Numerical Study On Nonlinear Aerodynamic Forces And Flutter For Long-span Suspension Bridges With Steel-box Girder

The increasing span of the suspension bridges can significantly intensify the structure's flexibility.However,varies of experiments and theories show that conventional flutter analysis,which is based on the linear theory,cannot satisfy the wind resistant for the design of long-span bridges.Because of the flaw of the linear theory,more and more researchers in recent years are trying to explore the nonlinearity of the wind-induced responses of the bridges.In this context,the CFD(Computational Fluid Dynamics)software was adopted in this paper to simulate the nonlinear aerodynamic force,which was systematically studied to summarize its intrinsic nonlinear features.In order to consider the nonlinear vibration of the bridge structure,the differential equations for the single-span and double-span suspension bridges were deduced.With the differential equations of the structure and the proposed nonlinear aerodynamic load,the nonlinear structure-wind system is built and can be used to study the wind-induced responses of suspension bridges.Details of this study are summarized as followings:Varies typical sections of suspension bridges,including flat steel box girder,separated steel box girders and the Tacoma's H-shape girder,were adopted to generate the aerodynamic force with CFD.The influence of bridge surfacing,facilities and the amplitude on the proportion of nonlinear components were studied.A model of nonlinear self-excited aerodynamic force was set up based on self-excited aerodynamic force's feature “multiple frequency”.This model's resolution,parameter identification method and parameter-amplitude correlation were analyzed.Differential equations of suspension bridge's vibration were set up.Differential equation of single-span suspension bridge's vibration is familiar in many papers.So differential equation of double-span suspension bridge's two-degree vibration was extended from it.Self-excited responses of suspension bridge were solved by differential equations.A new method for searching suspension bridges' flutter-critical wind speed was set up based on differential equations.Comparing the results of linear and nonlinear self-excited aerodynamic force,the conclusion is “the necessary condition of ‘soft flutter' is nonlinear self-excited aerodynamic force”.Modes transition phenomenon of MaAnShan Bridge was studied by modal truncation method,including first step of girder-bending mode,first step of girder-torsion mode and second step of girder-torsion mode.When give an initial exciting to step of girder-bending mode,first and second step of girder-torsion modes will exist half-phase difference.It can proof energy exchange between the two modes.