Will a crack prevent the footing from being able to support the load that is on it? If so, probably. If not probably not.
Why would the footing crack? If shrinkage, how big is the footing?
What other reinforcing steel is in it (if any)
I was looking at the ACI crack control measures per 10.6.4 where the rebars have to have the spacings given by two equations. My footing has a dimension 6.5'x6.5' and I was thinking about putting #6 @9" at bottom as required by flexure and #5 @ 12" for shrinkage.
Crack control is not required for the base of concrete footings. I have even seen this requirement waived for concrete with at least 500mm of earth cover.
Often a footing has to be relative thick to limit deflection for an unrelated reason. Since the intent is to distribute the applied loads UNIFORMLY to the soil under the footing, deflection is minimized (by making the footing thick - increasing the crossectional moment of inertia) to make this happen. Checks for both punching and beam shear often point to "thick" footings, also.
A "fringe benefit" for "typical" sized footings (like you are describing) is that cracking because of flexure is normally not a problem. Wise to check, however.
Why would you have different reinforcement in each direction in a square 2 way flexural footing? It should be reinforced like a 2way slab! And as it is symmetrical, so should the reinforceemnt be.
Or am I mis-understanding where you are putting the shrinkage reinforcement.
You have not said what the exposure is. If the soil contains anything corrosive, then yes, I would be checking for crack control. In fact, I would probably do it anyway as it could possible be permanently wet.
houseguy,
It is a flexural footing. They crack because they are flexural reinforced concrete and bending and shear stresses are induced in them from the pressure from the soil.
rob7204 - To reduce the likelyhood of field construction errors, rebar size and spacing for top and bottom mats is often made the same. Unless you have a lot (say dozens) of duplicate footings, the cost differential is minor. Also cutting down on the number of diffferent rebar sizes is usually a benefit to all parties (Owner, Engineer, Contractor, Rebar Detailer & Supplier, etc.)
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