Influence of Steel Reinforcement on the Performance of Elastomeric Bearings

Abstract

Seismic isolation is an effective measure for mitigating earthquake hazard on important structures such as bridges, hospitals, and nuclear power plants. Seismically isolated structures should remain essentially elastic for the design earthquake, experiencing minimal or no damage. Steel-reinforced elastomeric bearings are seismic isolation devices that are used extensively for the protection of bridges and other critical infrastructures. Because isolators are critical elements of an isolated structure, they should experience limited, if any, damage during earthquake shaking that exceeds the design level. Damage associated with yielding of the reinforcing steel shims in seismic isolation elastomeric bearings has received limited attention in the literature. In this context, this paper investigates the effect of the steel reinforcement characteristics on the behavior of rubber bearings under combined axial load, shear displacement, and rotation. The potential of damage in the steel shims under design-level shear strains is investigated using a damage factor, Ω, with emphasis placed on the thickness of steel shims. The investigation is carried out using advanced finite element modeling in Abaqus. It is observed that, under large axial loads, thin steel shims (1 and 2 mm) experience extensive yielding regardless of the rotation angle of the top anchor plate; whereas thicker steel shims (4 and 6 mm) remain elastic when the top anchor plate experiences no rotation but can yield when the top plate rotates. Yielding of the steel shims causes permanent deflections and local damage that alter the properties of the isolators.