[Cob] Cob -rusty nails vs transfer of loads to/from cob

[otherfish]

David, Brad,
Concrete is very strong in compression, and considerably less strong (weak)
in tension.   In reinforced concrete it is the compressive capacity of the
concrete pushing on the deformations (bumps) on the rebar that transfers
tension loads in the concrete to the high tensile capacity steel rebar.  Any
surface bond between the concrete and the steel rebar (even if it is
augmented by surface rust as you correctly point out) would be in the form
of tension & therefore relatively weak in comparison to the compressive
ability of concrete to transfer loads to the rebar.

We can speak similarly for cob  as it (cob) is even lower in tensile
strength than concrete.  The development of tensile bond between cob & other
materials is weak ( I'm guessing, negligble).  But even beyond this, cob's
compressive strength is so much lower that concrete ( appx 1/10 ) that cob
cannot exert sufficient compressive force on the rebar deformations to be
able to transfer any tension loads into the rebar.  The transfer of loads by
cob to another material needs to be via compression.  Otherwise, it
essentially will not happen.

So lets think about using nails to connect wood members to cob.

Consider a 16 penny nail studded piece of wood embeded in cob.  It will be
the surface area of the nail pushing against the cob that will transfer
loads into or out of the cob.  A 16 penny nail embeded 1.5" into wood can
transfer appx. 108 pounds of load either to or from the wood.  A nail thus
embeded in wood  will have appx 1/4 square inch of remaining surface area
acting perpendicular to the nail with which to contact the cob.  With 200
psi cob (which is typical for cob) this means this nail can transfer 200/4
psi or roughly 50 pounds of load perpendicular to the long axis of the nail.

Given that cob weighs roughly 120 - 130 (+ -) pounds per cubic foot, figure
that you need about 2.5 nails for every cubic foot of cob wall the loads of
which you want to transfer to the wood menmber.  Or vice versa for loads
which you wish to put into the cob.

How we analyse the magnitude of the loads involved & whether they need to go
to the wood or the cob is another story...........

john fordice

   

on 1/16/04 4:59 AM, David Knowlton at pilot1ab80 at hotmail.com wrote:

> rebar in concrete is allowed to rust. rough oxide provides a nice
> grip for the concrete. just a thought
> 
> 
>> From: Buckaroo Bonzai <tsuchimono at yahoo.com>
>> To: umbrella at netspace.net.au, coblist at deatech.com
>> Subject: Re: [Cob] cob in post and beam
>> Date: Thu, 15 Jan 2004 23:59:31 -0800 (PST)
>> 
>> Brad,
>> as I recall, Darel mentioned this before, you can
>> use the wattle and daub method where you can anchor
>> some boards, willow branches, or stips of bamboo
>> across the beams and even woven amongst themselves.
>> Twine can be used for the inter crossections and nails
>> for the endpoints.
>> 
>> I also recollect, and have to agree with Darel that
>> stainless steel nails or otherwise rust free nails
>> shoud be used for these walls.
>> 
>> You could easily work with this. Lared
>> 
>> -------------------------
>> 
>> 
>> --- Brad Calvert <umbrella at netspace.net.au> wrote:
>>> How is cob best tied into a post and beam frame?  I
>>> have been thinking of a
>>> sort of mud and stud wall and perhaps having largish
>>> nails partly hit into
>>> the sides of the studs, the protruding ends of the
>>> nails then embedded into
>>> the cob.
>>> 
>> 
>> __________________________________
>> Do you Yahoo!?
>> Yahoo! Hotjobs: Enter the "Signing Bonus" Sweepstakes
>> http://hotjobs.sweepstakes.yahoo.com/signingbonus
>> 
>> _______________________________________________
>> Coblist mailing list
>> Coblist at deatech.com
>> http://www.deatech.com/mailman/listinfo/coblist
> 
> _________________________________________________________________
> Learn how to choose, serve, and enjoy wine at Wine @ MSN.
> http://wine.msn.com/
> 
> 
> _______________________________________________
> Coblist mailing list
> Coblist at deatech.com
> http://www.deatech.com/mailman/listinfo/coblist