Cob: Berkeley living roof & wall loading vs. earthquakes

[John Fordice]

Hi all cobbers,

Shabari Bird & I had this brief communication last month & I'm finally
getting around to bringing it to the cob list."
This is intended to discuss the issue of cob walls being constructed as
either "load bearing" or "infill".

The exchange (in reverse order) went:

Shabari Bird wrote:
> 
> I would like to have this discussion on the list
..........
John Fordice wrote: 
> > Shabari,
> >
> > Question:  Where are you located?  I ask because of concern for the roof
> > first approach it sounds like you are using.  An ok technique in non
> > earthquake areas, but a bit suspect in my opinion if you are located in
> > EQ county.  Testing/observation has shown that earthen walls which are
> > carrying an evenly distributed roof load will survive an earthquake
> > better than an infill earth wall system in a post and beam structure.
> > Am I correct in the latter is what you are doing ?
> >
> > I would like to move this discussion to the open list if it's ok with
> > you, as it's an important issue that I'd like to see others join in to.
..................
> >
> > > Shabari Bird wrote:
> > >
> > > We are currently installing a living roof.  We will be cobbing the
> > > room in August.  OUr builder is building the roof this coming week.
> > > The room is 35 feet by 13 feet.  We have the liner left over from our
> > > pond installation.  We plan to put bales of straw and hay on top of
> > > the liner instead of struggling with getting large amounts of soil up
> > > to the top,.
> > > We will then wait until nature makes the soil and then we plant with
> > > creeping rosemary.
....................

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.

I welcome comments & reactions to this.

Re the Berkeley living roof:  The EPDM pond liner roof membrane went up
pretty much without a hitch two Saturdays ago.  The one problem was that
due to the slope of the conic section form of the roof, the EPDM tended
to slip down the roof & create a bit of tension.  We solved this
hopefully by attaching it temporarily at the top & then releasing it &
reattaching once we had the soil in place.  In hind sight, it would have
been a good thing to include some sort of adhesive attachment of the
underside of the EPDM to the roof sheathing at least down the center of
the roof slope. Next time.

The light weight soil mixture as placed the next day oh Sunday with the
help of several folks who responded to the Cob list & ADPSR email call
for help.  Thanks to those who came to help, we were able to make a
bucked brigade & place the 5 cubic yards of soil in about one hour.  We
did it as two layers of appx 3" with a layer of plastic bird netting in
between the two layers to serve to tie it all together on the compound
sloping roof.  Once the native grasses & wild flowers take root, we
should have a well tied together and stable layer of soil on the roof.
The soil mix and selection of grass & flower seeds was done by Heather
Koch. 

Thanks to all who made this milestone happen.

The recycled plastic bender board fascia is all that's left to be done &
the the roof will be complete.  The building should be finished in early
March.  It's been a long 2 1/2 years of Saturday sessions getting there,
but well worth the effort.  It's a beautiful little building.

john fordice
maker of cobbers thumbs
the cod code project