Deviations from nominal design values of mechanical characteristics of seismic isolators are inevitable due to uncertainties and/or errors in material properties, element dimensions, construction methods, quality control, and/or installation steps, etc. In addition, due to uncertainties existing in active fault mechanisms affecting the site and local soil conditions, earthquake ground motions show record-to-record variability. Moreover, environmental conditions and/or time-dependent factors such as aging, temperature, and travel, etc. may also cause additional deviations in nominal values of characteristics of seismic isolators and thus in structural responses over the service life of structures. Therefore, in order to capture the dynamic behavior of base-isolated buildings realistically and design reliable base-isolated buildings, influence of such uncertainty should be taken into account. In this study, seismic reliability of realistic fully three-dimensional benchmark multi-story buildings equipped with nonlinear isolation systems is investigated under historical near-fault earthquakes. In order to take into account the uncertainties in the isolation system characteristics, pre-yield stiffness, post-yield stiffness, and yield displacement parameters are assumed as random variables, while the record-to-record variability nature of the ground motions is considered by using large sets of historical near-fault ground motions with or without forward-directivity effects. Two different levels of nominal isolation periods are taken into account while three different levels of uncertainty are considered for the random isolator characteristics. The reliability of the buildings is investigated in terms of structural integrity, isolation system safety, and the safety of the vibration-sensitive contents of the buildings using the peak bearing displacements and the peak floor accelerations obtained from the nonlinear time history analyses conducted in the framework of the Monte Carlo simulations. The plots including the average reliabilities for the displacement and the acceleration limit values provide a comprehensive picture in terms of the reliability level of the structural systems and of the vibration sensitive contents.

Introduction

Seismically isolated structures are generally mission-critical structures housing vibration-sensitive equipment which must go on serving even during an earthquake. However, such equipment may fail if floor accelerations sustained during such motions exceed certain limit values. In addition, the isolators may fail different failure modes [1] and endanger safety of the whole structure. Isolation system displacements may exceed the design limits for the base displacements and therefore cause buckling and/or rupture of isolators may occur [2]. Buckling is a specific concern for stability of elastomeric isolators when the critical axial load capacities of the isolators are exceeded along with the increases in the horizontal isolator displacements [3,4]. In such a case, the subject isolator displacements become unconstrained and increase freely [5]. In another failure mode, cavitation damages occur at a critical tensile stress value inside a rubber layer and gain importance for horizontal displacement levels corresponding to shear strains above 100% [6,7]. Finally shear failure could be observed at very high shear (i.e., on the order of approximately 300–۵۰۰%) strain levels [4,8,1]. Above all these, in case one of the abovementioned failure modes occur in a significant number of isolators almost simultaneously, this may lead to the collapse of the whole isolation system [1]. Therefore, for obtaining reliable seismically isolated structural systems, it is economically and sometimes vitally important to determine the actual mechanical properties of isolation elements, which are used in the dynamic analyses of the subject systems, and take into account the factors that may lead to deviations in those mechanical characteristics.