Shear Friction
Shear Friction
(OP)
When doing new concrete to old, I uderstand that you have to transfer shear. Most say to roughen the old to 1/4" amplitude, and also use dowels to clamp the slabs together. Would the dowels be necessary if you can use shear keys? Something that would lock them together? I still see how they could pull apart, so maybe someway of a dovetail type keyway?
What do you guys and gals think?
What do you guys and gals think?






RE: Shear Friction
RE: Shear Friction
I'm in the same boat. I'm still a rookie with just a year and half in the real world of engineering lol, and I need to look into the same thing. In school it was easy to design these as it was always brand new lol. I have evaluated an existing spread footing, which doesn't work, so we're gonna enlarge it and tie everything together with dowels and rebar. The question I have, which is probably a dumb one so I apologize already, is that to examine this new larger footing for shear and moment at the interface of old and new, we simply assume that through the epoxy and such they act rigidly together and we design it as a plain old spread footing?
I know we need the shear friction and the Plain Concrete stuff in Chapter 22 of ACI, but is there like an example out there for this type of situation, as I assume it's quite common?
Again I apologize for the rookie type question but this retrofitting stuff is still new and makes me appreciate those green field designs so much more!
Thanks
RE: Shear Friction
RE: Shear Friction
Frozenman if you add to an existing footing you have to be sure that bending and shear work across the full width of the new footing width. Likely what you will find is that the moment at the end of the epoxy dowels is larger than the capacity of the developed portion of the original reinforcement. You will have to embed dowels far enough that this condition is satisfied.
RE: Shear Friction
RE: Shear Friction
RE: Shear Friction
RE: Shear Friction
RE: Shear Friction
RE: Shear Friction
Why cant you just analyze the rebar like a bolt in shear?
RE: Shear Friction
RE: Shear Friction
Simply gluing the bar into the existing concrete is not sufficient unless you either take it a full development length with proper cover and confinement, or meet 318 Appendix D (as the only adopted standard for post-installed anchorage to concrete.) Consider that without sufficient embedment, the likely failure mode will be removal of a chunk of concrete as the two sides pull apart.
As for shear on the steel cross section of dowels, without sufficient clamping forces, pins fail in bending. Bolts fail in true shear because they provide their own clamping action. Nails and dowels fail in bending as the parts separate. Once you reach full development, deformed dowels provide good resistance to withdrawal. They could then would work in shear, but you would reach crushing loads on the concrete in bearing, resulting local failure of steel and/or concrete. Once concrete crushes and steel tries to bend through the crushed material, failure is common. This is one failure mode for hooked rebar where there is not sufficient embedment length (ldh) before the bend.