Log Cabin Foundation

jakez470 · June 24, 2018, 09:26:40 AM

I am currently building a 14x16 log cabin in the Upper Peninsula of Michigan. I'm in the early stages of this build and would like some advice on the foundation. I would like it up off the ground so some sort of pier setup is what I will use. I was talking to a guy that basically said do not dig down below the frost line. He said to just pour a 3x3 pad and build a pier off of that? He said that way the cabin will move with the ground and I wouldn't have to worry about the piers cracking. And if they did its a easy fix, unlike one that would be below the frost line. Any recommendations are welcome here. Thank you!

ChugiakTinkerer · June 24, 2018, 10:26:10 AM

Up here in the land of perma-frost that is known as a pad foundation. The structure basically floats on top of the ground and moves as the ground moves. Besides being cheap and quick, that's about the only benefits it offers. The house is not coupled to the ground like with piers, and it definitely does not benefit from the stability a perimeter foundation offers.

What is the frost depth in your area?

P.S. Welcome to the forum!

Don P · June 24, 2018, 12:58:53 PM

First check and see what the rules are regarding habitable buildings in your county, likely this won't work if they are enforcing building codes. If you are flying under radar don't build more than you can lose.

Piers are an engineer only foundation in the codebook because so many of these methods have failed. Piers offer little lateral resistance when the wind hits the side of the building or if the soil moves say when it gets saturated or frost heaves, they simply topple. They also tend to settle differently. All of this is why a perimeter foundation with a continuous footing is the standard method.

If piers are cracking under this guy setting them above frost line where they are more likely to heave up and down is somehow going to make them crack less? That doesn't make much sense. The better question might be, why are pier foundations cracking.

R404.1.9 is the section on piers, your exterior walls are the braced wall panels.
https://codes.iccsafe.org/public/document/VRC2012/chapter-4-foundations

jakez470 · June 24, 2018, 06:21:01 PM

Frost depth is about 60".

rjwoelk · June 24, 2018, 07:17:38 PM

I set 3x4x 5 inch thick pads out of a shoping mall floor. Onto packed gravel. My log cabin is 22x26. I then cut 8x16 perimter beams as foundation. This was all ok by the inspector. Year 3 and it is still level .

jander3 · June 24, 2018, 10:51:19 PM

Foundation for the Stump Ranch. Dug down 4' (frontline here is 42") . Filled with rock, sack-crete and rebar. Drilled stumps for rebar, coated them with oil and called it good. Roof keeps the rain off. Foundation still strong and level after 10 winters.

Don P · June 24, 2018, 10:58:09 PM

We had a discussion here on the home front a week or so ago that sort of pertains to what we are talking about here especially pier footing width and thickness. A wide footprint is a good thing but the footing also needs to be thick enough to support the pier safely. I had done the main house a few years ago and Rick is bringing in a modular house for an aging parent. It will sit on a perimeter crawlspace foundation with a row of piers down the middle inside the foundation. The inspector was kicking things up to an engineer as per a note on the plans and I was asked to comment. Anyway some of this might help with how you think through a foundation.

I'm just a bystander on this one, Rick and Will are the guys on the ground so they rule.

I think he just needs a sealed okey dokey letter. I don't know if you can do it by letter (that would be my choice if it'll fly) mark up the existing plan or will have to generate your own.

In layman's terms I would call the soil decomposed foliated rock, the striping was still quite clear digging out the basement of the house. >2,000 psf easy, that would be my default soil bearing in lieu of testing. It is better than double that.

My take on it, footings, reference table R403 and figure R403.1(1). That would be a 6"x12" footing.

I typically use 8x16 footings with 2 strands of #4 rebar under something like this.

Foundation walls, specify plain masonry (unreinforced) 8" block, max 6' tall, 5' max unbalanced fill. Reference R404.1.1(1)

Note the 13.5k pier, I'd specify a 32"x32"x12" thick footing w/2#4's ea way with a minimum of 3" of concrete cover pier solid grouted and call out dimension on the other piers, 2'x2'x8", double stacked crossed block, watch your heights unless grouted. reference R404.1.9 .

That is the best load detailed foundation plan I've ever seen....

Holler if I can be of any more nuisance,
Don

Will

Jun 13 (11 days ago)

to Rick, Lukas, me

I was thinking jimmy told me that the footing was always 2x wall thickness x at least 8" thick. Not sure what his hold up is. Footings on my house are 2'x8" with 8" steps to stay below the frost. Fireplace footing was 2' wider and 1 foot deeper than the shell will be. That's what he told me to make it. Lukas had it drawn out for me though. Btw jimmy was really bragging on my plans. Way to go Lukas.

Don & Michelle

Jun 13 (11 days ago)

to Will, Rick, Lukas

That 2x wall thickness is typical but not always. The 6x12 I quoted from code is a minimum, I've never done one that small.

The soil bearing capacity and load determines minimum footprint to avoid sinking. We usually have very good soils till you get into the silty bottoms. The top right pic is a typical masonry foundation, P cannot be less than 2". P cannot be greater than T so for an 8" thick footing the projection P can't exceed 8" or you are in danger of "punch through". another way to think about it is the load leaves the foundation wall and flows through the footing on the way to the ground at no shallower than a 45 degree angle... you see that logic throughout the code in wood as well, keep loads stacked or within 45 degrees through materials.
If for instance in a heavily loaded pier you have to go wide to get enough soil bearing, thicken the footing enough to keep P less than or equal to T so the pier doesn't punch thru the footing. The rebar grid 3" off the bottom is also always a good idea.
On that pier supporting the ridge LVL's the load is about 13,500 lbs, divide by 2,000 lbs per square foot allowable soil bearing capacity = 6.75 square foot of footing required. 32" square is 7 square feet or 14,000 lbs capacity, check. a doubled block pier is 16" wide which leaves P=8" of footing projecting out on all sides of the well laid out and centered pier. I would go a little over on a heavy point load and make the footing under that pier 12" thick and pour that pier solid. That is the one pier to pay attention to in the interior pier line.

Look at the bottom right pic, if the soil is competent enough under a concrete wall for the loads, you don't really have to have a spread footing. Quantify loads and provide resistance.

PA_Walnut · June 25, 2018, 06:22:50 AM

Quote from: jander3 on June 24, 2018, 10:51:19 PMDug down 4' (frontline here is 42") . Filled with rock, sack-crete and rebar. Drilled stumps for rebar, coated them with oil and called it good.

Nice idea! I am curious, as I'm about to begin a small cabin project. Did code/inspection have to approve this or wasn't it required or "asked for"? :D
Love the log idea.