Evaluation of Seismic Performance Indices of Rubber-Friction Bearings in an Isolated Highway Bridge under Near Fault Ground Motion

The past near fault seismic events demonstrate the important effect of vertical components on response of the highway bridges. Due to the high seismicity of the country and number of active faults near the major cities, the probability of occurrence of near-fault earthquakes with significant vertical component in such area is high. Most of the design codes and regulations neglect the vertical component of the event in base isolated structures or over-simplify it. In this research, to determine the effects of the vertical component of earthquakes on such a structure, a bridge model is analysed and designed using rubber-sliding bearings in series known as EDF isolators. The model was analysed once using two horizontal components and again using two horizontal and vertical components. The modelling and analysis was performed in Open Sees Software. Effects of variation of sliding velocity and input vertical component were included in coefficient of friction in sliding part of the model. Nonlinear dynamic time history analysis was carried out using near-fault records of 14 seismic events. The deck midspan moment and deformation, axial force of the piers, and shear force and deformation of isolators have been calculated. It is shown that shear force, deformation and axial force of isolators increase when the vertical component of the ground motion is included. The results also show that the mentioned responses are acceptably approximated by linear regression with spectral value of the excitation at the frequency corresponding to the mode effective on vertical response of the structure.