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).
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.
Technical Specifications Snapshot
Metric
Standard(s)
Unit
Example Value (approx)
Notes
PS
ASTM, SANS
psi or kPa
50 psi (~348 kPa)
Force/Displacement per length
STIS
ISO, EN
kPa
6.5 kPa (from 50 psi)
Also called Ring Stiffness
SN
EN
SN (value)
SN4 (~4 kPa)
Nominal Stiffness (STIS in kPa)
Solid Wall STIS (UPVC SDR41)
ISO
kPa
>4.0 kPa (SN4)
Theoretical ~3.9 kPa, measured higher
FAQ Section
What is the difference between PS, STIS, and SN values?
PS (Pipe Stiffness) and STIS (Specific Tangential Initial Stiffness or Ring Stiffness) are both measures of a pipe's resistance to deformation under external load, used primarily for non-pressure applications14.... They are related by a conversion factor43. PS is typically used in ASTM and SANS standards (units psi or kPa), while STIS (in kPa) and its numerical equivalent SN (Nominal Stiffness) are used in ISO and EN standards21....
Why do standards for non-pressure pipes specify minimum stiffness rather than just wall thickness?
For non-pressure pipes, particularly structured wall types, the shape of the pipe wall contributes significantly to stiffness beyond just the material amount11. Specifying minimum stiffness (e.g., SN values) allows manufacturers flexibility in design and material distribution (like in structured walls or using fillers) while ensuring the pipe meets the required performance for resisting external loads27....
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Nick Belshof
