Seismic isolation of structures, Part II: A case study using the RNC isolator

The present paper is the second of two companion papers. The objective of this paper is to minimize twist of isolated asymmetric structures, together with their torsional pounding with adjacent structures, considering insufficient seismic gaps and strong near-fault ground motions. Concisely, the present study attempts to provide efficient seismic isolation under the above challenging conditions. The used isolation system is referred to Roll-in-Cage (RNC) isolator. Among the features of the RNC isolator are two characteristics that help achieving the objectives of the paper. The first is the independency of its bearing and pre-yield stiffness mechanisms. Such independency allows for accurate tuning of the isolators pre-yield stiffness to shift their center of rigidity, at the isolation level, to coincide with the asymmetric superstructure's center of mass above that level. This allows for minimizing the structural twist of an isolated asymmetric structure. The second feature is the inherent buffer mechanism of the RNC isolator, which draws down any possible seismic pounding of the isolated superstructure, with adjacent structures, to occur only within the isolation bearing itself. This leads to seismic pounding-free superstructure under limited seismic gaps. The obtained results show that utilizing the RNC isolator this way is able to minimize, or even eliminate, the out-of-plan displacement responses of asymmetric isolated structures under severe near-fault earthquakes, and consequently, minimizes a major cause of structural damage due to structural torsional pounding with closely spaced adjacent structures under such destructive ground motions.