Compared to traditional cold-formed thin-walled steel (CFS) opening section, CFS beams with built-up double-limb ∑-shaped sections (CFSBDS) exhibit significantly greater torsional rigidity. The addition of web and flange stiffeners in CFS members not only increases the section moment of inertia but also mitigates web buckling issues. Consequently, incorporating stiffeners effectively enhances the ultimate bending moment capacity of bending members. The flexural behavior of CFSBDS was investigated using numerical analysis methods. Two types of built up section beams were considered: CFSBDS flanges with and without stiffeners. Finite element (FE) models were developed using ABAQUS software, and their accuracy was validated by comparing them with experimental results reported in the literature, considering failure modes, moment capacities, and moment vs. deflection curves. To explore various combinations of web stiffener depth, flange stiffener width, and thickness of the built-up closed section, a numerical parametric study was conducted using the validated FE models. The FE analysis results indicate that increasing the ratio of web stiffener depth-to-beam width from 0 to 0.5 leads to a 27% increase in ultimate strength. Additionally, raising the ratio of flange stiffener depth-to-beam width from 0 to 0.33 results in a 45% enhancement in ultimate strength. There is no significant variation observed in the flexural capacity of built-up closed section beams due to changes in the width of flange stiffeners. The suitability and effectiveness of design methods outlined in Chinese GB/T50018-2025 and AISI S100-16 for calculating bending moment values of CFSBDS were evaluated, revealing that these prediction methods tend to be conservative compared to FE analysis.
