The use of steel braces equipped with dissipative devices (e.g. hysteretic dampers, HYDs) proved effective for seismic retrofitting of framed buildings. In a previous work (Mazza and Vulcano 2008) an effective procedure was proposed for the design of hysteretic damped braces (HYDBs) following a Displacement-Based Design approach, which combines pushover analysis of the actual structure with response spectrum analysis of an equivalent SDOF system. However, further studies are needed to validate the design procedure.

A crucial point is the selection of a suitable value of the equivalent viscous damping of the damped-braced structure, x_eq (see Mazza and Vulcano 2010 for its calibration). Different assumptions regarding the distribution law of stiffness and strength of the HYDBs should be examined comparing actual and target values of performance parameters. In particular, the stiffness distribution is assumed by either a proportional stiffness criterion (i.e., the same ratio between the lateral stiffness of the HYDBs and that of the unbraced frame is assumed at each storey) or a constant drift criterion (i.e., an inverted-triangular first vibration mode of the damped braced frame is assumed). Moreover, the distribution law of the HYDB strength is assumed either similar to that of the elastic force induced by the lateral seismic loads (aiming to dissipate energy at all the storeys) or according to a constant shear-ratio criterion (i.e., the same ratio between the ultimate and design shears of the damped braced frame is assumed at each storey).

To check the reliability of the design procedure, a numerical investigation is carried out considering 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 new Italian code (NTC08) in a high-risk zone. Then, the nonlinear dynamic responses of unbraced and damped-braced structures are compared under sets of real and artificially generated ground motions, whose response spectra match those adopted by NTC08 for different performance levels. For this purpose, the r.c. frame members are idealized by a two-component model assuming a bilinear moment-curvature law or, alternatively, a degrading moment-curvature law, to evaluate the influence of the beam model on the design-procedure reliability (e.g. with regard to the selection of the x_eq value). The effect of the axial load on the ultimate bending moment of columns (M-N interaction) is also considered. Finally, the behaviour of an HYDB is idealized by a bilinear law, providing that the buckling be prevented.

The results allow to check the reliability of the design procedure comparing the values obtained for many parameters (i.e., storey drifts, ductility demand for frame members and dampers) with the corresponding target values.