## 4 Seismic Performance of Isolation Systems Equipped with SLDTFPB and SLDRFPB
### Seismic Response of Isolation Layers Equipped with SLDRFPB and SLDTFPB  
Seismic response of the superstructure represents the isolation effect of bearing. Besides, the seismic response of the isolation layer represents safety and resilience of the isolation system. Figure 17 shows the relations between maximum displacements of isolation layers equipped with SLDTFPB of different depths or SLDRFPB and PGA. The vertical axis in Figure 17 denotes the maximum value of the response among seven ground motions, so that it represents the worst case of lateral deformation of the isolation layer. As shown in Figure 17, maximum displacement of the isolation layer subjected to a very-rare earthquake is more than 1 m. This displacement exceeds the horizontal deformation capacity of classic FPB, but it is much less than the deformation capacity of SLDRFPB and SLDTFPB. Large deformation of the isolation layer will not destroy the structure. In this sense, SLDRFPB and SLDTFPB are particularly suitable for isolation structures which need sufficient seismic performance when subjected to very-rare earthquakes. In addition, since SLDTFPBs with different equivalent radii and SLDRFPB have similar and sufficient deformation, the design requirements of deformation of the isolation layer do not limit the parameter selection of SLDRFPB and SLDTFPB. Figure 16: Maximum vertical accelerations of superstructures equipped with SLDRFPB or SLDTFPB. Figure 15: Maximum vertical displacements of superstructures equipped with SLDRFPB or SLDTFPB. It can also be seen in Figure 17 that the increase of displacement is uneven. There is no obvious pattern between the increment of displacement of the isolation layer and that of the isolation effect. This is because the isolation effect comes from the relative movement between the superstructure and the isolation layer, but the displacement is also determined by direction of movement. Superstructures may bond at random locations when the instant seismic load is not strong, while the sliding direction of superstructures at the next moment is uncertain. Moreover, the uncertainty is more obvious when subjected to small earthquakes, due to larger probability of a stick situation. Another reason is that the maximum value of maximum displacements of seven waves instead of the average value of them has been described in Figure 17. The earthquake wave which leads to the maximum seismic response determines this value, so that these curves in Figure 17 look more random than those in other figures. Figure 18 shows the residual displacements of these isolation bearings. Reset capability represents isolation resilience of SLDRFPB and SLDTFPB. As shown in Figure 18, residual displacement is less than 0.15 m in case of very-rare earthquakes. When only considering SLDRFPB or SLDTFPB as a foundation of structure, and ignoring other functions of SLDFPB except isolation ability, bearing capacity is not affected by residual displacement, and residual inclination of the superstructure is tiny. In this way, residual displacement of the isolation layer will not affect the use of the superstructure. In particular, mechanical properties and the isolation effect of SLDFPB with residual displacement are basically unchanged. In case of bearing with initial displacement, extra residual displacement is always smaller than that of the bearing without initial displacement. As a result, residual displacement can be neglected in real-life engineering application. It can be found that residual displacement of SLDTFPB with a smaller equivalent radius is smallest among the three models of the developed SLDFFB in this section. The reason is that this bearing has the smallest maximum displacement and the smallest maximum theoretical residual displacement. Maximum theoretical residual displacement is defined as the position where the restoring force equals the friction force.