Keywords: concrete filled tubular (CFT), high strength concrete, non-linear finite element analysis, buckling.

In recent years an increase in the utilisation of concrete tubular columns occurred due to their high stiffness, ductility and fire resistance. On the other hand the use of high strength concrete (HSC) is more common due to the advances in the technology. The use of this material presents different advantages, mainly in elements subjected to high compressions as building supports or bridge columns. However, there is a notable lack of knowledge in the behaviour of high strength concrete filled tubular columns. Hence the existing simplified design models for normal strength concretes are not valid.

Concerning the numerical models, it can be stated that there are not a lot of specific studies where the finite element method or sectional analysis is applied to this type of structure. Most of them: Hu et al. [1], Huan et al. [2], Lu et al. [3], and Shams et al. [4], study normal strength concretes. Only, recently Varma et al. [5] have implemented a fibre model applied to square tubular sections but for short columns, without taking into account the buckling. If a good sectional characterization (moment-curvature) was performed, it can be inferred that the actual simplified methods are valid as a first approach to study the strength of these supports. A few months ago, Zeghiche and Chaoui [6] published a study for circular sections following this procedure. It is important emphasize the last conclusion: "More numerical and experimental tests should be performed to check the validity of the buckling design methods of the EC4 for high strength concrete and double curvature".

In this paper a nonlinear finite element numerical model for circular concrete filled tubular sections is presented. The method has to be computationally efficient and must represent the behaviour of such columns, taking into account the effect of high strength concrete and second order effects. The model was compared with 78 experiments from different authors. The experimental tests selected corresponds to circular tubular columns filled with concrete (CFT) with pinned supports at both ends subjected to axial load and uniaxial bending. In these tests the eccentricity of the load at the ends is fixed and the maximum axial load of the column is evaluated. In 52 of the selected experiments of the bibliography, the eccentricity is equal at both ends while in 26 tests the applied eccentricities are different , where is the ratio between both eccentricities.

The novelty of the model is focussed in the numerical integration of the cross section using the Gauss-Legendre quadrature. The steps followed, start by decomposing the section into wide layers, reducing a double integral into a path integral and to evaluate them using a Gauss quadrature.

Two alternatives to integrate the stress field for the tubular columns are evaluated: (a) By superposition two circular sections with a radius and respectively (b) By using direct integration of the annular section with an external radius of and an internal radius of . It was demonstrated that the first alternative is more efficient (speed and accuracy).

This integration procedure, also for the circular and the annular section, due to its accuracy, efficiency and continuity is ideal for its implementation into nonlinear analysis structural programs.

1

Hu H.T., Huang C.S., Wu M.H., et al., "Nonlinear analysis of axially loaded concrete-filled tube columns with confinement effect", J. Struct. Eng. - ASCE, 129 (10): 1322-1329, Oct 2003. doi:10.1061/(ASCE)0733-9445(2003)129:10(1322)

2

Huang C.S., Yeh Y.K., Liu G.Y., et al., "Axial load behavior of stiffened concrete-filled steel columns", J. Struct. Eng. - ASCE, 128 (9): 1222-1230, Sep 2002. doi:10.1061/(ASCE)0733-9445(2002)128:9(1222)

3

Lu X.L., Yu Y., Kiyoshi T., et al., "Nonlinear analysis on concrete-filled rectangular tubular composite columns", Struct. Eng. Mech., 10 (6): 577-587, Dec 2000.

4

Shams M., Saadeghvaziri M.A., "Nonlinear response of concrete-filled steel tubular columns under axial loading", ACI Struct. J., 96 (6): 1009-1017, Nov-Dec 1999.

5

Varma A.H., Sause R., Ricles J.M., et al., "Development and validation of fiber model for high-strength square concrete-filled steel tube beam-columns", ACI Struct. J., 102 (1): 73-84, Jan-Feb, 2005.

6

Zeghiche J., Chaoui K., "An experimental behaviour of concrete-filled steel tubular columns", J. Constr. Steel Res., 61 (1): 53-66, Jan 2005. doi:10.1016/j.jcsr.2004.06.006

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £135 +P&P)