Sliding versus Rubber Bearings: Exploring the Difference in Collapse Probability

While collapse mechanisms have received significant attention for conventional buildings, they are less well understood for isolated buildings. Recently, there has been a more concerted effort to quantify the collapse probability of isolated structures; however, the majority of the research has explored the behavior of buildings isolated with concave sliding bearings, also referred to as friction pendulum (FP) bearings. Isolated buildings are expected to perform similarly under defined ground motion levels regardless of the type of bearing used. Yet the collapse probability of isolated buildings is directly dependent on the bearing failure characteristics, which differ by bearing type. Therefore, employing different isolation systems while following the same design guidelines may result in different collapse probabilities. In this study, the collapse probabilities of a 3-story buckling restrained brace frame isolated with either double-concave FP bearings or lead rubber bearings are compared. Different designs at maximum displacement are considered including use of moat walls versus allowing failure of the bearings (or impact of the restraining rims for FP bearings). In the absence of the moat wall, the system-level failure using both bearing types is triggered by exceeding defined displacement capacities. In contrast, with the moat wall, the system-level failure is dominated by either axial component-level failures or excessive yielding of the superstructure. However, when the moat wall limits ultimate displacement, the difference in collapse probabilities is small.