EQUIVALENT ELASTIC SEISMIC ANALYSIS OF BASE-ISOLATED BRIDGES WITH LEAD-RUBBER BEARINGS

The specifications for the seismic analysis of base-isolated bridges have recently been provided by the American Association of State Highway Transportation Officials (AASHTO). The specified effective stiffness and equivalent damping ratio for an equivalent elastic system to the base-isolated bridge are evaluated. Based on this study it is found that these parameters of the equivalent elastic system are unrealistically represented. An alternate approach derived from an empirical method is then proposed for the determination of the effective period shift and equivalent damping ratio. The proposed empirical method is validated using an inelastic seismic analysis method. In addition, an analysis procedure incorporated with the empirical method is formulated to determine the maximum inelastic responses of the base-isolated bridges with lead-rubber bearings subjected to the AASHTO design earthquake and recorded ground motions. The effects of the stiffness (or flexibility) of bridge abutments and column bents on the seismic responses of base-isolated bridges are considered in the formulation. The results obtained from the proposed analysis procedure are compared with those determined from an inelastic seismic analysis method.