## 4 Observation of LRB Failure Mode through Dynamic Test
### Discussion of Failure Response Characteristics in Dynamic Test  
To observe LRB dynamic characteristics due to the extreme load, shaking table tests as the last step of Test-II in Table 7 were conducted for the uniaxial seismic input at the upgraded level of 0.6 g simulating the BDBE. In this case, ZPA level of FRS for the working floor 137 ft high was amplified to almost 2.6 g by the plant building model. Figure 16 shows the displacement-time history, including the slip behavior, of the dummy mass following the test, and explains its behavior after the total break of the LRBs. The dotted line indicates the estimated time of failure initiation. Under the influence of a large input earthquake, partial failure among the LRBs by excessive shear displacement is detected at an early stage, at approximately 13 s. After a few seconds, the all of the LRBs supporting the dummy mass completely ruptured and slipped in a limited range on the table. The maximum shear displacement of the dummy mass before the break was approximately 100-110 mm (over 600% of the designed shear) and was reciprocated. The post-rupture behavior showed a slip response of approximately 70 mm in both directions on the shaking table after the center of mass moved approximately 80 mm. A detailed review of the experimental video record shows that the post-rupture behavior agrees with the response trend in Figure 16, and a small noise occurs near the point of complete break. The image in Figure 17 shows the seismic isolator LRBs supporting the dummy mass completely ruptured and moved with some rotation from the original position.  
Figure 14: Effect of isolation in the acceleration response of 10 Hz beam as a secondary structure (BDBE: 0.5 g): (**a**) response spectra; (**b**) time history.  
Figure 15: Response of 10 Hz beam structures before and after isolation system break (BDBE: 0.6 g).  
It is noteworthy that, even after the LRB was completely broken, the dummy mass continued to slip within a limited range due to friction. Therefore, even if the LRB is fractured due to the occurrence of a large earthquake exceeding the DBE, there is an additional secondary seismic isolation effect due to the slip. Therefore, this should be considered in the design of isolation systems, including LRBs, superstructures, and moat margins to ensure seismic safety, and prevent potential impact with interfacing neighboring structures [21].  
Figure 18 shows the cross-section shape of the fully fractured LRB after the extreme dynamic test. Figure 18a is a side view of the failed LRB with cracked cover rubber, and Figure 18b shows the different colors of the scratched surface, which are friction traces due to the slip after a fracture. Similar observations were made on the cut surfaces of the deformed lead core and rubber plate. The fractured laminate occurs near the top of the LRB laminate, which is similar to the point of maximum stress in the simulation analysis in Section 4.1.  
Figure 16: Displacement–time history of dummy mass before and after the LRB break.  
Figure 17: Shape of dummy mass on the totally ruptured LRB after testing.  
Figure 18: View of the LRB failure surface after break: (**a**) front view; (**b**) top view.