Seismic Performance of Buckling-Restrained Braces in an Irregular Three-Dimensional Frame Structure with Predominantly Single-Bay Frame Lines

Although buckling-restrained braces (BRBs) have been widely investigated for regular multi-span frame systems, their application to irregular three-dimensional frame structures composed predominantly of single-bay frame lines remains insufficiently understood. This study systematically evaluates the seismic performance of BRBs in an irregular three-dimensional frame structure composed predominantly of single-bay frame lines. A typical school building in Bao'an District, Shenzhen, characterized by plan and vertical irregularities, discontinuous floor slabs, and pronounced translation-torsion coupling, is selected as a case study. A refined finite element model is developed and subjected to multiple representative ground motions with distinct characteristics. The structural responses, including story shear, story displacement, interstory drift ratio, damage distribution, and energy dissipation, are comparatively examined to clarify the seismic mitigation mechanism of BRBs in irregular three-dimensional frame systems dominated by single-bay frame lines. In addition, several BRB layout schemes are evaluated. The results show that BRB installation markedly reduces the overall interstory drift ratios, with maximum mitigation rates of 12.59% in the X-direction and 11.67% in the Y-direction under maximum considered earthquake (MCE) excitation. A supplementary elastic torsional assessment shows that the torsional-to-translational period ratio decreases from 0.863 to 0.782, while the maximum floor and interstory torsional displacement ratios decrease modestly, by up to 0.7% and 3.1%, respectively. Under increasing seismic intensity, the BRBs exhibit a staged response mechanism, transitioning from stiffness contribution to limited energy dissipation and finally to pronounced energy dissipation, thereby improving the global energy-dissipation capacity of the structure. Damage analysis further indicates that BRBs reduce the spatial extent of concrete compression damage and limit high reinforcement strain demands, particularly under DBE and MCE excitation. The comparative assessment of the examined layout schemes shows that lower-capacity BRBs activate earlier and provide greater drift reduction within the investigated, non-normalized parameter range; this result is interpreted as case-specific rather than as evidence of universal superiority. These findings elucidate the seismic mitigation mechanism and comparative layout behavior of BRBs in irregular three-dimensional frame structures with predominantly single-bay frame lines, providing a theoretical basis and practical guidance for the seismic design and retrofit of complex irregular structures.