This paper considers the classification of vortex-induced oscillations and the geometrical shape effects on them. The following points are considered : 1) the flow pattern around the section, 2) the relation between the onset critical reduced velocity and the critical reduced velocity which is defined as the reciprocal of the Strouhal Number and 3) the relation between the two onset critical reduced velocities of vortex-induced oscillations (in the heaving and the torsional mode). Also, an estimation of “the onset critical reduced velocity” and “the allowable amplitude concerning the fatigue failure of steel” of vortex-induced oscillation is described
