Concrete-filled double skin steel tubular (CFDST) beam–columns subjected to cyclic bending

In recent years, it was proposed by several researchers that concrete filled double skin steel tubes (CFDST) be studied for their strength as a column or a beam. Advantages of CFDST over fully concrete filled steel tubes (CFST) include: increase in section modulus; enhancement in stability; lighter weight; better damping characteristics and better cyclic performance. It is thus expected that concrete filled double skin steel tubes (CFDST) have the potential of being used in building structures. This paper provides new test data pertaining to the seismic behavior of CFDST beam–columns. The test parameters included the section types (circular and square), the core concrete strength (fcu), and the axial load level (n)(n). Twenty-eight CFDST column specimens, including 16 specimens with SHS (square hollow section) outer and CHS (circular hollow section) inner, 12 specimens with CHS (circular hollow section) outer and CHS (circular hollow section) inner were tested under constant axial load and cyclically increasing flexural loading. The CFDST beam–columns were found to have significant increase in strength, ductility, and dissipated energy over the outer jackets. In general, the ductility and energy dissipation ability of specimens with circular sections are higher than those of the specimens with square sections. The mechanics model which was developed by the authors for concrete filled steel tubular (CFST) beam–columns subjected to constant axial load and cyclically increasing flexural loading, is used to analyze the behavior of CFDST beam–columns. It is found that the predicted cyclic responses for the composite beam–columns are generally in reasonable agreement with test results. Finally, comparisons are made with predicted beam–column flexural stiffness using the existing codes.