Shear lug separation from anchors re: ACI 318

[engjg]

I am considering a shear lug to alleviate baseplate shear on anchor rods causing bending failure (bending in anchor rods another good topic for debate...). Consider a four anchor bolt pattern 1-1/2" diameter rods securing an 1.5" plate with 2.5" of grout planned to allow for leveling nut adjustment. Thinking of a cruciform shape of 1.5" thick 4" x 4" x 5" tall. Concerned about proximity of cruciform to anchor rods as i note they it would be closer than the minimum spacing for cast in anchors of 4xD per ACI 318, does ACI speak to this? Anchor rods are seeing significant tension.

 

[KootK]

1) I know of no formal limitation for how close your anchor bolts can be to your shear key grout pocket. That said, the whole point of the grout pocket is to give the key something solid to push against. As such, I'd provide as much space between the bolts and the pocket as you need to give you confidence that you'll have decent concrete in there given your aggregate size etc.

2) There are two things that you probably want to avoid in this situation:

2a) You don't want to sacrifice resistance to bursting stresses developed at the anchor ends because some of those bursting stresses wind up close to your pocket.

2b) You don't want to sacrifice concrete breakout resistance because your presumed concrete failure frustum winds up passing through/near your pocket.

I would recommend dropping your anchors low enough below the pocket that you can credibly say that the neither of these issues comes into play.

 

[engjg]

Thanks KootK. I was going to make the grout pocket the same size or larger than my baseplate so the anchor rods would be in the recess to be grouted too...you see any issues with this approach?

 

[KootK]

I've never, ever seen it done that way but that fact alone doesn't mean that there's anything wrong with your approach. It does mean that you won't be able to use shim stacks. So will you accomplish your column plumbing with leveling nuts and support any pre-grout axial load on those? I suppose that another issue is ensuring that the ring of non-grout pier concrete surrounding the pocket is capable of transferring the shear load given it's much reduced width. I'm not sure that it's appropriate to just treat the pier concrete and pocket grout as a single monolith without taking special measure to ensure that.

 

[Klitor]

KootK,
I am eagerly waiting for your opinion on attached sketch (capacity, feasability, aesthetics...
Its a contractors proposal I received some time ago (both anchor bolts are to be placed in a common pocket and grouted after leveling.

 

[klaus.]

Looks pretty wrong to me

 

[Klitor]

Since you bit, please do elaborate further.
It isnt a KootK exclusive deal,everybody is welcome

 

[Celt83]

I'm not seeing a lug in your sketch.

If your anchors are in tension then your limiting factor is the bond between the grout and adjacent concrete, which is probably not great.

For shear in the anchors your failure will now be based on the grout pocket strength not the concrete, also probably not great.

If these are post installed anchors anyway you'd probably be better off using epoxy anchors.



Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[KootK]

It isnt a KootK exclusive deal,everybody is welcome

I feel so much less special now...

Aesthetics. Beauty is in the eye of the beholder. No comment.

Feasibility. If it's been proposed by your contractor, I would assume that they have vetted the detail from this perspective and that it is quite feasible. I would, however, take care to ensure that you have an arrangement and a grout specification that will give confidence that you'll get quality grout down in that pocket where it will be critical.

Capacity. Most of my thoughts mirror Celt's:

- Since you have no shear key proper, I assume that this connection does not see appreciable shear?

- If this is tension & compression only then you might have something viable here. Like Celt, my concern would be finding a way to ensure that the grout "plug" acts as though it were monolithic with the surrounding concrete rather than pulling out of the key as an independent mass.

- When the contractor turns the levelling nuts to plumb the column, what will prevent the bolt and it's base plates from just rotating? With conventional details, the anchor bolts are prevented from rotating (hopefully) by their bond with the concrete. You don't have that here.

When I get to 1000 stars, I get to meet BAretired for lunch. Current star count = 790. Just sayin'...

 

[KootK]

When the contractor turns the levelling nuts to plumb the column, what will prevent the bolt and it's base plates from just rotating?

Actually, I see no bottom side leveling nut. Are you planning to weld the lower of the two nuts to the base plate and turn the bolts from the top to plumb the column? If so then a) clever and b) take care to ensure that the welds between the nuts and base plate can support any construction load that may be on the column prior to grouting.

When I get to 1000 stars, I get to meet BAretired for lunch. Current star count = 790. Just sayin'...

 

[Klitor]

The welded leveling nuts thingy really is clever (construction loads are small).

After a friendly phone call they agreed to change to epoxy bolts so I never had to try hard to justify any grout-block-pocket capacity of the detail... Just posted this if someone has some extra time on their hands and nothin better to do