Loss Functions for the Risk Assessment of Residential Buildings

Within the ReLUIS 2019–2021 research project (Network of the University Laboratories of Seismic Engineering) supported by the Civil Protection Department (DPC), the Work Package WP4 “Seismic Risk and Damage Maps at National scale—MARS” aimed to update the 2018 version of the National Risk Assessment (NRA). One of the goals of the WP4—MARS was to collect, analyse and upgrade loss models to estimate direct economic losses, unusable buildings and casualties (Task 4.4—“Risk: evaluation of consequences and economic losses”). Task 4.4 aimed to carry out a critical review of the models available in the literature and subsequently specific comparisons to better understand the capability and reliability of the different models, in particular those adopted to calculate the seismic losses in the 2018 NRA. Furthermore, the derivation of new models in order to improve the results of the previous study was another goal of the project. The paper focuses on the assessment and calibration of models and tools to derive human casualties (deaths and injured) and usability rating as well as repair costs and population assistance costs associated with residential buildings damaged by seismic actions. In detail, the available literature on Casualty Estimation Models (CEMs) is analysed, and an updated model is calibrated to estimate human casualties for the Italian context. The model emphasises a relationship between the casualties and the severity of the damage. A strong correlation exists between the post-earthquake usability of buildings, damage state (DS), repair costs and time required to assist the population. According to data collected in the reconstruction processes of recent Italian earthquakes, fragility curves in terms of loss-of-usability for partially unusable and unusable buildings and the correlation between usability ratings (immediately available post-earthquake) and DS (easily computable by using existing empirical- or mechanical-based fragility functions) are defined. Finally, consequence functions able to associate direct costs (repair costs) and indirect costs (population assistance) have been calibrated as a function of buildings’ usability or damage state.