Long joists on a post and beam foundation

[Drew]

Hi all,

I am planning to build a straw bale building with external dimensions of 18'x24'.  I want to do a post and beam foundation because of the remote location of the site and the fact that my family and I have some but not a lot of building experience.

It's going to be non-load bearing using bale wrap for framing, so the bales will sit on the outside of the framing timbers.  With an 18" two-string bale, I get 15' of the 18' width inside.  I mention this to explain why I want 18'.

I've read the Small Home Plans and checked out the joist span calculators.  Accordingly, I would need to put my 12"x2"x18' joists on 12" centers (This is with a 10/40 dead/live load - I still get to calculate my requirements).

Long joists are expensive and difficult to transport to the site.  Can you folks give me an idea on some options I should look into?  Should I try any of the following?  I have no idea if any of these are reasonable practices.

a.  Incorporate a third beam in the center of my joist span, effectively reducing the centering and lumber size requirements .
b.  Use shorter joist lumber and join them in the center of the span with hardware.
c.  Don't try to make an 18' wide post and beam foundation - They are only feasible up to X feet.

Thanks in advance for the advice.

Drew

[glenn kangiser]

I don't have the complete picture, Drew.  Could you post a sketch - MS Paint will work for that - JPEG to Photobucket then paste the bottom link from Photobucket here.

Do the bales sit on the ground around the framing?  What keeps them off the ground and dry.  A beam in the middle could cut way down on joist sizes.

Welcome to the forum.

[Drew]

Thanks for the quick reply, Glenn.

I'll get the picture posted once I get to my home machine.  In the meantime, I'll try out my descriptive powers...

The framing and the bales will sit on the 18'x24' floor, which sits atop the foundation beams.  Nothing but the foundation footings touch soil.

The bale wrap framing technique is simply where the 18"x14"x36" bales are stacked on the "outside" of the framing timbers.  When finished, you'll see the bare timbers inside the house and the plastered bales pressed up against them.  This is different than infill framing where you stack the bales in between the framing members and plaster over everything.

Bale wrap framing is good for post and beam because the floor is cantilevered over the beams.  If I cantilever by 18" (Plus another 2 inches for my plaster), I can place my framing timber on the floor right above the foundation beam (transferring the load to the foundation footing), and place my 18" bale on the outside of the timber so the bale and plaster will be flush with the edge of the floor.

And I read your reply for the third time: "A beam in the middle could cut way down on joist sizes."  Well, there you are.

So if I read you right, I can use the joist spans between the beams for my joist size calculations, right?  In other words, if I have two beams 15' apart, I need to size my joists for a 15' span.  If I add a third beam in the center, I only need to size my joists to support a 7 1/2' span?

(Yeah, it seems obvious, but let me tell you about the lumps I got from the "obvious" )

Thanks again!

Drew

[glenn kangiser]

That's right, Drew.  We have others who have taken that option.  One thing to consider on your arrangement is that you will have a pretty significant load out on the cantilever, so be sure to anchor the inboard ends of your joists -- so they don't try to tip out.  I would think you would have about 200 lbs per foot of straw - about 1/2 of that to consider lifting up on the joist in the center of the room.  Weight and leverage of the joist will probably handle it but the force will be there.  Probably about a 7 to 1 leverage so about 20 lbs uplift in the center --for the straw.  Mud  will increase that by about 150 lbs per cubic foot with more reacting against the center of the room end of the joist -- since the weight is concentrated at the outside , probably a full 7th or so will lift up in the center -- probably around another 30 lbs -- I just do round figures then if it's too close I check things out better.

Flooring will probably splice the joists good enough but you could splice or bracket the centers.

PEG and others will probably have more suggestions -- I just throw out a few ideas.

[John Raabe]

From the point of view of your floor joists, and assuming the 18' width, you could build a centerline beam and then do standard joists - I don't have my span charts with me here on vacation but you can probably do 2x6 @16" o/c depending on the grade and species your lumber yard has. More critical to the design will be the spacing of the piers and the size and support of the perimeter beams between your posts. If you are using straw bale as infill walls you may need to satisfy racking resistance in some way if you have wind or earthquake forces to deal with. You may be able to do this with diagonal timber braces.

Here in the NW straw bale requires a complete structural bracing system equal to a frame house - and this usually means something like 4' wide plywood braced wall panels at each corner. These can be buried in straw of course but this adds to the complexity. At least here, a post and beam straw bale house is more expensive to build and more complex for new builders than a well insulated frame building.

I've worked on several as a volunteer. (This is the prime advantage of straw bale construction - you can get volunteer help nearly free! )

If you are building a one-story load bearing straw bale structure in a dry climate and there are no codes, then these can be pretty simple easy-to-build structures. You probably know the history of the Nebraska straw bale houses. (Most of these structures have a trench footing and dirt or concrete floors with the bales bearing on masonry.)

[Drew]

The structure is going to be non-load bearing, so the frame will carry the load instead of the bales and plaster.  We went to a workshop at the Solar Living Institute (Hopland, CA), which was lead by a building inspector.  The non-load bearing approach seems to fit our needs better and has fewer requirements with the county.

Our building will be built under an ag permit, but I think ag roofs hurt as much as residential roofs if they fall on your head.    

Our county (Butte, CA) does not have additional code for straw bale construction.  While I understand the plaster will lend to the sheer strength of the building, I planned to use simpson straps on the framing timbers.  Using plywood at the corners sounds like a good idea.  Would that provide better strength?

I get to do my calculations around a seismic zone 3 and 75 mph wind design speed.  I'm using your foundation plan from the Small House designs as a starting point but certainly plan to double check the spacing and beam size before calling it "final".  I found the joist span/sizing calculator.  Is there a similar tool for post spacing and beam sizing?

You're right about the volunteers.  My neighbor can't wait for us to get started so he can help!

[glenn kangiser]

A post and beam frame can get expensive by the time you pay an engineer.  John mentioned framing with standard  brace panels - 4' plywood both sides - nailed or nailed and glued to meet standard bracing construction requirements, then going with the entire central side and endwall sections between the braced panels as a header beam, and do as you wish with the strawbales.

In this way you avoid the cost of the engineered frame but still get to do your strawbales.  Maybe this structure would come in much cheaper.  It makes the entire structure standard construction but non bearing on the interior.  You could still do your cantilevered strawbales around the perimeter and openings in any other areas you wanted.