I have been given most of a metal polytunnel frame. The design is 8 hoops, each hoop is composed of two sections, which are identical curves. The design is that you plant short, straight metal tubes in the ground and then slot one end of each curved piece in, and the curved pieces meet in the middle of the arch and are joined by a connector, which also has a slot for a ridge pole. However there are no ground tubes and no connectors. That is not a problem, I can easily work something out. The bigger problem is that if I use it as intended, it is too wide for my space. However each curve (1/2 of a hoop) on its own is the perfect width. So I am thinking of bending the ends to a steeper angle, and planting tall uprights in the ground - at a slight inwards angle - to which I will bolt the curve. So each hoop would consist of two slanting uprights and a single metal arch across the top.
The person who gave me the frame said that I shouldconcrete the ground tubes in because it is very windy here, and with the wind they could jiggle around and get loose or sink further into the ground. I assume this would also be a potential problem if we use my adaptation plan and used metal uprights. So I was wondering if instead of metal uprights/ground tubes, I could instead plant round wooden fenceposts, and bolt the metal arch onto those. This would be tricky to get the posts at the right angle, but I think it's possible. Those posts sure are secure if you get them 18-24 inches in - strong enough for stock fencing. But would they be strong enough to take the stress of a fairly large polytunnel in the wind? (I am trying to build a tunnel about 12' wide and 45' long).
So some good points of my idea:
fairly cheap (wooden fence posts are cheap here, we have the metal curves already), the posts are relatively easy to get in, gets me a perfect size tunnel for my space
My major concerns are:
being able to get the fence posts in at the right angle, and strength in the wind.
I know that this works, our neighbour has one, and obviously then it's easy to get the size just right. But I still face the issue of securing the posts/ground tubes (concrete?) and it seems silly not to use the metal frame I now have, if I can work out a way.
Use those babies as half hoops! This will result in a much better greenhouse and it will also fit your space from the sounds of it. With the half hoop design all of your glazing (the part that lets light/heat through) will be facing south. In northern latitudes all north facing glazing does is let heat out. The only time the sun shines on it is when the greenhouse is too hot already. This way you also end up with a longer greenhouse (east-west) which provides better solar gain.
I would build a north wall that is insulated, even better if it contains thermal mass inside the insulation. Then put post poles into the ground with a base plat for the southern wall. The hoop attaches at ground level to the south and then on top of the north wall. If the post poles are well compacted and settled (you could set them in tamped crushed stone with sand as aggregate) and a sturdy base plate you shouldn't have a problem. I like to use two boards for an L base plate and drill a hole for the poles in the bottom of the L.
Here's what I did with the metal hoops that I salvage from an old state seed bank...
Thanks for your reply, it's another interesting idea! the space where i want to put the tunnel would make it running roughly N-S with full sun almost all day even in winter. It is windy but has some shelter in a few directions.
I also have several earth banks, about 6' high. One runs roughly N-S, and then curves around a corner to another one running E-W. What I eventually would love to do is to build a cob roundhouse into the corner of this bank, so it would have the earth bank to the north and west and having the north facing slope of the roof as a green roof for insulation, and the south facing slope of the roof and the south and east facing walls mostly glazed. I think this would offer something really special with shelter and insulation from the earth and cob while still getting a lot of light. But that's a long, long, long term project!
In the meantime I have been wondering about using the frame I have as a 'lean-to' half-hoop design against the bank. but I don't want to mess around with stuff too much since it's an intermediate step.
I'm not sure about using the half-hoop/lean-to design freestanding, ie, not against the berm, because with the set up I have that would mean the vertical wall on a long side would either be to the east or west - there's no way to do it to the north as a freestanding structure.
I will suppress my every urge. But not this shameless plug: