Civil Engineering Reference
In-Depth Information
Beam-columns
7.1Introduction
Beam-columns are structural members which combine the beam function of
transmitting transverse forces or moments with the compression (or tension)
memberfunctionoftransmittingaxialforces.Theoretically,allstructuralmembers
may be regarded as beam-columns, since the common classifications of tension
members, compression members, and beams are merely limiting examples of
beam-columns. However, the treatment of beam-columns in this chapter is gen-
erally limited to members in axial compression. The behaviour and design of
members with moments and axial tension are treated in Chapter 2.
Beam-columns may act as if isolated, as in the case of eccentrically loaded
compression members with simple end connections, or they may form part of a
rigid frame. In this chapter, the behaviour and design of isolated beam-columns
are treated. The ultimate resistance and design of beam-columns in frames are
discussed in Chapter 8.
It is convenient to discuss the behaviour of isolated beam-columns under the
three separate headings of In-Plane Behaviour, Flexural-Torsional Buckling, and
Biaxial-Bending, as is done in Sections 7.2-7.4. When a beam-column is bent
about its weaker principal axis, or when it is prevented from deflecting laterally
while being bent about its stronger principal axis (as shown in Figure 7.1a), its
actionisconfinedtotheplaneofbending.Thisin-planebehaviourisrelatedtothe
bendingofbeamsdiscussedinChapter5andtothebucklingofcompressionmem-
bersdiscussedinChapter3.Whenabeam-columnwhichisbentaboutitsstronger
principal axis is not restrained laterally (as shown in Figure 7.1b), it may buckle
prematurely out of the plane of bending by deflecting laterally and twisting.This
actionisrelatedtotheflexural-torsionalbucklingofbeams(referredtoaslateral-
torsional buckling in EC3) discussed in Chapter 6. More generally, however, a
beam-column may be bent about both principal axes, as shown in Figure 7.1c.
This biaxial bending, which commonly occurs in three-dimensional rigid frames,
involves interactions of beam bending and twisting with beam and column buck-
ling.Sections7.2-7.4discussthein-planebehaviour,flexural-torsionalbuckling,
and biaxial bending of isolated beam-columns.
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