[Strawbale] Load Bearing Multi-storey Straw Bale Tower House Castle

Chris Mowatt ChrisMowatt at i12...
Wed Jul 30 14:01:10 CEST 2003


Thanks for your comments. Its good to see that our little project is
provoking some interest. It was particularly nice that you have constructive
ideas rather than doubts and criticisms. It's good to be positive.

> 1) Build the two bottom stories with jumbo bales. They will easily take
> load. Build the other two either with normal bales or even  wood on the
> floor.
We have considered using mini hesston bales (8' x 3' x 2') for all or part
the building and certainly a hybrid design has appeal, but my rough
calculations suggest it shouldn't be necessary. I think the compressive
of 'standard' straw bales, laid conventionally, should be sufficient to
the building. The completed building would weigh approximately 100,000 kg
(60,000 kg walls + 40,000 kg roof, floors etc) which will bear on 25 square
metres of straw bale on the first course. This represents a loading of
approximately 4,000 kg per square metre, which, according to the Ghailene
Bou-Ali research of 1993, is less than one tenth of their capability.
> 2) I suggest you make an architectural detail at each floor break to keep
> rain off the walls. (like a 500-600mm skirt use thin slate to keep it
> but authentic). An alternative is to let successive floors jut out over
> previous. Both solutions offer easy access to the necessary jack lifting
> structure.
I like this idea provided we can make it look authentic. The planners might
object if it's not authentic. Certainly tower castles usually had an
just below the battlements. I'm not so sure about lower level skirts (or
We planned to make the conventional pitched roof project beyond the walls
on all sides to provide shelter for the walls and disguise this with
battlements above the roof pitch. The trouble with this approach and any
attempt at sheltering the wall is that we are subject to wind driven rain at
this location and so any skirt or extended roof would fail to shelter the
walls. If you frequently have driven rain is there any point in trying to
the wall? Would it not be better to concentrate our efforts on coating our
straw bales with a material that resisted water penetration and allowed
moisture to escape?

> 3) Don't discount the weight of plaster it is huge. We made a 2m^2 SB
> plastered sample it weighed in including frame at almost 400kg. ie at 20m
> high this means 4000kg per meter wall length without any other structure.
> It is quite a weight but doable.
The figure I have is 1 sq meter of straw weighs 75kg. It's not clear whether
that is with or without plaster. If the weight is 200kg per sq m then I'll
to recalculate.
> 4) I suggest you build a story at a time and then jack it up, which can be
> done in stages. As SB is much more forgiving then most other building
> materials you don't need super precision.
Certainly this is an option and our first inclination. It would certainly be
quicker. We felt that perhaps it might be more dangerous because people
would be working under a propped structure with no fail safe. Jacking it
up a course at a time will take longer but equiment failure would be less
likely to result in catastrophic collapse. There is also the added bonus
the weight of the completed structure can be used to compress the wall
a course at a time, which should lead to greater stability.
> 5) I also suggest you use some method to keep the walls as flat as
> thus reducing the amount of stucco needed. (you also need good tight bales
> which also require less settling)
Jacking the walls a course at a time would help, because when placing bales
you would have two points of reference, above and below, to ensure the
bales are vertically aligned. Minimising stucco/render/plaster might also be
an argument for laying bales on their side as the rendered surface would be

> 6) Make sure all bearing portions are symmetrically placed otherwise the
> building is sure to slowy become lopsided.
This is a problem with our current design, because, although the tower is
square, we plan to have a large part (50 percent) of the south wall curtain
wall glazed. We planned to have internal straw bale walls to maintain the
structural integrity. It would be a U shaped tower with glazing across
the top of the U. We might decided to build a square tower and fit the
glazing retrospectively.

> 7) I am very wary of the battlements. We all know what to what grief the
> Sante Fe style leads to in SB construction it is asking for trouble. The
> parapets are a sure source of leeks. If you have to make the battlements,
> fake them ie not SB and with the roof going all the way out to and
> including the previously suggested rain skirt.
You'll notice I said simulated battlements. We quickly abandoned the idea of
real battlements because of the problems of keeping moisture out. This is
why we've gone for a conventional pitched roof (which is still authentic -
most Scottish tower castles have a pitched roof garret, presumably also
to keep the rain out). I see no reason why the simulated battlements
couldn't be straw bale, so long as they are not structural. In fact their
condition, given their lack of protection, would be a good forewarning
about the condition of the rest of the walls.

> Besides the structural measures I didn't do any calculations. I think
> probably point 2 is the most important measure you should take. Keeping
> rain off the walls is essential for longevity.
With driven rain, I'm not sure about the practicality of keeping the rain
the walls. If anyone has experience of unprotected straw bale walls
moisture penetration we would be very interested. Lime render and lime wash
seems the best bet so far. We might be better off looking at ways to apply
regular coats of lime wash to seal any cracks.

Thanks for your help.


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