Embed with welded rebar connection to concrete footing

[JT-1995]

I am looking to design a horizontal embed flush with the top of my concrete footing at a tight elevation tolerance. The load on the future column that will be field welded to the embed is small, particularly in the shear and uplift. But the elevation is something I want to make as easy as possible for the contractor to hold. In past details (constructability and holding an elevation was an afterthought) I have simply seen designed nelson studs on the embed plate and away you go. Today I started a search for info on embed anchor design and specifically weldable rebar as the connection to the concrete. That search revealed clearance distances from the embed anchor to the reinforcement. I did not know that existed BTW, we always tied the rebar to the nelson studs (why not??). What I would like to do is bend a couple lengths of reinforcement into something approximating a "rebar standee" and tie the 2-3 lengths of rebar to the footing top and bottom rebar to help hold an elevation during the pour. In my mind the contractor could adjust the flair on the "legs" of the standee to make adjustments in the elevation until it was within tolerance and then tie it off. My questions are:
- does tieing the rebar embed to the footing rebar violate the ACI and IBC embed/rebar clearance requirements?
- I have found some prescriptive language that the weld between the weldable rebar and the bottom of the steel embed should be 'an all around 7/16" fillet weld', although I can not find a detailed discussion of describing the basis of this remark. Is this a code description? For my situation that weld would be extreme, can someone confirm that it is sufficient to simply design the weld as any other eccentrically loaded weld to determine its throat and length?

Thanks for your thoughts in advance.

 

[KootK]

- does tieing the rebar embed to the footing rebar violate the ACI and IBC embed/rebar clearance requirements?

I know of no such prohibition. Is it bothering you that this setup means that your footing rebar is electrically connected to a piece of metal that is exposed to the elements?

Is this a code description? For my situation that weld would be extreme, can someone confirm that it is sufficient to simply design the weld as any other eccentrically loaded weld to determine its throat and length?

I'm unfamiliar with that provision. Can you post it for us here to review or, at the least, direct us to where we might find it ourselves?

Reinforcement that is welded to embeds usually takes the form of deformed bar anchors rather than conventional rebar. And, where the bars would be perpendicular to the embed, the bars are usually gun welded to the embed in a semi-automated procedure. For these reasons, one usually welds the bars to the embed such that the tensile capacity of the bars is fully developed. I'd recommend checking out the PCI manual as a reference for this as the precast world is where most of the research and literature on this kind of thing lives.

...but the elevation is something I want to make as easy as possible for the contractor to hold.

I think that you really need to ask yourself what your tolerance are on this with respect to:

1) Elevation.
2) Pitch.
3) Roll.
4) Column plumbing.

Normal column base connections provide tolerance mechanisms to deal with precisely these things. I've seen what you're attempting done successfully in the past but I definitely think that it needs to be done with care. The welds can take up a little bit of slop but not much. And one doesn't really want erectors yanking the tops of columns too far laterally with the column bases welded up tight to a rigid base.

 

[MotorCity]

You could certainly weld rebar or studs to the bottom of the baseplate. If given the choice, I'd much prefer welding F1554 Grade 55 anchor rods (with supplement S1 to make sure they're weldable) instead of welding rebar. Is there any reason you cannot use cast in place anchor rods with a few leveling nuts to install the baseplate?

 

[JT-1995]

Thank you for your thoughts!

The tolerance is tight because we am supporting a precise and heavy piece of machinery 12ft above the embed plate so we are trying to keep the column as plumb as we can so the support plate 12ft AFF is flat and ready for the final machine alignment (it will be aligned to within 0.005"). There is a good amount of adjustment capability in the machine supports, maybe I am trying too hard to be precise at the footing. I am just not liking the idea of "hanging" a 24"x24"x3/4" embed plate in air over the footing reinforcement while the pour is placed. Then, when I saw ACI stating "clearances" between a stud and the reinforcement I have under the embed I started thinking how would it hang there then? (envisioning a wood "bridge" across the footing pit that inherently will be in the way during the pour). That is why I wanted to weld an embed anchor that would double serve as a stable place to support the plate in place.

* stud tolerance to rebar ACI 117-10 (rev 2015) Section 2.3.1. Old code(?) but this is where the rabbit hole started.
* the weld spec I read was in PCI literature. I will look to re-find it, maybe the comment was more specific to the case being discussed rather than a prescriptive directive from PCI. Thanks.

- I am not bothered by any thought of tiening the embed anchorage to the rebar. In fact, I would like to tie my anchorage to the footing rebar but I have been derailed by the tolerance spec in ACI.

- I agree using a different stud material would be more typical, but I am interested to see if there may be a slicker way to keep the embed in place during the pour. We have used (or seen used by others) anchor bolts and grout but the experience has not been great. There are a fair amount of dynamic loads and vibration so the installers or the operators often foul the threads on the anchors after install to keep the nuts from walking off. Also, the construction has to go fast (we need to get the operation back in service ASAP) so grout hindered the speed. Perhaps the biggest deterrent to anchor bolts was an experience the client had with them being misplaced and they would prefer we weld the column to an embed instead of rely on the perfection of the concrete guys to get everything in the right place. (the embed is oversized, so for final location adjustment the column can be slid around to the correct location and welded in place.)

One last thing. I am toying with the idea of shipping the column 6" long and prepping the bottom of the post to weld to the embed in the field. That does not sound fast does it, ha ha? But if I did that I could account for some unlevelness in the embed...maybe. Hmmm, maybe it is better to rely on the embed to be well placed and shop fab the column with a nice square base plate to have a simple place to surface weld the column base to the embed. This feels a little like "belts and suspenders" though. If you have any feelings on this idea I would appreciate it.

I appreciate your time and thoughts.