Composite Columns - Cased strut method.

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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