Keywords: high strength concrete, column, nonlinear analysis, confined effect, FRP, FEM.

With the increase in the strength of concrete, the ultimate strength of concrete columns shows the high load carrying capacities under axial compression. However, in general, a concrete shows low tensile strength coupled with high compressive strength. Therefore, a concrete column sometimes exhibits a brittle failure. Also, a column is subjected to an eccentric loading due to structural arrangements. In this case, the column is under combined bending and compression. To obtain a high ultimate strength and a ductile behavior of a concrete column under such loading conditions, it is important for concrete to resist both an axial load and an additional flexural compressive load. Such a problem plays an important role in the realisation of light weight and the economical structures.

To improve the strength and the ductility of concrete, the confined effect of concrete is an important factor and has been studied by many researchers [1,2,3,4]. This effect is achieved by resisting a lateral expansion due to the Poisson's effect on loading and acts on the concrete with a compressive force. It is well known that concrete columns represent the high strength and the sufficient ductility by utilizing such confined effects. However, research studies have been done under a concentric loading condition. Li and Hadi [3] and Hadi and Li [4] applied these confined effects to high strength concrete columns under combined bending and axial compression by experimental analyses and reported that these effects do not enhance the ultimate strength than those obtained by the concentrically compressed columns. To clarify these mechanisms, it is important to represent the stress status and failure mechanisms of both the concentrically and the eccentrically compressed columns up to failure.

In this paper, numerical analyses of high strength concrete columns with internal helical reinforcement or externally wrapped sheets are undertaken considering the experimental results. High strength concrete columns are pin supported at both ends and are subjected to concentric or eccentric axial compression. In numerical analyses two types of columns are considered. One has reinforcement internally and the reinforcement is placed in both axial and hoop directions. The other column has FRP wrapping without reinforcement.

In numerical analyses, the finite element method is adopted and a parallel procedure is applied based on an element-by-element (EBE) solution procedure. The R/C column is divided into hexahedral elements. Drucker-Prager yield criterion is adopted for concrete in compression and the tension cut off is also adopted for concrete in tension [5]. Reinforcement and FRP sheets are represented as thin sheets with uniaxial material properties. Incremental load is applied to the numerical model under the same conditions as the experiment and the calculation is controlled under both the displacement and the load increment.

From the numerical analyses, the computed results demonstrate that the failure mechanisms of the concrete column with external stiffening using several materials are simulated well. When the failure modes between numerical and experimental results coincide, the same ultimate strengths are obtained. In the eccentrically compressed concrete columns, an additional bulge, arises around the centre of the column, and decreases the ultimate strength of the column.

From the numerical results of the concentrically compressed column stiffened by the external wrapping, the failure is restricted around the ends of the beam. The failure modes are quite different from that of the plain concrete column. To improve the compressive strength with expecting the confined effects, an additional stiff sleeve must be applied.

From the numerical results of eccentrically compressed column stiffened by the external wrapping, almost all of concrete columns fail by the local rupture at the ends of the column. Therefore, extreme improvements of the ultimate strength are not obtained. In the eccentrically compressed concrete column, some kind of strengthening methods must be developed.

1

Razvi S.R., Saatcioglu M., "Strength and deformability of confined high-strength concrete columns", ACI Structural Journal , 91(6), 678-687, 1994.

2

Demer M., Neale K.W., "Confinement of reinforced concrete columns with fiber-reinforced composite sheets - an experimental study", Canadian Journal of Civil Engineering, 26(2), 226-241, 1999. doi:10.1139/cjce-26-2-226

3

Li J., Hadi M.N.S., "Behavior of externally confined high-strength concrete columns under eccentric loading", Composite Structures, 62,145-153, 2003. doi:10.1016/S0263-8223(03)00109-0

4

Hadi M.N.S., Li J., "External reinforcement of high strength concrete columns", Composite Structures, 65, 279-287, 2004. doi:10.1016/j.compstruct.2003.11.003

5

Hinton, E. and Owen, D.R.J., "Numerical Methods and Software for Dynamic Analysis of Plates and Shells", Pineridge Press, Swansea, U.K. 1986.

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