Cob: cob/strawbale combo and devil grass questions

[John Fordice]

Sandy,
In terms of combining strawbale with cob - a word of caution,  be aware
that bale structures and cob structures are quite different from one
another in how they react to forces.  Bale walls (or vaults) are very
flexible, while cob is (relatively) quite rigid.  If / when you combine
the materials, do so in a manner that this difference in flexibility
does not compromise the structure.  This will be particularly true in
locations subject to seismic forces.  The beautiful vaults you show on
your website I believe take this into account in that the cob end walls
are enclosed by the bales and hence lend a stability to the building
which is perpendiculer to the bale vault structure ( where it is
needed), and the bale vault which is more stable parallel to the vault,
in turn lends "out of plain" stability to the cob.  If however you
choose to build so that the cob and bales are subject to "out of plain"
forces in the same direction, then the difference in flexibility will
work against them at the point they join.

Cob's ability to accept vertical gravity load is quite good, but the
load must be as evenly distributed as possible.  Strawbale can also
accept evenly distributed loads, but will compress downward under those
loads, while cob essentially will not.  Concentrated loads are not a
good thing in either strawbale or cob.

In designing connections between cob and bales, keep this "distribution
of loads" character of the two materials in mind.  An embedment of many
small diameter saplings, willow or bamboo well into the cob which in
turn can be tied to a similar embedment into the bales will ( I surmise
) serve well.  The many small diameter embedments deep into the cob will
distribute any transfered forces into the cob well, and at the same time
will allow the bales to flex, which is what they want to do.  The area
of cob into which the embedments are made should be rather thick and
braced by cross walls, butress or a larger pilaster.

Hope this helps.

john fordice
maker of cobbers thumbs
TCCP

artlady wrote:

> I hope to do a strawbale/cob combination with the strawbale's insulative
> advantages on the shade side and because strawbale might be faster to
> construct.  Any advice on connecting them to each other and the roof?  I
> can't decide on the best roof. I have yet to attend workshops described
> on my website for Vaulted Straw Bale, but I love the sample photo with
> it's cob front: http://www.imagegypsy.com/sbvault.htm
> Sandy Rice