Interpretation of probability in structural safety – A philosophical conundrum

The term probability is essential in the domain of structural safety and yet its interpretation is, even after more than 50 years of application, still a subject of discussion. For instance, the probability of failure of structures belonging to the same cohort for a specific period of time, is often understood in a pure frequentist way as an observable average number of failure events for that period and portfolio. By contrast, the Bayesian interpretation considers probability as a degree of belief and a reflection of the state of information to the best belief or knowledge of the decision maker. In the field of structural reliability, depending on the type of decision problem, probabilities are often referred to as nominal (or notional) measures of uncertainty to emphasize that these values are conditional on a model and available observations. Probabilistic methods then serve primarily to undertake the book-keeping required to assign probabilities to different possible outcomes or decisions in consistency with models, available observations and other relevant information. This paper discusses the background of these interpretations and the degree to which correspondence between reliability calculations and observations of failures can be expected and/or achieved. Recommendations corresponding to the JCSS line of thinking will be summarized in Section 8.