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.

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Structures are subjected to cyclic loads that can vary in direction and magnitude, causing constant amplitude mode I simulations to be too simplistic. This study presents a new approach for fatigue crack propagation in ductile materials that can capture mixed-mode loading and overloading. The extended finite element method is used to deal with arbitrary crack paths. Furthermore, adaptive meshing is applied to minimize computation time. A fracture process zone ahead of the physical crack tip is represented by means of cohesive tractions from which the energy release rate, and thus the stress intensity factor can be extracted for an elastic-plastic material. The approach is therefore compatible with the Paris equation, which is an empirical relation to compute the fatigue crack growth rate. Two different models to compute the cohesive tractions are compared. First, a cohesive zone model with a static cohesive law is used. The second model is based on the interfacial thick level set method in which tractions follow from a given damage profile. Both models show good agreement with a mode I analytical relation and a mixed-mode experiment. Furthermore, it is shown that the presented models can capture crack growth retardation as a result of an overload.@en

Markov-based models for predicting deterioration for civil infrastructures are widely recognized as suitable tools addressing this mechanism. The objective of this paper is to provide insights regarding a network of orthotropic steel bridges in terms of degradation. Consequently, a model combining a dynamic Bayesian network and a Markov chain is first introduced that builds up the network in a concise way. In an attempt to represent a network composed of two general classes of orthotropic steel bridges, the classical method of structured expert judgment is carried out as a quantification procedure. The first objective is to indirectly elicit transition probabilities for a Markov chain that probabilistically describes how each type bridge temporally deteriorates. Second, experts are asked to provide estimates on required conditional probabilities related to the Bayesian network. An in-depth analysis of the results is presented so that remarks and observations are subsequently pointed out and, finally conclusions are drawn.@en