Cyclic Performance Evaluation and Retrofitting of Coupling Double-Beam Constructed with Reinforced and Fiber-Reinforced Concrete: An Experimental and Numerical Study

This study investigates the seismic performance and deformation mechanism of coupling double-beams (CDBs) through experimental testing and numerical modeling. The proposed mechanism assumes that a CDB initially behaves as a conventional coupling beam under small deformations, but transitions into two slender beams at large drift, shifting from shear-dominated to flexure-dominated behavior. This transformation mitigates sliding shear failure at the beam-to-wall interface, facilitating more effective retrofitting. Two geometrically identical CDB specimens, one with conventional RC and the other with steel fiber–reinforced concrete (SFRC), were subjected to cyclic displacement-controlled loading up to 6.0% drift. Compared with the RC CDB, the SFRC CDB exhibited a 14.8% increase in peak load, delayed initial cracking (1.75% versus 0.35% drift), and enhanced crack control, enabling efficient early-stage retrofitting. Both damaged specimens were retrofitted with steel jacketing and tested up to 8.0% drift. The retrofitted RC CDB regained its original strength with a 13.2% improvement, while the retrofitted SFRC CDB achieved the highest energy dissipation (43.44 kNm). Numerical simulations in ABAQUS captured peak strength within 0%–9% deviation, initial stiffness within 2%–24%, and energy dissipation within 9%–55%, confirming model accuracy. A comparative study of 55 coupling beams from the literature showed that the normalized shear stress coefficients of the tested CDBs (1.06–1.53) consistently exceed the standard limit of 0.83fc′ MPa, indicating the need to revisit code provisions for beams with enhanced detailing and retrofitting. The combined use of SFRC and steel jacketing presents a practical, thermally stable, and constructible retrofit approach, offering an effective solution for upgrading RC wall systems in seismic regions.