, which seem prima facie to be an appropriate solution, have not been investigated hitherto in depth. Thus, the behaviour of UDCSBs with steel-concrete decks when used for road construction is investigated, and the work provides a comprehensive set of design criteria. A numerical study on the behaviour of UDCSBs is presented. A benchmark bridge model is used as a reference to study the response of UDCSBs in different stages of the life of the bridge and under different load conditions. Moreover, multiple parametric analyses are performed from the benchmark model to investigate the influence of different geometric and mechanical properties of the bridge and to obtain meaningful conclusions of this structural typology. The current work also includes a detailed literature review of studies concerning UDCSBs with steel-concrete composite decks. The stability of the deck during the construction stage is studied initially. The compressive forces introduced by the stay cables when being pre-tensioned may trigger instabilities, and thus, both buckling and post-buckling behaviour are studied. An analytical model based on potential energy principles is presented. This model allows to study the influence of multiple parameters in the response. The response is intrinsically governed by the ratio of the flexural stiffness of the deck to the axial stiffness of the cable-staying system. Moreover, highly unstable interactive buckling is triggered under certain circumstances. Recommendations to achieve a safe construction process are devised and provided. Secondly, the behaviour during the service life of the bridge is characterized and optimal configurations are identified. The corresponding serviceability and ultimate limit states are studied to find the governing limit states for each of the elements in UDCSBs. While struts are governed by the ultimate limit state of axial forces, the design of the stay cables is governed by the fatigue at the anchorage sections. Nevertheless, the maximum slenderness of the deck is controlled by the serviceability limit state of vibrations under the action of traffic load. Moeover, it is found that the dynamic response is highly influenced by the amplification and cancellation speeds for the second flexural mode of the deck. The effect of nonlinearities is also investigated, through the analysis of geometric nonlinearities due to the large displacements and the material nonlinearities due to the nonlinear stress-strain relationships of the materials. The influence of nonlinearities is found to be relevant during construction and practically negligible in service under permanent and traffic loads. Additionally, the influence of time-dependent effects is studied, the outcomes not being negligible. In conclusion, some design criteria are provided. Recommendations regarding both the design and construction are presented. Moreover, some dimensions for UDCSBs with steel-concrete composite decks are suggested. This may certainly facilitate the design of UDCSBs with steel-concrete composite decks to achieve efficiency and sustainability.
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