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Author Topic: Poured Pier Foundation for Small Log Cabin  (Read 2527 times)

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Offline kantuckid

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Poured Pier Foundation for Small Log Cabin
« on: December 30, 2017, 02:51:54 PM »
I'm building a small log cabin ( about 14' x 20' or 24') on a remote site where I'll be pouring sonotube piers below the frost line .
 I'll be sawing 6" thick "D" logs for the walls and know what I need to know about the floor system & other construction above the piers-no need for that type of info, thanks.

The soil is clay and my tractors 3pt auger is 12" diameter so I could go 8", 10" or 12" with the holes. I'll be mixing the poured piers on site. The wise folks say 10" for gravel/sand soil and 12" for clay, 8" requires bell footers. I want to stick with the 12" auger holes as no backhoe or shovel work needed that way.
I have no idea how much such a cabin weighs nor do I want to calculate the whole shebangs weight.
I don't know the compaction for my area either but having dug around on this land for a long time I know it's rocky and not soft or swampy for sure! I have one son that does civil engineering who can tell me what I don't know about soil tables for this place but he's not the builder on this, that's me, myself and I...
I'm thinking I'll use 9 piers plus a few more for the porch.
Ideas on choosing the pier size? Thanks!  and  :new_year:
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Offline Don P

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #1 on: December 30, 2017, 04:19:36 PM »
If you have a civil in the bag there's the hot ticket. Vertical is easy, ask him how to take care of lateral loading. That is the reason they were changed to an engineer required foundation.

Offline kantuckid

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #2 on: December 30, 2017, 04:33:07 PM »
FWIW, I live in "no building code landia", so it's all about doing it right & to satisfy me.
 I looked up the soil type and emailed it to him but my 40 years living here tells me it's heavy clay after you get past the woods soil on top, rocks here and there too. 
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Offline MbfVA

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Re: Poured Pier Foundation for Small log der eh der eh Cabin
« Reply #3 on: December 30, 2017, 07:24:12 PM »
And I thought pig Latin sounded funny when I was in high school

 Allow me to alliterate:
 Famously factual is the finding that failure to follow up fundamentally & fortuitously wif da inganeer about your foundation fairly phenomenally & forcefully facilitates future failure.

Offline ChugiakTinkerer

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #4 on: December 30, 2017, 09:07:03 PM »
kk,

You can find this covered in depth over at the CountryPlans Forum.   

For clay soil you should probably use a value of 1500 psf for load bearing strength.  The total of your piers need to have enough square footage to support the weight of the cabin.  How heavy is the cabin?  The simple way to address that is to use the design loads for the floor and roof.  A living space should be designed for 40 psf live load and a 10 psf dead load.  The roof will also have a dead load of 10 psf and a minimum live load of 20 psf, higher if you have a snow ground load.

A 14x20 cabin is 280 sf, so the total load of the living space is 280 sf x 50 psf = 14,000 lbs.  Assuming a 2' overhang on eaves and rake, then the roof will have a footprint of 18' x 24' = 432 sf.  With the minimum roof design load of 30 psf that totals 12,960 lbs.  So the cabin weight you want to design your piers to accommodate should be at least 26,960 lbs.  Adjust that weight based on your planned dimensions for floor and roof.

Divide that by the 1500 psf rating for clayey soil and you arrive at a total area of 18 square feet.  If you have five piers on each side and an extra pier supporting the center of each short wall that is a total of 12 piers.  Each pier needs a minimum area of 1.5 sf.  That's if the loads are all perfectly distributed, but they aren't.  You should increase that by at least 50% so that any given pier will be able to support a concentrated load.  So for a circular pier to have a footprint of 2.25 square feet, you need a diameter of 20".

If you want to stick with a 12" auger, each pier is only good for about 0.8 square feet of area.  It's going to take a lot of piers at that diameter to get up to 18 sf of footing area.  If you can get your hands on an 18" auger bit you'd be just about spot on.  I would then design for an additional pier along each long wall to ensure adequate footage.

