Civil Engineering Reference
In-Depth Information
AREMA (2008) recommends that
L
r
min
≤
100 for main compression members,
(6.38)
L
r
min
≤
120 for wind and sway bracing compression members.
(6.39)
Example 6.6
Determine the design criteria for load case A1 (Table 4.5) for member U3-
50 ksi) steel with
ultimate stress,
F
u
, of 70 ksi (see Chapter 2).
Maximum forces in member U3-U4 (see Chapter 5) due to
Dead load force
98.10 kips
Maximumliveloadforcewithloadmovingacrosstruss
=
DL
=−
=
LL
max
=
0.00 kips
Minimum live load force with load moving across truss
=
LL
min
=
−
557.2 kips
Maximum live load impact
156.38
Figure 4.5)
=
20.75% (
L
=
P
max
=−
98.10
+
1.21
(
−
557.2
)
=−
771.2 kips.
Stiffness considerations:
L
r
min
≤
100
(
19.55
)(
12
)
100
r
min
≥
≥
2.35 in. (main member of truss)
KL
r
min
≤
75 (bolted end connection).
Strength considerations:
F
cr
(
min
)
=
0.60
(
50,000
)
−
(
30.17
)
3
/
2
(
75
)
=
30,000
−
12,430
=
17,570 psi
at
KL/r
=
75
(
Figure 6.11)
and
771,200
17,570
43.9 in.
2
A
g
=
=
(stability) (for maximum allowable slenderness).
6.3.3 A
XIAL
C
OMPRESSION
M
EMBERS IN
S
TEEL
R
AILWAY
B
RIDGES
Itisoftennecessarytofabricaterailwaybridgecompressionmembersofseveralstruc-
turalshapesduetolargemagnituderailroadloadsandpotentialforinstability.Bending
effects and connection geometry constraints may also dictate the use of built-up com-
pression members. The components must be adequately fastened to ensure integral
behavior of the compression member. In cases where a box-type member is undesir-
able, such as where the ingress of water is difficult to preclude, open compression
