Influence of the initial amplitude on the flutter performance of a 2D section and 3D full bridge with a streamlined box girder

The flutter performance of a streamlined box girder under different external excitations is experimentally examined in this study using a free-vibration wind tunnel test of a section model. The dependence of the critical wind speed on the initial torsional amplitude is observed in the test. The nonlinear flutter derivatives as functions of the reduced wind speed and vibration amplitude are then identified using a forced-vibration wind tunnel test. The results reveal the close relationship of the torsional-motion-related flutter derivatives with the nonlinearity emerged in the flutter of the girder. The mechanisms leading to the dependence of the flutter response and critical wind speed on initial conditions is investigated through a flutter analysis in which the modal properties are expressed as functions of the wind speed and amplitude. The results indicate that the uncoupled aerodynamic negative damping plays an important role in weakening the flutter performance of the girder. Finally, the influences of the modal coupling effect and the spanwise variation of the self-excited forces (or equivalently, the flutter derivatives) on the modal properties and the flutter performance of a full bridge are discussed. The results show that the modal frequency of the full bridge is mainly affected by the spanwise variation of the nonlinear flutter derivatives, while the modal damping ratio (or equivalently, the critical wind speed) is affected by both the spanwise variation of the nonlinear flutter derivatives and the mode coupling effect.