Probabilistic usability-based fragility curves for Italian reinforced concrete buildings using census and post-earthquake data

Abstract

This study develops empirical fragility functions for reinforced concrete (RC) residential buildings expressed in terms of post-earthquake usability. The analysis uses more than 10,300 inspected buildings from the 2009 L’Aquila (Italy) earthquake, integrated with census data to represent uninspected constructions. Buildings are classified according to three parameters - construction timespan, state of repair, and number of stories - allowing the derivation of usability-based fragility curves for eight typological classes. A lognormal model is adopted, with parameters estimated by means of maximum likelihood. Among the building attributes, the most influential predictor is the state of repair, which in the AeDES form used for the inspections reflects not ordinary maintenance levels but the actual damage condition of the building prior to the earthquake. This distinction is crucial, as pre-existing damage strongly affects the likelihood of usability loss. Results show that this parameter is a significant factor influencing usability, followed by building height, while the construction timespan exerts a secondary yet consistent effect, with older buildings resulting more vulnerable. Taller and poorly maintained buildings exhibit systematically higher fragility. The strong effect of the state of repair emerges clearly, although the limited occurrence of poorly maintained and/or pre-damaged buildings suggests a cautious interpretation of the curves. Compared with previous usability models for masonry buildings, the RC curves show both common and material-specific patterns: the state of repair remains the dominant factor in both systems, while height plays a much stronger role in RC. This new usability-based RC-building model complements the existing masonry-building one, providing a coherent framework based on an operational metric aligned with inspection outcomes, allowing for large-scale seismic risk analyses, supporting loss estimation, recovery planning, and national-level assessments.