Weld Design Between Post and Post Cap

[cgstrucg]

Hello,

A contractor for a project sliced down a post and need to design a weld between the new post top and post cap. See attached. I have designed mostly fillet weld till now and it doesn't look like we can use fillet weld in this situation. The hook which you see in the image is to pick up the post along with some things. The total axial load will be 12kips.

Does anyone have any suggestion on the weld type I shall use for this scenario? I was going through AISC and thought that maybe CJP weld will be the best option. But I don't know how do we design the thickness for the same.

Thanks

 

[Ron]

As a pre-qualified weld joint under AWS D1.1 (assuming steel posts since you mentioned AISC), this would be considered a corner joint. You would prepare the post with a single bevel joint. A complete joint penetration weld is likely not necessary, but you don't give the thicknesses of the materials or the diameter of the post. The weld area for design would be the depth of penetration that creates the throat of the weld, times the average circumference of the weld. This is the failure plane cross section.


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[cgstrucg]

I have asked the contractor about the material thickness. It looks like its a 0.25" thick post. It's diameter is approx 6.5". Can you tell me which weld you are looking at. If I look at AISC 14th ed then are you talking about Pg 8-61?

 

[CANPRO]

A single bevel on a 1/4" wall isn't going to get you much of an effective throat, granted you don't need much for your load. If you want a CJP (which appears to be overkill for the size/loads given) you would either need a backing bar or qualify a full pen weld from one side and no backing bar - which is very difficult.

Preferred route for me would be to oversize the cap plate and use a fillet weld. Can you use a cap plate with a 7" diameter and provide a 1/4" fillet all around?

 

[chucklesNOLA]

If you can't oversize it as CANPRO suggested (which would be my preference as well), is the "eyelet" small enough that you can set the closure plate inside the tube and fillet the top of the plate to the inside wall?

Would slotting the wall of the post and dropping the "eyelet" plate in like a gusset work for your application? That would give you plenty of length to fillet the plate-to-tube connection.

 

[cgstrucg]

Actually, this model is already built on site. Contractor had to cut down the post because of which they are asking for welds there. So we can't oversize the post cap. I don't think they can put an inside plate because of the same reason to weld from inside as there might be no space.
Contractor told me that it looks like initially there was a full pen weld 1/2". I think because of constraints we might have to go for full pen weld only. Can someone tell me how we design a full pen weld. I haven't designed a CJP weld thickness before and for some reason there are less online resources for them.

 

[CANPRO]

Now I'm confused. Are you designing a new cap plate or evaluating an existing cap plate? Why do you mean by full pen weld 1/2"? I thought you said the wall was 1/4"? Are you designing the weld between the cap plate and post, or between the cap plate and the lug on top of the cap plate?

 

[cgstrucg]

So I am designing weld for an existing cap. There is an existing post and post cap for which contractor reduced the size by cutting short the post. Initially, there was a 1/2" total pen weld between post and post cap as per the contractor. Now as the post is cut short, he needs a new weld between post and post cap. Post tube is 1/4" and the Post cap is 1/2" as per manufacturer. Contractor gave me rough dimensions previously and given proper dimensions now. I should have mentioned that. Sorry for the confusion. Weld to be designed is between the cap plate and post joint.

For the lug on top of the cap plate and cap plate, I have already designed the weld. It's between the cap plate and post which confuses me.

 

[Ron247]

Can you lengthen your lug and slide your lug into the pipe and weld it to the inside? Then you can use 2 half-circle caps for looks only.

 

[CANPRO]

I still don't understand. Is the cap plate currently welded to the post and you're evaluating that weld? Or you need to weld on a new cap plate and you're using the original condition as a reference? I don't know what you mean by "now the post is cut short". Are you trying to extend an existing post by splicing in a new section? It would be helpful if you could post a sketch of the original condition and what you're trying to achieve.

If your cap plate is 1/2" and the tube wall is 1/4", the full pen weld is based on the 1/4" wall, not the 1/2" plate.

 

[chucklesNOLA]

OK, so if I'm understanding correctly it sounds like the contractor cut off the post, leaving you with a post cap (the lifting lug, a plate & maybe some portion of the post hanging below?) and a post that's now some amount shorter that it was before. Is that correct?

It also sounds like you're trying to sort out how to reattach the lifting lug/plate/post segment to the now-truncated post below?

There's not a good way to do this short of either a PJP groove weld (depending on load characteristics like cycling and magnitude) or a CJP weld, for which you probably need some sort of backer (maybe a consumable like they use in pipe welding?), as CANPRO noted before.

Design of a CJP weld is (typically) based on the base metal thickness and strength, since the welding electrodes are (generally, but not always) over-matched. The design limit states are pretty clearly stated in AISC 360 Chapter J. I'm not sure what resources you're looking for exactly.

I'm not sure why there would have been a 1/2" "wide" CJP previously, since that seems awfully wasteful of material. Even at a 45° bevel on the end of the pipe that would have left almost 1/4" throat to be filled.

You mentioned a manufacturer somewhere in the thread. Was this post/lifting lug assembly a pre-manufactured part that was cut apart? What does the manufacturer think about all of this reworking of their component?

 

[cgstrucg]

Yes, chucklesNOLA. That is what happened. So I am asking the contractor to provide a backing ring and do a CJP. I am following AISC and designing as per 1/4" thick metal as mentioned by CANPRO and in AISC manual. Yes, this was a pre-manufactured part that was being cut apart. For some reason, we as consultants have taken responsibility to provide a weld for this even though I also thought that the manufacturer should work this out with the contractor.

 

[chucklesNOLA]

It's always fun fixing someone else's mistakes and then backing the solution with your E&O insurance. ;-)

 

[Ron247]

I would have the manufacturer design the repair if that is what happened. If they would not, then I might design it myself, but prefer they did.

 

[dhengr]

Cgstrucg:
I’d have a machinist cut the remaining pipe off the cap pl. and clean it up. I’d have him cut a .125” high round land on the bottom of the .5” pl., just a bit smaller in dia. than the i.d. of the pipe. I would also have him cut a .25” by 45° chamfer around the bot. of the pl. This will give you a full pen. weld, although I hardly think you need it to get enough weld btwn. the pipe and the cap. You didn’t tell us the dia. of the pipe, which is an important part of the design problem info. I don’t like the way the lifting pl. (lifting lug) loads the weld btwn. the cap pl. and the pipe. The weld stresses well be very high right under the lifting pl. and very low out away from it.