Civil Engineering Reference
In-Depth Information
Designers of railway bridges that carry high loads (e.g., double-stack rail cars) should
review this recommendation.
Example 4.14
Determine the AREMA (2008) recommended design wind force for the
unloaded girder span of
Figure E4.8.
50
(
1.5
)(
15
)(
125
)
1000
F
D
=
=
140.6 kips.
Example 4.15
Determine the AREMA-recommended design wind force for the unloaded
truss span of
Figure E4.9.
50
(
0.25
)
[
(
25
−
3
)
+
25
]
(
200
)
F
D
=
=
117.5 kips.
1000
4.4.2 L
ATERAL
V
IBRATION
L
OADS ON
S
TEEL
R
AILWAY
B
RIDGES
A dynamic analysis of lateral vibration similar to that performed in
Section 4.3.2.1.2
for vertical vibration of the superstructure may be conducted. However, in order
to simplify design procedures and ensure global rigidity of the superstructure, the
AREMA (2008) recommendations include a notional vibration load to be resisted by
the lateral bracing. Since the purpose of this design load is to ensure adequate lateral
bracing (lateral stiffness to resist vibration from live load), it is not combined with
other loads and forces (Waddell, 1916). Therefore, it is to be applied to the lateral
bracing as an alternative to the wind load on a loaded railway bridge. This notional
load is taken as 200 lb/ft on the loaded chord or flange of the superstructure (e.g., the
top flange of a deck plate girder span) and 150 lb/ft on the unloaded chord or flange
(e.g., the top chord of a through truss span).
Example 4.16
Determine the design wind load (including the notional vibration load) for
the top and bottom lateral bracing of the 125 ft long ballasted deck steel deck
plate girder railway span shown in
Figure E4.8.
Unloaded span
:
W
T
=
(
3
+
6
)(
50
)(
1.5
)
=
675 lb/ft wind at the top lateral bracing,
W
B
=
(
6
)(
50
)(
1.5
)
=
450 lb/ft wind at the bottom lateral bracing.
Loaded span
:
W
T
=
(
3
+
6
)(
30
)(
1.5
)
+
300
=
705 lb/ft wind at the top lateral bracing,
W
B
=
(
6
)(
30
)(
1.5
)
=
270 lb/ft wind at the bottom lateral bracing,
V
T
=
200 lb/ft vibration load at the top lateral bracing,
