Nonlinear seismic behavior of an existing RC building retrofitted with BRBs

The paper presents the seismic performance assessment of a four-storey reinforced concrete (RC) existing framed structures designed for gravity loads only. The sample structure exhibits high seismic vulnerability, i.e. low lateral resistance and limited translation ductility. The structural assessment and retrofitting was deemed necessary due to a variation of the building occupancy. An extensive experimental test program was carried out to estimate the mechanical properties of the concrete and steel reinforcement in the existing RC building. Natural frequencies and damping ratios are experimentally investigated by operational modal analysis; the estimated modal properties were employed to calibrate the numerical finite element structural model. The rehabilitation scheme utilized for the sample existing is also presented herein and the retrofitting strategy emphasizes the use of Buckling Restrained Axial Dampers (BRADs) that are a type of buckling restrained braces (BRBs) in which the dampers are used in series with traditional capacity-designed steel braces. Such braces can be conveniently installed along the perimeter frames of the multi-storey buildings to lower the seismic demand on the existing structure and regularize its dynamic response. Local strengthening can, however, be necessary. The adopted design approach assumes that the global response of the inelastic framed structure is the sum of the elastic frame (primary system) and the system comprising perimeter diagonal braces (secondary system); the latter braces absorb and dissipate a large amount of hysteretic energy under earthquake ground motions. Comprehensive nonlinear static (pushover) and incremental dynamic (response history) analyses were carried out for both the asbuilt and retrofitted structures to investigate the efficiency of the adopted intervention strategy. A suite of codecompliant natural strong motion records was selected and employed to perform inelastic response history analyses. The outcomes of the inelastic analyses show that the existing structure exhibit extremely poor strength capacity and energy dissipation. It is also demonstrated that, under moderate and high magnitude earthquakes, the damage experienced by the retrofitted structural system is concentrated in the added dampers and the response of the existing RC framed structure is chiefly elastic.