need help calculating butt-weld length

Yes, your calculations seem correct.

Just a few things:

1. Are you looking at a 'T' joint with fillet welds? (I am just trying to see why you went with 0.707 of throat thickness)
2. The amount of weld material seems so low because the loading for that size of plate is low. See 3 below.
3. Normally welded joints are designed/welded so that they have a minimum strength of the base material. I guess a general point is - if that is the maximum loading for that plate, why so large/thick? Could you not get away with smaller plate to save money? ........... (you didn't mistake 20 MPa loading for 200 MPa, did you?)
4. Are there any other applicable loadings? How about fatigue considerations?
5. If this is a true butt-weld (not a fillet welded 'T') it would be general practice to fully weld out the prepped edges of the plates (as per 3 above)
   

oh yeah - another thought ........

If this is a fillet welded 'T' - then are you sure of the direction of the loading? Is that 20 MPa in pure tension on the weld/plate, and not with any form of bending?

Hi - "Guest" here again.

I was thinking a little more - would it be possible to attach a little sketch? Something seems a little off, because normally in these configurations, the loadings are expressed with forces and not pressures (unless you just already calculated the pressure loading).

If you are prosperous enough to use 32 mm plate for only 20MPa resultant loading -- go all the way to make a full penetration 32mm butt weld-- AND FULL LENGTH OF IT.

Use Mig/Mag or Submerged Arc using 1 or 2 600Amps Mig/Mag welding sets simultaneously.

Why make the weld seam the weakest part of your system?

MM

Thankyou very much for you replies, I wasn't expecting any so I'm very happy with that, thanks.

The question is actually theoretical because it has appeared on an assignment I am completing. It has thrown me a little that the load was given in MPa because, as you say, loads are normally in kN. I'm thinking because butt-welds have to endure tensile loads mainly that is why the load is given as the tensile stress within the plate. With only 20 MPa stress the plate if fully welded would have a safety factor of 20, the weld material UTS being 410 MPa. It may be an exercise in demonstrating that only the required amount of weld need to be used. Therefore saving costs in joint preparation, weld metal, time, etc. It has confused me because the equation for a butt-weld is just f=F/A, or f= F/ (l*t), and throat thickness (t) in a butt-weld application would be a similiar size to an ordinary fillet weld I'd imagine. So it works out to be the same thing.

I was going use three passes per side with a 12mm electrode and double "V" joint preparation. But it wouldn't be needed for 20 MPa stress with a static load. In reality, at the design stage a much smaller area plate coud be used.

Thanks for the feedback and help, its nice to be able run a strange result by someone else.