Alan was a post frame building contractor for years, prior to becoming a Building Designer for Hansen Pole Buildings. If I had to estimate, I’d venture Alan constructed well over 200 of our buildings.
Recently, Alan had a client question the thickness of the concrete footings, beneath the columns, used to support the pressure preservative treated columns. It seems Alan’s client had engaged a local engineer to do the site design and she had put some ideas in client’s mind of our footings being inadequate.
15,000 buildings – I suppose I was due for the first client to question this one!
Throughout the industry, a nominal six inch thickness of concrete (actual thickness is 5-1/2 inches) poured beneath columns is pretty well accepted as being adequate. Many individual Building Departments provide handouts for non-engineered post frame buildings, none of which I have ever seen as providing for a footing of greater than six inches of thickness. Personally, I have never heard a report of a column supporting a post frame building having “punched” through the footing beneath it.
In the case of the engineers for Hansen Buildings, they are using a design with a full eight inches of concrete under each column – over 45% thicker than would be the common industry standard.
But – is this actually adequate? Good question, so I started doing the Google thing.
From decks.com…”This footing type involves pouring a pad or “cookie” footing at least 12” thick at the bottom of your hole below the frost line.” No basis, in their website, for where this thickness came from.
Fao.org (Food and Agriculture Organization of the United Nations) … “Isolated piers or columns are normally carried on independent concrete footings sometimes called pad foundations with the pier or column bearing on the centre point of the footing. The area of footing is determined by dividing the column load by the safe bearing capacity of the soil. Its shape is usually square and its thickness is governed by the same considerations as for foundation footings. They are made not less than 1 1/2 times the projection of the slab beyond the face of the pier or column or the edge of the baseplate of a steel column. It should in no case be less than 150mm thick. As in the case of strip footings, when a column base is very wide, a reduction in thickness may be effected by reinforcing the concrete.” For those of us who have forgotten everything we were ever taught about the metric system, this would be a minimum of 5.9 inches thick.
Now if this was a stick framed building, a nominal eight inch wide concrete foundation wall will support a two story structure, with a 16 inch wide by eight inch thick continuous footing below.
From ConcreteNetwork.com…. (in reference to footings under framed walls)…”When a footing must be widened to boost bearing ability, it should also be reinforced or deepened. An unreinforced footing that is too wide may crack close to the wall, overloading the soil beneath. Without reinforcement, codes say the thickness of the footing should be at least as great as the distance it projects next to the wall.
If you increase the footing width, the code requires an increased thickness as well. That’s because a footing that’s too wide and not thick enough will experience a bending force that could crack the concrete. The projection of the footing on either side of the wall is supposed to be no greater than the depth of the footing. So, for example, a 32-inch-wide footing under an 8-inch wall would need to be at least 12 inches thick. Instead, however, you could rein-force the footing with transverse steel (running in the crosswise direction, not along the footing). In most residential situations, #4 rod at 12 inches o.c. will be plenty for 8-inch-thick footings up to 4 feet wide. The steel should be placed about 3 inches up from the bottom of the footing.”
If the same was to be held true for a post frame building, the maximum diameter of an eight inch thick concrete pad, under a nominal six inch square column (5-1/2” square actual), would be 21.5 inches, without adding rebar.
At jjgarcia.com/webengineer/footing.html, an Individual ‘Pad Footing” Table is provided. Most jurisdictions accept a design maximum soil bearing pressure of 2000 psf (pounds per square foot). From the table using 3000 psi (pounds per square inch strength concrete) a ten inch thick footing (this happens to be the minimum footing thickness in the table) and three feet square with four Number three (3/8” diameter) rebars will support 17,000 pounds of load. This load is roughly the equivalent of a 60’ clearspan pole barn, with columns spaced every 12 feet and a roof load well in excess of 40 psf! For most post frame buildings, this would be a bit on the overkill side.
The same site’s recommendation for rebar placement: “The placing of the rebars are to be a minimum of 3 inches from the dirt on the bottom of the Pad Footing and 3 inches clear of the side dirt walls. The bars are placed in a checker board pattern and tie together with bailing wire at the points where they intersect. The spacing between each rebar should be equal.”
If all of this information left my readers as confused as it left me – then we are sailing on the same ship. For now – I’ll leave it in the good hands of our engineers. They have the knowledge and experience from successfully designing thousands of post frame buildings.
Good info.Noit so from 2012 codes.
