Experimental Modal Analysis and Fatigue Assessment on the Lagoscuro Viaduct

Abstract eng:
The functionality maintenance of infrastructures like bridges is acquiring more and more importance due to the huge economic losses related to the interruption of their regular service. This critical aspects is more stressed considering that new and existing bridges will be included in the new Trans- European Transport Network. In particular, fatigue represents one of the more diffused failure modes occurred in steel and composite steel-concrete bridges. Several studies were performed in the past in order to assess the fatigue resistance of steel and steel-concrete composite bridges; such studies were the base of modern codes and standards. Despite of these efforts, the fatigue assessment of railway bridges both considering the design of new bridges and the assessment of existing ones is one of the main issues in current practice. In fact, phenomena like “vibration induced” and “distortion induced” fatigue are still partially uncovered by actual design codes and represent critical aspects for the assessment of existing bridge remaining life and for the design of new bridges. In this context, the FEDLESS 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 train passage. 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 to control such effects during bridge design or assessment. In this paper, the preliminary results concerning the fatigue assessment of Lagoscuro railway viaduct will be described. The Lagoscuro viaduct is composed of 2 steel railway viaducts running parallel one to each other and crossing the Po river. The first viaduct was built in 1948 and it is composed by 9 single span truss-girder bridges; the upper and the lower chord, diagonals and stunts of the main truss girders are composed of 4 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 potentiate the railway line; the same geometry is adopted but, differently to the old viaduct, the truss girders are composed by H-shaped elements welded together in the joints. First, experimental dynamic tests are performed in order to identify the modal properties of both viaducts. Then, FE models are corrected in model updating procedures, in which the uncertain model properties are adjusted such that the numerical predictions correspond as closely as possible to the measured data, in term of modal frequencies and mode shapes. Starting from these models, a preliminary fatigue assessment is carried out following the procedure defined in EN1993 – Eurocode 3, part 1-9. In order to obtain the stress ranges on each element, dynamic analyses are performed with traffic mixes defined EN 1991 - Eurocode 1. Stress range spectra are determined and according to the Miner role the damage indexes are evaluated. Results obtained from old riveted viaduct are finally compared with those obtained from welded one.

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