Moment frame restraint in "unrestrained" slab?

[vato]

I'm designing a stem wall foundation with an interior slab, 50'by70'. I need to restrain the moment frame thrust between piers. It's 25kips. I am proposing to tie the piers together with #5's across and through the slab. Everywhere but at the piers the slab edge has an expansion joint. I'm concerned that I am creating some cracking with some of the slab tied into the foundation and the rest free to expand and contract. I'm using .2% reinforcing in the slab based on temperature. Trying to keep this simple without a separate bond beam under the slab or fancy joints parrallel to the tension bars. thanks.

 

[ishvaaag]

Without seeing the particular of the setup, and since the load is moderate, my recommendation is to examine if both the introduction of the load and the load paths result in some reasonable behaviour. If so, you may retain your idea, otherwise change to either a structural slab without joints (I would then recommend doubly reinforced) or your separate grill between foundation hardpoints.

 

[msquared48]

Don't place the tie beam integral with the slab. Instead, install the tie beam beam under the slab to serves as the tension tie between the piers. That way the slab will still be unrestrained, but the piers tied.

Mike McCann
MMC Engineering

 

[vato]

If I'm tied through the slab at the piers, with no construction joints across it, then won't that area just show fewer cracks than the unrestrained portions? I'm talking micro cracking, nothing severe. I am leaning toward separating the tie beam and slab. It is really nice grabbing the anchor bolts at the slab level with the ties, but it could be lower.

 

[msquared48]

The plinth should be able to transfer the 25 kips with no problems to the tie beam if the top of the beam is only 6" to a foot below the bottom of the slab. You can still bend the tie beam steel around the anchor bolts.

I do this a lot for Horse arenas where there is no slab. I just bury the tie beam a couple of feet and it works fine.

Mike McCann
MMC Engineering

 

[vato]

Thanks Mike,
I had a meeting with the contractor this morning, leaving this option out there as a possibility, and I'm drafting it up now before we issue. So now I'm putting a control joint over this area. I'm a little concerned with the rest of the slab settling over time in relation to these tie beams. I imagine the slabs rotating in opposite directions over the stiff beam. These will be the high areas between drains.

 

[dcarr82775]

I see the tie steel in the slab-on-grade crossing those 1/2" joints all the time. It is commonly done, and 25 kips isn't much. If your only concern is shrinkage and temp in the slab I wouldn't be too concerned. If you have expansive soils I would go with the tie beam.