Cob: Re: Earthquake.

[Robert Bolman]

>  How do cob buildings do in Earthquakes?

The defense of cob typically given by cob enthusiansts WRT earthquakes is
that:
1. Cob walls won't fail because they're curved.
2. Cob walls won't fail because they're wider at the bottom than they are at
the top.
3. Cob walls won't fail because they are a monolithic, homogenous mass
unlike adobe where the blocks can separate at their mortar joints.

These are all valid points WRT cob and earthquakes, but I feel that they'll
only hold up for so long.  I see where a lot of cob builders will take off
building with all their enthusiasm and whimsey and neglect some very simple
engineering & physics principals.

One of the most troubling things that I see is where, in an effort to make
the building as "natural" as possible, the builders will refuse to use any
portland cement in the foundation - making it instead out of dry stacked
rocks.  I worry that in the event of an earthquake, the earth will move
beneath the building and the massive walls will stubbornly try to stay right
where they are.  The loose rock foundation will serve as "ball bearings" to
facilitate this.  When the dust has settled, the house will no longer be
sitting quite on top of what was once the foundation.  The walls may be
cracked or worse yet, fallen in crushing those inside.

Another common principle of engineering that often falls through the cracks
when designing and building a cob building is that of LATERAL BRACING.
Especially along the south wall, the tendency is to install a lot of glass
when really, along all walls (including the south) there ought to be, at
some point, an uninterupted panel of either cob or serious wood construction
to provide lateral bracing - in other words, somethign to assure that the
wall will not flex, buckle, fold, distort, etc.  With a standard stick-built
house, they typically require that each wall have one full 4 x 8 foot
plywood panel at each end that is profusely nailed to the framing which is
bolted to the foundation.  Stick-built houses are very safe in eathquakes
because they are very light, structurally very rigid and bolted to a
foundation.  Cob houses are not light.  What rigidity the walls do posess is
dwarfed by their weight.  And they typically just sit on their foundation
which itself is often questionable.

All this said, I do just adore cob.  It's beautiful, very very
environmentally friendly and makes perfect sense in many applications.  I
just feel that the above issues should be thought of more.

Robert Bolman