Conventional composite beam construction suffers from deficiencies and challenges associated with the interaction between the steel section and the concrete parts and their connection where shear studs and connectors were used. Therefore, there was a necessity to find simple, efficient, and feasible construction methods that satisfy the target requirements and get rid of the shear connection components. This study introduces the flexural behavior of composite beams partially encased with cellular steel sections where the main flexural reinforcement is positioned using the available openings. The nonlinear finite element (NLFEA) simulation was adopted in this study after well validation against experimental tests from the literature. The effect of the concrete compressive strength (20, 35, and 50) MPa was investigated along with the effect of the composite beam width (150, 225, and 300) mm while the depth was kept constant at 300 mm. Results were linked together using the relative ratio between the beam’s dimension and the constitutive material properties, providing a wide vision on the improved composite system suitability and abilities. Therefore, outcomes are of major importance for engineers in the industry and academic fields.