[Cob] Hybrids - structural flexibility issues

[otherfish]

Scott,
Jonathon,
Jilly,

There is nothing wrong with, and alot to be gained in building hybrid
structures.  Stabilized or protected earth bag, cob, and strawbale  all have
a lot to offer and can make an excellent building if done in a manner that
takes into account the nature of how each acts structurally.

A very critical part of that consideration, and one which I've not seen in
this current discussion, is what can be called the structural period of a
material.  As has been already acknowledged, they each have a different
level of felexibility ( ie: period ).  Altho I don't know for sure, I
suspect that earth bag  & cob are probably fairly close in flexibility with
earth bag being a bit more flexible than cob.  However, there is a great
difference between the inherent flex of a strawbale wall versus that of cob.
The point of this is that when subjected to seismic or wind forces, each of
these materials is going to act differently.  So if you build a structure in
which you have any of these three wall materials (and whatever roof or floor
structures each material is supporting) relying on onother of these
materials for lateral (sideways) structural support, it's asking for
trouble.  

In the event of severe ocurrences of these natural forces upon a building
there will be concentration of these differential flexibilty reactions at
the joints between these  materials.  There is the potential for failure of
the structure at these points.  This failure could spread into the adjoing
material if you are relying on one to buttress another.  There is also the
potential for parts of the building to pull away from the other parts for
the same reason.  Buildings gain their stability via  CONTINUITY between the
structural components / systems.  If you build in a way that compromises the
continuity of the structure, you create the potential for failure and
possible collapse of the building as a worst case.

So think upon this when you go hybrid and make sure that each of these
different wall materials is built to be integral unto itself, or if it is
relying on another differently acting material for support, then DO NOT have
that reliant material act as the structural support for anything other than
itself.  Even then, be careful that this reliant material is not potentially
hazerdous if it should separate ( & hence potentially fail structurally )
from the material upon which it is relying.

cob on !
john fordice





on 11/8/03 12:15 PM, kupononaturalbuilders at kuponobuilders at snowcrest.net
wrote:

> I have built two hybrid structures.  While I don't have 100, 200 or 500 years
> of feedback I feel that they will structurally sound for many years to come.
> The first structure was a small studio utilizing earth bags from Cal-Earth as
> the foundation.  We decided to follow their lead and stabilize our fill with a
> little cement ( we did test blocks and went with a 10:1 ratio).  We then built
> walls with normal cob.  We hammered in large stakes and built keys made from
> two foot long earth bags to help lock the cob to the foundation.  This worked
> very well and the walls were very stable.
> 
> The second structure is a cob strawbale hybrid.  We have sandwiched rice straw
> bales between an exterior and an interior wall of cob.  The exterior is about
> 4 to 2 inches thick while the interior is thicker, 4-6 inches.  As the cob
> dries and shrinks it squeezes the bales.  This combo allows for the best of
> both worlds: structural strength and insulation.  Then as you build benches or
> other things in the interior, you build with cob or other heavy materials that
> can act as your thermal mass.  This is insulated from the outside by your
> thick bales walls, resulting in slow heat loss or cold bank.  You could make
> adjustments as you deem necessary,ie.. post and beam structure, load bearing
> walls, cob columns to support roofing beams,etc...  My wife learned about this
> while she was apprenticing at Cob Cottage.  We were really happy with the
> building that we are finishing this year and are planning on building with
> this design again.
> 
> Also, with the type and number of your questions Jilly (and anyone new and
> considering building), I would strongly recommend taking a natural building
> workshop.  There are workshops being held all over the country, covering all
> aspects of natural building.  These workshops allow for hands on experience
> with the material as well as, if not more importantly, meeting with and
> networking with other builders.  It would be one the most important
> investments one could make in the entire building process.
> 
> Jonathon
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