## 4 Observation of LRB Failure Mode through Dynamic Test
### Review of Shaking Table Test-II  
As explained in Section 3.1, the seismic input was adjusted for the LRB to lead to crack initiation at the laminated rubber surface and to investigate the response trend in the partially damaged LRB. The second table test was performed to understand the response characteristics of the entire isolation system from partial damage to complete failure of the LRBs. For this purpose, BDBE inputs at the 137 ft level were directly applied to the 3D shaking table. The floor response in this case was already amplified through the building analysis model of a Korean standard nuclear plant. In the second experiment, three designed beam structures were additionally installed on the shaking table and on the upper surface of the dummy mass to examine and compare the seismic response characteristics of the secondary structures inside the isolated facility. The test on the shaking table is shown in Figure 12, which includes the dummy mass supported by four LRBs in the bottom corners, and serial numbers of acceleration sensors with predetermined natural frequencies in parenthesis. To observe the seismic isolation effect on the secondary structures, two sets of beam structures with natural frequencies of 5, 10, and 20 Hz were installed on the shaking table bottom and on the upper part of the isolated primary structure. The accelerometer attached at the top of the beam compares the seismically isolated and non-isolated beam responses. The experimental sequences are listed in Table 7.  
The second shaking table test was used to observe the behavioral characteristics before and after the isolation system failure during BDBE. The input earthquake for the test was the same as that given in Figure 2, whose ZPA was highly amplified from the 0.3 g DBE to 1.2 g. As shown in Table 7, the experimental sequence proceeded using two identical sets of specimens (OPT-2A, OPT-2B) for three input earthquakes of 0.4, 1.3, and 2.2 g of ZPA, followed by another identical specimen (OPT-2C) tested by applying the largest ZPA of a 2.6 g earthquake as a BDBE. The test was performed with a one-axis excitation in the X direction. Figure 13 shows a comparison of the acceleration response measured on the shaking table for the DBE 0.3 g (ZPA of FRS: 1.3 g) test of OPT-2A with the response measured on the isolated dummy mass. As expected, the peak of acceleration response of the dummy mass appears around 2.3 Hz, which is the design frequency of the LRB. It gradually decreases to around 20 Hz, and then maintains a stable acceleration response in the remaining frequency range. The maximum acceleration of the dummy mass as a superstructure largely decreased in the range of about 4.5 to 100 Hz by seismic isolation, as intended. Figure 14 shows the seismic isolation effect of the beam structures installed on the shaking table and dummy mass. Figure 14a,b shows the acceleration spectrum and time history of the secondary structure at 10 Hz in the facility. Again, as expected, a peak appears at 9 Hz, which is near the design natural frequency, and, except for the region below 3.5 Hz, which is slightly higher than the isolation frequency of the LRB, the isolating effect of reducing the response is significant. The response reduction rate is relatively larger than that of the 5 Hz beam. The characteristic of the acceleration response versus frequency is similar to that of the 5 and 20 Hz beams; a peak appears near the design natural frequency, and the seismic isolating effect of reducing the response appears reasonable. Summarizing the above results, the seismic isolation performance of the LRB for small facilities developed in this study were confirmed once again, because the acceleration response of the secondary structure showed a seismic isolation effect similar to that of the primary structure. Figure 15 shows an example of the acceleration response history of beams when the primary structure experiences partial and complete failure in the isolation system owing to the ultimate load.