Torsion members - The Behaviour and Design of Steel Structures to EC3 - page 466

may be quite large, as discussed in Section 10.6). On the other hand, thin-walled
open-section members are comparatively flexible, and special measures may be
required to limit their rotations.

10.5 Torsion and bending

10.5.1 Behaviour

Puretorsionoccursveryrarelyinsteelstructures.Mostcommonly,torsionoccurs
in combination with bending actions. The torsion actions may be classified as
primaryorsecondary,dependingonwhetherthetorsionactionisrequiredtotrans-
fer load (primary torsion), or whether it arises as a secondary action. Secondary
torquesmayariseasaresultofdifferentialtwistrotationscompatiblewiththejoint
rotations of primary frames, as shown in Figure 10.31, and are often predicted by
three-dimensional analysis programs. They are not unlike the secondary bend-
ing moments which occur in rigid-jointed trusses, but which are usually ignored
(aprocedurejustifiedbymanyyearsofsatisfactoryexperiencebasedonthelong-
standing practice of analysing rigid-jointed trusses as if pin-jointed). Secondary
torques are usually small when there are alternative load paths of high stiffness,
and may often be ignored.
Primary torsion actions may be classified as being restrained, free, or destab-
lising, as shown in Figure 10.32. For restrained torsion, the member applying the
torsion action (such as ABC shown in Figure 10.32b) also applies a restraining
actiontothememberresistingthetorsion(DECinFigure10.32b).Inthiscase,the
structureisredundant,andcompatibilitybetweenthemembersmustbesatisfiedin
theanalysisifthemagnitudesofthetorquesandotheractionsaretobedetermined
correctly.Freetorsionoccurswhenthememberapplyingthetorsionaction(suchas

Joint rotation of
flexible portal
frame

Differential end
rotations induce
secondary
torsion

Negligible rotation
at end wall

Figure 10.31 Secondary torsion in an industrial frame.