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How thin can cob load bearing walls be?  RSS feed

 
Gilbert Fritz
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Location: Denver, CO
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I am building a small greenhouse, 16 feet by 8 feet, and 8 feet tall. The North and West walls will be cob, and about 18 inches thick. I will be inserting some pieces of concrete as we go.

Do I need to add buttresses to this wall to keep it up, or is it thick enough? The North and West walls already form a corner, so they provide at least a little Lateral support to each other.

 
Gilbert Fritz
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I should mention that these walls are straight, not curved.
 
Terry Ruth
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Gilbert, nobody can answer that question with the info you provided. You really need to hire a PE to learn what questions to ask. Pieces of concrete unless they provide a continuous (homogeneous) evenly distributed load paths from the roof to all walls evenly, and to the foundation makes matters worse (dead useless weight that can shift under load, voids, delams, etc). The buttress(s) depends highly on the weight of the upper wall(F=MA), wind and seismic loads you did not provide, and the attach structure of the roof to the wall and foundation/ground. How thin a cob wall can get depends on how well those forces are being reacted. In high winds, seismic, snow (like certain parts of CO), load regions strapping or mesh at critical joints can allow or show good by analysis with a decent safety factor applied to thin walls, or may be needed more than buttress to react over turning moments, not just planer shear. 16' span is getting up there, especially if your designed roof pitch is less than 6:12. For your own safety hire an Engineer to look at the complete design.

BTW: Curved structure can reduce loads significantly. If you want to make an Engineers job easier do that

Well it actually makes it more complex to analyze and perhaps build but, you have a better chance at thinning the walls down, perhaps reducing the cost of the foundation and wall-to-roof details.

I don't think alof of thick wall designers understand how to robustly design to less material and weight. A big reason COB is "labor intensive" that can pay the cost of a good PE to reduce material. People just beef it up which can do more harm than good in certain cases, especially at top(concrete bond beams, etc) and weight to the ground which can cause uneven settling loads and collapsing of structure with people in it.
 
Gilbert Fritz
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It does not seem very economically practical to hire an expert to help design a structure which will end up costing less then 1000 dollars. I could easily spend more on the professional. Besides, how could he analyze such variable materials? A rubble stacked foundation of broken concrete and salvaged brick, earth walls with a mix of earth, sand and straw that varies slightly from batch to batch, a roof and walls of salvaged lumber, and a glazing of salvaged glass of unknown quality. To make matters worse, the walls vary in thickness, each course will dry under different weather conditions and may be packed differently, and they may even have little niches set into them.

People who have money hire professionals like yourself, who do high quality work for a proportionally high price. People like me, who do not have money, build things themselves, for free with a proportional result. Amateurs built the Ark, Professionals built the Titanic.

So, for us amateurs, the only guide is what has been done before, and how long it has stood up.

I guess I should amend my question; is there a precedent for a cob structure with 18 inch load bearing walls? I am not in a seismic zone.

Sorry if I sound sarky, you provided a great answer, it just didn't fit my question.
 
Gilbert Fritz
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By the way, I do understand your concerns about inexperience amateurs doing stupid things and killing themselves. Especially when the start building domes and vaults, or underground structures. I think, though, that a solid, fairly low wall of cob might be a good place to start experimenting.
 
Terry Ruth
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I could easily spend more on the professional. Besides, how could he analyze such variable materials? A rubble stacked foundation of broken concrete and salvaged brick, earth walls with a mix of earth, sand and straw that varies slightly from batch to batch, a roof and walls of salvaged lumber, and a glazing of salvaged glass of unknown quality.


Structures such as these are analyzed all the time, it is quite easy actually. There are sites all over the internet with mechanical properties of earth, wood, etc, and composites. In the case of composites such as COB (clay, sand, straw) walls one would start by assigning a density 106 LB/FT3(see below) and double it. By doubling it we answer your question of "how could he analyze such variable materials?" That 2x safety factor is used extensively in construction by PE's and building safety codes all over the world to account for errors, omissions, and unknowns.

