“…In the above papers, the complex modulus was considered to be independent of frequency, and the damping was simulated as a hysteretic damping model. Stevens and Evan-Iwanowski (1969) and Stevens (1969) presented analytical and experimental results to discuss the effect of the behavior of a viscoelastic material on the instability regions of a column, when the complex modulus of the viscoelastic material is influenced by frequency. Steidel (1989) pointed out that the damping ratio is a constant for hysteretic damping model; increases with frequency for a viscous damping model, and decreases as the frequency of actual damping increases.…”