Investigation of Near-Fault Earthquake Vertical Component Effect on the Seismic Behavior of Isolated Bridges

In bridge design codes such as AASHTO, there is not a direct and explicit relation for considering the effects of the earthquake vertical component in the design of elastomeric supports. In lieu, AASHTO recommends a 20% reduction or increase of the dead load amount in its seismic isolators design guide for considering the earthquake vertical component without taking into account the acceleration vertical component, soil type, distance to fault, etc. While the earthquake vertical component in near-fault areas are strong and significantly increase responses in the bridge. In this study, Sadr bridge located in northeast of Tehran in which low damping rubber seismic isolators are used, is investigated under the effect of the vertical component of some earthquake accelerograms with near-fault properties. Besides, the non-linear time history analysis is performed for each of them. Results show that the earthquake vertical component in near-fault areas will significantly increase the response of bridge members such as the increase of the maximum axial force of the bridge piers, deck acceleration, shear and bending moment in the deck cross section and is much higher than the 20% increase or decrease of the dead load given in the code. In order to investigate the seismic isolators behavior due to the increase of the axial compressive force under the effect of the earthquake vertical component, a simple model of isolators with the capability of considering the stiffness variations in the vertical and horizontal directions and performing the buckling stability analysis was prepared, and the isolators responses for three different lateral loadings including the application of monotonic transverse displacement, cyclic loading and earthquake loading were obtained. For all three lateral loadings, the results show that the increase of the compressive axial force decrease the isolators’ lateral stiffness and increasing the lateral displacement will increase the isolators’ axial deformation. Besides, the hysteresis curve of the isolators will experience a reduction in the stiffness and lateral strength. In the following, the buckling capacity amount obtained from the isolation model with the capability of buckling analysis is compared with the buckling capacity amount obtained from the conventional recommended relations. The results show that the conventional relations slightly overestimate the buckling capacity and the use of buckling analysis models is preferable. In order to decrease the effects of the earthquake vertical component in the isolated structures, three-dimensional isolation is recommended through simultaneous application of vertical and horizontal flexibility in the isolators. The results show that the decrease in bridge response such as the midspan vertical acceleration, deck shear and bending moment has a descending trend with the isolators’ vertical stiffness reduction.