Greenhouse Structures and Design
Greenhouse Structural Components
The greenhouse is a structure made by assembling different parts or components (Figure 1.13). Each part has a specific role in the greenhouse structure. The main components of a greenhouse are its structural components, including trusses, purlins, and side posts. Most frames are made from steel, aluminum, plastic, or wood. There are several materials to consider when deciding the kind of foundation to use for the greenhouse. Wood timber, concrete or concrete block, and brick are all suitable choices. The covering material of the greenhouse, also known as glazing, is usually the most expensive component of the structure. It is also one of the most important because the glazing is what allows the entry of light and heat into the greenhouse.
Greenhouse Framing Members
Trusses are the fundamental vertical support structure of a greenhouse, which supports the weight of the roof. They provide strength to the basic structure and are hence placed at a distance of 2, 3, 4 feet or as required. The truss is made up of rafter, strut, and chords. Struts are support members under compression while chords are supports members under tension. Trusses are connected by the roof ridge and the purlins which run the length of the greenhouse. The ridge is the top of the greenhouse. Purlins are horizontal supports of a greenhouse.
Greenhouse Framing Materials
A greenhouse frame is an important part of any greenhouse because it serves to provide both strength and insulation. Metal frame greenhouses are ubiquitous throughout the world as standard greenhouse structural material. Metal frame structures pair well with polyethylene sheet glazing or polycarbonate panels because the glazing can be attached directly to the exterior face of the frame. A wider span can be achieved through metal frame design, creating a larger and less-obstructed area for growing.
Greenhouse Foundation
As for any building, a proper foundation is essential for the usefulness and life of a greenhouse. Essentially, the foundation is the complete system on which the greenhouse structure sits. A greenhouse foundation resists the forces acting on itself, such as the weight of the greenhouse as a building, snow, wind, and its contents. One of the key components of a foundation is the footing. The footing refers to the point at which the structure meets the soil. Footers help to prevent sagging or movement of the structure’s walls. The type of footing used is determined by the style of the greenhouse and building codes. The greenhouse foundations may be flagpole footings placed directly in the ground, spread footings, or continuous concrete footings-type foundations.
Greenhouse Walls
The end and sidewalls are typically covered with one of the rigid plastics. The structured acrylics and polycarbonates will provide better insulation and security against theft or vandalism but at greater initial costs than fiberglass.
Curtain Walls
Curtain walls usually comprise the first 2 to 3 feet (0.6–0.9m) of the sidewall above the soil line. They are made of concrete blocks, cement, and other non-transparent materials. Curtain walls do not support the weight of the greenhouse; the side posts carry out that function.
Design Loads
All buildings, including greenhouses, headhouses and storage buildings are affected by external and internal loads. These loads are transferred through the frame members and foundation to the ground. Loads are commonly classified as dead loads, live loads, snow loads, wind loads and, in some locations, earthquake loads. It should be kept in mind that the most unfavorable effect of the load combination in the structure, foundation, or member under consideration will govern the situation.
Dead Loads
Dead loads are the weight of all permanent construction material, including but not limited to floor, walls, roofs, framing and glazing materials, and fixed service equipment
Live Loads
Live loads are the temporary loads generated by the use and occupancy of the greenhouse and do not include the dead load or wind and snow loads. Live loads, both static and dynamic loads, can be exterior or interior loads resulting from the use and occupancy of the greenhouse. The static loads result from the weight and pressure from equipment, plants, stored products, and the materials used for maintenance activities.
Snow Loads
Snow load is a vertical load, which is based on expected ground snow accumulation at the greenhouse site, roof slope, covering material, and type of greenhouse structure, that is, whether the greenhouse is a single span or a gutter-connected greenhouse and whether it is a heated or an unheated greenhouse. Snow can be light and fluffy with a water equivalent of 12 inches (30.5cm) equal to 1 inch (2.5cm) of rain.
Wind Loads
Wind design pressure, that is, the equivalent static pressure, is used for determining wind loads on greenhouses. The pressure due to wind load is assumed to act in a direction normal to the surface under consideration, either as a positive pressure directed toward the surface or as a negative pressure (suction) directed away from the surface.
Greenhouse Flooring
Growers may choose from a range of floor surfaces for container-grown crops, from gravel to concrete. The actual floor design will be dependent upon the type of production being planned and the available capital.
Soil Floors
Soil floors are rarely found in large greenhouse operations, except for small-scale vegetable operations. Soil floors are a major source of pests and pathogens as they can survive for extended periods of time in soil and residue and can easily be transferred to container-grown plants.
Gravel Floors
One of the simplest floor systems is a gravel floor. A thickness of 6 to 8 inches (15–20cm) allows for good drainage while helping to block out weeds. They can easily be sanitized as well with a mild bleach solution in case of disease. This is a low-cost floor production system, and the gravel allows for excellent drainage. However, weeds may grow in the gravel, and sanitation can be difficult on gravel floors. It may also be difficult to have small containers sit upright and level on gravel floors.
Concrete Floors
Concrete floors are very common in commercial greenhouses. In part because environmental legislation requires growers to capture drainage and runoff and also to accommodate mechanized equipment and general cleanliness. Where concrete is used, it should be slightly sloped to allow water to drain off. Concrete floors prevent weeds and are generally easy to clean. They are durable and long-lasting and provide the advantage of maximizing space usage efficiency as well as flexibility. There are two basic types of concrete that may be used for greenhouse floors.
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