Direct loss-based seismic retrofit of reinforced concrete frames

In earthquake-prone areas, structures not compliant with modern design codes significantly contribute to seismic risk. Therefore, risk mitigation strategies (e.g., seismic retrofit) should be employed to reduce the expected economic and human losses. This paper introduces a procedure for the design of retrofit solutions for reinforced concrete (RC) frame buildings to achieve - rather than be bounded by - a desired target level of earthquake-induced loss for a given site-specific seismic hazard profile. The presented methodology is “direct” because the designer and/or client can set a loss target in the first step of the procedure and no design iterations are virtually required. Direct loss-based seismic retrofit (DLBR) relies on a simplified loss assessment methodology enabled by a surrogate probabilistic seismic demand model. This defines the probability distribution of seismic deformation demand of single degree of freedom (SDoF) systems conditioned on different shaking intensity levels. The proposed design methodology enables designers to account for risk/loss-based considerations from the conceptual/preliminary design phase, thus facilitating the choice among different retrofit solutions. Starting from two under-designed case-study buildings, four illustrative applications of the procedure are provided. They involve considering different economic expected annual loss targets and different retrofit solutions involving the addition of RC walls and RC column jacketing. Benchmark loss estimates are calculated using non-linear time-history analyses of refined, multi-degree-of-freedom models showing satisfactory results: the simplified loss estimate introduces an overestimation maximum equal to 15.4% among the four illustrative applications.