Flexural design of lightweight concrete-infilled hollow flange CFS beams with web openings

Concrete-filled hollow flange cold-formed steel (CF-HFCFS) beams have recently gained attention in the construction industry due to their structural benefits and improved performance by reducing buckling instability. However, current literature does not adequately address the challenges associated with CF-HFCFS beams containing web openings, which are essential for service integration. Therefore, this study aims to investigate, for the first time, the flexural behaviour of lightweight concrete (LWC) infilled CF-HFCFS beams with web openings. Experimentally validated three-dimensional finite element (FE) models are developed for a detailed analytical investigation. The study evaluates the influence of key parameters such as, dimensions and strength of CF-HFCFS beams and web opening sizes on the ultimate moment capacity. A total of 120 FE models were developed and analysed, comprising 24 without openings and 96 with openings. Results show that CF-HFCFS beams with large web openings experience a capacity reduction of up to 13.8% compared to those without openings. However, when the web opening is below 30% of the section height, the reduction in capacity is generally negligible, particularly in smaller sections. Additionally, beams with higher yield strength demonstrate improved resistance to capacity loss due to web openings. Based on the parametric study results, a simplified design equation is proposed to predict the ultimate moment capacity (M_ult) of CF-HFCFS beams with web openings. The proposed equation demonstrates high accuracy, with a mean value of 1.00, a coefficient of variation (COV) of 0.036, and a reliability index of 0.917, indicating excellent predictive performance. Therefore, this can be implemented in in modular construction systems, service-integrated floor systems and lightweight composite beams in low-to-mid-rise buildings.