My foundation plan, hopeing for a quick check before I start

[Micah]

Here is a google sketchup of what I have in mind.



The cabin is going to be 12x16 with a gable roof. Its going to be and open floor plan with small loft. Im going to use the same foundation that the Little House Plans use. Deck blocks over pavers. The cabin isn't going to have plumbing, electric, or drywall so alittle frost lift isnt going to a big deal. Im going to use 4x4 PT for the beams. I got a heck of a deal on them, $2 a piece! I plan to use 2x6x12's for the floor joists. Will they safely span 7'4" or should I jump up to 2x8's?

[Native_NM]

PT wood is not always the best species for framing in my experience.  4x4 seems a little small in my opinion also.  I'll look at the tables and see what I can find.

[Micah]

I agree I hate working with PT lumber but with how close to the ground its gonna be Im gonna have to.

I hope the 4x4 will work. I don't what to have to spend another $50 on 2x6 to make a 4x6 beam. Then again its not like I can't use the 4x4 else where. Like on a deck or something.

[dug]

There is a beam calc. here- http://countryplans.com/smf/index.php?topic=10511.0

Though it only goes down to 20 ft widths it can at least give you an idea. At that building width your beam would only span about 2 ft. Two 2 by 12's would be needed to cover the 5 and 1/2 ft span.

I really think the 4 by 4's are dangerously undersized for the job, I would think 4 by 8's at least- but that's just a semi educated guess. 2 by 6 joists should be fine for that span.

[Micah]

Now this may sound stupid believe me I debated myself about asking this, so if it sounds dumb I was only joking!

What if I where to put 2x4 on top of the 4x4? That would make a 4x6. I could glue it and lag screw it or nail the heck out of with some big zinc coated nails. Or is my best bet gonna be to suck it up and make a 4x6 build up beam?

[Native_NM]

There are no stupid questions.  The stupid thing is to NOT ask the question, and regret it later.  

The issue with building the beam in that manner is the direction of the grain.   Wood strength varies depending of how the load is applied relative to the grain:  perpendicular or parallel.  


↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓
=============
=============  
=============    

is stonger than:

↓↓↓↓↓↓↓↓↓↓
----------------------
------------------
----------------

The problem with most PT wood is they use the center of the tree, which results in generally weaker lumber for certain applications depending on the specific piece of wood, the slope of grain, and a few other factors. PT 4x4's are usually used as posts for that reason.

You might get away with it, but attaching the 2x4 to the top still has the grain running the wrong way.  You can buy 16' lumber at the big boxes.  (4) 2x8 x 16' DF in Albuquerque will run you about $65.  The piers you plan on using will keep it off the ground enough to eliminate the need for PT wood, unless there is something we are not seeing.  Find some scrap plywood and make 2 laminated beams for $100.  The foundation is not the best place to try and save a buck in my opinion.  

[Micah]

That explains alot. I didn't even think of the grain. I was just looking of thickness. Im very glad I asked.

Your right about probably not needing to use PT. I can make the height off the ground anything I want (with in reason). I have to use 8' pieces because I don't have anyway to transfer 16'. At my local lowes all I can get is this

http://www.lowes.com/pd_36774-99999-LBR-36774_4294807182_4294937087_?productId=3177231&Ns=p_product_prd_lis_ord_nbr|0||p_product_qty_sales_dollar|1&pl=1&currentURL=%2Fpl_Dimensional%2BLumber%2B_4294807182_4294937087_%3FNs%3Dp_product_prd_lis_ord_nbr|0||p_product_qty_sales_dollar|1&facetInfo=

Is whitewood #2 going to be strong enough? Everything I have read calls for DF #2.

[davidj]

If you assume something like 40psf live load, 10psf dead load on the floor and 30psf live load, 10psf dead load on the rooof, then you have 90psf load.  Those 4x4 sections are each carrying 6' x 5 1/2' = 33 sq ft of load (6' because the other beam carries the other half of the width).  I.e. a distributed load of maybe 3000lb.  That's per beam section.  So one way of thinking about it is there's a mid-sized car on each section.  Personally I'd be uncomfortable hanging my car from a 5' 4x4!!

Usually engineering calculations are pretty conservative and on a small cabin/shed then you can probably get away with a fair bit less than code would insist on and it won't break.  But you are off by a large amount here, to the point that it very well have problems.

Also it's worth remembering that a 4x8 is not just twice as strong as a 4x4, it's many times stronger (too late on a Friday to do the math!).  And putting two 4x4's on top of each other doesn't make an 8x8 because the 4x4s can move relative to each other.

