Dowels for shear transfer only - no epoxy
Dowels for shear transfer only - no epoxy
(OP)
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?






RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
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?
RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
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.
RE: Dowels for shear transfer only - no 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
RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
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.
RE: Dowels for shear transfer only - no epoxy
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.
RE: Dowels for shear transfer only - no epoxy
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.
RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
BA
RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
RE: Dowels for shear transfer only - no epoxy
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.