EFFECTS OF MASONRY INFILLS ON THE NONLINEAR SEISMIC ANALYSIS OF RC BUILDINGS WITH DAMPED BRACES

Recent design procedures of hysteretic damped braces (HYDBs) for the seismic retrofitting of buildings have faced the problem of the selection of proper performance levels in order to substantially limit structural as well as non-structural damage. For the latter, attention is generally focused on the in-plane (IP) behaviour of masonry infills (MIs), while their out-of-plane (OOP) response has been overlooked. Ex-post checking on effectiveness of the HYDBs against the OOP seismic collapse of MIs is generall y made. To overcome this current limitation, this work proposes a displacement-based design procedure of HYDBs, updated to include the effects of the OOP nonlinear seismic response of MIs. A five-storey reinforced concrete (RC) hospital building infilled with MIs placed in the interior bays of the perimeter frames is preliminarily designed in a medium-risk seismic zone. Then, retrofitting of the test structure in a high-risk seismic zone is carried out, incorporating diagonal steel braces with HYDs in the perimeter frames of the building plan. Specifically, the retrofit 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, respectively, are considered. Nonlinear static analyses are carried out assuming linear and uniform vertical distributions of the seismic load, proportional to the floor masses and the concentrated OOP mass of MIs. A lumped plasticity model is adopted for RC frame members of the superstructure, based on a piecewise linearisation of the axial load-biaxial bending moment elastic domain of the cross-sections; a five-element macro-model of an MI is considered, composed of a central IP truss element and four diagonal OOP beams that govern its inelastic response. Then, HYDBs are modelled with truss elements characterised by a bilinear force-displacement law, without considering the flexibility of the supports. Real accelerograms, matching, on average, the design response spectra for serviceability and ultimate performance levels, are employed to carry out nonlinear dynamic analyses before and after retrofitting to ascertain the reliability of the HYDBs.