[Strawbale] 3-string bales...

[Marcus]

Jeff Ruppert wrote:

> It is interesting that Matt described the 2-string walls
> as being plastered better than the 3-string, because I do
> not recall this in such stark contrast..  I do recall
> both common and different people working on the 2-string
> bales, but the level of quality on all of the walls was 
> sub-par IMHO.  I do remember walking between two of the
> 3-string walls and seeing bare straw after the first coat
> on one of the walls.  I think we looked at this and
> realized that the spacing was so close it was difficult
> to see with the lighting in the lab.  However, when we
> applied the second coat, it went on ok and the overall
> thickness was similar on all the walls.  That's just my
> humble opinion from partial (maybe selective) recall
> after 7 years (I think I am out of my mind at times).

Heh, that's what breathin' straw dust does to us. Just going
by what I saw in scanning the report the impression was that 
the 2SB walls were plastered by (mostly?) an experienced 
crew. This suggests that they knew enough to push the 
scratch coat well into the straw to get effective keying. 
That some spots were completely missed on the 3SB walls 
suggests that the inexperienced folks were just lightly 
smearing the scratch coat over the straw. Adding more stucco 
over that wouldn't seem to get much keying although building 
up the thickness would help. FWIW, I see this tendency to 
'frost the cake' in workshops with novice plasterers.

If this was what actually happened the skins would have been
much better supported on the 2SB walls. If the skins on the
3SB walls were actually very lightly attached, buckling of
the plaster would take relatively little force. With the
load not being supported by the skins, the bales would take
it and deform leading to further buckling.

I don't know if this is actually what happened but it would 
tend to explain the different load values. Even if this 
isn't what actually happened, it's worth pointing out to 
those who haven't thought about it: the plaster skins take 
the load. If the skins are not supported (by keying the 
whole first coat of plaster well into the straw) they just 
become skinny columns and will tend to buckle with 
relatively less force. In the case of severe loading 
(seismic &c) the skins will likely fail and the straw 
becomes a back up shock absorber system that will probably 
keep the roof from crushing the occupants.

So, my impression remains that what caused the 2SB walls to 
test stronger than the 3SB walls was not the bales 
themselves but that the 2SB wall systems were better assembled.

I'm not trying to knock the test, just trying to look at the 
results from outside the box. I'm grateful for any testing 
that happens. It would be sweet if we could find an angel to 
fund a lab specifically for SB testing. Some large (and well 
lighted!) warehouse where equipment could be set up and 
dozens of wall assemblies could be tested. Tests analyzed 
and repeated until consistent results were achieved. Yeah, 
well, just dreamin'...

Best,
Marcus