EFFECTIVE STIFFNESS AND EQUIVALENT DAMPING OF BASE-ISOLATED BRIDGES

In this paper, the guide specifications for bridge base isolation publ ished in 1991 by the American Association of State Highway Transportat ion Officials (AASHTO) are briefly summarized and discussed. The effec tive stiffness and equivalent damping ratio given in the AASHTO isolat ion specifications are evaluated. Based on this study, it is concluded that the equivalent damping ratio determined using the AASHTO procedu re may decrease with respect to the increase of the inelastic deformat ion of lead-rubber bearings. In addition, the equivalent damping ratio s calculated for a single-degrees-of-freedom system subjected to a gen erated ASSHTO design earthquake may be larger than 30% for a wide fund amental 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 analy sis could be a dilemma for the current bridge engineering design pract ice. An an alternate approach, an empirical method is proposed for the determination of the effective period shift and equivalent damping ra tio.