Wind Tunnel Investigations On The Nonlinear Aerodynamic Forces Of The Hangers Of Suspension Bridges

As the bridge engineering develops rapidly all over the world,advance design idea and method are applied to the construction of bridges.The length of hanger or cable increases as the span of bridges becomes larger and larger.Due to its characteristics of light mass,high flexibility and small damping,as well as its diversity arrangements,it is subject to a variety of wind induced vibration,which will affect the comfortableness of driving and safety.Referring to the reports and field measurements all over the world,many large span bridges suffer from wind induced vibrations of lar:ge amplitude,including the Great Belt East Bridge,the Akashi Kaikyo Bridge and Xihoumen Bridge.There are many reasons that cause the wind-induced vibrations of hangers,only the aerodynamic interference between multi-strand is investigated in this paper.According to the phenomenon observed on the bridge,the multi-strand subjected to the wake induced vibration could be simplified as twin-cylinder model,of which the windward strand keep fixed and the leeward strand vibrates.The quasi-steady theory and unsteady theory are used to make the analysis.The main contents are as follows.(1)Two smooth and rough ridge circular cylinder models were made to carried out wind tunnel tests to measure the aerodynamic forces of leeward cylinder in several cases.The spatial distributions of the mean drag coefficient and the mean lift coefficient were obtained.The Routh-Hurwitz stability criterion was derived to calculate the instable region of wake galloping.The results showed that the mean aerodynamic coefficient of smooth and rough leeward cylinder have the same chaage rule.CD is symmetrical concerning the centerline of the wake,while CL is antisymmetrical.They both changed rapidly within the distance of 2D from the wake centerline.The instable region of wake galloping of the leeward strand appears to be a strip shape with a width of 1?2D,while the instable region of rough cylinder is closer to the centerline of wake.(2)Referring to the unsteady method used in the fluter analysis in bridge,the idea of aerodynamic derivative is put forwarid,which could be applied to the analze the unstead aerodynamic force.The research result of aerodynamic derivative was introduced briefly.The time-domain method to identify aerodynamic derivatives by forcd vibration tests is introduced.At last,the procedure of aerodynamic determination and the dispose of data are declared.(3)According to the results of nonlinear numerical simulation,several cases of smooth and rough cylinders were taken to carry out the forced vibration wind tests.The relationship between the reduced wind velocity and eight aerodynamic derivatives of the cylinder in the wake were identified.The result showed that the aerodynamic derivatives H4*and H4*have large positive values,which could offer negative aerodynamic stiffness,while the rough cylinder does not have such result.This showed the difference between their instable regions of wake galloping.The test result also prove the accuracy of the aerodynamic derivative identification.