Reducing storm damage to your greenhouses

"39 pounds per square foot – That’s the weight of snow I had in my yard in Ashford after the January 2011 snow and rain storms," writes John Bartok, Agricultural Engineer at the University of Connecticut. "The Connecticut Building Code calls for a design load of 30 pounds per square foot (psf) and many greenhouses are designed for significantly less as the code assumes that the greenhouse will be heated to a minimum 50ºF during a storm.

"There are many reasons for structural failure but let’s review the type of loading that greenhouses are subject to first. Loads are commonly classified as dead loads, live loads, wind load, snow load and in some locations earthquake loads."

Dead loads – are gravity loads that are constant and include the weight of the structure, equipment and long term crop such as tomatoes. Values for wind uplift must not exceed the dead load of the structure otherwise the greenhouse could be lifted out of the ground. Hoophouses, for instance, are very light structures and have little weight.

Live loads – Short term hanging plants are considered live loads which could add 3 to 5 psf. A maintenance crew working on the roof or a hanging trolley conveyor is also a live load.

Wind loads – Loading from the wind can come from any direction. Basic wind speeds for design are 90 miles per hour (mph) in the northern part of Connecticut to 120 mph on the immediate coast. These are usually considered to act in horizontal direction against the walls. They are adjusted for such factors as site exposure, height and shape of the structure, roof slope and a use factor. For example, a 10’ x 100’ polycarbonate wall subjected to a 60 mph wind would have to withstand a force of about 10,000 pounds. The wind tries to overturn the greenhouse.

Greenhouses are also subject to uplifting due to the wind blowing over the roof. The shape of the roof gives it an airplane wing effect. A 60 mph wind will create a lifting force of about 18,000 pounds on a 25’ wide hoophouse and each post will have to withstand an uplift force of about 350 pounds. This is reduced somewhat in air inflated roofs as the plastic deflects and temporarily relieves the pressure.

Snow loads – Snow can be light and fluffy with a water equivalent of 12” equal to 1” of rain. It can also be wet and heavy with 3”-4” equal to 1” of rain. Snow having a one inch rainwater equivalent will load a structure with 5.2 psf. This amounts to about 6.5 tons on a 25’ x 96’ greenhouse. Temporary 2 x 4’s placed under the ridge every 16’ – 20’ carry considerable load and make a good safety measure. Locate them under a hoop or major frame member and secure them with a strap or wire.

Read more tips at the UMass Amherst website.