Exploring the Low Shape Factor Concept to Achieve Three-Dimensional Seismic Isolation

The current state-of-practice in the U.S., and elsewhere, for designing elastomeric seismic isolation bearings utilizes closely spaced intermediate steel shim plates with thin rubber layers that result in shape factors ranging from 15 to 30. Such high shape factors (HSF) produce a vertical stiffness several thousand times larger than the horizontal stiffness thereby providing isolation only in the horizontal plane (2D). While the large vertical stiffness has been thought to be desirable to minimize rocking in slender structures with an elevated center of mass it also results in a lower period of vibration in the vertical direction that can align with the dominant frequency content of the vertical component of earthquake ground shaking. A low shape factor (LSF) bearing concept to achieve three-dimensional (3D) isolation was explored in the past for the nuclear industry. Though this research demonstrated, through analysis and a prototype design, that the LSF concept could effectively provide isolation in both the horizontal and vertical directions, system level testing and implementation were never realized. This paper presents the results of an analytical, parametric, study aimed to further explore the low shape factor concept to achieve three-dimensional isolation. The results of this study suggest that 3D isolation might be achieved for low and mid-rise structures using the LSF concept if the bearing shape factors are less than four and supplemental vertical damping is included at the plane of base isolation.