A parametric analysis of typical base isolated bridges was conducted. The bridges were located in different soil types and were subjected to three different earthquakes (recorded on soft and medium soils). The work had two main objectives: to asses the effects of the nonlinear behavior of the isolation pads of the bridges on the seismic responses (accelerations, displacements, and pier seismic forces), and to study combined effects of base isolation and inertial interaction due to the presence of flexible foundations. The analytical models used for the study were selected on the basis of initial evaluation of different models proposed in the literature to represent a bridge structure and to evaluate the isolation pads’ nonlinear behavior. The bridges studied were developed with a three-dimensional model. After completing the studies, 2 degree of freedom models were used to investigate more general trends of the inertial SSI effects for the base isolated bridges. The results of the work show the efficiency of base isolation pads in improving the seismic performance of bridges in most cases. They suggest that the inertial SSI effects will not be generally important for bridge foundations designed with a factor of safety of 3, with more than one line of piles in either direction since they will be very stiff foundations. But they also showed that for slender piers it is important to carefully evaluate the translations on top of the piers due to the rocking effects of the foundation.