Concrete pier reinforcement advice needed

Zachary Hoyt · Mar 13, 2015

This spring and summer I plan to replace a poorly built pole barn with a real building. I asked a year ago about the concrete piers I was planning to use and was given some good advice about them including the name of the shape in the thread at the link below. Basically they will be a tapered footing 22x22" on the bottom and 8x8" on the top and 5' tall, set 4' in the ground. I am now wondering about reinforcement for these piers and have been doing some research without finding out anything very definitive. There is no code here for agricultural buildings so I don't have to worry about that, just about what will actually work well. I have three older books on concrete but none addresses this situation and online information has been contradictory. There will be 20 of these piers in a 28x48 small barn-type building with a loft. I am thinking of using 4 lengths of #4 rebar per pier, roughly paralleling the four corners about 4" inside the concrete and coming together at the top. Then I could set an anchor bolt in the top of each pier to fasten the 8x8 posts on. I am wondering if this seems reasonable and what I would need to tie the rebars together lower down in the pier. I could make a square or a ring out of rebar or I could just use some wire to hold them while the concrete is poured. Any advice will be much appreciated.
Zach

Zachary Hoyt · Mar 13, 2015

I forgot the link. Here it is.
Zach
Frustum of pyramid

dpendzic · Mar 13, 2015

the 4--#4 rebars sounds reasonable---it is also preferable to put a small 90 degree bend in the ends also for better anchorage of the rebar.
also rectangular ties around the # 4 bars at say 18 inches on center are usual practice--could even be a #3 rebar

Zachary Hoyt · Mar 13, 2015

Thank you, that sounds good and the #3 should be quicker to bend into a square. The bend in the end of the #4 bars will work out just right because a 20' piece of rebar will make 4 5' pieces and if each one has a bend that will keep them far enough away from the dirt on the bottom and give a better foot to sit on the pieces of block at the bottom of the hole prior to the pour.
Zach

Bob Harvey · Mar 13, 2015

Probably use less concrete with a 2' x 2' x 8" footing and a 10" 'Sono' tube with 4 - 5/8" rebars (L at bottom) and 4 rings spaced 16" O/C tied to verticals, saddle on top. HTH

phil_n · Mar 13, 2015

I worked at a place that installed heavy concrete 'sound barrier' fences where the posts were sunk up to 8' deep and surrounded with concrete. Most jobs had to be ok'd by the local engineering dep't. We often used reinforcing that looked like square fence wire (not sure of the correct name) rolled into a cylinder and fastened together with wire ties - sometimes of surprisingly light guage like 10 ga or 1/4 inch. According to the engineers it was the circle part that was the most important to them, the vertical pieces were mostly there to make it easier to hold the rings in place. Also the reinforcing had to be covered completely with concrete so moisture couldn't penetrate and start rusting the wire/rebar. We did use a lot of rebar also on larger jobs. I'm no engineer so there may be other opinions on this.

phil n

Janicholson · Mar 13, 2015

I agree with 4 - #4 vert. bars and #3 welded loops around them Be sure to keep the metal away from the exterior at least 2 inches. Or epoxy coat the rebar. Rust swells the metal and spalls the concrete. I teach Concrete and Masonry classes. Jim

paul · Mar 13, 2015

That's a lot of piers/ concrete for a rather small building, every 5 feet a pole down each side? Nothing wrong with overbuilding, but that's a lot.

A pole building uses the pole as a stiff structural member from below the frost line to the top of the eve to resist racking and folding over from wind, snow load, etc. make sure your joint between the concrete and the pole is sturdy. You want it to be strong enough to be a solid column, not a weak joint that could act as a knee...... If this is a basic pole design with concrete footers - if it is a different, braced in all directions design then never mind.

Paul

Zachary Hoyt · Mar 13, 2015

It would use less concrete but I would have to buy the sonotubes and I can form the tapered square piers for almost nothing with low grade wood from our sawmill.
Zach

Zachary Hoyt · Mar 13, 2015

Thanks, that's interesting. I have used mesh in slabs but am not
familiar with how it is used in a column.
Zach

Zachary Hoyt · Mar 13, 2015

Thank you very much. I will have to look into the loops and whether I can get them welded. Welding is a skill I am sad to have not learned yet, but I am hoping to try to learn one of these years. I was thinking that if for the lowest loop I made it 1 foot on a side and doubled the last side I would use 5 feet of rebar per loop and if I wired the doubled side together it would probably be good enough, but I can see that welding would be better. I had read online that 4" from the outside was a safe distance but it is good to know that it can be as low as 2". The rebar at our local yard is stored outside and is always rusty, so I hope it is not too much of a problem.
Zach

