Keywords: fatigue, bridge, ambient vibration test, long term monitoring, dynamic analysis, FADLESS.

The fatigue analysis of steel and composite bridges under real traffic is a challenging task. In situ monitoring of all the details of the structure is not economically (and even technically) feasible and consequently, numerical models are usually employed for that analysis. The common assumption is that if the numerical model reproduces well the real behaviour of the structure at some reference points, extrapolation of the numerical model results to other points is reasonable and accurate, from an engineering point of view. This paper, explores the different aspects of the fatigue analysis of the new Alcácer do Sal bridge, in Portugal, in the context of the FADLESS European project.

In Section 2 a description of the bridge is given, while in Section 3 the numerical model of the bridge is presented. Section 4 focuses on the experimental work developed with the aim of characterising the structure. In this context, an ambient vibration test of the bridge was performed and a large number of vibration modes were extracted, both with the EFDD and the SSI-UPC merged test setups algorithms. The latter algorithm was especially efficient and allowed the extraction of different vibration modes contained in very narrow frequency bands. The results of the AVT will be used on the calibration of the finite element model of the bridge. A long-term monitoring system was also built to characterise the traffic and its corresponding effects at some fatigue relevant points of the structure. The system was applied both in a static load test and in operation. The traffic characterisation system was of paramount importance, as it allowed the comparison between the experimental and numerical results, which were, generally, in close agreement.

Finally, Section 5 presents some of the results of the fatigue analysis performed on the structure, according to the Eurocode 3 [1]. The most critical detail was the top extremity of the diagonals of the last diaphragm of the bridge. For that detail, the worst loading case was the crossing of two standard fatigue trains at the last span of the bridge. Even so, low damage levels are foreseen. The global modes of vibration, with lower frequencies, mostly determine the total response of the bridge.

1

Eurocode 3, "Design of steel structures. Part 1-9: Fatigue", EN 1993-1-9. European Committee for Standardisation, 2005.

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