LB1 canopy goes up
Post date: 01-Feb-2015 10:11:38
We learnt a lot from this process about low-cost lightweight greenhouse support structures using plastic pipe and ETFE film.
Prior to erecting the new canopy, some earthworks were necessary to form a more or less (mostly less) regular perimeter shape where the new canopy would meet the ground.
Note the old poly pipe hoop canopy in this picture was 12 years old and the PE film was starting to disintegrate.
apart from the extra thermal mass, I wanted to get more thermal mass around the perimeter of the prototype LifeBubble.
I wanted much more thermal mass but unfortunately, I couldn’t go any deeper into the ground as there is solid basalt bedrock only a metre down.
My thermal mass exchange working hypothesis is that the higher the thermal mass surface area ratio is in relation to solar input energy, the more stable the LifeBubble temperature. I'm hoping to do some real monitoring at Yangpo Henan province China to confirm the actual numbers in this ratio over the next year or two.
Next, I erected 7 m high iron bark pole and fixed a welded 1.8 m diameter steel ring on top.
This hole will enable me to control the upper hot air ventilation once a cover is placed over it.
The polypropylene pipe is threaded into 32mm steel sockets welded onto the steel hoop and so allows the 57mm diameter canopy support ribs to be slid over the polypipe and simply fixed into place with a screw. This is less mucking around up a ladder than trying to fix into steel sockets.
Working with polypipe like in the original canopy shown above quickly teaches you that it easily softens and becomes more flexible in the sun over time, and although it is cheap, it isn't structural.
PP is better at holding its properties at temperatures up to about 100°C but some real-life tests showed that it sags too much in the sun too.
Conclusion - heavy walled 40 mm polypropylene pipe is too flexible and heavy to support its own weight over a 10 m span but is good and great to work with for the short webbing struts.
DWV pipe is engineered to take full sun and remain strong but as you can see from this picture if its too small a diameter it still goes wonky in the sun over time. Valuable lesson learned.... 57 mm DWV pipe sags over time in the heat of the day and gradually creeps so is also not suitable as a canopy rib support for the film over a 10 m span.
Better to skip 65 mm DWV and jump up to 100 mm diameter DWV..... but I had already bought my 57 mm at a bargain price!
57mm DWV with an 85 mm x 10 mm form ply rib under it that has been fibreglass encased and bonded to the pipe is stiff enough and doesn’t sag over time. (for anyone interested I can provide more details, but really if you value your time it would be better and faster to just go for 100mm DWV pipe and be done with it - you live and learn!)
The only advantage of the form-ply ribs is that it locks in the correct radius on all the ribs. Note this is not the same.
I was concerned to keep the supports narrow to minimise shading in the greenhouse but I'm convinced now that this was misguided. It wouldn’t matter if the pipe was 100 DWV even as some shade is an advantage - not the problem I envisaged after all.
Brad and Madhu help me get the ribs up and braced radially by the PP webbing struts prior to fixing the ETFE film.
The DWV pipe is fixed at the base by sliding it over star pickets driven into the ground at a slight angle.
The radial Webbing and the film had to be fixed to the ribs and then painted with a UV protective water-based paint using a cherry picker. The cherry picker hire was costly and it was very difficult to get back up that path and through the door frame because of its weight, we basically had to winch it out as it is powered mainly by lead-acid batteries and so is very heavy!!
For future LifeBubble canopies, I think it will be better to design and cast a very rigid hollow or foam filled fibre-cement shelled rib that can be used as its own work scaffold. I will do a separate web page on this later and include CAD drawings and construction methods that structural engineers can comment on so we can refine it to really suit LifeBubbles.
This is the finished structural frame with the ribs and webbing in place. Notice the first ETFE panel is also up above the lower door is not fully transparent but is slightly "milky". All the same, I love being able to look up and see the moon and stars at night.
Now for the most expensive component - 100um ETFE film. This cost just over $5300 AUD but still a fraction of the cost of a glass structure.
ETFE is like Teflon and not easy to glue, so I had to experiment with some common long life sealants and found that the best "off the hardware shelf" is a clear copolymer sealant. I was always going to have to staple to mechanically fix the film with 10mm stainless steel upholstery staples, but some tests showed that this combination was pretty reasonable. Time will tell! As you can see, for an amateur it is really difficult to avoid getting wrinkles in the film. I had an ex- sailmaker friend Ben help with the canopy and some panels turned out pretty well, but where adjacent ribs are of different diameters we just had to learn to live with "fair enough"! See the discussion in the FAQ section on better canopy materials
