Page 146 - Buried Pipe Design

120 Chapter Three

A discussion of radial tension in curved members can be found in most
advanced solid mechanics texts.
Delamination may also be caused by chemical action. A prime exam-
ple is the corrosion of reinforcing steel. When corrosion takes place,
corrosion products produce interlaminar pressure which can result in
delamination. Reinforcement is usually protected and will not corrode
except in the case of product misapplication.

Safety factors
The need for selecting a design load that is less than the performance
limit load arises mainly from uncertainties. These uncertainties exist
in service conditions, loads, uniformity in materials, and assumptions
made in design. Thus, a reduction factor is needed and is usually
referred to as a safety factor or factor of safety.

Rigid pipe. The safety factor for rigid pipe is usually based on the per-
formance limit of injurious cracking.

W f load to cause failure (cracking)
W w safe working load
Wf
W w
SF
SF safety factor
The acceptable safety factor is

SF 1.5
Thus, if a load of 2000 lb/ft will cause cracking, the safe design load
should be 2000/1.5 1333 lb/ft.

Flexible pipe. Performance limits for flexible pipes are usually deflec-
tion related. Safety factors are then often based on deflection instead
of on load. For example, if a cement-lined pipe has injurious cracking
at 3 percent deflection, a design deflection of 2 percent would be based
on a safety factor of 1.5. The design engineer has the responsibility to
design the installation (pipe, bedding, backfill, and so forth) so that the
calculated design deflection does not exceed 2 percent.
Each product will exhibit different performance limits, and the fac-
tor of safety is usually 1.5 or greater. For flexible products which
exhibit only deflection as a performance limit, the design deflection is
7.5 percent, and the factor of safety is 4 or greater.
The inexperienced design engineer should consider each possible
performance limit, in succession, until the performance limit which