[Cob] finding stats

[Shannon Dealy]

On Sun, 20 May 2007, claysandstraw wrote:

> Sara- your first clue: when looking for cob stats use the word "adobe".  My
> favorite structural engineer in Santa Fe considers them identical... he did
> admit that monolithic adobe could be stronger than assembled blocks.  I have

While this may be fine for getting you past the code, it will in many if 
not most cases cause you to do things which are structurally unnecessary.
The problem comes down to the one phrase above: "could be stronger" until 
this engineer sees data that says otherwise, he is going to treat it as 
adobe.  The fact is that cob IS MUCH STRONGER, if you doubt this, take a 
thoroughly dried adobe brick and a cob brick made with the same materials 
plus high straw content and try breaking them.  I have done this at many 
sites with a variety of mixes and different types of straw and grass.  I 
have never seen a cob brick that wasn't significantly stronger and though 
the variation has been large over the years (some I could break with my 
hands, some required repeatedly striking over the edge of something solid 
like a rock, concrete block or 55 gallon drum), the primary determining 
factors in how strong the cob brick was have consistently been the 
quantity, quality, and length of the straw used in the brick.  The clay in 
the soil certainly made a difference too, but the straw made a much larger 
one.  If you find that there is little difference between an adobe or cob 
made out of your local soil, take a good hard look at the straw you are 
using.

[snip]
> 1. structurally, bond beams are worth it

I would say yes, if needed, in many cases where adobe needs it, cob would 
unquestionably not need it.

> 2. the distance that headers bear on walls beyond the window opening is
> quite crucial

Agreed, though that distance is in large part a function of the 
compressive strength of the material, and again, cob's compressive 
strength (as well as shear and tensile strength) is greater than adobe.

> 3. stem walls should rise above interior floors
[snip]

This needs to be kept in context, the only reason I know of for stem walls 
to rise above the interior floors is to keep water away from the base of 
the walls in the event of flooding due to a pipe breaking or some other 
plumbing problem.  In a building that will never have plumbing, I know of 
no reason to apply this rule.  There may be a good reason to do so, but I 
have yet to hear it.

It is important to always try to learn where the general rules come from 
and why they are used, otherwise, you will find yourself doing things that 
make absolutely no sense, simply because you were taught to do things that 
way and never learned why.  On a regular basis, I find myself doing things 
that at some point I realize make no sense, it is important to question 
things often, because if you don't really understand the rules, you can 
waste alot of time and money, and more importantly, if you don't 
understand, you may blindly follow the rules when they are insufficient 
for what you are trying to do and the results could be disasterous.

I hope this doesn't come across as a rant, and if you can get your cob 
building passed by using the adobe codes, great!  But keep in mind, you 
may be using cement and steel that are not needed (depending on your 
location) if you go this route.  Cob is not adobe, as a structural 
material it is superior in every aspect, so building cob to adobe codes 
means over building (which I don't object to, always err on the side of 
safety), and probably using unnecessary steel and cement (which I do 
object to).  You will also probably spend alot of extra money on materials 
and construction costs using the adobe codes, but it is (at this point) 
most likely more than made up for by the savings in money and headaches of 
trying to get cob past the local officials.

FWIW.

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
    or: (541) 929-4089 |                  www.deatech.com