Page 272 - Buried Pipe Design

Design of Pressure Pipes 243

restrained length on each side of the joint is L. The frictional resis-
tance and bearing resistance are given by F and R respectively.
s
b
Summation of forces results in the following:
1
PA sin F s L cos R b L cos
2 2 2 2
or
PA tan ( /2)
L
F s R b /2
where P internal pressure
A cross-sectional area of pipe
F s frictional force
R b bearing force
For a cohesionless soil, the friction force F s may be calculated as
follows:
F s W tan

where W 2W e W p
f
W e total soil load
W p weight of pipe plus water
f
friction factor between pipe and soil

internal friction angle of soil

The above method will generally produce conservative results. If
cohesion is present, cohesive forces will also be involved which will
make results even more conservative. However, since cohesive forces
are time-dependent, it is recommended that they be neglected.

Safety factors
Design of pressure pipe is based upon certain performance limits such
as long-term hydrostatic burst pressure and/or crush load acting
either independently or simultaneously. The allowable total stress or
strain is equal to the failure stress or strain reduced by a safety factor.
For example,
f
f
A or A
SF SF
where A allowable stress
f failure stress
A allowable strain
f failure strain
SF safety factor