Multi-mode vortex-induced vibrations characteristics based on a full bridge aeroelastic model

An increasing number of real bridges experience multi-modal vortex-induced vibrations (VIVs). In this paper, the multi-modal VIV characteristics of flexible suspension bridges are investigated based on a full-bridge aeroelastic model in a wind tunnel. First, the generalized free decay method is employed to determine equivalent linear damping ratios and amplitude-dependent damping ratios. It is found that the higher order structural and aerodynamic damping ratio exhibit a significant nonlinearity increase as the amplitude increases. The time-frequency-energy spectrum method is used to capture the mode-switching/competition phenomenon, and it is analyzed quantitatively in terms of modal weight ratios and energy transfer. The results indicate that the multi-modal VIV amplitude is not only related to the Scruton number (Sc) and mode shapes but also directly associated with the modal weights and energy ratio. Other things being equal, higher modal weight ratios and concentrated energy lead to larger amplitudes. In addition, mode competition occurs mainly during mode switching, and its competitiveness is related to the damping ratio. The main features are as follows: in temporal and spatial terms, the response exhibits a quasi-stable periodic beating motion characterized by significant nonlinear effects; in the frequency domain, there is a common check and balance of two or three adjacent frequencies, with the modal weight ratios alternately dominating; and in terms of energy, mode-to-mode energy is transferred and balanced dynamically, which can lead to a decrease in the VIV amplitude during the transition.