Civil Engineering Reference
In-Depth Information
Therefore, for slip-resistant connections,
2
τ
bv
f
bv
2
σ
bt
T
bP
A
b
+
=
1.0.
(9.42)
The elliptical Equation 9.42 may be simplified by a straight-line approximation
(dashed line for ultimate stress values in
Figure 9.15)
as
τ
bv
f
bv
σ
bt
(T
bP
/A
b
)
+
=
1.0
(9.43)
and may be rearranged as
τ
bv
≤
f
bv
1
−
.
σ
bt
(
T
bP
/A
b
)
(9.44)
If
τ
bv
is taken as the allowable shear stress,
f
bv
, when combined with tension,
Equation 9.44 becomes
f
bv
1
,
σ
bt
(T
bP
/A
b
)
f
bv
≤
−
(9.45)
which is the allowable shear stress for combined shear and tension recommended by
AREMA (2008).
9.3.4.1.1.6 Allowable Fatigue Stresses in Bolted Connections
The allow-
able fatigue stress of bolted joints depends on whether the bolts are loaded primarily
in shear, such as in lap and butt joints (
Figure 9.10a-g)
or tension. AREMA (2008)
recommends that all joints subject to fatigue by cyclical stresses must be pretensioned
slip-resistant connections.
9.3.4.1.1.6.1 Allowable Shear Fatigue Stress in Bolted Connections
Bearing-type connections are subject to fatigue damage accumulation and crack ini-
tiation at the edge of, or within, holes due to localized tensile stress concentrations.
Slip-resistant connections are subject to fretting fatigue.
AREMA (2008) recommends the allowable stress range, based on Fatigue Detail
Category B, of 18 ksi for the base metal of slip-critical connections subjected to
2 million cycles or less or 16 ksi for the base metal of slip-critical connections sub-
jected to greater than 2 million stress range cycles (see Chapter 5). Bolts will generally
not experience fatigue failure prior to the base metal and, therefore, AREMA (2008)
contains no recommendations concerning allowable shear stress ranges for bolts.
9.3.4.1.1.6.2 Allowable Tensile Fatigue Stress in Bolted Connections
The stress range in a bolt of a slip-resistant connection is affected by the pretension
applied to the bolt and the rigidity of the connection joint.
The stress range is typically considerably less than the nominal tensile stress
in the bolt in relatively rigid slip-resistant connections with small prying forces.
