Comparison of Seismic Behavior of Buckling-restrained Braces and Yielding Brace System in Irregular and Regular Steel Frames under Mainshock and Mainshock-Aftershock

Due to low stiffness of braces after yielding, the structures with buckling-restrained braces (BRBs) experience high residual drifts during an earthquake, which can be intensified by aftershocks and causes considerable damages to structures. Also, due to poor distribution of stiffness, this problem is exacerbated for irregular structures. Recently, the yielding brace system (YBS) has been introduced; which is an alternative to BRBFs to solve this problem. YBS has a secondary stiffening part in its hysteresis behavior which can prevent excessive deformations. Therefore, structures with YBS are expected to show a better peroformance in seismic sequences compared to BRBs.  However, the seismic behavior of the YBS system in irregular structures has not been studied so far. On this basis, this paper investigates the seismic behavior of frames with BRB and YBS in regular and irregular structures under seismic sequences. Twenty four 4-, 8-, and 12-story frames with these two systems were designed and evaluated. First, a nonlinear dynamic analysis was conducted on the frames under mainshocks, and the maximum interstory drift and the residual interstory drift of the frames were compared. Then, using incremental dynamic analysis and fragility curves, the behavior of the frames under mainshocks was investigated. Afterwards, using incremental dynamic analysis and fragility curves, the behavior of the structures under mainshock-aftershock in three performance levels was aledso investigated. The results showed that YBS bracing, especially in low rise structures, leads to far lower maximum drifts and maximum residual drifts than BRB braces, which can reduce the probability of the occurrence of soft stories in structures.