Cob Bond Beam? - Non-member submission from [John Straube <JFSTRAUB at BRIDGE.watstar.uwaterloo.ca>] (fwd)

[Shannon C. Dealy]

---------- Forwarded message ----------
Date: Tue, 9 Dec 1997 10:12:37 EST
From: John Straube <JFSTRAUB at BRIDGE.watstar.uwaterloo.ca>
Subject: Re: Cob Bond Beam? 
To: Speireag <speireag at linguist.dartmouth.edu>
cc: strawbale at crest.org, The Cob Mailing List <coblist at deatech.com>,
        Glaiseun: ;

On Sun, 7 Dec 1997, Speireag wrote:

>     Many straw bale builders use a concrete bond-beam at the top to level
> things out and hold things together.  What would be the difficulties with
> making that bond-beam from cob, instead?

>     Does cob have enough strength in tension?  Could you reinforce it with
> rebar to achieve the same result which rebar does in concrete?  Would the
> cob resist the flexion produced by further uneven settling?  Concrete
> appears to do so; why?  Because of the rebar?  Does concrete crack anyway,
> a little bit, but it's under the eaves and so doesn't matter?

(BTW Has anyone told you Speireag that you have a very fertile mind?)

The advantage of a concrete bond beam, whether in sb or brick, or adobe, 
is that it is strong and stiff, so it distributes uneven loads around.  
SB and adobe tend to be weaker and more flexible materials -- the walls are 
strong because they use a lot of area (thick and continuous walls).  So, 
concentrated load from rafters, settling, heaving etc are taken by the 
strong and stiff conc beam and distributed to a larger area of the weaker 
and more flexible wall.  Cob is similiar to the other weak and flexible 
materials I mentioned -- it is strong when used in mass, but it will not 
function as a bond beam does.

So, as a structural engineer, I really like the idea of a strong bond 
beam since it allows for a lot of redundancy.  This does not mean it is 
imperative, just that it is easy and simple to do.

If the steel in reiforced concrete is to be useful, the concrete must 
crack.  Typically the cracks are very small (<0.3 mm is the standard 
value used in codes) and well distributed, but almost all the reinf conc you 
see is cracked if it is carry its load.  Reinforcing cob is difficult for 
2 reasons 1) the steel must bond to the cob, and this requires a 
considerable amnunt of tension strength that cob is unlikely to have 2) 
the soft and weak cob will probably compress too much to be of a lot of 
use.  Both probelms can be overcome by proper design in many siutations, 
but it aint gona be easy.

To save concrete I would make the bond beam smaller, say 6" or 8" square 
or something similiar.  Wrapping stucco mesh up and over the bond beam 
from the bales to the beam would nicely tie it all togehter.  Proper 
filling with loose straw at this juncture would also stop the bond beam 
from acting as a thermal bridge.


-----------------------------------------------------------------------
John Straube
Civil Engineering Dept.              Building Engineering Group
University of Waterloo               Webpage http://sunburn.uwaterloo.ca/beg
Waterloo, Ontario, CANADA N2L 3G1
UW Phone 519 888-4567 Ext 2378       UW Fax 519 888 6197
Personal Phone 519 741 7920          Personal Fax 519 885 5193
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