Civil Engineering Reference
In-Depth Information
where
F
y
is the tensile yield stress of steel,
F
u
is the ultimate tensile stress of steel, and
φ =
0.85 is the connection strength capacity reduction factor (Salmon and Johnson,
1980).
AREMA (2008) uses a safety factor of 9/5 which, when substituted into
Equation 6.1, results in
0.56
F
y
A
g
≤
0.47
F
u
A
e
.
(6.2)
The net area,
A
n
, is determined from the gross area,
A
g
, with connection holes
removed, and may require further reduction to an effective net area,
A
e
, to account
for the effects of stress concentrations and eccentricities at connections. Therefore,
the allowable strength of the tension member,
T
all
,is
T
all
=
0.55
F
y
A
g
(6.3a)
or
T
all
=
0.47
F
u
A
e
.
(6.3b)
The design of the tension member should be established based on the lesser
T
all
given by Equation 6.3a or 6.3b.
6.2.1.1 Net Area,
A
n
, of Tension Members
The net area,
A
n
, is determined at the cross section of the member with the greatest
area removed for perforations or other openings in the member.
∗
The gross area,
A
g
,
across a bolted tension member connection is reduced by the holes. The net area at the
connection is at the potential tensile failure line,
w
nc
, of the least length. The length
of potential failure lines at connections is (Cochrane, 1922)
N
b
−
1
N
b
s
j
4
g
j
w
nc
=
w
g
−
d
b
+
,
(6.4)
i
=
1
j
=
1
where
w
g
is the gross length across the connection (gross width of the axial member),
N
b
is the number of bolt holes in the failure line,
d
b
is the effective diameter of
bolt holes
1
/
8 in.,
s
is the hole stagger or pitch (hole spacing
in a direction parallel to load), and
g
is the hole gage (hole spacing in a direction
perpendicular to load).
The net area is
=
bolt diameter
+
⎛
⎞
N
b
−
1
N
b
s
j
4
g
j
⎝
⎠
(t
m
)
,
A
n
=
w
nc
(t
m
)
=
A
g
−
d
b
−
(6.5)
i
=
1
j
=
1
where
t
m
is the thickness of the member.
∗
Perforations and openings are stress raisers, and require consideration in fatigue design.
