Engineering and Construction

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The traditional method of designing cased columns, presented in BS 449: 1969, can be very conservative but is readily accepted as a method of simple design.

The minimum width of the concrete casing, according to BS 449, should be the flange width b of the steel section plus 100mm.At least four reinforcing bars are to be located in the concrete at a cover not exceeding 50mm, to which 5mm diameter steel stirrups are attached at spacings not exceeding 200mm.The normal aggregate size is 10mm and the minimum concrete grade is 21N/mm2.

To establish the axial capacity of the cased column (or strut), the radius of gyration of the solid section is taken as 0.2 (b + 100) mm, or alternatively that of the major axis of the steel section. The cross section excludes any concrete cover  in excess of 75mm from the overall dimension of the steel section. The net area of concrete is replaced by an equivalent area of steel by dividing by a modular ratio of 30.

Therefore, knowing the permissible axial stress, as a function of the effective slen- derness of the cased column, and the equivalent area of steel, the axial capacity of the section can be easily calculated. However, an onerous limit on the use of this method is that axial load on the cased column should not exceed twice that permitted on the uncased section.

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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).

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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The traditional method of designing cased columns, presented in BS 449: 1969, can be very conservative but is readily accepted as a method of simple design.

The minimum width of the concrete casing, according to BS 449, should be the flange width b of the steel section plus 100mm.At least four reinforcing bars are to be located in the concrete at a cover not exceeding 50mm, to which 5mm diameter steel stirrups are attached at spacings not exceeding 200mm.The normal aggregate size is 10mm and the minimum concrete grade is 21N/mm2
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To establish the axial capacity of the cased column (or strut), the radius of  gyration of the solid section is taken as 0.2 (b + 100) mm, or alternatively that of the major axis of the steel section. The cross section excludes any concrete cover  in excess of 75mm from the overall dimension of the steel section. The net area of concrete is replaced by an equivalent area of steel by dividing by a modular ratio of 30.

Therefore, knowing the permissible axial stress, as a function of the effective slen- derness of the cased column, and the equivalent area of steel, the axial capacity of the section can be easily calculated. However, an onerous limit on the use of this method is that axial load on the cased column should not exceed twice that permitted on the uncased section.

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In many cases of design, concrete or some other protective material is required around steel columns for reasons of fire-resistance and durability. It would seem appropriate, therefore, to develop composite action between the steel and concrete, thereby taking advantage of the inherent compressive strength of the concrete, increasing the compressive resistance of the section and leading to considerable savings in the cost of the steelwork. Even ignoring this composite action, the slenderness of the steel column in lateral buckling is reduced, thereby increasing the compressive stress that can be resisted by the steel section.

Much research has gone into the behaviour of concrete-filled tubular sections.

Architecturally, tubular columns have many attractive features; concrete filling has no visual effect on their appeal.The advantages from a structural point of view are, first, the triaxial confinement of the concrete within the section, and second, the fire resistance of the column, which largely depends on the residual capacity of the concrete core.

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Composite columns comprise steel sections with a concrete encasement or core.

Encased columns usually consist of standard I-beam or H-column sections with a rectangular or square concrete section encasement to form a solid composite section. Additional reinforcement is placed in the concrete cover around the steel section in order to prevent spalling under axial stress and in fire.

Concrete-filled columns consist of circular, square or rectangular hollow sections filled with concrete.

Additional reinforcement is not normally required, except in columns of large section. Examples of these different types of composite columns are shown in Fig. 22.1.

The steel structure is normally constructed in advance of the formation of the composite section.The minimum size of the steel element is therefore controlled by the construction condition. For practical reasons it is normally necessary to fill the tubular sections shortly after their erection but I-sections can be encased much later.

However, there are practical problems in concreting around a column with the floors in place. Alternatively, columns can be pre-encased in the factory and in situ concrete used to fill the zone around the beam–column connections.

Fig. 22.1 Examples of composite columns: (a) RHS and CHS concrete-filled section, (b) concrete-encased section