Structured Wall Pipes of PVC

Structured Wall pipes represent a design approach primarily used for non-pressure applications where bending load is dominant, such as drain, waste, venting (DWV), and sewers under soil loads. Unlike pressure pipes that rely on uniform wall thickness for tensile strength, structured wall pipes utilize shape to achieve stiffness.

Design Principle

The fundamental principle behind Structured Wall pipes is to move material away from the center of the pipe cross-section. This increases the moment of inertia (I) of the pipe wall. Since pipe stiffness is proportional to I, this design increases stiffness without significantly increasing material weight. The resistance against bending is proportional to the wall thickness to the power of three. Leaving out the middle of the pipe wall offers a significant weight saving while retaining a large portion of the stiffness; for example, a 50% weight saving might cost only 12.5% of stiffness. The outer 50% of the wall is more effective for stiffness than the inner 50%.

Examples of Structured Wall pipes include Cordrain pipe, Double Wall Corrugated pipe (DWC), Ultrarib pipe, Wavihol pipe, and Foam core pipe. They are considered light weight pipes.

Standards and Requirements

Standards for Structured Wall Pipes include EN13476 (for U-drain and Sewerage), ISO 21138, and AS/NZS 126027. These standards are often functional, prescribing a minimum pipe stiffness rather than a fixed wall thickness, although a minimum wall thickness base limit may still exist. For example, EN13476 prescribes a minimum stiffness of SN4 for pipes OD 500 mm and smaller. Some standards like AS/NZS 1260 specify minimum PVC content in the core (e.g., 80%) compared to others (e.g., 60% in EN13476).

Challenges and Risks

• Using low density cores or thin-walled profiles in structured walls can present risks:
• The skins might act independently if the core lacks sufficient resistance against shear forces, leading to lower actual stiffness than calculated.
• Maintaining tolerances for rubber ring chambers can be difficult.
• Axial leakage through foam cores is a risk; the ASTM standard includes a requirement for closed cells to prevent this.
• Double Wall Corrugated pipes with deep profiles may show lower than theoretical stiffness in practice due to susceptibility to deformation under load. Creep in the thin side walls of the deep profile can lower contact pressure in rubber ring joints, affecting long-term tightness.

Despite challenges, structured wall designs like DWC are used for applications like storm water.

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