Torsion members - The Behaviour and Design of Steel Structures to EC3 - page 435

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(a) Eccentric load (b) Bending (c) Torsion (d) Distortion

Figure 10.3 Eccentric loading of a thin-walled closed section.

and for members of high-torsional rigidity, for which the rates of change of the
angle of twist rotation vary only locally near points of concentrated torque and
warpingrestraint.Thissimplemethodofanalysisusuallyleadstosatisfactorypre-
dictionsoftheanglesoftwistrotation,butmayproduceunderestimatesofthelocal
stresses.
In this chapter, the behaviour and design of torsion members are discussed.
UniformtorsionistreatedinSection10.2,andnon-uniformtorsioninSection10.3,
while the design of torsion members for strength and serviceability is treated in
Section 10.4.
Structural members, however, are rarely used to resist torsion alone, and it is
much more common for torsion to occur in conjunction with bending and other
effects.Forexample,whentheboxsectionbeamshowninFigure10.3aissubjected
to an eccentric load, this causes bending (Figure 10.3b), torsion (Figure 10.3c),
anddistortion(Figure10.3d).Theremayalsobeinteractionsbetweenbendingand
torsion.Forexample,thelongitudinalstressesduetobendingmaycauseachange
in the effective torsional rigidity. This type of interaction is of some importance
in the flexural-torsional buckling of thin-walled open-section members, as dis-
cussed in Sections 3.7.5 and 6.10.Also, significant twist rotations of the member
maycauseincreasesinthebendingstressesinthin-walledopen-sectionmembers.
However, these interactions are usually negligible when the torsional rigidity is
very high, as in thin-walled closed-section members. The behaviour of members
under combined bending and torsion is discussed in Section 10.5.
The effects of distortion of the cross-section are only significant in very thin-
walled open-sections, and in thin-walled closed sections with high-distortional
loadings. Thus, for the box-section beam shown in Figure 10.3, the relatively
flexible plates may bend out of their planes as indicated in Figure 10.3d, causing
the cross-section to distort. Because of this, the in-plane plate bending and shear
stressdistributionsarechanged,andsignificantout-of-planeplatebendingstresses
may be induced. The distortional behaviour of structural members is discussed
briefly in Section 10.6.