Research On Longitudinal Vibration Characteristics And Restraint Measures Of The Stiffening Girder Of A Long-span Asymmetric Highway Suspension Bridge

With the wide applications of the long-span highway suspension bridges,floating systems or semi-floating systems are always adopted as the structural forms,which has the characteristics of low longitudinal rigidity and high structural flexibility.As a result,the problem that the longitudinal vibration of the structure under wind load and traffic load(such as wind-vehicle-bridge coupling vibration)is fairly prominent.However,the current researches on the longitudinal vibration and control of the main girder under the combination of the wind load and traffic load during operation phase are relatively limited.Therefore,it is necessary to research on the longitudinal movement characteristics and restraint measurements of the stiffening girder of the long-span asymmetric highway suspension bridges.To study the longitudinal vibration characteristics of the long-span highway suspension bridge under the combined actions of wind load and traffic load,taking Yunnan Honghe super bridge as the engineering background,three-dimensional finite element analysis model of the bridge is established by the finite element analysis software ANSYS.The improved stochastic traffic flow is simulated based on the concept of cellular automation(CA)as well as the existing traffic model,and the stochastic wind field is simulated as the stationary Gaussian processes.The numerical framework systematically incorporates the dynamic interactions among wind,traffic flow and bridge,which is solved through the universal programming language MATLAB.With the established numerical framework,the longitudinal displacement time histories,the maximum longitudinal displacement as well as the cumulative displacement of the stiffening girder under individual wind load,individual traffic load and the combined wind and traffic loads are thoroughly investigated.In order to research on the longitudinal vibration control of the long-span highway suspension bridge under stochastic wind and traffic loads,based on the existing wind-vehicle-bridge coupled dynamic system,the fluid viscous damper is integrated into the system.And the longitudinal displacement time history and spectrum of the stiffening girder,parameter sensitivity and optimization analysis of the fluid viscous damper under individual wind load,individual traffic load and the combined wind and traffic loads are deeply investigated.The research results can indicate the following conclusions:(1)The maximum longitudinal displacement of the stiffening girder under low wind speed(less than 5m/s)is much lower(less than 5%)than that induced by the traffic load.Nevertheless,as the wind speed increases to 20 m/s,the wind induced maximum longitudinal displacement of the stiffening girder increases to about 57.7%and 24.2%of those induced by the free-flow traffic load and moderate-flow traffic load respectively.(2)The coupling effects due to wind-vehicle-bridge interaction is significant,and the use of superposition approach by ignoring the coupling effects could lead to significant underestimation of the maximum longitudinal displacement of the stiffening girder.(3)The maximum longitudinal displacement of the stiffening girder under individual wind load follows the log-normal distribution,while the maximum longitudinal displacements under individual traffic load or combined wind and traffic loads follow the generalized extreme value distribution.(4)Although the contribution of wind load to the maximum longitudinal displacement of the stiffening girder is insignificant compared to that induced by the traffic load,the contribution of wind to the cumulative displacement cannot be ignored.This is due to the fact that the vibration frequency of the wind-induced longitudinal displacement time histories is much higher than those induced by the traffic load.(5)After implementing the fluid viscous damper,the amplitude of the longitudinal vibration of the stiffening girder under various loading scenarios can be mitigated,while whether the vibration frequency can be reduced depends on the dominant frequencies of the longitudinal vibration.(6)Both the maximum longitudinal displacement and the longitudinal cumulative displacement of the stiffening girder under wind only,traffic only or the combined wind and traffic loading scenarios can be reduced effectively after the implementation of the fluid viscous damper;and the performance of the fluid viscous damper in mitigating the longitudinal vibration increases with larger damping coefficient and smaller damping exponent.(7)The influence of the fluid viscous damper on the tower bending moment depends on the loading type;the use of the fluid viscous damper could enlarge the tower bending moment under wind-only loading scenario.(8)For the bridge case researched in this paper,the response surface method is employed to seek the optimum fluid viscous damper parameters for the purpose of optimizing the structural mechanical behaviors as well as reducing the longitudinal vibration of the stiffening girder under stochastic wind and traffic loads.The optimization results show that the optimum damping coefficient ranges from 500 to700k N/(m/s)~α,and the optimum damping exponent ranges from 0.3 to 0.5.