Experimental investigation on innovative lightweight steel exoskeleton for seismic resilience of masonry-infilled reinforced concrete frames

The seismic vulnerability of existing reinforced concrete (RC) structures is a pressing concern, driven by factors such as substandard material quality, non-compliance with modern seismic design provisions, and insufficient maintenance. Although masonry infills are typically neglected in design and assessment, their influence on the seismic response of RC frames is increasingly recognized. Recent earthquakes have highlighted the critical need to improve the safety of aging and deteriorating building stocks, particularly in seismically active regions. This study introduces the Resisto 5.9 Tube, a pioneering strengthening system for retrofitting of masonry-infilled RC frames through a lightweight, cost-effective, and sustainable steel exoskeleton. Unlike conventional methods, this novel system employs a grid of thin C-shaped steel profiles reinforced with diagonal braces, installed exclusively on the building’s outside to minimize disruption. Its groundbreaking features include: (i) seamless integration of seismic enhancement with energy efficiency, (ii) negligible added mass, (iii) modular adaptability that preserves existing openings, and (iv) a scalable design unmatched by traditional retrofitting techniques. To assess its effectiveness, an experimental campaign was conducted, involving quasi-static cyclic testing of two full-scale masonry-infilled RC frames constructed with low-quality materials representative of deficient Italian building practices. One frame served as as-built control, while the other was retrofitted with the proposed system. Experimental results demonstrated that the steel exoskeleton significantly enhances lateral load resistance, increasing peak force and displacement capacity by up to 50 % and cumulative energy dissipation by over 60 %. Additionally, the system mitigated relative displacement between the masonry infill and concrete elements, reducing pounding effects and the risk of brittle failure. By replicating conditions of existing vulnerable structures, this investigation underscored the strengthening technique’s potential to markedly improve seismic performance. These findings offered a practical, economical solution for retrofitting aging RC buildings without extensive reconstruction, contributing valuable insights for structural engineers and policymakers aiming to enhance the resilience of seismically vulnerable infrastructure.