Page 248 - Buried Pipe Design

222 Chapter Four

Ductile iron. The design approach for ductile iron pressure pipe is given
in AWWA C150. Various thickness classes are available. The required
thickness is determined by considering stress due to internal pressure,
ring deflection, and earth loads separately and independently.
Calculations are made for the thicknesses required to resist the
bending stress and the deflection due to trench load. The larger of the
two is selected as the thickness required to resist trench load.
Calculations are then made for the thickness required to resist the hoop
stress of internal pressure. The larger of these is selected as the net
design thickness. To this net thickness is added a service allowance
and a casting tolerance to obtain the total calculated thickness. The
standard thickness and the thickness class for specifying and ordering
are selected from a table of standard class thicknesses. The reverse of
the above procedure is used to determine the rated working pressure
and maximum depth of cover for pipe of given thickness class.
Trench load P v . Trench load is expressed as vertical pressure, in pounds
per square inch, and is equal to the sum of earth load P e and truck load P t .
Earth load P e . Earth load is computed as the weight of the unit prism
of soil with a height equal to the distance from the top of the pipe to
the ground surface (prism load). The unit weight of backfill soil is taken
to be 120 lb/ft . If the designer anticipates additional loads due to frost, the
3
design load should be increased accordingly.
Truck load P t. The truck loads are computed using the surface load
factors for a single AASHTO H-20 truck on unpaved road or flexible
pavement, with 16,000-lb wheel load and 1.5 impact factor.
The stress of the pipe invert produced by the total external loading
is limited to 48,000 lb/in . This stress is the sum of the bending stress
2
and the wall thrust stress. Wall stresses due to external loads are a
function of this type of installation. Various installation types are con-
sidered in AWWA C150 with associated tables for stresses as a function
of depth of cover.
The ring deflection is limited to 3 percent. This is a design condition
independent of wall stress. Most ductile iron water pipes have a cement
mortar lining. The 3 percent limitation is to protect that lining from
cracking or spalling.
The wall stress due to internal pressure must be equal to or less
2
than 21,000 lb/in . The yield stress in tension for ductile iron is approx-
imately 42,000 lb/in . Thus, a design stress of 21,000 is based on a
2
safety factor of 2.0.
The above procedure is not based on a combined loading analysis,
and a combined loading criterion is not recommended. Each perfor-
mance criterion is evaluated separately, and the controlling parameter
dictates the design thickness.