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 | | From: | Gordon Couger | | Subject: | Re: Triaxial polytunnels | | Date: | Sat, 11 Dec 2004 03:05:53 -0600 |
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 | Oz wrote:
> Tim Tyler writes
>
>>Basically, I built a model of a triaxial weave
>>polytunnel, photographed it, wrote some material
>>about its significance - and then uploaded the results.
>
>
> A nice idea but as a polytunnel it has the problem that its not
> triangulated perpendicular to the axis of the tunnel. As a consequence
> it will be prone to flexing if the wind hits the long side. You could
> fix this by making it a dome, but that's a difficult shape to cover
> neatly with plastic sheeting and has a rather inconvenient internal
> space.
>
> Others have commented on the durability vs cost of the plastic covering
> for conventional polytunnels. One can buy 'transparent' tarpaulins with
> a very open weave fibre reinforcement welded between two layers.
> Doubtless significantly dearer than plain clear polythene, but should be
> very much more tear resistant. In particular they are very much less
> elastic so that deformation in a high wind, which allows movement,
> should be significantly less. I would expect this to confer a
> considerable increase in life under windy conditions.
>
At Ag Engineering we built an air supported green house with
double walls for insulation that was supported during erection
and and when there was a tear buy a simpler structure nd more
expensive than the poly tunnel. The internal structure also
supported the second skin and the inflated wall protected the
inner skin from the wind.
Our Oklahoma weather is pretty rough on such structures. Any
wind over 50 mph was likely to cause a problem. Fortunately
those winds usually come on cold fronts from the west not the
aortic ones and you usually have little time to fix the
structure before the temperature inside falls too low.
The Poly tunnel looks promising from a cost stand point but a
building made with 2 CATENARY arches and two anchors and cables
should cost less but fitting the skin is more difficult.
I built a model that looked promising. It used two Catenary
arches at each end and cable were laced from an anchor in the
ground on each end and run thorough eyelets in the archs in a
pattern that was offset from one arch to the next so a sway back
roof was formed.
All the load was carried by the cable anchors and the vertical
load on the arches. I can't find an example of the structure but
if anyone is really interested I can find the software that
calculates the arch and you would probably have to do a bit of
experimenting on a model to get the lacing right.
The structure is inexpensive as cable and steel even at today
prices are cheap but building the skin would be a problem.
Computers could develop the pattern but the actual cutting and
sealing the skin together requires a clever solution I never
found to make it work on the farm.
Of course using concrete as a skin it should work great but that
would put the price out of range for this kind of discussion and
rather hard to use as a green house.
http://www.canopyaccess.com/CCA%20pdf/Designing%20Canopy%20Walkways.pdf
Our experience was the failure point were where the lactic
attached or touched the frame.
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| | From: | Derek Moody | | Subject: | Re: Triaxial polytunnels | | Date: | Sat, 11 Dec 2004 12:21:22 +0000 |
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| In article , Gordon Couger
wrote:
> Oz wrote:
> > Tim Tyler writes
> The Poly tunnel looks promising from a cost stand point but a
> building made with 2 CATENARY arches and two anchors and cables
> should cost less but fitting the skin is more difficult.
>
> I built a model that looked promising. It used two Catenary
> arches at each end and cable were laced from an anchor in the
> ground on each end and run thorough eyelets in the archs in a
> pattern that was offset from one arch to the next so a sway back
> roof was formed.
How critical is the siting? I'd worry that it could be dangerously close to
an aerofoil profile from some angles.
> All the load was carried by the cable anchors and the vertical
> load on the arches. I can't find an example of the structure but
Loosely related to a clothed suspension bridge?
> if anyone is really interested I can find the software that
> calculates the arch and you would probably have to do a bit of
> experimenting on a model to get the lacing right.
>
> The structure is inexpensive as cable and steel even at today
> prices are cheap but building the skin would be a problem.
> Computers could develop the pattern but the actual cutting and
> sealing the skin together requires a clever solution I never
> found to make it work on the farm.
Heat sealing in the open with added dust - I can see the problem.
> Of course using concrete as a skin it should work great but that
> would put the price out of range for this kind of discussion and
> rather hard to use as a green house.
You can fix that with a pot of paint ;-)
(In the same manner as red barns get to be red)
> Our experience was the failure point were where the lactic
> attached or touched the frame.
OK, bluesky suggestion here:
With a double walled inflated structure why not glue the inner to the
supports? ( maybe glue->insulating_pad->support )
That way there is no chafe, no heating friction and the outer layer can
still deform to accommodate the wind.
Cheerio,
--
>> derek@farm-direct.co.uk
>> http://www.farm-direct.co.uk/
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| | From: | Jill. | | Subject: | Re: Triaxial polytunnels | | Date: | Sat, 11 Dec 2004 10:09:20 -0000 |
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| Gordon Couger wrote:
>
> Our Oklahoma weather is pretty rough on such structures. Any
> wind over 50 mph was likely to cause a problem.
That would be a real concern up in our neck of the woods then - winds over
50mph can /do happen alot in most directions and times of year :~((
> The Poly tunnel looks promising from a cost stand point but a
> building made with 2 CATENARY arches and two anchors and cables
> should cost less but fitting the skin is more difficult.
>
> I built a model that looked promising. It used two Catenary
> arches at each end and cable were laced from an anchor in the
> ground on each end and run thorough eyelets in the archs in a
> pattern that was offset from one arch to the next so a sway back
> roof was formed.
I find the ideas going around really interesting
--
regards
Jill Bowis
Pure bred utility chickens and ducks
Housing; Equipment, Books, Videos, Gifts
Herbaceous; Herb and Alpine nursery
Holidays in Scotland and Wales
http://www.kintaline.co.uk
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