Seismic retrofit of existing frame buildings using fluid-viscous dampers according to different damper configurations

The paper presents a practical design procedure for the seismic retrofit of existing frame buildings using fluid-viscous dampers in two configurations: (i) inter-storey dissipative braces and (ii) dissipative links connecting the structure to an external stiff tower, called a Mass Proportional Damping (MPD) tower. The “Direct Five-Step Procedure,” originally developed for new buildings, is extended to existing ones by including hysteretic energy dissipation in ductile structural elements. For new buildings, viscous dampers are sized to keep elements elastic under rare earthquakes. However, existing buildings designed only for vertical loads typically cannot remain fully elastic even after damper installation. Hence, the procedure accepts limited plastic hinge formation in the structural elements, ensuring the ductility demand does not exceed the available (though limited) ductility capacity. The design starts from a target response reduction factor defined on the basis of the global capacity-to-demand ratio (e.g., the ratio of the maximum base shear the building can sustain to that of a new elastic design). It then determines mechanical properties for selecting suitable commercial non-linear viscous dampers. An application example involves an 11-storey reinforced concrete building in Bologna (built 1980-81) with monodirectional frames. Time-history analyses assess two damper configurations for several target viscous damping ratios: inter-storey and MPD tower. Results show the procedure reliably achieves performance targets, often conservatively, in reducing base shear and roof displacement. A performance comparison between the two damper arrangements is also provided.