Page 220 - Buried Pipe Design

194 Chapter Four

P burst strength of pipe with no external load, lb/in 2
p internal pressure at failure under combined internal and
external loading, lb/in 2

Schlick’s research was carried out on cast iron pipe and was later
shown to apply to asbestos-cement pipe. Neither of these piping mate-
rials are currently manufactured in the United States. However, these
materials are available in other countries.
An example of the Schlick method of combined loading design for a
rigid pipe is as follows: Suppose a 24-in asbestos-cement water pipe has
a three-edge bearing strength of 9000 lb/ft and a bursting strength of
2
500 lb/in . Figure 4.5 shows graphs of Eq. (4.12) for various strengths
of asbestos-cement pipes. The curve for this particular pipe is labeled
2
50. If this pipe were subjected in service to a 200 lb/in pressure (includ-
ing an allowance for surge) times a safety factor, this graph shows the
pipe, in service, would have a three-edge bearing strength of 7000 lb/ft;
2
for an internal service pressure of 400 lb/in , the three-edge bearing
strength would be 4000 lb/ft; and so on. The three-edge bearing
strength must be multiplied by an appropriate load factor to obtain the
resulting supporting strength of the pipe when actually installed.

Flexible pipes. For most flexible pipes such as steel, ductile iron,
and thermal plastic, a combined loading analysis is not necessary.
For these materials, the pipe is designed as if external loading and
internal pressure were acting independently. Usually, pressure
design is the controlling factor. That is, a pipe thickness or strength
is chosen on the basis of internal pressure, and then an engineering
analysis is made to ensure the chosen pipe will withstand the exter-
nal loads.
An exception to the above statement is fiberglass-reinforced thermal-
setting resin plastic (FRP) pipe. This particular type of pipe is designed
on the basis of strain. The total combined strain must be controlled to
prevent environmental stress cracking. A recommended design proce-
dure is given in Appendix A of AWWA C950. The total combined strain
in this case is the bending strain plus the strain due to internal pres-
sure. Some FRP pipe manufacturers recommend all components of
strain be added together to get the total maximum strain. The follow-
ing is a list of some loadings or deformations that produce strain.

1. Internal pressure
2. Ring deflection
3. Longitudinal bending
4. Thermal expansion/contraction