Anchor Shear Reinforcement for Anchors in Wall

[JR55]

I'm attempting to clarify the shear anchor reinforcing in ACI 318-19 section 17.5.2.1 for anchors into the top of a 16" wide wall. Because of the width of the wall, I don't have room to develop hairpin bars, so I am wondering if stirrups are the only option as the hook and lap would take care of the development length. However, I'm unsure if that would meet the requirement of developing the reinforcement on both sides of the breakout surface. As long as the depth of the stirrup is outside of the breakout cone am I okay?

Is there a better way to preclude shear concrete breakout in a wall without a slab to tie into? It doesn't seem like the continuous horizontal rebar outside of the anchors, or horizontal ties closely spaced at the top can be considered to prevent shear breakout correct?

 

[driftLimiter]

Might have to think about strut and tie model for this. Those horizontal stirrups could be useful as well if you get them close enough to the anchors.

The anchor reinforcement needs to be developed on each side of the breakout cone.

When the loads start getting large and turn 90 deg, we start to think about a concrete compression strut hitting that node. That compression strut needs a tension tie at the other end as well.
Which only 16" to work with you might take a look into that.

THere is an API anchorage document that uses Strut and Tie to justify the load path of the anchorage reinforcement.
Link

 

[KootK]

Some engineers, in some situations, will take advantage of the beam stirrup detailing convention that implies that a stirrup is developed at the start of the bend if:

1) The stirrup is #5 or small and;

2) The stirrup wraps around a transverse bar.

That said, this feels like a particularly aggressive use of that trick for some reason.

Those horizontal stirrups could be useful as well if you get them close enough to the anchors.

That closeness would be very important to me as well. In fact, I might tie a #3 stirrup to either side of the anchors.

 

[KootK]

Is there a better way to preclude shear concrete breakout in a wall without a slab to tie into?

Meh. Depends how badly you want it I guess. You could:

1) Use an embed with studs to better engage the rear anchors.

2) Reinforce the wall edge with an angle embed welded to deformed bar anchors.

I'd expect both of those solutions to be less popular than what you originally detailed however.

I am wondering if stirrups are the only option

I think that hairpins would work as well as stirrups here, better if they were 135 hooks at the ends.

I feel that downward opening U-bars would also work as well as stirrups. And that would probably be my first choice.

 

[JR55]

Thanks you all. KootK, for the downward opening U-bars, to me this sounds similar to what is detailed for tension reinforcement in Figure R17.5.2.1a. Can the same U-bar be considered to contribute to tension breakout in the vertical leg and shear breakout in the horizontal element?

 

[KootK]

Can the same U-bar be considered to contribute to tension breakout in the vertical leg and shear breakout in the horizontal element?

You bet. The tension in the U-bar legs probably enhances the shear capability of the horizontal segment a bit.

 

[JR55]

One question I think I know the answer to - If the U-bar encompasses the edge reinforcement bar (typically assumed to have 3" clear cover), this would make the clear cover of the U-bar less than 3". Assuming it is cast against earth, is there any way to get around this rather than moving all of the wall reinforcement in to increase the clear cover?

 

[driftLimiter]

Is it really going to be cast against earth. Normally in a footing only the bottom surface is cast against earth. The sides are backfilled.

 

[jayrod12]

Agreed with driftLimiter, where they form the edges, you don't need the 3" clear cover. As I understand it, the 3" is to account for potential variations in the earth form, thereby allowing some areas with less than the 3" of cover, but maintaining at least 2" of cover everywhere.