The Effects of High Isolation Damping on the Performance of Base Isolated Structures

This study investigates the influence of isolation damping on the response of multi-storey steel moment frames isolated by lead-rubber bearings (LRB) and high damping rubber bearings (HDRB) subjected to near-fault ground motions. The LRB is modeled as a bilinear system and the HDRB is modeled as an equivalent linear system with viscous damping. The structures meet the gravity and seismic requirements of IBC 2006 and are analyzed in SAP2000 by the Fast Non-linear Analysis method. Past studies have shown that although the base response decreases effectively with increasing isolation damping, the super-structure response is not steadily decreased; there exists a value of damping for which the superstructure response- displacement and acceleration- of a given system attains a minimum and then starts increasing. As the isolation damping is increased, the imposed ground acceleration increasingly influences the dynamic modes of the structural response via the interaction terms. The results of this study show that the significance of the modal interaction terms is almost singularly dependent on the characteristics of the ground-motion. Some ground motions coupled with increasing damping in the isolation significantly increase the super-structure response, other ground motions coupled with increasing damping in the isolation effectively decrease the super-structure response, and still others show a combination of both behaviors. Further studies are necessary to characterize the specific traits of the ground-motions that adversely effect the interaction terms. Important precautionary steps can be taken in the interim to lessen the negative impact that certain ground motions present to heavily damped base isolated structures. For lead-rubber bearings the damping should not exceed 25%. For a range of near fault ground motions, hysteretic damping in the isolators up to 25% can show significant benefits in lowering the base response as well as the response in the superstructure; exceeding this magnitude can exacerbate the unfavorable behavior some ground motions present to any hysteretic damping. HDRB or LRB with a low value of the post-yield stiffness ratio with stiffening measures in the super-structure are strongly recommended to reduce the floor accelerations and inter-storey drifts respectively. In addition, a low value of the effective stiffness is one of the most effective ways to minimize the increase in floor acceleration and inter-storey drift due to high isolation damping. An effective strategy to moderate the base displacement and the isolation damping is to select the lowest value of the effective stiffness, which will allow the base displacement to stay within necessary limits, for the damping levels recommended.