[Cob] cob and earthquakes

[Shannon C. Dealy]

On Wed, 18 Feb 2004, Kyle Towers wrote:

[snip]
> information on adobe (with straw) tensile strength.  The building code calls
> for 50psi.  A company with a block press, using 4000psi forming pressure, is
> bragging about getting 100psi.  As tensile strengths go, these numbers are
> puny.

You don't mention if the amount of straw in the tested adobe was given,
but my past reading on the subject generally puts adobe at somewhere
between zero and 2% straw, where cob typically runs 10% or higher
(sometimes much higher).  Based on the enormous difference in straw
content, any measurement of adobe tensile strength is meaningless when
applied to cob unless they pointedly ran the tests with much higher straw
content than normally occurs in adobe.

> And don't think that you gain much by building more massive.  In an
> earthquake, the loads are inertial loads, i.e., they are proportional to the
> mass.  So if you double the size to double the strength, you double the
[snip]

Testing of earthen structures seem to indicate otherwise, making the
structure more massive significantly improves it's ability to withstand
the stresses of earthquakes.  The weakness appears to be a function of
wall height relative to thickness, roof strength and load, and probably
other things like bond beam (or lack there of).  This makes sense from a
purely intuitive perspective (at least to me), imagine a block of cob
which is 8' high x 8' wide x 8' deep  versus a block that is only 1 foot
deep, it seems obvious that the one foot thick block will be far more
likely to break apart when subjected to an earthquake.  History (and
the recent earthquake in the middle east) back this up, the earthen
buildings with the thickest walls consistently survive far better than the
ones with thinner walls.

>From a more physical perspective it also makes sense in that the one foot
thick wall will flex/bend far more easily relative to the amount of mass
it contains, and bending it sufficiently will break it, where a much
thicker wall will not bend as much, so the stresses will be more limited
to shear, which (for every material I can think of) is generally much
higher than the bending strength.  I'm probably not describing this very
well, but I'm trying to keep it reasonably comprehensible and also avoid
doing alot of math that you don't want to see and I don't want to do (I'm
way to out of practice on it :-)

Shannon C. Dealy      |               DeaTech Research Inc.
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