Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
b
Py
Py
O.45fc ______
Introduction 653
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). Formulae are given in BS
5400: Part 5, Appendix C. 1 The interaction between axial load and bending moment
can be considered in terms of a simplified interaction formula (section 22.2.2).
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.
stress
O.45f
P - axial force P = 0 P > 0
M - major axis moment
M =
Fig. 22.2 Plastic stress blocks acting on cross-section of composite column subject to
moment and axial force