A risk-based definition of the confidence factor for the seismic assessment of URM existing buildings

The paper critically examines the current approach adopted in Standards to face the incomplete knowledge that constitutes one of challenging issue involving the seismic safety assessment of existing buildings. As known, the current approach is based on the confidence factors approach, whose value is related to the knowledge level achieved, aimed at penalizing the verification procedure when knowledge is limited, thus leading to a more cautionary assessment. Such an approach is applied in the paper to a prototype representative of existing unreinforced masonry buildings, for which the possible uncertainties compatible with the different knowledge levels, according to the Standards definition, are considered. More specifically, the criteria recommended in the Italian Structural Code issued in 2018 and the directions outlined in the draft of Eurocode 8 (Part 3), that is currently under review, have been tested in the paper. To establish a reference solution to assess the reliability of these approaches, nonlinear static analyses are carried out to quantify the propagation of uncertainties by using a Monte Carlo sampling (100 models); the uncertainties deriving from the mechanical parameters or masses and those associated with drift are distinguished. Having verified the unreliability of the current approach, a practice-oriented procedure is proposed in the paper, based on a codified sensitivity analysis, which allows to derive the confidence factor from the dispersion of the outcome of the assessment, which is the input acceleration compatible with the limit state under verification. Therefore, the proposed procedure is consistent with a probabilistic safety check, even if based only on few deterministic evaluations. (c) 2023 The Authors. Published by Elsevier B.V.