Improvement of the seismic performance of r.c. framed buildings with soft storeys by hysteretic damped braces

A suitable insertion of steel braces equipped with dampers can be very effective in retrofitting of r.c. framed buildings in order to attain a designated seismic performance for a given intensity of the ground motion. In particular, this represents an easy and less expensive solution, in comparison with other retrofitting techniques, to overcome the vulnerability problems resulting from an irregular layout of masonry infill walls as in the case of soft storeys. Currently a wide variety of energy dissipating devices (e.g. friction, metallic, viscous or viscoelastic dampers) are available. In this paper hysteretic (metallic) dampers are considered, but, with a suitable modification, the results can be extended to other kinds of damping devices. Current seismic codes allow for the use of the above devices but few codes provide simplified design. A Displacement-Based Design (DBD) procedure, in which the design starts from a target deformation of an equivalent elastic linear system with effective properties (i.e. secant stiffness and equivalent viscous damping), is proposed in the present work for the seismic retrofitting of a r.c. framed structure with soft storeys. Hysteretic damping due to nonlinear behaviour of the masonry infills (MIs) is taken into account. The criteria for proportioning the hysteretic damped braces (HYDBs) are aimed to obtain a damped braced structure which can be considered globally regular with regard to stiffness. In detail, the stiffness distribution of HYDBs is selected assuming a same value of the drift ratio at every storey of the irregularly infilled building retrofitted by insertion of damped braces; moreover, the strength distribution of the HYDBs is assumed so that their activation tends to occur at every storey simultaneously, before reaching the yielding of the framed structure and/or a preset damage level of the infill walls. To check the effectiveness and reliability of the criteria proposed for proportioning the HYDBs, a numerical investigation is carried out with reference to a six-storey r.c. framed building, originally designed in compliance with a former Italian code [DM 1996] for a medium-risk zone. It is supposed that, after a change in use of the building from residential to office removing the masonry infills of the first storey, the resulting soft-storey building has to be retrofitted by inserting of HYDBs to attain a performance level imposed by the current Italian code [NTC 2018] in a high-risk zone. Nonlinear dynamic analyses of the bare framed building, infilled framed building with first soft-storey and infilled framed buildings with first soft-storey retrofitted by HYDBs having different properties are carried out for a set of artificially generated ground motions whose response spectra match that adopted by NTC 2018 for the life-safety performance level. To this end, r.c. frame members are idealized by a two-component model, assuming a bilinear moment-curvature law and considering the effect of the axial load in the columns. The response of a HYDB is idealized by a bilinear law, preventing buckling. A simplified diagonal pin-jointed strut model reacting only in compression takes into account the in-plane failure modes that can occur in an infill panel (i.e., compression at the center, compression at the corners, shear sliding and diagonal tension). More precisely, the response of a diagonal strut is simulated by a trilinear law with stiffness degradation and suitable hysteretic properties. The effectiveness of the criteria for proportioning the HYDBs, on which the proposed DBD design procedure is based, is checked for different properties of the HYDBs.