2 Post Lift and Control Joints

[loosegoosegarage]

30x50 slab was just poured with 12" deep digouts for 2 post lift.
They were going to do 1 control joint long way (50') and 4 across (every 10' from front to back) which would have ended in 10' x 15' foot sections.
They ended up switching to 2 joints front to back, still 4 across to create 10' x 10' sections, which I do prefer.
However, I had them skip the 10' section that would run in between the 2 lift columns, as I swore I had read that you did not want a seam in between them.
Now I am second guessing that since it is 10' x 20' which is larger than recommended and I don't want to risk it cracking.

Is it safe to cut a control joint in between 2 post lift columns? Not under the base plates, think of down the middle where you would pull a car into lift.

 

[wssix99]

Yes, it is safe to have a control joint between the posts. You should treat the saw cut joint as a "crack" regarding the guidance in the instructions. As long as you have maintained the proper distance from the control joints required from any other "crack," you are fine.

Your biggest challenge at this point is deeper pours under the posts. The lift instructions do not mention this for good reason. The practice is problematic and can cause cracking.

The deep sections will act as anchors in the room and as the slab shrinks, it could pull "away" from these posts and crack around them. Having proper control joints between the posts becomes even more important so the slab can pull away towards the posts.

 

[firebirdparts]

Any rebar? I think a crack right in the middle is far preferable to a crack close to a post, but I don't have a saw cut between my posts. I probably wouldn't cut it, but I can see the wisdom.

 

[Eddiear]

Thanks for posting this I had the same exact question for a slab being poured next week.

 

[Jetfixr320]

Yes, it is safe to have a control joint between the posts. You should treat the saw cut joint as a "crack" regarding the guidance in the instructions. As long as you have maintained the proper distance from the control joints required from any other "crack," you are fine.

Your biggest challenge at this point is deeper pours under the posts. The lift instructions do not mention this for good reason. The practice is problematic and can cause cracking.

The deep sections will act as anchors in the room and as the slab shrinks, it could pull "away" from these posts and crack around them. Having proper control joints between the posts becomes even more important so the slab can pull away towards the posts.

I don't recall seeing anything about having a thicker area being a problem.

 

[finn]

I don't recall seeing anything about having a thicker area being a problem.

I do recall reading that in one of the manuals it was stated locally thickened slabs aren’t recommended because of potential residual stresses that can lead to cracking. Specifically, if you do thicken, it should be the full width, and long. Stay away from thicker in just the area of the legs.

What is more important is to have a relatively large one piece slab to prevent tipping of the posts. Even a 4” slab is sufficient to support the weight.

 

[mikedodge]

The instructions usually note that the colums have to be a minimum distance away from cracks, cuts and perimeter, doesn't matter where the cuts are beyond that.