Fiutter Derivatives Research Of Large Span Bridges Under Skew Wind

On study the wind induced vibration of large span bridge under wind effects, only the wind whose direction is perpendicular to the bridge longitudinal direction is studied in conventional method. Therefore the conventional section model tests of bridge flutter tests are mainly conducted in the wind of normal direction. Whereas the wind in the bridge site is often not perpendicular to the bridge longitudinal direction and the corresponding study suggests that the most unsafe flutter state for bridge may happened under skew wind. Thus, it is necessary to conduct the identification of flutter derivatives research for large span bridge for the security of wind resistance. Based on the Forth Nanjing Yangtze River Bridge whose girder is flat closed box girder, the paper studies change rule of flutter derivatives and flutter critical wind speed under the skew wind effects.The paper first introduces the currently study of flutter derivatives under skew wind and underlies the necessity of the research. The origin and development of flutter derivatives are involved in the paper. The principles and characteristics of various methods for identifying flutter derivatives are analyzed and the program by least square method for the identification is tested.Next, Based on the Forth Nanjing Yangtze River Bridge,the fabricating and installation of skew section model and the testing equipments are introduced in detail. A series of section model of flat close box girder wind tunnel tests in different attack angles(-3°,0°, and3°) and yaw angles (0°,5°,10°,15°, and20°) are conducted in order to recognize the flutter derivatives. The flutter derivatives are identified by sub-interrelated MATLAB program and the change law between flutter derivatives and wind yaw angles is analyzed by the drawing.In the last, the flutter critical wind speeds of the Fourth Nanjing Yangtze Bridge in various wind attack angles and yaw angles are calculated by the identified flutter derivatives so that the influence of wind yaw angle on bridge flutter critical wind speed could be studied. The study suggests that there is a correlation between the change law and the bridge cross section’Relative aerodynamic shape’. The flutter critical wind speed increases with the increase of wind yaw angle when the value of attack angle is0°while a fluctuating change law without any regular pattern will be witnessed if the value of attack angle is not0°. The minimum flutter critical wind speed is often appears when the bridge suffers the yaw wind.