Equivalent linear analysis of seismic-isolated bridges subjected to near-fault ground motions with forward rupture directivity effect

This study encompasses the assessment of the equivalent linear (EL) analysis procedure for the design of seismic isolated bridges (SIB) subjected to near-fault ground motions with forward rupture directivity effect. The assessment procedure involves the comparison of the seismic response quantities obtained from EL analyses with those obtained from nonlinear time history (NLTH) analyses. The effect of several isolator and near-fault ground motion properties are considered in the assessment of the EL analysis procedure both individually and in the form of three dimensionless parameters. Regression analyses of the acquired data are then performed to appraise the effect of these parameters on the accuracy of the EL analysis results. It is found that the accuracy of the EL analysis results is affected by the magnitude of the near-fault ground motion, distance of the structure from the fault, post-elastic stiffness and yield strength of the isolator. Next it is demonstrated that the effective damping (ED) equation currently used in the design of SIBs must incorporate these parameters for a more accurate estimation of seismic response quantities. Finally, a new ED equation that includes such parameters is formulated and found to improve the accuracy of the EL analyses.