Page 273 - Buried Pipe Design

244 Chapter Four

The total working stress/strain must be equal to or less than the
allowable stress/strain. If a combined loading analysis is not required,
stresses due to internal pressure and external loads are evaluated sep-
arately, and the safety factor is applied to the largest value. For com-
bined loading, the safety factor is applied to the combined stress.
For nonlinear failure theories such as the Schlick formula, safety
factors must be applied to both internal pressure and external load.
These two factors of safety need not be equal.
For plastic pipe, the design is based on life rather than a failure
stress. As previously discussed in this chapter, a hydrostatic design
basis (stress) is established on the basis of a life of 100,000 h. The
design stress is the hydrostatic design basis reduced by a factor of
safety. A factor of safety of 2.0 will give, essentially, infinite life since
the stress regression curve is linear on a log-log plot (see Fig. 4.6).
Standards for each pipe product may list recommended safety fac-
tors. Also, manufacturers often recommend certain safety factors for
their products. The bases for the calculations of these are often quite
different. The design engineer should be aware of these differences
when comparing products and should always have the option of requir-
ing a safety factor which is different from the recommended value. The
need for safety factors arises mainly from uncertainties. These uncer-
tainties are due to causes ranging from the pipe manufacturer to pipe
installation conditions. The greater the uncertainty, the higher the
safety factor should be. The engineer should be very cautious in utiliz-
ing safety factors that are lower than those recommended by national
standards or by the manufacturer.

References
1. ASTM. 1976. Standard Method of Test for Time-to-Failure of Plastic Pipe under
Long-Term Hydrostatic Pressure, ASTM D 1598, Philadelphia.
2. American Water Works Association. AWWA Standards M11, M9, M23, C150, C200,
C206, C300, C301, C303, C400, C401, C402, C403, C900, C901, and C950. Denver,
Colo.
3. Andrews, James S. 1970. Water Hammer Generated during Pipeline Filling.
Master’s thesis. Fort Collins: Colorado State University.
4. Bair, D. A. 1984. Analysis of Strain vs. Internal Pressure of Buried FRP Pipe from
Tests and Finite Element Modeling. Master of science thesis. Logan: Utah State
University.
5. Bishop, R. R. 1983. Course Notebook. Logan: Utah State University.
6. Bowman, J. A. 1990. The Fatigue Response of Polyvinyl Chloride and Polyethylene
Pipe Systems. Buried Plastic Pipe Technology, ASTM STP 1093. Eds. George S.
Buczala and Michael J. Cassady. Philadelphia: American Society for Testing and
Materials.
7. Carlstrom, B. I. 1981. Structural Design of Underground GRP Pipe. Paper presented
at the International Conference of Underground Plastic Pipe, New Orleans. March.
8. Cole, B. W. and L. 0. 1981. Timblin, Jr. Strain Calculations for FRP Pressure Pipe.
Paper presented at the International Conference on Underground Plastic Pipe in
March.