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Improved assessment methods for static and fatigue resistance of metallic railway bridges in Europe
Laboratoire Central des Ponts et Chaussées, France.
Laboratoire Central des Ponts et Chaussées, France.
RWTH Institute of Steel Construction, Aachen.
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2006 (English)In: Bridge maintenance, safety, management, life-cycle performance and cost: proceedings of the Third International Conference on Bridge Maintenance, Safety and Management, Porto, Portugal, 16 - 19 July 2006 / [ed] Paulo J. S. Cruz; Dan M. Frangopol, London: Taylor & Francis Group, 2006, p. 751-753Conference paper, Published paper (Other academic)
Abstract [en]

A very important part of the bridges in the European railway networks are metallic bridges that have been built during the last 100 years (some of them are much older). The increasing volume of traffic and axle weight of trains means that for many structures the loads today are much higher than those envisaged when they were designed. This paper deals with the research program established to develop improved assessment methods for existing metallic bridges in the context of the work package 4 (Loads, Capacity and Resistance) of the 6th framework European project Sustainable Bridges. Three main research topics under development. The first topic concerns information about material properties of steel and iron used in old existing railway bridges. Any safety assessment for an old bridge requires the knowledge of the structural resistance and therefore the information of the material properties. For the resistance to static loads the material strength expressed as yield strength (fy) is the significant parameter. In order to ensure sufficient fatigue resistance, next to the classic fatigue methods using damage accumulation further assessment models have been established that are based on fracture mechanics. For the classic fatigue assessment the fatigue properties are needed. Fracture mechanical approaches, taking into account that crack-like defects are very likely to be in the structure, use the fracture toughness as material resistance. This is usually given by J-Integrals (Ji, Jc) or stress-intensity-factors (KIc). Also further crack growth parameters, e.g. threshold values for crack growth, are important. The early metal bridges in the 19th century were fabricated of cast iron or puddle iron (wrought iron). Puddle iron surpassed cast iron having a lower carbon content that goes along with a better ductility and it allowed forging and an easier workmanship. Yet at the beginning of the 20th century puddle iron was superseded by mild steels that obtained higher qualities concerning the chemical composition and cleanness as well as better technological material properties (e.g. weldability, strength). For these old metal bridges, that were built between 1870 and 1940 in particular, the material parameters are in many cases not available. One reason is that although steel production and construction technology (bolting and welding of joints) developed quickly, appropriate testing methods to examine relevant properties as toughness, fatigue, etc., were missing completely and were not available until many decades later. Therefore only fragmentary knowledge exists concerning early iron materials, complicating the handling and assessment of old metal structures. Therefore this research activity is based on the collection of material data from more than 120 old bridges from Sweden, France and Germany. The second topic is associated with the development of new assessment methods for resistance of riveted structures. This mainly concerns the study of resistance and deformation capacity of cross sections formed by riveted slender plates. Modern standards for design of steel structures like Eurocode 3 cover riveted structures but they do not give complete information. Old design standards on the other hand are quite incomplete concerning instability phenomena and they cover elastic design only. In this research Eurocode 3 is considered as the starting point and some additional information relevant for riveted structures will be developed. The cross section classes in Eurocode 3 are essential in defining the resistance to bending moment. They are defined for rolled or welded sections but those definitions are not sufficient for riveted girders. First the maximum distance between rivets in the stress direction has to be defined. Further, there are some beneficial effects of confinement of plates in certain cases. The traditional method for assessing the resistance of steel bridges is based on elastic analysis. In case the resistance in ULS is insufficient it is possible that allowing for plastic deformations gives a more favourable answer. This is very obvious if the girders are stocky enough for using plastic hinge analysis. This is rarely the case but also more slender girders have some plastic deformation capacity, which can be utilized for a limited redistribution of moments in the girders. The third research activity is related to the assessment of fatigue life of riveted and welded bridges. Fatigue related failures are the most common cause of failure of riveted bridges. Riveted structures were constructed over a period of more than 100 years up to the 1950s. A large number of riveted bridges (thousands) can be still found on the European railway networks. Constantly increasing loads and the fact that these bridges were not explicitly designed against fatigue raise questions regarding their remaining fatigue life. Economically its not justified to replace a bridge when it reaches the end of its design life. Often the design life it's an arbitrary value and there is considerable reserve. As is well known, metal fatigue exhibits high levels of uncertainty and can be influenced by a very important number of structure and environmental factors. An important amount of service life may be justified by a better knowledge of the fatigue behavior of riveted connections. Furthermore, the load history, which plays a main role in fatigue life evaluation, is largely unknown in most of cases. In that context, there appears to be a need to develop a comprehensive fatigue assessment methodology for riveted railway bridges. Since the 1950s welding become a useful procedure for assembling components of metallic bridges. In welded joints cracks are often localized at the welding. Indeed the welding process induce some defects which help small cracks to appear. These defects can growth under cyclical loading and can induce the joint failure, and depending of the redundancy degree of the bridge can lead to the failure of the structure. The conditions governing crack growth are respectively structural geometry, initiation site, material characteristics and loading. In general all these conditions are highly random. Therefore, an appropriate analysis of fatigue phenomena consists by treating the problem in a probabilistic manner. Fatigue durability and inspection planning are then very important issues in the design and scheduled inspection of welded bridges. Usually welded structures have to be designed for a finite life with an accepted probability of failure based on S-N approach. Consequently cracks may propagate and become critical during the estimated safe-life, unless detected in time and repaired. If fracture is not acceptable, supplementary safety measures must be taken through in-service inspection requirements specifying appropriated non-destructive inspection techniques (NDT) and inspection planning. This leads to the damage tolerance concept: a welded joint containing a crack has to resist the service loadings for some time. All through this time there must be a large probability that the crack can be detected (and repaired) before it becomes critical. In order to verify this probability, the reliability against such a failure have to be evaluated as a function of service time in support of inspection strategy. The fracture mechanics and the reliability theory provides the necessary tools for these calculations. This approach (so called probabilistic fracture mechanics) is used on the assessment of welded joints of metallic bridges. This kind of analysis is carried out using the first order reliability method (FORM) or Monte Carlo simulations, which have become the standard methods in structural reliability. A limit state is formulated by applying linear elastic fracture mechanics (LEFM) the uncertainties of the main parameters can be considered by treating them as basic random variables. One problem, which usually appears, is that the necessary statistical informations of these variables (mean value, standard deviation, and distribution type) are not known. Another problem is that this kind approach does not give the statistical distribution of the cumulated damage, or in different words, the statistical distribution of the crack sizes at a given time. This information is important to evaluate the performance of different NDT methods used to control welded joints in bridges. An alternative approach, based on concept of Markov chain is proposed.

Place, publisher, year, edition, pages
London: Taylor & Francis Group, 2006. p. 751-753
National Category
Building Technologies Infrastructure Engineering
Research subject
Steel Structures; Structural Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-31937Scopus ID: 56749183849Local ID: 642c1860-7acd-11df-ab16-000ea68e967bISBN: 0-415-40315-4 (print)ISBN: 978-0-415-40315-3 (print)OAI: oai:DiVA.org:ltu-31937DiVA, id: diva2:1005171
Conference
International Conference on Bridge Maintenance, Safety and Management : 16/07/2006 - 19/07/2006
Note
Godkänd; 2006; 20100618 (andbra)Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2017-11-25Bibliographically approved

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