This paper presents a few fundamental instability characteristics of vortex shedding excitations for closed cross sections of bridge structures by means of wind tunnel and water flume tests. Experimental results indicate that the hexagonal form of a cross section with spoilers and flaps is a satisfactory stable cross section for aeolian oscillations. It is also noted that there are multiple synchronized regions of wind velocities for vortex shedding excitations of such cross sections treated here. As far as induced drag forces acting on obstacles and shedding frequencies are concerned, the results by water flume tests well coincide with those by wind tunnel tests
