Study On Nonlinear Wind Load And Wind-Induced Vibration Of Long Span Bridges

With the increase in span length of bridges,wind-induced nonlinear vibration associated with vortex-induced vibration(VIV)and flutter and the stochastic buffeting response due to wind fluctuations have become the key factors in the design of long span bridges.Continued research efforts toward developing physically more meaningful and computational effective nonlinear aerodynamic force model and flutter analysis framework as well as the qerodynamic admittance function are required.This study presents a comprehensive investigation of the nonlinear vibration characteristics of vertical and torsional VIVs and coupled flutter of a streamlined box bridge deck section at different wind speed and angle of attack through wind tunnel test and theoretical analysis.A Hilbert Transform(HT)approach is proposed to determine the nonlinear aerodynamic damping and nonlinear flutter derivatives based on free vibration response.More reasonable analysis frameworks for both two-dimensional(2D)and three-dimensional(3D)of VIV and nonlinear flutter are established,which provides new insights that leads to improve understanding and quantification of nonlinear self-excited forces and their effects on bridge VIV and flutter.On the other hand,this study investigates the aerodynamic admittance function and the coherence of buffeting forces of a streamlined deck section at different turbulent wind fields and angles of attack through synchronous pressure measurement in wind tunnel.The influence of using different aerodynamic admittance and coherence function on buffeting response is analyzed.The main research contents of this thesis including the following aspects:(1)The nonlinear characteristics of vertical and torsional VIVs and couple flutter of a streamlined box bridge section at different angles of attack and wind speeds are investigated through wind tunnel test,where the section model is spring-supported in two degrees of freedoms(2DOFs)in both vertical and torsional directions.Based on the free vibration response and the proposed HT approach,the amplitude-dependent nonlinear aerodynamic damping and other response modal characteristics(e.g.,modal frequency,modal damping,amplitude ratio and phase difference between vertical and torsional displacements)are determined.The nonlinear aerodynamic damping at a given wind speed and angle of attack is modeled as a polynomial function of vibration amplitude.The model is proved to be more flexible and accurate in describing the amplitude dependence of nonlinear aerodynamic damping compared to currently widely used model.(2)An approach for extracting nonlinear flutter derivatives based on coupled 2DOF free vibration test of section model in wind tunnel.The nonlinear flutter derivatives associated with VIV and flutter at different angles of attack and wind speeds are subsequently extracted and modeled as polynomial functions of vibration amplitude.The nonlinear flutter derivatives are used to predict VIV and flutter amplitudes and then compared with wind tunnel data,which validates the accuracy of the proposed approach of extracting nonlinear flutter derivatives.The obtained nonlinear flutter derivatives can be used to examine the influence of various structural parameters and vibration control strategies on VIV and flutter.(3)With the extracted nonlinear flutter derivatives,an approach is proposed for predicting the 3D VIV amplitude response of long span bridges at different mode with consideration of both spanwise variation of vibration amplitude and spanwise correlation of aerodynamic forces,through which the VIV characteristics of a long span suspension bridge with a main span of 1700 m at different mode is analyzed.The effect of different structural damping and spanwise correlation function of aerodynamic forces on the VIV response is discussed.When the spanwise correlation function of aerodynamic forces is ignored,the 3D analysis leads to a significant higher VIV amplitudes as compared to 2D analysis.The significance of the consideration of spanwise variations and correlation of self-excited forces is highlighted.(4)An approach is proposed for predicting the 3D bimodal coupled nonlinear flutter response of long span bridges.The amplitude responses of 3D coupled nonlinear flutter of the1700 m long span suspension bridge is calculated and compared with the 2D estimation.Results show that the 3D analysis leads to a significant lower nonlinear flutter amplitudes as compared to 2D analysis.The nonlinear flutter characteristics and internal mechanism at different angles of attack are discussed from the perspective of contribution of different components of self-excited forces to system damping.(5)The two-wavenumber admittance based on the rapid distortion theory can be used to determine the single-wavenumber admittance and spanwise coherence of buffeting forces under actual turbulence that can have different characters from those in wind tunnel test.The characteristics of one wavenumber,two wavenumber and spanwise coherence of buffeting forces of a streamlined box bridge deck are investigated comprehensively at different turbulent wind fields and angles of attack based on synchronous surface pressure measurement in wind tunnel.The influence of model length or turbulence coherence on the one wavenumber admittance that obtained from force balance method in wind tunnel is calculated and discussed.The increase of model length in turbulent flow field will lead to the decrease of admittance that obtained from force measurement The buffeting response of a long suspension bridge with a main span of 1700 m influenced by using different aerodynamic admittance and spanwise coherence is analyzed and compared.