Reinforcing Coverage

[ars001]

I'm designing a foudation for a metal building. I need to provide hairpins at a few column locations. Which is pretty typical. I have a 4" slab and need #6 hairpins. Do I need to follow the ACI limitations for concrete coverage and provide a minimum of 3"(ACI 7.7)or do slab on grades not fall under this provision? Its been awhile since I've done a metal building foundation but it seems to me that I use to put the reinforcing in the center of the slab.

 

[JAE]

If you have a vapor barrier directly under the slab, the 3" clear would not apply (i.e. you could theoretically use 3/4" but I would stick to 2").

For the top of the slab, you need 3/4" clear.

If you have to, you should thicken the slab to accommodate the required clearances.

 

[archeng59]

I put the hairpins in the center of the slab thickness. ACI 318, section 7.7 gives the MINIMUM clear cover requirements.

 

[ars001]

archeng59, I'm not sure if I'm getting what you are saying. So if you are getting a min. of 3" cover you must be using around a 6" slab.

 

[archeng59]

ACI 318, section 7.7 recommends a minimum 2" cover when the concrete is exposed to the earth. As JAE noted, that cover is applicable with the vapor barrier/retarder cast under the concrete instead of the 3" clear cover for concrete cast against the earth. The minimum cover is 3/4" for slabs with rebar smaller than #11 not exposed to earth. The hairpin can be placed in the center of the slab, or slightly above center, for the 4" slab and still satisfy ACI 318 minimum cover requirements. At least that's how I interpret things. 4" slab minus 2" clear minus 3/4" diameter bar (#6) minus regular slab reinforcing bar (#3 or #4 bar) equals 3/4" minimum top cover.

If you don't have the vapor barrier, the 3" clear cover is applicable. Thicken the slab to at least 5" so you get the minimum clear cover below the hairpin and adequate slab thickness to get the minimum 3/4" above the hairpin.