The issue Don P mentioned has to do with the fact that sonotube-poured  uprights have no lateral bracing.  Depending on the ground conditions and the design of the structure, it could be at risk of racking and collapse.  One option might be to brace between the sonotube piers with a sheathed framing brace: http://countryplans.com/smf/index.php?topic=13659.msg178442#msg178442

Another way to deal with lateral loads is to use a continuous perimeter foundation.  If poured concrete isn't an option, you might consider a permanent wood foundation.  Of course, depending on the planned usage for the structure the lateral forces may not be a concern.  If this is a cabin that is occupied only during deer season, for example, you may be able to live with the chance of movement in you piers.  if this for a permanent residence, it's probably worth the effort to ensure a long-lasting foundation that will keep your family and belongings safe in all conditions.  That means a continuous perimeter foundation as per code, or something designed by an engineer to meet local conditions.
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Online rjwoelk

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #5 on: December 30, 2017, 09:57:23 PM »
When I built my 22x26 log cabin I had 9 pads 3x4 under it sitting on 4 to 6 inchs of packed gravel then 3 ft of clay under the gravel was packed as well. Not a single movement I  2 years,  building inspector had no problems with doing it that way. Then a 8x16 beam primeter foundation on the pads then a 2x12 floor the logs were stacked on it.
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Offline kantuckid

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #6 on: December 31, 2017, 08:55:13 AM »
Thanks for taking the time to make a fairly detailed reply.
My non-engineer mind :D was thinking 12" sonotubes with rebar in them and a framework of 6" x 8" white oak timbers bolted to embedded bolts in concrete, then my floor system fastened to that oak.
Based on above: .8 SF per 12" dia pier, the 12 in total w. porch make for 12 x .8=9.6 SF or ~ half what you say for the proper load bearing footprint.
I will say that this site is very sheltered as for wind given it's buried within a heavily wooded area and right below a steep slope too. The large trees directly above fell from an ice storm a few years back so no tree danger either.
The easier way to get the surface area up is a bigger auger, staying with what machine I own.
I see 18" augers are ~$200 for a 50" long tool. If I were to use 9 piers of 18" diameter= 16 SF plus 3 piers of 12" dia(porch) that gets me to 18.4 SF. 
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Offline Heartwood

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #7 on: December 31, 2017, 09:35:50 AM »
I wouldn't try to use the diameter of the pier to get the bearing area; use a spread footing instead (like rjwoelk did) and then you can use smaller diameter piers (concrete has 2000-3000 psi capacity). Also, a spread footing prevents uplift (assuming the pier is connected to it well with rebar or a continuous pour); without it a simple cylinder can be pushed up by the clay soils (and frost if you have it) like a champagne cork.
You could dig multiple holes next to each other; more hand work after to clear the hole but worth it, IMO

Offline kantuckid

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #8 on: December 31, 2017, 10:09:26 AM »
I was thinking along a similar line of thought but drilling an 18" auger hole and pouring a pad below frostline  with rebar then a leveled smaller pier on top with the rebar as the tie-in.  Less shovel work going into the ground! with increased pad size.
Another obvious plus is the bag count/price on cement goes down.
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Offline ChugiakTinkerer

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #9 on: December 31, 2017, 12:08:46 PM »
If you're thinking about nine piers as three rows of three, that could get you 15.9 square feet if using 18" footings.  But I think you may want to consider adding at least one pier along each long wall.  The issue isn't footing area, it's the spacing between piers.  With a 12' span between piers you'll be needing a very strong beam.

As a quick exercise in sizing the timber, look at the toolbox link on the left side of the page, bottom link under the sponsors.  Under Don P's Calculators select beam and column calculators, then select Uniformly Loaded with Dropdown.  We can estimate how much load a beam will need to support, then size it to fit.  For load, we go back to the design numbers from my prior post.  If you have the floor supported down the middle, then your long wall piers are only supporting 1/4 of the floor area.  For a 12' span by 3.5' that's 42 sf.  At 50 psf load the floor has a load of 2,100 lbs.  Assuming you have standard rafters and not a structural ridge, your long wall piers are supporting 1/2 the roof load.  With a 2' eave that gives a tributary area of 12' by 9' for a total of 108 sf.  At 30 psf roof load you have a total roof load on the beam of 3240 lbs.  Combine those together to get the load on the beam of 5,340 lbs.

Plugging that load into the calculator ( http://www.forestryforum.com/members/donp/beamclc06b.htm ) and a dead load of 1500 lbs, you will find that for a 144" span with White Oak #2 you can have a beam 8x10.  If you were to add a pier, so that you have 8' span between piers, then your load calcs drop by a third: 3560 lbs (1000 dead load) at 96" span only requires a 6x8 beam.

