Civil Engineering Reference
In-Depth Information
F
bU
f
bt
Bolt elongation
FIGURE 9.13
Bolt direct tension forces and elongation.
Rearrangement of Equation 9.28a yields
l
e
d
b
≥
0.87
f
B
F
y
+
0.5,
(9.28b)
1.4
∗
)to
which may be conservatively simplified (for
l
e
/d
b
≥
l
e
d
b
≥
f
B
F
y
.
(9.29)
2.0
†
provides the allowable bearing
Rearrangement of Equation 9.29 and using FS
=
stress as
l
e
F
y
d
b
l
e
F
u
d
b
(
FS
)
=
l
e
F
u
2
d
b
.
f
B
≤
≤
(9.30)
Equations 9.24 and 9.30 are the allowable bearing stresses on bolts recommended by
AREMA (2008).
9.3.4.1.1.4 Allowable Tension Stress in Connections
Figure 9.13 illustrates
that the behavior of a bolt under tensile load is elastic for small elongations. The
strength of a bolt loaded in direct tension is not affected by pretension stresses from
installation by a method that applies the pretension to the bolt from torquing (Kulak,
2002). This is because the pretension load is readily dissipated as the direct tensile
load is applied to a connection. Therefore, bolt strength is determined based on ulti-
mate tensile strength. The allowable tensile stress,
f
bt
, in anA325 bolt, using FS
=
2.0
and
A
st
=
0.75
(A
b
)
,is
0.75
F
bU
FS
f
bt
=
=
45 ksi.
(9.31)
∗
This is the case in practical structures.
†
A lower FS is used due to the conservative nature of the assumptions made to develop Equation 9.29.
