Pier foundation question 24' X 32' Gambrel roof cabin

Started by Ricky J, April 04, 2017, 09:57:28 PM

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Ricky J

I am building a gambrel roof cabin that is 24 X 32. I was thinking of 20 piers. 5 along the 32' length and 4 across the 24' width. Is this enough?
Should I put more ? if so placement or spacing suggestions.

ChugiakTinkerer

Welcome to the forum!

With that many piers you may as well put in a continuous perimeter foundation.  Beyond that, it's hard to make any recommendations without knowing what your soil conditions are, ground snow load, etc.  For a structure as large as you're describing, I suspect you are going to invest a lot of money.  You'll probably want it to last, and the best way to ensure that is to build a foundation in accordance with residential codes.  If you don't have to deal with a building code inspector and are interested in learning more about post and pier construction you should search this forum using the keyword piers.  Here are a few threads with more info...

http://countryplans.com/smf/index.php?topic=14659.0
http://countryplans.com/smf/index.php?topic=14630.0
http://countryplans.com/smf/index.php?topic=14593.0
http://countryplans.com/smf/index.php?topic=14470.msg189287#msg189287

Post and pier foundations are useful for small structures or ones that aren't full time residences.  If you go down the pier route, you'll need to have enough footprint to support the design weight of the structure.  Assuming a second story and no snow load, your design load will probably be in excess of 100,000 lbs.  If you are in decent soils that can support 2,000 lbs per square foot then you'll need at least 50 square feet of pier footprint.  You can go with many smaller piers or fewer larger piers.  Just remember that every post will require diagonal bracing.  The more posts, the more bracing.  At some point you may be better off moving to a continuous foundation such as poured concrete, masonry blocks, or permanent wood.
My cabin build thread: Alaskan remote 16x28 1.5 story


Ricky J

Piers seem more economical for a cabin. I am looking at 18" round footings and 10" sonotube piers. For 20 footings that gives me 35.34 ft^2 for bearing area. A 32 X 24 cabin is 768 ft^2. At 100# per square foot that gives me a soil bearing pressure of 2173" /ft^2. Sound reasonable?

azgreg

I'm not a fan of piers myself, but this build seems to be about the best of them I've seen on this forum.

http://countryplans.com/smf/index.php?topic=12576.0

Ricky J

Seesm like that pier foundation would withstand a nuclear blast. Had something much less involved in mind.


akwoodchuck

With that amount of concrete you could pour yourself a continuous footer....slap a treated stem wall on top (material you'd have to buy anyway if you're skirting it), and voila! Not a bad payoff for two days of work.... [cool]
"The lyf so short, the craft so long to lerne."

Don_P

A treated stemwall doesn't have to be on concrete, a gravel filled trench would suffice... a permanent wood foundation.
A post frame building is another way to achieve a braced structure. This is an old sketch of a smaller structure but the idea is the same. The perimeter posts extend unbroken from footing to plate. They are braced by the wall sheathing.


ChugiakTinkerer

It still goes back to what are you building and where?  If snow load is NOT a concern, you still need to allow for 50 lbs per square foot per level of living space, plus 30 lbs per square foot for the roof.  If you are building in an area that has a high ground snow load you would need to allow for yet more weight on the roof.  You can see what your snow load is at http://snowload.atcouncil.org/

Allowing 12" eave and rake overhang for the roof, your roof area is 26' x 34', or 884 s.f.  The minimum design load for the roof is 26,520 lbs.  Your 100 psf for the two floors comes out to 76,800 lbs.  Add them together and you get 103,320 lbs.  Divide that by 35.34 s.f. and your average load on the soil will be 2923 psf.  But houses don't distribute load evenly.  The middle piers may only be supporting the ground floor.  The side piers then take up more of the load, and you may end up with upwards of 5,000 psf on an individual pier.  If your soil is good for 2,000 psf do you think your footings will be adequate?

