Dowels for shear transfer only - no epoxy

[jeffhed]

I have a 36" diameter awning footing that was installed without engineering and is not deep enough to resist the lateral forces. The awning is already installed. Bearing is ok, but I also need more area for to resist uplift forces. To prevent tearing out the entire footing (and awning), I am going to just cast a concrete jacket around the existing footing to increase the diameter of the footing. My question is where I am doweling into the existing footing and only need the dowels to resist shear for uplift, how do I calculate the shear resistance of the dowels? I am thinking check bearing of the dowels and then the shear strength of the dowels themselves? Seems like there should be more to it than that, or maybe I am over thinking this?

 

[shobroco]

The shear strength of the dowels is unlikely to be a factor. Figure bearing of the steel dowel against the concrete and shear of the concrete section at each side of the dowel. Keep the specified hole size very close to the dowel size, & check the distance from the hole to the surface because you might simply blow out the top before you develop any strength if the rotary hammer fractures the concrete.

 

[jeffhed]

Shobroco,
I should look at it like a cast in anchor and run calculations according to ACI appendix D? Where do I look to determine the appropriate calculations?

 

[ron9876]

I would grout them and design with shear friction per ACI.

 

[PUEngineer]

ron, don't your bars have to be developed to assume shear friction is applicable? I would think grouting the dowels wouldn't be sufficient for that.

 

[jeffhed]

Ron9876,
What PUEngineer said about developing the bars is why I wasn't considering shear friction. The bars would have to go pretty deep into the existing footing to develop them. We don't usually use shear friction for any post installed dowels as getting the epoxy to develop the bar strength can be tough, especially if there are small edge distances.

 

[dik]

Unless the bars are really large, epoxy embedment would be relatively short... maybe in the order of a foot or so... To properly develop the bar, even with shear friction, it is likely that they should be embedded using epoxy.

If you try a plain bar without epoxy, it has to be a large diameter and you have to look at the confined bearing strength... I was involved in a legal case several years ago (40?) and the professor that modelled the dowel indicated that the bearing stress (using FEM) was over 13ksi... When I asked him (on the stand) what redistribution would occur after crushing, he couldn't answer it and he hadn't considered it. This occured at a time when the ACI had started to recognise shear friction... but it wasn't included in the Canadian standards...

Dik

 

[ron9876]

Yes they would have to be embedded enough to develop the required tension. Don't know how to design if they are just placed in a hole. Wouldn't there be some tension in that condition?

 

[PMR06]

I'd just use an epoxy and be done with it. Your time researching this has probably cost more than the epoxy anyway.

 

[TLHS]

Yeah, I'd say you have two options:

Long embedment:

Epoxy rebar and use shear friction

Short embedment:

Treat it like an anchor bolt looking independently at each piece of concrete. You'll still need epoxy or grout or something to fill your void to provide contact between your dowel and the existing concrete.

Honestly, I'd use epoxy either way probably.

There are other ways you could use the geometry of the new section to help tie things together, but I don't see any of them being cheaper.

 

[jeffhed]

Dik,
If i had some tension in the dowels then I wouldn't flinch at 12" embedment. However, 12" embedment with epoxy seems really extreme for only shear transfer. I feel it can be done with dowels without epoxy, but am unsure of the calculations. I mentioned in my original post that maybe I would only need to calculate the bearing on the concrete and the shear on the dowels. I'm just making sure I am not missing anything.

 

[jeffhed]

ron9876,
I don't really know how to design if they are just in the hole either. And I agree there would be some tension in that condition from pry out. That is what generated my question. Without epoxy how do you satisfy prying?

PMR06,
Using the epoxy does solve the problem of not having uniform bearing of the dowels in the existing concrete. Looks like I am going to just specify some epoxy as usual. I don't think I'll need many dowels anyway. If there was ever a situation for using dowels without epoxy, this would be it. I thought there may be some calculations to apply to that situation, but it doesn't sound like there is.

 

[hokie66]

I think your original idea is fine. Drill the dowels in past the pier reinforcement. Drive them in, which is fine for shear. Use circular ties for the "donut" jacket to ensure confinement. The shear force resisted by each dowel would be similar to that resisted by any post installed anchor. The uplift force of the dowels would be downward in the pier, but upward in the jacket, so the depth to the dowels will be controlled by the design of the jacket.

 

[TLHS]

Yeah, without some sort of fill material in the hole, either epoxy or grout, you're going to end up with possible large slippages, and unequal load distributions where some rods are contacting and some aren't. Plus if you end up at an angle your bearing assumptions are all going to go to hell.

 

[BAretired]

Use lots of circumferential reinforcement in the outer jacket. Roughen the surface of the existing concrete to ensure decent bond. Shrinkage of the outer jacket combined with circumferential reinforcement will ensure the new and the old concrete will bond and act monolithically, but to be safe, drill and epoxy a few radial bars to provide a shear/friction component of resistance.

BA

 

[jeffhed]

Thanks for all the responses. I ended using epoxy for the dowels and using circumferential reinforcing in the jacket as has been suggested.

 

[jeffhed]

Ok, one more question on shear friction, ACI 318 11.6.8 states that shear friction reinforcing shall be anchored to develop fy on both sides of the shear plane. Because my shear force is somewhat small, can I reduce the development length using As provided/As required? I can't seem to find anything that tells me either way. I think becuase i have provided the circumferential reinforcing that this doesnt matter in this case. This is probably more for my own education than anything. The development length for shear friction can sometimes be difficult to achieve. Would be nice if there is an acceptable way to reduce the required devrlopment.

 

[shobroco]

I'm going to throw something into the mix that you won't find in any literature. Drill a 5/8" hole 8" deep in concrete, and drive a 15M rebar into it with a sledge hammer until it bottoms. I will guarantee you that you will have better tensile resistance than any 5/8" epoxy anchor that you have ever seen installed (I mean actually installed, not manufacturer's design data) and the shear resistance will be as close to cast-in-place as you can achieve. When dowelling footings or walls of additions to existing structures, this is what I specify if I think that I really need to make a positive connection. I have personally installed both these and epoxy anchors & I can assure you that these are infinitely more likely to be reliable in a real-world situation. Most of the guys doing concrete work are not just filling time between brain surgery gigs.

 

[jeffhed]

shobroco,
I agree with you that you can achieve a solid connection this way. However, I don't feel comfortable specifying anything that is not backed up by calculations. Too paranoid I guess.