Remarkable damage to non-structural elements and sometimes to structural members was surveyed in buildings retrofitted with dissipative bracing (DB) technologies recently hit by moderate-to-medium amplitude earthquakes. Damage is a consequence of the delayed contribution of protective systems to the seismic response of the buildings, caused by too high activation forces of dissipaters. In view of this, a sizing procedure for DB systems incorporating fluid viscous (FV) spring-dampers is implemented in this study. The procedure provides a simplified version of a recently proposed energy-based design criterion, and an extension of it by including a pre-evaluation of the activation force of the FV devices with respect to the normative Serviceability Design Earthquake (SDE)-related seismic demand. The sizing procedure is applied to the retrofit design of a demonstrative case study, represented by a school built in Italy in the early 1980s. Noticeable seismic vulnerabilities of the above-ground steel structure of the building are assessed in current conditions, highlighting local unsafety conditions of the profiles constituting the reticular steel columns starting from the SDE. A retrofit intervention consisting in the installation of a DB system equipped with FV spring-dampers is presented for the steel structure, designed by applying the proposed sizing method. The final verification time-history analyses confirm the activation of the FV devices at the SDE, and the attainment of the targeted elastic structural response up to the Maximum Considered Earthquake normative level.
