12x20 bunk house build

[Dgratiot]

Hey guys, I have been lurking this forum for some time now, and building a cabin has been a pipe dream...until now! Next summer we will be breaking ground on a 12x20 "bunkhouse" to house us while we build the main cabin. The idea is a 1 room post and pier foundation. That's where the questions start. First off, I plan to use 9 10" sonotubes sunk below frost line. On top of that the Simpson strong tie adjustable post brackets for a 6x6. Sitting atop the 6x6 will be a triple 2x10 beam running the full 20'. Three beams running the 20' length with 5.5' between them. I'll build the floor with 2x8s resting on the center beam, and hangers around the rim board. My question is is 3 piers per beam enough? There will be 10' between them. According to all the information I can find on span tables and beam tables this is acceptable. Anyone else have experience with this? My other option was sinking 4x6 posts under the frost line every 5'. I'm concerned about rot there but then again pole barns do this all the time! Any advice?!

[MountainDon]

Welcome.
1a.  You don't mention, so I will ask. Do you need to go through the building permit process and build to code?  Virtually all states have adopted the IRC for their statewide code. Some rural locations may not enforce code or insist on building permits. However, many do. If you have not checked into that perhaps you should.

1b.  Pier and beam foundations are not approved by the IRC unless an engineer certifies it is okay.  Many reasons for that, but if the local jurisdiction strictly follows code that may be a factor.

2.  With a 12 foot wide structure you do not need a center beam. A 12 floor span can be easily handled with dimensional lumber. The allowable span will vary some depending on species, grade and spacing between joists. For example S-P-F, grade #2, 16"OC can span 12'3" using 40 PSF as a floor load and a deflection of L/360. A stiffer floor would use 2x10's. Lots of room for insulation. Floor joist and rafter span calculator here

One of the potential issues with pier foundations is that one may sink more than others resulting in a need to shim the pier to beam connection at some later point in time. If that happens along a center beam that is more and harder work. IMO, since a 12 foot span is possible I would do away with the center beam

3.  I believe the 10 foot lengthwise pier spacing is too great for a triple 2x10 built beam. I would suggest that there should be 4 piers down each 20 foot side, with a 2x10 triple-layer beam on top.  Floor joists could then be placed on top of the beams.  That would support a single story, floor, walls and roof, as long as the earth can support that PSI.  Splices in a built up beam should be made only directly above a pier. No glue needed; just a waste of materials. IRC Table 602.7(1) has some girder (beam) sizing info that is meant for figuring allowable spans above doors and windows. It can be useful for the DIY builder. Spans depend on the building width and can be extrapolated down or up to accommodate unlisted building widths.

4. You mentioned the piers would go below frost depth. Good, but how deep is frost depth?  If frost depth is relatively shallow (like 18 to 24") that is not deep enough to provide lateral support to the piers. That is another reason piers are not approved for use by code unless an engineer is involved.   Also, piers act as independent components. They may not sink or rise but may also rotate laterally around some point along the buried length. There is no rule of thumb, unless the thumb belongs to an engineer with a calculator  .  Generally, there will be much more pier buried than sticking above grade.

5. You mentioned the possible use of 4x6 wood posts. Use a 6x6, nothing smaller if you go that route. And look for foundation grade pressure treatment, although those can be hard to find in many places.

6. How far above grade is the beam placement planned to be?  If the built up beams have their lower edge closer than 12" to the ground the wood should be pressure treated. If the floor joist bottom edges are closer than 18" to the ground the joists should be PT. That is code. 

I hope that helps.

[NathanS]

Lots of good info from Mountain Don.

Location and code dependent -

One option would be to scrape away top soil, bring in a couple loads of stone, and build right on top of that. 12x20 is pretty small, just lay a couple of ground contact 8x8 - 20' in parallel and frame right on top of that. If something moves (unliklely) you could always jack it up and shim to level. Or pour a slab.