Probabilistic fatigue life updating accounting for inspections of multiple critical locations

Many steel structures contain multiple fatigue sensitive details that have similar geometries and are subjected to similar load fluctuations. Examples are orthotropic (bridge) decks and stiffened (ship) hulls where tens to thousands of similar details are present in one structure. Generally only visual inspections on fatigue cracks are considered for these structures because more accurate techniques are considered to be too expensive and time consuming when so many details need to be inspected. Visual inspections are known to have a low probability of detection. Consequently Bayesian update techniques usually show a marginal effect of the result of visual inspections on the reliability of structures with respect to fatigue failure. On the other hand the inspection result of one location also provides information on similar details at other locations and thus the significance of the inspection result may be larger if multiple potential crack locations occur and cracks are not detected at any of these locations. This paper provides a probabilistic fatigue crack growth (fracture mechanics) model of a system containing a fatigue sensitive detail at multiple locations that accounts for the results of inspections. Spatial correlations of loading, resistance, and uncertainty variables between the different locations are evaluated and estimated through a literature review and are accounted for in the model. The model is demonstrated on a practical example of an orthotropic bridge deck containing a detail at 100 locations. The paper shows that visual inspections may be effective provided that a certain minimum inspection reliability can be guaranteed, that the structure is relatively tolerant to large cracks, and that the geometry and loading conditions are similar for a large number of locations

Optimization of steel monopod offshore-towers under probabilistic constraints

In this work, economical design implementation of a circular steel monopod-offshore-tower, which is subjected to the extreme wave loading, is presented. The mass of the tower is considered as the objective function. The thickness and radius of the cross-section of the tower are adopted as design variables of the optimization. Moreover, stress or buckling is specified as probabilistic constraints. The numerical strategy employed for performing the optimization uses the IMSLLibraries routine that is based on the Sequential Quadratic Programming (SQP). The FORM is used for the reliability calculation from a specified limit state function based on the stress or buckling. A demonstration of an example monopod tower is presented

Overview of structural reliability methods

In the past decades an impressing and useful set of operational methods for probabilistic structural analysis has been developed. Based on SAFRELNET activities in Task 5.1 the paper provides a structured discussion of these methods and indicates the various fields of application. The Level II time-independent component-reliability methods (FORM/SORM) are summarized first. Consequently, the Level III reliability methods are presented briefly. Then, time-independent system reliability methods are outlined for parallel and series systems. For complex reliability problems, techniques of the SFEM are summarized. Regarding time-dependent reliability methods, attention is given to the transformation of time dependent failure modes into a corresponding time independent mode for load combination models, for which the Turkstra's rule and Ferry Borges-Castanheta load model are highlighted. Having outlined the outcrossing approach for both discrete and continuous processes with lower and upper bounds of a time-interval failure-probability, some examples are presented for demonstration purposes

Life cycle design: principles and models

Tunnels and other underground structures are designed for lifetimes of at least 50 to 100 years. A tunnel can probably last even longer from a technical point of view, providing the original functionality it is designed for does not change too much. In order to avoid high maintenance costs a careful design procedure is necessary. For this kind of problems, a life-cycle costing (LCC) approach is an absolute necessity to ensure an integral view on the various costs that have to be spent in different time frames to operate the infrastructure

Typen onzekerheden bij bepaling overstromingskansen

Onzekerheden zijn er in verschillende soorten en maten. Dit is van invloed op de interpretatie van de resultaten van kansberekeningen en ook op de beslissingen die men op basis daarvan wil nemen. De meest gebruikelijke hoofdindeling van onzekerheden is die in "natuuronzekerheid" en "kennisonzekerheid". De natuur of inherente onzekerheid kan geassocieerd worden met dobbelsteenworpen, rivierafvoeren, ruimtelijke fluctuatie van grondeigenschappen, enz. Hiermee hebben de meeste mensen weinig moeite. Het gaat om variabiliteit die zichtbaar en meetbaar is en, althans in beginsel, in objectieve termen te beschrijven valt. De tweede categorie, de kennisonzekerheid of epistemische onzekerheid, is de onzekerheid die ontstaat door een tekort aan informatie. Voorbeelden zijn een beperkt aantal waarnemingen of een sterk geschematiseerd rekenmodel. De interpretatie daarvan levert in de praktijk vaak grote interpretatieprobleem op. Er is per definitie geen volledig objectieve beschrijving mogelijk en men is in ieder geval gedeeltelijk aangewezen op "intuïtie", "expert opinion", "engineering judgement" en dergelijk

Nieuwe brugbelastingen

De brugbelasting in Nederland wordt geregeld via de VOSB. De nu nog officieel vigerende versie dateert van 1963 en is hoognodig aan herziening toe. Aanvankelijk was het de bedoeling een volledig nieuwe Nederlandse NNI-Norm te maken. Ter voorbereiding daarvan werd in 1991 door TNO een studie uitgevoerd, voornamelijk op basis van metingen in 1978. Toen echter bleek dat de Eurocode voor Brugbelastingen op vrij korte termijn beschikbaar zou komen, werd besloten geen nieuwe Nederlandse norm te maken, maar uit te gaan van de Eurocode. Dit Eurocode-deel kwam als Voornorm (ENV) gereed in 1995 en in 1997 werd begonnen met het schrijven van een zogenaamd National Application Document (NAD). In een NAD kan ieder land naar eigen inzicht bepaalde wijzigingen aanbrengen. Intussen waren er ook enkele nieuwe metingen ter beschikking gekomen (uitgevoerd op drie locaties in 1995 en 1997) en bleek bovendien dat een aantal stalen bruggen vermoeiingsproblemen vertoonden. Met beide zaken wordt bij het opstellen van het NAD rekening gehouden. Eurocode en NAD zullen uiteindelijk gezamenlijk worden uitgegeven als NEN 6707. De verwachting is dat dit in 1999 zal gebeuren

Reliability based structural design

According to ISO 2394, structures shall be designed, constructed and maintained in such a way that they are suited for their use during the design working life in an economic way. To fulfil this requirement one needs insight into the risk and reliability under expected and non-expected actions. A key role in this respect is played by the structural reliability analysis (SRA). In this paper the present state of the art will be summarised, including the simplifications to semi-probabilistic calculations as being used in daily practice. Although in principle the adopted Bayesian reliability approach should be able to take care of all uncertainties involved, present practice still uses other safety concepts like Robustness Design and Quality Assurance as tools for achieving the safety objectives of design and assessment. © 2014 Taylor & Francis Group, London.