(3) Determine coefficients Cm and Cd as follows:

(a) For most design situations, and for cars with either 2-axle or 3-axle trucks, the rail moment

coefficient (Cm) can be taken as 0.8.

(b) The load coefficient Cd can be taken from the following:

X1 =

28

30

32

34

36

38

40

Design

66:

1.24

1.30

1.36

1.44

1.54

1.62

1.70

Wheel Spacing

75:

1.08

1.15

1.22

1.30

1.36

1.42

1.50

g. Check Rail Bending Stress. Use equation 6-7 to check that rail bending stress is less than 32,000:

Eq 6-7

fo =

Maximum flexural stress (psi).

Pd =

Dynamic (design) wheel load (lb) from table 2-2.

Cm =

Moment coefficient.

X1 =

Distance from wheel load to point of zero bending moment (inches).

Zb =

Section modulus of rail base, from table 6-7 (cu in.).

h. Ties and Tie Spacing.

(1) Choose a trial tie spacing and calculate the maximum rail seat load using equation 6-8:

Eq 6-8

qo =

Maximum rail seat load (lb).

Pd =

Dynamic (design) wheel load (lb) from table 2-2.

Cd =

Load coefficient for adjacent wheels.

S=

Tie spacing (inches).

X1 =

Distance from wheel load to point of zero bending moment (inches).

(2) Select tie size, either 6-in. x 8-in. x 8.5 ft, or 7-in. x 9-in. x 8.5 ft.

6-18

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