Concrete-filled steel tubular (CFT) columns have become increasingly popular in structural applications due to their high strength, high ductility, and large energy absorption capacity. These structural properties are the result of integral behavior of the concrete core and the steel tube. This research provides an analysis of the interaction of the concrete core and the steel tube in CFT columns.

While there have been many experimental studies of CFT columns, there has been no analytical work modeling the confining effect of the steel tube on the concrete core. Furthermore, current design procedures tend to underestimate the ultimate strengths of CFTs since they neglect the confining effect of the steel tube on the concrete core. Since experimental programs are expensive, time consuming and limited to small ranges of parameters, it is important to develop an analytical model for CFT members.

In this study emphasis is placed on the nonlinear response of CFT columns subjected to axial and combined loadings. A three-dimensional finite element model is developed for CFT columns. The concrete core is modeled with 3-D solid elements which use the Pramono-William concrete model and the steel tube is modeled using shell elements which allow large deformation and contain a von Mises plasticity model with kinematic hardening. Gap elements are used to model concrete and steel interface. The finite element model is calibrated against existing experimental results.

Analyses of columns under axial loading indicate that the stress-strain properties of the concrete core are highly affected by the geometrical configuration of the columns as well as material properties of concrete core. CFT columns exhibit high flexural capacity with a hardening type of behavior due to higher ductility and larger compressive capacity of concrete core which is provided by confining effect of steel tube. The circular CFTs show a hardening type of response under axial loading while, a degrading type of behavior is observed in square columns.

Simple models are developed in order to define the axial and flexural capacity of CFT columns. The proposed methodology is calibrated against experimental data.