Goal: To see if either caterpillar tunnel version (with, or without purlins) could withstand winter snow load.
Results: Very interesting. We picked two of the four tunnels that were built in May, 2010, planted onions in them, and buttoned them up for the winter. Both tunnels were structurally the same, with a center ridgepole 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, were used in the summer 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. Doing so would have greatly reduced the pressure bearing in on the sides (and therefore the plastic's weight felt on the top). 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 snow load insurance and peace of mind.
You can view the complete gallery of this trial below:
The 26-page illustrated manual for the bender used to make these tunnels may be downloaded on the Quick Hoops™ High Tunnel Bender's product page.
Johnny's Tools & Supplies Manager