Planning new building, slab foundation questions

[misterfixit]

Been a while since I've posted here, been reading though. My last build was our home "Barndominium", and then just a few years later we added on. That is now our home. The built in 'garage' of the Barndo has been outgrown and it's time to build a separate dedicated shop building. For some particulars, I will be building in Kaufman County Texas, basically next door to Dallas. I have about 30 acres in the country, outside the city limits. There are no permits and no inspections, I say that because often folks assume that it will have to pass a certain code or to satisfy the inspector but in this case it only has to satisfy me. I don't say that to make you think I want to cut corners or do shoddy work, I want this to be perfect, it's just a reality that no one besides myself will be overseeing or even care what or how it gets built.

I'm planning on a 30x50 shop. I will have a slab poured and then build up myself. Columns will be steel 3x3 heavy square tubing, perlins will be 1.5x4 "C" channel. Trusses will be 1.5x1.5" sq. tubing. Building will be insulated with a 'solar guard" roll insulation, outer skin will be 26ga R-panel. This will be a workshop with both woodworking and metalworking equipment. I don't do cars, no lifts will be added.

I'm getting bids much higher than what I anticipated, like 2 times as much or more. I've done a bit of everything and have poured some concrete, although I lack the tools to finish it. I've looked and it appears guys are marking up both steel (rebar) and concrete substantially. I've had guys suggest just a 4" floating slab is enough, which I don't think it is.

I've priced materials myself and am thinking of setting forms and steel myself and paying someone to pour and finish. I can get pour an finish done for about $1.50/sq ft.

My question is what slab design do I set up? I can set up to pour a 4" slab with perimeter beams as well as cross beams. The beams actually use more concrete than the 4" flat part of the slab. I could actually set up and pour an 8" thick floating slab (3000psi mix) with no beams, reinforced with #5 steel for less than what most contractors have bid for a simple 4" floating slab.

We don't really get ground freezes here, except maybe once a year for a day or so.

So, what would folks think about a 6" or 8" thick floating slab for a simple metal building?

Before anyone says 'Pole Barn', I have considered it. My thoughts are this: anything in the ground, be it round concrete support for pole, or beams on a foundation are going to be subject to the swelling and movement of the soil trying to heave it up when wet and letting it fall when dry. Would a floating slab not alleviate that issue?

Love to hear thoughts and suggestions

 

[matt_i]

I think your 4" center is enough....unless you plan on moving heavy equipment with forklifts. I planned to move up to 10k machinery which puts slightly less than 20k on the front tires of the forklift. So with that I went to 6" thick,4ksi.

I think you could do a thickened edge, say 12" square cross section, around the perimeter. With rebar "belts" running thru the thickened edge.

How are the columns attached? Expansion/wedge anchors or are these cast-in L-shaped bolts?

I cant comment on building/construction services pricing. They may be still pricing as if they will be super busy with work thru the remainder of the year or they may be looking at a huge slowdown and are trying to hook any fat fish they can right now. Its too early to tell.

If you are going to air condition, I think the bubble wrap insulation isn't enough. I would want to go thicker.

 

[misterfixit]

I think your 4" center is enough....unless you plan on moving heavy equipment with forklifts. I planned to move up to 10k machinery which puts slightly less than 20k on the front tires of the forklift. So with that I went to 6" thick,4ksi.

I think you could do a thickened edge, say 12" square cross section, around the perimeter. With rebar "belts" running thru the thickened edge.

How are the columns attached? Expansion/wedge anchors or are these cast-in L-shaped bolts?

I cant comment on building/construction services pricing. They may be still pricing as if they will be super busy with work thru the remainder of the year or they may be looking at a huge slowdown and are trying to hook any fat fish they can right now. Its too early to tell.

If you are going to air condition, I think the bubble wrap insulation isn't enough. I would want to go thicker.

The columns will be attached to weld plates which are set in the wet concrete. Basically a 6x6 square steel plate with 3-4 pieces of rebar welded to one side in a "J" shape to hold it in.

As for the insulation, supplier told me that the Solar Guard foil faced one side has a higher R-value than the 3" white faced fiberglass they sell. 3" fiberglass roll has r-10, solar guard has r-11. They sell both, so not sure why they would fib on specs.

 

[pmiranda]

No worry about frost, but how is the shrink/swell there? If you have a lot of clay, then the beams and rebar do most of the work.
I think the beams (and interior beams) are what keeps the columns where they're supposed to be, so I wouldn't assume an 8" slab is always better. Is this a pre-designed building engineered for your wind, snow, seismic, and collateral loads, or are you building it based on your past experience and calculations?
As a point of reference, my 40' span building has beams full of rebar running cross wise over 2 feet deep.

