Many seismic protection techniques have been suggested over the years to mitigate the damaging effects of earthquake shaking. Seismic isolation is an earthquake-resistant design approach where a horizontally flexible layer is introduced at the base of a structure to decouple the structure from the motion of the ground. This is achieved through the installation of special devices called isolators at the base of columns. The concept of using isolators as a practical method has evolved into reality with the development of multilayer elastomeric bearings. The elastomeric bearings sometimes feature thick steel end plates and are connected to the superstructure and substructure by mechanical mean named bonded application. In buildings, traditional practice places the isolation system at the foundation level and calls for the construction of rigid diaphragms above and below. The flexural rigidity of these diaphragms prevents the isolators from experiencing rotations. However, in some application such as bridge applications, isolation of high-rise buildings and mid-height isolation, it is possible for elastomeric bearings to experience rotation. In another application, the bearings do not have end plates, and shear forces are transferred from the bearing to the superstructure and substructure by the frictional force that develops along the rubber-to--concrete or rubber-to-steel interface named unbonded application. This shear resistance is necessary to provide the bearing with an adequately large vertical stiffness. Since friction plays a dominant role in holding unbonded bearings in place, it is important to predict the effect of slip in this application. The main objectives of this study are to investigate the rotation effects on the horizontal behavior of bonded elastomeric bearings and study the effect of slip on the vertical behavior of unbonded bearings. To provide a better understanding and quantitative characterization of the behavior of elastomeric isolators under combined loading, an extensive nonlinear 3D Finite Element Analysis (FEA) study has been done. Moreover, three mechanical models available in the literature were modified to capture the effect of rotation on the lateral behavior of elastomeric bearings. In addition, a new macro model to find the lateral stability limit proposed and the results were compared against FEA. In order to study the axial-shear-bending interaction in elastomeric bearings, a setup was built at Applied Dynamic Laboratory to conduct the experimental tests on a 1/4-scale circular bearing. Furthermore, to study the effect of slip on the vertical behavior of unbonded bearings, a closed--form solution including the effects of the elastomer’ s bulk compressibility was provided and compared against FEA result.

Doctor of Philosophy (PhD)

Thesis