[Cob] supporting a one ton tank - engineering needed!

[Ocean Liff-Anderson]

Well, even if cob is only 1/10 as strong as concrete, typical 2-foot  
thick walls should be adequate to support a 2000 tank. Consider even  
at 24" x 24" column.  This is 576 square inches x 100 psi = 57,600  
pounds load capacity!  Of course, the cob column must support its own  
weight too - here's where the engineering is beyond most of us.  How  
do you calculate the load rating of say, at ten foot high wall?

Most Ianto-style cob buildings are built by lay people who haven't  
any information regarding engineering calculations.  The gift of cob  
is its ridiculous psi ratings support typical roof loads without any  
trouble.  Adding 2000 pounds of water, and I would definitely  
recommend getting experts involved in design.  Ask Janet Standeford  
who she used for engineering advice - she's got an approved structure  
going up in Southern Oregon.




On Jul 6, 2011, at 11:04 PM, Shannon Dealy wrote:

> On Wed, 6 Jul 2011, Ocean Liff-Anderson wrote:
>
>> Actually, cob has incredible compressive strength, similar to a  
>> concrete wall.  I don't know the numbers - Shannon, can you pitch  
>> in some figures? But I think the walls circling a small bathroom  
>> and closet, if made thick enough, would be able to support your  
>> water tank.  But best to get some engineering...
>
> Well, the numbers I have are nowhere near a concrete wall.  A study  
> done by an engineering student a few years back using samples  
> provided by experienced cobbers in this area gave a range from 65  
> to 129 psi.  His search of the literature found that most past  
> testing by others had ranged up to 175 psi, but as far as I can  
> tell, most of it seems to center around 100 psi except for one  
> outlier which cited a range of 400 to 610.  In otherwords, there is  
> a lot of variation.
>
> It is important to note that cob is built with local materials and  
> optimized for a variety of tradeoffs which may not include  
> maximizing tensile strength.  The soil and sand available where I  
> live give a mixture that is more ductile and with much lower  
> compressive strength than mixes I have worked with at other  
> locations (some of which are probably easily over twice the  
> compressive strength of my local mix which is around 100 psi).  Of  
> course if I had need of higher compressive strength, careful  
> selection of materials for the mix and thicker walls would allow  
> for the handling of quite substantial loads.
>
> Shannon C. Dealy      |               DeaTech Research Inc.
> dealy at deatech.com     |          - Custom Software Development -
> Phone: (800) 467-5820 |          - Natural Building Instruction -
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