SEISMIC RETROFITTING OF FRAMED STRUCTURES BY DAMPED BRACES CONSIDERING THE OUT-OF-PLANE RESPONSE OF MASONRY INFILLS

Design procedures of hysteretic damped braces (HYDBs) for the seismic retrofitting of buildings are generally focused on the in-plane (IP) behaviour of masonry infills (MIs) while neglecting their out-of-plane (OOP) response. Ex-post checking on effectiveness of the HYDBs against the OOP seismic collapse of MIs is generally made. To overcome this limitation, a displacement-based design procedure of HYDBs is updated by including the effects of the OOP nonlinear seismic response of MIs. Specifically, the retrofit design target displacement is derived from the capacity curve of the original infilled structure, in which IP and OOP contributions of MIs parallel and perpendicular to the direction of seismic loads are considered, respectively. To this end, a computer code for the nonlinear static analysis of spatial framed structures is enhanced introducing a five-element macro-model of a MI, composed of a central IP truss element and four diagonal OOP beams that govern its inelastic response. Modal and uniform load distributions are considered, proportional to the floor masses and the concentrated OOP mass of MIs. A five-storey reinforced concrete hospital building with regular plan and elevation is preliminarily designed in a medium-risk seismic zone. Bare and infilled structural models are examined, the latter including MIs in the interior bays of the perimeter frames. Then, retrofitting of the test structures in a high-risk seismic zone is carried out, incorporating diagonal steel braces with HYDs in the perimeter frames of the building plan. Finally, nonlinear dynamic analyses are performed before and after retrofit to delve further into the reliability of the HYDBs, also including nonlinear IP and OOP response of MIs.