Page 144 - Buried Pipe Design

118 Chapter Three

Poisson’s circumferential strain

(longitudinal strain) (3.23)

where b bending strain
c ring compression strain
p internal pressure strain
circumferential Poisson’s strain
t wall thickness
D diameter
y vertical deflection
P v vertical soil pressure
E Young’s modulus
p internal pressure

Poisson’s ratio

Longitudinal stresses. Installation design and construction should be
such that longitudinal stresses are minimized. Rigid pipe products and
many flexible pipe products are not designed to resist high longitudi-
nal stresses. Longitudinal stresses are produced by

1. Thermal expansion (contraction) (major design consideration in
welded steel lines)
2. Longitudinal bending
3. Poisson’s effect (due to internal pressure)

Thermal stresses in welded steel lines are often produced by weld-
ing the pipe during the high-temperature period in the day. Cooling
later can cause extremely high tensile stresses. These stresses can be
minimized by providing closure welds during cool temperatures or by
the use of expansion joints.
Some of the major causes of longitudinal bending or beam action in
a pipeline area are
1. Differential settlement of a manhole or structure to which the pipe
is rigidly connected
2. Uneven settlement of pipe bedding or undermining, e.g., erosion of
the soil below it into a water course or leaky sewer
3. Ground movement associated with tidal water
4. Seasonal rise and fall of soil effected by changes in moisture content
(e.g., most expansive clays)
5. Nonuniformity of the foundation
6. Tree-root growth pressure