Footing Cover

[scottiesei]

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!

 

[scottiesei]

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

 

[JKW05]

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)

 

[scottiesei]

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.

 

[DaveAtkins]

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

 

[MiketheEngineer]

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

 

[scottiesei]

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.

 

[scottiesei]

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.

 

[UcfSE]

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

 

[csd72]

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

 

[DaveAtkins]

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

DaveAtkins

 

[JKW05]

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?

 

[civilperson]

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

 

[scottiesei]

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.

 

[JKW05]

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.

 

[scottiesei]

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?

 

[JKW05]

Scott, check this out as an example....

http://nrmca.org/aboutconcrete/cips/29p.pdf

 

[scottiesei]

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

 

[UcfSE]

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

 

[PMR06]

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".