Displacement-based design procedure for the seismic retrofit of existing buildings with self-centering dissipative braces

Existing framed buildings, especially those designed according to outdated seismic codes, often offer inadequate seismic protection levels due to material degradation, low amount of reinforcement and poor construction details. Among the possible retrofit strategies, elasto-plastic dissipative braces (EPDBs) can be installed to provide supplemental stiffness and energy dissipation in the main resisting frame. Since they are based on the plastic deformation of sacrificial steel components, EPDBs are inherently affected by scarce recentering capability, i.e., presence of residual deformation at the end of the mainshock that could impair their effectiveness for aftershock events. As demonstrated in recent seismic sequences, there might be not enough time to restore (e.g., though hydraulic actuators and a reaction frame) the undeformed configuration of the structure before a subsequent shock. This issue can be overcome through self-centering dissipative braces (SCDBs), i.e., braces equipped with dampers characterized by a "flag-shaped" hysteretic behavior, which inherently facilitates the immediate re-centering of the frame. Despite this evident benefit, there is still lack of practical design procedures for the seismic retrofit of buildings through SCDBs. To fill this gap, a direct displacement-based design procedure for SCDBs tailored to building frames is here conceived and illustrated. The proposed method aims at achieving simultaneously two performance requirements: (i) a target maximum roof displacement under the ultimate-limit-state seismic event; (ii) the absence of significant residual inter-storey drifts at the end of the shaking. The implementation of the procedure is described by two applications to a steel and to a reinforced concrete case-study building. Nonlinear time-history analyses are carried out to prove the effectiveness of the proposed method, along with a comparison of the seismic performance offered by SCDBs versus traditional EPDBs implemented in the same case-study buildings.