A new study demonstrates how near-surface-mounted (NSM) reinforcement can effectively mitigate the shear loss caused by vertical holes in concrete beams, providing safer and more flexible design solutions for modern buildings.
Study: Shear behavior of reinforced concrete beams with vertical holes externally strengthened with near surface mounted bars. Image Credit: WHYFRAME/Shutterstock.com
Published in Scientific Reports, the study investigates how vertical openings in reinforced concrete (RC) beams (often necessary for routing mechanical, electrical, and plumbing systems) affect structural performance. While RC is widely valued for its strength and durability, these openings can create shear zones prone to early failure.
Traditional methods like externally bonded fiber-reinforced polymers (FRP) or steel plates are commonly used for strengthening, but they often suffer from debonding issues under high stress.
NSM reinforcement offers a compelling alternative.
By embedding steel bars or cold-formed sections into shallow grooves on the concrete surface and securing them with epoxy, this technique improves bonding and load transfer while reducing the risk of premature failure. Although NSM has been studied in various contexts, its role in strengthening beams with vertical holes remained underexplored - until now.
Experimental Setup: Testing Shear Performance
The team assessed the effects of vertical openings placed within the shear zones of RC beams and examined how these openings influence shear behavior and overall structural performance.
The experimental program consisted of ten beam specimens, each with an effective span of 1000 mm, a total length of 1100 mm, and a rectangular cross-section measuring 120 mm by 200 mm. Nine beams contained vertical holes with diameters of 20 mm, 30 mm, or 50 mm, while one specimen without an opening served as the control.
To assess the effectiveness of strengthening strategies, several specimens were reinforced with NSM bars, including horizontal bent-end steel bars, horizontal and vertical straight bars, and cold-formed steel (CFS) sections. Grooves were cut into the concrete surface, cleaned, filled with epoxy, and fitted with NSM bars before sealing.
All beams were tested under a concentrated mid-span load, with deflections, cracking behavior, and failure patterns monitored using Linear Variable Displacement Transducers (LVDTs) and visual inspection. This setup allowed a comparison between unstrengthened beams and those reinforced with NSM bars, providing insights into how openings and strengthening techniques affected shear capacity and failure modes.
The study also varied the length of the NSM strengthening bars relative to the opening size (3D, 5D, and 7.5D), showing that longer bars increased strengthening effectiveness.
Key Findings: How NSM Reinforcement Performs
The outcomes showed clear effects of vertical openings on the shear behavior of RC beams. Increasing the hole diameter reduced shear capacity, with the 20 mm, 30 mm, and 50 mm openings resulting in decreases of approximately 4.5 %, 9 %, and 20 %, respectively, compared to the control beam. Larger openings also triggered earlier shear cracking and more severe crack propagation, contributing to premature failure.
NSM strengthening techniques were also found to significantly improve shear performance. Beams reinforced with horizontal NSM bars recovered much of their lost capacity, with some specimens achieving ultimate strengths comparable to the control. Strengthened beams also demonstrated enhanced ductility and improved energy dissipation, shifting their failure modes from shear-dominated to more favorable flexural behavior. However, the degree of improvement varied by configuration.
Vertical NSM bars and CFS sections were the only methods that fully restored or exceeded the control beam’s shear performance, while horizontal bent-end and straight bars did not completely recover the strength of beams with larger openings.
Among the strengthening configurations, CFS sections provided the most notable improvement. When used around 50 mm openings, they increased shear strength by 195 % compared to unstrengthened beams with the same opening size, and by 26 % compared to the solid control specimen. The failure modes were classified into three categories based on the dominant crack types: F+S1 (flexural cracks dominant), F+S2 (combined flexural and shear cracking), and F+S3 (shear cracks dominant).
The study also found that longer NSM bars were more effective at suppressing shear cracks around the opening, particularly in beams with smaller and medium-size apertures.
Design Implications for Real-World Structures
This research has significant implications for the construction of RC beams in buildings where utility openings are unavoidable. The use of NSM bars demonstrates a practical method for restoring and enhancing the shear capacity of beams weakened by vertical apertures. By integrating these techniques, engineers can maintain the structural integrity of concrete elements while accommodating essential systems required in modern architecture.
The findings also guide future design practices. Understanding how opening size affects shear performance can help engineers make informed decisions during planning and detailing, reducing the need for costly retrofitting later on.
The study additionally validated its experimental findings against theoretical predictions using national design equations, showing close agreement between predicted and measured loads. Incorporating NSM reinforcement into both new construction and strengthening projects supports safer and more flexible architectural designs, ensuring that RC structures continue to meet performance and safety standards as urban environments evolve.
Conclusion and Next Steps
In summary, this study provides a detailed assessment of the shear behavior of RC beams with vertical openings and demonstrates the effectiveness of NSM reinforcement in restoring lost shear capacity. The findings indicate that while openings reduce structural performance, appropriately designed NSM bars can mitigate these effects and improve beam behavior. Vertical NSM bars and CFS channels were particularly effective, especially for larger openings, offering practical strengthening options for real-world applications.
However, the study also highlights the need to further explore NSM applications in more complex structural configurations, including beams with multiple openings or varied geometries.
Future work should evaluate the long-term durability of NSM-reinforced beams under different conditions to support practical implementation. Overall, this research will help bridge the gap between architectural functionality and structural safety, ensuring that RC beams can accommodate essential utilities without compromising performance.
Journal Reference
Nawar, M.E., &. et al. (2025). Shear behavior of reinforced concrete beams with vertical holes externally strengthened with near surface mounted bars. Sci Rep 15, 41746. DOI: 10.1038/s41598-025-27250-z, https://www.nature.com/articles/s41598-025-27250-z
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