Civil Engineering Reference
In-Depth Information
F
bv
f
bv
Bolt elongation
FIGURE 9.11
Bolt shear stress and elongation.
AREMA (2008) recommends only slip-resistant connections and, therefore, does
not provide an allowable stress,
f
bv
, based on shearing of the bolt shank. In slip-
resistant connections, service loads are transmitted by friction and bolt shank shearing
will not govern the design.
9.3.4.1.1.2 Allowable Effective Shear Stress in Slip-Resistant Connections
The shear slip force,
P
bv
,is
n
P
bv
=
mnf
bv
(A
b
)
=
k
s
m
α
T
bP
i
,
(9.20)
i
=
1
where
f
bv
is the effective allowable bolt shear stress;
k
s
is the shear slip coefficient
of steel connection;
m
is the number of slip planes (faying surfaces);
n
is the number
of bolts in connection;
T
bP
i
is the specified pretension in bolt
i
;
T
b
i
/T
bP
i
;
T
b
i
is
the actual pretension in bolt
i
. Therefore, the effective allowable shear stress (which
is based on the magnitude of the prestress force and the shear slip coefficient) is
α =
k
s
α
i
=
1
T
bP
i
nA
b
f
bv
=
.
(9.21a)
0.75
(A
b
)
, Equation 9.21a for A325 bolts, when the specified preten-
sion in each bolt,
T
bP
i
, is equal, becomes
Using
A
st
=
k
s
α
T
bP
i
A
b
k
s
α
(
0.70
F
bU
A
st
)
A
b
f
bv
=
=
=
63
α
k
s
.
(9.21b)
In tests done to establish an empirical relationship for the effective allowable bolt
shear stress, the slip probability level, mean slip coefficient,
k
sm
, and bolt pretension
arenotexplicitlydetermined.Theyarecombinedintoaslipfactor,
D
,thatincorporates
the
k
s
and
k
sm
relationship, and
α
of Equation 9.21b as
k
s
α
T
bP
i
A
b
f
bv
=
=
(
0.53
)Dk
sm
F
bU
=
63
Dk
sm
.
(9.22)
