Experimental investigation on the behavior and strengthening of opened joints in stepped reinforced concrete beams using CFRP, steel plates, and NSM reinforcement

This study experimentally investigates the structural behavior of reinforced concrete (RC) stepped beams with opened joints, commonly used in sloping roofs, ramps, and stairs. Unlike conventional beams, stepped beams develop combined axial force and bending moment at the joint, resulting in complex normal and shear stress interactions. The experimental program comprised two phases: Phase I examined the effect of reinforcement detailing on performance, while Phase II evaluated three strengthening techniques—externally bonded CFRP sheets, steel plates, and near-surface mounted (NSM) steel bars—on six beams. Results showed that proper reinforcement detailing, particularly extending to the upper joint edge, significantly enhanced ultimate load and ductility, whereas insufficient development length reduced performance. Among strengthening techniques, steel plates provided the highest improvements in ultimate load (up to 69.7%) and ductility, while NSM bars were least effective. The most efficient configuration used 100 mm wide, 2 mm thick steel plates. Strengthening also reduced vertical deflection and horizontal displacement. Cost–benefit analysis indicated steel plate strengthening as the most economical solution. The study highlights the critical role of joint reinforcement detailing in stepped beams and demonstrates the effectiveness of targeted strengthening techniques in restoring and improving structural performance.