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RE: GSBN:Compression

Hi Derek & Maren,

Using the polyester strapping with 1,100 kg breaking strain and extra
heavy duty buckles we are consistently achieving 7% compression which
equates to approximately 160mm on a 2450 (7 bale on flat) wall. We have
done little with bales on edge, but have not found them to respond not
too differently to those laid on flat.

Regarding the compression of bales, the report on the tests done at
Sydney West University in Australia indicate that the straw bales are
more than capable of carrying the load rather than the render. As a
conventional builder I can understand where the theory that the render
is supporting the load comes from but it is unnecessary to rely on that.
I have also discussed this with our engineers and Dr Zhang who also
agree that it is unnecessary to rely on the render to support the load.
We believe that not only is unnecessary, but unwise, as to rely on a
material that can be degraded by weather is fraught with danger.

The principle applied to this application are the same as those of a
hollow core door with honey comb cardboard in the center and a thin ply
skin. The core has no strength of its own but in combination with the
ply it has strength. If however the core of the door were made of solid
timber the ply on the outer edge would be for decorative purposes and or
to extend the life of non weather resistant timber in the core. Whilst
it could still be considered that the strength is in the ply because it
is fixed to a spacer, it would not be calculated as so, as the core has
its own strength. This I believe is the same with straw bale
construction when appropriate pre compression is achieved. Whilst
builders may understand the implications of relying on the skin, owner
builders probably wont, which is likely to lead to misapplication of the
theory and will most likely lead to serious problems, as the following
example shows.

I recently hear of a house being built where the roof was fitted to a
straw bale wall with insufficient pre compression resulting in the wall
sagging and the loss of a straight and level roof line. Their solution
was to jack the roof up and render the wall in an attempt to support the
roof. I can only assume that this was working on the principle that the
render supports the roof load. This approach frightens me, as I believe
it is like putting a Band-Aid on the wound from open heart surgery. You
don't have to be a rocket scientist to realize what will happen if the
render is not maintained, and given that the bales have already sagged
under the load it would be considered that the walls had failed. I
wonder whether they had an engineer come and assess all of the
implications prior to applying this solution. Probably not. My engineer,
when told of this solution, simply smiled and said there is no way he
would put his name to it.

When relying on the strength of the bales to support the load, it is
essential that the bales are precompressed to a consistent tension
around the complete building, rather than to compress to a wall level.
If compression is done to a wall level, and there are soft bales in a
section of the wall it is likely to drop when the load is applied, which
is probably what happened in the example above. Once compressed to a
consistent tension it is easy to pack up the top boxing to achieve a
consistent and level pitching point for the roof.

I trust this is of help rather than simply controversial.

Kindest rregards

Brian Hodge
Anvil Straw
Building Consultants

-----Original Message-----
From: GSBN [<a  target="_blank" href="mailto:GSBN@...";>mailto:GSBN@...] On Behalf Of Derek Roff
Sent: Thursday, 19 April 2007 5:02 AM
Subject: Re: GSBN:Compression

Hi, Maren,

Can you explain more fully the information that you are looking for?
Reading your question, I am unsure of what you want.  Obviously, bales
can be compressed different amounts, using different compression
systems, such as straps, threaded rod, pneumatic tubes, etc.  People
often refer to these methods as pre-compression.  Is that what you are
thinking about?

In theory, unplastered bales under load will compress indefinitely.  In
a wall assembly, the engineers assure us that the plaster takes the
load, and plastered bales won't compress any further.


--On April 18, 2007 6:47:20 PM +0000 Maren Termens
autoconstruccin@... wrote:

> Hi all!
> Does somebody know the % of compression of bales layed on side?
>  Thanks, Maren

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek@...