Han, Huang, and Zhao 2009
This paper discusses the analytical behaviour of concrete-filled double skin steel tubes (CFDST) beam columns subjected to constant axial load as well as cyclically increasing flexural loading. Moment vs. curvature, lateral load vs. lateral displacement, and the ductility coefficient for the specimens is recorded in a parametric analysis.
Theoretical Discussion and Model
It was assumed that the concrete in between the steel has the same behavior as concrete in a fully in-filled steel tube. The steel properties were determined from a typical hysteretic stress-strain curve, and strain hardening, as well as the Bauschinger effects were taken into account. A constraining factor was used which takes the composite action into account, and includes the nominal steel ratio, the yield stress of the steel as well as the compression strength of concrete. The monotonic stress-strain curve was used to determine the skeleton curve of concrete. The stress states of each element were based on the previous stress state, such that the loading history is recorded for every element. The predicted results for load vs. lateral deflection were compared with test results, and it was found that the experimental test results agreed with the model. The moment curvature response included six stages: elastic stage, elastic-plastic stage, un-loading stage, elastic stage of reverse loading, elastic-plastic stage of reverse loading, and the re-loading stage. Stages 1, 3, and 4 displayed linear, or nearly linear behaviour, whereas stages 2, 5, and 6 displayed non-linear behaviour. The influence of various parameters, such as axial load level, nominal steel ratio, strength of the outer and inner steel tubes, strength of concrete, width-to-thickness ratio of inner steel tube, and hollow ratio on the moment vs. curvature were investigated. It was found that the stiffness of the moment vs. curvature curve increased as the nominal steel ratio increased. The yielding moment increased with an increase of nominal steel ratio, yield strength of the outer steel tube, and the strength of the concrete.
Reference
Han L.-H., Huang H., Zhao X.-L. (2009). "Analytical Behaviour of Concrete-Filled Double Skin Steel Tubular (CFDST) Beam-Columns under Cyclic Loading," Thin-Walled Structures, 47 (6-7), pp. 668-680. doi:10.1016/j.tws.2008.11.008