3S for secondary stress, Q

[b5quaj]

When secondary membrance stress plus bending occurs,
3Sm may be used.
In this case, 3S is greater than its yield stress.
Does It means yield or deformation of shell.
Secondary is self-limiting.

I considered the load which are acting on shell to be attached lifting lug and tailing lug
as secondary on discontinuity
So, I used WRC-107 and 3S as allowable stress of shell.

But a engineer think deformation will happen for lifting vessel,
becase 3S is greater than yield stress.
I am wondering if his explaination is correct or not.

I would like to take your opinion.

 

[prex]

To be more precise, here are some comments:
1)the 3S_m limit is valid for primary plus secondary stresses
2)before checking the 3S_m limit you need to check the 1.5S_m limit on local primaries (assuming in the lifting operations the general primaries are zero)
3)the local primary component is the membrane stress in the periphery of lug (or pad plate)
4)yes, some yielding occurs, but no gross distortion (you can't notice it) if the method is correctly applied.

prex

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[b5quaj]

1) In erection condtion, Can load acting shell be considered as secondry?
2) 3S is greater than Sy, Does it mean yield occurs?
3) What is self-limiting(or self-equilibrating for secondry?
4) What is gross distortion?
5) If yield occurs to deformation,
why is it invisible?

Thank you.
your answer is very helpful.
More exact reply is needed.

 

[prex]

1)If you don't know how to classify primary and secondary stresses, please refrain from doing any calculations involving a 3S limit.
By no means that classification may depend on the erection or service condition.
2)Yes
3)Self limiting is the base condition for a stress to be secondary. You need an in depth knowledge of some concepts before starting to use them. Can't be more specific here: a book would be required, not a short message.
4)Gross distortion, as referred to in ASME VIII Div.2, is a deformation not acceptable for a vessel component: the limits set forth on stresses avoid this.
5)Yield starts at some 0.2% strain, you can't normally see such a small deformation.

prex

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[warrenw]

How can a lifting load be self-limiting?
There is no way that I would use a local stress of 3S at a lifting lug.

 

[jte]

b5quaj-

Listen to prex - he has said lots of good things to you. Your case sounds like a typical situation in the early life of a vessel: The vessel gets designed, material has been ordered and perhaps already fabricated... Then someone decides to think about how to lift the blasted thing. People seem to forget that lifting can actually be the governing load case for things like the skirt and base ring, and occasionally insert plates are useful for lugs or trunnions. Read the introduction to WRC-107. My copy's at home, so I can't quote it, but it says something like: "Some of these results might be off by a factor of 2." Now consider the impact to cost and schedule and safety for the client to have a critical lift go bad. Be conservative with your lifting calc's.

Prex-

I've sometimes argued that gravity loads should be classified as causing secondary stresses: The loads on the lugs or trunnions are self limiting when the column hits the ground!

jt

 

[prex]

jte,
the classification of stresses near a lifting lug I've always used is as follows:
-the membrane component is a primary local stress (if it satisfies the extension condition to be local, as it normally does)
-the bending component is a secondary stress.
BS (or PD) 5500 App.G has a detailed treatment of local lug stresses and the stress limits are consistent with the above classification, though with cofficients a little safer than with Div.2 (1.25S and 2.25S, if I recall correctly, in place of 1.5S and 3S)

prex

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[bvi]

Prex is correct. What you need to understand, is that while the load may not be self limiting, a component of the stress may be. For example, the load may be carried by membrane stresses, and the bending stresses may simply be a result of compatability of deformation. This is laid out in the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, Appendix 4. However, this is not something to be learned from reading a dozen sentences in this forum. Take is as a tip, as a pointer towards what you should be learning from books and technical papers.