Buckling strength of concrete filled steel tubular columns is calculated in two different ways. One is performed using the tangent modulus theory( Nsc), considering steel portion and concrete portion behave as a one body. The second one is the method of superposed strength in which a simplesum of separately computed buckling load of steel and concrete columns is regarded as a bucklingstrength(Ns+c). By comparing with two strength, the Ns+c/Nsc ratio almost ranges from 0.9 to 1.05.Inconservative differences( Ns+c/Nsc>1.05) are found in case that strength of concrete (csB) and steel tube ( σy) is quite different such as in combination with cσB=900kg/cm2 and σy=2.4t/cm2 or cσB=300kg/cm2 and σy=6.0t/cm2.
