Pier foundation, 14x30 cabin

[midrover170]

Hello again.

The cabin I'm planning is 14' x 30'. There's is an 8 x 13 attachment for bathroom and utilities, but let's leave that out for now.

My question is this: for a 1 1/2 story cabin of this size I'm planning two rows of girders running the length of the build. Each row would have five piers, with approx 7ft spacing. The rows themselves would be nearly 12 feet apart OC. Also planning 12" inch piers with bell at bottom going three feet deep (I'm in Idaho). Girders would be 2 2x10s or twelves.

Does this sound adequate? I'm close to just hiring an engineer because the foundation is, well, important. I'm struggling to calc loads on my own and this bit of the build is just stressing me out.

Thanks.

[cbrian]

I did one like this myself, you can find all the span tables and girder sizing requirements online. No one on here will be able to answer your questions, it would only be a guess. You either have to know what your looking at as you build it, or hire an engineer that knows. Every building site is different, and if you are considering piers, soil conditions have got to be just right. You also have to think about wind, snow, and earthquakes stresses put on the structure, and know how to counteract those forces.

With that said, no, what you have there does not sound adequate. Going three feet in the ground is not a magic number. On my pier foundation some holes where 3 feet into solid bedrock while others where 8 feet deep, it all depends on the soil. You have to know what your looking at as you go or hire someone who does. As far as your girders and spacing, that doesn't sound like enough for a 2 story structure. Mine is a story and a half split level, my spacing between each post is 6', and all my girders are triple 2 x 12's. Again, that is for my house, and according to tables created by professionals, and some free opinions from an engineer, your house may be totally different.

If you do go for it regardless, read about soils, spans, loads, and review a continuous footing to tie all your piers together if soil conditions require it, and for the love of God, brace the thing like there is no tomorrow. Also review shear wall retrofitting and that will require you to start out with a continuous footing. Again all I can do is tell you to build it like it sits on a volcano because I don't know your conditions, maybe I am offering overkill advice, but like I say anything anyone tells you without seeing it and having the knowledge or degree required is just guessing.

[John Raabe]

An engineered pier foundation design is now required in most locations. This is because the bracing and load distribution on this type of foundation is more dependent on soil and site conditions than the standard concrete stem wall (crawlspace) foundation.

Here's an example of what one engineered project looks like...

[midrover170]

That looks really slick, John. Will likely do some x-bracing like that when I'm ready. I was at lot today and marked out all the footings. Talked to a geotech engineer last week and he said with the type of soil in the area, the post/pier plan I have should work well. He did recommend doing something like a Bigfoot footing under each, which gets me probably way more load bearing capacity than I need. My rows will be about 11'10" apart, with posts about every 6'6" along the row.

Digging footings in two weeks but probably no concrete going in this year.

-D

[cbrian]

If you can swing the extra cost I would put the rows closer together so you don't have bounce in your floor, unless you are planning on using engineered floor trusses. I hear you can span them for over 20'. Not to mention the closer the spacing between your rows the more effective your cross bracing will be. I also waiting on putting concrete in my footings, worst mistake I ever made, ended up having to dig them all out when they all sloughed back in.