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Steel Designers' Manual - 6th Edition (2003)
454 Beams
4,'
M for compound section
for base section
Fig. 16.11 Selective increase of moment capacity by use of a plated UB
development of this idea is the use of tapered sections fabricated from plate. 11 To
be economic, tapered sections are likely to contain plate elements that lie outside
the limits for compact sections.
Because of the interest in developing longer spans for floors and the need to
improve the performance of floor beams, a number of ingenious arrangements have
developed in recent years. 12 Since these all utilize the benefits of composite action
with the floor slab, they are considered in Chapter 21.
16.7 Cold-formed sections as beams
In situations where a relatively lightly loaded beam is required such as a purlin or
sheeting rail spanning between main frames supporting the cladding in a portal
frame, it is common practice to use a cold-formed section produced cold from flat
steel sheet, typically between about 1 mm and 6 mm in thickness, in a wide range of
shapes of the type shown in Fig. 16.12. A particular feature is that normally each
section is formed from a single flat bent into the required shape; thus most available
sections are not doubly symmetric but channels, zeds and other singly symmetric
shapes. The forming process does, however, readily permit the use of quite
complex cross-sections, incorporating longitudinal stiffening ribs and lips at the
edges of flanges. Since the original coils are usually galvanized, the members do not
normally require further protective treatment.
The structural design of cold-formed sections is covered by BS 5950: Part 5, which
permits three approaches:
(1) design by calculation using the procedures of the code, section 5, for members
in bending
(2) design on the basis of testing using the procedures of section 10 to control the
testing and section 10.3 for members in bending
(3) for three commonly used types of member (zed purlins, sheeting rails and lattice
joists), design using the simplified set of rules given in section 9.