ACI 318-14 Sect 22.9.4 - Nominal Shear Strength

[hemiv]

Does this section apply to cases where concrete is poured against concrete without any reinforcing across the joint? Say, a slab poured against a concrete retaining wall? That is a shear plane, and I'd like to considered the ability for this plane to transfer load.

 

[JAE]

No - it only applies where you have reinforcement across the joint as stated in 22.9.4.1.

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[hemiv]

That section in my ACI copy, 318-14, just says "Value of Vn accross the assumed shear plane shall be calculated in accordance with 22.9.4.2 or 22.9.4.3. Vn shall not exceed the value calculated in accordance with 22.9.4.4"

It mentions minimum shear-friction reinforcement areas, but doesn't say anything about shear across unreinforced areas. I suppose this value is normally ignored. In my case, I have a slab at the bottom of a hole with concrete walls. I would like to count on some unreinforced friction to resist the possibility of small uplift forces.

 

[JAE]

Section 22.9.4.1 references the two other sections that explicitly define the amount of shear you can estimate based on reinforcement areas. There's limits on those two values based on concrete area but there is no shear friction where you don't have reinforcement per ACI and this section.

Concrete shrinks so a slab "against" a wall may not really be tight to it for the long term.

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[hemiv]

Ok, fair enough. Makes sense to me, thanks!

 

[KootK]

I would like to count on some unreinforced friction to resist the possibility of small uplift forces.

If the retaining walls push into the slab such that you had reliable compression in the slab under uplift, you could use that compression in place of the clamping reinforcement required by ACI. Estimating the reliable compression in this case would be a pretty difficult task, however, so I'd be inclined to find another solution. Chances are that you're dealing with a four sided shaft where the walls may be spanning horizontally rather than vertically anyhow.

Is the slab existing?

 

[hemiv]

No, slab is not existing. Circular shaft with secant pile walls. I'm going with dowels to the wall at this point. Although it would be nice to count on friction, I think JAE is right about shrinkage.

 

[KootK]

Oh yeah, dowels all the way.