If you want to have a rough guide as to the difference between beams on joists, just remember that for projects like this a beam should give the impression of being about ten times stronger than a joist.  If your joist feels like it would be fine with a few people bounding up and down on it, then your beam should feel like it could handle a few elephants (or F150s!) bouncing up and down on it.

[Don_P]

I didn't check real deeply but the 4x4 girder will fail miserably in every check, bending, deflection and shear. A double 2x8 #2 SPF or better will pass at David's load.

For bending strength the section modulus times the design strength of the wood gives the allowable stress in a beam. The formula for determining the section modulus is (breadth x depth squared)/6... notice how heavily depth is favored here. A 3.5x 3.5" beam has a section modulus of 7.14"^3 where a 3.5x 7" beam would be 28.58"^3... 4 times as strong. Or, doubling the depth quadruples the strength. But stacking 4x4's one above the other is not doubling the depth of the beam, you have 2 beams of the lower section modulus.

[Native_NM]

Micah,

The consensus is the 4x's are too small.  There are places to scrimp; the foundation is not one of them.  Now, about those deck piers...

[Micah]

Thanks everyone. Im glad I asked before I started. Ill be going to lowes and getting some 2x8 and a sheet of 1/2 osb.

[John Raabe]

The Little House plans include a similar foundation for a 12' wide rustic cabin. This Little House foundation is the minimal structural configuration I have found for a durable structure. It uses 4x10 beams by the way and has other details you should consider. For instance, you could build-up a beam from 3 PT 2x10's - see: http://www.countryplans.com/builtupbeam.html (leave out the plywood for your girder).

RULE of the FOUNDATION: The more money you are going to put into the house and site work the more you should invest in a solid permanent foundation.

If you are putting in a well or water system, a septic system and building a cabin with electricity, heat and the other trappings of modern life, then you will be investing many thousands of dollars. In that case a conventional perimeter concrete foundation may be a good investment and will protect the money you spend on the full house above it. Site built houses, done properly, appreciate over time. Trailer houses, shacks and less solidly built cabins decrease in value over time because they are not seen as permanent.

That said there is a place for buildings that can be put up quickly and inexpensively and provide a more rustic living environment. Many site members have built versions of the Little House plans and enjoyed the process and the structures they build.

Here's one of similar size that was recently sent in. (Built in Fiji - click the image for an info thread.)

[MountainDon]

I might have missed it but don't think I saw a reply to the question of "PT or not to PT" Answer is ... beams should be PT if closer than 12 inches to the ground. Joists should be PT if closer than 18 inches to the ground. (by code)

Making the cabin higher off the ground by increasing the length of those little posts that sit on the concrete blocks can lead to issues with instability. A post resting on a concrete bock/pad creates two hinge points. One at the concrete to post and the other at the post to beam. Lots of bracing in both the N-S and the E-W directions in any event and higher needs more.


Frost movement will/can still affect windows and doors.

G/L

[Micah]

Its been awhile since I replied to this post. I decided to go with PT all the way around. I think I might even do the floor joists PT aswell. I decided to go with a 4x8 build up beam. I only have one last question before I start digging. Are 4x8 beams going to be strong enough with only 3 piers per beam? Would upping it to 5 piers per beam be a good idea or would that be over kill?

[MountainDon]

To answer that correctly I'd have to re read the entire thread to see what we're dealing with. Sorry, no time or energy for that right now. Building size, number of floors, that sort of thing..... Does the soil have sufficient bearing capacity for three piers?

Do you have actual purchased paper plans? What do they say?

I don't mean to be rude or to blow you off but it's either ask that or say nothing at all right now.

[Micah]

Hey Don I completely understand!

I actually own the Little House Plan. I'm building 12x16 single story with 12/12 gable roof so I can have a small loft. The 12x18 plans call for 6 piers and 4x10 beams. Im doing a 12x16 with 6 piers and 4x8 beams. I was up there today and once you get through the ground cover vegetation the dirt seams to be perfect. My plan was to dig 12-16 inches down back fill with 12-14 inches of gravel top that with a 12x12 inch paver then put the dek bock on that.

I had seriously considered just doing sona tubes but after digging a few holes around the building site I decided against it. I was pulling huge granite boulders out about the size of watermelons. Its gonna take me forever just to dig the 6 shallower holes.

[MountainDon]

Here's a quick answer.

I see back in this thread Don_P figured a doubled 2x8 should be okay. If you want to be more assured for the strength I have 2 simple solutions.

1. Use one more post per beam, spacing them at approximately  4 1/2 to 5 feet apart.
2. Use 2x10 for the built up beam.

G/L

Be sure to install bracing (piers) before getting too far along. Best is to do that before you start the wall framing.