Keywords: composite, confined-concrete, modelling, width-to-thickness ratio, concrete-filled tube, finite element.

Concrete-filled tube (CFT) columns are increasingly used in common engineering practice; CFTs possess favourable characteristics in terms of strength, stiffness, and ductility; and are therefore recommended to be used in high-rise buildings and mega structures. Another advantage of the system is the natural formwork provided by the steel tube.

An analytical study on the behaviour of circular concrete filled steel tube columns concentrically loaded in compression is presented in this paper. The analytical portion of this study was needed to develop calibrated nonlinear finite element models of the CFT specimens and to further investigate the adequacy of the design provisions. Nonlinear finite element models were built using the nonlinear analysis software "ADINA" [1]; the models were developed and verified using experimental results [2]. The concrete core and the steel tube were modelled using four-node two-dimensional solid axisymmetrical elements. The proposed two-dimensional solid axisymmetrical element is assumed for its simplicity and less computer memory is required.

The primary purpose of the numerical investigation was to investigate the effect of the depth to tube wall thickness (D/t) ratio on the axial compressive strength of the CFT, and to extrapolate these effects to the large dimensioned columns used in construction. Twenty-five different models were analyzed and tested to investigate the effect of steel wall thickness on the ultimate strength of composite columns. The strain hardening behaviour for the steel stress strain relation or an elastic-perfectly plastic relation is carefully considered in the analysis to simulate the actual behaviour.

Confinement of the concrete core provided by the circular tube shape was also addressed. The interface of friction between the two materials is represented in the proposed model, friction coefficients varying between 0.0 and 5.0 were considered.

Results from this study showed a good agreement between the numerical and previously carried experimental results. The results also indicated that circular tubes offer substantial post-yield strength and stiffness as well as better ductility. Ultimate strength results were also compared to current specifications [3] governing the design of concrete filled steel tube columns.

A comparison between the percentages of material increase (smaller D/t for a specified diameter) and yield strength increases showed that a 45% increase in steel material increased the results by 18% for strength gain. It is deduced that by keeping the thickness of the CFT column as low as possible, and that by using higher D/t ratios, will results in material saving.

1

ADINA, "Automatic Dynamic Incremental Nonlinear Analysis", Theory and Modeling Guide, Report ARD 00-7, August 2000.

2

H.S. Basha, A.C. Masri, M.A. Sabra, "Behaviour of Axially Loaded Concrete Filled Steel Tubes", American Concrete Institute, Kuwait Chapter, Proceedings of the ACI-KC 2nd International Conference, 125-136, 2007.

3

AISC, "Manual of Steel Construction, Load and Resistance Factor Design (LRFD)", American Institute of Steel Construction, 1994.

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