Civil Engineering Reference
In-Depth Information
CJP welds made in accordance with the AREMA (2008) FCP will be of the same or
greater fatigue strength than the base metal.
However, for PJP groove or fillet welds subjected to shear, axial, and/or flexural
tensile stresses due to live load, the allowable fatigue stress range for the appropriate
Fatigue Detail Category and equivalent number of constant stress cycles (Chapter 5)
must be considered. The allowable fatigue stress ranges may be small and govern the
required weld size for weak fatigue details such as transversely loaded fillet or PJP
welds and fillet or groove welds used on attachments with poor transition details.
∗
9.2.4.2
Fatigue Strength of Welds
Stress concentrations are created by welding processes. These processes may intro-
duce discontinuities within the weld, distortion of members, residual stresses, and
stress raisers due to poor weld profiles. Stress concentration factors for butt joint
welds typically range from 1.0 to 1.6 and from between 1.0 and 2.8 (or more) for
other joints (Kuzmanovic and Willems, 1983). These stress concentration effects are
included in the nominal stress range fatigue testing of many different weld types, joint
configurations, and loading directions. This provides the design criteria, in terms of
the allowable fatigue stress ranges, for the various Fatigue Detail Categories rec-
ommended in AREMA (2008). Further discussion of allowable fatigue stresses for
design is contained in Chapter 5.
9.2.4.3
Weld Line Properties
It is intuitive and convenient to design welds as line elements. The effective weld
area,
A
e
(on which allowable stresses are assumed to act), is
A
e
=
t
e
L
w
,
(9.1)
where
t
e
is the thickness of thinner element for CJP groove welds, or depth of the
preparation chamfer less 1/8 in. for PJP groove weld root angles between 45
◦
and
60
◦
(for SWAW and SAW welds), or depth of the preparation chamfer for PJP groove
weld root angles greater than or equal to 60
◦
(for SWAW and SAW welds), or the
throat length equal to 0.707
a
(for fillet welds with equal legs,
a
)
†
and
L
w
is the length
of the weld.
If we consider the weld as a line, the allowable force per unit length,
F
w
,onthe
weld is
F
w
=
t
e
( f
all
)
,
(9.2)
where
f
all
is the allowable weld stress, and, from Equation 9.2,
F
w
f
all
t
e
=
.
(9.3)
∗
Generally, such details should be avoided to preclude low allowable fatigue stress ranges, which may
render the superstructure design uneconomical.
†
Fillet weld throat length is sometimes increased by a small amount to recognize the inherent strength of
fillet welds.
