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Steel Designers' Manual - 6th Edition (2003)
(1) the rafter must be a UB
(2) the haunch flange must not be smaller than the rafter flange
(3) the distance between tension flange restraints must be stable when checked as
a beam using the procedure of section 16.3.6.
Equation (18.12) is less sensitive than Equation (18.11) to changes in x, with the
result that it gives an average value for Ls/ry of about 65. It is often regarded as good
practice to provide bracing at the toe of the haunch since this region corresponds
to major changes in the pattern of force transfer due to the change in the line of
action of the compression in the bottom flange. In cases where the use of clause
5.3.4 does not give a stable haunch because the length from eaves to toe exceeds
Ls, Appendix G may be used to obtain a larger value of Ls. If this is still less than
the haunch length, then additional compression flange restraints are required. 6
18.7.4 Bracing
Special design methods for members in portal frames 531
The general requirements of lateral bracing systems have already been referred to
in Chapter 16 – sections 16.3, 16.4 and 16.5 in particular. When purlins or siderails
are attached directly to a rafter or column compression flange it is usual to assume
that adequate bracing stiffness and strength are available without conducting specific
calculations. In cases of doubt the ability of the purlin to act as a strut carrying
the design bracing force may readily be checked. Definitive guidance on the appropriate
magnitude to take for such a force is noticeably lacking in codes of practice.
A recent suggestion for members in plastically-designed frames 7 is 2% of the squash
load of the compression flange of the column or rafter: 0.02pyBT at every restraint.
In order that bracing members possess sufficient stiffness a second requirement that
their slenderness be not more than 100 has also been proposed. 7 Both suggestions
are largely based on test data. For elastic design the provisions of BS 5950: Part 1
may be followed.
When purlins or siderails are attached to the main member’s tension flange, any
positional restraint to the compression flange must be transferred through both the
bracing to main member interconnection and the webs of the main member. Both
effects are allowed for in the work on which the special provisions in BS 5950: Part
1 for tension flange restraint are based. 8 When full torsional restraint is required so
that interbrace buckling may be assumed, the arrangement of Fig. 18.17 is often
used. The stays may be angles, tubes (provided simple end connections can be
arranged) or flats (which are much less effective in compression than in tension).
In theory a single member of sufficient size would be adequate, but practical considerations
such as hole clearance 6 normally dictate the use of pairs of stays. It
should also be noted that for angles to the horizontal of more than 45° the effectiveness
of the stay is significantly reduced.
Reference 9 discusses several practical means of bracing or otherwise restraining
beam-columns.