Page 147 - Buried Pipe Design

Design of Gravity Flow Pipes 121

occurs at the lowest load or deflection is arrived at. The factor of safety
is then based on that performance limit. Literature published by the
pipe manufacturer is very helpful in assessing the capabilities and
limitations of pipe products.
When a pipe deflects under load, bending strains are induced in the
pipe wall. These strains vary linearly through the pipe wall. Somewhere
within the wall section (usually about the center) these bending strains
will be zero.
Profile-wall pipes are designed and manufactured to minimize the
use of material by increasing the section modulus of the pipe wall.
Profile-wall is a relatively new designation, but the concept is not new.
Corrugated steel pipe is truly a profile-wall pipe. Some of the newer
plastic products introduced in the last several years are of this type.
That is, the plastic is placed primarily at the inside and outside walls
or in ribs for higher pipe stiffness. Many of these products have been
shown to perform with the profile section acting as a unit as designed.
For adequate safety, for any such product, the design should include
sufficient plastic between the inner and outer walls and/or between
the ribs to carry shear and to ensure that the profile section indeed
acts as a unit.
The placement of a rigid-like filler material between walls as a sub-
stitute will impart a brittle-like behavior to the pipe and will interfere
with the pipe’s ability to deflect without cracking. Such pipes often
deflect as a flexible pipe and have a brittle behavior and crack under
deflection. Some pipes manufactured in this manner are sometimes
referred to as semirigid. This is simply a misnomer. Many solid wall
PVC and ductile iron pipes are actually more rigid and still behave as
flexible pipes.

Parallel Pipes and Trenches

When buried pipes are installed in parallel, principles of analysis for
single pipes still apply. Soil cover must be greater than minimum.
However, the design of parallel buried pipes requires an additional
analysis for heavy surface loads. Consider a free-body-diagram of the
pipe-clad soil column between two parallel pipes. See Fig. 3.26. Section
AA is the minimum cross-section. This column must support the full
weight of the soil mass, shown cross-hatched, plus part of the surface
load, W shown as a live load pressure diagram. The soil column is crit-
ical at its minimum section AA at the spring lines.

Definitions of terms
D outside diameter of pipe 2r
A pipe wall area per unit length of pipe