Greetings, here's my idea for building my future home and my questions are pertaining to any potential freezing and possibilities of combining footings here and there under the floating slab. I just can't find info anywhere that helps me understand better. House would be 40' x 56' on concrete piers calculated to be 18" diameter and 20" diameter. I calculated entire totals of weight bearing at each pier and want to add either a few more piers in the middle of floating slab to hold the 2nd floor mezzanine. 40' across and 20' wide each mezzanine at each end. spanning 13ft each of 3 beams with floor joists going 20' to gable ends so each quadrant would be 13x10x50lbs/Linear foot making each pier load of about 6500 lbs. the floating slab: remove organic layer, compact 8" granular A gravel, vapor barrier, 2" rigid XPS under the slab and also vertically around 4' straight down against the grade board. Ottawa frost line is 4'6". concrete slab having a bag of fiber mesh/cubic meter. so the question is....with the slab also radiant floor pex inside the 4" and perimeter insulation straight down... the slab should never come close to ever freezing? therefore wouldn't any footings or piers placed within (under) the floating slab only need to go down 12" instead of crazy deep like the perimeter piers going around 6'? so maybe i'd make each pier 20" diameter and 12" deep. With soil being 3000 psi that makes the pier 6540 lbs load bearing. Another question I have is that I haven't got a clue how much weight a 4" floating slab can actually hold. Probably order the concrete to be 3000 psi but does that actually mean if I put an interior wall it could hold the mezzanine (worst case scenario 50lbs L and Dead load) 10ft (half of floor joist or tributary width). I found these charts which show 2x4 and 2x6 load bearing walls and spacing. https://www2.owenscorning.com/around/sound/loadbearing.asp but my other question would be....maybe just through some rebar for 16" wide that strip under the interior load bearing wall and make the concrete maybe 8" there instead of 4". its that or just go with the piers 20" and put beams spanning the piers to hold the floor joists. Nevermind comments about P.Eng. stamps etc. I'm just learning here and of course any plans I make will be reviewed and stamped. thanks for any POSITIVE feedback.
floating slabs residential
- Thread starter tserkva
- Start date
dcg9381
Well-known member
We see questions about loads and mechanical engineering all the time and I'm 100% certain we have some structural and mechanical engineers here, but you generally won't see those questions answered when it gets this specific.
What's complex in your case is it's not just about the load on the slab from the mezzanine, it's that (in my experience) ALL of foundation engineering starts with what is under the foundation that is TBD by a soils test... Without that test, you can't engineer squat or answer any questions about how things should be built...
I think your questions CAN be internet answered if you're asking about wall loads, joists, spans for the mezzanine, even roof loads...
But not foundation engineering... Not without knowing what's under it.
I've got a house that was built with a 4-5" slab, 24" perimeter beams (pretty common here). But the garage, damn thing needed 12-16 piers over 10' deep. Why? Because the soils test came back saying that what was under the garage was not terribly stable.
Others will have good design ideas on insulation, heating, and freeze.. I'm in the south, we just throw down a moisture barrier and call it good. No one heats the floor.
wssix99
Well-known member
Your solution is going to be straight-forward:
- The load bearing wall idea in in the Owens-Corning example is going to be crazy expensive for you, because you'd have to have an additional footer under the load bearing wall. A beam over columns and piers should be much more economical. (Per your question above, the concrete floor doesn't hold any weight. It transfers load to the ground below. ...This is why you will need the foundation.)
- Yes, your piers inside the foundation still need to go to the frost line. You can't 100% count on your heating and insulating system working and the structure needs to be durable in the event the house were to be unplugged. The soils at the frost line are also going to have a higher bearing capacity, which you will probably need.
Your footing and piers will not be tied into the floating slab. They will all be isolated from the slab with isolation joints. (Using materials similar to those that section off sidewalks.)
The penetrations from the piers inside the foundation will induce cracking in the slab, so the piers will be poured in a diamond shape and your saw-cut control joints will radiate out from those piers:
The floating slab will crack (as it should) along the control joints, the floating floor will be free to move around the diamond-shaped piers and your radiant system will be happy and normal.
- The load bearing wall idea in in the Owens-Corning example is going to be crazy expensive for you, because you'd have to have an additional footer under the load bearing wall. A beam over columns and piers should be much more economical. (Per your question above, the concrete floor doesn't hold any weight. It transfers load to the ground below. ...This is why you will need the foundation.)
- Yes, your piers inside the foundation still need to go to the frost line. You can't 100% count on your heating and insulating system working and the structure needs to be durable in the event the house were to be unplugged. The soils at the frost line are also going to have a higher bearing capacity, which you will probably need.
Your footing and piers will not be tied into the floating slab. They will all be isolated from the slab with isolation joints. (Using materials similar to those that section off sidewalks.)
The penetrations from the piers inside the foundation will induce cracking in the slab, so the piers will be poured in a diamond shape and your saw-cut control joints will radiate out from those piers:
The floating slab will crack (as it should) along the control joints, the floating floor will be free to move around the diamond-shaped piers and your radiant system will be happy and normal.