Annual reliability requirements for bridges and viaducts

Abstract

In the assessment of existing structures, such as bridges and viaducts, reliability requirements are used to decide if a structure is sufficiently safe, even when subjected to degradation. The reliability requirements may be expressed in different ways but should ultimately result in similar reliability performance of the structure. Most of the current assessment rules follow from a maximum allowable failure probability, depending on the reliability class, within a fixed period of time (the reference period: e.g. 15, 30, 50 or 100 years). A reliability requirement expressed using a fixed reference period fits the design of new (to be built) structures, but it is problematic for existing or temporary structures. In the case of existing structures, when for instance the design life has passed, the assessment should be flexible with respect to the expected or desired remaining life. Other factors, such as deterioration or changing loads, may also call for an assessment with a smaller time period since the failure rate could increase with time which makes a reliability requirement for a longer period less useful. Using fixed reference periods therefore could lead to suboptimal solutions where the economy and environment are unnecessarily hurt. Reliability requirements formulated on an annual basis provide a solution to this problem. In this article, reliability analyses of several typical bridges are performed to quantify the requirements in such a way that assessments based on annual reliability result in performance similar to the current practice. Special attention is paid to minimising and highlighting the cases where the annual requirements may lead to a trend that breaks with the current reliability requirements.