Study on Foundation Constraint Modeling of a Sea-Crossing Cable-Stayed Bridge Under Combined Wind–Wave Actions

Foundation constraints are commonly defined according to the deformation characteristics of the supporting system; however, structural deformation is also strongly affected by external loads. Compared with inland bridges, sea-crossing bridges experience much larger horizontal loads under combined wind–wave actions, and whether foundations in hard-soil conditions can be simplified as rigidly fixed still requires verification. In this study, the m-method is used to determine the equivalent spring stiffness of each soil layer from soil parameters, and a spring-based soil–foundation interaction model is established. This spring-based model is taken as the reference to evaluate the applicability of the rigidly fixed foundation assumption. Using the Qiongzhou Strait highway–railway combined cable-stayed bridge as the engineering background, both rigidly fixed and spring-based foundation models are developed to simulate foundation constraints. The dynamic responses of a single bridge tower and of the entire bridge system under combined wind–wave loading are computed. The influences of foundation constraints on tower-top displacement, foundation reaction forces, and bending moments are investigated. The maximum discrepancy between the two approaches reaches 7.83%, providing a rational basis for selecting foundation constraint conditions in dynamic analysis and design of sea-crossing bridges.