Seismic retrofitting strategies for unreinforced masonry buildings in Europe: a review of techniques for improving seismic performance

Abstract Unreinforced masonry (URM) buildings were commonly constructed before the introduction of modern seismic design provisions; thus, they often exhibit inadequate detailing and poor seismic performance. As a significant portion of the existing building stock in many seismic regions consists of URM structures, improving their performance through appropriate retrofitting strategies remains a critical challenge for both researchers and practitioners. Over the past decades, numerous strengthening techniques have been proposed and experimentally validated. However, the available literature is widely dispersed across experimental studies, analytical investigations, and case-specific applications, hindering the development of a concise and up-to-date synthesis of current knowledge. On these premises, this paper presents a critical literature review of seismic retrofitting strategies for URM buildings, with a specific focus on historic constructions and European masonry typologies. The review examines techniques reported in 192 studies, that are then grouped into four main categories: (i) interventions to increase connections and guarantee a box-like behavior, (ii) interventions that enhance horizontal diaphragm stiffness, (iii) techniques that improve masonry quality and through-thickness integrity, and (iv) strategies involving the addition of new structural elements, including steel and timber bracing systems. A structured methodology was applied to guide this review, based on the definition of research objectives, inclusion and exclusion criteria, and a search and screening process supporting the selection of the final corpus of articles to examine in detail. The review highlights the mechanical principles governing the effectiveness of these interventions, discussing their advantages and limitations in terms of structural performance, feasibility, durability, and compatibility with heritage conservation requirements. By integrating findings from experimental investigations, numerical studies, and practical applications, this study aims to provide a framework for understanding current URM seismic retrofitting techniques and identifying key directions for future research and engineering practice.