Page 142 - Buried Pipe Design

116 Chapter Three

do. Thus, a deflection consideration must be made in design. For such
products, bending stress and bending strain may also become limiting
performance criteria. Such products are often cited as having only the
positive attributes of both rigid and flexible pipe. However, tests have
shown that these same products can and do exhibit the combination of
performance limits of both rigid and flexible pipes which makes design
analysis more complicated.
The calculated design deflection should always be equal to or less
than the design deflection limit for the particular product. The design
deflection is calculated by one of the methods described under the flex-
ible pipe analysis section of this chapter. Traditionally, Spangler’s
Iowa formula has been used. Finite element methods are starting to be
used and will be the method of the near future.

Reversal of curvature. Reversal of curvature is a deflection phenomenon
and will not occur if deflection is controlled. A reverse curvature perfor-
mance limit for flexible steel pipe was established shortly after publica-
tion of the Iowa formula. It was determined that corrugated steel pipe
would begin to reverse curvature at a deflection of about 20 percent.
Design at that time called for a limit of 5 percent deflection, thus pro-
viding a structural safety factor of 4.0. From this early design consider-
ation, an arbitrary design value of 5 percent deflection was selected.
Buried PVC sewer pipe (D 3034 DR 35), when deflecting in response
to external loading, may develop recognizable reversal of curvature at
a deflection of 30 percent. This level of deflection has been commonly
designated as a conservative performance limit for PVC sewer pipe.
Research at Utah State University has demonstrated that the load
carrying capacity of PVC sewer pipe continues to increase even when
deflections increase substantially beyond the point of reversal of cur-
vature. With consideration of this performance characteristic of PVC
sewer pipe, engineers generally consider the 7.5 percent deflection
limit recommended in ASTM D 3034 to provide a very conservative
factor of safety against structural failure.

Strain limit. The strain must be limited in certain pipe materials, such
as some fiberglass-reinforced pipes. This limit is necessary to prevent
strain corrosion. Strain corrosion is an environmental degradation of
the pipe material which takes place in a finite time only after the pipe
wall strain is greater than some threshold strain. Proper design calls
for the design strain to be lower than this strain limit with some safety
factor.
Strain is related to deflection. Therefore, most manufacturers of
such products will propose installation techniques for their particular
product which will limit deflection and thus limit the strain. Usually