Strain-Based Experimental Investigation of Load Transfer and Infill–Frame Interaction in Low-Strength RC Frames Under Cyclic Loading

Reinforced concrete (RC) infilled frames are widely used structural systems; however, seismic design provisions often idealize masonry infill as non-structural, leading to uncertainty in performance assessment. This study experimentally and numerically investigates the role of unreinforced masonry infill in RC frames, focusing on load-transfer mechanisms, strain evolution, and energy redistribution. Two 2/3-scale single-bay, single-storey RC frames (bare and fully infilled) were tested under constant axial load and quasi-static reversed cyclic lateral loading. Reinforcement strain gauges were used to capture local deformation demands, and a nonlinear macro-model was developed and validated against experimental results. Results show that the presence of masonry infill significantly increases ultimate strength, initial stiffness, and energy dissipation capacity, in comparatively more brittle post-peak cyclic behavior and accelerated stiffness degradation that leads to more abrupt post-peak degradation. Strain measurements provide clear evidence of a staged interaction mechanism: at low drift levels, the infill governs lateral resistance through diagonal compression strut action, limiting reinforcement demand in the frame; with increasing drift, progressive cracking and crushing of the infill promote a gradual transfer of forces to the RC frame, reflected by increasing reinforcement strains and stiffness degradation. At higher drift levels, the system transitions to frame-dominated behavior with localized strain concentration and shear failure at column bases or joints. These findings demonstrate that infill significantly modifies structural response and highlight the importance of incorporating strain-based mechanisms in the seismic assessment of infilled RC frames.