A series of wind tunnel sectional model dynamic tests were conducted to examine the effects of torsional-to-vertical natural frequency ratio on the aerodynamic and dynamic properties of bridge decks. The natural frequency ratios tested were around 2.2:1 and 1.2:1 respectively, with the fundamental vertical natural frequency held constant for all the tests. Two 2.9 m long sectional bridge models, one simulating the cross-section of a twin-deck bridge with a gap-width to total chord ratio of about 16% and the other with zero gap-width to approximate a thin plate, were tested to determine whether the effects of frequency ratio are similar for bridge decks with different cross-section shapes. The results of wind tunnel tests suggest that for the model with zero gap-width, the flutter derivatives are relatively independent of the torsional-to-vertical frequency ratio for a relatively large range of reduced wind velocities. However, for the model with a relative large gap-width (around 16%), the flutter derivatives are evidently dependent on the frequency ratio for most of the tested reduced velocities. A comparison of the modal damping ratios also suggests that the torsional damping ratio is much more sensitive to the frequency ratio, especially for the model with a 16% gap-width. The results of this investigation show that the torsional-to-vertical natural frequency ratio is an important structural parameter, which can significantly affect wind-structure interaction.

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