[Cob] How wide should a cob wall be in earthquake zones?

[john fordice]

Amanda,
There are no dumb questions.

The deformations on ordinary rebar are designed to transfer tension loads in
the concrete mass into the rebar via the compressive strength of the
concrete.  Concrete has a compressive strength of 2000 psi minimum.  Cob has
a compressive stgrength of more like 200 psi, so the deformations will do
nothing to transfer the tension loads out of the cob to the rebar.  As you
surmise, there might actually be a negative displacement effect to the
surrounding cob by the deformations when the rebar moves inside the cob in
the event of seismic motion.  So the SMOOTH rebar is intended to act as a
tension tie only between the bottom footing and the wood bond beam embeded
in to of the wall. It's this tenson systwm which in effect captures the cob
wall between the stronger top bond beam & bottom foundation tension
accepting elements & thus keeps the building intact in the event of fracture
due to seismic forces.  The supposition is that in an earthquake, the cob
wall WILL break & the tension system is there to keep it from falling apart
& thus prevent a resulting collapse of the building.
Placing the tension system on both sides of the exterior of the wall as is
done in strawbale construction should also do the same thing, but is double
the material & then the tie points at the turning point laps of the double
plate wood bond beam embeded in top of the cob will need to be accomplished
in some other way.

john fordice - cob architect/builder


on 6/16/06 9:37 PM, Amanda Peck at ap615 at hotmail.com wrote:

> 
> Dumb questions--why the smooth reinforcing on the vertical parts of the
> walls?  Just so they're not like the allthread that might act as a saw, or
> smoother than the standard rebar?  And do these go in the center of the wall
> thickness or the outside, covered with plaster the way people seem to be
> reinforcing straw bales?
> .............
> 
> john fordice - cob architect / builder wrote:
> 
> 
> 3/8² SMOOTH rebar vertical ties at 24² spacing in full height wall sections
> - embed in footing & hooked to a #4 cross bar in footing - full height of
> wall - attach to bond beam atop wall - thread top
> 
> 
> 
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