Composite columns are a combination of concrete and steel columns combining the advantages of both types of columns. According to Eurocode 4, the design of composite columns has to be carried out for the ultimate limit state by using a simplified design method. For concrete-filled hollow sections, the plastic resistance of the cross-section is given by the sum of the components or by taking into account the effect of confinement in the case of circular sections. In this study on the basis of constitutive relationships for material components, the stress state in composite columns is determined taking into account the dependence of the modulus of elasticity and Poisson's ratio on the stress level in the concrete. It is determined that the effect of confinement occurs at a high stress level when structural steel acts in tension and concrete in compression, and the ultimate limit state of material strength is not attained for all parts simultaneously. In order to optimize the working conditions of a composite column and to prevent the possibility of column failure in the case of a small thickness of structural steel, large eccentricities and fire, the appropriate strength classes of concrete and steel have to be used.