High-wind warning

It seems that climate change is creating greater extremes in weather with many sections of the country experiencing an uptick in severe weather events.

Hurricanes, tornadoes, microbursts and thunderstorms all produce damaging high winds that can result in structural damage to your greenhouse. But there are steps you can take to avoid wind damage.

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.

Use these tips to minimize storm damage and be prepared before a storm strikes:

• 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.
• Inspect for dry or weak tree limbs that could fall on the greenhouse. Use common sense and safety equipment when working in trees.
• 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.
• 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.
• 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.

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.

Structural considerations

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.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com