1 DISPLACEMENT-BASED SEISMIC DESIGN OF DAMPED BRACES FOR RETROFITTING IN-ELEVATION IRREGULAR FRAMED STRUCTURES Fabio MAZZA 1 , Mirko MAZZA 2 and Alfonso VULCANO 3 ABSTRACT A Displacement-Based Design procedure is proposed for proportioning hysteretic damped braces (HYDBs) in order to attain, for a specific level of seismic intensity, a designated performance level of a reinforced concrete (r.c.) in-elevation irregular framed building which has to be retrofitted. To check the effectiveness and reliability of the design procedure, a numerical investigation is carried out with reference to a six-storey r.c. framed building, which, primarily designed according to an old Italian seismic code (1996) for a medium-risk zone, has to be retrofitted by insertion of HYDBs for attaining performance levels imposed by the current Italian code (NTC08) in a high-risk zone. To simulate a vertical irregularity, a change of use of the first two floors, from a residential use into office, is also supposed; moreover, masonry infill walls, regularly distributed along the perimeter, are substituted with glass windows on these floors. Nonlinear dynamic analyses of unbraced (UF), infilled (IF) and damped braced infilled (DBIF) frames are carried out considering sets of artificially generated and real ground motions, whose response spectra match those adopted by NTC08 for different performance levels. To this end, r.c. frame members are idealized by a two-component model, assuming a bilinear moment-curvature law whose ultimate bending moment depends on the axial load, while the response of an HYDB is idealized by a bilinear law, providing that the buckling be prevented. Finally, masonry infills are represented as equivalent diagonal struts, only in compression, with an elastic-brittle linear law. INTRODUCTION Vertical irregularities in buildings are now having a lot of interest in seismic research area, because many structures are designed with significant change in stiffness, strength and mass distribution in- elevation due to architectural views and functional reasons. Specifically, low- and medium-rise reinforced concrete (r.c.) framed buildings are often built with irregularities in elevation due to soft- storeys, unsymmetrical layout of infill walls, setbacks. However, recent earthquakes confirmed that r.c. buildings having such irregularities were often severely damaged. Among the techniques of passive control that have had real application for the seismic retrofitting of framed structures, in the last two decades that based on the dissipation of a large portion of the energy transmitted by the earthquake to the structure can be considered very effective. Currently a wide variety of energy 1 Researcher, Dipartimento di Ingegneria Civile, Università della Calabria, Rende (Cosenza), Italy, fabio.mazza@unical.it 2 Research fellow, Dipartimento di Ingegneria Civile, Università della Calabria, Rende (Cosenza), Italy, mirko. mazza@unical.it 3 Full professor, Dipartimento di Ingegneria Civile, Università della Calabria, Rende (Cosenza), Italy, alfonso.vulcano@unical.it