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Steel Designers' Manual - 6th Edition (2003)
436 Beams
proprietary nature, the manufacturers normally provide design information, much
of it based on physical testing, listing such properties as moment capacity. In the
absence of design information, reference should be made to Part 5 of BS 5950 for
suitable calculation methods.
Although the part of the web between the flange and the horizontal edge of the
castellation in a castellated beam frequently exceeds the compact limit for an outstand,
sufficient test data exist to show that this does not appear to influence the
moment capacity of such sections. Section classification should therefore be made
in the same way as for solid web beams, the value of Mc being obtained using the
net modulus value for the section at the centre of a castellation.
16.3 Basic design
One (or more) of a number of distinct limiting conditions may, in theory, control
the design of a particular beam as indicated in Table 16.3, but in any particular practical
case only a few of them are likely to require full checks. It is therefore convenient
to consider the various possibilities in turn, noting the conditions under
which each is likely to be important. For convenience the various phenomena are
first considered principally within the context of using standard hot-rolled sections,
i.e. UBs, UCs, RSJs and channels; other types of cross-section are covered in the
later parts of this chapter.
Table 16.3 Limiting conditions for beam design
Ultimate Serviceability
Moment capacity, Me (including Deflections due to bending
influence of local buckling) (and shear if appropriate)
Shear capacity, Pv
Twist due to torsion
Lateral – torsional buckling, Mb Vibration
Web buckling, Pw
Web bearing, Pyw
Moment – shear interaction
Torsional capacity, MT
Bending – torsion interaction
16.3.1 Moment capacity, Mc
The most basic design requirement for a beam is the provision of adequate in-plane
bending strength. This is provided by ensuring that Mc for the selected section
exceeds the maximum moment produced by the factored loading. Determination of
Mc, which is linked to section classification, is fully covered in section 16.2.