Analysis of rectangular vessel with ASME stress limits

Dear sir,
We are in the process of design of rectangular vacuum vessel subjected to external pressure. The vessel is provided with stiffeners. The analysis was carried out with ANSYS(FEM) Package. We want to classify the stresses as primary membrane, local primary membrane and secondary stressess etc according to ASME section-VIII, Devision-2 rules. We are facing the problem in classifying the local primary membrane stress for rectangular vessel since the region for the definition of local membrane stress is given interms of radius of the vessel in ASME code. How to apply this definition for rectangular vessels which does not have radius to define this local region? Please suggest the suitable method for classification

 

[prex]

The use of the stress classifications per Div.2 in cases not covered by the code requires a knowledge in depth of priciples for such classification, that primarily come from limit analysis.
In your case, apart from local discontinuities such as close to openings, you only have primary membrane (of probably minor importance) and primary bending.
Concerning the corners, you need to decide wether you want to count on the mutual clamping effect of the walls joining in the corner, or to consider the corner as a simple support for both walls. In the first case the bending stresses near the corner will be primary, in the latter secondary.
Also keep in mind that FEM gives you a total stress (including peak stresses, if you use elements with more than one node through the thickness): you must be able to separate the different contributions before going into Div.2 criteria.
prex
motori@xcalcsREMOVE.com

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