Load sharing in double stud walls
user-1137156
| Posted in Energy Efficiency and Durability on
The advice given here repeatedly is that only one of the walls in a double stud assembly should be load bearing. OK so I’ll put all the necessary structural ” stuff” in one and the other will be built as light as possible. Must anything be done to PREVENT load sharing?
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Jerry,
Q. "Must anything be done to PREVENT load sharing?"
A. Not that I know of. I have never heard of any downside to overbuilding (from a structural standpoint, that is -- we all know that some types of overbuilding carry an energy penalty and are wasteful from a materials standpoint).
When it doubt, consult an engineer.
I assume that, basically and legally, load sharing is okay, but each wall has to be strong enough to carry the full load alone.
If you intend only one wall to be load bearing, and make it strong enough to do so, and place a second wall alongside of it that is not strong enough to bear the load; then there is the possibility that shrinkage could take the load off the load bearing wall and transfer it to the non-load bearing wall. This is where it gets tricky.
Theoretically, 1/64” or less reduction in height of the load bearing wall could transfer 100% of the load to the non-load bearing wall. However, does it really matter if you load the non-load bearing wall if the loading can only act within a 1/64” or even 1/8”?
Shrinkage is not going back off the support of the load bearing wall for much distance. If it backs off further, then it might cause damage elsewhere, even without a second non-load bearing wall, and therefore, there must be a fundamental problem with the load bearing wall.
Overall, if you have a load bearing wall cabable of bearing 100% of the load, and a non-load bearing wall alongside of it, I do not see any practical problem with placing the load on both of them and letting them decide how to share the load.
Ron,
Is there any way to predict which will shrink least? Will the colder wall shrink less? Or more? Making both strong enough to support the whole load may be adding unnecessary material if we can be sure which will shrinks least the other would need considerably less "reinforcement". In my particular situation the amount of cross grain and parallel grain wood is identical they sit on a common sub floor and are on the same floor truss with the foundation under the inner The outer will on average be a good bit colder than the inner If I deliberately built the load bearing one taller and filled the gap with foam, how much height difference would be necessary to accommodate lifetime shrinkage?
Jerry,
I think it would difficult and impractical to design around splitting of hairs over the amount of shrinkage. Seasonally the wood changes with moisture content. It would also change by thermal expansion / contraction, and that would be a larger cycle for the outer wall by some small increment. But these are tiny amounts. A larger shrinkage is liable to be the initial moisture loss after the lumber is installed (in a relatively cold climate) especially across the width of planks.
But, as I mentioned, if you have one wall load bearing, and other not; and if loading inadvertently falls on the non-load bearing wall, if won’t matter if the load bearing wall will re-assume the load once it drops the load by just 1/16 or 1/8 inch.
If, as you suggest, you were to gap the non-load bearing wall to make sure that it never assumed a load, I would think that ¼” gap would be adequate and reasonable. That wall would need stabilization to prevent swinging, but that could be obtained by a cross linkage with the load bearing wall.
One technical code issue that I see is this: If you have two walls sharing the load (whether intentionally or not), the outer one is likely to be sitting on the floor joists directly above the foundation wall, and the other may be sitting on the floor joists offset inward from the foundation wall. So if inner wall bears load, it will be supported by the span of the floor joists cantilevered off of the support of the foundation wall— as opposed to being supported directly by the foundation wall vertically through the floor joists.
Therefore, this cantilevered loading of the inner wall actually adds loading to the floor joists beyond their normal code loading of the floor only. However, as you know, the loading of the floor joists poses the greatest challenge to their midpoint, and since the inner wall will place that extra loading only a slight distance from the ends of the joists, it won’t matter much from a practical standpoint. However, I suspect that it may be technically outside of the code.
In my opinion, this concern about how the double walls will share the load, though worth considering, is of no concern compared to the prospect of placing the foundation on foam, under the expectation that the settling will only be ¼”, and that it will be uniform throughout the foundation. That is what I would call skating on thin ice.
