Plug Welding Rods Into Base Plate

[mogug]

I have a situation where I need to attach a light pole (HSS 4x4) to the top of an existing railing post (HSS 8x6). The light pole needs to be removable. The pole bolt pattern and existing post size do not allow for a simple plate to plate bolted connection. One solution would be to weld a larger plate to the pole base plate and bolt that plate to a plate welded to the top of the post.

Another solution recommended by my boss would be to plug weld threaded rods into a plate welded to the top of the post that that pole could connect to. Am I correct in having concerns about this (plug weld in tension)? Is there another way to do this using the same concept (Nelson studs)? I worry fillet welds might prevent full contact between plates.

 

[Ron]

Yes, you are correct to have concerns. A plug weld will have limited weld area to resist shear, brought about by tension in the rods.

Welding two properly sized base plates is your better option, using fillets all around. Keep in mind however, that the lower base plate that is welded to the top of the existing post will be welded "out of position", meaning an overhead fillet weld. Make sure the welder is certified for such a weld.

I'm assuming both plates are relatively thin (< 1/2"). Depressing the rod in the hole would only yield about a 1/4" fillet weld. If the rod is maintained flush with the plate bottom, then you will only have a perimeter butt weld of limited penetration and placed completely in shear at the rod's perimeter.

 

[StrPE]

Per AISC 13th, J3.b, "Plug ... welds are permitted to be used to transmit shear ... or to prevent buckling ... and join component parts of built-up members." i.e. They are not approved for tension.