Composite Columns - Principles of Design.

The axial resistance of a stocky column, defined as a column that is not subject to the effects of instability, is determined by adding the ultimate compressive strengths of the steel and concrete components. In traditional reinforced concrete design, a ‘short’ column is defined as one whose effective height to least cross-sectional dimension is less than 12, and a nominal allowance is made for eccentricity of axial load.

The axial resistance of concrete-filled sections is greater because the concrete is not able to expand laterally (Poisson’s ratio effect) under load, and triaxial stresses are developed in the concrete. This causes an increase in the compressive strength of the concrete by an amount dependent on the proportions of the cross-section.

The hoop tensions created in the steel have a small adverse affect on its strength.

The effect of eccentricity of axial load is to develop a bending moment in the section. The moment resistance of the section (in the absence of axial load) can be calculated considering plastic stress blocks (see Fig. 22.2).

Plastic stress blocks acting on cross-section of composite column subject to moment and axial force
Fig. 22.2 Plastic stress blocks acting on cross-section of composite column subject to
moment and axial force

The interaction between axial load and bending moment can be considered in terms of a simplified interaction formula.

Slender columns require a more refined treatment. The effective slenderness of a column is determined from the proportions of the composite section. The second moment of area is obtained by adding the second moments of area of the steel and concrete (divided by an appropriate modular ratio). This represents a considerable increase over the properties of steel alone.The axial stress that the section can resist is then determined from the column buckling curves for the steel section under consideration.The resulting axial stress, relative to the yield stress of the steel, is effectively a resistance reduction factor to be applied to the ‘stocky column’ compressive resistance.

It should be noted that the above approach assumes that loads are not applied laterally over the column length. Concentric and eccentric axial loads cause relatively low interface shear stresses between the steel and the concrete. Lateral loads cause greater shear stresses and may necessitate the introduction of shear-connectors.

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