# Seismic Response and Mitigation Analysis of a Subway Station in the Site with Weak Interlayers
## 3 Seismic Mitigation Effect of FPB in Weak Interlayer Site
### The Seismic Mitigation Effect of FPB  
The dimensions of the FPB used in this study are shown in Figure 22. The radius of the bottom plate sliding surface is 1000 mm and the friction coefficient of the FPB is assumed as 0.1. The upper support surface of the FPB is anchored to the bottom of the beam and the lower support surface of the lower support block is anchored to the top of the central column. The specific finite element model is depicted in Figure 25. The boundary condition of the soil layer and the contact between the soil and the station are consistent with the calculation model in Section 2. Here, the Chi-Chi earthquake is selected as the input earthquake, only considering the horizontal ground motion. Calculations are performed for the scenarios listed in Table 2. The seismic response of the subway station structure with and without the FPB is analyzed to assess the seismic mitigation effects of the FPB. In this section, the subway station structure without the FPB model is referred to as the original structure, while the subway station structure with the FPB is referred to as the seismic mitigation structure. Figures 26-28 present the differences in maximum displacements of the subway station structure's side wall and central column under various conditions. A positive difference in displacement indicates that the FPB can reduce the structure's displacement, while a negative difference suggests that the FPB may increase the structure's displacement. In some of the figures, red boxes are used to highlight and magnify specific areas, which are also shown within the same figure. From Figures 26-28, it is evident that the seismic mitigation structure has slightly increased the maximum displacement of the side wall compared to the original structure, but the increase is relatively small. On the other hand, the subway station structure with FPB has led to a reduction in the maximum displacement of the central columns. This reduction is relatively small under minor seismic forces (0.05 g, 0.1 g) but more significant under severe seismic conditions (0.2 g, 0.4 g); the maximum reduction could reach approximately 36 mm. The displacement values of the side wall of the subway station structure with FPB on the top of the central column are slightly bigger than that of the original structure, which is due to the decreased lateral stiffness of the structure with FPB on the top of the central column. The reduction in central column displacement is attributed to the change in boundary constraints on the top of the central column, transforming it from a fixed connection to an approximate hinge. This change substantially releases the horizontal displacement of the central column, resulting in the displacement of the top slab of the seismic mitigation structure not being transferred to the upper central column effectively. Figure 25: Finite element model of friction pendulum bearing. Figure 27: Difference in the maximum displacement of the side wall and column at different positions. (**a**) Side wall. (**b**) Upper column. (**c**) Lower column. Figure 26: Difference in the maximum displacement of the side wall and column with different thicknesses. (**a**) Side wall. (**b**) Upper column. (**c**) Lower column. In order to gain a deeper understanding of the deformation characteristics of the subway station structure, Figure 29 illustrates the inter-story displacements for various conditions. As shown in Figure 29, the inter-story displacements of the subway station's structural layers remain well below 1/250 under seismic forces, which meets the design standard. Importantly, the implementation of FPB on the top of the central column leads to an increase in inter-story displacements. The maximum inter-story displacement angle occurs at the lower level of the subway station structure. The increase in the inter-story displacement angle of the subway station structure is attributed to the release of horizontal displacement in the central column by the FPB, resulting in an overall reduction in the station's lateral stiffness.