Harmonizing structural safety levels with life-quality objectives

Abstract

Life-quality objectives are identified as an essential element in design decision making. Of particular concern is the question of optimal safety levels that are consistent with reasonable expectations of individuals in a present-day society. Using sound principles of decision analysis and utility theory, a lifetime utility function is developed. It is shown to be related to human consumption, life duration including the cumulative effects of mortality and discounting, and the relative amount of time spent on work versus leisure. Questions regarding the acceptability and affordability of changes in life quality can be addressed using the utility functions developed. As an application, design safety levels for the Confederation Bridge are examined and discussed. Life-quality objectives can also be included in a life-cycle cost optimization. This allows us to perform a level IV probabilistic design approach including costs and consequences without having to estimate the value of human life, but instead including the effect of consequences on changes in life quality of individuals at risk. This results in a useful tool to determine optimal limit state design safety levels, as is illustrated in a parametric analysis in the case of a single limit state.Key words: lifetime utility, life-quality index, risk acceptance, limit states design, target reliability levels, risk reduction, minimum life-cycle cost, structural safety.

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