Dynamic characterization of complex bridge structures with passive control systems

The article focuses on dynamic identification strategies for complex bridge structures equipped with passive vibration control systems. In detail, a recently built curved twin-deck cable-stayed footbridge was analysed, whose structural complexity required wind tunnel tests and a specific design for vibration reduction. A passive control system was deemed necessary in order to: (i) meet wind safety requirements owing to premature aeroelastic instability, and (ii) mitigate pedestrian vibrations. Uncertainties in numerical modelling and changes during its construction suggested a modal testing campaign in order to check the effectiveness of the vibration absorption system. Different excitation sources related to output-only techniques were exploited, including ambient noise and free-decay oscillations through released masses. Several sensor set-ups were deemed necessary in the view of the structure complexity that exhibits numerous close modes and modal couplings between the two decks. The effect of the dampers was analysed by performing the testing campaign for two distinct configurations: (i) disconnected dampers, and (ii) connected dampers. The dynamic properties of the cables were also investigated in order to complete the whole dynamic characterization of the structure. Two time-domain techniques were applied and compared under different excitations. The dynamic identification procedure provided consistent results and highlighted that full functionality of the damping system was realized only for high vibration levels. Finally, time–frequency instantaneous estimators were applied in order to analyse both the modal frequency and the damping time-variation. These results revealed amplitude dependent behaviours as well as dynamic deck-cable interactions.