 

[misterfixit]

No worry about frost, but how is the shrink/swell there? If you have a lot of clay, then the beams and rebar do most of the work.
I think the beams (and interior beams) are what keeps the columns where they're supposed to be, so I wouldn't assume an 8" slab is always better. Is this a pre-designed building engineered for your wind, snow, seismic, and collateral loads, or are you building it based on your past experience and calculations?
As a point of reference, my 40' span building has beams full of rebar running cross wise over 2 feet deep.

This will be as you say based on past experience and calculations.

As far as the soil, the top roughly 12" is sandy loam, under that it starts getting into clay.

 

[ConCretin]

I would expect that the loads imposed by your steel structure would require footings that are deeper than 8". This would lead me to a 4 or 5 inch thick slab with thickened footings and tie beams. I realize your building won't have much of a load on the perimeter but I like to see a thickened edge anyway.

I'd reinforce the footings and tie beams as required and throw a couple bars in the perimeter haunch to help hold everything together. I'd use properly supported wire mesh for the slab itself. This seems to me like the most efficient design.

 

[misterfixit]

I would expect that the loads imposed by your steel structure would require footings that are deeper than 8". This would lead me to a 4 or 5 inch thick slab with thickened footings and tie beams. I realize your building won't have much of a load on the perimeter but I like to see a thickened edge anyway.

I'd reinforce the footings and tie beams as required and throw a couple bars in the perimeter haunch to help hold everything together. I'd use properly supported wire mesh for the slab itself. This seems to me like the most efficient design.

I'm not arguing, but I guess I cant understand why pole barn foundations are ok, but this type isnt enough. Each column on a pole barn has a 'foundation' that supports itself alone, and folks don't seem to worry about them. Regardless of how deep the hole for the pole support is it only is supported by the cross-sectional size, in other words it is only supported by the diameter of the hole you dig and fill with concrete. Even if each hole is 12-14" diameter, that isn't much compared to a full size slab spreading out the support for all the columns.

What do I not understand?

 

[ConCretin]

I'm not arguing, but I guess I cant understand why pole barn foundations are ok, but this type isnt enough. Each column on a pole barn has a 'foundation' that supports itself alone, and folks don't seem to worry about them. Regardless of how deep the hole for the pole support is it only is supported by the cross-sectional size, in other words it is only supported by the diameter of the hole you dig and fill with concrete. Even if each hole is 12-14" diameter, that isn't much compared to a full size slab spreading out the support for all the columns.

What do I not understand?

That's a good question. I'm not an engineer and can only speak from my experience as a contractor. We rarely see column footings less than 12" deep. Obviously a steel building transfers loads differently than a pole structure but I can't argue with your logic.

Maybe someone else will weigh in on the engineering but even if you end up with 8" footings, I'd still thicken the slab where necessary and place a 4 or 5 inch slab everywhere else.

 

[Marctrees]

"As for the insulation, supplier told me that the Solar Guard foil faced one side has a higher R-value than the 3" white faced fiberglass they sell. 3" fiberglass roll has r-10, solar guard has r-11. They sell both, so not sure why they would fib on specs."

I don't think he is lying to you... But, I do think he is passing on to you something he mistakenly believes.

Google about the bubble or FG filled foil... read the objective scientific info, not the sellers info.

You will quickly learn it is not as you have been told.

It will reduce radiant gain, but ONLY if properly installed, draped loosely.

You will have to somehow make SURE the contractor does that.

But it still will not be as good as encased FG rolls.

Study into this.

Marc

 

[pmiranda]

I'm not arguing, but I guess I cant understand why pole barn foundations are ok, but this type isnt enough. Each column on a pole barn has a 'foundation' that supports itself alone, and folks don't seem to worry about them. Regardless of how deep the hole for the pole support is it only is supported by the cross-sectional size, in other words it is only supported by the diameter of the hole you dig and fill with concrete. Even if each hole is 12-14" diameter, that isn't much compared to a full size slab spreading out the support for all the columns.

What do I not understand?

Poles are usually set a good 2 feet into packed earth. (Don't quote me on that, just a vague memory from last year) The hole diameter doesn't have to be huge but it has to bear against something to not push out when you get side loads. A flat slab without foundation beams would bear against nothing, and only the rebar would resist the columns trying to move sideways. Also, once that clay a foot or two down starts moving around with changes in moisture, it won't evenly support a flat slab.
I'm sure other folks know alot more about this.

 

[Marctrees]

Actually, PB posts are set more like 4'deep in most cases.

Freezing MN, or Scorching TX both.

Marc

 

[Marctrees]

I would feel way more comfortable doing a simple proven properly designed and constructed Pole Building.

But, If you are happy w the system you used for the first Barndominium 9 yrs or whatever later, well then do same for shop.

Replied to your PM.

Marc