New building codes are mostly CYA type by suppliers waningt to sell construction crap. This is abt 80% overkill of excessive work and expenditure.
Woody ~ While I can appreciate your feelings, experience tells me otherwise. I joined what is now the ICC back in 1987 and I can vouch for changes in the Codes being due to real life circumstances as well as testing. For the most part – all anyone is trying to do is protect the occupants of buildings. In the end, it is less expensive to construct Code compliant buildings, than it is to rebuild the ones which do not meet the current Codes. It amazes me when I hear resistance to spending an extra $500 per house to make buildings more resistant to high winds…..what is the value of a human life?
$500.00??? Really. I wish. But since you brought it up I’ll address it. What is the value of a human life? Well.. you answered your own question… $500.00. But why not $600.00 or 10,000. Now do you see the slippery slope you and guys like you often create with that “if It could just save one life” BS. 40 years of designing and building and I’ve learned enough to know when someone is creating a job for themselves and creating self importance.
I don’t need someone looking out for me. I’m as smart as you are and employ self preservation on a daily bases.
As to experience being the bases for code, everything has a threshold. It is unreasonable to try to prevent all things particularly for things that may never happen or rarely happen.
Since this article does not mention any dollar amounts in it, your commentary has no background basis. Building Codes do not prevent all things – and certainly not buildings from failing, they are designed to prevent loss of life. While you may very well be smarter than I, adherence to Building Codes and sound engineering practice could save you from being sued in the event your self preservation design methods fail and you have a client injured or killed as a result.
So I’m going to build a RV garage to store my boat so when I get off the ocean I can wash it. And be out of the weather I’m going to pour a concrete foundation my building is going to be 20′ x 50′ how thick should I make the slab. The type of structure is going to be made out to those metal buildings not too heavy I believe it’s going to be a solid foundation
My recommendation is going to be to construct a post frame (pole) building – where the building goes up first, then the concrete slab is poured inside of the building. This will eliminate all of the extra unneeded expense of a concrete foundation. As to the thickness of the slab, if the site has been properly prepared and well compacted a nominal four inch thick slab will do everything you need it to do.
How deep do u need to dig to pour a concrete footer? How do u know u are deep enough?
The depth of the footings will be determined by the engineer who designs your building and is based upon numerous factors including but not limited to – design wind speed and exposure, eave height, roof height, footprint of building, is it fully or partially enclosed (or just a roof), are the endwalls enclosed to the ground?, strength of soil, etc. As you can see – it is more than just a one size fits all and should not just be guessed at. In any case, the depth should be at least as deep as the frost depth in your area.
Hello everyone I’m wanting to do a 30×60 concrete slab with 30×30 of it closed off as a building. I’m lost as for footing on the 30×30 enclosed part of it. It’s all gonna be the metal building packages online. The other 30’ of it will be just extended roof over the concrete but my question is the 30×30 that will be enclosed as the shop what kind of footer do I need? Location is Clewiston Florida
Building the building first, with embedded properly pressure preservative treated columns in the ground, once the building is up you can then pour the slabs.
I AM CONSIDERING A POLE BAR STYLE HOME. TINY HOUSE DESIGN NO BIGGER THAN A 32 FT RV. I AM PLACING THE POLES 4 FT APART. AFTER READING THIS I AM INCLUDING 12 IN FOOTING W/ REBAR PLACEMENT. MY ? IS HOW DEEP. I AM IN THE MTNS OF TN. SO I WILL NEED TO USE THE “LIQUID” DYNAMITE FOR HOLE DIGGING. ITS JUST TO COSTLY TO DRILL. SHOULD MY HOLES BE 6 FEET?
From the sounds of it, you are winging it – trying to arm chair engineer your own building. Poles every four feet is entirely overkill – when 10 or 12 foot spacings will probably do everything you need to do to properly support your new building. You also will not need to have them six feet in the ground. A Hansen Pole Buildings’ Designer will be reaching out to you shortly – we should be able to save you enough money in reducing the number of columns and depth to be able to have someone with a rock bit auger come dig your holes.
It was reassuring to read that you’ve never heard about a supporting beam “punched” through a circular footing beneath it. My wife and I have been thinking about adding a room addition to our house outside of our living room. We were thinking that if we got a good quality formatube to help form and pour the footing, it would be enough to support the new room.
