Target reliability levels for assessment of existing structures

Abstract

Specification of the target reliability levels is one of the key issues of assessment of existing structures. ISO 13822 (2003) and ISO 2394 (1998) indicate a procedure for specification of the target reliability levels by optimisation of the total cost related to an assumed remaining working life of a structure. This approach is applied to estimate the target reliability levels of an existing structural member. Obtained results are critically compared with recommendations given in present standards and available literature. Two target reliability levels are considered when assessing existing structures—a minimum level below which the structure is unreliable and should be repaired, and the level indicating an optimum repair strategy. Both these levels seem to be primarily dependent on failure consequences and on the marginal cost per unit of a decision parameter; repair costs, remaining working life and discount rate seem to be less significant. It appears to be uneconomical to require the same target reliability levels for existing structures as for new structures. failure consequences and costs of safety measures, is also indicated by CEN/TC 250 & JRC (2009). The target reliability levels recommended in EN 1990 (2002) related to consequences of failure, are primarily intended for new structures. More detailed classification is given in ISO 2394 (1998) where relative costs of safety measures are also taken into account. The target reliability levels provided in both documents are partly based on calibrations to previous practice and should be considered as indicative only. ISO 13822 (2003) indicates a possibility to specify the target reliability levels by optimisation of the total cost related to an assumed remaining working life. The submitted paper attempts to apply this approach for existing structures. An example of existing building with the remaining working life of 15 or 30 years, moderate costs of safety measures and moderate failure consequences (“model structure”) is considered throughout the paper to clarify general concepts. 2 TARgET RElIABIlITy lEvElS IN COdES EN 1990 (2002) recommends the target reliability index for two reference periods (1 and 50 years), see Table 1. A couple of β-values given in Table 1 for each reliability class corresponds approximately to the same reliability level. For example, considering a structure of RC2, the reliability index β = 3.8 should be used provided that probabilistic models of basic