Effective stiffness of rectangular concrete filled steel tubular members

Concrete filled steel tubular (CFST) members are efficient structural members and have been widely used in seismic areas. The effective stiffness of CFST members is important in the structural design for determining the buckling capacity of a member, and evaluating the elastic deformations and internal forces of a structure, but current understanding is not sufficient on it. To address this, a theoretical modal was developed on the basis of a perfect bonding assumption and assumed strain distributions. Design equations were derived and errors induced by the assumptions were corrected using finite element analyses. The proposed equations for the effective flexural and shear stiffness of section consist of the full stiffness of steel tube and a reduced stiffness provided by the concrete infill. The reduction factors for the concrete contribution are related to the level of axial force and the ratio of axial stiffness of steel to concrete. From investigation of available test results, it was found that the proposed equations could well predict the stiffness of CFST members under different loading conditions, and could be used in engineering practice.