Experimental Investigation Of Post-flutter Behavours Of Bridge Structures

Wind-induced is one of the key issues during the design of long-span flexible bridges,and is of the utmost importance for wind-resistant design of bridges.The linear framework for bridge flutter established in 1960-1970 s is based on linearized aerodynamic forces and also does not consider structural nonlinearity,which leads to infinite amplitude of vibration of bridges once flutter occurs.In reality,the windinduced flutter is generally featured as limit cycle oscillation(LCO)due to presence of both nonlinearities in aerodynamic forces and structures.Due to complicated features of nonlinear vibration,the post-critical behaviors of flutter are seldom addressed.In this paper,the vibration mode and mechanism of the bridge's post-critical flutter are studied by the wind tunnel test of the sectional model and the aeroelastic model,and the study on the post-critical flutter of the nonlinear aerodynamic model is carried out based on the quasi-steady theory.The main contents and conclusions of this paper are as follows:(1)The existing linear flutter theory of the bridge and the research status of the post-critical flutter are reviewed,and some existing problems are summarized,and the main contents of this paper are put forward.(2)Based on elastic suspension test of the sectional model,the post-critical flutter response of blunt body section is studied.First,the nonlinear characteristics of the frequency and damping ratio of the sectional model under the state of elastic suspension are analyzed.With the amplitude increases,the vertical and torsional frequencies changes very little.With the amplitude increases,the damping ratio of the torsional and vertical increases greatly.The high added steady angle of attack will occur at the high wind speed,and the aerostatic displacement has a great influence on the flutter response.This effect can not be ignored in the post-critical flutter response.Even for the bluff body section,with the wind speed increasing to a high level,the vertical oscillation is inclined to play more and more important roles in the vertical-torsional vibration.The influence of damping ratio on structural vibration is discussed.It is concluded that structural damping has a positive effect on the critical velocity and torsional amplitude.This provides alternative engineering measures for improving the flutter performance of bridges.(3)Taking the Zhangjiajie glass bridge as the engineering background,the wind tunnel test of the full bridge aeroelastic model was carried out,and the flutter characteristics of the open and closed box girder sections were studied.The results show that the critical velocity of the opening section is lower than the closed box girder section,but with the increase of wind speed,the torsion amplitude increases more slowly than the closed section.It shows that the blunt body section with strong aerodynamic nonlinearity has better post-critical flutter characteristics,which will provide guidance for the selection of the girder section of long-span bridges.(4)Based on the quasi-steady theory,the post-critical flutter study of the nonlinear aerodynamic model of the two-dimensional sectional model is carried out.The post-critical flutter characteristics of the rectangular section with the height to width ratio 1:5 are analyzed,and the effect of the damping ratio on the amplitude of the post-critical flutter is studied.The above research results will help to improve the existing bridge flutter analysis theory system,and provide reference for further evaluation criteria of flutter amplitude on the basis of the critical velocity of flutter in the future,and provide alternative engineering measures for the future improvement of bridge flutter performance.