In most sections of the United States, the ‘wind load’ is the greatest force that a greenhouse will be subject to. Wind load can occur from hurricanes, tornados, or a sudden squall from a passing weather front.
Wind loading is a complex phenomenon. The wind forces that act on the greenhouse are influenced by numerous factors, including the basic wind speed, building orientation and exposure, height and shape of the greenhouse, and doors or vents that may be open at the time of the wind occurrence. The wind passing over a greenhouse creates a positive pressure on the windward side and a negative pressure on the leeward side. These can combine to create a force that results in the collapse or overturning of the structure. It can also create a force similar to an aircraft wing that moves to lift it off the ground.
Wind speed
The basis for wind load design is the wind speed map that has been developed by the American Society of Civil Engineer (ASCE-7) and is based on National Weather Bureau data. It is made up of wind speed measurements at 33 feet above the ground, in open terrain, based on a probability of recurrence every 50 years.
Basic wind speed varies from 85 mph in the western part of the United States, to as much as 150 mph along the Gulf Coast and southern Florida. Each state develops its own wind speed chart, by municipality, that is appended to the building code. Either the International Building Code (IBC) or the National Fire Protection Association 5000 Building Code is the basis of structural design in all states. The National Greenhouse Manufacturers Association (NGMA) has developed standards and recommendations that are used in conjunction with the above codes.
Velocity pressure
In greenhouse design, the basic wind speed is converted to a velocity pressure in pounds per square foot (psf). The velocity pressure varies as the square of the velocity. This value is modified to take into account that the wind speed is reduced for buildings less than 33 feet high and for areas where obstructions such as trees or hilly terrain reduce exposure. It is also modified by an importance factor (I) that takes into account hazard to human life. Except for greenhouses in hurricane areas, this factor is 0.87 for production greenhouses, and 1.0 for retail sales greenhouses.
To illustrate with an example: the velocity pressure for a production greenhouse with a 12 foot effective building height, located in a suburban setting, in a non-hurricane area, with potential 90 mph wind would be about 14.4 psf.
Force coefficients
The full velocity pressure does not normally occur on all surfaces of the greenhouse because of shape and orientation. The wind generally hits the surfaces at some angle, such as a roof, depending on the building profile. This creates an aerodynamic effect. Force coefficients have been developed for these different surfaces and affect the loading. These are also increased by a factor that accounts for wind gusts.
Using the above example, the velocity pressure, on a 30 foot x 100 foot, free-standing glass greenhouse, with the wind perpendicular to the sidewall would be about +14.9 psf for the windward vertical sidewall, -7.3 psf for windward facing roof, -14.0 psf for the leeward facing roof and -5.6 psf for the leeward facing wall.
On a 30 foot x 100 foot free-standing hoophouse, the loadings would be +11.2 psf for the lower section of the windward sidewall, -5.1 psf for the top ½ of the roof and -9.4 psf for the lower section of the leeward side.
The structure has to withstand various loads depending on the direction of the wind and the shape and size of the structure. When the area of the different surfaces is multiplied by the loading, the total force that the surface has to withstand can be determined. For example, the 10 foot x 100 foot windward sidewall of the glass house would have to withstand a load of 10 foot x 100 foot x 14.9 psf = 14,900 pounds. In many parts of the U.S., the magnitude of the largest forces are comparable to the snow load.
The forces on gutter-connected houses are calculated the same way. They are the greatest on windward section and less in subsequent sections. With taller gutter- connected greenhouses being built today, the forces can be very large.
The loads on individual structural members and the greenhouse glazing also have to be calculated. The structural engineer does this during the design process.
The loading can become even more critical if a large opening such as a door or vent is left open on the windward side of the building during high winds. If this happens, a positive pressure builds up inside the greenhouse, so that not only is there an uplift suction force on the outside due to the aerodynamic effect, there is also pressure on the inside, equal in magnitude, acting in the same direction. This force could be large enough to lift the greenhouse off the ground, unless it is anchored well.
From the above discussion, it is apparent that a good design for the structure and foundation are necessary to get good service over the long life of the greenhouse. Before signing a contract for a new structure, check that the design will meet the building code in your area.
Connecticut Design Wind Loads
References
Bartok Jr, J. W. (1991). Reduce Storm Damage to Your Greenhouses. Yankee nursery quarterly (USA).
Greenhouse Engineering - NRAES 33 by Robert Aldrich and John Bartok - available at https://ecommons.cornell.edu/handle/1813/69429
Energy Conservation for Commercial Greenhouses - NRAES-3 by John Bartok available at https://ecommons.cornell.edu/handle/1813/62126
National Greenhouse Manufacturers Association. (2015). Structural Design Manual. NGMA, Harrisburg PA. Available at https://ngma.com/.