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Steel Designers' Manual - 6th Edition (2003)
Chapter 16
Beams
by DAVID NETHERCOT
16.1 Common types of beam
Beams are possibly the most fundamental type of member present in a civil engineering
structure. Their principal function is the transmission of vertical load by
means of flexural (bending) action into, for example, the columns in a rectangular
building frame or the abutments in a bridge which support them.
Table 16.1 provides some idea of the different structural forms suitable for use
as beams in a steel structure; several of these are illustrated in Fig. 16.1. For modest
spans, including the majority of those found in buildings, the use of standard
hot-rolled sections (normally UBs but possibly UCs if minimizing floor depth is a
prime consideration or channels if only light loads need to be supported) will
be sufficient. Lightly loaded members such as the purlins supporting the roof
of a portal-frame building are frequently selected from the range of proprietary
cold-formed sections produced from steel sheet only a few millimetres thick, normally
already protected against corrosion by galvanizing, in a variety of highly efficient
shapes, advantage being taken of the roll-forming process to produce sections
with properties carefully selected for the task they are required to perform. For
spans in excess of those that can be achieved sensibly using ready-made sections
some form of built-up member is required. Castellated beams, formed by profile
cutting of the web and welding to produce a deeper section, typically 50% deeper
using the standard UK geometry, are visually attractive but cannot withstand high
shear loads unless certain of the castellations are filled in with plate. The range of
spans for which UBs may be used can be extended if cover plates are welded to
both flanges.
Alternatively a beam fabricated entirely by welding plates together may be
employed allowing variations in properties by changes in depth, for example, flange
thickness, or, in certain cases where the use of very thin webs is required, stiffening
to prevent premature buckling failure is necessary. A full treatment of the specialist
aspects of plate-girder design is provided in Chapter 17. If spans are so large that
a single member cannot economically be provided, then a truss may be a suitable
alternative. In addition to the deep truss fabricated from open hot-rolled sections,
SHS or both, used to provide long clear spans in sports halls and supermarkets,
smaller prefabricated arrangements using RHS or CHS provide an attractive alternative
to the use of standard sections for more modest spans. Truss design is discussed
in Chapter 19.
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