It seems that climate change is creating greater extremes in weather with many sections of the country experiencing severe weather. Hurricanes, tornadoes, microbursts and thunderstorms all produce damaging high winds that can result in structural damage and power outages. Wind, however, causes the most concern to the greenhouse industry.
Site exposure, height, shape and roof slope affect the wind load that a greenhouse can take without sustaining damage. Although most greenhouses are designed for a minimum 85-mph wind load, gusts can damage large sidewalls at even lower speed. I have seen both hoop houses and gutter-connected greenhouses that have been damaged by winds that were estimated to be only 60 mph. I also have pictures of both styles of greenhouses that were overturned by similar-force winds.
Although we have no control over the force or direction of severe winds, here are a few tips to help minimize storm damage:
1. Check the area for loose objects. Anything that can be picked up and hurled through the glazing should be secured or moved indoors. Boxes, flats, pieces of sheet metal or plywood can create a large hole that will allow the wind and rain in. Metal chimney (stove pipe) sections should be braced and secured with sheet metal screws.
2. Inspect for dry or weak tree limbs that could fall on the greenhouse. Use common sense and safety equipment when working in trees.
3. Close all openings including vents, louvers and doors. You double the effective force of the wind when it is allowed inside the building. The wind on the outside puts a pressure or lifting force on the structure. The wind that gets inside tries to force the walls and roof off
4. Check all structural joints and bracing. Tighten the bolts on collar ties, truss supports, purlins and foundation brackets. It is surprising how many of these loosen over time. On hoop houses and gutter-connected houses inspect diagonal bracing and tighten bolts or cable turnbuckles. This is important to prevent the greenhouse from racking end to end. If your greenhouse does not have bracing, install 2 x 4s, tubing or cables from eave to foundation. The bracing should be secured to each frame with bolts or clamps.
5. Inspect glazing splices and barcaps. Check that screws are in place and tight. There is considerable lifting force on large sheets of glass or plastic panel
6. On poly houses, there are several additional measures that can be taken:
- Increase the inflation pressure slightly by opening the blower’s intake valve. This will reduce rippling in the plastic. Be sure the plastic is attached securely and that any holes are taped. Use double wiggle wire or battens if necessary
- Disconnect the arm to the motor on all ventilation-intake louvers and tape the intake shutters closed. Then turn on enough exhaust fans to create a vacuum in the greenhouse. This will suck the plastic tight against the frame. All doors should be securely closed.
Growers should also start their generators to see that it is working properly. Also, take care not to backfeed to utility company power lines.
Supplies to have on hand:
- Several days worth supply of fuel for your backup generator
- Duct tape for tears in plastic, broken glass, etc.
- Plywood and fasteners for doors and vents that may blow off
- Flashlights and spare batteries
- Battery-operated radio
An 80-mph wind can produce a pressure of 16 pounds per square foot. For example, the 12’ x 100’ sidewall of a gutter-connected house would have to resist a 19,200-pound load. An 80-mph wind blowing perpendicular to the side of a 28’ x 100’ greenhouse can create a lifting force of 220 pounds per square foot of length or 22,000 pounds on the whole structure. When you consider that the total weight of the greenhouse is about 6,000 pounds, each foundation post must have a withdrawal resistance of about 300 pounds.
Although we can’t presently harness the energy of severe weather, we need to recognize the large forces that are generated. Preparation before a storm strikes can help to keep damage to a minimum.