is it recommended to use liberal stress allowance for underground piping systems? knowing that the mass of the system gets completely ignored in a buried model, the allowable stress due to displacements could spike up to 1.25(Sc+Sh).

Hi, there,

I believe you refer to ASME B31 design codes (B31.4 or B31.8). Having this assumption, it depends now how you qualify the Code stresses: are you going to treat buried pipe as fully restrained pipe, or do you intend to treat it as unrestrained pipe (e.g. since the pipe-soil interaction is accurately modeled, the axial force & pipe "limited freedom" effects are quantified properly)?

As per both B31.4 and B31.8 codes, for fully restrained pipes, the displacement stress range (SE) versus Allowable Stress Range (SA) approach does not exist and does not apply.

If Code stresses are quantified and qualified using "unrestrained" pipe approach, than the latest (2012) editions of ASME B31.8 & B31.4 Codes employ "Liberal Stress" approach only. However, the 1.25*(Sc+Sh) limit is not practically used, because of longitudinal pressure stresses (e.g. accurately quantified based upon soil-pipe interaction model).

My current approach is to use the "unrestrained" pipe method, and to employ Bourdon effect ("Translation Only" option) too, which may have some impact for high pressure piping.

Cheers and good luck!

Dear Dorin,

I think I need to clarify my situation; I am using B31.3

Doesn't matter, the philosophy is same as for "unrestrained" buried pipe as per B31.8 or B31.4 Codes. Liberal stress is a valid option, but 1.25*(Sh+Sc) limit cannot be practically used, because of longitudinal pressure stress.

Actually, ASME B31.3 with 2012 Ed. updates represents a very good option for buried pipe analysis, assuming the pipe-soil interaction is modeled properly.

Regards,