Seismic retrofitting with buckling restrained braces: Application to an existing non-ductile RC framed building

• Published in: Soil dynamics and earthquake engineering (1984) 2010-11, Vol.30 (11), p.1279-1297
• Main Authors: Di Sarno, L., Manfredi, G.
• Format: Article
• Language: English
• Subjects: Ductility; Dynamical systems; Earth sciences; Earth, ocean, space; Earthquake construction; Earthquake design; Earthquakes, seismology; Engineering and environment geology. Geothermics; Engineering geology; Exact sciences and technology; Internal geophysics; Limit states; Natural hazards: prediction, damages, etc; Nonlinear dynamics; Reinforced concrete; Retrofitting; Seismic phenomena
• ISSN: 0267-7261; 1879-341X
• DOI: 10.1016/j.soildyn.2010.06.001

This paper assesses the seismic performance of typical reinforced concrete (RC) existing framed structures designed for gravity loads only. The sample two-storey structural system exhibits high vulnerability, i.e. low lateral resistance and limited translation ductility; hence an effective strategy scheme for seismic retrofitting was deemed necessary. Such a scheme comprises buckling restrained braces (BRBs) placed along the perimeter frames of the multi-storey building. 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 dynamic (response history) analyses were carried out for both the as-built and retrofitted structures to investigate the efficiency of the adopted intervention strategy. A set of seven code-compliant natural earthquake records was selected and employed to perform inelastic response history analyses at serviceability (operational and damageability limit states, OLS and DLS) and ultimate limit states (life safety and collapse prevention limit states, LSLS and CPLS). Both global and local lateral displacements are notably reduced after the seismic retrofit of the existing system. In the as-built structure, the damage is primarily concentrated at the second floor (storey mechanism); the computed interstorey drifts are 2.43% at CPLS and 1.92% at LSLS for modal distribution of lateral forces. Conversely, for the retrofitted system, the estimated values of interstorey drifts ( d/ h) are halved; the maximum d/ h are 0.84% at CPLS (along the Y-direction) and 0.65% at LSLS (yet along the Y-direction). The values of the global overstrength Ω vary between 2.14 and 2.54 for the retrofitted structure; similarly, the translation ductility μ Δ -values range between 2.07 and 2.36. The response factor ( R- or q-factor) is on average equal to 5.0. It is also found that, for the braced frame, under moderate-to-high magnitude earthquakes, the average period elongation is about 30%, while for the existing building the elongation is negligible (lower than 5%). The inelastic response of the existing structure is extremely limited. Conversely, BRBs are effective to enhance the ductility and energy dissipation of the sample as-built structural system.