ape2010:
You have a very nice CAD drawing of a symmetrical spreader beam, very short, very light, and non-compact too. I sure would like to see the design calcs. for that spreader beam, including some consideration of stability and buckling. Then, let’s see your rationalization of the load path and stresses, at each level, from the top lifting lug (at 16kips), through the light flg., to the light web, through another light flg., to the two bottom load lugs (at 8kips ea.). It’s actually a fairly complicated little structure, which causes very complicated load paths, and direction changes, through a bunch of potential hard spots and stress raisers. These are really very complex, tri-axial, stress conditions at a point. The old std. beam design approach we learned in college does not always lend itself well to this type of equip., but isn’t to be ignored either. The beam with its light flanges and web and the web stiffener welds which you didn’t ask about present some real potential problems too. And, you are wondering about the weld size btwn. the lifting lug and the 1/4" top flange, which seems to me to be the least of the potential problems?
You are most likely to get a poor HAZ, creators, undercuts, etc. at the start and stop of the weld of the lug to the top flange, these are all stress raisers, four of them at the lug corners. That’s right where you have an identical welding condition and stress raiser on the underside of the flange at the web stiffeners. These are awful, hard spots, right where the stresses are transmitted through the flange, in the through thickness orientation. Otherwise, it is assumed that the lug welds transmit their load through the bm. flanges acting as short cantilevers off the web and its radius to the flange. But, this bending load transfer, in the flange, won’t happen until you get some considerable yielding, or maybe worse, at the four hard spots, at the corners of the lug.
Pin bending and bearing stress (Hertz stresses) and the real max. stresses in the pin plates are important to look at too. You will quick discover, that for a lot of equipment like this, it really doesn’t pay to try to use structural shapes, right out of the book, they just don’t meet our needs very nicely. Built-up sections, where we can pick our web and flange size often fit the bill better.
Some engineers and specs. are inclined to weld everything with full penetration welds, at great cost, and potential detriment to the structure, when there is nothing about the loading or stresses which require this. Imagine what a pretzel that light beam would look like after welding those 1.75" lugs with full penetration welds. To your original question; two 5" fillet welds would be fine, except for the issues above. Thus, (load factor)(16k)/(2)(5") = shear flow (kips/inch)/ (allowable weld stress) = (design throat of weld)/.707= filet weld size. That being said, it is important to pay attention to OSHA, ASME and the like, but they are not design guides, they are requirements, to be met. Do offshore lifting specs. really require full penetration welds, which might be detrimental to the structure, or seal welding of all joints?
Why not consider using a 3/4" plate, or some such, 38.25" long and 16"± high, flame cut it to a shape that the loads and stresses would tend to take, btwn. the three load pin points. A boomerang shape comes to mind, and that way it would come back to you when they’re done with it. At the three load points (pin holes) apply 1/2" stl. pl. doughnuts, (6 total), fillet weld all around, with a 6" O.D.; the 1&5/16" I.D., for your 1.25" bolts, is drilled or line bored after everything is welded up. I wouldn’t change hole sizes on the links either, that just makes the machinist change tools. You sometimes need some web stiffening flange plates on an arrangement like this. These are just some ideas for you to think about, you do your own design.
