MECHANICAL RESPONSE OF FIBER-REINFORCED ELASTOMERIC ISOLATORS UNDER VERTICAL AND LATERAL LOADING

The response of unbonded fiber-reinforced elastomeric isolators (FREIs) under lateral loading has been extensively studied and is characterized by an initial softening due to detachment from the supports, known as rollover, and a stiffening at large displacement after contact of the originally vertical faces of the isolator with the supports. During a seismic event, however, FREIs in a base isolated structure will be subjected to varying lateral and vertical loads due to overturning effects and vertical ground motions. The response accounting for this combined action of vertical and lateral loads has been studied to a much lesser extent. Some experimental programs have been developed, while very few studies have attempted to analytically characterize the behavior by adapting approximate formulations developed for steel-reinforced elastomeric isolators (SREIs). However, a rigorous mechanical description of the response remains elusive. Understanding and modeling the behavior of the bearings under this combined loading is critical to properly assess the response of a structure isolated with FREIs under seismic loading. This paper describes the mechanical response of unbonded FREIs under the combined action of vertical and lateral loads under no initial lateral displacement. The isolator is treated as a short one-dimensional beam element which considers warping deformations due to shear. A simple closed-form expression for the horizontal stiffness of the bearing under vertical loading is systematically derived and shown to be analogous to that used for SREIs, albeit with a different buckling load due to the cross-sectional distortions caused by shear. Three-dimensional finite element analyses are presented, which validate the effectiveness of the formulation in characterizing the mechanical response of the isolators.