Hi, I am building a stick frame Gazebo 22 ft x 12 ft. Front has four 6×6 post with a 6×10 beam on top. The other three walls are stick framed. Florida is very concerned about ‘Uplift’. I am planing to do 10 inch x 4 ft piers with a flair at bottom for the front. And for the rear, 8 inch x 4 ft piers. both with 2 number #4 rebar, 3 inches from bottom and sides. The Gazebo has a metal roof. I am inserting framing anchors into the top of each pier. My question is, are these Piers deep enough or to deep ? and is the diameter correct ?
You really need to invest in the services of a Registered Professional Engineer. He or she can do an analysis of your proposed building and provide sealed drawings and calculations which will meet with your Building Official’s approval.
I am trying to repair some rotten 6” posts on a 54’ x 24’ barn (18 poles making five sections if that makes sense). We are on heavy plastic clay and the poles are 6’ in the ground. I am struggling to find a repair option that makes practical and fiscal sense to me. I want to stitch 4” angle iron to the posts and pour concrete pads under them. Should I be digging down 6’ again? Frost isn’t really an issue here and the site is sheltered as it is on the leeward side of woodland. Any advice welcome!
4″ angle iron is highly unlikely to be adequate. My best advice would be to have a competent engineer review your building and design a properly engineered repair.
I would recommend Perma-columns. look up their website. If its just the bottom of the posts that are rotten, you can cut off the bottom of the post, dig out rotten section and install a perma-column. They are a concrete post with steel brackets that you bolt to the existing post
Trying to “armchair engineer” a 512sqft building. I’ve calculated a dead load + live load @ 50,176 lbs and decided on 15 poles spaced 8ft apart. Given local soil conditions, I determined a minimum footing width of 15.5″ or diameter of 17.5″ for a circular footing, and I’m going to upsize that to 24″ because that’s what they sell at the building store. The poured concrete (in sonotubes) will be rebar reinforced (non-tensioned); 48″ tubes with 16″ above GL and 32″ down. The only thing I can’t find is any design specs for the diameter of the beams themselves! They’re holding 2x10s at an 8′ span. I know this is all over-engineered, but the cost difference between meeting code and exceeding code is negligible so far – where it gets really expensive is if I only need a 8″ diameter pole and I pour a 12″! Someone please help me find this design spec.
I strongly recommend you do not attempt to armchair engineer your own building – invest in the services of a Registered Professional Engineer. Chances are excellent they will save you more money from an efficient design, than what they charge for their services.
im building (6 ) 6ft brick pillars,16 inches wide, i have firm rocky soil,what should be the depth of the footing ? pillars will only by grouted 1/2 full so not a ton of weight..
Code requires footings to be at least below frost line or down to solid bedrock, whichever is least. Your building’s engineer can fully detail footing requirements for you.
The ground I am building on has a 2500 pounds per square foot capability.
Knowing what a post or wood column required for support. You can then calculate how thick and wide the concrete “cookie” has to be.
A 6” cookie is only 28.26 square inches. A square foot is 144 square inches.
My property is 2500 pdf. So, I have .19.625 percent of a foot. 19.625 times 2500 is: 490 pounds of bearing pressure.
My calculations are you need a cookie with the radius about 6.8 inches. The cookie with a radius of 6.8 inches gives you a square foot of bearing pressure. Or 145.27 square inches.
145.27 square inches times the 5.5” thickness of the cookie, gives you about 1/2 cubic foot of concrete. Or 0.461 cubic feet.
I don’t know the square inches of the wood columns or the bearing pressure they require.
I better get back to work.
To find the required bearing capability needed you need to account for roof loads (live plus dead) x column spacing x (1/2 truss span plus any side overhangs in feet) plus building weight attributed to this particular wall column (weight of column plus framing, siding, wall insulation and interior finishes)
A 6” circular cookie is only 28.26 square inches. A square foot is 144 square inches.
My calculations are you need a cookie with the radius about 6.8 inches. The cookie with a radius of 6.8 inches gives you a square foot of bearing pressure. Or 145.27 square inches.
145.27 square inches times the 5.5” thickness of the cookie, gives you about 1/2 cubic foot of concrete. Or 0.461 cubic feet.
Doing a 9’x9′ Bathroom addition to my 1930s pier and beam foundation house. I’ve got sandy soil and was wondering about the depth of and thickness of the piers needed.