We know your proposed wall is 1.5x8x16 or 192 ft3. The weight of the wall is therefore 212x192=41,704 lbs, ~ 20 tons. Now we convert that to square feet by distribution of weight over the foot print area= 1.5x16=24 SF......

41,704 lbs/24 SF = 1,700 LBS/SF

Now look at the attached soil compression load allowables from dead weight in lbs/SF. Your walls alone are getting past the code min of 1500 PSF( if you are using an expansive wall clay the loads case is higher (use a SF of 3 above). We need to add the rubble trench, Your ground snow loads in CO are 15-20 Lbs/SF that need to be converted to a vertical additive load......along with the weight of the roof by again assigning a density and 2x saftey factor. I don't have time I'l let you finish. Look at your neighbors exact same 1.5x.8x16 cob wall won't work since that soil allowable could be entirely different than yours. That can change drastically on your own lot, hence why many jurisdictions require samples at different locations within the sites perimeter.

I guess I should amend my question; is there a precedent for a cob structure with 18 inch load bearing walls? I am not in a seismic zone.


See the map CO is in seismic zone B or C.....If in C (dark area) there are some things about structures you need to know or one could get hurt. All of CO is in a basic 90 mph wind that gets factored up or down depending on location and surrounding terrain. These lateral loads need to be looked at to determine shear and bending at the joints I mentioned above. That is simple force times distance then looking the allowable up in tables.

As the "aspect ratio" foot print on the ground (EG: 2' x 16' vs 1.5' x 16') increases reduces bending at the foundation, increasing weight at the top of the wall with a heavy bond beam makes the reaction at the foundation worse, when the ground accelerates and the top of the wall lags behind casing a collapse from high shear.

CO is also in a "Severe Weather" climate zone, frost heaving, etc.....

There is no differentiating these requirements for an "amateur or professional" building or builder. Many jurisdictions require a PE or code compliance for a reason. I hope now you can see your OP provided more unknowns of a greater concern than your structure and I said no one can answer without having all the facts. If you want to build prescriptively do so to code (New Mexico, AZ, CA has it) or guess at your own risk.

I am pretty sure I answered all your question in my first post, if not I illustrated the answers in this one. Many reports of building collapse on the internet regardless of size or cost from people that design structures that should not be or, they thought they could copy their neighbor or some build on the other side of the world. Pay a PE now or pay a doctor later.


https://ecosenseliving.files.wordpress.com/2011/09/science-research-report_sept_1.pdf

Good luck and be safe.
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Gilbert Fritz
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Hello Terry,

Thanks for the detailed answer. I will go through it more thoroughly when I have time.

Just a quick clarification; so you are saying that I have exceeded the bearing limit for the underlying clay soil?
 
Terry Ruth
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Gilbert Fritz wrote:Hello Terry,

Thanks for the detailed answer. I will go through it more thoroughly when I have time.

Just a quick clarification; so you are saying that I have exceeded the bearing limit for the underlying clay soil?


Bearing is a good word and used often but we are looking at compression, sort of the same thing if you consider the bearing area is 1.5 x 16 = 24 SF

Yes and no, I'm saying you exceed the allowable for sandy, silt clay of 1500 PSF max...I don't know, you need to see what your frost line depth is there and your rubble trench footing needs to be a good 6" inches below that. At that location look at the soil to ID a type using the table I gave you. That will determine the total compression the building can put on that type of soil at that depth (EG: 30" is our frost depth here in KS so we go to 36"). All new construction regardless of size here has to have lab test at least at two perimeter locations to determine a plastic index of the soil which is an indication of how expansive the soil is that can cause issues. That test does not cost much and the PI needs be below 16 otherwise I would have you tie reinforcement into the cob and solid rock ground(pile drivers), or modify the soil. I be looking for undisturbed 3000 PSf sandy gravel to support my rubble trench and building if I were you.

