ASME VIII DIV. 2 PAR. 5 - SELF LIMITING STRESSES

[Anthony Aponte]

Dear members,

I am designing the top support of a column with ASME VIII div. par.5 elastic approach. Idea of the system is attached. Building the FEA model is not a problem: my concern is on theory.

Loads on my support are the sum of a primary component ( weight of pipe ) and a secondary component ( thermal force between top nozzle and support itself due to difference in expansion ). I have the output of stress analysis so I know exhactly how much of the load belong to the first component and how much the second.

When I consider the primary part of the piping load it is all clear to me: local stress need to be less than SPL for both membrane and membrane + bending. At a distance of square root of Rt the stress need to relax to Pm. Is ok: done.

When I consider the total load ( so weight of pipe + thermal ) allowable for membrane and membrane + bending is SPS. The spirit of the code is racheting here but what about large deflection? I understand that secondary loads are self limiting but this information can't be enought because you can't - in advance - know how much mm of deflection are needing to relax your component! In my case I know there are 10 mm so use SPS also for membrane will not lead to a big mistake

On general cases why ASME accept a membrane stress bigger than Sy? How can code know that this assumption will not lead to large deformation?

Thanks

 

[TGS4]

A couple of points:

1) There is no "elastic approach" within ASME Section VIII, Division2, Part 5. There are multiple Failure Modes that need to be protected against. Each failure mode analysis has an elastic stress analysis method.

2) Within Protection Against plastic Collapse, please read paragraphs 5.2.1.2, 5.2.1.3, and 5.2.1.4. The answer to you question about separating the primary and secondary stresses lies in there. Additionally, please understand the serviceability criteria does not differentiate between loads that cause primary stresses and loads that cause secondary stresses.

3) The second failure mode that you list above is protection Against Cyclic Loading: Ratcheting. Please make a note of the "Cyclic Loading" part of the title. Do you understand what the ratcheting failure mode is? And do you understand why the limit is twice yield? And why the limit is twice yield on the primary-plus-secondary membrane-plus-bending stress range?

Happy to discuss further.

 

[Snickster]

The Pressure Vessel Handbook by Bednar goes into great detail in first couple of chapters about primary versus secondary stresses and allowables per B&PV Code.