ASME BTH-1 fillet weld stress equation 3-55 3

[SteveGregory]

The 2014 ASME BTH-1 fillet weld stress equation 3-55 does not have a different allowable for a fillet weld loaded transverse versus one loaded longitudinally. The AISC 360-10 equation J2-5 shows a 1.5X factor for a 90 degree transverse fillet weld. Is there a good reason for not using this factor for a lifting device designed to meet the ASME BTH-1 standard?

 

[JAE]

I'm not sure directly why the BTH writers ignored it - but could guess a bit. The AISC commentary for J2 suggests that it has a lot to do with the load deformation compatibility between the weld and the connecting material. It also is of course affected by the angle to the weld axis (See Figure C-J2.13 in the 360-10 specification).

It may be that for below-the-hook applications, taking advantage of the direction of load wasn't considered in order to keep the BTH spec somewhat direct and conservative - as sometimes in-the-field load applications can be more chaotic than in direct structural design.

 

[SteveGregory]

Thanks JAE!
Being conservative in design of a lifting device is wise. However, sometimes we get pushed into "reverse" engineering calculations to justify a shop-made device that needs an engineering seal to back it up. Sometimes when new staff come on board, they start asking questions like "Do we have documentation for this device?". In these cases, we might have to sharpen our pencils a little.

 

[dhengr]

SteveGregory:
Many times on an assignment like this, evaluating a piece of exiting lifting equipment, for its allowable load limit, there are all kinds of problems other than nit-picking the allowable weld stresses. With all of the reduction factors, amplification factors, load and resistance factors, nominal design factors, and nominal weld tensile strengths, etc., being tossed around these days, we analyze the hell out of stuff and then allow details and workmanship which is certain to cause problems. You are likely to find all kinds of difficult shop-made, welder or fitter made, not necessarily engineered details which give you some trouble, load path issues, and poor details and poor welding which cause stress raisers and the like, rather than some small average weld overstress, giving you problems. Look this thing over with a fine toothed comb, for these kinds of things, and justify them or fix them, and most of the rest will fall into place. If you don’t have calcs., materials lists and drawings from the original fabrication, you have to measure everything, including weld sizes. They (you) likely don’t have a good handle on the material grades used or the weld materials used, so you may end up making assumptions there too. The factors of safety used in BTH-1 are quite conservative, so it usually isn’t the end of the world if you exceed the allowable stresses by a few percent, in a worst case scenario, as long as you have good clean load paths, details and welding. Then, be sure that the design contemplates and only allows the proper misc. lifting hardware to be used with it. And finally, you should load test the piece of equip. to 1.25(safe working load) which is the ultimate proof of the pudding. The equip. piece should then be marked with that safe working load. Then, using the equipment safely and as intended, in its work setting, is very important too.

 

[SteveGregory]

Great advice, dhengr!

I got lucky on this assignment. The device has not been constructed yet and the manufacturer that I am working with has a good team of certified welders. However, the manufacturer wants me to "only check" their design as opposed to doing my own design. The device is to be used "in house" only.

 

[JStephen]

My guess is that they went for simplicity.
In the 05 version, weld strength is same as BTH, but then "Alternatively...strength of welds...is permitted to be determined", and you get the complicated version.