In concrete-filled columns,bond occurs through the interlocking of concrete in two types of imperfections of steel,namely the surface roughness of steel,and also the variation in the shape of the tube cross-section. In steel encased columns,a reinforcement cage is used to contain the lateral expansion of the concrete,and also to prevent the premature spalling of the concrete. Composite columns are characterized by several typical failure mechanisms.Failure due to combined compression and bending could occur,together with the phenomena that characterizes the behavior of slender beam-columns(imperfections and residual stresses).The shear interaction mechanism could also be present, especially for stocky elements.Local buckling is usually prevented.  In concrete-filled columns, bond occurs through the interlocking of concrete in two types of imperfections of steel, namely the surface roughness of steel, and also the variation in the shape of the tube cross-section.  In steel encased columns，a reinforcement cage is used to contain the lateral expansion of the concrete, and also to prevent the premature spalling of the concrete.  Composite columns are characterized by several typical failure mechanisms. Failure due to combined compression and bending could occur, together with the phenomena that characterizes the behavior of slender beam-columns(imperfections and residual stresses). The shear interaction mechanism could also be present, especially for stocky elements. Local buckling is usually prevented