Figure 6-10. Example Load Distribution along the Track

CEMP-E

TI 850-02

AFMAN 32-1125(I)

1 MARCH 2000

5. TRACK STRUCTURE.

a. Design Objective. Among its various functions, the track system serves to distribute the large,

concentrated wheel loads longitudinally, laterally, and vertically away from the wheel contact area on the

rail surface. A well designed, constructed, and maintained track will distribute the loads in a relatively

uniform fashion, with each component supporting its share of the load. The role of the designer is to

select a track structure (or changes to an existing track) so that the intended wheel loads will be properly

supported, without overstressing any of the four main track system components: rail, ties, ballast, and

subgrade.

b. Load Distribution. As figure 6-10 shows, when a wheel is centered over a tie, the tie directly beneath

the load will generally carry less than half of that wheel load, with the remainder supported by two ties on

either side. Beneath the wheel, the pressures are distributed (reduced) approximately as shown in figure

6-11. With a wheel-rail contact area of about ½ sq in., stresses are reduced from 60,000 psi at the top of

the rail to about 10 psi at the depth of the subgrade surface. While the actual load and pressure

distributions will vary with wheel load, track design, and track condition, the two figures illustrate a realistic

case for good track.

Figure 6-10. Example Load Distribution along the Track

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