Effects of steel shim geometric characteristics and imperfections on the behavior of unbonded elastomeric bridge bearings subjected to large lateral displacements

Unbonded laminated elastomeric bridge bearings are designed to accommodate service-level deformations, but during an earthquake they can experience deformation demands that are significantly larger than those for which they were designed. Because these bearings are critical elements of a bridge, they must perform reliably even under seismic-level demands. The effects of the characteristics and condition of the steel shims on the seismic behavior of these bearings have received limited attention in the literature. Yet, the manufacturing process of these bearings sometimes introduces imperfections. For instance, during vulcanization, the steel shims might get rotated or bent. In this context, this paper investigates how manufacturing imperfections and the thickness characteristics of steel shims affect the behavior of unbonded elastomeric bridge bearings subjected to successive cycles of large lateral displacement. Finite element analysis is used to study the deformation pattern of these bearings, their hysteretic response characteristics, and the damageability of their steel shims. It is observed that imperfections can increase a bearing’s roll-off and risk of instability. Imperfections can also increase a bearing’s effective lateral stiffness and damage, quantified though a specific plastic-energy-based damage factor. Plastic damage in the shims is found to be appreciably dependent on their thickness. Under cyclic displacement, damage manifests itself more in the form of spreading of the yielded area rather than increased peak strain ductility. Plastic strains in the steel shims do not get close to the fracture limit, and all the bearings exhibit fairly stable hysteretic loops under the adopted loading protocol.