[Cob] Embedding metal

[Shannon Dealy]

On Wed, 18 Apr 2007, Georgie Donais wrote:

[snip]
> Especially in an unheated building, it seems a questionable idea to embed 
> metal support posts in cob, due to their different expansion/contraction 
> rates. I can imagine strips of cob popping off. Comments?

I always recommend when posts like this are to be embedded that the cob on 
at least one side of the post be the full thickness of the rest of 
the wall, otherwise, you are creating a natural fracture point in the 
wall, regardless of what the post is made of.  I also usually recommend 
that the wall weave in and out around the posts so that the cob is 
inherently stable on it's own, rather than having a straight wall and 
relying on the posts.

> He is requiring a bond beam around the top of the cob as it meets the roof, 
> as per New Mexico's adobe building code. He is balking at my suggestion that 
> it is not needed. Comments?

Assuming he is a competent engineer, he should be willing to run the 
numbers if you run structural tests on samples of the cob mixes you are 
using to determine if it can meet the codes on it's own without the 
reinforcement.  Ideally, he should be able to tell you what the 
minimum structural characteristics he needs it to meet are, allowing you 
to adjust the mix to try and meet his requirements.  Unfortunately, many 
engineers aren't particularly competent and either would not or could not 
give you the numbers (or worse will simply make up some ridiculous 
numbers that far exceed what is actually needed).  There may also be 
problems with the testing requirements in that either the engineer, or 
your local building department may require the tests be run by a 
recognized national testing lab, rather than accepting simple and 
inexpensive tests done locally.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
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