welding to epoxied anchors - heat damage 1

[ggcdn]

The contractor for one of my projects missed a bunch of embed plates for canopies at the ground floor. Because the plates are exposed finish, they really want a fix that does not have bolts+nuts showing, and are asking for something like a plug weld to epoxied anchors.

My immediate concern is that the heat of welding will compromise the epoxy and possibly reduce the anchor capacity. Anyone familiar with this scenario? Are my concerns unfounded?

Another solution could be to pre-weld dowels to the plate and epoxy the entire thing at once, but then we can't get pull tests done.

I'm leaning towards just telling them to suck it up, but interested in hearing others opinions.

 

[TehMightyEngineer]

I've welded some angle brackets onto a plate with embedded studs before and had similar concerns so I had the precaster load test the embeds and it checked out fine at greater than 100% the design capacity. Not sure if epoxy instead of cast-in embeds would have any issues but I suspect that concrete is just such a huge heat sink that you'd need to have quite a lot of weld heat to make it an issue. Epoxy might be more sensitive to heat but I'd surmise it's a short-term loss of strength. I'd definitely see if the epoxy manufacturer has any data on short-term heat damage.

Overall I'd say it's a concern but likely acceptable for some circumstances. For example you might be able to get them to setup a trial test of the setup in a some scrap material and load test two samples or something. I'd definitely not put yourself in a position that you're putting your neck out to save a contractor for their mistake.

Are you able to use mechanical anchors rather than epoxy?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL, CO) Structural Engineer (IL, HI)

 

[KootK]

You may be off reservation right out of the gate if the plug welds would be in tension as I believe that most steel codes only allow them to be used in shear.

Maybe you bolt something on and then weld on a structural cover plate with a backside recess to receive the nut.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[KootK]

The interweb seems to actually recognize weld heat as a method of uninstalling anchors which is not encouraging in this context: Link

What's the scale of these canopies?

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[XR250]

You may be off reservation right out of the gate if the plug welds would be in tension as I believe that most steel codes only allow them to be used in shear.

Damn! Did not know that. Learn something new everyday.
Thanks.

 

[Once20036]

Do you know where that is in the code Koot? I don't use a lot of plug welds, but feel like that tidbit is worth remembering.

I believe that the last time I called Hilti with this concern, they didn't have any strong objections. The heat based removal might involve A LOT more heat than a plug weld. I`m on board with the "do a sample & tension test it" option. I think you`ll be fine, assuming plug welds are allowed in tension.

 

[CANPRO]

KootK is correct, plug welds are for shear only. My Canadian friends can reference CSA W59 clause 11.3.2.4.

If you need the anchor to take tension, you could bevel the end of the rod or the edge of the hole and provide a partial penetration weld between the anchor and the plate. You might need to upsize the anchor diameter to make this detail practical. That detail would need a bit more thought and maybe a conversation with the fabricator to see what weld procedures they have that will work in this scenario.

You could also look at using countersink expansion anchors.

 

[sbisteel]

AISC J2.3 only allows plug welds to transmit shear forces in lap joints.

Outside of plug welding, perhaps epoxy threaded inserts to the concrete and provide countersunk bolts to get the desired flush look?

There are also expansion bolts on the market with countersunk heads as well.

 

[KootK]

Thanks for tending to the code clauses gentlemen.

1) It's a weird thing, really, given that:

a) If your hole is big enough to fillet weld the perimeter, you can use those welds in tension. And, intuitively, it's pretty tough to buy that a well made plug weld isn't at least as good as a fillet weld around the perimeter. That said, there must be a fusion quality rationale etc for the the limitation. Otherwise, why have it? Unless it's really just to prevent dumb asses from thinking that the tension capacity of the plug is the As x Fy of the plug area.

b) For better or worse, there is a rather substantial history of folks doing exactly this in the realm of base plate anchor bolt repair.

2) How would one go about tension testing the post-weld anchors? Just proof load the entire canopy to 1.5 x design load? Certainly, I'd be satisfied with that if it can be sold to the build team successfully.

3) I wonder if any adhesive strength lost as a result of heat would subsequently be regained when the heat is removed and the adhesive allowed to cool? No doubt there a good deal more material science involved than just melt/freeze but, still, there may be an element of that.

4) I've pitched another idea below which is a refinement of my previous one. I suspect that much will depend on how successfully the final product can be finished after all of the welding etc. What are we dealing with here? Stainless? Galvanized? Powder coated?

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[ggcdn]

Thanks guys, lots of good discusion here

TehMightyEngineer, I was aiming to use weldable grade rebar as my anchors. But a trial is a great (and fun) idea.

Koot, the reactions on the embed produce shear and tension. These are for 6ft long structural steel outriggers supporting glazing and snow load. Plug welds aren't something I've ever looked at before, so I didn't realize the no-tension limitation. Theoretically, couldn't one design a fillet weld with legs equal to the radius of the hole, and then add 'filler' weld over top? Not that I want to do that... If I had to, I think I would use a chamfered hole, protrude the anchor a bit beyond the plate surface, do a bevel groove weld like CANPRO suggested, then grind smooth.

I'm going to call Hilti later and see if they have any comments about their adhesives getting heated from welding.

 

[CANPRO]

KootK, I had a similar thought to your last sketch with the cover plate to provide a clean finish.

On the issue of the plug weld not taking tension, I don't know the official reason, but I suspect it is because of this: For arguments sake say you have a 1/4" plate and a 5/8" hole for a plug weld. When you first strike an arc in the hole, you can direct your electrode at the root of the weld and get proper fusion, but at at the same time, due to curvature of the hole and size of electrode, you'll also start to weld the face of the 1/4" plate in other portions of hole...but your electrode will not be directed in a manner that will provide proper fusion. So in the end you'll end up with a blob of weld fused to the underlying plate, but with proper fusion in only part of 1/4" plate - this provides good shear resistance, but poor tensile resistance.

 

[KootK]

Theoretically, couldn't one design a fillet weld with legs equal to the radius of the hole, and then add 'filler' weld over top? Not that I want to do that..

Feels that way doesn't it? I've seen some stuff in modern steel construction articles etc that seems to indicate that, barring a pretty large hole and/or a thin plate, it's pretty tough to get the weld access needed to make something that we're willing to call "fillet weld" instead of a plug weld. And, at some point, I feel as though there ought to be some reasonable limit on the minimum amount of linear..ness (?) we're willing to accept before it just gets silly to be calling the thing a fillet weld. Heck, the minimum weld length of 4t might limit you depending on the proportions of things.

These are for 6ft long structural steel outriggers supporting glazing and snow load.

That'll fall into the "too minor to care much" bucket for me. You may now consider your plug weld idea to be KootK approved if you wish/care. I feel that your focus can reasonably shift away from public safety in favor of keeping your clients happy.

@CANPRO: thanks for the explanation, that sounds quite plausible.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235