Study On The Aerodynamic Admittance Function Of The Buffeting Response Of Long-span Cable-stayed Bridge

The developmental trend of modern bridges is towards larger span and higher flexibility, which makes the wind-resistant design one of the key factors in bridge design. A topic of great concern is whether supplementation and improvement should be made to the currently widely adopted theoretical frame for bridge wind vibration established by Scanlan and Davenport in 1960s to suit the wind-resistant design of super-span bridges. To conduct refined analysis on the wind resistance of large-span bridges, aerodynamic admittance function is an important parameter in the refined study of bridge buffeting. This paper presents the following studies:(1) Unconstrained optimization of the residual objective function was made with variable metric method using the quasi steady force model of the lift force and moment in Scanlan two-dimensional flow field; following that, a theoretical MITD-optimized identification method of flutter derivative was established.Then its effectiveness and adaptation degree were verified through numerical simulation of the plane model and analytic solution comparison. The flutter derivative of model one (closed streamlined flat steel box girder model) was identified in the wind tunnel test and then aerodynamic admittance function identification was conducted using the relation between flutter derivatives and aerodynamic admittance.(2)Through discussion on the identification theory and method for aerodynamic admittance function with fluctuating wind-induced unsteady air buffeting force model, an identification method for aerodynamic admittance of buffeting force spectrum was established Comparison between the aerodynamic admittance of the quasi flat plate model measured in the wind tunnel test and the Sears admittance function indicated the closeness between them, which showed that the identification method in question was feasible.(3) Experiments on the identification of aerodynamic admittance in the wind tunnel were carried out relating to three typical bridge section models: model one (closed streamlined flat box girder), model two (open streamlined flat box girder) and model three (bluff main girder). Experiment contents included: 1) simulation and measurement technique of fluctuating wind field and the establishment of a grid turbulent flow field with uniform turbulence in the wind tunnel; 2) introduction of the force-measuring principle of high-frequency-force-balance, establishment of the mechanic model of high-frequency-force-balance and then verification on the precision and accuracy of 5-component high-frequency-force-balance. Analysis of the experimental results showed that: 1) no obvious effect on the aerodynamic admittance function was caused by the variation in model's aerodynamic configurations such as with open or closed bottom place, with or without railings and so on and slight variation in attack angles; 2) all 6 aerodynamic admittance functions of 4 models identified with buffeting force spectrum method were considerably identical to Sears admittance function; all being smaller than Sears function in the low-frequency section of the testing section, but almost coincident in the high-frequency section; 3) the effect of such variations in the model's aerodynamic configuration as with or without railings, with open or closed bottom plate and gradual changing from quasi flat plate to bluff body on the value of aerodynamic admittance had a characteristic that the bluffer the model configuration was the bigger the value of aerodynamic admittance would be, but not exceeding 1; and in high-frequency section there was no obvious effect onχLu2,2χDu andχMu2.(4) Buffeting frequency domain analysis was conducted on the Beicha Cable-stayed Bridge with 720m main span of Xiamen-Zhangzhou Cross-sea bridge applying multi-mode coupling buffeting response calculation method for cable-stayed bridges with aerodynamic admittance functions and the buffeting responses under various aerodynamic admittance functions were obtained. Then, the adaptation and reliability of applying the identification results in actual bridge engineering were verified by comparing the results with those of the full-bridge aeroelastic model buffeting experiments.(5) By calculating the buffeting response of the Beicha Cable-stayed bridge with the 720m main span of Xiamen-Zhangzhou Cross-sea bridge, the buffeting responses were obtained respectively when the value of the aerodynamic admittance was 1 and when Sears function was adopted. This showed that the buffeting response of this bridge was overestimated. The closeness between the buffeting responses calculated by the aerodynamic admittance fitted in the experiment and that of the full bridge aeroelastic model experiment indicates that the aerodynamic admittance results fitted in the experiment discussed in this paper is reliable to a certain degree.