Agree, in a structural application without knowing the mechanical properties of perlite it is too risky. We do know the properties of Portland cement, and you could use perlite as the aggregate. Without a drawing I have no comments on the details, looks like you are getting better at this. You say you have an “allowable” of 4500#/linear ft of wall load, meaning I assume, the load from the floor-roof is split evenly to the inner and outer wall your actual “load” is lower than 4500#/ft. If you have a positive margin of 2 the actual load is 2250#/ft and each wall is seeing 1125#/ft. You looked at your compression allowabes for boxed assembles with sheathing, per code and see how much compression they can take. The best design distributes the load evenly to each wall, then to a large sole plate for even distribution to concrete without a bending moment from uneven loads from double walls. The edge slab aspect ratio now takes into account this 2250#/ft + the dead weight of the walls as additional load and distributes it evenly to soil . The soil takes that load and the dead weight of the edge slab you show good with a +2 margin @ 3000PSF. Assuming you did your math or load over area calculations correctly, P=F/A. For robust design to cost you don’t want to keep adding margins, you design to limit load add a 2x margin at the end. That is just static analysis you show good. Now you have do a dynamic analysis at each joint(node) based on code driven seismic and wind zones. For most, hiring a good PE can pay for itself in added weight and cost of over designing, or hopefully not under designing which can result in death.

There is a BIG difference between margin and redundancy…. NO per your comment above, having another wall should not provide a margin, it provides another failure path if you think you need it like high seismic zones. In that case, you design both walls to take all the load with no or little margin but that will cost more, or you design per above to split the load and create a cost effective lean structure as possible.

The great thing about having the large foot print of a double stud wall is it provides to the ground a low aspect ratio (width/height) compared to single . Check with helix for edge thickness and effective area for sizing, they should have data based on the loads you are seeing at their helix per your area dimensions. As far as bearing/compression from lateral pressure loads, the concrete anchor’s/sole plates should provide that not a lip on concrete. More P=F/A, you know the compression allowable across the grain of wood, you know the effective bearing-compression area is half the diameter of the anchor, add the summation of forces, look up the load, add a margin of 2. Look at code on how to integrate PWF to slab.

If you have not dug the basement out yet and do not understand the soil check this thread out:

check this thread out

Posted By Liebler on 10 Jun 2014 07:08 PM
TLP,
All I'm getting better at is anticipating your concerns and covering them in advance.

Really? Na, if you look at what you posted in your OP you have come a long way in looking at structure first, not insulation value. Perlite is a fluffy insulation material not to be mistaken with stone that is denser and proven to work under load for centuries.


Perhaps you will get better at not taking what I wrote out of context. You did not post a drawing I asked for so, I was not sure what you meant by concrete "lip"? No dimensions, so how is anyone suppose to know what YOU are talking about? You are looking at one design 'guide' an approach of many. You can design as you want, as long as the design takes out the loads, that simple! I did say this,


"Check with helix for edge thickness and effective area for sizing, they should have data based on the loads you are seeing at their helix per your area dimensions. As far as bearing/compression from lateral pressure loads, the concrete anchor’s/sole plates should provide that not a lip on concrete. More P=F/A, you know the compression allowable across the grain of wood, you know the effective bearing-compression area is half the diameter of the anchor, add the summation of forces, look up the load, add a margin of 2. Look at code on how to integrate PWF to slab."



Not hard to post drawing's, they are worth 100 words ;)? Single wall 2006 IRC, this is great why you did not mention? Because the highest pressure are found at the bottom of the wall and a concrete edge is good in compression. This is the first area you should have worked out in your OP, perlite under here and a smaller thickness of it will NOT work.....so yes as I said, "your are getting better at designing structure first, not insulation".  If I put the sole plate on the slab with a thickened edge, I determine that by "helix design guides" (not someone elses) because as we said concrete is good in compression, the helix adds tensile torque better than rebar in some cases. The steel anchor-concrete node same look at helic compression guides. The wood is where the fail path begins, it is not as good as steel (80+ KSI, concrete 3-6KSI, compression-bearing), so hence I said look at the wood transverse grain compression allowable. In this design you could anchor your inner wall to slab, or outer.

