Slab options for two post lift

[ericm]

The concrete guy I want to use for my shop says he usually does a nominal 4" slab which comes out to 4-4.5" as he works to guarantee the 4" minimum. He says he normally puts in 3'x3'x8" footers where the two post lift will go. The lifts I want to use spec 4.5" depth. If you have to cut out a slab because its not deep enough they want a 4'x12'x6" footer. I want to make sure I meet the lift requirements but I'm also concerned about cracking due to differences in cross section. Another option is to ask for nominal 4.5" or 5" for the whole slab, which would be 40x56.

Which is best and most cost effective?

Thanks.

 

[ericm]

Oops I forgot to mention that I was going to ask for two bays to be two post ready, in case I move the lift from one to another.

 

[u2slow]

Find out the concrete specs for the lift you want. Also consider you get some blow-out on the underside of the slab from drilling.

The lifts I was looking at wanted 5", so I got 6" slab placed knowing that typical variation is +/- 1".

 

[f121]

My ramp guy wanted 6” so did the whole slab in 8” to be on the safe side, as I wasn’t sure where the ramp was going until the walls were up

 

[OX1]

I did 12" deep, 4' x3' on 5" pad about 12 years ago. No cracks

 

[Mattilac]

The key is to gradually ramp between different thicknesses.

Or do what I did and just pour the whole thing 6"+.

 

[mike93lx]

I'd probably do the whole slab at 5". Deep pours at the posts are not really helpful as long as the pad is thick enough

The bigger factor are your control cuts. You have to make absolutely sure that they are far enough away from where the lift will go.

 

[AC-WC]

1) follow BendPak concrete guide.

https://www.bendpak.com/media/wysiwyg/pdf/Concrete-Slab-Footer-Recommendations-BENDPAK-2023-Version2-Rev._B.pdf

2) Here's what I did for my retro-fit 3'X12'X1'. Technically I could have done 2 separate 3X3 pads but by the time I worked around all the expansion joints it was easier to just do 4 cuts. I used fiberglass rebar instead of steel. 50% stronger but 25% more cost than steel. They will never rust! Maybe spent $100 more in material but labor and time saved more than made up for it.

 

[ConCretin]

Thickened slabs have been used on just about every commercial and industrial slab we've ever done and I'm talking millions of square feet and I've never seen a crack. The key is to gradually transition the base 30 degrees or so rather than have an abrupt drop.

Thickening the slab under a lift just provides a little margin for error and make absolutely sure you have enough concrete depth given normal variations in base and top of slab elevations. If the concrete is 1/4" low and the base happens to be 1/4" high where you want to place your lift, you're down to 3 1/2" of concrete.

The lift's anchors only care how much concrete there is above the anchor not below so there is no need to go crazy. Thicken the slab under the lift to 5 or 6 inches and carry on.