[Cob] Re: How about another BC test, on sheer strength?

[ocean]

Hi Ian,

Thanks for elaborating on your test.  Sounds very thorough!  I wonder 
if the skinny walls on your scale model actually were weaker than 24" 
full sized walls, given that real walls have much more straw in cross 
section than your scale.

While we're on the subject of materials testing, another test I'm very 
interested in seeing done isn't seismic, but rather sheer.  Just how 
good is straw fiber in preventing sheer failure?  How about building a 
very beefed up cob beam, maybe 2'x2' cross section and seeing what its 
sheer strength is?  Since many of us are building in the range of 18" - 
24" thick walls, this info might be of great import, esp. when the 
question of rebar gets raised (and I am so often asked by the general 
public if our cob walls at the restaurant have rebar in them!)

Oh yeah, since the roof didn't collapse on your test, I'd be very 
interested in the overbuilt roof connection - how did you attach?  
Deadmen to round rafters?

Ocean

On Dec 11, 2004, at 6:41 PM, Ian Marcuse wrote:

> We used alot of straw. Test bricks were made prior to building and the 
> strongest was used. We also did a number of materials testing to 
> examine different ratios of everything. The clay we used on the model 
> was local and contained a certain low percentage of silt. This was a 
> concern, but we went with what is commonly representative of many clay 
> samples in our region. The building went up quickly so as to avoid 
> drying between layers, but it does look like some of the failures 
> occured between layers. I also suspect that alot of cob buildings that 
> take years or months to build have these less integrated layers so 
> perhaps a degree of less integrated layers is representative. We used 
> cobbers thumbs and I would say that we worked the walls way more than 
> I ever saw anyone else doing it. We really wanted to make sure that 
> the straw was well connected.
>
> At any rate, this test was of course not definitive. I would say that 
> for a model, it was very well built and engineered. To some extent it 
> was overbuilt, particularly the roof connection. It would be great to 
> see other tests done, but  It was an incredible amount of work done to 
> in the end have to see fall apart