CEMP-E
TI 850-02
AFMAN 32-1125(I)
1 MARCH 2000
(4) The AREMA track design method uses the beam-on-elastic-foundation model. In this model, the
track has two components: the beam, which is the rail, and the elastic foundation, which represents
everything below the rail combined. The basic expression in the model relates three main variables: the
load on the rail, the stiffness of the track system, and the amount of vertical rail deflection, as shown in
equation 6-4.
P
Y=
(64 EIu3 )0.25
Eq 6-4
Y
=
vertical rail deflection at a point (inches).
P
=
applied wheel load (including contributions from adjacent wheels) (lb).
EI =
stiffness of the rail, where:
modulus of elasticity for steel (30 x 106 psi).
E
=
vertical moment of inertia of the rail section (inches4).
I
=
u
=
stiffness of rail support, or track modulus (psi).
In this equation, EI represents the stiffness of the "beam" and u the stiffness of the "elastic foundation."
c. Design Load and Wheel Spacing.
(1) Select the design wheel load from table 2-2, based on the most common, heaviest car expected
to travel over the track.
(2) Figure 6-13 shows the two most common wheel configurations. Most cars have two-axle trucks
and the design wheel configuration is that in drawing (a), with an average 75-in. wheel spacing. The 140-
ton series flat cars (for carrying M-1 tanks and other heavy vehicles) and some 100-ton flat cars have 3-
axle trucks; their design wheel configuration is that in drawing (b), with 66-in. wheel spacing.
6-15
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