Analysis of effect of vertical ground shaking in bridges isolated with spherical sliding bearings

Abstract

Studies have shown that in structures isolated with spherical sliding bearings, the seismic base shear increases when the vertical component of ground shaking is explicitly considered, since the dynamic variation of bearing axial force is passed on to the base shear due to the friction mechanism. In this paper, the effect of vertical shaking is systematically investigated in multi-span concrete box-girder highway bridges isolated with triple pendulum bearings. Grounded in the theory of bearing mechanics, a simplified method to predict the amplification of base shear with vertical ground shaking intensity in such bridges is developed and evaluated. The simplified approach is potentially applicable for bridge design based on equivalent static analysis. At low to moderate vertical intensities, the simplified method can capture the observed variation in the base-shear coefficient with isolation system parameters. The differences in base-shear amplification for bridge superstructure parameter variations are insignificant, and thus the method is widely applicable. At high intensity vertical ground shaking, the base shear coefficient can be affected by an uplift and impact phenomenon. Thus, application of the simplified method should be limited to motions with peak vertical acceleration up to 1 g; simulation with 3D motions is recommended for intensities beyond 1 g.