Zachary Hoyt · Mar 13, 2015

The design is for 13 foot posts every 12 feet down the long sides and another row of 8 foot posts 8 feet in from each side. These shorter posts will support upper 8 foot posts on top of the cross members which will support a lengthwise beam to hold up the rafters. It will be a slightly modified post and beam type structure which I am hoping will be more durable than a pole barn. Every post will have diagonal bracing both ways, probably made from 2x6. All of the lumber will come from our sawmill so it will be full dimension size. The existing building that the new one will displace is supported on 10 6x6 treated poles and while it has not technically fallen down it is far from satisfactory.
Zach

phil_n · Mar 13, 2015

If you roll hoops, or form up squares, you don't need to weld - some engineers don't even like them welded - not sure why. Just overlap the ends by at least 6" and wire them together. Rusty rebar is no problem - ours was always rusty. Epoxy coated is being banned in some states for infrastructure as they have found that the concrete doesn't stick to it.

dpendzic · Mar 13, 2015

I agree---no need to weld if there is a sufficient lap and tied together

dpendzic · Mar 13, 2015

2 inches clear if exposed to the elements but 3 inches if exposed to soil which holds moisture and doesn't ever dry out for the most part.

VADAVE · Mar 13, 2015

Your really over building it! It appears you have your basic design and are working out the details.
Suggest you figure out the price BEFORE you start.

ss55 · Mar 13, 2015

Keep in mind that concrete is very strong in compression but weak in tension. Steel reinforcements are added to concrete to carry the tension loads and prevent the concrete from pulling apart. Look where the concrete is most likely to pull apart and add steel to those locations, (most likely where it is bending in tension and around the perimeters).

A pier will likely be carrying a vertical compression load pushing it down into the ground rather than any vertical tension trying to pull it out of the ground. So there should be little need for vertical reinforcement, mostly for horizontal reinforcements and just enough vertical supports to keep the horizontal rings in position during concrete pouring.

Zachary Hoyt · Mar 13, 2015

Price will be about $600 for concrete, maybe $200 for rebar and duplex nails and wire and such, maybe $300 for 150 pounds of 4" and 50# of 6" nails and enough screws to do the roof. I already have the roofing from the old building and will reuse it as it still looks good. I have all of the hinges I need for the doors from auctions and such. I will need to buy in a couple of dump truck loads of fill to level up the floor as one end of the site is about a foot lower than the other. I will make up most of the difference with rocks which we have in abundance in piles and just need to move. Most of the framing lumber is already cut at an estimated cost of 2-4 cents per board foot for gas and blades. We have a 40 foot shipping container which is currently under the pole barn which I hope to sell for $2,000 as it is the tall size which is more desirable. The sale of the container should more than offset the cost of construction. I am overbuilding the frame because I am using aspen lumber for most of it and it will not be graded so it is best to leave a comfortable margin of strength.
Zach

nontypical · Mar 13, 2015

Zach, You almost describe my barn that I built about 17 years ago. Same size, 17 concrete piers, almost exactly like yours. I didn't use any rebar in the piers. But when I backfilled the piers, I wrapped them in plastic to prevent the frost from clinging to them and pulling them up. I used angle iron embedded in the piers to fasten my poles. We have quite a problem with frost here (central Wisconsin) and my piers and barn seem to be holding up well. I also used most of my own lumber that I sawed with my bandsaw mill. Good luck with your project.

Bob Harvey · Mar 13, 2015

I'm sure you are right, BUT, make sure that wooden form is VERY strong or you could easily have a failure. Properly braced 'sonotubes' are pretty much bullet proof - simple box at the bottom and simple box (braced ) at the top. I understand the need for thrift, so why not just make square forms (in lieu of tubes) with your lumber ? Just checked, 12' Sonotubes are $30 ea. A LOT of labour saved. Your call of course. Just trying to be helpful. (retired bldg. contractor)

Zachary Hoyt · Mar 13, 2015

I like the angle iron idea, maybe that would be better than anchor bolts. The only problem I can see is that with my design the top of the pier and the bottom of the post are the same size so there wouldn't be much there to work with.
Zach

dpendzic · Mar 13, 2015

the wind load on the barn will produce some horizontal load on the piers--which means that there may be tensile loads developing on one side of the pier--hence the rebars to take the tensile stress

sms · Mar 14, 2015

I like these wet set anchors.
Pole anchors

Geo-TH,In · Mar 14, 2015

I would have concerns. First wind shear, second weight of pier, last how would you get then level in in place.

