Civil Engineering Reference
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ismuchmoredifficultthanplasticanalysis,andalsobecauseequation10.78often
needs to be checked at a number of points along the member.
10.4.7 First yield design
The use of first yield design should be limited to Class 1, Class 2, and Class 3
memberswhichcanreachfirstyield.Themaximumdesignuniformtorques
T
t
and
bimoments
B
canbedeterminedbyelasticanalysis.Thememberissatisfactoryif
2
2
T
t
T
t
,
Rd
,3
B
B
Rd
,3
+
≤
1
(10.79)
is satisfied at all points along the member. This first yield method is the same as
that specified in EC3 except for the neglect of the warping shear stresses
τ
w
.Itis
just as difficult to use as the first hinge method.
10.4.8 Local buckling design
Class 4 members should designed against local buckling. The maximum design
uniform torques
T
t
and bimoments
B
can be determined by elastic analysis. The
member is satisfactory if
2
2
T
t
T
t
,
Rd
,4
B
B
Rd
,4
+
≤
1
(10.80)
is satisfied at all points along the member.
10.4.9 Design for serviceability
Serviceabilitydesignusuallyrequirestheestimationofthedeformationsunderpart
oralloftheserviceabilityloads.Becausetheseloadsareusuallysignificantlyless
than the factored combined loads used for strength design, the member remains
largely elastic, and the serviceability deformations can be evaluated by linear
elastic analysis.
There are no specific serviceability criteria for satisfactory deformations of a
torsion member given in EC3. However, general recommendations such as 'the
deformations of a structure and of its component members should be appropriate
tothelocation,loadingandfunctionofthestructureandthecomponentmembers'
may be applied to torsion members, so that the onus is placed on the designer to
determine what are appropriate magnitudes of the twist rotations.
Becauseofthelackofpreciseserviceabilitycriteria,highlyaccuratepredictions
of the serviceability rotations are not required.Thus the maximum twist rotations
ofmembersinnon-uniformtorsionmaybeapproximatedbyusingequation10.56.
Usually, the twist rotations of thin-walled closed-section members are very
small, and can be ignored (although in some cases, the distortional deformations
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