Since the 1920s, American National Standards Institute — now the American Water Works Association — Standards Committee A21 has been responsible for this series of standards on gray and Ductile iron pipe. Since Ductile iron pipe was first introduced in 1955, the Standards Committee on Ductile Iron Pipe and Fittings has been provided with extensive data on trench loading tests, strength tests, corrosion resistance, tapping strength, flow characteristics, impact resistance, lining and joint integrity, and virtually all aspects of the material that can affect its performance.
The design procedure for Ductile iron pipe includes:
- Design for internal pressures (static pressure plus surge pressure allowance).
- Design for bending stress due to external loads (earth load plus truck loads).
- Select the larger resulting net wall thickness.
- Add an 0.08-inch service allowance.
- Check deflection.
- Add the standard casting tolerance. This procedure results in the total calculated design thickness, from which the appropriate pressure class is chosen.
Design of Ductile Iron Pipe
With more than six decades of outstanding field experience, Ductile iron pipe is widely recognized as the industry standard for modern water and wastewater systems.
The basis of the design standard for Ductile iron pipe is that when subjected to internal pressure and underground loading conditions, it behaves as a flexible conduit and rerounds under pressure. Therefore, the pipe is designed separately to withstand external loads and internal pressure.