Modal testing and FE model tuning of a lively footbridge structure

Despite huge advances in numerical modelling of civil engineering structures in recent decades, finite element models for footbridges should still be developed and used with caution when evaluating modal properties of these structures. This is due to some inherent modelling uncertainties related to a lack of information on the as-built structure, such as boundary conditions, material properties and the effects of non-structural elements. These are difficult to deal with at the design stage. A common method to rectify this problem is vibration testing of these structures after construction. As footbridges are unique prototype structures, testing at this late stage does not help very much in the design of the actual structure. However, combining testing and analysis improves understanding of its vibration behaviour, helps future designs of similar structures and provides key information for the design of remedial measures, if required.This paper describes a lively full-scale footbridge, its numerical modelling and dynamic testing. This was done using state-of-the-art procedures available nowadays for finite element modelling and frequency response function based modal testing. The time efficiency of the testing and parameter estimation procedures carried out without formally closing the footbridge is demonstrated as well as good quality of results achieved. The identified vibration parameters compare well with those from an ambient vibration survey where only the bridge responses were measured. Also, it was demonstrated that properly planned testing can be performed successfully even with some limited facilities, such as only two accelerometers available. The correlation between a very detailed finite element model and experimental results is then studied. For this particular structural system, stiffnesses of girder end supports in the longitudinal direction and bending stiffness of inclined columns were identified as the modelling parameters which influenced most strongly the vertical and the horizontal modes of vibration, respectively.