Cob: Seismic Effects

[John Fordice]

John G. & all cobbers

Here is a repost of my views re post & beam  with cob infill, and
seismic stability of cob.
...........
So, as I've stated above,  the sometimes adopted technique of building
cob as an infill in a post and beam structure has a basic structural
problem to it. 
 
Post and beam structures are by their very nature, a series of straight
walls with the loads of the roof being carried by the beams to the posts
and thence to the ground.  This eliminates the roof load from going into
the cob walls. A reasonable technique to structurally satisfy a building
department that does not understand cob, or to speed up a cob building
while protecting the cobbing by placing it under an already standing roof.
The rub comes in the nature of the inherent strength of cob, and how
earthquakes (EQ) act on straight walls.  Cob is very strong in
compression. One way earthquakes act on a wall is what is called "in
plane".  This is when the EQ forces push & pull in line with the length
of the wall & in that direction there is lots of cob to resist the
forces and alot of these forces can be taken up in compression of the cob.

The other way EQs act on a wall is known as "out of plane", or sideways
to the wall.  In this case, there is very little wall cob to resist the
EQ forces, and the perpendicular direction of the load essentially puts
the forces into the cob as tension, and tension is where cob is weakest.

ALWAYS REMEMBER, COB IS STRONG IN COMPRESSION AND WEAK IN TENSION.

The advantage of loading a cob wall at the top is that you will create a
system of restrain for the "out of plane " forces at the top of the
wall, and should the wall fracture from the forces of the EQ, you
additionally will have gravity loaq forces pushing down on the wall
which will help keep the fractured portions of the wall in place.  It is
best if the roof loading is as uniform as possible (cob doesn't like
highly concentrated loads) and will be further helped of you have some
sort of a "bond beam" atop the wall which serves to grip the top of the
wall and resist the "out of plane" forces at the top of the wall while
at the same time serving as a mechanism to evenly distribute the many
point loads created by the roof rafters where they bear on the wall.

This is not just pie in the sky theory.  Observation of failure of earth
walls due to EQ forces as shown this to be true, and the system of a
wall top bond beam which is well attached to the wall AND to a strong
roof diaphragm has been tested on adobe walled structures an proven to
resist strong EQ forces.  This added to the apparent( though as yet
unproved) superior strength of cob over adobe will impart a substantial
level of ER resistance to a well built cob structure.
.....................

In addition here is some writing about my work in pursuit of
incorporating earthquake stability in cob buildings in lieu of any
proven methods SPECIFIC TO COB

......................
In June if 1998, I worked with Katie Jeane to help her get a building
permit for her cob house near Willits, California.  This is in Mendocino
County, which has the good fortune of being one of the three counties in
California which allow “Class K” (owner builder) Building Permits. 
Under this type of permit, the owner-builder is exempted from many of
the normal UBC permit requirements. The County, due to a lack of
understanding of cob, required Katie to have an Architect of Engineer
sign her plans, and this is how  I became involved.  The task as I saw
it was how to fulfill my legal and moral obligations as a California
Licensed Architect (i.e. make sure the building design was structurally
sound) and at the same time not subject Katie to the cost of hiring a
Structural Engineer.  On searching around, I found the Getty Report 
which lays out a set of simple Seismic Stability techniques for use on
historic adobe buildings.  As this was the closest thing going to a
tested and proven method of assuring that an earthen building will in
fact withstand a severe earthquake, I elected to employ  these
techniques in the plans for Katie’s building.
Katie was thus able to get a permit for her building ( the first cob
specific building permit in California, to my knowledge), and the last I
heard, her building was nearing completion in the fall of 1999.
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.
The stability methods tested in the Getty report and their earthquake
severity performance results are (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 August of 1998 I began the construction of a cob toolshed at the
Northside Community Garden in Berkeley, California.  This a also small
(<120 s.f.) building in an urban setting and we have not sought a
building permit on that basis.  Roughly triangular in plan, it employs a
concrete bond beam designed to grip the top of the wall,  1/4” dia.
vertical wall ties @ roughly 24” o.c., and a mortared urbanite base wall
on a reinforced concrete footing.  The cob was mixed with the
combination of a small cement mixer for the earth, sand & water,  and
foot stomping the straw on tarps.  The cob walls and bond beams are
completed to date and roof framing is just starting.  The project has
been done by all volunteer labor of a Saturday morning basis.  We expect
completion of the living roof by fall.
....................

P.S.
The Berkeley project is essentially completed.  The roof facsia, and
hanging the doors & windows are all that remain to finish.  There wil be
a dedication on this coming May 13th & all are invited.

Hope this is helpful.

john fordice
maker of cobbers thumbs
The Cob Code Project

John Gorman wrote:
> 
> 
> I would very much like to see you re-post the information about
> strengthening cob structures.