Barn - concrete opinions?

[RichT]

Building a steel barn for my tractor (and a few other things). Foundation is a concrete pier/slab design, ten 24" diam x 42" deep piers for the barn columns topped with a 25'x37' 3.5" slab. I'm doing all the excavation and form work myself. I'll hire some pros for the pour/finish, approx 15yds. Welcome opinions on plans as follows:

The barn plans do not specify any steel in the piers/slab. I plan to run a 5/8" rebar perimeter on the slab, connecting to 2 or 3 vertical rebars in each of the piers, with 6x6 steel mesh throughout the slab. Since I'm using a backhoe, I'm using sonotubes for the concrete piers.

I'm in SOCAL with hard clay soil. For the slab underlayment, I plan to lay down a moisture barrier, then about 2" of tamped 3/4- gravel.

The barn plans specify using hilti bolts for the anchors. I asked the plan engineering firm why hiltis instead of wet embedded anchors. They said mostly because of the precision needed for placement. There are a total of 26 anchors. I plan to set the 10 main anchors (for the steel connector columns) using 5/8" J bolts embedded 6" ahead of the pour using anchor guides attached to the forms. The remaining 16 anchors will be hiltis.

Given the size of the slab, should I set up screed pipes to help the finishers?

 

[BB_TX]

Shifting and cracking foundations is a well known problem in this area. I put up a 30'x40' metal building 4 yrs ago. I hired a concrete contractor with a good reputation for my slab. It is beam and slab rather than pier and slab. By beam, I mean they dug a trench 1'+ wide and 2'+ deep around the perimeter of the slab site. Then they dug a trench down the middle front to back and another at the middle side to side. The trenches are filled with concrete as the slab is poured to form beams. In the beams (trenches) they put 8 rows of 1/2' rebars. For the slab they crisscross 3/8" rebar every foot or so. They also poured a 20'x3' apron in front and a 10'x3' apron in the rear. All total, they poured 39 yards of concrete. After they set the forms and before they poured, I made them remeasure to ensure they were correct and square (and they had to readjust a little). Then I fabbed some anchor bolt holders from some 1" boards cut to size to hold my 20 anchor bolts and firmly attached them to their forms. Then I remeasured all that. That way I knew everything was going to be correctly in place. The result came out great. (Well, I did have to torch about a half inch out of one hole in one column.) After 4 years, the slab still looks great. Some very minor cracks, but nothing that even remotely appears that it will become a problem. Attached is an outside view.
Do your plans call for a shelf around the perimeter of the slab to prevent water from getting under the steel siding?

 

[Banjo]

When I poured the slab for my 16x30 workshop I followed the advice of the rep from the Portland Cement Association. He recommended to go with more, higher strength concrete rather than steel, especially mesh in the slab. His reasoning was that most times the mesh ends up at the bottom where it doesn't help you. Of course this advice is consistent with selling concrete, not steel /forums/images/graemlins/smirk.gif.

I went 6" thick everywhere (no thickening at the edges) with crack control groves to split the slab into 8 sections. Down the center I embedded a 1" angle iron, flat side up so create the groove and to give me a place to tack weld things to if I needed them to stay put. He also recommended a few bits of rebar to bridge across the crack grooves, so that if (when) it cracks along them, the pieces still stay together. The idea apparently is that if it is going to crack, it goes in a predetermined place. I seem to recall that I welded the rebar links to the angle iron down the center to keep it in place afterwards, and the reinforcing there during the pour. Cut the other grooves a few days later with a skill saw and abrasive blade.

I don't remember the strength of concrete I ended up with, but it was higher than "normal".

For site prep, I stripped off the topsoil and brought in a triaxle and tandem load of "pit run" gravel. Packed it with the tractor and that was it.

I built the shop 17 years ago and there haven't been any cracks or discernable movement. This is in Eastern Ontario, so there's pretty deep frost.

HTH (and makes at least some sense /forums/images/graemlins/confused.gif). Andrew

 

[RichT]

The slab has about a 5in "overhang", that is, extends beyond the steel sidings 5in on all sides. I plan to caulk the area where the steel meets the concrete to help prevent water intrusion.

Plan to do as you have described, fab some anchor bolt holders and place them - carefully. After talking to the barn design engineer more, I'm convinced they recommend hilti bolts because they worry wet embedded might not get the precision needed. I'm working on the theory that wet embedded anchors (at least on the wall columns) will increase the pullout strength significantly...and I live in a high wind area (90mph rated).

I've been giving some thought to the 3/8 rebar crisscross pattern you describe, instead of the 6x6 mesh, then maybe add some fiber to the mix.

