Flutter Mechanism of Flat Box Girder under Different Attack Angles

The effect of an additional attack angle induced by aerostatic force on flutter instability of long span bridges is significant, thus the study regarding the flutter mechanisms of a flat box girder under different attack angles is important and necessary. Flutter derivatives have been identified using forced vibration devices and the critical velocities have been calculated based on the bimodal coupled flutter analysis method. Based on this analysis scheme, the roles of aerodynamic damping, phase lag, and modal frequency, which aid to better understand the flutter mechanism of the girder under different attack angles, are intensively analysed. The study results indicate that uncoupled aerodynamic forces provide energy to enhance the stability of the systems that result in a higher critical flutter speed at 0° and 3° compared with the speed at 5° The negative aerodynamic damping induced by the coupled force increases rapidly while the positive damping provided by the non-coupled force decreases significantly at 5° attack angle, which results in the weakened flutter performance. The phase lag of coupled flutter is the primary parameter for increasing the negative damping, and the effect of aerodynamic amplitude is negligible. The mechanism indicates that the weakening flutter performance of a flat box girder at large attack angle is increased primarily owing to coupled motion, not induced by negative damping generated in the torsional flutter.