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Structural Plain Concrete

Structural Plain Concrete

Structural Plain Concrete

I am checking a slab founded on rock loaded by steel strand anchors for two way shear (punching shear). Analysis shows #9 bars @ 12" in both directions are required to resist flexure. This does not meet the requirements of ACI Eqn. 10-3 for min reinforcement of flexural members but does satisfy ACI 10.5.3.

The question I have is should this slab be considered as Structural Plain Concrete since it does not have minimum reinforcement? By definition, plain concrete is structural concrete with no reinforcement or with less reinforcement than the minimum amount specified for reinforced concrete. I am unsure if by following ACI 10.5.3 does this then classify the slab as reinforced concrete?

RE: Structural Plain Concrete

I think a sketch would get you more responses. If you have a slab on rock, how do you have punching shear?
If #9's at 12 inch doesn't meet ACI 318, how thick is the slab? Back figuring, that comes out to about 2'-3". And the minimum of 10-3 can be exceeded if you have 1/3 more reinforcing than is required by analysis.
But to answer your question, since the slab is on rock, it shouldn't see bending. So the bending reinforcing minimum doesn't apply. I would call it reinforced concrete as long as it meets the temperature and shrinkage reinforcing limits.

RE: Structural Plain Concrete


Thank you for your response. I have attached a picture of the structure for clarification. I understand your question of punching shear when the slab is on rock, however, the forces (P) are very high, in the order of 1900 kips (unfactored). The thickness used for analysis is 4.5' and the remaining is used for cover. The situation to consider, as you pointed out, is will the slab "bend?" If not, then this question resolves itself.

RE: Structural Plain Concrete

If the rock can carry the loads within the punching shear outline, than you have no bending (and no shear either, just bearing). It looks like your shear cones overlap, so I suspect that's the case. If you need to activate the whole slab to resist the downward forces (the shear cones are not enough), then you'll have bending, although with a 54 inch deep section, it should be pretty easy to reinforce.
Doing a rough calculatrion, the shear cone would have a perimeter of at least 9' x 9'. So 1900 kips divided into 81 square feet is 23 ksf, unfactored. What does your Geotechnical Report say about the rock capacity?

RE: Structural Plain Concrete

The issue arises that, in plan, an anchor is located 5' from the edge of the slab on two sides. This does not allow us to use a method similar to that of punching shear for a footing because d/2 is outside of the outer block. WRT the rock capacity, I will have to speak with my Geotech section.

RE: Structural Plain Concrete

Unless you have really poor rock, fissured, fractured, or whatever, 23 ksf does not seem very high... we have limestone at about 60 ksf, in these environs... likely little flexure... also, up to TS, it is common to increase the flexural load by 1/3 and use less than flexural rfg... If it works in bearing, there should be lots of reinforcing... might even cut some out!


RE: Structural Plain Concrete

Due to a fault located in the rock, I was told to assume that the rock is compressible.

RE: Structural Plain Concrete

Yes, everything is compressible. Ask him or her what capacity it has.
Deflection is a seperate issue. With that thick of slab, everything is going to deflect equally, so it's just whether you can live with some overall deflection.

RE: Structural Plain Concrete

Thank you all for your comments. You have been of great help.

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