SB: [Fwd:] SB Vault Questions

["Robert W. Tom" <ArchiLogic@yahoo.ca>]

I'm afraid that my back won't let me sit-up for long enough to answer 
Doug's questions so perhaps others could ?  I don't know if Doug is 
on the SB List so it'd probably be a good idea to copy him directly.
--- * ---
Rob Tom
Kanata, Ontario, Canada
<ArchiLogic@CHAFFyahoo.ca>
(winnow the "chaff" spamguard from my edress in your reply)

Please visit http://www.theHungerSite.com daily

------- Start of forwarded message -------
From: Doug  Nuttall <dnuttall@rcii.com>
To: "'archilogic@yahoo.ca'" <archilogic@yahoo.ca>
Subject: Fwd: More curiosity about straw bales
Date: 11/22/02 11:31:12 AM

About a year ago I dropped you a line at a different email address, 
asking
about straw bale construction, and you replied that I should do some
research at CMHC.  I've done some research on permeability and 
structural
elements, and I know I have to learn a lot more about all of this.  I am
currently trying to research straw bale arches & vaults, and I am 
having
problems finding an appropriate buckling theory for straw bales, with
plaster on the inside, and steel mesh on the outside.

Do you know of any specific research on arch buckling, composite 
buckling,
and/or straw bale buckling theory?  Thus far I've found theory on 
circular
arch buckling, but I'm thinking of either parabolic or catenary arches, 
and
the theory falls apart if the arch deviates significantly from the circular.

I've asked a friend who is a structural engineer to run a straw bale 
vault
through his S-Frame software, using straw, steel, and plaster 
elements, but
I don't know things like how much tension to put in the steel & how 
thick to
make the plaster on the inside to maximize the span and minimize the 
cost.
I may be able to get him to run a variety of models, but I don't want 
to owe
him that much beer.

I've looked at the circular arch designed and tested by Skillful Means, 
and
although it produced some spectacular results, I'm envisioning a 
different
concept.  I want to do an arch that has tapered bales, so there is no
plaster between the bales, and use pure tension on the outside, pure
compression on the inside, just to keep it simple.  Get the angles right,
and the whole thing should work well.

I'm also envisioning a completely different cladding concept, using
geotextiles over the outside to keep water out while allowing the outer
surface to breathe, rather than stucco.  My moisture transport model 
showed
me that in a typical Ottawa winter, condensation would occur if there 
was
almost any resistance to water vapour on the outside (tyvek, for 
instance).
Preventing that with a plastic sheet on the inside surface would cause
problems in air-conditioning weather.  I'm still working on the 'best'
solution that prevents moisture building up in the walls, has low 
embodied
engergy and resources, and is generally simple to construct.  Thus far 
I'm
at a drain-type geotextile (Tenax Tendrain Geonet, for example), 
covered by
an impervious layer, covered in soil or stone - I like the concept of a
living roof, but it's heavy.  The air entering the home would be drawn
through the drain material, regaining heat and moisture that are being 
lost
through the walls.  I have a problem with the amount of head required 
to
drive the air system, due to the amount of moisture that would have to 
be
picked up and the (lack of) ability of cold wet air to gain more 
moisture.
Still working...

I'm also looking for a relationship between bale density, compressive
strength, and modulus of elasticity.  I'm pretty sure they should have 
that
at CMHC, but I haven't spent enough time there to find what I'm 
looking for.
What I want is a relationship that will predict long term performance 
of
bales, and how it would change if I re-packed them to make them 
denser.  Do
you have any suggestions of what I should look for?

One of the other things I'm looking at is a straw bale silo, say 4 
storeys
tall, using bales with their strings parallel to the ground, trimming and
retie-ing the bales to make them tapered, and wrapping the entire 
structure
in steel mesh.  Do you know of projects that have done this?  I have 
seen
photos a 2 storey structure that was about 3/4 around, on the corner 
of a
rectangular structure.  There was no live load at the second storey 
(big
open space), and they bent the bales by beating on them.  Seemed to 
work.

Any suggestions you can offer would be appreciated.  Most of my
investigations are intended for my own use at this time.  Perhaps I will
know enough that I could offer professional services to the straw bale
industry, but I'm not there yet.


Thanks for your time;

Douglas Nuttall, P.Eng.
ROBINSON CONSULTANTS INC.
Kanata, ON.

-------- End of forwarded message --------




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