For stress analysis of HDPE pipe, you need the following information from vendor pertaining HDPE material properties :
Sh, r=Sa(0:1)/Sa(2:1), Eff (f2), Fac (k), Emodulus axial, E modulus hoop, Shear modulus, Poisson's ratio Axial/ hoop, Density & Expansion coefficient.
In CAESAR II setup - FRP Properties field enter the relevant data. In spread sheet select material no.20 and design code as UKOOA. Enter material properties in the respective field. Then check stress in operating case only. Suitable support span to be maintained to keep sagging less than 1mm. The analysis can be carried out based on BS7159 or ANSI B31.3 with minor changes in method.
Regards
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Manoj Sarkar