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)
In up-to-date analysis packages, the application of these NHFs is invariably allowed
for when the load cases and load combinations are being derived.
In addition to the use of NHFs in the assessment of frame strength, they are also
used in ascertaining the stiffness of frames relevant to in-plane stability checks. This
latter consideration is covered in greater detail in 1.4.1, below.
1.4 Design of common structural forms
1.4.1 In-plane stability
Design of common structural forms 13
Without doubt, one of the most significant changes in the recent amendment to
BS 5950: Part 1 is the provision of in-plane stability checks to both multi-storey and
moment-resisting portal frames.
The somewhat simple procedures of Section 2 of BS 5950: Part 1 cannot be
utilized when considering the in-plane stability of portal frames since they do not
consider axial compression within the rafters. Axial compression in the rafters has
a significant effect on the stability of the frame as a whole.
In-plane stability checks are required to ensure that the load that would cause
buckling of the frame as a whole is greater than the sum of the applied forces.
Unlike a beam and column structure, a single-storey, moment-resisting portal
frame does not generally have any bracing in the plane of the frame. As such,
restraint afforded to individual columns and rafters is a function of the stiffness of
the members to which they connect. In simplistic terms, rafters rely on columns,
which in turn rely on rafters. The stability check for the frame must therefore
account for the stiffness of the frame itself.
When any structure is loaded it deflects. To this end, the deflected shape is different
from the idealized representation of the frame when it is deemed unloaded
or ‘at rest’. If a frame is relatively stiff, such deflections are minimized. If, however,
a frame is of such a ‘small’ stiffness as to induce significant deflections when loaded,
‘second-order’ effects impact on both the frame’s stiffness and the individual
members’ ability to withstand the applied load.
Consider the example of a horizontal, axially loaded strut as shown in Fig. 1.8.
Prior to application of the axial force, the strut would deflect under its self-weight.
If the strut was relatively stiff, the self-weight deflection, D, would be small. On application
of an axial force, P, a bending moment equal to P.D would be induced at
P I P
Fig. 1.8 Horizontal, axially loaded strut