With that said your 1.5' wall should work, and if you have alot of lateral loads add more straw and use a high binding strong clay on the bottom half of the walls.... Use a 24' thk wall, corner buttress w/45 deg 2':1' taper bottom to top, light weight bond beam, hurricane ties if you have high winds or are in the C seismic zone, and do no exceed 8' in height or a 16' span unless broken with an interior footing/buttress and/or do not tie the floor joist/ceiling rafters into the walls, and 2x6 roof rafters can clear span 16' depending on grade and species. Use the 20 PSF table for a 6:12 or less roof pitch. The tables are based on a horizontal projected length in your case 16'.

http://publicecodes.cyberregs.com/icod/ibc/2012/icod_ibc_2012_23_par194.htm
 
Gilbert Fritz
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Hello Terry,

The sub soil is 25 percent clay, 45 percent coarse sand, and 30 percent fine sand/ silt. It is extraordinarily hard, a pickax just bounces off. I have been told that while parts of the Denver metro have expansive clay soils, Littleton is not one of them. I capped the rubble trench with a reinforced concrete bond beam, and if anything probably went overkill on the rebar. I am currently building the "rock" rubble stem wall on top of that.

We can get high winds here, but this is a highly built up area, with buildings, high fences, and dense tree lines in every direction. If 90 mile per hour winds were actually hitting the greenhouse we would have a problem; our fences, house, and the neighbours line of pine trees would have all disappeared.

I can't get any seismic data for Littleton as such. The main map for the nation shows us at Zone 1 = 0.075g, but it is not at a very useful scale. The historical earthquake page of Colorado does not mention any damaging earthquakes in this area, though some are recorded further north, near the old Rocky Mountain arsenal. (The highest was 6 on the scale and broke windows.)

 
Terry Ruth
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I see....your foundation sounds solid. You are in S_ZONE C in code that is the first level there are adjustments to tables to elevate the requirements, the next level is Do, D, towards CA. There may be some ground shifting you don't feel or see that no matter how strong the foundation it can move it. It is an acceleration force that gets applied you won't find easy. It usually results in more rebar and deeper footing's if using code which sounds like you have.

Follow the other advice I gave above....the 1.5 wall should do fine, buttress won't hurt as long as the trench supports it.

The 90 MPH winds noted in code are 3 sec gust.

I have to get back to designing a 3200 SF home and stop with the greenhouse. Good luck!
 
Terry Ruth
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Just to clarify my last post I still recommend obtaining a PE. I believe from looking at the map this build is in SZ_C which is moderately severe. COB or rigid concrete will not do well in seismic zones unless it has an alternate continuous load path (metal mesh, wood, etc). Without seeing prints above my assumptions are as follows,

1. All the walls are of the same density and distribute load from the roof to the foundation evenly. Combining different wall types can get dangerous especially if the COB sees more load or uneven point load and needs a PE.

2. Most of the weight of the walls are in the lower half and the roof is light weight. This lowers the center of gravity of the wall, reduces the lever arm and bending moment at the foundation.

3. The lateral out-of-plane unsupported span does not exceed 16', wall height 8'. Unsupported wall spans should not exceed 10 the thickness of the wall, otherwise a buttress or interior load bearing wall is required.

4. In a seismic load event walls are not connected by ceiling rafters or floor joist and laterally react independently. Unless analysis shows otherwise. A light weight ring or bond beam should be employed. In severe S_Zones an interconnection to continuous vertical and horizontal primary structure should perform this function.

5. The structure is built square and plumb.

6. Clay has a PI less than 16 and core samples are taken to a lab.

7. Wall opening's are centered and small(not to exceed 1/3 the wall length).

8. Built on solid foundation per local code.

A composite structure of strawbale core and earth skins would perform better due to the high flex modulus of the of the core COB and other rigid structure does not have unless it has a continuous alternate load path especially in severe seismic zones. Even this structure can require alternate redundant load paths at the foundation and roof.

I think too often permies see a cost saving's with materials like COB without knowing enough about structures and environmental loads to choose the right path or hire the proper professionals. Not to discourage anyone since a cost saving's can be had, but not at the cost of safety.



