[Cob] supporting a one ton tank

[Henry Raduazo]

Talking about the compressive strength of cob is pointless. Note for example: suppose I were to support a one ton water tank on a one square foot column. (Something I would never actually be stupid enough to do.) A square foot of cob has 144 square inches. 2000lb./144 = less than 14 pounds per square inch. Of course cob will support that!
	Throw away the numbers. You are going to build a support that is so wide and so massive that the the compressive strength of the cob per square inch is insignificant. Cob walls are often made 2 feet thick (24 inches) How big is a 2000 square inch wall? 2 ft x 84 inches you would be supporting one pound per square inch. 
	The real question is how high are you supporting this above the ground? Are you going to be living underneath the water tank? Is your support going to be subject to erosion from the weather? 
	I went through this agonizing thought process when I was deciding weather or not to place a 12 foot/ 50 pound per square foot living roof on top of two 7 inch thick walls.  I figured that it is only about 3 pounds per square inch and 6 pounds per square inch at the overhang at the end of the wall. Then I wondered about the point loading where rafters are imbedded in the wall. And then I decided to heck with all this agonizing and I built it. I plastered it and limed it. Two years ago Virginia had a record snowfall and there are no cracks anywhere even around rafters above windows. The lime finish is such that it would show even the smallest cracks.
	The one foot overhang keeps the wall bone dry most of the time. I noticed a couple of windy rain events where a little water got on the wall but it is quite difficult to hydrate a totally dry cob wall covered with lime.
	If you are still worried? Put a couple of pieces of wood under the tank to further distribute the weight. Another possibility is that you can test the strength of the walls by filling the tank to the top but set the pump to maintain the tank only half full. 

Ed
 
http://www.habitat.org/cd/cwp/participant/participant.aspx?pid=93541387

On Jul 7, 2011, at 2:04 AM, 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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