[Cob] cob and earthquakes

[Kyle Towers]

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.

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.

----- Original Message -----
From: "Shannon C. Dealy" <dealy at deatech.com>
To: "Kyle Towers" <ktowers at locl.net>
Cc: <coblist at deatech.com>
Sent: Thursday, February 19, 2004 1:20 AM
Subject: Re: [Cob] cob and earthquakes


> 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.
> dealy at deatech.com     |          - Custom Software Development -
>                       |    Embedded Systems, Real-time, Device Drivers
> Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
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>
>
>