Ambient and free vibration tests of the Millau Viaduct: Evaluation of alternative processing strategies

The Millau Viaduct is an outstanding multi-span cable-stayed bridge located in southern France that opened to traffic in 2004. It is the tallest vehicular bridge in the world, with the top of a pylon rising at 343 m above the river level, and due to its total length of 2460 m, it also the longest cable-stayed bridge in the world. The present work aims to demonstrate the usefulness of ambient vibration tests performed at the commissioning stage and is specially focused on the evaluation of the efficiency and accuracy of alternative state-of-the-art processing strategies for identification of the modal parameters of the tested structure. Since both ambient and free vibration tests were performed, the modal damping ratio estimates obtained with the two testing approaches are compared. Furthermore, the natural frequencies and mode shapes provided by the ambient vibration test are correlated with modal estimates derived from a numerical model developed at the design stage. At the end, it is demonstrated that the data collected during the ambient vibration test, which only took some hours and very few resources, processed with adequate algorithms provided very useful information. It enabled the experimental identification of more than 20 natural frequencies and mode shapes that permitted the validation of their numerical counterparts and provided modal damping ratio estimates in good agreement with the values derived from the time series collected during the less economical and practical free vibration tests.