[Strawbale]Some loadbearing compression ideas/questions

Stewart Hargrave Stewart at Hargrave....uk
Thu May 5 13:02:06 CEST 2005

On 27 Apr 2005 at 11:15, rikki nitzkin wrote:

> HI all,
> Some thoughts about compression for you tecnical people.  I have been 
> thinking about these techiniques that are popping up as ways to reduce 
> compression on loadbearing walls.  Specifically the techniques like those 
> used by Tom Rijven, or the man in ávila:  they use clay paster on the walls 
> before or during the wall-raising.  This solidifies the walls and they don´t 
> compress.  This has obvious advantages because it protects the wall at once, 
> and you don´t have to leave gaps above or below the windows and doors, and 
> if you build the wall well you don´t have to worry about uneven compression, 
> etc.  SOunds good, huh?  But the other day I was thinking (I do that once in 
> a while) and I thought to question:  A compressed wall is sure to be 
> stronger, no?  it may be more comfortable avoiding compression, but will it 
> not make the walls weaker and capable of bearing less weight? On my 
> load-bearing walls I have a clay-tile roof, I don´t know if I could 
> recommend putting a heavy roof on an "uncompressed" loadbearing wall . . 
> .But in the testing people have done they say that the plaster bears more 
> weight than the bales themselves, some maybe my doubts are irrelevant . . 
> .What do you all think?

I was reflecting the other day about the construction of a Formula 1 car. These 
are made out of thin carbon fibre - the strongest parts of the car are made from 
two thin layers of carbon fibre separated by a weak core material a few mm thick, 
making the whole very much stronger than the sum of the parts. The essential 
element is the gap between the two thin layers - on their own the thin layers will 
bear much less compressive and shearing forces, but as long as a stable gap 
can be maintained between them, their combined strength is very much greater.

It seems to me that in some respects, this is not so different to a load-bearing SB 
structure - two thin layers of stucco separated by a core of straw. Now carbon 
fibre is much stronger under tension that stucco, but that may not be an issue 
under load-bearing conditions.

The point is, much evidence suggests that the stucco takes most of the load, 
rather than the straw, so once two layers of stucco are applied, the tendency for 
the straw to settle down may not be as much of a factor as you may first think, 
and the two layers of stucco acting in conjunction with the straw core provides a 
strong enough support.

I should point out that this is not an expert opinion, just an idea I had.

Stewart H.

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