Author Topic: 20x32 in Alaska  (Read 1345 times)

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Offline Backcountry Joe

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20x32 in Alaska
« on: February 12, 2017, 09:02:27 AM »
Hello all,

Well we bought the land and the plans and are ready to rock and roll as soon as breakup comes. We're building about 100 miles south of Fairbanks in Denali borough so building code. We'd really like to stay as close to the design as possible but there are a few things that are real expensive this north so I have some questions about substitutes. I need to go down about 6 feet to get past the frost line so I was looking at using railroad ties for my foundation posts. Does anyone have any experience with this or a reason not too?

Also, can I use 2x12 20ft for the floor in place of the I beams?

Offline ChugiakTinkerer

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Re: 20x32 in Alaska
« Reply #1 on: February 13, 2017, 09:41:23 AM »
I think most folks would just go ahead and put in a concrete pier using a sonotube.  If this will be your full time residence you might consider how much time and effort you will be investing, and perhaps it is worth spending a little more on a foundation that is known to last.  For a shed or weekend cabin it may be worth the small savings to use ties.  I recall seeing on the web somewhere some pole barns built using railroad tie piers, perhaps it was on the TractorByNet forum.

2x12 joists could possibly span the 20' floor in place of I-joists.  It would depend on the species and grade of wood for just how long it can span.  The international code for wood floors is here: http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%205.html
It shows the allowable span table.  You want to scroll down to the one for living areas, and select the appropriate dead weight column.  If you have dead weight of 10 lbs per square foot and put your joists on a 12" spacing then you can span up to 20'-11" with a 2x12 Doug-Fir graded at #2 or better.  If your lumber yard sells southern pine, a 2x12 graded at #2 will only span 19'-1".  But putting joists at 12" spacing means you will need 33 joists to cover the 32'.  While it might be a little less expensive buying the 2x12, it will make your work a heck of a lot harder if you are running any plumbing or electrical lines through the joists.  Insulation in the floor could be a little more challenging too.

I-joists can span the floor on a 16" spacing.  There are different sizes and strengths, but take a look at the Boise-Cascade BCI 6000 in a 11-7/8" size.  With 16" spacing it can span up to 21-6".  At 16" spacing it would take 25 I-joists to cover the 32'.  If your prices are comparable to the Spenard Builders Supply online store, the I-joist has a minimum length of 24' and costs $55.99.  For 25 joists the total is $1399.75.  SBS sells the 2x12 DougFir @ 20' for $37.99 so the cost for that is $1253.67.  That is a difference of $146.08 which may or may not be worth the savings.  I have never built with I-joists so I don't know what's involved in their installation, whether they are easier or quicker to build the floor compared to sawn lumber.  If your plans include any utilities in the floor then you may need the extra 4" just to make installation possible.

You could probably get by with 2x12 on a 16" spacing if you can get Select graded lumber.  Such a board in DougFir, HemFir, or SprucePineFir would have the strength to span 19-6".  You would need to avoid southern pine.  If you can source Select 2x12 at 20' at price equal or less than the I-joist it would be a no-brainer.  Alternatively, you could go with a different floor design that does not have joists free-spanning the entire 20'.  You could place a few piers in the interior to support a beam that runs the length of the floor.  With a long center beam each joist would only have a free span of under 10'.  With that span you could go with 2x8 on 24" spacing and have just as rigid a floor.  In such a scenario, the costs for floor joists would be 17 of 2x8 @ 20', which with a price of $23.99 each totals $407.83.  That $800 to $1,000 savings doesn't come free though, as you would need piers and a beam to support that floor.

Edit to add: Bear in mind the span tables are based on the allowable deflection ratio of L/360.  This is usually fine for most flooring.  If you might have tile flooring in part of the building, you will want a stiffer floor, and it would also add to the dead weight of the floor.  I would have to look it up, but I think the standard for tile is L/480, or perhaps L/720.  In this case the L is a length of 20' and the allowable deflecton for L/480 is 1/2 inch.  If your floor has too much bounce or flex, then ceramic or stone tiles will crack.  So if you want tile flooring you'd have to throw out all my estimates above and go with something more conservative.
My cabin build thread: Alaskan remote 16x28 1.5 story

Offline Backcountry Joe

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Re: 20x32 in Alaska
« Reply #2 on: February 26, 2017, 09:46:52 AM »
Thanks for the info Sir.

I've built remote cabins in the past but this is the first run at a full time place. I'll look at the price of concrete but it isn't a common material used around here so I figured there was a reason.

Joe

Offline ChugiakTinkerer

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Re: 20x32 in Alaska
« Reply #3 on: February 26, 2017, 12:05:58 PM »
Lots of things to worry about, I know.  You might consider going to a narrower floor plan.  A 16' width gives you a lot more flexibility.  If you have good soils then perma-frost shouldn't be too much of a concern.  A foundation that rests on top of the ground is used where there is potential for thawing of permafrost.  That's what I'm looking at for my remote build.  An excellent example of a railroad tie cribbing foundation is here: http://forums.outdoorsdirectory.com/showthread.php/144384-Logs-have-arrived!?p=1547095&viewfull=1#post1547095
My cabin build thread: Alaskan remote 16x28 1.5 story

Offline Backcountry Joe

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Re: 20x32 in Alaska
« Reply #4 on: March 17, 2017, 01:17:04 PM »
I looked at several examples of that style and talked to a lot of folks around here. Where I'm at the frost line is 4 foot with snow and about 7 foot without. Some of the man camps around use a pad and pier foundation but I just can't wrap my head around that for long term.

