Page 224 - Buried Pipe Design

198 Chapter Four

where ( L ) max maximum longitudinal stress
stress riser
Lb stress due to longitudinal beam action
L longitudinal stresses due to Poisson’s effect
LT longitudinal stresses due to temperature
th longitudinal stresses due to thrust

Design Bases
Each piping material has criteria for design such as a limiting stress
and/or a limiting strain. Also, each product may be limited as to spe-
cific application in terms of fluids it may carry or in terms of temper-
ature. Usually these limiting conditions are translated into codes,
standards, and specifications. Such specifications will deal with specific
acceptable applications, permissible soil load or depth of cover, inter-
nal pressure, safety factors, methods of installation, life, and, in some
cases, ring deflection. The limiting parameters for a given product when
considered together form the basis for design.

Rigid pipes
The use of pressure pipe constructed wholly from rigid material is
rapidly becoming history. Cast iron pipe has been replaced with duc-
tile iron, which is considered to be flexible. Asbestos-cement pressure
pipe is still in production in some countries, but is rapidly losing out
in the marketplace. Concrete pressure pipe, which is really steel pipe
with a concrete liner and a concrete or cement grout coating, is usually
considered to be rigid.

Asbestos cement. Design information for asbestos-cement pressure
pipe can be found in AWWA C401 and in AWWA C403. A combined load
analysis using the Schlick formula is required. This method is discussed
under the combined loading section of this chapter. Equation (4.12) is
repeated here.

P p
w W (4.12)
P
or

w
p P 1 2 (4.13)
W
It is generally considered desirable to use the thick-walled formula
for ratios of diameter to thickness exceeding 10. Equations (4.1) and (4.2)