ASME Section VIII, Division 1, UG-36(a)(1)

[pperlich]

UG-36(a)(1) states, "When the long dimension of an elliptical or obround opening exceeds twice the short dimensions, the reinforcement across the short dimensions shall be increased as necessary to provide against excessive distortion due to twisting moment."

I've read enough here and other places to know that I will probably being doing FEA to show that a design is adequate. But I'd like to avoid that if possible.
My question is: Where does the twisting moment come from? I have a vertical vessel that has 3" x 12" sight windows, major axis vertical, so there are no external loadings. I read somewhere that the internal pressure causes this twisting moment, but how? (From differences in static pressure?)

 

[ronfrend]

This was addressed in the COMPRESS CODEWARE forum
thread1259-288691

You are right it is internal pressure but you will have to do FEA.

Sorry

Ron Frend

http://www.predicon.net

 

[pperlich]

Thanks for the reply, Ron.

I did read that post, and would have posed my question within that thread if it were not closed.
I am struggling with the "action of the shell under internal pressure." How does this action generate a twisting moment and what is the direction of the twisting moment?

 

[pperlich]

Also, I'm pretty new to the Code. How did engineers deal with this requirement before FEA, or is this relatively new?

Thanks, Phil

 

[ronfrend]

Hi Phil,
There is not much written on this subject but consider the membrane stresses. If you have a circular opening the membrane stresses are fairly equal around the opening (taking into consideration the difference between hoop and longitudinal stresses). With differences in the length of the axes you will have different stress concentrations and therefore strain. And if you get different strain, you get twisting.

How did we do FEA in the "old days" By hand - with a slide rule. It took FOREVER.

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Ron Frend

http://www.predicon.net

 

[pperlich]

Thanks Ron.
I can somewhat visualize what your saying now. I was really, really hoping that there was some way to avoid FEA with a hand calculation, even if its ultra-conservative.
FEA reports for CRNs are a chore. In the future I will be pushing our customers towards round sight windows.

Phil