Compressive behavior of low shape factor lead-rubber bearings: Full-scale testing and numerical modeling

Abstract

Low shape factor lead-rubber bearings, defined as bearings with relatively thick rubber layers, have been confirmed by several studies to be feasible for vertical seismic isolation because of their flexibility in the vertical direction. However, information on various behavior aspects of low shape factor lead-rubber bearings remains inadequate. With much greater vertical deformation under working conditions than conventional high or medium shape factor bearings, low shape factor lead-rubber bearings display certain unique mechanical behaviors due to the nonlinearity and inelasticity of lead plugs. The applicability of existing design equations to the compressive stiffness of low shape factor bearings remains unknown. Although it has been demonstrated that the axial load has an insignificant effect on the compressive stiffness of lead-rubber bearings with high or medium shape factors, few studies have referred to the influence of the axial load on the vertical stiffness of low shape factor bearings. To investigate the unique compressive behavior of the low shape factor lead-rubber bearings, bearings with various shape factors were tested under various axial loads to compare their compressive behavior in terms of compressive stiffness and force-displacement relationships. The obtained results indicate that the existing design formula for compressive stiffness is not applicable for lead-rubber bearings with a shape factor lower than 11. Although the axial load has a minor influence on the compressive stiffness of lead-rubber bearings with high shape factors, it has a non-negligible influence on that of low shape factor bearings. In addition, a unique behavior characterized by hardening in compressive stiffness is found in the force-displacement relationships of the bearings. Numerical models were developed and calibrated with the experimental results. A parametric study of the numerical models shows that the hardening phenomenon is caused by sliding between the lead-steel interfaces and the lead-rubber interfaces. This sliding decreases the vertical stiffness and reduces the energy dissipation of the bearings. An increase in the coefficient of friction between the interfaces can increase the compressive stiffness of bearings with relatively high shape factors, but bearings with low shape factors are insensitive to the friction coefficients.