The pier in the center I assume is supporting just the floor and not the roof.  With just one interior pier, you are again supporting your floor with a beam with a 12' span.  In this case the pier must support 1/4 of the floor load, or 3500 lbs.  Going back to the beam calculator, you need an interior beam of 6x10 to support the floor.  If you opt to have two interior piers, then the span drops to 8' and the load drops by a third.  For a floor load of 2333 lbs (467 dead load) you only need a beam 4x8 to support the floor.
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Offline kantuckid

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #10 on: December 31, 2017, 12:47:47 PM »
I was (suggesting is the word) using a span of 10' twice for the 20' length of cabin (3 piers) and span of 7' twice for the 14' width( again 3 piers) of cabin plus an 8' span for the front porch with 3 more piers across the front porch edge. Again, note that i'm gonna stick with 20' length, not 24' as considered earlier.
 It then seems that the 6 x 8 white oak beams will carry the load OK based on your notion? All the beams would lap or be jointed and be bolted down to piers.
 The center pier would help stiffen the floor system via two 10' oak timbers under the center.
 Twelve piers in total.
Hey, I'm open minded! Lets hear anyone's thoughts on this and thanks so far!
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Offline ChugiakTinkerer

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #11 on: December 31, 2017, 01:00:47 PM »
I guess I jumped to a 24' length right off the bat.  If you go with 20' then the loads would be smaller and you'd probably be alright for vertical load with 9 piers on the cabin.  The calculations I did are all assumptions and envelopes, but I might as well be thorough.  Will the roof be a gable roof, with rafters supported by the side walls?

Also, do you have any snow to worry about?  Snow load should be taken into account if there is more than 20 pounds per square foot in your area.  See your area at http://snowload.atcouncil.org/
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Offline kantuckid

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #12 on: December 31, 2017, 01:13:19 PM »
I live in snow country but since moving to KY I've only seen two winters, both in the late 1970's that it was near 3 ' accumulation and a few times of 1-2' since then. Since climate change (I'll not go there really) not much snow load to talk about.
Gable roof with sawed timber frame assembly on walls to hold it up.
 I'm now thinking I can come up with enough pine to build my walls and at that time of sawing my wall logs I'll know if I can get the roof structure from the same logs-if not I'll saw yellow poplar for roof frame. Thinking 4 x 6 roof framing as that's about as big as a one or two man show can handle, esp. an old guy.  :D
I had been considering oak wall logs as no question as to having enough oak to build with.
I put in the geo figures for my area and it says 15 PSI snow load for here.
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Offline ChugiakTinkerer

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #13 on: December 31, 2017, 02:13:03 PM »
 When snow load is so light, we go with the minimum roof design requirement of a live load of 20 lbs per square foot.  I took the liberty of calculating loads on each pier for a 14x20 with nine piers supporting the main structure.  Here are the tributary areas and their respective loads:

 

Piers 1,3,7, and 9 each see a load of about 2,032 lbs.  Piers 2 and 8 have a load of 3,801 lbs.  Piers 4 and 6 see 1722 lbs, and pier 5 sees 3,220 lbs.

The greatest load is on piers 2 and 8.  For clay soil with a compressive strength of 1500 lbs per square foot (psf) it should have a footing area of 2.53 square feet.  An 18" diameter footing only provides 1.75 sf.  In theory, with a fully loaded cabin your footing could be sinking into the ground.

Edit to add: I don't claim any particular expertise, and it's a great idea to double-check anything you read on the web.  If I were building with piers in your area I'd either add some footing square footage or get a soil test to know just how much load the soil can bear.
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Offline ChugiakTinkerer

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #14 on: December 31, 2017, 02:25:14 PM »
A beam running from pier 1 to pier 2 will be supporting all the load at 1 and half the load at 2.  That adds up to 4,132 lbs.  Using the beam calculator, with a span of 120" a 6x8 will fail.  A 6x10 or 8x8 will pass.

The beam running from 4 to 5 has a load of 3,332 lbs.  A 6x8 beam fails, but a 4x10 passes (barely).
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Offline kantuckid

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #15 on: December 31, 2017, 02:50:13 PM »
Some non-scientific facts-
My wood shop, 24 x 36 is a pole building done on PT rough sawed 6x6's in very similar soil. It was built around 1982-3 and shows no sign of settling. I bored the holes, poured around the posts sitting on a concrete discs/pads. Nearby is a storage building, same size that uses 4 x 6 PT posts I built in ~ 1998 and it hasn't sunk any either. They are both board and batten walls so nowhere near as heavy as a log wall and both have conventional truss roofs, asphalt shingles.
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Offline kantuckid

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #16 on: January 05, 2018, 01:14:24 PM »
My son did the math and stayed away from the thread to avoid pollution of his calculations until after he was done. He got very close to the same figures as Chugiak above. His figures came to 21.84 final, thus my 18"dia piers (sort of guessed at- :D) were shy of best practice.
We decided to go with 24" BigFoot or Redi-Bases and 10" sonotubes for piers.
For the purpose of clarity in the discussion I'll add that he does this stuff everyday and is licensed in GA & TN for Civil, maybe somewhere else too? but not in KY where he grew up.
   
