Seismic risk mitigation at national scale requires the vulnerability assessment of the built stock, which is generally based on the definition of appropriate fragility curves by means of different methods. A first type of methods uses an empirical approach with calibration from observed damage. A second group of methods uses mechanical models suitable to describe the structural behavior; however, because an in-depth geometric and structural characterization of the buildings is very time-consuming at the territorial level, these models must remain "as simple as possible" to limit the number of input parameters. The aim of this work is to estimate the fragility of the Italian masonry buildings stock grouped in macro-typologies ISTAT (National Institute of Statistics), i.e. defined by construction age and number of stories; this is essential for deriving damage scenarios at territorial scale based solely on ISTAT information. Vulnus Vb 4.0 (2009), a software developed at the University of Padova, was found to be very useful for this purpose as it provides fragility curves of masonry buildings assessing the in-plane (IP) failure, the principal out-of-plane (OOP) mechanisms and the main typological-structural characteristics; judgments on the quality of information are also used to provide an upper and lower fragility limit. More than 500 buildings were examined with this software, appropriately chosen to guarantee a good statistical representation of the built stock. The information for each building was retrieved directly from the related projects and, in the case of missing information, reference was made to design manuals and codes, and to specific literature. Finally, the fragility results were processed to provide a mean fragility curve (representing the trigger of an IP-OOP mechanism) and a range of possible dispersion for each ISTAT macro-typology. These results can also be used to calibrate macro-seismic models of vulnerability in the literature, if the will is to represent this mechanical fragility in a more distributed way, i.e. with more damage states.

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The earthquakes of the last decades have shown that the Italian residential masonry built heritage has high seismic vulnerability, in particular when considering structures built before 1919. For this reason, it is necessary to develop effective large-scale risk mitigation strategies in order to reduce the huge losses that could occur in the aftermath of an earthquake. In this paper some retrofit interventions applicable mainly to old buildings are presented, explaining their advantages and potential. These interventions are then implemented, through Vulnus 4.0 software, on a database of 205 buildings built before 1919, previously analyzed in their as-built state. Fragility curves are then developed for each building, and are processed in order to create a vulnerability model for different construction periods that takes into account the possible retrofit intervention strategies. Therefore, this procedure allows a comparison between pre and post retrofit intervention fragility, and the results in terms of curves can be used for large scale damage and risk simulations.

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