Page 303 - Buried Pipe Design

274 Chapter Five

Thus, the required three-edge bearing service load is defined in
terms of the D load to produce a 0.01-in crack. The D load required for
a particular pipe soil installation is determined as

W e W L W w
(W D load ) 0.01 (5.14)
B f D i
The 0.01-in crack criterion is not applicable to nonreinforced con-
crete pipe. These pipes are at the W 3-edge strength limit when the first
crack occurs. Although nonreinforced pipe may still be able to perform
after cracking starts in field installations, a safety factor from 1.25 to
1.5 is normally provided against flexural cracking at the service load.
Thus, the required ultimate D load strength for nonreinforced concrete
pipe is

1.5 (W e W L W w )
(W D load ) ult (5.15)
B f D i
Note: W L (live load) is frequently neglected when the height of earth fill
above the pipe is more than 8 ft or the outside pipe diameter,
whichever is greater. Note: W w (hydrostatic load) is frequently
neglected, especially for small-diameter pipe.
The required ultimate W 3-edge strength for reinforced concrete pipe is
given by Eq. (5.15), except for the use of a reduced strength factor for
pipe strength classes higher than class 4. For typical pipe strength
classes up to class 4, the ultimate strength design limit is 1.5 times the
required service load W 3-edge strength. This strength factor is 1.25 for
class 5 pipe strength and is linearly interpolated between 1.5 and 1.25
for any strength classes between 4 and 5. An ultimate strength design
limit is also defined for reinforced concrete pipe designed by the indi-
rect design procedure. Such a design limit is included in ASTM speci-
fications for reinforced concrete pipe.
Design requirements for nonreinforced pipe with specified W 3-edge
strength requirements are given in ASTM C 985. Design W 3-edge
strengths for nonreinforced concrete pipe having various standard
diameters and wall thicknesses are given in ASTM C 14. The D load
definition of three-edge bearing strength is not used in this standard.
Design requirements for reinforced pipe with specified D load
strength requirements and special reinforcement designs are given in
ASTM C 655. Design W 3-edge strengths for reinforced concrete pipe hav-
ing various standard diameters, wall thicknesses, concrete strengths,
and reinforcement requirements for standard strength classes are
given in ASTM C 76, C 506, and C 507.
Specifying agencies sometimes require W 3-edge tests for proof of
design. More frequently, they may require W 3-edge tests for proof of quality