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Footing Cover

Footing Cover

Footing Cover

(OP)
Gentleman,
What would be the minimum depth for the footing of a wall?  Could it be 8"?  Footing is directly cast againist earth.  I am looking at ACI318 req. of 6" of cover ABOVE reinforcing.  So if I use a #5 bar, with the req. 3" of cover on the bottom and 6" on the top, I find 8" impossible.  Am I missing something?
Thanks!

RE: Footing Cover

(OP)
Let me also clarify and say that this is a privacy wall, not a wall for a building....

RE: Footing Cover

If you have determined that the local building code references ACI 318, then I would say you are correct. For your example, it would seem you need at least 10 inches.  Have you also considered overturning in the footing design?
(Even if ACI isn't refernced, it seems like a good idea)

RE: Footing Cover

(OP)
The footings are strip footings 24" to 36" in width.  Calculation wise 8" will work, the wall is 6 to 8 feet in areas.  The building code is FBC which references ACI 318.  In ch 15 of ACI, it states that the provisions are good for "Isolated" footings, I'm not sure that my "strip" wall footing is an isolated footing.

RE: Footing Cover

I am not sure they meet Code, but 8" X 16" wall footings are used A LOT.  For design, they are generally considered unreinforced, so perhaps that is how they meet Code.

DaveAtkins

RE: Footing Cover

What about frost heave??  Around here - St Louis - you should be at least 30'' deep.

RE: Footing Cover

(OP)
I think most 8"X16" wall footings have vapor barriers under them allowing for a 1-1/2" cover? 1.5+5/8+6= about 8"?  And they are typically perscriptive as well.  Agreed that a plain footing would not be required to have bottom reinforcement and therefore the 6" top cover would not apply.  6" top cover makes sense though, they are in effect saying that the minium d=6"+1/2 bar dia.  I think I will go with 10" as being the minimum h for the wall.

RE: Footing Cover

(OP)
Florida is only 12" below grade for frost.  These guys use an 8" stem covered with dirt so the bottom of the footing is about 16"-20" down.

RE: Footing Cover

I would go with the cover at bottom, bar diameter, plus hook development length.  That's at least 10".

RE: Footing Cover

Florida has a minimum frost depth!? Code officials with too much time on their hands.....

RE: Footing Cover

I have never heard of a vapor barrier under a wall footing.  Are you talking about a thickened slab edge?

DaveAtkins

RE: Footing Cover

I've never heard of vapor barriers under wall footings either.  Does the footing project far enough from the wall to require transverse flexural reinforcing to resist the overturning moment from the wind loads?  

RE: Footing Cover

3" of concrete below reinforcement is not used for structural calcs, it is assumed contaminated and below the required strength.

RE: Footing Cover

(OP)
The vapor barrier reference was for houses in which the 8"X16" footings are so often used.  I wasn't implying that a stand alone wall like mine would need or have one.  Just recognizing the difference in cover requirements for the different situations.

RE: Footing Cover

Still never heard of a vapor barrier under a separate footing.  Under a thickened slab edge per DaveAtkins...yes.

I don't believe the concrete is not used because it is "contaminated"  The 3" coverage is for protecting the rebar. Yes, the concrete in tension below the effective rebar depth (below the neutral axis, for that matter) is not considered in the analysis.

RE: Footing Cover

(OP)
I guess I have never paid attention on a stem wall pour.  A mono one yes.  Where would the vapor barrier end on a stemm wall foundation then?

RE: Footing Cover

(OP)
Around here (central Florida) we use a chair block and pour the slab with the wall though.

RE: Footing Cover

Typical central Florida residential has the vapor barrier stop at the stem wall.

RE: Footing Cover

We always use 12" thick footings, and never get any argument.

Just to be clear (pun intended?) the 6" requirement is above the bottom reinforcement, not necessarily clear cover for top reinforcement.  For example, if you have top steel in a footing to resist uplift, the cover is not required to be 6".

RE: Footing Cover

Sorry, I think this is another regional thing :)  What is a "chair block"?

RE: Footing Cover

Thanks Scott.

For that condition, I think I would lap the vapor barrier into the block ~2"

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