In this paper, based on the finite element analysis model verified in previous studies, a new model of a buckling restrained brace reinforced with a steel plate was proposed. A design formula was proposed for the new model to dissipate energy without buckling the steel core under load protocol, and the performance of the model satisfying the design formula was evaluated by comparing it with the previous model through the results of hysteresis loop, bi-linear curve, cumulative energy dissipation capacity, and equivalent viscous damping.

This research presents that seismic performance of steel moment resisting frame building designed by past provision(UBC, Uniform Building Code) before and after retrofitted with BRB (Buckling-Restrained Brace) was evaluated using response modification factor（R-factor). In addition, the seismic performance of the retrofitted past building was compared with that specified in current provision. The past building considered two different connections: bilinear connection, which was used by structural engineer for building design, and brittle connection observed in past earthquakes. The nonlinear pushover analysis and time history analysis were performed for the analytical models considered in this study. The R-factor was calculated based on the analytical results. When comparing the R-factor of the current provision with the calculated R-factor, the results were different due to the hysteresis characteristics of the connection types. After retrofitted with BRBs, the past buildings with the bilinear connection were satisfied with the seismic performance of the current provision. However, the past buildings with the brittle connection was significantly different with the R-factor of the current provision.

The conventional brace system is generally accepted lateral load resisting system for steel structures due to efficient story drift control and economic feasibility by frame materials decrease. But the lateral stiffness of the brace decreases following buckling in this system and buckling causes unstable structures with strength deterioration hysteresis performance. Buckling restrained brace system that performs stable behavior after yielding of core element prevented from buckling by tube element is better than conventional brace system in point of earthquake energy absorbing capacity. In this study, the seismic performance of the multi-story steel frames applied for brace and buckling restrained brace is respectively analyzed, so that, the damage of two systems is quantitatively evaluated by analyzing energy absorption capacity.