Representation of concrete-filled steel tube cross-section strength

This paper presents the development of a polynomial equation to represent the three-dimensional (3D) cross-section strength of square or rectangular concrete-filled steel tube (CFT) beam-columns. The equation provides an accurate representation of the cross-section strength of a CFT subjected to a combination of axial force, strong axis flexure, and weak axis flexure. This expression is verified against the results of a detailed fiber analysis formulation, and against experimental tests of shea, square CFTs available in the literature. This CFT cross-section strength equation forms a compact expression of the failure surface of square or rectangular CFTs having a wide range of cross-section dimensions and material strengths. The cross-section strength surface also provides the basis for a bounding-surface concentrated plasticity model in 3D force space for CFT beam-columns, presented in related work by the writers. This beam-column model, in turn, is suitable for conducting monotonic static, cyclic static, or transient dynamic seismic analysis of complete composite unbraced frame structures composed of steel I-girders framing biaxially into CFT beam-columns.