In this study, a rational and efficient optimal seismic design method for bridge system subjected to devastating earthquakes considering performance at ultimate state is proposed. The bridge system consists of superstructure, rubber bearings, RC piers and cast-in-place concrete pile foundation. In the proposed optimum design method, the optimum solutions for the heights of rubber bearings, cross-sectional dimensions and amount of steel reinforcements for RC piers and the detail of concrete pile foundation are determined for several allowable ductile factors of RC piers considering the constraints on the relative horizontal displacements of rubber bearings to the both bridge and transverse directions, the ductile factor of RC piers, and the constraint on the cast-in-place concrete pile foundation. From the practical design the heights of rubber bearings can take continuous values, but the other variables must be selected from discrete variable sets. Therefore, the construction cost minimization problem can be expressed as a mixed discrete-continuous problem. This problem is transformed into a convex approximation problem with the estimation formulae by using the experimental design, and the dynamic behaviors and those sensitivities are calculated analytically by using the estimation formulae without analyzing the structures. The optimum design problem is solved by a classical branch and bound method with dual algorithm. In the numerical design examples, it is emphasized that the optimum solutions can be obtained efficiently by using the experimental design. It is also demonstrated that the reductions of the heights of rubber bearings and mcross-sectional dimensions of RC piers can be observed by increasing the allowable ductility factor
