Fragility curves for residential unreinforced masonry buildings prone to out-of-plane mechanisms: the case of the historical center of Sora

The earthquake events of the past and recent years highlight as the seismic protection of historical towns is a crucial and current issue in Italy, but also in many other earthquake prone areas of Europe, where severe damages and collapse of buildings have been observed after seismic events. In particular, significant damages have been detected in historic centers of small and medium towns, mainly characterized by unreinforced masonry buildings, highly vulnerable with regard to out-of-plane collapse mechanisms. In this context, the assessment of the seismic vulnerability of unreinforced masonry buildings is a fundamental issue, in that they represent the most widespread construction typology in Italian and European small and medium towns. One of the most effective approaches in predicting potential damages of building typologies, is based on the derivation of fragility curves. An important aspect that affects the construction of fragility curves is the choice of response spectra to define the seismic demand; both Italian code and European guidelines allow the use of natural records, but also propose to employ smoothed spectra, in that the first one is hardly applicable by practitioners. Nevertheless, the employment of the smoothed spectra does not allow for adequate considerations about the influence of record-to-record variability. Downstream of this brief discussion, the paper presents the evaluation of fragility curves for unreinforced masonry buildings typical of the Central Italy, prone to the occurrence of out-of-plane collapse mechanisms, by considering both the spectral-shapes directly derived from the Italian codes and spectra obtained by the selection of natural accelerograms.