Torsion members - The Behaviour and Design of Steel Structures to EC3 - page 462

togenerally, onlythefirstyielddesignresistanceisspecificallydiscussedfortor-
sion members. Further, there is no guidance on section classification for torsion
members, nor on how to allow for the effects of local buckling on the design
resistance.
In the following sub-sections, the specific EC3 provisions for torsion analysis
and design are extended so as to provide procedures which are consistent with
those used in EC3 for the design of beams against bending and shear.
First the member cross-section is classified as Class 1, Class 2, Class 3, or
Class 4 in much the same way as is a beam cross-section (see Sections 4.7.2
and 5.6.1.2), and then the effective section resistances for uniform and warping
torsion are determined. Following this, an appropriate method of torsion analysis
(plasticorelastic)isselected,andthenanappropriatemethod(plastic,firsthinge,
firstyield,orlocalbuckling)isusedforstrengthdesign.Allofthestrengthdesign
methodssuggestedignorethewarpingshearstresses τ w becausethesearegenerally
small, and because they occur at different points in the cross-section and along
the member than do the much more significant uniform torsion shear stresses τ t
andthewarpingbimomentnormalstresses σ w .Finally,amethodofserviceability
design is discussed.

10.4.2 Section classification

Cross-sections of torsion members need to be classified according to the extent
bywhichlocalbucklingeffectsmayreducetheircross-sectionresistances.Cross-
sections which are capable of reaching and maintaining plasticity while a torsion
plastic hinge collapse mechanism develops may be called Class 1, as are the
correspondingcross-sectionsofbeams.Class2sectionsarecapableofdeveloping
afirsthinge,butinelasticlocalbucklingmaypreventthedevelopmentofaplastic
collapse mechanism. Class 3 sections are capable of reaching the nominal first
yield before local buckling occurs, while Class 4 sections will buckle locally
before the nominal first yield is reached.
For open cross-sections, the warping shear stresses τ w are usually very small,
and can be neglected without serious error. The uniform torsion shear stresses τ t
changesignandvarylinearlyacrossthewallthickness t ,andsocanbeconsidered
tohavenoeffectonlocalbuckling.Theflangeelasticandplasticwarpingnormal
stress distributions are similar to those due to bending in the plane of the flange,
andsothesectionclassificationmaybebasedonthesamewidth-thicknesslimits.
Thus the flange outstands of a Class 1 open section would satisfy

λ ≤ 9

(10.65)

in which λ is the element slenderness given by
λ = ( c / t ) √ f y / 235 .

(10.66)