I finally found a rental source for the 24" auger at Franklin Rentals, Lexington,KY- they have multi locations but new there-it's $50 day and can pickup day before take back another day or $100 weekly.
Mixer I'm going to buy, not rent.

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Offline peterpaul

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #17 on: January 10, 2018, 08:51:50 AM »
I no longer use any Sonotubes in favor of pre-cast piers such as these.  http://www.diversifiedprecast.com/index.php?n=precast_concrete_piers  Less work, no cement mixing, no piers out of place.  Less time from excavation to build.  The precast piers have an Engineers Stamp so our Building Inspectors allow them in our area but you should check local codes.  They are readily available in our area as well, not sure in yours.  Accurate layout and excavation is critical regardless of what you do.  To assist in layout, I made up a number of batter boards (like an "H") out of pipes with a sliding sleeve secured with a thumb screw on the horizontal leg. A layout string attaches to the thumbscrew.  This allows me to slide the string along the pipe and remove and replace string during excavation without affecting layout.  This allows for precise final pier location.  Do not over dig the hole so as to not disturb the naturally compacted soil.  I dig a hole below the frost line (4') in our area so I use 5' piers.  The bottom of the hole is critical.  I take a little pea stone (1" or so) for leveling with a 2' level and shoot the final elevation of the hole with a laser so that the top of pier is at the proper elevation.  I then set the pier in the hole using an eyebolt in the tapped hole in the top of the pier.  Setting the pier in the precise location is really easy as the pier can be moved horizontally after it is set in the hole.  By taking care to make the bottom of the hole level, the top of the pier is also dead level.  I backfill by hand around the base of the pier and walk around the pier to compact the fill soil.  Frost uplift is minimized/eliminated due to the smooth surface and taper of the pier.  Using a Simpson Strong Tie base such as an ABU ** insures that building uplift is also addressed by securing the beams to the pier.   https://www.strongtie.com/retrofitpostbases_postbases/aba-abu-abw_productgroup_wcc/p/aba.abu.abw

If I am pouring a foundation or slab and have a cement truck at the site anyway, then I will use a Bigfoot.  Also depending on the amount of moisture in the ground, the Sonotubes must be filled soon as possible after backfilling otherwise the sono tube may collapse.  In our area, per code, there is supposed to a pre-pour inspection which complicates using Sonotubes as well.  The only time I pour piers is if I am pouring a foundation and have a cement truck on site then I use Bigfoot(s) and pour them along long with the piers. 
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Offline kantuckid

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #18 on: January 10, 2018, 10:01:17 AM »
Thanks but nope.
 Pre-cast might be great when the cost is passed onto someone else's pocket but my budget is me, myself & I.
"Me-cast is way cheaper than pre-cast".
 Even more importantly I want the flexibility to choose my distance above grade AFTER I dig the holes! The possibility of depth I can get is a big variable on this build site, meaning to rock or rocks too large to auger past them. Only paper tubes give me that flexibility to choose after learning whats down there and to adjust the distance above grade as well so my log building has no splashback problem.
I get what your suggesting and can see from your well put description that it works for you.
In a related way I spent a good bit of yesterday fetching a Kushlan cement mixer in OH for a bargain.
Thanks!
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Offline ChugiakTinkerer

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Re: Poured Pier Foundation for Small Log Cabin
« Reply #19 on: January 10, 2018, 10:47:33 AM »
I had to laugh read I read about this thread polluting your son's calculations.  :D :D

Glad to hear you've got a path forward.  My cabin build will be on a remote piece of land that is only accessible by snowmobile in the winter and float plane in the summer.  I also want to build in accordance with best practices and meet the code as best I can, but my logistics will force some compromises that normally wouldn't fly in residential construction.  Having an engineer you can call on will go a long way in making sure any compromises you have to make will still result in a safe and sound building.
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