Offline ChugiakTinkerer

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Re: 20x32 in Alaska
« Reply #5 on: March 17, 2017, 04:21:30 PM »
If permafrost is not a concern, a frost-protected shallow foundation might be an option. 

http://www.cchrc.org/sites/default/files/docs/Draft_Final_Report_FPSF_CCHRC-6-15-08.pdf
My cabin build thread: Alaskan remote 16x28 1.5 story

Offline Backcountry Joe

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Re: 20x32 in Alaska
« Reply #6 on: March 20, 2017, 09:23:50 AM »
Thanks, I'll take a look at that.

If I were going to do a pad and pier foundation what size would the pads need to be? I was thing 24"x24" 6" deep but really have no idea.

Offline ChugiakTinkerer

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Re: 20x32 in Alaska
« Reply #7 on: March 20, 2017, 10:49:38 AM »
It all comes down to what the design load of the house is and what the load-bearing strength of the soil is.  A useful way of determining the load is to add up the design loads for all floors plus the roof.  Living spaces are typically built to withstand a live load of 40 pounds per square foot (psf).  The weight of the floor construction itself is what is called a dead load.  If you know what that weight is you should use it, but the minimum to use is 10 psf.  Combine those and you are looking at a total load for the ground floor of 50 psf spread over 640 sf, totaling 32,000 lbs.  You do a similar calculation for the roof.  The ground snow load for your area ( see http://snowload.atcouncil.org/ ) might be as high as 80 psf.  Add in 10 psf for the dead load of the roof.  Allowing for 24" overhang of the eaves and 12" overhang of the gable your roof has an area of 28x34.  Putting up to 90 psf on an area of 952 sf gives a total roof load of 85,680 lbs.  Add the roof load to the floor load and you are looking at a total of 117,680 lbs.  If this is a two-story house, add another 32,000 lbs for the design load of the second floor.

Unless you are in a bog, your soils should be able to support at least 1,500 psf ( see https://www.nachi.org/structural-design-foundations-home-inspector.htm ).  Using 1500 for a worst case estimate, the pads and footings must have a big enough area to support the house.  A load of 117,680 lbs on soils of 1500 psf strength means you need a foundation area of 78.5 square feet.  If you are on gravel or sandy gravel, the presumed load bearing strength is 3,000 lbs per square foot, so then you would only need 39.3 sf of foundation support.  Knowing the area you need and deciding on how many posts you want to support the beams will guide you on how big the pads need to be.  You can make up for smaller pad area by having more of them.

If you haven't seen it before, have a look at http://www.cchrc.org/sites/default/files/docs/DesignManualforNewFoundationsonPermafrost.pdf for recommendations on foundations in permafrost areas.  The post and pad foundation starts at page 73.  One important thing to notice is that the organic material is removed and replaced with non frost susceptible (NFS) material such as sand & gravel.  Filling in with gravel and compacting it gives you a larger load bearing surface.  The load from the pad is distributed down and out at 45 degrees, sort of like a pyramid.  If the gravel is 6" deep, it extends the load bearing area outward by 6".  If the gravel is 8" thick it extends the load area out by 8", etc.  For a 24"x24" pad, if it is on a 6" thick gravel base of 36"x36" then the pad has a load bearing area of 9 square feet.


My cabin build thread: Alaskan remote 16x28 1.5 story

Offline Backcountry Joe

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Re: 20x32 in Alaska
« Reply #8 on: March 20, 2017, 04:34:52 PM »
Just so I'm tracking, a 24"x24" pad, if it is on a 6" thick gravel base of 36"x36" then the pad has a load bearing area of 9 square feet and if I needed a foundation area of 78.5 square feet, I would divide 78.5 by 9 which would tell me I need at least 8.72 pads to support the load? 

Offline ChugiakTinkerer

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Re: 20x32 in Alaska
« Reply #9 on: March 20, 2017, 06:35:49 PM »
Yes indeed, you are correct.  To look at it another way, if you spaced posts every 8 feet, there would be 5 posts along each 32' wall.  With 10 posts, each pad would only need to have a load bearing area of 7.8 square feet.  That is a square 33.5" on a side, or a circle 38" in diameter.

Pad and post foundation is a great way to cope with the gravity load stresses when the ground is expected to move, either from frost heaving or permafrost thawing.  It is quick to build, relatively inexpensive, but it terrible in resisting lateral forces or uplift.  For a remote cabin the cost of foundation failure isn't really a big deal in terms of financial cost.  If you are investing a huge amount of your time and money into building a full-time residence, you really owe it to yourself and your loved ones to build something that can withstand an earthquake or fierce wind storm.  You are way more likely to get that with some form of continuous perimeter foundation, either concrete, concrete blocks, or permanent wood.  I would only go to piers or pads if ground conditions made all the other options impractical.
My cabin build thread: Alaskan remote 16x28 1.5 story