Investigation of three-dimensional static wind nonlinear effects on nonlinear flutter of long span bridges using a multimodal approach

To investigate the influence of three-dimensional (3D) static wind nonlinear effects on the nonlinear flutter of long span bridges, a multimodal coupled nonlinear flutter analysis method considering 3D static wind nonlinear effects is established, taking a double-deck steel truss girder suspension bridge as a case study. First, amplitude-dependent flutter derivatives under different wind angles of attack (AoAs) are obtained by identifying free-vibration wind tunnel test results of a section model. Additionally, the 3D additional wind AoAs for the main girder are derived from analyzing the bridge's 3D static wind response. Finally, the influence mechanisms of multimodal coupling and 3D static wind nonlinear effects on the bridge's nonlinear flutter performance are investigated through 3D multimodal coupled nonlinear flutter analysis. The results show that lateral flutter derivatives and the first-order symmetric lateral-bending mode influence the 3D nonlinear flutter response, with their effects modulated by the initial wind AoA. The two-dimensional (2D) closed-form solution based on the “strip assumption” significantly underestimates the steady-state amplitude by neglecting the 3D spanwise amplitude effect in large-amplitude nonlinear flutter analysis. Higher-order vertical modes influence the steady-state amplitude by altering modal damping while also modifying the amplitude distribution pattern. Modes coupled with a weak torsional mode suppress the torsional amplitude. The 3D static wind nonlinear effect impacts nonlinear flutter performance by introducing nonuniform 3D static wind additional AoAs. Moreover, this effect exhibits significant dependence on the initial wind AoA, demonstrating substantial influence on large-amplitude nonlinear flutter in high wind speed regions under an initial AoA of 3°.