Non-pressure pipes, used in applications like drain, waste, venting (DWV), and sewers, are primarily designed to handle bending loads, such as soil loads. Unlike pressure pipes, which have a simple, uniform shape, non-pressure pipes benefit significantly from optimized wall shapes to increase stiffness. Moving material away from the center of the pipe wall is a key principle for weight saving while maintaining stiffness.
Key Design Considerations
The resistance against bending is determined by the moment of inertia (I) of the pipe wall's cross-section. I is proportional to the wall thickness cubed. This principle leads to Structured Wall pipes with various profiles, including Double Wall Corrugated pipe, Ultrarib pipe, and Foam core pipe. Foam core pipe resembles a solid wall pipe but replaces the inner material with foam for weight saving.
Pipe stiffness is a critical parameter for non-pressure pipes. It is defined as Pipe Stiffness (PS) or Ring Stiffness (STIS). PS is measured under parallel plate load. Formulas relate PS and STIS to the material's E-modulus, the moment of Inertia (I), and the pipe's diameter. For solid wall pipes, STIS is proportional to E divided by (SDR-1)^3.
Standards for structured wall pipes (e.g., EN13476, ISO 21138, AS/NZS 1260) are functional, prescribing minimum pipe stiffness rather than fixed wall thicknesses. This allows for flexibility in design and material use, such as higher CaCO3 levels to increase E-modulus and potentially reduce wall thickness. Minimum stiffness requirements exist, like SN4 for certain pipes in EN13476. Standards for foam core pipes include ASTM F891, JIS 9798, and SANS 1601.
Risks associated with a low-density core include inadequate shear resistance leading to lower actual stiffness, challenges meeting rubber ring chamber tolerances, and potential axial leakage through the foam. Some standards require closed cells to prevent leakage.
Why are non-pressure pipes designed with different shapes compared to pressure pipes?
Non-pressure pipes primarily handle bending loads like soil weight, where optimizing the wall shape (structured walls) is much more effective for increasing stiffness and saving weight than a simple thick wall used for tensile pressure loads.
What does the moment of inertia (I) signify in non-pressure pipe design?
The moment of inertia (I) of the pipe wall cross-section is proportional to the pipe's resistance against bending. A higher 'I' means a stiffer pipe for a given material.
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Michan Katerbarg
