Structural Slab Design -- Need Help

[nolimits76]

I'm helping a buddy with a lake house project.

No building codes are being enforced!!! Freeze table depth is 20". He desires a concrete slab on grade, 4" thick. Slab is L-shaped. Construction will be modest (frame siding, trusses, etc). He is trying to self-perform as much as possible, and play general contractor for the rest.

Can someone provide a little guidance on the slab?

My initial thoughts was a 1' x 2' perimeter footing. Slab would be 4" thick w/ 12" thickened edges to transition to the footing. Use fill sand under the main slab w/ waterproofing. Is this sufficient? Better ideas?

I am not 100% certain on reinforcing use. I'd say #4 on 12" centers both ways for the main slab, which would allow min. 1.5" clearance from top & bottom. Not sure on slab edge reinforcing sizes/spacing/layout, nor the footing itself. Seems logical to tie it together so using an L bar to connect footing and slab.

 

[kd3pc]

no mention of the soil type? Dry? Compaction OK? Stable? Etc.

Regardless of code enforcement or not, one needs to keep physics and engineering in mind.

The "footer" needs to go below frost line or the pour will sit on top (float),
float seems to be what you are attempting, but the ability to "float" depends on the characteristics of my first line above. If those are weak or not achievable, then you need to drop back to a footer/floor monolithic, or footers done, then back fill to leave 4"+ for concrete to go inside that. Footer needs to go below the frost line to prevent heaving

Once this is decided, then you get to the "reinforcement", as without the proper foundation, reinforcement will do nothing for the integrity of the pour.

Others will chime in, choose wisely....
...it may be prudent to speak with a local concrete mason/team to see what is "normal" in your area.

 

[nolimits76]

Thank you for a quick and honest response, much appreciated.

I also asked about soil when we first started discussing this. I was told "somewhat rocky". His property is near Grand Lake, OK (NE corner). Attached is a soils map I found (dated 2011) and I put a white box up in the vicinity.

My company builds bridges and we do drilled shafts. We've encountered some hard stuff in the area. But according to my buddy, he had hired out a guy to dig his septic and came in UNDER budget and added EXTRA work giving him (and me) the indication it wasn't as hard as originally anticipated.

But no real engineering data is available. I am used to boring logs, geotech reports, etc. I don't have that.

That being said -- I put together a quick sketch of what I think you mean in regards to the footers and slab. The new design would be monolithic and "floating" in a sense. Is that also what you envisioned?

 

[readhead]

Your sketch numbers don't add up. You need 6"-8" above grade and a couple of inches below frost. That would be about 30".
I would suggest two #4 in the turndown, one high and one low and #4 24" o.c. in the slab with 3500 psi mud. I would probably add a 6 mil vapor barrier under the slab.

 

[kd3pc]

readhead has pointed out the correct numbers....and I concur on the turnndown iron

Where we are, undisturbed soil, then some clay/sand mix (depending on the base soil) compacted, then gravel compacted, then vapor barrier or foam insulation - if you are going to heat the slab.

Then concrete.

 

[ConCretin]

First off, you are not building a structural slab - the slab you describe is ground bearing. A structural slab is designed to span between supports.

With regard to frost. Will the structure be heated? Are you building it as a frost protected shallow foundation where insulation will keep the frost from getting under the slab? If you answered 'no' to either question, then the slab will 'float' on the frost so the depth of the perimeter beam is irrelevant.

Nothing wrong with this as long as you're building on a well compacted, free draining granular base. If the soils under your structure don't retain water, then there's nothing to freeze and heave.

I've built many such structures and they work just fine. I would suggest #4 bars at 18" centers for the slab and 2, #5 bars in the bottom of the haunch. I'd construct the haunch to a total depth of 12" with a width of at least 8" excluding the taper. I like the idea of some bent bars around the perimeter that turn down from the slab into the haunch.

I also like the idea of a concrete or CMU curb wall around the perimeter to lift the wood framing above the exterior grade and create a stronger perimeter beam to support the structure. If you do it out of concrete, stand a 24" form, place the slab and then stand a 12" interior form and place the curb. Tie the curb wall to the slab with dowels and add a couple longitudinal bars and anchor bolts in the top of the curb.

 

[OHSCrifle]

Your sketch numbers don't add up. You need 6"-8" above grade and a couple of inches below frost. That would be about 30".
I would suggest two #4 in the turndown, one high and one low and #4 24" o.c. in the slab with 3500 psi mud. I would probably add a 6 mil vapor barrier under the slab.

Want to repeat what readhead said. Any wood siding needs to be at least 8" higher than final grade outside the house. This rule exists where codes exist, so wood siding doesn't rot when rain splashes up on it.

 

[radrush]

But no real engineering data is available. I am used to boring logs, geotech reports, etc. I don't have that.

That being said -- I put together a quick sketch of what I think you mean in regards to the footers and slab. The new design would be monolithic and "floating" in a sense. Is that also what you envisioned?

Engineering data is WAY over rated so we don't use it any more...and that geotech stuff is borderline voodoo! (just kidding)

But seriously, in your sketch I think that 45-degree angle under the transition from turndown to SOG is too sharp and the detail should probably show a vapor barrier.

 

[radrush]

Here's another similar detail.

 

[radrush]

No building codes are being enforced!!!

That's an odd statement. At a minimum the owner can enforce whatever "code" he wants regardless of what the county/state enforces. The owner pays the bills so nothing can happen without his say so.

This is why I prefer the "code+1" contract.

 

[nolimits76]

Thanks for the input everyone.

To clarify, I messed up on my sketch with the dimensions. I do realize you need 8" from ground level to top of slab. Add to the 20" frost line and that is minimum 28". If you take it 4-6" down below frost line, the correct dimension would read 32-34". In reality, I'd shoot for 36" total.

Also, radrush, you are correct -- the owner can enforce any codes he wants. This is just unique because I've never been associated with a project that doesn't have some city or county code to follow.

Lastly, LLWillysfan, sorry about the mislabeling of my thread title. You are correct, it would be ground bearing. In OK, it's pretty rare to see heated floors. My buddy hasn't mentioned them so I assume there will NOT be any. However, he has mentioned putting some insulation under the slab. I haven't done this before myself, so maybe you can explain how that will change things?

 

[Daniel Dudley]

First off, you are not building a structural slab - the slab you describe is ground bearing. A structural slab is designed to span between supports.

With regard to frost. Will the structure be heated? Are you building it as a frost protected shallow foundation where insulation will keep the frost from getting under the slab? If you answered 'no' to either question, then the slab will 'float' on the frost so the depth of the perimeter beam is irrelevant.

Nothing wrong with this as long as you're building on a well compacted, free draining granular base. If the soils under your structure don't retain water, then there's nothing to freeze and heave.

I've built many such structures and they work just fine. I would suggest #4 bars at 18" centers for the slab and 2, #5 bars in the bottom of the haunch. I'd construct the haunch to a total depth of 12" with a width of at least 8" excluding the taper. I like the idea of some bent bars around the perimeter that turn down from the slab into the haunch.

I also like the idea of a concrete or CMU curb wall around the perimeter to lift the wood framing above the exterior grade and create a stronger perimeter beam to support the structure. If you do it out of concrete, stand a 24" form, place the slab and then stand a 12" interior form and place the curb. Tie the curb wall to the slab with dowels and add a couple longitudinal bars and anchor bolts in the top of the curb.

Great post. ^^^^^