Applicability of Combined Seismic Isolation Device in Seismic Design of Railway Simply Supported Beam Bridge

Friction pendulum bearings (FPBs) can effectively improve the seismic performance of bridges in class II sites. However, for class III and IV sites, using only FPBs under large earthquakes can easily cause significant displacement of the main beam, leading to beam collapse. In order to improve the seismic performance of railway simply supported beam bridges under poor geological conditions, this study proposes a new type of combined seismic isolation device, which extends the vibration period of the bridge through hyperbolic spherical bearings and provides energy dissipation through circular steel dampers. Based on the relevant design parameters of the steel damping and the bearing, their mechanical models are calculated and superimposed to obtain the mechanical model of the combined seismic isolation device, and the model is verified through experiments. Then, a bridge model using this device is established using OpenSees, and the effects of pier height, pier height difference, and far-field long-period seismic motion on pier bottom bending moment and support displacement under class III and IV sites are analyzed. The damage status and indicators of the combined device were provided, and the fragility of the device was analyzed. The results show that under design displacement (300 mm), the hysteresis curves of the combined seismic isolation device are with good consistency in mechanical properties in all directions and strong energy dissipation capacity, and the applicable pier height range of the device is determined under class III and IV sites. This study can provide a reference for the seismic isolation design and practical railway simply supported beam bridges.