Nonlinear behavior of fixed-end RC beams with circular openings based on experimental and finite element analysis

Reinforced concrete (RC) beams often require service openings due to architectural and utility demands. However, such openings lead to stress concentrations caused by the reduction in the concrete cross-section and the absence of transverse reinforcement, which can adversely affect the structural performance of beams and lead to premature failures. Despite extensive research on RC beams with openings, most studies have been conducted under simply supported conditions, which do not accurately represent real structural behavior. This study aims to investigate the structural behavior of RC beams with circular mid-span openings under fixed-end (encastre) boundary conditions through both experimental testing and numerical analysis. Openings were introduced in the bending region at 20%, 40%, and 60% of the beam height. Full-scale beam specimens were tested under four-point bending, and detailed nonlinear finite element models were developed using ABAQUS software. The results show that openings exceeding 40% of the beam height significantly reduce ductility and lead to sudden load drops. Additionally, when openings exceed 20%, the remaining concrete section around the opening plays a critical role in load transfer and bond performance. The numerical results exhibit strong agreement with the experimental findings, demonstrating that the proposed modeling approach can reliably capture the structural behavior of beams with openings. The findings provide important insights into the realistic behavior of fixed-end RC beams and contribute to the safe and efficient design of structural members with service openings.