Torsion members - The Behaviour and Design of Steel Structures to EC3 - page 467

Torsion
member
DCE

Rigid joint C
requires compatible
twisting

Q

A

B

D

Q

C A

C

B

Loading
member
ABC

E

(b) Restrained torsion

(a) Torsion and loading members Cantilever BC permits

lateral deflection of
torsion member

Q

Reaction eccentricity at
pin connection C causes
torque

B

D

Q C

C A

C

B

E

(c) Free torsion (d) Destabilising torsion

Figure 10.32 Torsion actions.

ABCinFigure10.32c)doesnotrestrainthetwistingofthetorsionmember(DEF),
but does prevent its lateral deflection. Destablising torsion may occur when the
memberapplyingthetorsionaction(suchasBCinFigure10.32d)doesnotrestrain
either the twisting or the lateral deflection of the torsion member (DCE). In this
case, lateral buckling actions (Chapter 6) caused by the in-plane loading of the
torsion member amplify the torsion and out-of-plane bending behaviour.
The inelastic non-linear bending and torsion of fully braced, centrally braced,
andunbracedI-beamswithcentralconcentratedloads(seeFigure10.33)havebeen
analysed[25],andinteractionequationsdevelopedforpredictingtheirstrengths.It
wasfoundthatwhilecircularinteractionequationsareappropriateforshortlength
braced beams, these provide unsafe predictions for beams subject to destablis-
ing torsion, where lateral buckling effects become important. On the other hand,
destablisinginteractionsbetweenlateralbucklingandtorsiontendtobemaskedby
thefavourableeffectsofthesecondaryaxialstressesthatdevelopatlargerotations,
and linear interaction equations based on plastic analyses provide satisfactory
strength predictions, as shown in Figure 10.34. Proposals based on these find-
ings are made below for the analysis and design of members subject to combined
torsion and bending.