Design of L or J shaped concrete anchor

[patelam]

Has anyone designed the pre-installed (L-shape/J-shape or) or post-installed (adhesive) concrete anchors with the bent shaft? Like the one shown in the image. Does anyone have the design calculation for this type of anchor that they can share?

 

[jjl317]

Where is the edge of concrete relative to the bolt?

And some reason at 30 degrees? ACI provide pretty clear guidance on a 90 degree condition, and some of the software from the anchor manufacturers (i.e. Hilti Profis, Simpson Strong-tie Anchor Designer) can calculate for you.

 

[patelam]

Bolt is 8" away from edge. I dont know a reason for 30 degrees, but this is an existing anchor installed at site. Does Hilti Profis design the anchors with bent or inclined shaft? I thought it only designs for vertical shafts.

 

[jjl317]

In terms of the concrete edge, what I meant by my question was where is the concrete edge (horizontal in your sketch) relative to the bend in the bolt? Poorly worded on my end.

But I don't believe any of the software packages have provisions for inclined rods. The only option for analysis that comes to my mind is a "first principles" approach based on ACI 318 Chapter 17/ Appendix D (depending on relevant code), with conservative assumptions on the influence of the bend.

 

[TLHS]

why are you doing this?

 

[dauwerda]

As far as I know there is no testing or literature for this type anchor.
My gut tells me it would reduce the capacity of the anchor in tension as you would get high stresses resulting in local concrete crushing at the bend.

 

[BAretired]

As far as I know there is no testing or literature for this type anchor.
My gut tells me it would reduce the capacity of the anchor in tension as you would get high stresses resulting in local concrete crushing at the bend.

Has anyone designed the pre-installed (L-shape/J-shape or) or post-installed (adhesive) concrete anchors with the bent shaft? Like the one shown in the image. Does anyone have the design calculation for this type of anchor that they can share?

 

[BAretired]

I believe the bent anchor has merit. It's true there is a lateral force at the bend point, but the lower end has much better tensile resistance than either a L or J anchor.

 

[icebloom]

I believe the bent anchor has merit. It's true there is a lateral force at the bend point, but the lower end has much better tensile resistance than either a L or J an

The net force on the engaged segment of concrete is still F acting towards the edge regardless of how it is internally resolved by the kinking of the anchor. The segment of concrete engaged will be based on the embedment depth of the anchor, which if anything would be deeper were it straight rather than bent at 30 degrees.

Perhaps you can somehow justify engaging a larger tension cone (dubious ?) or a that the component of force at the bend is resolved in some other way (also dubious). I echo dauwerda in that there is neither design guidance in codes nor information in the literature about these types of anchors. I really don't think its worth trying to develop some theory on this in order to try justify what is potentially only minimal improvement in your tension capacity via concrete failure - especially given that the formulas for tension cone failure as well as pullout capacity of anchors rely heavily on empirical data. There is no way a 30 degree bend will give you enough improvement in capacity to be worth the pain (/risk).

You have to be careful with bent anchors too - if they are high grade steel and cold bent it can induce cracking at the joint which will compromise the capacity of the anchor in tension (steel failure mode). 30 degrees is probably not as bad as the full hooks you see on J bolts, however when you look at design guides they tend to warn that unless the bending is performed under certain conditions, it potentially can damage the steel. I am of the belief it is usually better to use rebar with either a 90 degree bend or larger (compliant bend radii and length etc) welded to a cast in plate if you want to start having hooked fixings - or alternatively if you want to use anchors, a headed anchor (with an additional washer or just a larger head if you really need large pullout capacity).

If its just some nominal connection taking minimal loads you can probably just make some conservative assumptions and it will be fine - if its taking serious loads I'd reconsider.

The only place I can think you may find some guidance is from suppliers if they have done their own testing. I don't think HILTI would have anything as you can't post fix a bent anchor like this. You could try some of the others, Simpson for example has a curved anchor.