In the original post, you ask if 20 piers is enough.  With the size of pier you want to use, the answer is no.  I would look into a permanent wood foundation to get a continuous perimeter foundation that will provide support along its entirety and give the bracing needed to prevent racking from winds or ground movement.  It would definitely be faster than pouring concrete and wouldn't surprise me if it comes out less expensive as well.
My cabin build thread: Alaskan remote 16x28 1.5 story

Ricky J

Upstate NY. Frost penetration of 48". Snow load of 40 PSF.I was looking at one floor and a half loft. A gambrel roof with no overhang. Soil is mostly clay with channers (flat rocks) all over the place. I can't put a shovel in 8" without hitting a rock. You never see wood foundations around here. Piers, perimeter  block or poured concrete. Have to go down below the frost line.


ChugiakTinkerer

Will the loft be free-span or will you have load bearing support under it?  Same with the roof.  If the side walls are bearing most of the load you will want to consider a bigger pier footprint.  Even with rocks in the ground, if you have a lot of clay the soil strength will be on the low side.

The way to look at load on a given pier is to add up the loads from the tributary area of each surface the pier supports.  It sounds like your loft is free span and you do not have a structural ridge.  That is the scenario that would yield max load, well also if your floor is free span but it sounds like you'll have center support.  A BOTE estimate for the max load will be on the pier that is supporting the floor, the loft, and the roof and not on a corner.  Your floor tributary area will be 8' x 4' @ 50 psf for a total of 1600 lbs.  Your loft tributary area will be 8' x 12' @ 50 psf for a total of 4,800 lbs.  Your roof tributary area will be 8' x 12' @ 50 psf (40 snow load and 10 psf dead load) for another total of 4,800 lbs.  Add those up and you're looking at 11,200 lbs.  Assuming a 2,000 psf soil strength, you should have a pier footprint of 5.6 s.f.  That equates to a circle with diameter of 32".

If you want to go with piers, you may need to have a structural ridge, where you have the center of the roof being supported by walls or posts that provide a direct line of support all the way down to the ground.  That will reduce the tributary load from the loft and floor significantly.
My cabin build thread: Alaskan remote 16x28 1.5 story

SouthernTier

Quote from: Ricky J on April 06, 2017, 11:43:02 AM
Upstate NY. Frost penetration of 48". Snow load of 40 PSF.I was looking at one floor and a half loft. A gambrel roof with no overhang. Soil is mostly clay with channers (flat rocks) all over the place. I can't put a shovel in 8" without hitting a rock. You never see wood foundations around here. Piers, perimeter  block or poured concrete. Have to go down below the frost line.
Another upstater here, although "upstate" can mean many different things depending on where you are in the state.  You soil sounds like mine.

I personally haven't seen any PWD foundations, but one guy here has done one in upstate NY and it appears from his thread that is is working well.

http://countryplans.com/smf/index.php?topic=10671.msg139962#msg139962

Also this one, but his photos have disappeared (but his build came out great).

http://countryplans.com/smf/index.php?topic=10791.msg140128#msg140128

Both these guys are in the Adirondacks, which is different than, say, the Southern Tier.

I was considering a PWD for part of my cabin, but now am thinking something different (need to get a post up soon).

jsahara24

If you're set on piers you may want to look into "bigfoots", basically a spread footer that connects directly into sonotubes.  I'm building in upstate/central NY, I can't figure out where that line is, and most of the cabins up there use piers against the recommendation of most on this forum.  Our soils are sand/gravel which is going to give you pretty much the highest soil bearing you can get except for bedrock, however bedrock has its own issues.  Clay would be the worst and likely around 2000 psf. 

Its doubtful you will put in a continuous perimeter foundation and regret it, however I am sure many have regretted piers.  I understand cost and ability is a concern which is why many lean toward piers.  Good luck with either route and keep us up to date!

NathanS

All of NYS is under the IRC code which does not allow pier foundations.

SouthernTier

Quote from: NathanS on April 06, 2017, 04:42:02 PM
All of NYS is under the IRC code which does not allow pier foundations.
...unless under a design stamped by a PE

Nathan: great build in your main thread.  Lots of great stuff I can use as I *finally* get going on my cabin this summer.


