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CRACK CONTROL IN FOOTINGS
6

CRACK CONTROL IN FOOTINGS

CRACK CONTROL IN FOOTINGS

(OP)
I am working on the design of a footing that has two way flexure. Do I have to check the footing for crack control?

Thanks,

Rob

RE: CRACK CONTROL IN FOOTINGS

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)

RE: CRACK CONTROL IN FOOTINGS

(OP)
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.

RE: CRACK CONTROL IN FOOTINGS

Hi rob7204

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.

Regards

VOD

RE: CRACK CONTROL IN FOOTINGS

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.

www.SlideRuleEra.net reading

RE: CRACK CONTROL IN FOOTINGS

2
Section 10.6 applies to "Beams and One-Way Slabs".  Footings don't fall under this category.

RE: CRACK CONTROL IN FOOTINGS

(OP)
Thanks guys. I really appreciate your valuable inputs.

Rob

RE: CRACK CONTROL IN FOOTINGS

rob704

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.

RE: CRACK CONTROL IN FOOTINGS

Also check out ACI 318, 10.5.4.

RE: CRACK CONTROL IN FOOTINGS

(OP)
rapt,

The reinforcement is same in both directions. I missed a word "top" in my previous reply, #5 @ 12 is at the top for shrinkage.

RE: CRACK CONTROL IN FOOTINGS

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.)

www.SlideRuleEra.net reading

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