Optimal design of cable-stayed bridge and its approach spans under spatially varying ground motions

The out-of-phase vibration between neighboring structures owing to their different vibration characteristics and spatially varying seismic excitations may result in strong impact between the adjacent structural components, which might result in detrimental damage to the structures. To preclude such damage, the gap size of the expansion joint and the vibration period ratio between the adjacent structures should be carefully selected. Herein, the framework of performance-based seismic design (PBSD) is extended to optimize the gap size and period ratio between the cable-stayed bridge and its approach spans under the spatially varying ground motions. Considering the computational burden, an equivalent cable-stayed bridge model is proposed and verified, and then adopted in the optimization process. Based on the multi-stripe analysis (MSA) method, the component fragility curves (pylons, bearings, and local damage) for the typical cable-stayed bridge are derived, and then the corresponding structural fragility functions, namely, repair cost ratios (RCRs) are derived. The optimal gap size and period ratio between main bridge and approach spans could be identified by the proposed optimization framework under both uniform and spatially varying excitations. The analytical results demonstrated that the optimal gap size and period ratio depend on the earthquake characteristics and the excitation methods. The optimized RCR values of the bridge subjected to the spatial varying ground motions are much larger than those of bridge under the uniform excitation.