Numerical investigation on performance of concrete-steel composite beams incorporating multi-transverse holes

The integration of web openings in reinforced concrete (RC) beams for building services severely compromises shear capacity by disrupting load paths and creating critical stress concentrations. While previous research has focused on external strengthening of traditional RC beams, a significant gap exists regarding the performance of composite beams with embedded steel sections near openings. This study introduces a novel strengthening strategy using internally built-up I-section and T-section steel elements as shear reinforcement. The methodology integrated experimental testing with a validated nonlinear 3D finite element model in ABAQUS to conduct an extensive parametric study. Key investigated parameters included I-section web thickness (0.1-2.5 mm) and flange width (16-64 mm), and T-section compression and tension flange widths (0-64 mm). The key findings were substantial: web openings caused a 14% reduction in strength but a 41% increase in ductility, indicating a brittle failure mode. The incorporation of steel sections effectively reversed this; the I-section (176 × 2 mm web, 40 × 2 mm flanges) in beam BI-W2.0 yielded a remarkable 53.4% increase in ultimate load capacity, outperforming the original solid beam. An optimal I-section web thickness of 2.0 mm was identified, with diminishing returns beyond this point. For T-sections, the tension flange width was far more influential than the compression flange on strength recovery. A fundamental finding was that even a simple steel web alone provided a 43% strength gain, highlighting the critical role of bridging the opening. The reinforcement trade-off was a controlled 16-22% reduction in deflection, enhancing stiffness while maintaining structural safety. The research provides optimized, practical design guidelines for utilizing built-up steel sections to ensure structural integrity in perforated beams, effectively bridging architectural functionality and engineering safety.