Mechanism And Aerodynamic Model Of Nonlinear Flutter Of Long Span Bridge

With development of bridge engineering,the span of bridges has been continuously improved,and the long-span suspension bridges have been widely used ’all over the world.As a typical flexible structure,the long-span suspension bridges are very sensitive to wind loads.The wind-induced vibration of bridges has been regarded as significant factor which has to be considered in the design of longspan suspension bridges.The flutter of long-span suspension bridge is a very harmful wind-induced vibration,which possibly cause the damage of bridge structures and even directly collapse.Therefore,it is extremely necessary to develop the flutter research of long-span bridges.The flutter is divided into two categories in general views,that is,the classical divergent flutter and the post-critical limit cycle vibration(LCO)of flutter.Moreover,in the present study,we experimentally found that a nonlinear hysteresis phenomenon will take place under large angles of attack or some nonlinear structural damping conditions.In fact,the linear theory of classical divergent flutter has been applied well to predict the flutter critical wind speed in the bridge engineering.However,for the post-critical LCO and flutter hysteresis phenomenon,there is no universal flutter model to describe the complicated and interesting phenomenon.There is a great significance to carry out a further research to explore the mechanism of the post-critical LCO and hysteresis phenomenon.In the paper,the spring-suspended segment model is adopted to conduct the wind tunnel test.The aspect ratios of the rectangular section models,the structural damping conditions and the wind attack angles are adjusted to induce different types of flutter behaviors and investigate the mechanism difference between them.The detailed content of this thesis is listed as follows:Ⅰ.The nonlinear dynamic parameters of the spring-suspended segment model system are investigated.The nonlinear dynamic parameters of the system are divided into the mechanical part and the still-air aerodynamic part.The identification of dynamic parameters under zero wind speed are carried out through the free vibration tests,The identification results pointed out that the natural frequency of the dynamic system is little affected by the amplitude,and the stiffness nonlinearity can be ignored.Nevertheless,the mechanical and still-air aerodynamic damping are greatly affected by the vibration amplitude,and the nonlinear effects have to be considered.Ⅱ.The time histories of flutter vibration are measured by two laser displacement meters.The relationship between amplitude and wind speed and the characteristics of the phase space and frequency domain are analyzed and the influence of the model aspect ratios and the wind attack angles on flutter behavior are investigated.Ⅲ.the transformation of flutter type is explored by changing the mechanical damping of the system.The mechanism of flutter hysteresis phenomenon is discussed on terms of the changing rules between the aerodynamic damping and mechanical damping of the dynamic system.Finally,a single-degree-of-freedom nonlinear flutter model is built to descript the complicated flutter hysteresis phenomenon.