Adequacy of a single confidence factor to account for epistemic uncertainty in the seismic assessment of existing structures

Abstract eng:
Eurocode 8 Part 3 (EC8-3) is devoted to assessment and retrofitting of existing buildings. In order to take into account the uncertainty in the knowledge of structural properties, EC8-3 defines, in addition to the ordinary material partial factors, an adjustment factor, called “confidence factor (CF)”, whose value depends on the level of knowledge (KL) of properties such as geometry, reinforcement layout and detailing, and materials. This solution is plausible from a logical point of view but it cannot yet profit from the experience of its use in practice, hence it needs to be substantiated by a higher level probabilistic analysis accounting for and propagating epistemic uncertainty (i.e., incomplete knowledge of a structure) throughout the seismic assessment procedure. The paper investigates the soundness of the format proposed in EC8-3 and pinpoints some problematic aspects that would require refinement. The approach taken rests on the simulation of the entire assessment procedure and the evaluation the distribution of the assessment results (distance from the limit state of interest) conditional on the acquired knowledge. Based on this distribution a criterion is proposed to calibrate the CF values. The obtained values are then critically examined and compared with code-specified ones. The methodological significance of the work extends beyond the assessment procedure in EC8-3, since similar factors appear in other international guidelines (e.g. the knowledge factor of FEMA356).

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