 
Gilbert Fritz
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Hello Terry,

1. The walls are not all the same; front and east walls are frame. However, they don't have to be rigidly attached to the cob walls. The cob walls should distribute weight evenly.

2. Most of the wight is in the lower half; I am tapering the wall to 12 inches at the top. The roof is light weight, polycarbonate panels over light wood framing.

3. "lateral out-of-plane unsupported span" is this the rafter length, say? If so, it is OK; the longest rafter is 8 feet. The height is 7 and a half feet, not counting the stem wall. The longest wall span is 16 feet, so a little over, but I am adding a buttress.

4. The roof frame is attached to all the walls, and dead maned into the cob ones. I am using a wooden bond beam, and not just inserting joists into the cob.

5. The structure is built square and plumb.

7. There will be only one small window in one of the cob walls. No doors, they are in the frame wall.

8. Foundation has a reinforced concrete bond beam.

Thanks for taking all this time to advise me, and sorry if I sounded snappy earlier.

I may consider doing straw bale with a heavy clay plaster for thermal mass and comprehensive strength.
 
Terry Ruth
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Gilbert, no worries. Perhaps I was looking at that zoning map wrong. Double check me it does not look like you are the darkest shaded areas of CO or C? It appears the #17 loft that separates A & B( the next two lighter shaded areas) is very close to Littleton. If you are in one of those zones A-B the concern is low, and you say your wind gust speeds are blocked by terrain, so lateral forces are low. We are Z-A with low seismic requirements, same wind zones as you.

Still would not hurt to incorporate some of C+ requirements into the greenhouse if you can afford it,

The "bond beam" being referred to is at the top of the wall normally referred to an upper sill in light weight construction. Some folks use concrete coupled with thick walls that is when there is too much weight at the top. The bond beam plays a critical role in the design. Simply put it is used to resist racking of the upper structure in a seismic or wind gust event. So the goal is to stabilize the upper beam/wall in three dimensions which stabilizes the COB, helps keeps it from going into bending, tensile cracks, shear.

You have a lower concrete "bond beam" you referred to. What is stopping you from attaching a lower sill plate to it and erecting minimum rectangular frame work around the building? You would then in-fill the COB where you want it. When the COB is wet you attach the bond beam (wood, etc) to the upper wall that provides a "bonded" shelve to attach the 8' rafters to that bears down on the COB as much as possible. That shelve distributes load 'evenly" to the COB wall from the roof rafters (dead weight, snow loads, walking loads, etc). Make sense? That is what I meant by "even" distribution. If you drilled some 16 or 24 oc holes in the bond beam and drive some wood dowls into the COB that makes it even stronger.

The upper bond beams resist racking in two ways, it is tied to frame work of the other walls, and by the COB.....this is "horizontal" support....

This video below explains alot of what we have been discussing. Go to lateral forces at time = 10:00 and look at the pictures of the three modes of failure. https://www.youtube.com/watch?v=xTXVBxIIG5g

Vertical corner post tied to the upper and lower sills or bond beams through the COB resist bending and shear at the corners which are critical.....now you don't really need corner buttress in your design since vertical, horizontal, and racking(shear) loads are resisted. If you add a post in the middle of the 16' span now you don't really need a buttress there. Mind you, buttress won't hurt it increases the safety margin we defined earlier.

So in essence you created some minimum 3d frame work to support the COB and the COB supports it lowering the structural requirements than the other framed walls have. Make sense? At the same time by using a light weight roof and upper bond beam sill you have lowered the CG, resisted any lateral loads, and eliminated the need for tapered walls (which are not a good idea unless they are done with form work and an Engineer verifies the thicknesses and taper angle).

Right, your roof rafter spans are 4' my bad use the table I provided to handle your live and dead loads and check that your sheathing spans can handle 20 PSF. Panels are rated 24/16 (left # is max spans for roofs and walls, right floors). The minimum thickness for wood is 3/8. If the poly panels are not rated do not use them.

Adding more straw to the mix will lighten it and help resist tensile cracking, as long as the clay holds it together.

 
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