The construction in Portugal of a relatively long and slender footbridge prone to vertical and lateral vibrations induced by the action of pedestrians motivated the full assessment of the corresponding dynamic behaviour with the purpose of installing a control system. The current paper describes the studies undertaken at a design stage and the results of a thorough experimental assessment of the constructed bridge properties This experimental test allowed in particular the identification of modal parameters and the observation of the lateral "lock-in" effect under the crossing of the footbridge by pedestrian streams prior to installation of control devices The number of pedestrians to trigger lock-in was found to verify the formula developed within the Millennium bridge studies. Furthermore, the full characterisation of the bridge behaviour was of utmost importance in the design of a control system based on tuned mass dampers that is discussed in a companion paper.
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