Page 226 - Buried Pipe Design

200 Chapter Four

Reinforced-steel cylinder pipe is designed on a maximum combined
stress basis. The procedure is to calculate stresses in the steel cylinder
and steel reinforcement produced by both the external loads and inter-
nal pressure. The combined stress at the crown and invert must be
equal to or less than an allowable tensile stress for the reinforcing
steel and steel cylinder. See AWWA C304-92 and AWWA M9 for details
of current recommended design procedures.
Many engineers are more familiar with the simplified design proce-
dures as found in pre-1997 versions of AWWA C300 and pre-1992 ver-
sions of AWWA C301. In these standards, prestressed concrete pipe
was designed for combinations of internal and external loads by the
following cubic equation:

P o p
3
w W o (4.14)
P o
where P o internal pressure which overcomes all compression in the
concrete core, when no external load is acting, lb/in 2
W o 90 percent of three-edge bearing load which causes
incipient cracking in the core when no internal pressure
is acting, lb/ft
p maximum design pressure in combination with external
loads (not to exceed 0.8P for lined cylinder pipe)
o
w maximum external load in combination with design
pressure
The value of W can be determined by test, and the value of P can
o
o
be either determined by test or calculated. With these parameters
known, w and p can be calculated using Eq. (4.14) in a manner that
is similar to the use of the Schlick formula for asbestos-cement pipe.
Further information concerning the combined loading analysis
using the cubic parabola Eq. (4.14) is available in these previous
standards.
Pretensioned concrete cylinder pipe is considered by many to be a
rigid pipe. Truly, it does not meet the definition of a flexible pipe (must
be able to deflect 2 percent without structural distress). The limiting
design deflection for pretensioned concrete cylinder pipe ranges from
0.25 to 1.0 percent. AWWA Manual M9 indicates that this type of pipe
is semirigid. However, the recommended design procedure found in
AWWA C303 and AWWA C304 is based on flexible pipe criteria. The
recommended procedure is to limit stresses in steel reinforcement and
2
the steel cylinder to 18,000 lb/in or 50 percent of the minimum yield,
whichever is less. The stiffness of the pipe must be sufficient to limit
2
the ring deflection to not more than D /4000 where D is the nominal
inside diameter of the pipe in inches.