(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.

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