Keywords: fatigue analysis, dynamic identification, railway, steel viaduct, riveted joint, model updating, evolutionary algorithm, local vibrations.

The FADLESS research project aims to define innovative technical guidelines for the assessment and control of existing and new bridges with regard to fatigue phenomena induced by vibrations and distortions produced by the passage of trains. The project will employ the most recent experimental and numerical techniques in order to identify the most typical details frequently subjected to high fatigue effects and to draft technical guidelines suitable of controlling such effects during bridge design or assessment.

As part of the research project, this paper describes some results concerning the fatigue assessment of the Lagoscuro railway viaduct. The Lagoscuro viaduct is composed of two parallel steel railway viaducts crossing the Po river. The first viaduct was built in 1948 and is composed of nine single span truss-girder bridges; the upper and the lower chord, diagonals and struts of the main truss girders are composed of four L-shaped steel elements, riveted together by means of plates; stringers and additional elements supporting the railway lines are also riveted. The new Lagoscuro viaduct was recently built in order to improve the railway line; the same geometry has been adopted but, different to the old viaduct, the truss girders are composed of H-shaped elements welded or bolted together in the joints.

First, the results of the experimental dynamic tests [1] are performed in order to identify the modal properties of both viaducts. Then, global finite element models are corrected according to the model updating procedures, where the uncertain model properties are adjusted in order to obtain numerical predictions as close as possible to the measured data, in terms of modal frequencies and mode shapes. An evolutionary algorithm [2] is used. After having identified the members potentially subject to fatigue or local vibrations, a substructure finite element model containing the specific member has been studied. The detailed substructure model has been then excited by prescribing at the boundaries the displacement time histories obtained in the corresponding nodes of the global model. From the stresses obtained in the detailed finite element model, the fatigue assessment is carried out following the procedure defined in EN1993 - Eurocode 3 (part 1-9) and the results are compared to those obtained with the procedure proposed in [3]. In order to obtain the stress ranges on each element, dynamic analyses are performed with traffic mixes defined according to EN 1991 - Eurocode 1. Stress range spectra are then determined and the damage indexes are evaluated according to the Miner role.

1

B. Cauberghe, "Applied frequency-domain system identification in the field of experimental and operational modal analysis", PhD thesis, Vrije Universiteit Brussel, 2004.

2

L. Vincenzi, M. Savoia, "Improving the speed performance of an evolutionary algorithm by second-order cost function approximation", Proceedings of 2nd International Conference on Engineering Optimization, Lisbon, Portugal, 6-9 September 2010.

3

L. Georgiev, "Fatigue life analysis of an old riveted bridge based on S-N lines", Proceedings of 7th International Conference on Bridges across the Danube, Sofia, Bulgaria, 14-15 October 2010.

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