# Effect of Subsequent Subgrade on Seismic Response of the High-Speed Railway Track-Bridge System
## 5 Seismic Response Analysis of Each Critical Component in HSTRBS
### Seismic Response Analysis of Bearing  
Bearings play a pivotal role in the seismic analysis of bridges, significantly impacting the bridge structure's internal forces, resultant displacements and deformations, and the dynamic characteristics of the structure itself. The comparative analysis of the longitudinal deformations and shear forces endured by the bearings in the two bridge models, BMCS and BMWS, under various PGAs, is meticulously illustrated in Figures 7 and 8.  
Figure 6: Vertical displacement of the main beam under varying PGA.  
Figure 7: Longitudinal deformation of bearings under varying PGA.  
The analysis reveals that the longitudinal constraint effect of the SSTS can reduce the longitudinal displacements of all the bearingss under the combined effect of longitudinal and vertical ground vibration. Notably, the longitudinal deformations of the fixed pier bearings (bearings 3# and 4#) are less impacted by the longitudinal confinement effect of the SSTS compared to other pier bearings (bearings 1#, 2#, 5#, 7#, and 8#). Additionally, the longitudinal confinement of the SSTS results in a decreased shear force on the fixed bearing, while the shear force on the sliding bearings remains relatively unaffected. This is attributed to the fact that bearing 3# is a fixed bearing, immobile in both longitudinal and transverse directions, primarily bearing the longitudinal seismic inertia force of the main beam. In contrast, the rest of the bearings are sliding bearings, which are movable in the longitudinal or transversal direction. Their longitudinal stiffness is less than that of fixed bearings. The longitudinal constraint effect of the SSTS has a significantly greater effect on the longitudinal stiffness of the sliding bearing than that of the fixed bearing. As a result, the longitudinal deformation changes generated by the sliding bearings are more obvious when the seismic response generated by the bridge structure is transferred to the bearing structure.  
As the PGA incrementally rises, the longitudinal constraint effect of the SSTS affects the bearing's longitudinal deformation to a progressively greater extent. Specifically, the longitudinal deformation of the sliding bearing displays an almost linear escalation. At a PGA of 0.4 g, there is a notable surge in the longitudinal deformation of the fixed bearing, beyond which the longitudinal shear force ceases to increase. This phenomenon occurs because at PGA values up to 0.2 g, the longitudinal deformation of the fixed bearing remains below its damage threshold. In this scenario, the bearing remains intact, with the seismic response of the bridge structure primarily borne by the fixed bearing. However, at a PGA of 0.4 g, the fixed bearing enters a yielding state, causing the seismic response of the bridge structure to be transmitted through the superstructure of the track to adjacent abutments and subgrade structures. To prevent the risk of beam collapse due to excessive longitudinal slip in the bearing superstructure, the longitudinal restraint effect of the SSTS comes into play. This demonstrates that the SSTS's longitudinal confinement effect on the seismic response of the bearing is a non-negligible factor in the face of strong seismic activity. With a PGA of 0.2 g, the longitudinal confinement effect of the SSTS reduces the longitudinal displacement of the fixed bearing by 3.9 mm and diminishes the longitudinal shear force by 139 kN.