Structural behavior of reinforced concrete beams with longitudinal voids incorporating embedded steel tubes

The integration of utility services in modern buildings often requires longitudinal openings within reinforced concrete (RC) beams. However, these voids, particularly in the compression zone, significantly reduce structural capacity and ductility by decreasing the effective concrete area and inducing stress concentrations. This study investigates the structural behavior of RC beams with longitudinal voids incorporating embedded steel tubes as internal composite reinforcement. An experimental program comprising eleven RC beams tested under four-point bending was conducted and validated using three-dimensional nonlinear finite element analysis. The study examined the effects of void geometry, tube shape, tube orientation, and reinforcement ratio. Results showed that introducing an unreinforced void reduced the ultimate load capacity by 27.5% and caused substantial losses in stiffness and energy absorption compared to the solid beam. In contrast, embedded steel tubes significantly enhanced structural performance through composite action. The best rectangular double-tube configuration increased the ultimate load capacity by 150% relative to the unreinforced void beam, while the circular double-tube system achieved an improvement of 180%. Circular tubes exhibited slightly superior performance due to more uniform stress distribution, whereas horizontally oriented rectangular tubes outperformed vertical ones because of their higher moment of inertia. The developed finite element model accurately predicted the experimental behavior and failure modes. The findings demonstrate that embedded steel tubes provide an efficient and practical solution for improving the performance of RC beams with service-integrated longitudinal voids.