Loss-based seismic retrofitting design for existing RC structures through loss-displacement demand transition

In recent years, seismic retrofitting of existing buildings has become a primary approach for urban renewal. In retrofitting design, effectively incorporating economic considerations and the initial damage state of existing structures remains a significant challenge. To address this issue, this study proposes a seismic retrofitting design method for existing reinforced concrete structures based on loss-displacement demand transition, in which initial damage is explicitly incorporated into the design process. The method consists of three parts: the conversion from loss demand to displacement demand, the conversion from displacement demand to force demand, and the customized design of the retrofitted structure. The proposed method is applied to two reinforced concrete frames with five and ten stories under a moderate damage state. Two retrofitting strategies, namely column section enlargement and the addition of buckling-restrained braces, are considered. The results indicate that initial damage reduces the seismic capacity of the structure and leads to higher expected annual losses. The retrofitted structures designed using the proposed method achieve expected annual losses below the target limits at all limit states, thereby satisfying design requirements and validating the effectiveness of the method. Moreover, the expected annual losses at lower limit states are greater than those at higher limit states, indicating that improving structural performance at lower limit states requires higher economic investment. The present study provides valuable guidance for developing rational and economically informed retrofitting strategies for existing reinforced concrete structures.