I would think of using a 12 inch post hole digger. Make a 4 ft hole, then make it cone shaped at bottom, about 24 inches. Then fill hole with concrete, add re-bar. I would still be concerned with attaching post to pier.

So, what's wrong with your old pole barn design??

Zachary Hoyt · Mar 14, 2015

Looks like an interesting idea.
Zach

Zachary Hoyt · Mar 14, 2015

I want the piers to have smooth sides so the frost can't grab them and lift. I will get the forms in position and then connect them together using some of the 2x8 floor joists I have cut. I figure if they are all connected in a grid pattern with diagonals added for rigidity it should be strong enough so they will stay put. The existing pole barn was a bad idea that was poorly executed. It was built in 1994 or 95 by our predecessors who set untreated red pine logs in the ground and built their own trusses to go on top of them. The trusses began to sag and had extra wood scabbed onto the bottom chord sometime before we came in 2001. By 2006 the poles on the south side had rotted through below ground level and the south side sagged about 2 feet in the winter and was being held up by the 40' container. I jacked it back up and replaced the poles with treated 6x6. Two years later the north side began to sag but I caught it before it went down as far and replaced the poles. The only good things about the pole barn are the roofing steel and the headers, which are hardwood 2x12 and will be reused. I am not a fan of pole barns in general after the experiences we have had with them here. I have built pole wings onto buildings but I have done them with treated 6x6 and solid rafters and I do not expect them to last forever.
Zach

Janicholson · Mar 14, 2015

The link below is for a solution from concrete to wood interface. I like your Ideas so far. Look up some info on epoxy rebar and comparison to galvanized or stainless. It is a reasonable choice and is not subjected to salt. Getting the loops welded after you bend them is cheap and an overlap of 6 bar diameters would be easy to weld. (not a but weld) Using a laser Level (cheap) to assure uniform pier elevation is easiest. Jim
Post anchor

Zachary Hoyt · Mar 14, 2015

Thank you very much. The Amish neighbors I talked to about this last year stick a piece of rebar in the top of the pier and set a 2" thick piece of wood over it and then bend the end of the rebar down on top of the wood. Then they toenail the bottom of the 8x8 to that piece of wood. I had been thinking of an ordinary L shaped anchor bolt set in the concrete instead of the rebar, but that design you linked to looks handy. I will have to see what kinds of rebar are available locally. I have a cheap laser level that should come in handy, though I have never used it before. I am hoping to get the piers all with their tops level but if I end up with some that are too high or low after they are cast I can always compensate by adjusting the length of the posts slightly.
Zach

Geo-TH,In · Mar 14, 2015

I've built decks. Dug a hole below frost line, filled hole with concrete and while wet planted a post in the concrete. Never had a problem with frost pushing concrete out of hole.

Don't know where you live. You would have to have a frost line below 4 ft to cause heaving. Not to mention making the hole cone shaped should hold the pier in hole. Ground freezes from top down.

Most of the time people on YT ask for advice then do what we already have our mind set on.

Good luck, expecially lifting and setting the piers level.

ObsoElitist · Mar 14, 2015

There is no code here for agricultural buildings so I don't have to worry about that, just about what will actually work well.
Zach

While you may not be required to follow codes its not a bad idea observe them. Different regions have different codes for very good reasons

Zachary Hoyt · Mar 15, 2015

I would never set a post in concrete for a building as I have found they tend to rot out because the concrete holds water around the post. For a deck it wouldn't matter so much but I personally would still not consider it. I live in northern NY and I have been told that if there are irregularities in the concrete surface that the frost can use them to push the pier up. As you said, the ground freezes from the top down so if the ground near the surface can freeze and grab hold of the pier then when the lower levels freeze and expand they can push the pier up, as I understand the theory. 4 feet should be safe as our frost line is officially 3 feet.
Zach

bjb in TX from Ne · Mar 16, 2015

The reason there are no guidelines is that the size and number of rebar is highly dependent on the loads you are expecting not only from the downward weight but also the shear (lateral) from the wind and the soil qualities. You might want to find a structural engineer in your area to get a better feel on the design. Even though there are no building codes specified, they can still design it to the ASCE loads for your area and ACI concrete code.

phil_n · Mar 16, 2015

And if it ever came to it in the future it wouldn't hurt the resale value to have a drawing with an engineers stamp on it. Or maybe for insurance.

Concrete pier reinforcement advice needed

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