Analytical and Numerical Study of the Axial Stiffness of Fiber-Reinforced Elastomeric Isolators (FREIs) under Combined Axial and Shear Loads

Featured Application This paper provides an analytical formulation to account for the vertical stiffness reduction of fiber-reinforced elastomeric isolators under combined axial and shear loads. The analytical method can be used in design practice to estimate the vertical stiffness of these seismic devices. Fiber-reinforced elastomeric isolators (FREIs) are rubber-based seismic devices introduced as a low-cost alternative to steel-reinforced elastomeric isolators (SREIs). They are generally used in unbonded applications, i.e., friction is used to transfer the lateral loads from the upper to the lower structure. Under combined axial and shear loads, the lateral edges of the unbonded bearings detach from the top and bottom supports resulting in a rollover deformation. Due to increasing horizontal displacement, the overlap area of the bearing decreases; thus, the vertical properties of the device are a function of the imposed lateral deformation. This paper introduces a closed-form solution to derive the vertical stiffness of the bearings as a function of the horizontal displacement. The variations of the vertical stiffness and of the effective compressive modulus of square-shaped FREIs are given in this work. The analytical results are then validated through a comparison with the outputs of a parametric finite element analysis of FREIs, including different mechanical and geometric parameters.