Development and verification of simplified expressions for shear strain in rubber layers for use in design of elastomeric bearings
Ecker Lay, Ricardo Alberto
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Seismic isolation is a technique used in seismic design of new or retrofit of existing structures. It basically consists in mounting the structure on horizontally flexible bearings with the purpose of shifting the natural period of the system to a higher value and of enhancing the capability of the system to dissipating energy. Elastomeric bearings represent a widely used system for seismic isolation. Also, elastomeric bearings are used as regular bridge bearings for accommodating bridge movements due to effects of temperature changes, traffic and creep and shrinkage of concrete. Also, elastomeric bearings are used to provide vibration isolation from ground borne vibration in buildings. In general, the construction of elastomeric bearings is similar regardless of the application-consisting of alternate layers of rubber and steel shims. However, depending on the application, the geometry and thickness of individual rubber layers differs. These differences result in substantial differences in the distribution of strains in the rubber and in the capacity of the bearings to sustain load under deformation. The design of elastomeric bearings is based on the calculation of rubber strains due to the combined effects of compression, and of shear and rotation of the top of the bearings with respect to its bottom. Current design specifications for elastomeric bearings utilized simplified equations for the calculation of rubber strains which do not exactly apply for the range of geometries and loading conditions utilized in seismic isolation and other applications. The work presented herein reformulates equations for the maximum shear strain in rubber layers of elastomeric bearings in a form that is practical for use in design and which is also theoretically correct. The validity of the derived results is investigated by finite element analysis of representative rubber bearings.