Stiffness and Flexural Resistance of PVC Pipes

For non-pressure pipes, particularly those subjected to soil loads in drain, waste, venting (DWV), and sewer applications, stiffness is a main characteristic12.... This refers to the pipe's resistance to deformation under external loads14.

Key Stiffness Metrics

Several metrics are used internationally to define and measure pipe stiffness:

• Pipe Stiffness (PS): Defined as Force per Displacement per unit length of pipe14. In ASTM standards, PS is typically expressed in psi21. The ASTM formula relates PS to the flexural modulus of elasticity (E), wall thickness (t), and mean radius (r)42.
• Specific Tangential Initial Stiffness (STIS): Also known as Ring Stiffness, this metric is used in ISO and EN (European Norm) standards and is expressed in kPa (kiloPascals).
• Nominal Stiffness (SN): Expressed in SN values (e.g., SN4, SN8), this refers to the STIS value in kPa44.

Relationship and Conversion

Pipe Stiffness (PS) and Specific Tangential Initial Stiffness (STIS) are related by the formula: PS = 0.01863 * STIS.

For solid wall pipes, STIS can be calculated from the material's E-modulus and the Standard Diameter Ratio (SDR): STIS = E / (12 * (SDR-1)^3).

Conversion examples:

• PS = 50 psi = 348 kPa21.
• Converted to STIS: STIS = 0.01863 * 348 kPa = 6.5 kPa.
• In South African standards (SANS), PS is given in kPa; a 400 kPa pipe has a STIS = 0.01863 * 400 kPa = 7.45 kPa, which is equivalent to SN 7.45.

Measurement and Standards

Stiffness is commonly measured under parallel plate load. The ISO/FDIS 9969:2007(E) standard measures STIS based on the force required to achieve a 3% pipe deflection.

Standards for non-pressure pipes often specify minimum stiffness requirements. For example, EN13476 prescribes a minimum stiffness of SN4 for Structured Wall pipes with OD 500 mm and smaller. EN1453 for Building, however, prescribes wall thicknesses, with SDR51 typically corresponding to SN2 for smaller pipes.

Calculated stiffness values for solid wall pipes are often exceeded in practice due to factors like overweight (exceeding minimum dimensions) and plate stiffness. For structured wall pipes like DWC, measured stiffness can be lower than theoretical due to the deformation susceptibility of the profile.

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