Cob: RE: Cob codes, etc.

[John Schinnerer]

Aloha,

-----Original Message-----
From: Kelly, Sean [mailto:SKelly at PinpointTech.com]
>OK... this is some info I am pretty interested in (or is the jury still out
>because of necessary testing?).  Which Earthen construction methods are
>suited for which applications, locations and/or climates?  Anyone have this
>information?

Well, here's a few examples to illustrate why I wouldn't lump all the
aforementioned earth-based methods together in testing or formal or informal
codes.

Structurally, earth-filled bags stabilized between courses with four-point
barbed wire (a la CalEarth) are a different animal in terms of engineering
physics than a monolithic hand-formed mass consisting of clay, sand and
straw moistened with water for mixing (i.e. cob).  They do not serve equally
well in all shapes and configurations; earth bags seem to do great for
vaults and domes whereas cob is less suitable for these shapes.  Although
both might make nice curved walls, two "identical" such walls will not
behave the same under stresses such as earthquakes or the lateral load from
roof rafters.

A third case would be rammed earth, which structurally speaking has none of
the tensile fibers such as straw (used in cob) or poly bags and barbed wire
(used in earthbag construction), but may be more dense due to being rammed
into forms instead of shaped and pressed together by hand (cob) or filled
and stacked/tamped by hand (earth bags).  It may also use binders or
stabilizers not used in cob or earth bags.  So it too will have distinct
behaviors under various stresses such as earthquakes or roof loads.  It is
also most suited to straight walls, since it needs forms, yet appropriately
curved walls are stronger/more stable under certain kinds of stress than
straight ones.  Angular junctions between materials (as in straight walls)
concentrate stress at the joint whereas curved transitions distribute it
through/across more material.

An earthen structure that is fired becomes yet another species of material -
fired earth has physical characteristics significantly different from
unfired earth (ask any ceramicist!) and will behave differently under the
various stresses encountered during life as a building.

In any earth-based method, all else being the same, the actual constitution
of the earth used in a particular case (percentages of sand, silt, clay,
gravel, rock; type of clay (red, green, gray, etc.)...) will also play a
role in the structure's integrity.

There's a few touches on materials differences...then there's local/cultural
appropriateness.

If i've got straw and earth and some water, cob might be easiest and best.
If I haven't got straw or other suitable fibers, rammed earth might be
best...but have I got material for forms?  Maybe unformed or hand-formed
puddled earth is the only option.  If I want to fire my earthen building,
have I got enough of something to burn to do so?  Or would that material
better be used for roof beams, door frames, etc. in some unfired type of
earthen building?

If I haven't got poly bags and four-point barbed wire and/or rebar or bamboo
for pinning it, earth bags are pretty much out.  Vaults apparently need
forms, although domes can be built without.

If I'm building a vault or dome, how do I waterproof the outside (and is it
even necessary in the climate I'm building in)?  ...would it make more sense
to build vertical walls and some other sort of roof (shingles, shakes,
tiles...)?  

Some climates favor squat buildings with few openings, others favor tall and
open structures, some favor sqat and open buildings...etc. etc.  

Where did various methods and techniques of building with earth originate,
and what does that indicate about where they might be most suitable?

There's a few considerations in those domains...

I'm in favor of any testing results and "codes" that support people in
making informed decisions over the full range of their locally appropriate
shelter options and free them to build their own shelter in the manner of
their choosing after so informing themselves (If I wrote any "codes," they'd
be more like FAQs...).  They may choose to invite assistance from others
with more experience, of course.  Perhaps this is what the code enthusiasts
in natural building are up to after all...I hope so.

John Schinnerer