Page 249 - Buried Pipe Design

Design of Pressure Pipes 223

The following is a summary of the design bases for ductile iron pipe:
Soil loading
Prism load truck load (H-20 impact)

Deflection

(A) Iowa formula (3 percent limit)
Stress due to soil loading

2
(B) Calculate stress at invert (limit 48,000 lb/in )
Stress due to internal pressure
2
(C) S (limit 21,000 lb/in ) PD/2t (P working pressure surge
allowance)

Design procedure
1. Calculate the required thicknesses in steps (A), (B), and (C) above.
2. Subtract service allowance (corrosion) of 0.08 in from thickness
found in (A), and compare with thickness found in (B) and (C)
(largest value controls).
3. Add service allowance (0.08 in) for minimum thickness.
4. Add casting allowance for total thickness.

The net effect of the above procedure is to include a service
allowance for stress considerations but not for deflection. The ratio-
nale for not including it for deflection is that the deflection limit is
based not on the ductile iron, but on the lining.

Steel pipe. There are many types of steel pipe and many applica-
tions. AWWA M11 (steel pipe design and installation) adequately
covers the design and installation procedures. A brief review is
included here.
Steel pipe is, for the most part, designed for internal pressure and
installed in such a manner that other design considerations, or limits,
are met. The basic design procedure is as follows:

1. Design for pressure.
2. Calculate stiffness.
3. Design the soil system based on pipe stiffness, depth of cover, and
performance limits.