Existing reinforced concrete (RC) buildings designed according to outdated codes may lack sufficient strength, stiffness or ductility to meet the seismic performance criteria of current codes. To enhance the system stiffness and re-centering capability, an elastically designed supplementary steel frame (SF) is installed in parallel with the BRBs. Near full-scale cyclic tests are conducted on such retrofit schemes for performance evaluation. The retrofitted specimens showed stable hysteretic behavior up to the retrofit target story drift of 1/150 as proposed in the Japanese seismic regulations. Tests demonstrate that the proposed system is feasible and increases both strength, ductility, and damping to an adequate seismic performance level while the elastic steel frame is effective in providing post-yield stiffness and re-centering capability even when the RC frame is subjected to moderate inelasticity. Special emphasis is placed on the composite behavior of RC members and SF. A simplified composite interaction model is proposed and results from the developed model show good agreement with the experimental data. Ductility demands are shown to concentrate in the BRBs as per the design intent.