[Cob] cob and earthquakes

[Kyle Towers]

> Cob studies.  Links, bibliography, please.  I don't think I've seen them.
> There's that one book on structural engineering for straw and adobe, not,
I
> think much about cob in that, although I haven't seen it in a long time.


The most likely source of hard numbers is Earth Construction Handbook: The
Building Material Earth in Modern Architecture by Gernot Minke, which I see
is now up to $76.  I got it through interlibrary loan; you can do the same.

For a quick reference, it took me about 2 minutes w/ Google to find
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.  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
loads, too.  If you have a 7' tall wall that tapers from 2 1/2' to 1 1/2'
thick, you may survive a moderate quake with just cracks.  However, if the
ground moves horizontally several feet at an acceleration of 2 G's or more,
which happens, it's coming down like a house of cards.  Cob is better than
adobe in seismic events, but inferior to wood, steel, reinforced concrete,

Off-list, Jennifer Hileman noted that, "adobe is not the same methodology as
cob and the tensile and shear strength will not be the same it is a
monolithic construction and not blocks."  My response, "They tested a single
adobe in a tensile testing machine, which is functionally equivalent to
testing a piece of cob of the same dimensions, since no mortar joints are
involved."  Unless someone has evidence that cob is orders-of-magnitude
stronger in tension merely because it is made in bigger units, I think this
is pertinent data.  I also recall that the Minke book shows scale cob
structures being tested on a seismic table.

Kyle

----- Original Message -----
From: "Amanda Peck" <ap615 at hotmail.com>
To: <coblist at deatech.com>
Sent: Sunday, February 15, 2004 9:29 AM
Subject: Re: [Cob] cob and earthquakes


>
> Mass can get you into trouble.  Of course.  But looking at the pictures of
> the Bam earthquake, it looks like the fortress mostly stayed standing.  In
> other words the really really heavy, well buttressed building survived.
> With the newer stuff, some did collapse into power.
>
> Cob studies.  Links, bibliography, please.  I don't think I've seen them.
> There's that one book on structural engineering for straw and adobe, not,
I
> think much about cob in that, although I haven't seen it in a long time.
>
> In the Minke manual that I seem to post a link to EVERY WEEK, he says that
> he and his graduate student(s) concluded that skimping on building with
> earth (not particularly cob) was what leads to failure in an earthquake.
> Skimping on mass, lintels, buttressing (which can make up for the wrong
> shape, also a factor).
>
> http://www.gtz.de/basin/publications/books/ManualMinke.pdf
>
> In the interests of anti-true-believerdom--Minke has also published
> stuff--not readily available in English--on bamboo.  My German extends
about
> as far as "Guten tag" and "bambu."  Most of his field work seems to have
> been in South America, where both earthen and bamboo building are
possible.
>
>
> .............
> Exchange between Kyle Towers and someone (snipped and resnipped):
>
>  >that was one of the few buildings left
>  > standing after a major earthquake hit.  I think that the idea behind
>  > cob is that it is a monolithic building structure > The tensile and
shear
> strength of something built well seems phenomenal
>  > to me.
>
>      The mass, however, could easily be considered
> to be so.  Alas, it is often mass that gets a building in trouble in an
> earthquake.  The foundation moves and the massive inertia of the building
> causes it to be left behind.
>
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