 

[JD3130]

Rich,
If you are near the valley I'm supprised they let you get by with piers that shallow. That clay shifts with the moisture content, which is going to stress your slab! I know the codes okays 3 1/2" for garage floors, but if your tractor is very large like mine that's a lot of weight on one side. (12,000 in my case) don't cost but a few dollars more to add an extra 1" and use #3 rebar instead of that worthless wire. If you tie the rebar yourself, the cost won't be a lot higher. It will depend on the codes there for the spacing, I would think 16" would be okay maybe even 18" to to beat the wire, and it won't be on the bottom. Be sure they put the vaper barrier under it, will cut down on the sweating later, and allow you to paint the floor. It's probably better you go with the opinions of your local builders, than ours, they know your soil conditions. There are many factors involved in deciding which slab is best suited. Frost line, water table, soil type and depth to solid footing or undisturbed bed, live load, dead load, intend use and always the un-intend uses. Best advice any of us can give you is "Will you be happy with it later on!" I think most the guys here will agree, I did a dozen plus, projects, that I was happy with, then a year two years, I hear myself asking "Why didn't I do it this way?" or "Why didn't I make it 10' bigger or wider, longer, or higher?" So make it as big as you can afford, and place it on your lot so you add on later or add a bath in later years if you decide to. Couldn't hurt to stick a couple pieces of 2" PVC through the slab near one wall, just in case you decide to add a sink to wash the cat, or tractor seat, or your hands. Later you dig under the slab run the water line up through the PVC and the drain down the other one. Just a my 2 cents. Don't hurt to over kill or plan ahead!!!
JD

 

[BB_TX]

RichT - Here is a sketch of the shelf or ledge around the perimeter of mine. This method ensures that (short of a flood) water can't get under the sides. When they set the forms, they nailed a 2"x2" to the top of their forms all the way around the perimeter.

 

[dmccarty]

Rich,

I am planning on building a barn with concrete piers. What are your piers 24" in diameter? I was thinking that 12" diameter was overkill. I'm going to be using 6x6 PT as the posts/columns. I'll be using sonotubes or something similar since I'll be digging with a backhoe as well.

What are hiltis anchors. Are the drilled into the concrete after the pour?

My house has a finished concrete floor. The house is about 2400 square feet. The finishers did a decent job of getting a level floor. There are some wavy places that can be noticed when looking at the wall molding. I don't have a problem with this in the house and certainly would not have a problem in a barn/garage.

To "level" the floor the finishers simply took masons string from one edge of the slab to the other and pulled it tight. While it was held in place they used the twine to level the concrete. Simple and it work real well. Not sure if they would need to use the screed pipes. Might be tough to get out of the concrete and then patch back up.

We did have grooves cut into the slab the day after the pour. For the most part the cracks followed the grooves. Our house is L shapped with part of the corner cut out for a porch. The places where the concrete cracked are at corners under stress. So the porch corner caused a crack. We have a built in shower and we did not pour the shower during the slab pour since the shower floor would be built later. The "hole" for the shower caused stress cracks as well. The only other cracks we have is where the grooves where not cut deep enough. In one or two places the crack was following the grove when the groove became shallower and the crack would "pop" out of the groove.

Long way of saying to cut grooves in the slab or have it poured in sections.

Later,
Dan

 

[RichT]

JD, pier specs are part of overall barn design pkg, furnished to me by the barn company (MD Barns in Ontario). These are pretty common designs throughout the area. Heaviest thing in my barn will be a couple of cars (I restore Mopars). Tractor might spend some time in there, but its a lightweight (Kubota BX22). Good advice on the PVC water line, except I'd probably go ahead and lay the line so no need to disturb the slab later.

BB, thx for the pic...NOW I understand what you were saying before. Duh. Excellent idea...I'll consider it.

Dan, yes the piers are 24" diameter. Finding the sonotubes was a challenge. Finally found them at a commercial bldg supply. Local home depots, lowes, etc...biggest they carry is 8-12". Hiltis are drilled after the pour, usually a mechanical or epoxy anchor. Definitely will go with stress cuts after the pour.

Thx to all for your suggestions.

 

[JD3130]

Rich,
If you run the PVC lines yourself. Make sure you wrap the lines anywhere they will be in contact with the concrete. They make a rubber wrap just for that, but we always used the A/C insulation that is used on the copper pipes. That way if the slab ever shifts it will not crack the PVC. Or just run an over size PVC and run the 3/4" inside it.
JD

 

[keeney]

I would go a minimum of 4" thick on anything that will ever have a vehicle on it, 5" being better.

If you want it to last, upgrade the concrete to at least a 6-bag mix (4000 PSI?). Here in MN, the wire mesh is worthless. Any road salt that drips off of vehicles gets in the cracks and rusts through the thin gauge wire in a few years exactly at the spot where you want the reinforcing to hold the cracked sections together. I would add as much 1/2" steel as you can afford - spaced every 1' x 1' being "enough".

The concrete and steel are less than half the cost of the project when you add the labor to install it. Upgrading them doesn't add hardly any labor.

- Rick