ON THE VARIATION OF MAXIMUM ISOLATOR DISPLACEMENTS DUE TO GROUND MOTION DIRECTIONALITY

Abstract

In most of the cases, code specifications dictate the use of nonlinear response history analyses (NRHA) to estimate maximum isolator displacements (MIDs) of a seismically isolated structure (SIS). For this purpose, a set of ground motion records with similar characteristics needs to be selected. Then, the structure is analyzed bidirectionally by considering both orthogonal horizontal components of these records. However, there is not any provision regarding the ground motion directionality effect in the codes but simply use of as-recorded motions is encouraged. This study investigates the effect of ground motion directionality on variation of MIDs in case of bidirectional NRHA. Thus, a typical SIS, where the isolator units are composed of lead rubber bearings (LRBs), is subjected to ground motions rotated from their as-recorded original form by increments of 10o up to 360o. Here, LRBs are modelled by a deteriorating hysteretic representation in which the strength of the isolator reduces gradually due to the applied loading. In the analyses, first, the original as-recorded ground motion is applied to the SIS and the corresponding MID is noted. Then, the same structure is subjected to rotated versions of the same motion and again the MIDs are noted. To quantify the variation in the isolator displacement, analytically obtained MIDs are compared. Results showed that there is an amplification in MIDs due to change in ground motion direction.