## 3 Observation of LRB Failure Mode through Static Test
### Observation of Failure Characteristics in Static Test  
The static test of OPT1 of the small-sized LRB for equipment shows that the force-displacement response curve has hysteresis characteristics similar to the typical shear deformation behavior of the LRB for large structures until the shear strain reaches 300%, as shown in Figure 4. However, the shear force no longer increases near the shear displacement of approximately 60 mm (350% of shear strain), and the load decreases sharply without any strain change near a displacement of approximately 70 mm (400% of shear strain). This was considered to be caused by the initiation of buckling. However, in the opposite direction, the slope of the graph does not appear symmetrical, and the shear load hardly changes at a specific displacement range. When a shear force is applied in the opposite direction after a half cycle, the curve changes to show a negative shear stiffness from approximately 150% of shear displacement. Such a response trend indicates partial damage to the rubber part, when referring to a detailed review of the test video and internal fracture surface. This could be supplemented by observing the vertical section of the laminated surface of the tested specimens by water jet cutting after the test.  
In summary, in the case of the 400% shear test of LRB OPT1, which has a low shape factor, the initiation of internal buckling was estimated when the shear displacement reached approximately 350% of the design shear, but partial damage was assumed to occur in the sequential backward shear test.  
In contrast, the force-displacement response of OPT2 (Figure 5) shows typical hysteretic response characteristics with up to 500% shear displacement. OPT2 exhibits better response repeatability in the hysteresis curve than OPT1 due to the improved shape factor. In the 600% shear test, where the effect of bending moment by the vertical load is expected to increase significantly, the force-displacement response started to exhibit an asymmetric trend similar to the 400% case of OPT1. In the course of the 600% shear test, the shear force decreases sharply over 500% displacement and changes unstably as the bearing suffered partial damage inside, showing a non-linear negative stiffness before it reaches the maximum deformation. Near 100 mm (approximately 570%) shear displacement, the damaged area rapidly expanded along with buckling of the rubber disc inside the LRB laminate, leading to an almost zero slope in the opposite direction test. A similar response trend was observed in the shear test of other OPT2 specimens. This implies that the overall damage to the rubber laminate may proceed rapidly without prior signs and processes of partial buckling, and caution is required in the ultimate case, such as 600% shear in a BDBE. Detailed investigation of LRB sections with damaged surfaces by water jet cutting and test video review after the test supplements this estimation of the damage process. As shown in Figure 6a,b, in the damaged LRB, traces such as partial failure of the rubber disc of the laminate, excessive deformation of the steel plate, and plastic deformation of the lead core  
Figure 4: Hysteresis curve for OPT1 shear test.  
can be compared to the undamaged cross section. In conclusion, compared to OPT2, the excessive P-8 effect due to the poor shape factor brings forward the initiation of buckling earlier in OPT1. The dissipated energy in OPT2 shear test is estimated to be larger than that in OPT1 test because the area of hysteresis curve increased by the shear displacement and size.