Torsion members - The Behaviour and Design of Steel Structures to EC3 - page 469

10.5.3 Design of members without full bracing

Amemberwithoutfullbracinghasitsbendingresistancereducedbylateralbuck-
lingeffects(Chapter6).Itsbendingshouldbeanalysedelastically,andtheresulting
moment distribution used to determine its bending resistance M b , Rd according to
EC3.
Anelastictorsionanalysisshouldbeusedtofindthevaluesofthedesignuniform
torques T t and bimoments B . An appropriate torsion design method should be
selectedfromSections10.4.5-10.4.8andusedtofindtheloadfactor α x bywhich
the design torques and bimoments should be multiplied so that the appropriate
torsion design equation (equations 10.77-10.80) is just satisfied.
The member should then satisfy the linear interaction equation

M b , Rd + 1

M Ed

α x ≤ 1

(10.82)

There is no need to amplify the moment because of the finding [25] that lin-
ear interaction equations are satisfactory, even for unbraced beams, as shown in
Figure 10.34.

10.6 Distortion
Twisting and distortion of a flexural member may be caused by the local distri-
bution around the cross-section of the forces acting, as shown in Figure 10.3.
If the member responds significantly to either of these actions, the bending stress
distribution may depart considerably from that calculated in the usual way, while
additionaldistortionalstressesmaybeinducedbyout-of-planebendingofthewall
(seeFigure10.3d).Toavoidpossiblefailure,thedesignermusteitherincreasethe
strength of the member, which may be uneconomic, or must limit both twisting
and distortion.
Theresistancetodistortionofathin-walledmemberdependsonthearrangement
of the cross-section. Members with triangular closed cells have high resistances
because of the truss-like action of the triangulated cross-section which causes the
distortionalloadstobetransferredbyin-planebendingandshearofthecellwalls.
On the other hand, the walls of the members with rectangular or trapezoidal cells
bendoutoftheirplaneswhenunderdistortionalloadingandwhentheresistances
to distortion are low [26], while members of open cross-section are even more
more flexible. Concentrated distortional loads can be distributed locally by pro-
vidingstiffdiaphragmswhichreduceorpreventlocaldistortion.However,isolated
diaphragms are not fully effective when the distortional loads are distributed or
canmovealongthemember.Insuchcases,itisnecessarytoanalysethedistortion
of the section and the stresses induced by the distortion.
The resistance to torsion of a thin-walled open section is comparatively small,
and the dominant mode of deformation is twisting of the member. Because this
twistingisusuallyaccountedfor,theimportanceofdistortionliesinitsmodifying