A Study of the (Behavior of the) Isolated Bridges Using Elastomeric Isolaters Located Near the Faults

The present paper is a study of the (Behavior of the) elastomeric bearing isolated bridges located near the faults. The earthquakes near the faults have high frequency and narrow bandwidth. Such characteristics cause the strikingly high energy released due to production acceleration happening at the initial stage of the earthquakes to be imposed on the system. The present research undertakes to study the effects of the forces resulted by such type of earthquakes on the bridges. For this purpose, a three span highway bridge being of 60 meters length, for which were applied three types of lead rubber isolators in pursuant to the regulation envisaged in ASHTO standard, was designed. Using SAP2000, a 10.1 version was designed and the same was modeled in base-isolated and non-isolated modes. Considering the effect of three seismic recorders near the fault along longitudinal and latitudinal directions as well as in (YZ and XZ) directions, a nonlinear time history analysis was performed. The study of the behavior and load shift curves of the lead rubber isolators showed that initial strains and yield strength of such isolators cause significant reduction of base shear and bending moment in the bridge under study. Furthermore, simultaneous considering of the vertical and horizontal component of the earthquake in X and Y directions in the analysis showed increase compress or tension forces in column. These results show the shortcomings of the provisions recommended by, as instance, NEHRP and IBC about the isolated bridges in not considering the vertical effect of the earthquake. Such an effect should be taken into consideration as is the case with the non-isolated bridges.