Keywords: state-of-the-art, concrete filled tubular (CFT), high strength concrete, non-linear finite element analysis, numerical models.

This paper presents a state-of-art review of the numerical models for concrete filled tubular (CFT) columns paying special attention to the field of high strength concrete. The numerical behaviour of such columns is discussed with respect to the composite behaviour, geometry of the sections, type of loads, load application system, confinement effects, bond-slip, local buckling, second order effects and residual stresses. The paper reviews the research undertaken in the last ten years, and addresses future work in this interesting field.

The authors have found more than two hundred papers related to CFTs., but in the last decade, some papers have been published reviewing the state of the art of composite or concrete filled tubular columns.

Shams and Saadeghvaziri [1] presented the state of the art for concrete-filled steel tubular (CFT) columns including experimental and analytical work. They discussed the general response of CFT columns and the use of steel jacketing. They also presented an overview of analytical work for CFTs, including a comparison between the different design codes.

Hajjar [2] presented a review of the behaviour of circular and rectangular concrete-filled steel tube beam-columns and braces, and particularly focused on their behaviour when subjected to cyclic seismic loading. He explained the monotonic behavior of CFTs subjected to axial, flexural, and torsional loading, summarizing the effects of CFT behaviour including creep, shrinkage, composite action and residual stresses.

Gourley et al. [3] summarised the experimental tests on CFTs in a very important research report.

Spacone and El-Tawil [4] presented a state of the art of nonlinear analysis of steel-concrete composite structures. The work was focussed on frame elements, section models and fibber models, with lumped and distributed inelasticity, as well as models with perfect and partial connections.

In this paper, research done by the authors is summarized, extended and focussed on the field of high strength concrete. However, due to space limitations the numerical models for fire, connections, and seismic loading are not presented here because they require a special review for each of them.

The conclusions of the paper are that:

1. There is uncertainty about the effect of bond between the steel and concrete of slender columns for slender columns of HSC. The effect of the slenderness ratio for improving the bond between concrete and steel needs to be studied.
2. Most of the numerical models in the literature take into account separately the contact between the steel and concrete and the slippage. For the contact, a gap model is used and on the other hand the bond-slip is included with elastic-plastic springs in the longitudinal direction. None of the models have taken into account interaction between both effects. A bond-slip relationship for high strength concrete dependent on the lateral confinement should be provided.
3. The lack of information on the behaviour of CFTs arising from creep, shrinkage and residual stresses indicates a need for further research in this area.
4. The global buckling of CFT columns for high strength is not well predicted. Effects of important factors such as the slenderness ratio, aspect ratio, cross-sectional shape, and creep need to be investigated.
5. New designs method needs to be established for HSC because design provisions for composite structures have generally been extrapolated from provisions for traditional reinforced concrete or steel structures

1

Shams M., Saadeghvaziri M.A., "State of the art of concrete-filled steel tubular columns", ACI Structural Journal 94 (5): 558-571 Sep-Oct 1997.

2

Hajjar J.F., "Concrete-filled steel tube columns under earthquake loads", Prog. Struct. Engng Mater. 2: 72-81, 2000. doi:10.1002/(SICI)1528-2716(200001/03)2:1<72::AID-PSE9>3.0.CO;2-E

3

Gourley, B.C., Tort, C., Hajjar, J.F., and Schiller, P.H., "A Synopsis of Studies of the Monotonic and Cyclic Behavior of Concrete-Filled Steel Tube Beam-Columns", Structural Engineering Report No. ST-01-4, Department of Civil Engineering, University of Minnesota, Minneapolis, Minnesota, Version 3.0, December, 263 pp., 2001.

4

Spacone E., El-Tawil S., "Nonlinear analysis of steel-concrete composite structures: State of the art", Journal of Structural Engineering - ASCE 130 (2): 159-168 Feb 2004. doi:10.1061/(ASCE)0733-9445(2004)130:2(159)

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