PSI - Issue 78

Ingrid Boem et al. / Procedia Structural Integrity 78 (2026) 457–464

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1. Introduction The seismic vulnerability of existing unreinforced masonry (URM) buildings is a well-documented concern in earthquake-prone regions (Lagomarsino et al., 2021). These structures, often designed without seismic considerations, tend to perform poorly under seismic loads due to their poor materials, inadequate connections and brittle failure mechanisms. Among these, masonry school buildings represent a particularly critical subset (di Ludovico et al., 2019), due to their architectural and functional characteristics, such as greater inter-storey heights, wider spacing between structural walls, elongated or irregular floor plans, façades with several and larger openings, and internal partitions with minimal openings. Such features can significantly compromise their seismic performance, exposing to a heightened risk large numbers of students and staff. The assessment and mitigation of earthquake risk of these building asset at a territorial (or regional) scale is essential to support strategic, large-scale decision-making policies, by providing a probabilistic framework for understanding how the building stock is likely to perform and the large-scale effect of strengthening interventions. A very useful tool is provided by the fragility curves, that describe the probability of reaching or exceeding certain levels of damage, given a specific seismic hazard intensity. Within this context, the authors actively contributed to a multi-phase research project under the National DPC ReLUIS framework (http://www.reluis.it), aimed at assessing and mitigating the seismic risk associated with school buildings. As part of the 2022–2024 program (WP4 “MARS-2”), they drawn and analyzed the seismic fragility curves of a representative stock of 101 URM school buildings located in the Friuli - Venezia Giulia Italian region (Giusto et al., 2025). Under the ongoing 2024–2026 program (WP4 “MARS-CARTIS”), the analysis has shifted toward evaluating the large-scale effects of seismic retrofit strategies. Specifically, the impact of the Composite Reinforced Mortar (CRM) strengthening technique - which involves the application of fiber-reinforced mortar layers to masonry surfaces - on the seismic vulnerability of the same building stock is investigated. To that, the original analytical mechanical methodology has been revised and adapted to account for the improved mechanical behavior: the paper details the main features of the CRM system, the strategy for its inclusion in the fragility assessment, and the comparative analysis between as-is and retrofitted conditions, given a preliminary estimate of the large-scale effect of The study focuses on school buildings having an exclusive or predominant masonry construction system. In particular, the reference sample is composed of 101 URM schools representative of the regional asset of the Friuli - Venezia Giulia Italian region. For each building, detailed information on the structural characteristics is available, gathered from both research and analysis of design documentation, and on-site surveys and inspections. The collected data include the localization, the age of construction - and, if any, of the main interventions, the subdivision into structural units, the number and height of storeys, the masonry and floor type, the in-plan layout and thickness of the walls. To group the sample into sub-typologies, the taxonomy adopted is based on the number of floors (i.e. 1, 2, ≥3) and age of construction (≤1920, 1921-45, 1946-60, 1961-75, ≥1976). The construction age is approximately evenly distributed among the four periods before 1976, whereas new construction systems replaced URM in recent ages. Common features are recognized within the buildings of the sample: they are predominantly 2-storey (50%) and small in plan area (84% are < 1000 m 2 ), but have significant inter storey height (median value 3.6 m) and substantial distance between transverse walls (larger than in residential buildings). The floors are mainly of mixed type (>90% r.c. ribs with hollow clay blocks); the predominant masonry type is solid brick with lime mortar (54%), followed by hollow brick (25%) and stone masonry (21%). The planimetric shape is variegated, ranging from compact to elongated rectangular plans or irregular plans; internal walls are typically characterized by with very limited openings, while several large openings in the main façade. Generally, the percentage of load-bearing walls at the ground floor tends to increase with the number of stories, with median values of 3.5-4.6 6.2% for 1-2- ≥ 3 storey buildings, respectively. the CRM application. 2. Reference sample