[Strawbale] Re: SB arches

Rene Dalmeijer rened at cistron...
Sat Oct 25 00:16:07 CEST 2003


You will not get me phased. I like the challenges you put up every now and 
again. I was at the German second annual SB conference and many people 
there had been following the discussion about the LB SB 4 story castle very 
inspiring to all.

At 07:36 AM 10/24/03, you wrote:
>Has anyone got experience of building arches in a SB wall? What technique
>works best? Can SB arches be made genuinely load-bearing or do they need to
>be re-inforced in some way?

Most arches I have seen have not been load bearing but needed another 
structure to carry the load. The reason is simple. Bare bales have a too 
low modulas of elasticity (is not stiff enough) too work effectively as an 
arch in the way you intend them too. Once plastered the situation changes 
because the plaster has quite a bit higher modulas of elasticity then the 
bales. I have seen examples of post created arches based on this principle. 
One of the people I have spoken to said he had always struggled to make 
arches they were a pain in the neck (he is an accomplished SB builder) Till 
one day he decided to do away with the form board approach he was used to 
using. He built a full wall without an arch then after it was finished and 
plastered. He cut out the arch. Added some mesh and plastered it. ( the 
plaster was cement plaster hence the mesh but the mesh is very important 
here beause it will be taking the load due to its very high modulas of 
Elasticity. Very simple to do and no problems at all. The samples I saw 
were quite big but did not and would not take a serious load. I suggest you 
go for the same approach with  the addition of a supporting steel structure 
pushed into the void after you have cut it. You will have to support the 
wall whilst doing the cutting and latter pushing. I suppose the jack lift 
structure is still there when you do this. If you take this approach make 
sure the netting is tightly attached to the steel supporting arches or the 
whole SB structure will slowly do the splits over the arches. It is also 
prudent to use netting along at least the foot of the rest of the walls 
again to prevent undue spreading. You could see this as cheating but is you 
approach it in the following frame of mind you could see it as SB walls 
running over a flat foundation with a few oddly shaped arches in it.

>What type of SB arch is strongest and or easiest
>to build. Roman arches seem simpler. Norman arches built of masonry are
>structurally stronger and lighter than Roman Arches. Is the same true for
>SB? How is a Norman arch achieved? How do you calculate the correct radius
>for the curves in Norman arches and where do you position the centres?
>We are planning four largish(2m/7ft wide) arched windows or openings in our
>proposed four storey tower house castle. The floor structure of the floor
>above will spread the weight of the upper storeys, but the section that
>contains the arches will need to do its share of the load-bearing. Would it
>be prudent to have a squared floor to floor timber frame and add arched SB
>infill simply for aesthetics? Or can a SB arch be relied upon to take the
>load and distribute it into the neighbouring wall in the same way a masonry
>arch would? The arches could be deeper than the standard wall if necessary.
>This would add strength and make for an interesting internal feature, but
>we'd rather not if we don't have to. Likewise, we'd rather not have any more
>timber-framing than we have to. We are trying to take the load-bearing
>concept as far as we can.
>Isle of Arran
>I can already hear you sceptics out there sucking in your breath and saying
>"He's gotta be joking! Not only is he crazy enough to build a four storey SB
>building, but he's going to put damn great holes in it!". Well you guys have
>got a shock in store.

Rene Dalmeijer

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