The research objective of this project is to develop improved methods for modeling and predicting the stability limit of elastomeric and lead rubber seismic isolation bearings. Elastomeric and lead-rubber isolation bearings are used for seismic isolation of building and bridge structures to reduce the damaging effects of strong ground shaking. The Elastomeric bearings consist of alternating layers of rubber bonded to intermediate steel shim plates and the lead-rubber bearings have an additional lead plug inserted into a central hole. During severe earthquake ground shaking, these isolation bearings are likely to be subjected to simultaneous large lateral displacements and axial compressive loads that could cause bearing instability. This research will identify the fundamental mechanism(s) causing bearing instability through detailed nonlinear finite element analysis and component level testing. These studies will aid in the development of an improved macro-model capable of capturing bearing instability and the derivation of a mechanics-based closed-form expression for predicting the bearing stability limit state.
The results of this research will provide tools to improve the assessment of bearing safety for the design of isolation systems composed of elastomeric or lead-rubber bearings. This research will also develop improved macro-models for rapid numerical earthquake simulation that will provide new knowledge about the performance of isolated structures under extreme earthquake loads. This new knowledge would lead to innovative solutions to modify and enhance the response of individual bearings to achieve a desired system performance. The results will be disseminated broadly to the research and design communities through journal publications, seminars, conference sessions and design recommendations. The project will provide advanced training to graduate students with professional development opportunities of presenting their work at technical meetings and conferences. This project will also recruit undergraduate students from diverse backgrounds to participate in structural engineering research and to pursue a graduate education in civil engineering through Penn State's Office of Engineering Diversity's Summer Research Program (SROP) program.
Elastomeric bearings are widely used to protect important buildings, including the building’s contents, from the damaging affects of earthquake ground shaking. For instance, elastomeric bearings have been employed for the seismic isolation of hospitals, emergency response centers, museums and historic buildings in regions of moderate and high seismicity around the world and in the United States. A typical elastomeric bearing consists of a number of rubber layers bonded to intermediate thin steel shim plates. During earthquake ground shaking these bearings will be subjected to simultaneous large lateral deformation and compressive loads. However, elastomeric bearings exhibit complex behaviors including geometric nonlinearity, material nonlinearity, coupling between the horizontal and vertical degrees of freedom and instability. These complex behaviors have important implications for the design and probabilistic performance assessment of seismically isolated structures and therefore should be replicated by the mathematical models used to analyze and assess the adequacy of the isolation system design. Prior to this research the mechanism(s) responsible for these complex behaviors were not well understood. Furthermore, existing mathematical models for elastomeric bearings either had significant limitations such that any one model could not generally replicate all of the complex behaviors or required extensive experimental calibration. Using state-of-the-art sensitivity and simulation techniques, the research team discovered the fundamental mechanism controlling the stability of elastomeric bearings at large lateral displacement. Specifically, the mechanism controlling the stability at large lateral displacement is the development of tensile stress in the outer most rubber layers leading to a rotational hinge, loss of rotational stiffness and instability. Guided by this finding, the research team developed a mechanistic approach to model elastomeric bearings that is able to replicate the nonlinear response, coupling, and stability behavior without relying upon empirical parameters that must be experimentally calibrated. The mechanistic model developed in this research removes the calibration requirement by consisting entirely of engineering parameters related only to the geometry and material properties of the bearings and overcomes limitations of other models. The significance of the mechanistic approach is that by removing the need for calibration and other limitations, the mechanistic model produced in this research increases the accuracy of the analysis and assessment of the seismic demands imposed on the isolation system (e.g. peak lateral displacements) and surrounding components (e.g. foundation systems) thereby enhancing the reliability of seismically isolated buildings and other structures.
