Slab and truss question

[65cayne]

Central Oklahoma, 30'x45'x4" monolithic slab (long fiber mesh, 3500 PSI) with 12"x12" footer and 2x #5 rebar running around the footer, undisturbed clay soil with HARD pack about 12" down. Basically flat earth so no dirt work involved.

My plan was to compact a few inches of gravel on the surface, dig the footer, install the rebar, form, and pour. I will used 6x6 Simpson strong ties for each post. I am not burying the posts. Suggestions on the base/pour?


Second, I am planning on 7/12 attic trusses, 16" OC, which will provide a clear span rated at 40#/sf and meet code according to the company here. Is there any difference in using a triangular truss where the top/bottom cord meet at the eave versus a truss where the top/bottom cord meet at the top plate and the top cord hangs over the edge (12" eave) ? Hope that makes sense.

Thoughts/ideas?

Thanks,
-Kevin

 

[Falcon67]

We don't do dirt work here either. However, clay is expansive (got some of that too) and I would expect your local requirements to be similar to ours - bar 18" OC, footer, at least 2 ea 1/2" bar in the footers. I would also run a beam longways and a few across with bar in there. The more footprint in the ground the less likely it is to move around. you'll also want it up high enough that you can keep water from soaking around the footers because it will move the building.

 

[65cayne]

... bar 18" OC, footer, at least 2 ea 1/2" bar in the footers. I would also run a beam longways and a few across with bar in there. The more footprint in the ground the less likely it is to move around. you'll also want it up high enough that you can keep water from soaking around the footers because it will move the building.

Thanks Chris. What are you referring to with reference to the bar and beams? I assume on the beam, you mean running a trench/footer across the slab?

 

[Falcon67]

Yes - pic from my build. We have been pretty dry but in the last two years I can say that none of the standard shrinkage cracks have changed in size or shape. If you like I can find a copy of the local codes I used as a reference.

 

[lynnbilodeau]

Don't know where in central OK you are.

I am in Edmond, OK, and that footer wouldn't fly with the Edmond Municipal inspector. Here, they want the footer 12 inches BELOW the frost line.

Also, consider this: we have had many small earthquakes in the last few years (8 small ones last night). Code here calls for 24 inches deep on the footer. I did a room addition a couple years ago and because of the earthquake issue and the clay issue, went 32 deep and 24 wide on the footers except for the corners where I went 36 by 36 by 36. I also used 5/8 bar, and used double what the code called for. It was probably overkill, but seriously, if you are doing the work yourself, what is a few hundred dollars worth of concrete? Worth the peace of mind to me.

 

[teamextreme]

I would recommend checking with your building dept on the limits for a monolithic slab as well. In my city in Colorado you can't build anything with a mono slab over 600 sq ft.

 

[Bruce4310TX]

dont spend any money on fiber mesh it doesnt do anything put the money into rebar and footers and beams.

 

[ConCretin]

I agree with those who suggest checking with your local building authorities to see what if any requirements they might have.

The most important aspect to any slab is the base. I'm not familiar with soils in your part of the country but up here we wouldn't build on clay. The base material should be stable and well draining and clay can be neither. Check around to see what local practices are.

I wouldn't worry too much about frost lines especially if it's unheated. A monolithic slab floats on the frost by design.

Lastly, I concur with the suggestion to ditch the fiber and use steel reinforcing in the slab in addition to the perimeter beam. Because your slab is 'floating' on the soil, it can move around a bit. Properly supported rebar or mesh will help hold everything together - fibermesh won't.

I have a 28 x 32 garage built on a monolithic slab (12" perimeter beam with 3, #5's and and a 4" slab with #4's at 18" centers). We get 4' of frost up here and it's held up beautifully.

 

[Falcon67]

We have zero frost line here and if the OP is very close to the Red River, it's likely the same. I see where OKC requires a min 18" .

"Footing and Foundation Plan - Footing details or foundation plans should show what type of foundation is proposed and clearly show that it will comply with the minimum frost depth of 18". "

That's only 6" more, might as well get it all and be done with it. A sound foundation just makes everything that's put on it last longer and require less maintenance.

On your eves - I personally like eves as they shade the side walls a bit and push rain water out away from the wall. I came out about 12" to match the house and it's working well here.

 

[MrV]

Here in central texas we dont worry too much about freezing. Despite that,I went with perimeter beam that was 2 foot by 1 foot wide reinforced with 5/8 rebar. Also similar beams crossed in both directions meeting in the center (25X30 slab) and the slab itself had 1/2" rebar on one foot spacing. This was probably way over engineered for what i have but the slab was on a sloping grade (sloped about 2 feet from front to back) so I wanted to make sure it didn't go anywhere.

dean

 

[65cayne]

Don't know where in central OK you are.

I am in Edmond, OK, and that footer wouldn't fly with the Edmond Municipal inspector. Here, they want the footer 12 inches BELOW the frost line.

Also, consider this: we have had many small earthquakes in the last few years (8 small ones last night). Code here calls for 24 inches deep on the footer. I did a room addition a couple years ago and because of the earthquake issue and the clay issue, went 32 deep and 24 wide on the footers except for the corners where I went 36 by 36 by 36. I also used 5/8 bar, and used double what the code called for. It was probably overkill, but seriously, if you are doing the work yourself, what is a few hundred dollars worth of concrete? Worth the peace of mind to me.

I've been here 20+ years (moving from Massachusetts)...I assume 12" inches below the frost line means I have to dig down about 13" ?

Seriously though, those are valid points. Lots of "piece of mind" for not much more $$. I'll have to call the building inspector because I have looked and looked and cant decipher the codes without making many assumptions. The continuous references to section after section after section make me wonder if there is any valid info in there at all . Seems riddled with circular references.

I thought there was a difference in requirements between a dwelling and a barn.

 

[65cayne]

On your eves - I personally like eves as they shade the side walls a bit and push rain water out away from the wall. I came out about 12" to match the house and it's working well here.

I love eaves. They make all the difference aesthetically and I personally would not construct a building without them. What I am referring to though is the truss construction. I can get a truss where the top/bottom chord (cord?) meet at the eave, OR where the top/bottom chord meet at the top plate (wall) and the top chord continues over the wall to form the eave. The former would give me another 6-8" of ceiling height in the attic space and would not require additional framing for the soffit.

Just wondering what (if any) people's preference was and if there is any difference structurally.

Thanks for the input everyone.
-Kevin

 

[65cayne]

Re: Slab and truss question (follow up)

So I called the city and low and behold they do require an 18" depth on the footer and because I am using attic trusses it is technically a two story and thus requires a 16"x16" footing with 2x#5 rebar.

Question, with 6"x6" posts....would it make more sense to dig a hole for each post and install concrete columns/piers to support my posts and then pour a simple 4" slab? Or should I stick with the original plan and do a monolithic slab ?

I dont see an advantage of one over the other so I am now in the process of calculating cost of one method versus the other.

-Kevin