Seismic vulnerability assessment of the residential URM buildings built during the XX century in Florence

This work deals with the urban scale vulnerability assessment of the unreinforced masonry buildings with RC slabs built during the XX century in Florence. The public housing interventions, for their numerosity and the archive documentation of the design projects, have been chosen as representative of the coeval urban stock. A meaningful database with a large number of selected buildings was realized. Every construction has been firstly investigated adopting an empirical approach based on geometrical and mechanical parameters; houses have been divided into typological classes in function of geometrical and architectonical features. Then, a typology with a specific related case study has been selected and assessed by an analytical procedure. An equivalent frame modeling discretization has been adopted and the seismic performance has been evaluated by means of nonlinear static analyses. Both aleatory and epistemic uncertainties have been considered; then, their sensitivity has been studied. The aleatory uncertainties have been investigated adopting the star design with the central star approach, while the epistemic uncertainties have been modeled through a logic tree approach. Analytical fragility curves have been finally derived, considering both the dispersions in terms of capacity and seismic demand. The fragility curves pointed out the vulnerability of the case study and the related damage scenarios for different expected return periods. Specifically, they showed a high vulnerability of these buildings for the 475 and 975 years return period; for the Life Safety limit state (SLV), around 40% of probability to have DL4 and 40% to reach DL5 is expected. The results have been finally extended to the building class population through a simplified procedure calibrated on the analytical results. The results point out homogeneous outcomes, exhibiting a high vulnerability and a relevant brittle behavior in the plastic phase.