Weld Strength Paths

[BigTank]

This is going to expose my inexperience with the code at this level, but how else will one learn, eh?

I have a few questions regarding the weld strength paths described in Sect. VIII Div. 1, UG-43.

I understand the allowable stress modifiers that come from UW-15(c) and UG-45(c) and their use. I understand the calculation of the strength paths and what those values represent.

What I do not understand fully is the methodology illustrated by the examples in Appendix L in which they use these modified allowables in conjunction with some strange area calculations to find the allowable loads for these strength paths. Also, the types of failures do not seem readily appearant by these calculations.

For example: the areas calculated in L-7.2.9 are as follows:

(a) PI/2*nozzle O.D.*weld leg*Mod Allowable S
(b) PI/2*mean nozzle dia.*tn*Mod Allowable S
(c) PI/2*nozzle O.D.*t*Mod Allowable S
(d) PI/2*reinf. elem. O.D.*weld leg*Mod Allowable S

Because the type of failure aren't appearant, it isn't clear what area is being approximated by these calculations (i.e. the radial and longitudinal fillet weld size is the same for a 45° fillet, the groove weld in tension must mean the failure would be if the vessel wall pulled away from the nozzle wall, but that in combination with the fillet weld shear?). It took me a bit to understand why the geometry seemed to be divided by 2 in each case, but I believe I understand that correctly now (areas taken in cross section of the nozzle on BOTH sides of the centerline, meaning to get the full 360° area one need only 'revolve' 180°, thus PI/2).

So here are my questions:
1. why the area approximations? why not just accurately calculated areas? those are not difficult calcs.
2. what are the implied modes of failure for the welds? i believe more detail is needed than just 'fillet weld shear'. i.e. shear in what plane?
4. when it comes to adding the resultant allowable weld strengths, how do they come up with the combinations without detail the mode of failure for that particular weld?
3. is this methodology detailed anywhere else in the code aside from in the examples?
4. this detail seems to be treated poorly by the code. why?
5. is there more information to be found outside of the code, but still using the same methodology?

I apologize if this might seem a bit elementary, but I've already spent too much time trying to reverse-engineer this methodology already. Maybe some history of the code or another outside reference will put my thinking over the edge.

Happy Friday!

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Fitter, happier, more productive

 

[JStephen]

When I first started looking that this, I had to make some sketches trying to figure out exactly where the line of failure was going to be, and working with those sketches helped visualize what was going on.

I'm not sure what you mean by exact areas. But consider that normally, these welds are at the intersection of two cylinders, whereas the illustrations show a nozzle through a flat surface. So calculating "exact" areas could turn out to be rather involved.

 

[chicopee]

If I remember correctly from past ASME articles that I used to get as a former B&PV inspector, fillet weld failures would start at the toes of the fillet welds in the majority of cases.

Other than what JStephen tells about sketching which is an excellent tool to understand technical material, I would suggest two references: 1)Pressure Vessels- the ASME code simplified authored by Robert Chuses and Stephen M Eber 2)Pressure Vessel Design Handbook authored by Bednar.

The first reference will give you in addition to explaining the codes, an insight into the thinking of ASME pressure vessel committees when they write and modify their codes. The second reference is technically oriented with analysis including welding supporting the design of pressure vessels.

 

[BigTank]

JStephen: thanks for the advice on the sketching. i've tried that, but admittedly half-heartedly. i may dig a little deeper using that tool as you suggest.

as far as the areas go, yes, i guess my line of thinking was more that of a cylinder intersecting a flat plane, which could make the calcs simpler to conceptualize with the use of PI/4*(D^2-d^2). i would think that would be a more accurate approximation of area than what the code seems to be using in its place in the referenced example. i suppose that without doing some fancy calculus i'll just have to take their word for it that the area approximations used are 'accurate enough'.

that being said, i wouldn't question the approximations used so much if i knew the mode(s) of failure that these calcs are checking.

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Fitter, happier, more productive