Comparing the performance of retrofitting strategies in URM buildings through fragility curves

Recent earthquakes have highlighted the seismic vulnerability of existing masonry buildings, pointing out the urgent need to design high-performant and non-invasive strengthening interventions. To this aim, reliable numerical models and assessment procedures are necessary to evaluate their effectiveness and quantify the effects on the structural seismic performance of the building. In this context, the paper describes the derivation of fragility curves for a case study representative of an ordinary unreinforced masonry (URM) building placed in L’Aquila (Italy) and considered both in the actual state and retrofitted configurations. Its seismic performance was assessed through several nonlinear dynamic analyses performed according to the multi-stripes approach and employing an equivalent frame modelling strategy. Different strengthening techniques were investigated, designed using nonlinear static analyses to conform with the Italian Technical Code, and modulated to achieve a safety index higher than 1 or 0.8 (allowed by the code for existing buildings) as a function of two different soil conditions (stiff and soft soils). The resulting fragility curves were used to quantify the improvement of the seismic performance guaranteed by the different strengthening strategies and compare their effectiveness concerning two performance conditions (namely usability-preventing damage, and global collapse limit states) to identify the best solution for the two soil conditions.