[Cob] finding stats

[claysandstraw]

> > -----Original Message-----
> From: Shannon Dealy [mailto:dealy at deatech.com]
> Sent: Sunday, May 20, 2007 12:38 PM
> To: claysandstraw
> Cc: coblist at deatech.com
> Subject: Re: [Cob] finding stats

<snip: for structural calculations cob = adobe>
>
> 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.<snip>


...well the thing is that by the time your walls have enough mass you have
far far exceeded the structural requirements for either cob or adobe.  So if
the adobe standards are strong enough then why go any further for code
purposes?


>
> [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.


... I think "unquestionably" is a bit stong.  I can tell you based on my own
"shake the building test" that my cob columns were stonger after the bond
beam went on than before.  I'm not saying that the building would have
collapsed without a bond beam, I'm just saying it was clearly stonger once
the bond beam was on. Now it has been well argued that cob can form a bond
beam, from what I have seen this is true.  But 6" of concrete and steel will
always be stronger than 12" of cob... in extreme circumstances of roof load,
earthquakes etc. this *might* be an advisable compromise in the use of
concrete.


>
> > 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.


... I would consider the flex resistence of your header material (probably
wood) as the limiting factor in any window opening scenario.  I have yet to
see any headers crushing the wall below them, but I have seen several
headers deflected from the load above.  When a header extendeds into the
wall on either side: in order for the middle to bend downward the ends want
to lift up... the further the ends extended into the wall on either side the
more cob they have to lift before the middle can go down.


>
> > 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.


... well, first of all, how many cob buildings get a roof before their first
rainstorm?  ... and (secondly) what happends in 50 years when the roof gets
a leak and no one is around to fix it and water rains in?  or (thirdly) when
the roof burns and the fire deparment comes and hoses the hole thing down
and leaves 3" of standing water inside the house?

>
> It is important to always try to learn where the general rules come from
> and why they are used, <snip>



One of the great priviledges of spending time in rural NE New Mexico is the
abundance of 100+ year old earthen buildings in various stages of disrepair.
I have crawled through many a fence to poke through the remains of, or peer
through the windows of very old earthen structures.  In my previous e-mail I
should have said "things I learned from the living culture of adobe
construction, including the code, supported by historical examples."  I
could write ten pages on the things I have learned from my continuing
observations of the old buildings.  But I will spare you all the length of
specific examples, if you get the chance, go see them yourself!
Kindra