Author(s)
Sabry Fayed, Mohamed Ghalla, Yahia Iskander, Rabeea W. Bazuhair, Yahya M. Bin Mahfouz and Saad A. Yehia
Abstract
Modern buildings increasingly require mechanical, electrical and plumbing services to pass through structural members without increasing floor-to-floor height. One practical solution is to introduce longitudinal openings within reinforced concrete beams, allowing utility services to pass through the beam depth instead of beneath it. Although this approach improves space utilisation and architectural flexibility, removing concrete from the beam significantly weakens its structural performance, particularly when the openings are located in the compression zone.
This research investigates the effectiveness of embedded steel tube reinforcement as an internal strengthening solution for reinforced concrete beams containing longitudinal openings. Rather than relying on external strengthening systems, steel tubes are integrated directly into the beam, surrounding the service openings and working compositely with the concrete to restore structural capacity. The investigation combined laboratory testing with validated three-dimensional nonlinear finite element modelling to evaluate how void geometry, tube shape, tube orientation and reinforcement ratio influence structural behaviour. Eleven reinforced concrete beam specimens were tested under four-point bending to compare solid beams, beams with unreinforced openings and beams strengthened using embedded rectangular or circular steel tubes.
The results demonstrate the significant structural penalty associated with unreinforced longitudinal openings. Introducing a void reduced the beam’s ultimate load capacity by approximately 27.5%, while stiffness and energy absorption both decreased by around 41%. These reductions occurred because the opening removed effective concrete from the compression zone, increased stress concentrations and accelerated crack development, resulting in lower flexural resistance and reduced ductility.
Embedding steel tubes within the openings substantially improved structural performance through composite action between the steel and surrounding concrete. Even relatively modest reinforcement ratios restored the lost capacity and enhanced crack control, stiffness and deformation behaviour. Increasing the steel tube reinforcement ratio produced progressive improvements in ultimate load capacity, stiffness, energy absorption and post-cracking ductility, demonstrating that the embedded tubes remained effective load-carrying elements after concrete cracking.
The geometry of the embedded tubes also influenced performance. Circular steel tubes achieved slightly higher load capacity and energy absorption than comparable rectangular tubes because their smooth profile promoted more uniform stress distribution and reduced local stress concentrations around the openings. For rectangular sections, horizontally orientated tubes outperformed vertically orientated tubes owing to their greater moment of inertia, which provided more efficient resistance to bending.
To verify the experimental findings, a nonlinear finite element model was developed and calibrated against the laboratory results. The numerical predictions closely matched the observed load-deflection behaviour and failure mechanisms, confirming that the modelling approach can reliably assess similar composite beam configurations. A further parametric study also showed that increasing concrete compressive strength significantly increased beam load capacity, supporting future optimisation of this strengthening technique.
Overall, the study demonstrates that embedded steel tube reinforcement offers an efficient solution for strengthening reinforced concrete beams with longitudinal openings. By restoring structural capacity while allowing utility services to be integrated within the beam depth, this approach has the potential to improve structural efficiency, reduce overall building height and support more economical construction. For engineers interested in innovative structural systems, this research provides valuable insights into composite reinforcement strategies for service-integrated reinforced concrete design. It also complements broader guidance on reinforced concrete beam design and strengthening techniques.
READ MORE
Recent Comments