Ron
Thank you for great information! It appears there are two choices. 1. Assume that under peak loading conditions the non load bearing wall will experience sufficient deformation to transfer the load to the stronger load bearing wall. 2. Shorten the non load bearing wall so that regardless of any unequal shrinkage the taller, properly built, load bearing wall will always carry the roof loads. Have you evaluated perlite as a foundation insulator? It is inelastic and breaks down at over 300 psi. and is cheap and r3/"
If you're building the structural walls first, it's hard to imagine how you'd fit the non-bearing wall in tight enough to worry about that.
Jerry,
Some of this issue about how the double studs bear weight depends on how the wall is constructed. If the floor is framed with 2 X 10s, for instance, they might undergo initial shrinkage of as much as 3/8” across their width. If you have the inner wall sitting on the floor and joists, and the outer wall sitting directly on the foundation (as is done in some designs), you could get a lot of drop of the inner wall relative to the outer wall. I would avoid that scenario by placing both walls on the floor and also avoiding plank joists.
My preference for the two walls is to make them both load bearing in structure and placement. I would not make a gap under one wall to assure that it does not pick up loading. One point to consider about this issue is that there is no guarantee that the studs in a single wall will bear the same load, but it is academic as long as nothing can collapse or cause damage.
I would not place insulation under the footings because I believe the risk outweighs the benefit. Everywhere in the foundation except for under the footing, I would use insulation on the interior and exterior along with vapor barriers with the intent that the foundation does not contact the interior air. Under the footing, I would omit the insulation and use only the heavy duty vapor barrier.
Ron,
I certainly agree that both walls should sit on the same floor, while it does perhaps add a wood thermal path the structural benefit overwhelms any thermal concern. Even stopping the sub floor inside the outer wall isn't worth it. The concerns about "thermal bridging are, I'm convinced, misguided and the heat loss through wood structural elements can be made up for by slightly more insulation elsewhere.. In my specific case I'll need to use doubled LSL 2x4 s as studs in the inner load bearing wall to carry my 75 PSF snow load on a big roof. While the outer wall needs no more than ordinary 2x4 framing. It seems framing the walls the same height makes most sense. If the 3 plates of the inner wall shrink a bit more that the 3 in the outer wall, the outer wall will compress a bit, so what?
However I strongly disagree about insulation under footings, it is a significant heat path and needs some insulation. 6 " of Perlite concrete (1-8mix) gives r 10+ and has a compressive strength well above any soil.
Jerry,
I have not looked into using Perlite as under-footing insulation, but I can see that it might be work okay. What size granules would you use? What would be the thickness of the Perlite layer? How common is it to use Perlite under footings as insulation?
Although I consider omitting under-footing insulation, I would not leave the footings un-insulated. My approach would be to insulate the interior and exterior of the foundation, so the interior insulation will be good coverage of the footing even with no insulation in the ground bearing interface.
Perliite itself is loose granulus and flows quite easily so it alone is probably not suitable, under footings, but Perlite concrete mixes are quite common and used in industrial settings but are essentially unknown in residential construction. The grade used for filling concrete blocks is what is recommended as " sub slab insulation It comes in 4 cubic foot plastic bags and several passive houses have had it used, left in the bags under slabs. The bags naturally make an 8" thick layer about 2'x3' per bag. straight perlite is r 3/ " . For under footings I'm suggesting a perlite concrete as it eliminates any flow issue. The same grade is suitable as an aggregate. The highest ratio, recommended by the perlite institute, of perlite to concrete is 8/1 and it is inelastic with a compressive strength of over 80 PSI The cost of perlite from a typical expander ranges in the $4 to $6 per bag range. Here is a link to get you started.
http://www.perlite.org/industry/insulation-perlite.html
My plan< subject to change, FWIW is to use a 6" thick layer of 8 - 1 perlite concrete under the footings which will give about r 10 and 8" of bagged perlite under the rest of the slab then put a 2 bag stack around the slab before any backfill then continue above the bagged perlite with ^"6" mineral wool using Cascacia clips so that I can continue my thin brick below grade.
I changed my mind! I calculated my rim joist & sub floor using 3 layers of r 15 comfort bat. it worked out to a slightly disappointing r 38 Then I recalculated using r15 Roxul as a "thermal break" in the sub floor, I still have the sub floor under the outer wall but it is a 3 1/2" strip then a 3 1/2" space filled with Roxul then the sub floor continues under the inner wall. II came out r40! Definitely worth the trouble.