Dynamic Properties from Full Scale Recordings and FE-Modelling of a Slender Footbridge with Flexible Connections

At the opening of the Lardal Bridge large lateral pedestrian-induced vibrations caused by a relatively dense and continuous flow of people were observed, similar to the observations made at the Millennium Bridge in London and the Solferino Bridge in Paris. The observations made on the Lardal Bridge initiated extensive investigations, commencing with quantifying the structural properties such as eigen-frequencies, mode shapes and damping, with the idea of tuning a numerical model to the recorded events. This paper deals with the development of a Finite Element (FE) model, an unusually demanding task as the design contains connections and joints with considerable flexibility, for example, between the steel wind truss and the laminated wooden beams or the sliding joints at both ends of the mid-span. These are flexibilities that if unaccounted for will greatly overestimate the global structural stiffness. The FE modelling process showed that any small alteration of model input may imply large changes to the estimated values of the important structural properties. Furthermore, possible structural modifications to alleviate the problem for the Lardal Bridge have been presented, including measures that can be applied to the present design, as well as modifications that should rather have been made at the design stage.