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Shear Friction Old Conc to New Conc

Shear Friction Old Conc to New Conc

Shear Friction Old Conc to New Conc

(OP)
I have to provide a pipe anchor with 1500 lbs of shear force on to an existing seal slab (4" thick no reinforcing) that is under grating that construction cant access with equipment to bust out the concrete. Using ACI 318-11 do I have to provide shear reinforcing to gain any resistance or can I just count on the shear bond strength of the old concrete intentionally roughened to the new poured concrete?



R11.6.3
The relationship between shear-transfer strength and the
reinforcement crossing the shear plane can be expressed in
various ways. Equations (11-25) and (11-26) of 11.6.4 are
based on the shear-friction model. This gives a conservative
prediction of shear-transfer strength. Other relationships
that give a closer estimate of shear-transfer
strength11.16,11.45,11.46 can be used under the provisions of
11.6.3. For example, when the shear-friction reinforcement
is perpendicular to the shear plane, the nominal shear
strength Vn is given by11.45,11.46
Vn = 0.8Avf fy + AcK1
where Ac is the area of concrete section resisting shear
transfer (in.2 ) and K1 = 400 psi for normalweight concrete,
200 psi for all-lightweight concrete, and 250 psi for sandlightweight
concrete. These values of K1 apply to both
monolithically cast concrete and to concrete cast against
hardened concrete with a rough surface, as defined in 11.6.9.


11.6.5
For normalweight concrete either placed
monolithically or placed against hardened concrete
with surface intentionally roughened as specified in
11.6.9, Vn shall not exceed the smallest of 0.2fc′Ac,
(480 + 0.08fc′ )Ac and 1600Ac, where Ac is area of
concrete section resisting shear transfer. For all other
cases, Vn shall not exceed the smaller of 0.2fc′Ac or
800Ac. Where concretes of different strengths are
cast against each other, the value of fc′ used to evaluate
Vn shall be that of the lower-strength concrete.

RE: Shear Friction Old Conc to New Conc

I would carve out one or more keyways in the old concrete. If the new concrete will be confined between opposing faces, this would be sufficient. If there is no way to trap the new concrete tightly against the old concrete, there will not be much slip resistance without shear friction reinforcement. HOWEVER, you cannot sufficiently develop any sort of bar perpendicular into a 4" slab.

If you can swing it, maybe core drill or hammer a series of 4-6 inch holes through the slab to create anchors. Extend the holes deep enough to embed reinforcement sufficiently and grout into place.

RE: Shear Friction Old Conc to New Conc

1500 lbs is nothing. You could do a few things:

1) Look it as shear friction but only use the self weight as the clamping force. Essentially, this is just plain old Leonardo da Vinci friction.

2) Take your shear out in active / passive pressure against any adjacent soil and forget about the seal slab.

3) Install some dowels into the existing concrete and analyse it as true dowels rather than shear friction.

4) Detail it like a building housekeeping pad and never think of it again.

I'm flying a bit blind here as I don't fully understand the situation. I'm picturing steel pipe support fastened to new concrete slab of some thickness poured over 4" mud slab? I'm not an industrial guy.

In the Canadian code, you can use adhesion for shear friction resistance even if you don't have any reinforcing. I don't trust it yet though, particularly if the loading has a dynamic component to it.

As TX structural pointed out, ACI technically gives you nothing for shear friction if your reinforcement isn't fully developed on either side of the shear plane. In practice, engineers often cheat this and prorate the capacity based on the percentage of development length achieved.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

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