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Steel Designers' Manual - 6th Edition (2003)
654 Composite columns
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 shearconnectors
(as for composite beams – see Chapter 21, section 21.4).
22.1.4 Cased strut method
The traditional method of designing cased columns, presented in BS 449: 1969, 2 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 100 mm. At least four reinforcing bars are to
be located in the concrete at a cover not exceeding 50 mm, to which 5 mm diameter
steel stirrups are attached at spacings not exceeding 200 mm.The normal aggregate
size is 10 mm and the minimum concrete grade is 21 N/mm 2 .
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 75 mm 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 slenderness
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.
22.2 Design of encased composite columns
22.2.1 Axial load resistance
The design of composite columns is described in BS 5400: Part 5 1 and in Eurocode
4 (draft). 3 It is based on the method developed by Basu and Sommerville 4 and modified
by Virdi and Dowling. 5 The maximum compressive resistance (squash load) of
a stocky column is:
Pu = 0.45fcuAc + Aspy + 0.87Arfy
where Ac is the cross-sectional area of concrete
As is the cross-sectional area of the steel section
Ar is the cross-sectional area of the reinforcement
(22.1)