"Predicting how loads will be shared in a double stud wall system is at best a crude guess as the elastic properties of the multiple materials involved are simply approximations and subject to wide variation"

"Elastic property"? or "Modulus of Elasticity" ? Ahh, Stress/Strain. How elastic the materials are is VERY well known. Concrete is not that good, you can find the value easily by looking at code or concrete material data sheets. Wood not that great, either, steel pretty good! Not of utmost concern here unless you are in a seismic zone and need ductility or shock absorbtion, or low deflection of mason floor. If you want a low MOE design look at a foam(or any flexable) core with stiff reinforced stressed skin SIP, core is ductile, skins transfer load to the foundation. Straw bale-plastered designs have stood for centuries in high wind, seismic zones. Shaker table test two years ago finally proved this unknown elastic property of load bearing bale walls. The monolythic features of mechanical properties are robust as it gets. Strain from a gage is valuable at calculating the stress. You really want to look at compression (or bearing), shear, and tensile (failure mode from bending moments concrete and wood are not that good at), that is different than modulus of elasticity. All of the materials properties involved here are VERY well known, there are NO variations, we are NOT reinventing the wheel with some new composite technology. The structure could be designed very lean and cost efficently by someone that knows what they are doing, with a lean design-build plan.

"my plans will be reviewed by a PE before I address the permitting authorities"


Good plan!

 Check out the way the Swedish do it, double wall with the interior 2x2 non-load bearing utility chase. Less thermal bridges, I don't like the broken sole plate for transfering shear and compression to the foundation as much as the way we do it in America.

I see, now you are opening up a can of worms – ha! Ah, the wood. Here is what BCS has to say about wood based on hot box test I presume. Attached is one I have access to now, we do more metal I am quiting my day job in two weeks to go design-build some tight production homes this year. I’m not a big fan of wood or concrete nor the issues in mating these materials. I’m looking at different materials, but let’s look at them anyway.

“Wood is entirely unpredictable, if someone invented it today it never be approved as a building material. It’s strength properties depend on its orientation, no two pieces are alike, cruelly of all it expands and contracts based on relative humidity, it expands differently based on orientation, when in shrinks it expands, it shrinks and expands along the grain differently than perpendicular, shrinks and expands more at a right angle to the grain than along the grain, studs get shorter or longer, do not get thinner or thicker. It burns, rots, it can’t get worse but, after years of testing and use we understand it. “

That is why this industry designs with such high safety margins that fight themselves in weight and dead loads. There are far better material choices, better strength-to-weight ratios, and combinations that work well together.

The Hot Box – Basically there is a CO2 chamber outside the big hoses are running from to the sub-zero chamber that provides below freezing temps. We mix it with heat, humidity, and pressure from a pump to get the atmosphere the test calls for that is in the foil ‘hot box” to left, and we can cycle it between hot and cold at any humidity level we want. We can also add fungi spores. BCS did the same with their “Thermal Metric Test” on convective loops degrading r-value, and they probably uses strain gages on several samples and species of wood to determine the above statements we shall assume are accurate. Question I think you are asking is how do we design to the variables and unknowns.

All the time I got right now, more later….

Your walls are not both seeing the same loads, if they were the same size studs should be shown from top to bottom. Your outer wall is taking the bulk of the roof loads, the inner ceiling or floor. If you look @ load like water flow from top to bottom both outer and inner have kinks or (kick loads, or "couples") forces that cause local or nodal over turning moments. So your fasterens will be pre-loaded to take them out. I don't like the kick in your concrete either and I would not try and butt concrete and wood together for reasons already discussed. You have to be carefull with wood in basements.

You can get away with this, it could be better too. If you can afford it get a PE stress engineer to run a FEM (finite element model) and show you were the stresses differ and you'll see what I mean. Ask to do both static and dynamic loading. It will show stress at alot of important nodes, you can move things around to get the stress even and down, or to the outer wall, inner, or both. Once optimized it will show stud sizes and concrete thickness. It is all dependant on getting the correct loads in the model and the user's ability.