WIND TUNNEL TESTS ON MOTION-INDUCED VORTEX VIBRATION

摘要

In 2009, a crack was discovered in the bracing member around the intermediate pier of Ikitsuki Bridge, a truss bridge in Nagasaki Prefecture, Japan. As a result of a field oscillation measurement by Nagasaki University, the main cause of the crack was identified as the Karman vortex-induced vibration. However, aerodynamic vibrations were also observed even in the wind speed range lower than the resonance wind speed of the Karman vortex-induced vibration. Conventionally, according to the result of a wind tunnel test using side ratios of B/D=2.0-8.0 (B: along-wind length, D: cross-wind length), the motion-induced vortex vibration caused by unification of the separated vortex from leading edge and the secondary vortex at trailing edge was recognized. However, considering the fact that the side ratio of the section of the bracing member of Ikitsuki Bridge is B/D=1.18, there has been a little past research on motion-induced vortex vibration on a rectangular cross section of B/D of less than 2. Therefore, the authors moved forward with a research of the motion-induced vortex vibration, focusing on the rectangular cross sections with small side ratios including B/D= 1.18. In order to clarify the generation of motion-induced vortex vibration, a spring-supported test and a smoke flow visualization test were conducted on the said cross sections. A possibility was shown that without the formation of the secondary vortex at trailing edge, a motion-induced vortex vibration might occur. Based on the wind tunnel test results, this research aimed to verify whether or not the secondary vortex at trailing edge is essential for the generation of motion-induced vortex vibration. As a result, it was found that it is considered that the secondary vortex at trailing edge is not always necessary not only for rectangular cross-sections with less than a side ratio of B/D=2, but also for a cross section similar to the rectangular cross section at both small amplitude range at the onset reduced wind speed of motion-induced vortex vibration and a comparatively large amplitude range where the maximum response amplitude of motion-induced vortex vibration occurs.