| Summary: | 碩士 === 淡江大學 === 土木工程學系碩士班 === 93 === In wind engineering application, the conventional technique using free oscillation method to obtain flutter derivatives of bridge decks has become more mature. This method is easy to use and results are mostly satisfactory in the application. However, based on our past experimental experience, the test results from this method are sometimes unreliable under the circumstances when the influence of vortex shedding is significant or the operations during tests are not cautious.
To improve the reliability of test results, this thesis proposed a new approach of determining the flutter derivatives that uses forced oscillation technique. In the forced oscillation tests, the bridge deck section is connected to a two-axis shaking table driven by two servo-motors, which produces white-noise forced vibration actions in two degree of freedom to the deck. By measuring the response of bridge deck under various wind speeds, the aero-elastic transfer functions of the responses can be computed by using Fourier analysis. The results were curve-fitted to those from theoretical formulation by minimizing the error between each other, in which the Genetic Algorithm is used to locate the optimal parameters. Consequently, the flutter derivatives can be thus derived.
For demonstration, a chamfered bridge deck with width/depth ratio of 25 is constructed and tested in the wind tunnel following the proposed approach. The experimental results show that the flutter derivatives and are consistent for different wind speeds; in particular the aerodynamic damping matches Theodorsen function quite well. However, the flutter derivatives and are not consistent with the wind speed varied. In the future study, further investigations shall be conducted to clarify such an inconsistency.
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