Investigation of base-isolated structures during recent earthquakes and computer simulations utilizing near-source long-period ground motions

Abstract

Base isolation is a recently applied technology for building structures in the United States. To date, the three base-isolated buildings considered in this study have been subjected to earthquakes of varying magnitudes and epicentral distances. The records obtained from these instrumented buildings demonstrate low levels of excitation and small structural responses. In all cases, the maximum relative displacement of the roof to the foundation is less than 3 cm. However, an increasing quantity of near-source strong-motion records produces large spectral displacements of up to approximately 50-55 cm in the 2 to 2.5 sec period range for 15% damping. This suggests that long-period structures such as base-isolated structures would be vulnerable to these near-source ground motions. The current study contains two major parts. Part One consists of the identification and analysis of three existing base-isolated buildings in Southern California. The identification and analysis utilize the recorded motions of these structures from past earthquakes. System identification is useful for understanding the extent to which the structures enter the nonlinear realm and how much their properties change. Models are constructed assuming completely elastic three-dimensional superstructures, with idealized bi-linear hysteretic elements for the isolating bearings. The properties used in the bearing models were taken from tests of the actual bearings before installation. The models were then verified by comparing their responses computed using the various recorded foundation ground motions, with the recorded responses of the actual structures. The models were adjusted to minimize the error of several response quantities. Part Two contains computer simulations for the three structural models developed in Part One subjected to large-amplitude near-source ground motions. These structural models were subjected to two classes of ground motions. The first is a sampling of near-source recorded motion from past moderate-to-large earthquakes. The second is a group of synthetic near-source motions generated for a hypothetical M 7.0 earthquake. In some cases, the lateral response of the models exceeds the isolation gap, indicating that the displacement barrier would be impacted. In order to further study base-isolated buildings when the isolation bearings undergo large displacements, a typical base-isolated building (TBIB) model is used and the computer program 2D-BUMP is developed. This program includes the effects of a fully nonlinear superstructure, nonlinear springs acting as displacement barriers which engage at specified distances, and a tri-linear model for the elastomeric bearings. Using this model, several conclusions are drawn regarding the probable areal extent of damaging near-source ground motions from the M 7.0 event, as well as the behavior of base-isolated structures due to these near-source long-period ground motions.