## 4 Observation of LRB Failure Mode through Dynamic Test
### Review of Shaking Table Test-I  
As explained in the previous section, the 1 st shaking table test was conducted in the order of three uniaxial and one triaxial OBE tests, and a uniaxial SSE test, as shown in Table 6. In particular, the fifth experiment enabled observation of the response characteristics to extreme loads before failure. Figure 10 shows a graph comparing the results of the resonance test before and after the experiments.  
In Figure 10a, there is little change in the natural frequency in the X-direction before and after the tests. However, after the tests, the acceleration response at the top of  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**No.** & **Seismic Input [137 ft]** & **ZPA [g] of FRS** \\ \hline
1 & OBE-X & 1.3 \\
2 & OBE-Y & 1.3 \\
3 & OBE-Z & 0.4 \\
4 & OBE 3-Axis & - \\
5 & SSE-X & 2.6 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Shaking table test-I procedures.  
Figure 10: Peak response correlation affected by LRB damage: (**a**) resonance response in the X-direction; (**b**) resonance response in the Y-direction.  
the dummy mass was amplified in the peak region, and the acceleration at the bottom decreased, as shown in Figure 10b. This seems to be caused by the influence of rocking behavior on the X-direction translational response. In contrast, when comparing the responses before and after the whole test, the peak of the Y-directional acceleration moved to a lower value, indicating a decrease in stiffness due to damage. The observation of the specimen cross-sections by water jet cutting after the experiment shows some cracked areas in the rubber laminated surface inside the LRB. These response characteristics can be a good reference for evaluating the internal health and integrity of LRBs by analyzing the response records of seismically isolated facilities after an actual BDBE.  
Figure 11 shows the displacement response of the uniaxial SSE shaking table test. Figure 11a depicts the responses of the seismically isolated dummy mass in the horizontal X- and Y-axes before the main test. As anticipated, the Y-axis response is almost zero, indicating that it is hardly affected by the X-axis excitation. However, once the LRB has been damaged through the ultimate test of a one-directional SSE, the Y-axis displacement response is non-zero, as shown in Figure 11b, due to the inequality and eccentricity of stiffness inside the LRB in the X-direction. In the case of the isolated dummy mass displacement, the response of the damaged LRB in the orthogonal direction is no longer independent of the effect of rotational behavior. This is unlike the response trend between the two horizontal axes of the shaking table. Therefore, if the acceleration and displacement response can be relatively checked between the two horizontal axes after a BDBE, it may provide useful information about the structural integrity inside the LRB. A comprehensive review of the response correlation trend can determine the continuous reusability of LRBs for the residual design life and improve the estimation reliability of possible partial damage inside the LRB.  
Figure 11: Correlated responses of the damaged LRB in the SSE 1-D Test: (**a**) displacements of the LRB without damage; (**b**) displacements of the LRB with damage.