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# Mean Line Circumference?

## Mean Line Circumference?

(OP)
I've found that shell plate, if strapped after rolled, grows on the outside of the plate more than it compresses on the inside of the plate.  This is understandable since steel has more strength under compression than tension- also, I don't see any real problem as long as all the shell courses are calculated using the same mean line, all courses should be the appropriate length with relation to the next course.  What if you're dealing with a 4" thick wall vessel? -miscalculating the mean line may result in a large variation of capacity.  Should the mean line calculation used in determining plate length actually be closer to the inside of the shell rather than the center of the shell plate?  Is there a generally accepted standard?

### RE: Mean Line Circumference?

I would think for vessell capacity calculations you would strap the outside and deduct the plate thickness.  Strapping the outside is much easier than the inside.  Maybe i'm missing something in your post?

### RE: Mean Line Circumference?

(OP)
Good point- definitely strap the outside of a constructed vessel. I'm wondering about design-  how do you figure the plate length?  As I mentioned, if you miscalculate the location of the mean line when calculating theoretical plate length, the actual could be way off if the thickness was great enough and the diameter small enough.

### RE: Mean Line Circumference?

I've never done this but you could use the ratio of compressive to tensile strength to locate the neutral axis for your plate length calculations.  Just curious - what are you doing with 4" materials?

### RE: Mean Line Circumference?

(OP)
I design large diameter above ground storage tanks, and recently have come to question our methodology for calculating plate length.  After a few instances where our field crew had to trim a shell plate to complete the shell course, it was decided that inside diameter should be used to calculate total course circumference rather than meanline-  I don't agree with this logic.  Rather, I believe that with large diameter tanks the meanline should be used to calculate course circumference, or plate length (moot point if proper tank building techniques are followed- use chords, and 1/4 pts).  I used 4"t vessel wall,which I recently observed at a vessel manufacturer's shop headed for the Candian oil sands, as an example. Surely, meanline must be a major design consideration for some applications- its with these applications that I'd like to base my assumptions.  I like your idea IFR, "you could use the ratio of compressive to tensile strength to locate the neutral axis for your plate length calculations", I'll look into that.

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