Page 250 - Buried Pipe Design

224 Chapter Four

Pressure design. Pressure design is based on the thin-walled pressure
formula as follows:
PD PD
S or t
2t 2S

where S safe working stress (usually 50 percent of yield)
P working pressure plus calculated surge
t steel thickness

Determine stiffness. If the pipe is not cement-coated or lined, the stiff-
ness is easily calculated.
3

t
Stiffness EI E
12
where t thickness determined from pressure design and E is usually
2
30 10 lb/in .
6
For cement-lined and/or coated-steel pipe, the stiffness will be avail-
able from the manufacturer or can be determined experimentally. For
pipes that are lined after installation, only the steel should be consid-
ered in any stiffness calculation.
Soil system design. The known parameters at this point in the design
will be
1. Pipe performance limits, usually 2 percent deflection
2. Depth of cover
3. Pipe stiffness

The parameters to be determined are pipe zone soil type and soil
density in pipe zone—embedment techniques, and so forth.

Recommended procedures. Loads may be calculated by Marston’s Iowa
formula for flexible pipe or by the prism load method. See Chaps. 2 and
3 for details.
Deflection is determined by using Spangler’s Iowa formula,
Watkins’s soil strain method, or empirical data. Manufacturers’ rec-
ommendations should be given serious consideration in this regard
as many have developed tables for deflection from actual test data.
Also, other standards such as AWWA C200 and AWWA C206 provide
useful and pertinent information regarding installation design.
Buckling. Many steel pipelines are extremely flexible and may be
subject to buckling or collapse from external pressure or internal vac-
uum. The engineer should consider buckling in the design and take