NathanS


Ricky J

I was told by the local building inspector piers are acceptable. I am in Cattaraugus county, the south western part of the state. I was thinking of the 18" diameter bigfoot and 10" sonotubes. I see a lot of pier foundations on cabins around here, so I doubt they are not legal. I was told under 1500ft^2 no stamped plans. If you look at this link, you will see a bunch of their big barns on piers. http://buffaloshed.com/html/barns.html. NAthan, where in the IRC code say no piers? I see many images of building with piers on their site.
https://basc.pnnl.gov/images/typical-installation-double-section-modular-home-pier-and-ground-anchor-foundation
https://basc.pnnl.gov/images/polyethylene-completely-covers-floor-crawlspace-and-attached-walls-and-piers-well

Also virtually all of the state cabins at Allegany State Park near here are built on piers.
https://parks.ny.gov/PhotoGallery/Default.aspx?gal=Cabins%20and%20Cottages

SouthernTier

Quote from: NathanS on April 06, 2017, 06:59:05 PM
Thanks neighbor. :)
Your welcome.

But actually, Ricky sounds like an even closer neighbor.  I am in Cattaraugus Co. 

Yes, piers can work well in CattCo soil.  Check out my shed thread in my sig.  But I didn't need to get a building permit for that.

And I'm spending this coming weekend in one of those Allegany State Park cabins on piers.  They probably are either PE-stamped, or built a long time ago and were grandfathered in.

Ricky:  Stop in at the ASP summit cabins this weekend if you want to say hi.  Got a big group there for trail work.  We're grilling Saturday evening! (will be out in the woods during the day).

NathanS

That's interesting the code officers allow piers - that is really all that matters.

https://codes.iccsafe.org/public/document/code/446/7992266

This is the foundation section for IRC/NYS - no provisions for pier foundations. If you go to 403.1 it says the footing must be fully continuous.

With piers you are relying on the soil to brace your structure horizontally. Over here in Chenango and Otsego there are virtually no old buildings on piers, and the stuff that is 30-40 years old has moved a lot.

flyingvan

#18
  That's sure the case here.  No old buildings on piers.  When people build retaining walls, the soil quickly reaches the top of it---soil on slope is always flowing downhill.  Two of the things I've learned since haunting this site are, once people decide to go with piers, there's no changing their minds.  The other thing is, there are quite a few of us (myself included) with bad experiences with piers.  Saving a nice house with a failing foundation is no fun.  It will be fine in the short term.  My feeling is, I want the structures I put my heart and soul into to still be around in 100 years...
   I think the UBC designs stuff for a 30 year lifespan.  Depending on a whole bunch of variables, a pier foundation might last that long.  On bedrock, even longer.  There are other disadvantages besides outright failure though---earth/wood contact being one.  Airflow underneath a house really drives up heating costs.  All your plumbing has to be carefully freeze protected.  There have been breaches in home security by people coming up through the floor.  Homes in the wildland interface are difficult to insure, and difficult to protect from fires, trapping heat under a structure with plenty of airflow to cause problems from a small ember
Find what you love and let it kill you.

Ricky J

Southerntier I am in Lyndon (as I call it a suburb of Franklinville) 3 miles past Case lake. There is a cabin larger than what I am proposing on 6 X 6 wooden piers across the street from the lake up on a hill.


Ricky J

Thanks for the input. I am rethinking my choice. If I put up a perimeter wall I can go with 2 X 10's and still have a stiff floor If I run support up the middle parallel with the long axis. Next question. I must go down 4' so I am under the frost line. That means 8 courses of block. Should I just excavate the perimeter and then backfill? Open up the interior as a crawl space? kind of a shallow basement?

flyingvan

Find what you love and let it kill you.

Ricky J

No radon, a little cow manure next door, but no radon.

Don_P

Going down 4' you are beginning to see why a basement is common up north and a crawlspace is more common in the south where the frost depth is much shallower. The logic was, if you're down 4' what is 2 more feet, project 2' up and you have a useable full basement. As time has moved forward there are a couple more options, a frost protected shallow foundation is one, it allows for a shallower footing by insulating from the frost. A gravel trench and wood foundation is another, a precast foundation on a gravel footing is a third.

flyingvan

Without radon I'd go the basement route.  Great storage, a place that stays cool in the summer and freeze protected in the winter.  Plumbing, mechanical and some electrical will be accessible.  You might be able to put HVAC and water heater freeing up interior space. 
Find what you love and let it kill you.