[Cob] Re: Cob and Cobwood hybrid - a critique

[otherfish]

Teresa,
At the risk of being a nay sayer, I'll put forth the following analysis of
your proposed wall design:

1. cob gets it's strength from being built as MASSIVE walls.  Cob does not
have much in the way of tensile strength and whatever it does have is going
to be seriously lessened in a thinner, mortar like, application.  A use of
cob as mortar in cord wood will add essentially NO strength to the wall.
Cob does not have a significant ability to bond to other materials, so the
cob mortar will only act as infill between the wood pieces ( to the extent
it stays in the wall after it cracks at the narrow points in the mortar
application ) and it will only have strength in a compressive or gravity
driven ( ie downward ) direction.  In the type of wall you propose, cob will
probably be the weakest link in the structural system you are creating.

2.  The air passages of which Charmaine speaks will also be pathways for
rainwater to enter the wall.  This is a big red flag to my mind in that it
could potentially compromise either the cob, or the wood, or both via
moisture intrusion into the wall.  If you use the cordwood/cob combination
as the base wall or plinth, there is also the potential for rain runoff from
the roof to splash back and wet the lower part of the basewall and further
agravate the possiblity for water intrusion.  Moisture is something you most
definitely want to keep OUT of both earth and wood wall systems.

3.  The cob portion of the wall is going to be considerably heavier than the
cordwood portion.  There are three potential problems that I see here:

A.  You'll be supporting a heavy wall on a more flexible cordwood stemwall
base.   This is a setup for possible failure in the cob due to both future
settling and seismic motion.

B. There is also the potential due to possible water intrusion for the
cordwood wall to deteriorate from rot getting into the wood.  This could
potentially undermine the heavier cob wall above in the future.

C. You are putting the heavier wall higher in the building where any seismic
(earthquake) forces will be magnified due to it's distance above the ground
plane origin of potential seismic forces .  Even if you are in a low seismic
activity zone, don't assume that the potential of an earthquake is not a
factor.

Maybe I'm totally full of it & don't understand what you have in mind, but
based on the above issues, I'd rethink the wall system you mave in mind.
Whatever labor saving you may gain is ( in my mind at least ) probably not
worth the possibility for a compromised structure.

Regards & cob on !
john fordice





on 11/16/03 11:16 AM, Teresa Banks at tbanks98926 at yahoo.com wrote:
  
> a cordwood/cob mortar, and cob hybrid, with cordwood on
> the bottom, and cob on the top?  Since both can be used as loadbearing,
> I don't see why combining them should be an issue.  As far as the
> cob/mortar shrinking, or the wood shrinking, and letting in
> drafts/letting out heat, it seems that adding a little extra cob in the
> cracks (assuming you can find them) would be easy enough.  I wouldn't
> want to do a double wall, my intent is to cut down on the work, not
> increase it, as I will be doing much of the work by myself.
> 
> Thanks, Teresa