CEMP-E
TI 850-02
AFMAN 32-1125(I)
1 MARCH 2000
(3) Check tie bending stress using equation 6-9a or 6-9b, and ft = 1100 psi as a suggested limit:
qo
For 6 x 8 Ties : ft =
Eq 6-9a
29
qo
For 7 x 9 Ties : ft =
Eq 6-9b
52
ft =
Flexural stress on underside of tie below rail seat (psi).
qo =
Maximum rail seat load (lb).
(4) If ft > 1100, choose a larger tie size or decrease tie spacing.
i. Ballast and Subgrade.
(1) Determine ballast surface stress from equation 6-10, with pm = 65 psi as a suggested limit:
qo
pm =
Ab
Eq 6-10
pm = Ballast surface stress (psi).
qo = Maximum rail seat load (lb).
Ab = Effective bearing area of tie on ballast (sq in.).
For 6 x 8 ties, Ab = 270.
For 7 x 9 ties, Ab = 312.
(2) If pm exceeds 65 psi, choose a larger tie and/or decrease tie spacing.
(3) Select design subgrade bearing capacity (pc) according to results of soil tests or other data. (In
the absence of other guidance, the design bearing capacity for cohesive soils may be the same as the
unconfined compressive strength).
6-19
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