Bending our High Tunnel PipeMay 4, 2011
In our farming practices we try to align ourselves with natural processes, but sometimes we make the choice to move further away from them. This is the case for the high tunnel we are building. It adds the potential for year round growing and warmer overall temperatures throughout the year. Since I love experimenting and being cheap I decided to design my own high tunnel and build it myself.
For the unfamiliar a high tunnel is basically a unheated greenhouse. They come in many shapes and sizes and are normally covered in a polyethylene film. The have been used by grower to extend season of high value crops such as tomatoes or peppers. They have been more recently used to extend the growing season of many farms well into the winter for salad and cooking greens. Elliot Coleman is one of the main leaders for year round growing in New England. His book The Winter Harvest Handbook is a good introduction to winter production systems.
The two main shapes for high tunnels are quonset hut which is basically a half circle and gothic which has straight walls that curve up to a peak. I decided to go with a a gothic style for two reasons. The first is they are much better at shedding snow then the quonset style. This is due to the shape at the top of the two structures. The quonset style ends up have a pretty large flat surface on top where snow can collect which can collect of lot of snow. The gothic has a smaller or no flat spot on the top so snow cant build as much. The second reason is the vertical space in a gothic is much greater because of the wall design. This makes for more growing space inside the high tunnel compared to a quonset.
The problem with this is that there is not very much information about building gothic style high tunnels on the net. Almost all the information is for building quonset high tunnels. I ended up looking at lots of pictures and designing a basic framework in SketchUp a really easy to use 3D modeling program. It gave me a rough idea of how much pipe I would need and what degree bends I would have to do to make the correct shape. Of course the design changed many times before getting to the version we are actually bending now.
I contacted the folks at Ledgewood Farms who sell high tunnels and asked them for their metal supplier. They passed along information on Sonco Tubes where I made an order for lots of metal tubing. I ordered the pipe before we made the decision to move back to Tinmouth. I ended up storing the pipe till the fall because I didn’t think building it for one season then moving it to Tinmouth made much sense. This spring I started working on a new design for our new garden layout and then I had to figure out how we were actually going to bend this stuff. The final design is 22′ wide and 48′ long.
I found some information on bending pipe online using jigs made out of metal or wood. I figured it made a lot more sense to make one of these then buy a really fancy meaning really expensive tube bender. My design called for bending 1 5/8 inch tubing for the walls and 2 inch tubing for the peak. From my 3d design I knew I needed a curve with a radius of 5′ 3” for my wall bend. I put a piece of plywood onto an empty hay wagon and measured out 5’3” and made an arc with a pen and a length of string. I then made a jig out of pieces of wood along that arc and made a stop so the pipe would be bent in the same place every time. I did not account for spring back with this measurement. When ever you bend a tube the metal will rebound back after you make the bend. This distance back it goes depending on the metal and the type of pipe or tube you have. I made the radius of the bend smaller by moving the pieces of wood in away from my arc line the same distance as the spring back but in the opposite direction. Once I did that the tube bounced back to the 5’3” arc line and we were ready to bend.
The peak bend was a heck of a lot more challenging. 2” tubing is really tough to bend and its really really tough to bend to a small radius. The design called for a peak bend of 11” which is really tight for that size pipe. The smaller the radius the bend is the more likely you are to get a kink in the pipe. This makes it much weaker. I built a jig using layers of plywood. My plan was to bend smaller pipe to a slightly smaller radius and then use the pipe as a jig for the 2” tube. I ended up kinking the smaller pipe because I didn’t smooth the plywood jig enough. The degree of the bend on the out side of the bent pipe was still okay so then it was time to building a jig using these two new pipes. The picture shows our final version of the jig. There were many many many more that did not make it. Most exploded from the pressure of the bending tubing. I finally gave up using the 2” tubing and when to the smaller 1 5/8” which was a heck of a lot easier. Instead of doing one bend we made two bends one on each side of the middle. This gave us the smaller radius we needed without kinking the pipe.
We have one side of the bows bent and all the peaks are done. We are going to bend the rest of them next week. The high tunnel is going to be mounted on wheels that are made for gate fences. This will allow it to cover double the area at a lower cost. It also allows a more flexible planting schedule. We will have tubing on the ground that the wheels roll on and the high tunnel will be connected to support rods that will be hammered into the ground to keep it in place. They should be delivered next week hopefully along with the poly film covering.
More pictures in the Gallery
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