Civil Engineering Reference
In-Depth Information
Class 2 open sections would satisfy
9
<λ
≤
10,
(10.67)
Class 3 open sections would satisfy
10
<λ
≤
14,
(10.68)
and Class 4 open sections would satisfy
14
<λ
.
(10.69)
For closed cross-sections, the only significant stresses that develop during tor-
sionareuniformtorsionshearstresses
τ
t
thatareconstantacrossthewallthickness
t
and along the length
b
of any element of the cross-section. This stress distribu-
tion is very similar to that caused by a bending shear force in the web of an
equal-flanged I-section, and so the section classification may be based on the
samewidth-thicknesslimits[24].ThustheelementsofClass1,Class2,andClass
3 rectangular hollow sections would satisfy
λ
≤
60
(10.70)
in which
λ
is the element slenderness given by
λ
=
(
h
w
/
t
)
√
f
y
/
235
.
(10.71)
in which
h
w
is the clear distance between flanges. Rectangular hollow sections
whichdonotsatisfythisconditionwouldbeclassifiedasClass4.Circularhollow
sections do not buckle under shear, unless they are exceptionally slender, and so
they may generally be classified as Class 1.
10.4.3 Uniform torsion section resistance
The effective uniform torsion section resistance of all open sections
T
t
,
Rd
can be
approximatedbytheplasticsectionresistance
T
tp
givenbyequation10.29inwhich
the shear yield stress is given by
τ
y
=
f
y
/
√
3
(10.72)
TheuniformtorsionsectionresistanceofClass1,Class2,andClass3rectangular
hollowsections
T
t
,
Rd
,123
canbedeterminedbyusingthefirstyielduniformtorque
T
ty
ofequation10.31.TheeffectiveuniformtorsionsectionresistanceofaClass4
rectangularhollowsection
T
t
,
Rd
,4
canbeapproximatedbyreducingthefirstyield
uniform torque
T
ty
of equation 10.31 to
T
t
,
Rd
,4
=
T
ty
(
60
/λ)
(10.73)
The effective uniform torsion section resistance of a circular hollow section
T
t
,
Rd
can generally be approximated by using equation 10.31 to find the first yield
uniform torque
T
ty
.
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