Cob: Earthquake.

[John Fordice]

The reality of the "Cob versus Earthquakes" situation is that we don't
really know.  

Cob, in the model that we use, originated in England, an area which I
believe is not known for earthquakes. so it's success and long life
there seems not applicable to those of us who live and wish to build in
earthquake counrty.

There is the apparent success of Cob in seismically active New Zealand,
but the examples that have come to us here in North America are few.  It
seems to me that a better study of what has been built there that has
survived earthquakes is currently lacking.  New Zealand gives us
encouragement, but I am aware of no solid broad proof.  A New Zealand
architect knowledgeable in cob put it clearly: "Cob buildings that are
well designed, well constructed, and well maintained have survived earthquakes".

The catastrophic mass failure due to eartheuakes of earthwall buildings
in various parts of the world and the associated high numbers of
fatalities give us the clear message that we had best be aware what we
are about when we build with earth, lest we suffer similar consequences
to ourselves and upon those who use our buildings.

In seismacally active California, we do have earth walled buildings, in
the form of Adobes, to which we can look for clues as to how earth
walled structures have historically faired in earthquakes.  It is true
that these adobe buildings are not the same as Cob, and the supposition
is that Cob, due to it's monolithic nature is probably stronger than
Adobe.  Be that as it may, these buildings are all that we have to look
to here by way of example, and so can serve us well in increasing our
understanding of the seismic / earth wall dynamic.  Two further points
relative to the benefit of studying adobe buildings are: 1.) Adobes are
the buildings with a history of failure; 2.) If cob is indeed stronger
than adobe, then incorporating an understanding how to prevent such
failure in adobe buildings into building with cob will, in all
likelyhood, result in a superior form of cob.

In pursuit of this perspective, the following may be of benefit to our
thoughts about Cob and Earthquakes.

In 1995 The Getty Conservation Institute issued the report: “Seismic
stabilization of historic adobe buildings”.  In a nutshell, the report
gives us the following knowledge about adobe walls: 
adobe walls will crack in a severe earthquake;  
the wall height-to-thickness ratio effects wall performance;
a ratio of<4 will remain standing even if cracked by an earthquake; 
 a ratio of 4-6 (typical of historic adobe buildings) will usually
remain standing if cracked; 
a ratio of 6-9 lessens in stability as the ratio increases and is likely
to collapse in strong earthquakes;
a ratio of 9-12 is likely to be unstable and collapse;
a ratio of>12 is unstable;
walls supporting a load from above are more stable than those not
supporting a load.

As a part the this report, The Getty folks also published the results of
a series of seismic shaketable tests of model adobe structures which
were conducted and the seismic stability techniques which were
incorporated.  The following are the stability methods which were tested
and the earthquake severity performance of each: 
(in a range of 1  to 7 as  moderate to severe) 
a stiff bond beam is effective if it has a positive connection to the
wall top (1-4);
a flexible bond beam performs better than a stiff bond beam (2-5);
horizontal straps along the walls act effectively to restrain the wall
once it cracks (3-6);
a flexible bond beam with vertical ties performs best (1-7).
Of the model structures which were shake table tested, the best
performing stability methods were a roof diphragm combined with either a
flexible bond beam with center wall ties, or a flexible bond beam with
vertical and horizontal straps on the outside of the walls.  These
methods show a survival potential of almost 3 times that required by the
UBC.  This is promising indeed.

In the absence of similar tested and proven information relative to Cob,
it seems prudent to adopt these techniques into building with cob in
locations subject to earthquakes.  To do less is at the very least, to
proceed somewhat blindly in a blith assumption of cob's superiority over
the forces which can be generated in an earthquake, and at worst to be
totally irresponsible and capable of creating buildings which may infact
be dangerous to those who use them.

This is not to discourage anyone from building with cob.  I personally
am totally in awe of and in love with the mud and what it can do.  I
intend to keep building with cob whenever and wherever the opportunity
arises.  This is simply a call to all of us, as responsible cobbers, to
deal with reality.  Once we have been able to actually test cob and
establish it's limits. then we will truly know. I will be among the
happiest if cob is shown be able to resist earthquakes without the added
stability techniques espoused above.  But until this needed work as been
done, and a through and exacting testing completed to establish cob's
actual strengths and limits, erring on the side of prudence, caution,
and responsibility is truly called for.

Cob on !!!
john fordice