System Identification of Base-isolated Buildings Utilizing Records from Recent Southern California Earthquakes

Abstract

System identification is used in this study as a systematic approach to determine modal periods, mode shapes, and estimated damping for three Southern California base-isolated buildings. In addition, estimates of the level of response each building experienced during recent earthquakes can be evaluated. Of particular interest are the variations in modal parameters as a function of the level of excitation due to the non-linear nature of these structures. By using identification of each building for several earthquakes and dividing the time histories into short segments, structural response trends can be investigated. It is shown that structural period and damping vary with amplitude of excitation. Since the bearings are hysteretic and softening elements, under increased structural response, the periods of vibration as well as the equivalent modal viscous damping increase. It is also shown that although the 1994 Northridge California earthquake generated moderate levels of horizontal ground acceleration at building sites, the bearings were exercised well below their maximum design levels.