Flanged-tube strengthening of reinforced concrete beams with circular web openings: experimental and numerical investigation

Abstract Reinforced concrete (RC) beams are commonly used in modern construction because of their strength and flexibility. However, making web openings for services like electrical conduits, HVAC ducts, and plumbing lines often results in decreased stiffness, strength, and premature cracking. Several studies [1–6] have shown that these openings negatively impact the shear capacity, crack progression, and overall load-bearing ability of beams. To address these issues, various strengthening and retrofitting methods, such as FRP wrapping, steel plate bonding, and hybrid techniques, and have been examined [7–12]. While these methods provide some improvements, there are still challenges regarding cost, build ability, and long-term durability. This experimental study introduces a new retrofitting technique that uses flanged tube inserts to improve the performance of RC beams with circular web openings. A series of beam specimens, both with and without the retrofitted flanged tubes, were tested under four-point bending to assess load-carrying capacity, stiffness, deflection behavior, and crack patterns. The results showed that flanged tube retrofitting greatly improved shear resistance, lowered stress concentration around openings, and enhanced energy absorption compared to unstrengthened specimens. This technique offers a promising, practical, and cost-effective solution for retrofitting RC beams with service openings, ensuring better usability and structural safety.