Plug welds vs Fillet welds in Holes/Slots 1

[SeizeTheMoment]

Per AISC table J2.5, fillet welds in holes and slots are allowed in tension and is allowed to be neglected in design. However, plug and slot welds are not allowed to be designed in tension/compression. I'm having a hard time visualizing this. If a plug/slot weld was used, would it not be able to support a tension load and have a failure plane similar to a fillet weld? The way I see it, a plug/slot weld would fill up the entire hole while a fillet weld only at the edges - how will the fillet weld be able to support the axial loads with less weld, while a plug/slot weld which fills the entire hole cannot?

Don't bend to the stress, seize the moment!

 

[canwesteng]

Easier to inspect the fillet maybe. Don't think it would be pleasant to have to put the fillet in the hole or slot if it was sized for a plug weld either.

 

[dauwerda]

how will the fillet weld be able to support the axial loads with less weld, while a plug/slot weld which fills the entire hole cannot?

I think that's the point, you can't rely on all that extra weld metal to effectively transfer a tension load. If you can't get it done with a fillet weld, don't try to do it with a plug weld.

 

[SeizeTheMoment]

I think that's the point, you can't rely on all that extra weld metal to effectively transfer a tension load. If you can't get it done with a fillet weld, don't try to do it with a plug weld.

Hmmm, but for plug welds it does not allow any axial loads at all while it does for fillet. At the minimum, should it not allow axial loads equal to fillet welds?

Don't bend to the stress, seize the moment!

 

[IFRs]

Plug welds could be made from the middle out and the fillet area won't have any melt into the parents. Not saying would be but it could be and you could not really tell.

 

[dauwerda]

This is just speculation as I haven't found anything in a few quick google searches, but perhaps it is because it can lead to brittle failures? A plug weld will be in tension (both directions) in the plane of the weld because the weld metal will want to shrink when it cools and is fully restrained. Adding a tension load out of plane will put the plug into tension on it's third axis which can lead to brittle failures.

AISC/Duane Miller had a great presentation on this not too long ago:

https://www.aisc.org/education/continuingeducation/education-archives/ductility-another-view-e29/

 

[SeizeTheMoment]

I'm now under the impression that having the entire hole welded, but using the effective area of a fillet weld (times the circumference) would produce similar strength as to the "fillet weld inside a hole." So it's using a plug, with design strength of a fillet weld inside a hole.

Don't bend to the stress, seize the moment!

 

[dhengr]

SeizeTheMoment:
Small plug welds and slot welds are notoriously difficult to do well and to inspect properly. The depth of reach and the angle of the rod or gun are such that a good sound, penetrating weld at the root are suspect, otherwise the welder is just kinda puddling the hole/slot full, and often doesn’t get any/good penetration into the side walls. Thus the min. hole size vs. thk. to assure some min. accessability for better welding, and still not optimal. For shearing loads you at least have to shear the plug off the base plate, but for tension or compression loads you may not have good fusion/bonding to the side walls. I don’t have the latest few eds. of the codes so I don’t know some of the machinations they may have gone through to get their correction values or exact foot notes, but the above is the primary rational. A reasonably sized hole or slot quickly becomes too large to fill, so you end up with a larger hole into which you can properly apply a fillet weld.

 

[SeizeTheMoment]

I see now, that is the main difference. Thank you dhengr.

Fillet welds are constructed to penetrate through the side walls while plug welds are just "puddled" until full, therefore we can't rely on the welds attached to the side walls to support axial loads.

Don't bend to the stress, seize the moment!

 

[dhengr]

SeizeTheMoment:
No, you kinda misinterpret what I said/meant, please reread and read btwn. the lines a little too. Plug welds should penetrate both the base plate and the side walls too, but when they are small holes vs. the pl. thk. you can’t always get good access (a good rod/wire angle into the root btwn. the two pls.) for good penetration into the side walls. That first pass in a plug weld should be a good fillet weld at the root, all around, then fill the middle, partially remelting the previous fillet welds, then another fillet all around at the outer wall, and fill the middle, etc. Without sufficient access, the tenancy is to get what you described. And, the top surface of the welds can look o.k. but the lower passes are just chicken crap. A puddle weld is really a different animal, with the top pl. being pretty thin and its edge is just melted into the base pl. by the puddle weld. If you lay out the AWS min. plug/slot weld hole size recommendations to scale, you should be able to get about a 45̊ rod angle down into the root, that’s the reason for these requirements. But, it is still difficult for the welder to manipulate the rod tip 360̊ around the small hole to get a good root pass.