[Strawbale] 3-string bales...

[John Swearingen]

The render on the walls, being stiffer than the bales, is the first element
in the assembly to take compressive loads.  The strength of the render
varies depending upon what material you use (dirt, lime, cement).  With the
weaker renders (dirt), precompression would be a Good Thing.  With the
stronger renders (cement), the skins may be strong enough to take the loads
by themselves, and precompression is less important.

It's worth while to remember that the bales are already compressed, probably
tighter than you can do in the field, by the baling machine.  What we call
"precompression" would more properly be called "mushing", since you're
mostly squeezing the bales together, taking up the slack between courses of
bales.

This could be accomplished with mud packing between courses, or by various
other methods prior to stapling off the mesh, such as yanking down the mesh
with a pitchfork, resting a backhoe bucket on top of the wall, jumping up
and down, balloon inflation, hydraulic jacks, and herding moose families
along the top of the bale.  Using threaded rods and cranking the nuts seems
to me to be, well, boring and tedious, and limited in strength.

For out of plane loads, like wind, seismic and things that would make the
walls bend (trucks, moose, etc.) the assembly can act like an I-beam when
properly detailed, with stiff tension skins (render plus mesh) on both sides
and the compressive bale element in between.  Tying the two mesh layers
together, through the bales, will prevent the skins from popping off the
bales in bendig, and makes the assembly function like an I-beam.   External
pins, or exo-stiffeners (if it's external, it's not really a pin, is it), do
work, whether they are rebar, threaded rod or bamboo, but since you already
have mesh, isn't an exo-stiffener redundant?

The mesh is usually sufficient to hold down the bales from uplift, as well,
and we haven't threaded a rod since out first, almost 15 years ago.  Pins on
the outside are popular, but, again, the mesh will do the same when properly
detailed, so the pins become redundant.

We have used the bale/plaster/mesh assembly for in-plane loads and shear,
replacing x-bracing on occasion, but that's another topic.

John "Mesh Head" Swearingen




On 3/28/07, David Neeley <dbneeley at gmail.com> wrote:
>
> It seems a majority of the strength comes from the render on the walls,
> not
> the bales themselves--although when the render is properly keyed into the
> bales, the resulting stability is a very "good thing" as well.
>
> If you live in seismic areas, there will be code requirements for various
> kinds of stabilization, often metal bracing running diagonally along the
> wall (beneath the render). In some of these areas, bale infill with strong
> frame of wood or other materials may also be required.
>
> Some of the California members can tell you a great deal more about this
> than I can.
>
> Outside of active seismic areas, a well built load bearing strawbale wall
> has far more strength than is required for either one or two story
> residential construction and needs little else.
>
> Threaded rod is still used by some, but I would be surprised if it is
> 3/4".
> In recent years, pinning has gone from the original inside-the-bales to a
> surface pinning for most--often with bamboo.
>
> The primary utility of threaded rod lies in two areas (and not
> particularly
> to "keep the bales from bouncing apart": to precompress the bales prior to
> the roof going on and the render being applied, and to give a firmer
> connection to the roof structure to resist the upthrust from high winds
> over
> the roof--countering the Bernouli Effect, in fact.
>
> David
>
>
> On 3/28/07, Shody Ryon <qi4u at yahoo.com> wrote:
> >
> > Intuitively, to me, it seems like SBs would withstand
> > direct downward pressure well. I worry about bouncing
> > them appart though, ...
> > Does anyone install hold downs, such as 3/4" threaded
> > rod, connecting a top plate on top of the bales to the
> > foundation on a bale barring wall? It would seem that
> > such a hold down would increase sheer value too. Is
> > plaster generally relied on for this purpose? Perhaps
> > there would be a way to use bamboo instead of threaded
> > rod?
> > Shody
> >
> >
> >
> >
> >
> >
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-- 
John Swearingen
Skillful Means, Inc.
Design and Construction
www.skillful-means.com