Primary or Secondary Stress?

[DriveMeNuts]

If an attachment to a vessel has a sustained "load controlled" gravity type load that causes Pl+Pb in the shell to be just below Yield then plastic collapse will not occur and therefore this is acceptable. (Say this sustained load causes a deflection of the attachment from zero to 5mm.)

If the same attachment while having the sustained gravity load on it also has a displacement caused by expanding pipe work which has a displacement range between zero and 10mm. As the PL+Pb+Q across the displacement range is slightly less than two times yield, plastic deformation in the first cycle will occur as will shakedown, therefore this is also acceptable.

For the 10mm piping displacement to be considered as a "strain controlled" load, does the piping it need to be fastened to the bracket?
If the piping is not fastened to the bracket, then the piping is more like a "pusher" moving between zero and 10mm, coming into and out of contact with the bracket during each cycle.

If the stresses around the bracket are already close to yield with a gravity load and then a non-fastened "pusher" displaces the bracket further, are the stresses associated with the pusher displacement still Secondary, or do they become Primary because the pusher does not provide the bracket with a returning displacement?

Is it possible for the shell local to the bracket to plastically collapse if the piping is not fastened to the bracket as described above?

 

[KevinNZ]

Have a look at

https://www.eng-tips.com/viewthread.cfm?qid=438540

 

[TGS4]

You seem to have some fundamental misunderstanding about the nature of the failure modes of Plastic Collapse and Ratcheting, and how they are different and separate.

If you really can't figure out how to separate the primary and secondary stresses, then you ought not be using elastic analysis. Use elastic-plastic analysis instead. Include the attached piping.

 

[DriveMeNuts]

Thanks KevinNZ, the membrane stresses from the displacement are primary and therefore contribute to Plastic Collapse.

TGS4, EN13445 has the same material design margin as ASME VIII Div 2 of 2.4 and yet it allows membrane stress due to strain controlled loads to be secondary. Why do you believe this is fundamentally incorrect and potentially massively unconservative?

My interpretation of ASME VIII Div 2 is for protection against collapse:
Pl + Pb shall be less than Yield when the sustained gravity load only is applied.
Pl + Pb plus the additional Pl stress caused by the displacement load shall be less than Yield when gravity and sustained load are applied. The additional bending stress caused by the displacement load is secondary and therefore not included.
For the European code, the second assessment would not need to be conducted as both membrane and bending due to the expansion load are considered secondary.

For protection against Ratchetting:
Pl + Pb while the gravity load is applied plus Pl + Q for the displacement load shall be less than two times yield. Where Q is equals the total bending minus Pb from the gravity load.

So I guess for Div 2, the answer to my question is that when a strain controlled load is applied in addition to the load controlled gravity load then for ASME VIII Div 2 there is a need to conduct an additional Plastic Collapse assessment as the additional membrane stresses from the strain controlled load are Primary and could push primary stresses to be greater than Yield.

It is only an experienced Engineer that would know this as Paragraph 5.2.2.4 Step 3b) suggests that all stresses due to strain controlled loads are secondary however the experienced engineer needs to be aware of the small print in the definitions that states that all membrane stress is considered as Primary irrespective of the type of load.
5.2.2.4 Step 3b): "If a load case is analyzed that includes only “strain-controlled” loads (e.g., thermal gradients), the computed equivalent stresses represent Q alone...."
This is also different from the European code, which considers membrane stresses due to displacement loads to be secondary stresses and bending stresses due to load controlled loads to be Primary. The European code makes more sense, however which code reflects reality better.

 

[TGS4]

There's nothing simple about stress categorization. Making sweeping grand statements is just fundamentally incorrect. Please see the advice provided in 5.2.1.2, 5.2.1.3, and 5.2.1.4. If there is any doubt about the appropriate-ness of a particular stress categorization, then you are obligated to perform the elastic-plastic analysis.