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Buckling-restrained braced (BRB) frames are steadily replacing concentrically braced frames because they can yield without buckling when subjected to both tension and compression loads. Though BRB frames are being widely used in construction industry especially for building structures in high seismicity areas such as Iran, it is shown that at large strains, a considerable amount of permanent deformation is generated at the support connector between the brace and the frame. This drawback can be overcome by providing recentering capabilities to the braced frame system. By applying the concept of a recentering system to the design of BRB frames, we used braced frames that incorporate BRBs with superelastic shape memory alloy (SMA). Also, the use of SMA in the bracing system causes damping and reduction of residual deformation. BRBs are considered as lateral load-bearing systems due to their non-buckling in compression. But these braces also have disadvantages. Among these disadvantages is the creation of permanent deformation in the structure after the end of loading and also the costly replacement of these members after the failure and current of the steel core of these braces. Therefore, the application of SMA in BRB systems, given the specific characteristics of these alloys, can be an effective step in improving seismic responses. However, recent studies have shown that BRB frames are susceptible to residual deformations during earthquakes which makes them vulnerable to aftershock events. The effectiveness of SMA-BRBs in controlling the seismic response of a structure largely depends on the relative strength and stiffness of SMA bars and BRB core plates.  The aim of the current study is to investigate the aftershock collapse capacity of BRB frames with and without SMAs. In this paper, seismic behavior of frames with BRB’s and the effect of utilizing SMAs were studied. The selected models are three frames with 3, 6 and 9 story, which in different openings have BRBs in two states with and without applying shape memory alloys. These prototypes were modeled in OpenSees under nonlinear dynamic time history analyses. The results comparison was performed under three records including Main Shock-Aftershock Ground Motions. The results include comparing the seismic responses of structures with and without applying SMAs including maximum roof displacement, maximum interstory drift, maximum base shear, maximum acceleration of roof and hysteresis curves in structures with BRBs and SMAs rods. The results showed that by employing SMAs rods, seismic responses including roof displacement, interstory drift and base shear have been significantly reduced. By reviewing the results, it is clear that improvements in the 6 and 9-story frames compared to the 3- story frame is more tangible. Also, the analysis results showed by equipping the frames with SMAs, the energy dissipation concentration pattern has been changed. In the case of frames without SMAs, due to the greater absorption of lateral force (larger base shear), the amount of energy dissipation of BRBs was higher. In these frames, energy loss was in the first stories and in frames with SMAs, the energy dissipation concentration was in the final stories. Using a SMA in these frames can reduce the cost of restoring and recovering of damaged systems and make more resilience building system.

Keywords: Buckling Restrained Braces (BRB), Shape Memory alloys (SMA), Mainshock-Aftershock Ground Motions, OpenSees Software, Nonlinear Dynamic Time History Analysis.

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