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Steel Designers' Manual - 6th Edition (2003)
658 Composite columns
(22.6)
In design to BS 5400: Part 5, 1 slender columns subject to major axis moment are
treated as subject to biaxial moment by including an additional minor axis moment
of 0.03Pbc, where P is the axial load on the column. An interaction formula given
for combining major and minor axis effects is not considered further here.
Provision should be made for the smooth transfer of force between the concrete
and steel in cases where the section is subject to high moment. No mechanical shear
connection need be provided where the shear stress at the interface between the
concrete and the steel is less than 0.6 N/mm 2 K P P
M M M
K K
for encased columns.
=
Ê 1 - / u ˆ
pc £ 09 . pc
Ë 1 - 2 ¯
22.3 Design of concrete-filled tubes
22.3.1 Axial load resistance
The compressive resistance of a concrete-filled rectangular or circular section is
enhanced by the confining effect of the steel section on the concrete, which depends
in magnitude on the shape of the section and the length of the column. Buckling
tends to reduce the benefit of confinement on the squash load as the column slenderness
increases. To account for this, modification factors are introduced. In circular
sections it is possible to develop the cylinder strength (0.83fcu) of the concrete.
The ‘squash’ load resistance of a circular concrete-filled column is:
Pu = C1pyAs + 0. 87 fyAr +( 0. 83fcu/ g mc) Ac{ 1+ C2( t/ f)
[ py/ ( 0. 83fcu)
] } (22.7)
where t is the thickness and f is the diameter of the tubular section, and gmc is the
material factor for concrete (= 1.5). The terms C1 and C2 are coefficients which are
a function of the slenderness factor l of the column, defined in Equation (22.2); C1
is less than unity because of the effect of hoop tensions created in the steel. The
values of C1 and C2 are presented in Table 22.1.
The method derives from research6,7 carried out by CIDECT and CIRIA and has
been incorporated into Eurocode 4. 3 Limits to the use of the method are that the
term Aspy should represent between 20% and 90% of Pu. To avoid local buckling,
f £ 85te (where e is ( 275/Py ) ).
The effect of slenderness on the axial resistance of a concrete-filled column may
be treated as in section 22.2.1. The slenderness factor l may be determined from
Equation (22.2) as a function of Pu. This involves iteration as Pu is partly dependent
on l.
As a reasonable approximation, l may be determined assuming that C1
= 1.0 and C2 = 0.0. Having evaluated l,
the resistance reduction factor K1 can be
determined from the column buckling curve given in Table 27(a) of BS 5950: Part
1.