Experimental investigation on cyclic lateral response of masonry-infilled reinforced concrete frames strengthened with cold-formed lightweight external steel exoskeleton

The seismic performance of masonry-infilled reinforced concrete (RC) frames is strongly governed by infill–frame interaction, which enhances stiffness and strength but often limits deformation capacity and promotes brittle failure. This study investigates the effectiveness of an innovative steel exoskeleton system for improving the lateral response of existing non-ductile RC frames. An experimental campaign was conducted on five full-scale, single-story, one-bay RC frames representative of gravity-load-designed buildings. The test matrix included a bare frame, two unstrengthened infilled frames with slender and thick masonry panels, and their corresponding strengthened configurations. All specimens were subjected to constant axial load and quasi-static cyclic lateral loading up to ultimate conditions. The experimental program enabled detailed assessment of both global and local behavior, including the isolated lateral contribution of the infill and strengthening system, beam–column joint deformation, infill–frame interaction, and deformation mechanisms of critical exoskeleton components. Results show that masonry infill markedly increases initial stiffness and peak strength, with thicker panels providing higher capacity but exhibiting more brittle post-peak behavior. The strengthening system substantially improved both strength and deformation capacity, increasing peak lateral resistance by up to 90% for slender infills and 40% for thick infills, while displacement capacity reached levels comparable to the bare frame. Energy dissipation was significantly enhanced, with equivalent viscous damping ratios exceeding 10% at large drifts, and damage in both infill and RC elements was delayed and reduced. The exoskeleton response was governed by cyclic buckling and yielding of the diagonal braces, whereas primary steel members remained below yielding, confirming the system’s robustness and reparability. Leveraging the experimental evidence, a practical design strategy was developed to estimate the strength and stiffness contribution of the exoskeleton based on its configuration. By demonstrating improved load-carrying capacity, enhanced energy dissipation, and effective damage control, while also providing a basis for design, this study establishes a solid foundation for the adoption of the proposed strengthening system in the seismic retrofitting of deficient masonry-infilled RC frames.