PSI - Issue 78

S. Cattari et al. / Procedia Structural Integrity 78 (2026) 1577–1584 Author name / Structural Integrity Procedia 00 (2025) 000–000

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2. Key objectives of the ongoing survey on the most widespread retrofit strategies in Italian school buildings Whitin the MARS project a consensus-based vulnerability model, known as the “School-MARS model”, has been developed (Cattari et al. 2024). This model integrates fragility curves based on various approaches (i.e. empirical, mechanical-analytical, mechanical-numerical and hybrid) and is tailored to archetypes representative of the Italian school buildings, both URM and RC. The fragility curves have been defined referring to a specific taxonomy which combine the following main attributes: age of construction; number of stories; material; gross area; diaphragms type. As briefly summarized in Fig. 1, the model was defined starting from the original fragility curves developed using different approaches but consistent assumptions by various RUs and adopting a lognormal format and 5 damage grades. These curves were then converted into vulnerability classes compatible with those of EMS-98 (Gruntal et al. 1998), following the proposal by Lagomarsino et al. (2022). The final model thus incorporates also the epistemic model uncertainty that was founded higher for RC structures than the URM ones. This approach is designed to be flexible, allowing for updates over time to incorporate the development of new fragility curves and to integrate new approaches, too. However, as of now, it refers only to the current state of school buildings. The ongoing challenge of the project is to update the model so that it can also be used to develop risk assessments in the mitigated state. Given the wide range of possible interventions and the varying levels of reliability that different approaches may have in simulating their effectiveness, the chosen approach is to estimate modifiers to be applied to the as-is fragility curves and able to quantify their impact in terms of changes to the median values and dispersion of fragility curves.

Fig. 1. MARS-Schools vulnerability model (example for one subtype of URM school buildings): a) fragility curves derived by individual RU; b) conversion of fragility curves into vulnerability classes; c) final model.

A preliminary contribution in this direction in MARS project is the work of Follador et al. 2023 on residential URM buildings, which consider also different options and possible combinations for retrofit strategies. As an example, Fig. 2, adapted from the work of Follador et al. 2023, firstly illustrates a set of fragility curves for a residential building 2-storey typology dating from before 1919, in the as-built and retrofitted conditions. These curves were obtained using three different approaches: a mechanical-analytical model (UniGEb), a heuristic model (UniGEa), and a hybrid model (UniPD). Then, for two-storey buildings only, fragility curves in the retrofitted condition are shown, corresponding to different strengthening solutions (MSN2 –intervention on masonry strength; FLR – increase in diaphragm stiffness; TR – addition of steel tie-rods). Finally, the values of the modifiers required to transition from the as-is to the retrofitted condition are reported, expressed as the percentage increase in the median PGA as resulting from the three models. The latter may be also combined to define a single reference value eventually associated also to a dispersion. In order to define more realistic and practically useful mitigated-state scenarios for national-scale planning, it is essential to have information on the most commonly adopted retrofit interventions across the country. This requires a realistic overview of the most recurrent intervention strategies, which can then be used as the basis for the development of fragility curves. The analysis of real case studies initiated in this paper moves in that direction. As discussed in in §3, the analysis aims to provide an overview of interventions that combine effects on both vertical and horizontal load bearing structural components. Thanks to the set of archetype buildings previously identified and defined within the MARS-Schools project (Cattari et al. 2024) – which effectively represents the diversity of school building typologies