Dynamic tests on the Millau viaduct

In November 2004, the Public Works Regional Laboratory of Lyon has operated dynamic tests on the Millau viaduct. The purpose of these tests was to identify the modal characteristics as reference values for future monitoring and to compare them to the theoretical values. It was in particular the opportunity to assess the real structural damping. Measurements were carried out in partnership with three companies or institutes: ADVITAM, SITES and CSTB. Excitation was performed by a tended cable fixed under the deck, which was cut to provide decayed response. This cable was placed at the third part of the span (laterally shifted towards the bottom bridge deck) to create vertical and torsional movements. The equipment made it possible to record signals with a sufficient sensitivity level. By means of the vectorial random decrement technique, an advanced analysis of these recordings had been performed. The results show that 13 modes are lower than 1 Hz, some being very close or corresponding to multiple modes. The corresponding damping ratios are very high, highlighting the large capacity of structural dissipation. A complete examination of all the measurements carried out during the tests is presented.