A wind tunnel study was carried out to examine the low speed instability of rectangular cross sections, which could occur at a lower wind speed range than that of Karman vortex-induced oscillation, if the Scruton number Sc = 2mδ/(ρD2), in which m = the mass of the system per length; δ = the system damping measured in still air; ρ = air density; D = is a reference length, is very small. The low speed instability was found by the full-scale measurements at bracing members having a rectangular cross section of the long-spanned truss bridge in Japan. The Scruton number of the bracing members was estimated to be approximately 2–3, therefore the low speed instability was thought to occur. However, the occurrence depended on the wind conditions. Unfortunately there are few past researches on the low speed instability on a rectangular cross section of B/D of less than 2, which is the ratio of the width B and height D. In this study, the free oscillation experiment and unsteady aerodynamic lift measurement were conducted in order to clarify the effects of an angle of attack, a yaw angle, cross section shapes and the turbulence intensity on the low speed instability.

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