Page 263 - Buried Pipe Design

Design of Pressure Pipes 235

R R R P n

R 1 P n

P n
Figure 4.25 Rerounding coefficient from test data on both ductile iron and FRP
pipes.

3. Rerounding is generally insensitive to soil type, compaction, or pipe
stiffness, as shown by all data sets.
4. Rerounding increases steadily, as an approximately linear function
of internal pressure, as shown by all data sets. The rerounding fac-
tor R may be conservatively approximated as

P n
R 1 (4.21)
435
2
where P internal pressure, lb/in (0 < P < 435).
n
n
5. The rerounding coefficients reduce somewhat with time, as seen
from the Carlstrom data. 7
Discussion. A combined loading analysis is outlined in App. A of
AWWA C950-81. Two methods are given for calculating the combined
stresses and strains. One method has been attributed to Spangler and
the other to Molin. Both equations are to be used, and the lowest
value is the resulting combined stress and/or strain. This lowest value
must not exceed the long-term bending strength of the product
reduced by a design factor. The method permits the rerounding of a
deflected pipe due to internal pressure to be considered when com-
puting the total combined strain in the pipe. The combined strain is