Civil Engineering Reference
In-Depth Information
Stiffness considerations:
L
r
min
≤
200
(
27.25
)(
12
)
200
r
min
≥
≥
1.64 in.
Select a member with the following section properties:
Minimum Gross Area of the member
6.92 in.
2
(tensile yielding)
Minimum Gross Area for portions of the member connected
=
A
g
≥
A
g
≥
=
9.31 in.
2
(fatigue)
Minimum Effective Net Area of the member at the connection
=
A
e
≥
6.42 in.
2
(fracture)
Minimum Net Area of the member away from the connection
=
A
n
≥
6.21 in.
2
(fatigue)
Minimum Radius of Gyration of the member
=
r
min
≥
1.64 in.
Example 6.4
Determine the design criteria for load cases A1 and A2 (Table 4.5) for member
50 ksi) steel
with ultimate stress,
F
u
, of 70 ksi.
Forces in member U1-L2 are
Dead load force
=
DL
=+
54.56 kips
Maximum live load force
=
LL1
=+
332.4 kips
Minimum live load force
=
LL2
=−
7.4 kips
156.4
see Chapter 4)
Maximum live load impact
=
20.75% (
L
=
Mean live load impact
=
0.65
(
20.758
)
%
=
13.49% (see Chapter 4)
339.8 kips [AREMA
(2008) recommends that all stress ranges be considered as tensile stress
ranges, due to the potential for preexisting mean tensile stresses—see
Chapters 5 and 9].
Range of live load force
=
LL
range
=+
332.4
−
(
−
7.4
)
=
Load combinations:
P
rangeLL+I
=
1.135
(
339.8
)
=
385.7 kips
456.1 kips.
AREMA (2008) recommends that web members and their connections be
designed for 133% of the allowable stress using the live load LLT that will
increase the total maximum chord stress by 33% (see Chapters 4 and 5).
The maximum chord forces are
Maximum dead load force in chord
P
max
=+
54.56
+
1.208
(
+
332.4
)
=
98.10 kips.
Total maximum chord force (for Cooper's E80 live load)
=
=
P
chordmax
=
98.10
771.2 kips (same impact factor for chord and diagonal
members in 156.4
through truss) (see Chapter 5).
+
1.208
(
557.2
)
=
