Results: Were very interesting. We picked two of the four tunnels that were built in May, 2010, actually planted onions in them, and buttoned them up for the winter. Both tunnels were structurally the same, with a center ridge-pole made of chain link fence top-rail, but one of them also had purlins about three feet from either side of the ridge. These purlins in summer, were used to trellis vine crops, and carried that load quite well. They really stiffened up the whole structure. However, in summer, they tended to cause the plastic to collect rainwater. This made me think that the purlins would either make it so strong that it would better carry the snow load or they would catch more snow than the tunnel without purlins and cause a collapse. It looks like the latter is the case.
Summary and recommendations: It is important to note that we intentionally did not remove snow in this trial, as we were looking for structural failure in worst case conditions. Removal of snow even just next to the tunnel that failed probably would have prevented its failure, because the pressure bearing in on the sides (and therefore the plastic's weight felt on the top) would have been greatly reduced. If you intend to use a structure like this to overwinter crops in northern climates where snowfall is a concern, I would recommend a maximum of four foot bow spacing during construction, not using side purlins in winter tunnels because they catch snow, and of course the removal of snow whenever possible from the sides and top of the tunnel. It would also be prudent to shore up the bows internally with notched two-by-fours placed vertically under every other bow as a means of additional snowload insurance and peace of mind.
The 26-page illustrated manual for the bender used to make these tunnels and an MS Excel cost calculator spreadsheet may be downloaded on the bender's product page here.
You can view the complete gallery below.
Thanks!
Adam.
Adam Lemieux
Tools & Supplies Manager
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