Shear Friction 1

[ztengguy]

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?

 

[JAE]

Smooth dowels won't work. Reinforcing bars are necessary to develop shear friction. You don't get shear friction without them (by code). Dovetail key would require tension in concrete to work. Not a good idea.

 

[Frozenman]

Hi,
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

 

[dcarr82775]

The concept of roughening the concrete only provides shear transfer because the steel locks the two surfaces together. That is why controlling state is tension in the bar for the roughened surface. Shear keys will work just fine, but you still have to hold the two sides together.

 

[ron9876]

When you use a shear key you have to check the components of the key to be sure that they work for shear and bending. So it depends on how large the shear is and the size of the key.

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.

 

[ztengguy]

Yea, we are finding the same that bars are required. we have a thin section (6") and getting a bent bar developed (6" min for hook) we have to thicken the bottom section for development into that. We are using AASHTO code.

 

[Splitrings]

ACI 318 has a section on shear friction. I don't have it in front of me. I believe it states both ends of the bar need to be developed in order to resist the shear forces.

 

[Lion06]

Yes, both ends do need to be developed - 11.7.8

 

[ztengguy]

Yes, I understand that both ends have to be developed. I am looking to do it without reinforcing, or some other method to eliminate thickening the base to get the reinforcing developed.

 

[Lion06]

The key won't give you any shear friction, because there is nothing to create the clamping force.

 

[abusementpark]

I've used the shear friction method extensively, but I have always wondered:

Why cant you just analyze the rebar like a bolt in shear?

 

[ztengguy]

On thing we have looked at is using a concrete insert, and develop a bar into the second pour. If I can calculate how much steel I need, surely I can calculate how much capacity i need in the insert.

 

[TXStructural]

It's not shear friction without 1) fully developed reinforcement crossing the joint, and 2) sufficient friction, created by a roughened surface.
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.