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This paper presents a new vibration control method for long-span bridges using a complex damper system. The complex damper system has a simple mechanical configuration with oil and elastoplastic dampers, which are velocity and displacement dependent in absorbing vibration energy, so as to produce various damping forces against the external forces according to its parameter setting. The oil damper dissipates vibration energy for relatively frequent and small vibration amplitudes as in small-to-moderate earthquakes, whereas the elastoplastic damper system works for rare and large amplitude vibrations such as strong seismic events. In order to obtain the optimal performance of the damper, the preference-based optimization technique is applied. A numerical model is established for the complex damper system, and the response characteristics and effectiveness of the proposed system are presented through numerical simulations. Numerical results show that the proposed complex damper system can significantly improve the seismic performance of long-span bridge structures with the combined damping mechanism that is much more effective than the single conventional passive damper system.