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Predicting the usability of a building, i.e. its condition of being occupiable after a seismic event, is relevant both in a postemergency situation and within a risk-reduction policy. In the past an empirical model was proposed, involving the computation of a usability index based on macroseismic intensity and on seven building parameters, combined by means of regression coefficients and weights. The statistical model was calibrated on data of about 60 000 buildings affected by the 2009 L'Aquila earthquake in Italy. Therefore, it is useful to validate the model against data from the 2002 Molise earthquake in Italy. Good agreement between predicted and observed usability is shown, despite the fact that in 2002, macroseismic intensity was attributed to an entire municipality instead of a more limited area. Moreover, given the current availability of the shakemaps for the 2009 event, a novel model replacing conventional macroseismic intensity by an instrumental intensity measure is proposed. Three ground motion parameters are considered here: peak ground acceleration, peak ground velocity, and spectral pseudoacceleration at a period of vibration of 0.3 s. The model has been streamlined by reducing the building parameters from seven to five: building position within the structural aggregate, roof type, construction timespan, structural class, and pre-existing damage to structural elements. Peak ground acceleration and spectral pseudoacceleration are shown to be less effective than peak ground velocity in predicting observed usability. Therefore, usability probability matrices are computed in terms of peak ground velocity; the model is presented with all necessary coefficients and weights, and a worked-out example shows how to apply the procedure.

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Background: Seismic risk mitigation has become a crucial issue due to the great number of casualties and large economic losses registered after recent earthquakes. In particular, unreinforced masonry constructions built before modern seismic codes, common in Italy and in other seismic-prone areas, are characterized by great vulnerability. In order to implement mitigation policies, analytical tools are necessary to generate scenario simulations. Methods: Therefore, data collected during inspections after the 2009 L’Aquila, Italy earthquake are used to derive novel fragility functions. Compared to previous studies, data are interpreted accounting for the presence of buildings not inspected due to those being undamaged. An innovative building damage state is proposed and is based on the response of different structural elements recorded in the survey form: vertical structures, horizontal structures, stairs, roof, and partition walls. In the suggested formulation, the combination of their performance is weighted based on typical reparation techniques and on the relative size of the structural elements, estimated from a database of complete geometrical surveys developed specifically for this study. Moreover, the proposed building damage state estimates earthquake-related damage by removing the preexisting damage reported in the inspection form. Results: Lognormal fragility curves, in terms of building damage state grade as a function of typological classes and peak ground acceleration, derived maximizing their likelihood and their merits compared with previous studies are highlighted. Conclusion: The correction of the database to account for uninspected buildings delivers curves that are less “stiff” and reach the median for lower peak ground acceleration values. The building feature that influences most the fragility is the masonry quality.