hybrid strawbale/cob

[Mark A Hoberecht]

At 03:34 PM 12/1/96 -0600, you (Eric D. Hart) wrote:
>At 09:50 AM 11/22/96 -0500, Mark A Hoberecht wrote:
>
>>I built a hybrid cob/straw-bale structure this past summer in Ohio and
>>discovered some interesting characteristics. 

>        Was the cob or strawbales load bearing?  Was the whole wall assembly
>load bearing?  You mention wooden framing members so I am guessing that the
>cob and strawbales aren't load bearing. 

The wooden framing was a post-and-beam timber frame using dead-standing
trees from the site.  The bark was removed with a draw knife and the logs
then used as-is (no milling).

 I don't see why the cob couldn't be
>load bearing.  How thick would one of the cob walls have to be to be load
>bearing?  Six inches seems to be too thin to me.  Maybe a foot thick??

The timber frame carried the majority of the loads, but half the floor
joists rested in cob, as did one end of all the steps in the spiral
staircase.  As for thickness, even 4 inches works fine, as long as you can
eliminate any lateral loads and you're only dealing with compression.  You'd
be surprised at how hard cob gets when crusher fines are substituted for
sand in the recipe.  The knee braces in the timber frame took care of the
lateral loads. 

>Personally I would make the strawbales or the cob load bearing.  No use
>going to all that trouble with the cob and strawbales and then holding the
>roof up with wooden framing.  

I still would use a timber frame for support of at least a ridge beam (maybe
a single bent), but then let the cob support everything else, especially for
a tall building (mine was a full 2 stories).  The cob would need to be much
thicker if you wanted it to do the whole job.  Mixing and laying up cob for
that wall thickness (18-24 inches)would take a lot of time, even using a
mortar mixer.  For just straw bales, I imagine it would still be simpler
(with less wood) using a full or partial timber frame if you're dealing with
2 stories of bales.

Mark Hoberecht
scarecrow at lerc.nasa.gov

>Eric D. Hart			
>Community Eco-design Network	  
>Minneapolis, MN  USA		
>(612) 305-2899 
>erichart at mtn.org			
>http://www.tc.umn.edu/nlhome/m037/kurtdand/cen 
>