Civil Engineering Reference
In-Depth Information
1000
Reference values
∆
C
Reference values
∆
C
×10
6
∆
C
=
∆
when
N
= 2
500
1
4
0
1
1
2
9
0
7
1
5
6
4
5
3
6
1
6
0
1
2
5
1
0
0
8
0
6
3
5
0
4
0
Constant amplitude
fatigue limit
m
=5
1
Cut-off limit
m
100
50
m
=3
m
=3
10
55
5
2
5
5
4
6
7
8
10
10
10
10
10
Number of stress cycles
N
Figure 1.10
Variation of the EC3-1-9 fatigue life with stress range.
EC3-1-1 [8] does not provide a treatment of fatigue, since it is usually the case
that either the stress range
σ
or the number of high amplitude stress cycles
N
is comparatively small. However, for structures supporting vibrating machinery
and plant, reference should be made to EC3-1-9 [28]. The general relationships
betweenthefatiguelife
N
andtheservicestressrange
σ
forconstantamplitude
stresscyclesareshowninFigure1.10forreferencevalues
σ
C
whichcorrespond
to different detail categories. For
N
≤
5
×
10
6
,
m
=
3 and
K
=
1, so that
the reference value
σ
C
corresponds to the value of
σ
at
N
=
2
×
10
6
. For
5
×
10
6
≤
N
≤
10
8
,
m
=
5 and
K
=
0.4
2
/
3
≈
0.543.
Fatiguefailureundervariableamplitudestresscyclesisnormallyassessedusing
Miner's rule [29]
N
i
/
N
im
≤
1
(1.5)
in which
N
i
is the number of cycles of a particular stress range
σ
i
and
N
im
the
constant amplitude fatigue life for that stress range. If any of the stress ranges
exceeds the constant amplitude fatigue limit (at
N
=
5
×
10
6
), then the effects of
stress ranges below this limit are included in equation 1.5.
Designingagainstfatigueinvolvesaconsiderationofjointarrangementaswell
as of permissible stress. Joints should generally be so arranged as to minimise
stress concentrations and produce as smooth a 'stress flow'through the joint as is
practicable. This may be done by giving proper consideration to the layout of a
joint,bymakinggradualchangesinsection,andbyincreasingtheamountofmate-
rial used at points of concentrated load. Weld details should also be determined
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