Page 326 - Buried Pipe Design

Steel and Ductile Iron Flexible Pipe Products 297

It must be understood, of course, that this presentation applies only
2
for a given yield point stress. In this case, about 35 to 40 kips/in .
If the yield point were twice as high, all the allowable stress lines
would go up roughly twice as high. This is not precise, however,
because ring flexibility, soil compressibility, and ring deflection, as
well as yield point of the material, influence the allowable stress lines.
The test data indicate that the longitudinal seams do influence the
ultimate ring compression stress lines, but show that this influence is
much less significant than the soil compression and pipe wall crush-
ing strength.
2
1
Example 6.1 Suppose that a 48-in-diameter 2 / - by / -in corrugated steel
3 2
pipe is to be installed under 120 ft of soil embankment. The soil about the
pipe is to be compacted to 90 percent modified density (found to have a unit
3
weight of about 120 lb/ft ). Determine the pipe wall thickness (gage) if the
performance limit is defined as incipient ring failure (Fig. 6.4). Suppose that
H-20 loading will pass over the surface. If control of the installation is dubi-
ous, a safety factor of N 2 will be assumed.
The apparent vertical soil pressure on the pipe ring is
P D L 14.4 kip/ft 2
L
L
3
where D L H 120 lb/ft 120 ft
L L negligible
The apparent ring compression stress is

2
PD (14.4 kips/ft ) (4.0 ft) 28.8 kips/ft

2A 2A A
where A area per unit length

The apparent ring compression strength (based on 40 ksi yield point) is
f c 60 kips/in 2

which is the ordinate to the strength envelope shown in Fig. 6.4 corre-
sponding to soil density of 90 percent and a pipe diameter of 4.0 ft in a 2 / -by
2
3
1 / -inch corrugation. (Where the yield point is something other than 40 ksi,
2
the apparent ring compression strength f is modified proportionally.)
c
Equating stress to strength divided by safety factor yields
PD f c

2A N
or
28.8 kips/ft 60 kips/in 2

A 2