Codification of Design Load Criteria Subject to Modeling Uncertainty

Structural code development is greatly influenced by the requirement to take into account the effect of model imperfection and model uncertainty whenever it arises. The extent to which these uncertainties should be reflected in design specification factors such as partial factors or return periods is discussed. Code calibration techniques, which allow greater flexibility in the representation of model uncertainties in design check equations, are investigated from a decision theoretical point of view. A two-step calibration procedure is proposed that involves the use of two separate sets of design factors. The first set reflects the effects of basic variabilities. The second set consists of so-called ignorance factors that can be adjusted as a function of model quality. The general characteristics of suitable utility assignment functions are discussed and they are related to the calculation of safety measures under uncertainty. The advantages and the limitations of the proposed calibration procedure are discussed. A practical application illustrates the feasibility of the approach.