Page 219 - Buried Pipe Design

Design of Pressure Pipes 193

TABLE 4.3 Pipe Stiffness
F/ y, lb/in 2 D f
5 15
10 8
20 6
100 4
200 3.5

Total circumferential stress can be obtained by the use of the fol-
lowing:

T p c b
where T total stress
p stress due to internal pressure (static and surge)
c ring compression stress
b stress due to ring deflection (bending)

This total stress may or may not be necessary to consider in the
design (see the next subsection on combined loading).

Combined loading
A method of analysis which considers effects due to external loads and
internal pressure acting simultaneously is called a combined loading
analysis.
Rigid pipes. For rigid pressure pipe such as cast iron or asbestos
cement, the combined loading analysis is accomplished in terms of
strength. The following procedure was originally investigated and sug-
gested by Professor W. J. Schlick of Iowa State University. It has since
been verified by others.
Schlick showed that if bursting strength and the three-edge bearing
strength of a pipe are known, the relationship between the internal
pressures and external loads, which will cause failure, may be computed
by means of the following equation:

P p
w W (4.12)
P
where w three-edge bearing load at failure under combined internal
and external loading, lb/ft
W three-edge bearing strength of pipe with no internal pres-
sure, lb/ft