## 3 Test Results and Analysis
### Horizontal Shear Performance
#### 3.2.3 Vertical Pressure  
Due to the effect of the horizontal overturning moment and vertical seismic action, the vertical pressure on the rubber bearing will change considerably, which will have a certain influence on the shear performance of the bearing. Therefore, it is important to study the pressure correlation of the shear performance for the bearings. The three bearings were slowly and continuously loaded vertically with the vertical pressure loaded to 3400 kN, 4250 kN (1.25 P), and 5100 kN (1.5 P), respectively, and the pressure remained unchanged during the subsequent test. The shear strain was 100% and the horizontal reciprocating load was successively applied to each bearing at the loading frequency of 0.01 Hz. The hysteretic curves of the bearings obtained by the test are shown in Figure 15. The horizontal equivalent stiffness and equivalent damping of the bearings were calculated and are listed in Table 7.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Loading Frequency (Hz)**} & \multicolumn{2}{c}{**Equivalent Stiffness (kN/mm)**} & \multicolumn{2}{c}{**Equivalent Damping Ratio (\%)**} \\ \cline{2-7}  & **HDRB-LSF** & **TLRB** & **LRB** & **HDRB-LSF** & **TLRB** & **LRB** \\ \hline
0.01 & 1.09 & 1.35 & 1.22 & 9.26 & 38.89 & 31.75 \\
0.0082 & 0.97 & 1.01 & 1.21 & 8.90 & 38.29 & 31.15 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Horizontal performance of the bearings under different loading frequencies.  
Figure 14: Horizontal shear load-displacement curves under different loading frequency: (**a**) HDRB-LSF; (**b**) TLRB; (**c**) LRB.  
As can be seen from the figure and table, HDRB-LSF and LRB are more sensitive to the change of vertical pressure. The horizontal equivalent stiffness of HDRB-LSF increases and then reduces with the growth of vertical pressure, while that of TLRB and LRB gradually decreases with it, which is due to the axial compression and 100% shear deformation caused the out-of-plane distortion of the steel plates, thus reducing the horizontal constraint of the steel plates on the rubber pads. Therefore, the designed surface pressure of the bearing must be strictly limited when it is employed. The equivalent damping ratios of all the three bearings enlarged as the vertical pressure grew, owing to the fact that the higher vertical pressure boosted the triaxial stress of the rubber layer, making the rubber material inside the bearing denser, and magnifying the intermolecular friction of the rubber material, leading to the increase of the hysteretic curve area and equivalent damping ratio. Although the HDRB-LSF has a certain horizontal energy dissipation capacity, the values of the equivalent damping ratio for HDRB-LSF are rather low, owing to the fact that the equivalent damping ratio of high damping rubber material was set to be around 10% when  
Figure 15: Horizontal shear load-displacement curves under different vertical pressures: (**a**) HDRB-LSF; (**b**) TLRB; (**c**) LRB.  
fabricated and considering the constant cost as compared to TLRB and LRB. This paper emphasizes studying the isolation performance law of all the rubber bearings vertically and horizontally used in the base-isolation structures. Hence, the low equivalent damping ratio for HDRB-LSF is acceptable. Of course, a follow-up study of HDRB-LSF will enhance its equivalent damping ratio and reach the corresponding level of the bearings with a lead core.