This study investigates the flexural behavior of reinforced concrete beams encasing smooth and perforated cold-formed steel (CFS) sections. A comprehensive three-dimensional nonlinear finite element (FE) model was developed in ANSYS to simulate the load–deflection response and ultimate capacity of composite beams. Validation against experimental results showed that discrepancies in ultimate load and mid-span deflection were within ± 7%. A parametric study assessed the influence of CFS thickness, height, and configurations (back-to-back, front-to-front, U, and n-shapes). Results revealed that increasing CFS thickness and height significantly enhanced stiffness and ultimate load. While perforations slightly reduced the ultimate capacity in some cases, they significantly improved ductility through enhanced mechanical interlock. Finally, high-order polynomial empirical design models (Z-K and Z-M equations) were proposed to predict the ultimate load enhancement ratio (Z) with high correlation (R2 > 0.97). These formulas, validated for thickness ratios (K) up to 4% and height ratios (M) up to 0.9, provide a reliable and practical tool for the preliminary design and optimization of composite beams within these geometric limits.