This is a question best answered by the engineer who designed your structural plans for the addition and will be based upon climactic conditions at your site, frost depth and building heights.
new building in wenatchee wa state
Thank you for your interest in a new Hansen Pole Building. One of our Building Designers will be reaching out to you shortly, or call 1.866.200.9657 for immediate assistance.
Hello! Sorry for the long bump here. I’ve been looking at arched cabin kits, S-model Quonset huts, and pole barn construction. My project would be a lot smaller than almost anything I’ve ever seen on this site — a 16’x20′ “tiny house,” divided into 2 spaces – 16’x10′ living and 16’x9.5′ back partition (assuming 6″ interior walls) with the back partition further divided into a 4′-wide bathroom/laundry space and the remainder, about 11.5’x9.5′, being a bedroom. I would want a standard ceiling height of 8′ and a 3/12 or 4/12 pitch roof, with overhangs on all 4 sides to shed snow/water.
There is a *lot* I still don’t know. The chief concern is the foundation, but I also have questions about the framing material itself.
– Would using structural steel rather than structural wood work for this? Would it be any of stronger/cheaper/lighter/tougher?
– What spacing is acceptable for trusses? Can I get away with just the two gables and a third one smack in the middle at 10′ on center since the entire thing is only 20′ long? How thick should these members be, and in addition to the corners how many poles do I need? I am guessing 6 total, one for each corner and one to support each side of the center truss.
– What do I do about moisture and frost from below? Should I place a 15mil or so vapor barrier with holes cut for the footers, add EPS foam over that, and *then* pour the concrete overtop of that? I am thinking of essentially a 20’x24′ (when the skirt insulation is factored in) frost-protected shallow foundation but with the poles and footers integrated due to continuity with the poured concrete, i.e., the backfill and slab would be poured together and level out naturally since they’d be all the same pour.
– The frost line is a good 6′ below grade here. Do I need to sink 6′ round concrete footers to, let’s say, 2′ below frost line, then have the poles “float” 4′ down into the hole and backfill the 4′ space with concrete? This of course means 12’6″ members to produce the 8′ ceiling (6″ slab planned).
– How to underlay and insulate/waterproof the roof? Would 1/2″ plywood, then #30 felt or water/ice shield, then metal roofing work?
– Similarly: can I sheathe and insulate the walls on the outside? Would 2×6 bookshelf girts work for this if I attached Zip sheathing to these, then covered in Tyvek, then a vapor barrier, then metal siding?
I am a complete novice to all of this, so please forgive the massive wall of interrogation here! Looking forward to any insight I can gain about the build process.
Fully engineered post-frame will be your most affordable design solution when all is said and done.
You should have a single truss on each end and double trusses 10′ from each endwall. We can provide an engineered DIY wood truss that will be easily assembled and far more affordable than prefabricated trusses.
It will take six roof supporting columns.
Code requires a well-sealed 6mil under slab vapor barrier (thicker will be more resistant to puncture during concrete placement. It should be wrapped up and over pressure preservative treated splash planks as well as sealed to each column. EPS, XPS or Mineral Wool rigid under slab insulation on top of your vapor barrier is an excellent choice and is required if you will be doing radiant in floor heat.
Columns should be set so base of bottom collar is at or below frost line. Holes can be augered to this depth, then columns are floated in hole 8″ above bottom of hole, then bottom of hole is filled up 18″ with premix concrete.
If you are using radiant floor heat, only slab edge (and underneath) require insulation. If there is no floor heat, then slab perimeter can be insulated down 2′ and then out 2′ with R-0 insulation boards.
Roof steel with an Integral Condensation Control can be applied directly to roof purlins, there is no need for solid sheathing. Use raised heel trusses to allow for full insulation depth from wall-to-wall. Vent eaves and ridge to allow for proper airflow above insulation.
For walls – from out-to-in: Steel siding, Weather Resistant Barrier (Tyvek or similar), bookshelf wall girts, unfaced batts to fill cavity (we recommend Rockwool), well sealed vapor barrier, then interior finish.
Always keep in mind, building codes are the minimum safety requirements for a structure. And most codes departments are not held liable if your build fails. That’s really weird !
Excellent point sir!
Actually (best I can determine) all Building Departments disclaim any liability for deficiencies – even when they have done (and of course charged for) plans checks! This is yet one more reason I am a proponent of engineer sealed structural building plans. Engineered plans are like investing in a lifetime insurance policy for your building – with only a single payment! Bonus – a good engineer will save more money than they are paid, due to efficiencies in materials usage.