[Cob] cob and earthquakes

[Shannon C. Dealy]

On Thu, 19 Feb 2004, Kyle Towers wrote:

> 1)    Even if your assumptions are correct and you assumed 5 times more
> straw would give 5 times the tensile strength, 250psi is still puny for
> something so heavy.

Puny or not (which is an extremely relative term), it only has to be
sufficient to hold the wall together under the criteria it is designed to
meet, earthquake or otherwise.   It is important to keep in mind how
other common building materials are used when comparing them with cob,
materials like steel and wood are generally used in much thinner walls,
and as frameworks rather than monolithic which makes meaningful
comparison virtually impossible in any direct manner since this type of
usage distributes it's strength over a much larger area, and subjects it
to much higher bending stresses, while at the same time reducing the
amount of mass involved.  Ultimately, when you compare apples and oranges,
whatever their similarities, in the end they are still two different
fruits :-)

> 2)     Factors such as wall height relative to thickness are the
> compensating factors that I referred to originally, but a powerful quake
> with strong horizontal accelerations could collapse a wall that is almost as
> thick as it is tall.  For equations on seismic loading of stabilized earth
> structures, you might refer to Bruce King's "Buildings of Earth and Straw".
> The equations for cob would take the same form.

As I recall the title of this book included something about rammed earth
and strawbale, since I don't have a copy, I can't readily determine for
myself how applicable the equations would be to cob (since both strawbale
and rammed earth are radically different techniques), however, assuming
the equations are generalized enough to apply to cob, it might be
interesting to take some real world cob strength measurements and see what
his equations would really indicate for cob, and if they give any
additional structural insights.

Shannon C. Dealy      |               DeaTech Research Inc.
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