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Reliability levels related to different reference periods and consequence classes

The target reliability levels recommended in various national and international documents for new and existing structures are quite different and vaguely related to types of structures and consequences of failure. The optimum reliability levels can be obtained by considering the total expected cost of the structure over the design working life. Indicative values of reliability indexes are given in EN 1990 (2002) for two reference periods, 1 year and 50 years. The annual target reliability levels βt1 are presently indicated in the recent draft of EN 1990 (2017) without any explicit link to different reference period or design working life. When determining the target reliability index for the reference period of n years that generally differs from the design working life, correlation of failure events in subsequent years should be taken into account. Considering the annual reliability index β1 an approximate formula is proposed for the reliability index βnk related to the reference period of n years and independence interval of k years. The independence interval k ≤ n corresponds to the mean time period in years for which the failures in subsequent periods of k years are assumed to be mutually independent. The operational use of the proposed formula is clarified by practical tools. The reliability differentiation reflected in consequence classes is also discussed in this contribution.