Civil Engineering Reference
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interaction, I V ,or
I F =
RE
+
I V .
(4.3)
4.3.2.1.1 Rocking Effects
Railroad freight equipment will rock or sway in a lateral direction due to wind forces,
rail profile variances, and equipment spring stiffness differences. Rocking due to rail
and equipment conditions will affect the magnitude of equipment axle loads and is
considered as a dynamic increment of axle load by AREMA (2008). Rocking effects
are independent of train speed (AREA, 1949; Ruble, 1955).
The rocking effect, RE, is determined for each member supporting the track as
a percentage of the vertical live load. The applied rocking effect, as recommended
in AREMA (2008), is the force couple created by an upward force on one rail and
downward force on the other rail of 20% of the design wheel load, or 0.20 W , where W
is the wheel load (1/2 of axle load).The calculation of RE for an open deck multibeam
deck span is shown in Examples 4.1 and 4.2.
Example 4.1
A double track open deck steel multibeam railway bridge is shown in
Figure E4.1 . Determine the rocking effect, RE, component of the AREMA
impact load.
The applied rocking force is a force couple, R A =
W ,as
shown in Figure E4.2. If the vertical live load is equally distributed to three
longitudinal beams (AREMA allows this provided that beams are equally
spaced and adequately laterally braced), the applied rocking forces are
resisted by a force couple with an arm equal to the distance between the
centers of resisting members each side of the track centerline.
The resisting force couple (Figure E4.2) is R R =
0.20 W( 5.0 ) =
F R (6.00). Since R A =
R R , F R =
0.167 (W) and the rocking effect, RE, expressed as a percentage of
vertical live load, W ,isRE
=[
F R ( 100 )/W
]=
16.7%.
§ Bridge
§ Track
§ Track
7.5 '
7.5 '
9.0 ' Wood tie
2.5 '
6.0 '
9 @3.0 ' = 27.0 '
FIGURE E4.1
 
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