Effective Stiffness and Equivalent Damping of Base‐Isolated Bridges

Abstract

In this paper, the guide specifications for bridge base isolation published in 1991 by the American Association of State Highway Transportation Officials (AASHTO) are briefly summarized and discussed. The effective stiffness and equivalent damping ratio given in the AASHTO isolation specifications are evaluated. Based on this study, it is concluded that the equivalent damping ratio determined using the AASHTO procedure may decrease with respect to the increase of the inelastic deformation of lead‐rubber bearings. In addition, the equivalent damping ratios calculated for a single‐degree‐of‐freedom system subjected to a generated AASHTO design earthquake may be larger than 30% for a wide fundamental period range. As a result, a three‐dimensional inelastic time‐history analysis as required by the AASHTO provisions may be necessary. However, to require a three‐dimensional inelastic time‐history analysis could be a dilemma for the current bridge engineering design practice. As an alternate approach, an empirical method is proposed for the determination of the effective period shift and equivalent damping ratio.