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Buckling-restrained braced frames (BRBFs) are attracting interest worldwide by virtue of their extraordinary response reduction performance. The system relies strongly on the energy dissipation of buckling-restrained braces (BRBs). Therefore, the surrounding members (beams, columns, or gusset plates) must sustain the force imposed by a BRB. Notably, beams are not designed primarily to support the axial force. The sections consequently tend to become deep and slender to enhance the flexural strength and stiffness effectively, thereby leading to local buckling, which lessens the reaction force to the BRB. Moreover, the instability is activated by the compressive axial force transferred from the BRB. However, beams in the BRBFs generally become composite beams through an assemblage with a concrete slab by stud shear connectors. The concrete slab shifts the neutral axis of the beam, which imposes more strain on the bottom flange. The BRB axial force and composite effect therefore influence the steel beam buckling behavior. Eventually, the beam might be subjected to instability at a smaller-than-expected deformation. However, the rotation capacity of the composite beam in BRBFs has not been clarified yet. To address this concern, this study established a finite-element analysis (FEA) model referring to a former specimen and an experiment scheme. Additionally, a parametric study of the influential factors was conducted using the experimentally validated FEA model. This study examined a proposed evaluation equation of an apparent axial force issued from the composite effect based on the obtained results. An earlier evaluation index established for bare steel beams was ultimately modified to include the axial force influence. Eventually, the results demonstrated that the structural performance can be assessed appropriately using the existing evaluation equations proposed using a steel frame subassembly. [ABSTRACT FROM AUTHOR]