CEMP-E
TI 850-02
AFMAN 32-1125(I)
1 MARCH 2000
(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:
(0.318 Pd Cm X1 )
fo =
Eq 6-7
Zb
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:
0.39 Pd Cd S
qo =
X1
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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