I am trying to find some reasoning/history on why the equations to calculate for Principle Stresses have changed from the Section VIII Division 2 1995 edition to the new Section VIII Division 2 2010, 2011 addenda edition.

Primarily the use of the Natural Log to calculate the Hoop(Circumferential) Membrane Stress in a shell.

I am comparing my principle stress calculations using the 1995 edition with the 2010 edition and the new code returns a higher principle stress.

Any light shined on this would be appreciated.

Perhaps because there's 15 years between the two editions. There's been a lot of changes in methodologies and materials in the intervening years. Actually, there were a lot of changes made between the 2007 and 2010/2011 versions. You'd have to find a Code committee member, several of whom frequent these forums, but the main reasons seem to have to do with allowing thinner materials to be used.

By the way, I noticed that you posted both your questions in two different forums. You might want to look at the forum policies, in the FAQ following my signature.

Patricia Lougheed

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Because the natural log equation is more accurate, especially over a significantly larger range of D/t ratios.

As vpl pointed out, the 2007 Edition of Division 2 was a major rewrite. Everything was changed. I would recommend that you refer to ASME PTB-1 for a thorough discussion.