Section 5: Basic Structural Design Considerations
Many factors must be considered in the greenhouse structural design. It is difficult to give a specific set of requirements, as there are many exceptions to any rule. However, a structure must meet the building codes for the specific locality. Most greenhouses are now designed by engineering firms or are constructed from packages developed by engineering firms (see the Learning Links in this unit for examples). In many cases, the design firm will also build the structure in place for an additional fee. Larger installations are usually custom-designed and built by an engineering firm specializing in greenhouse structures. However, with this in mind, it is still valuable to understand some basic design considerations.
The primary objective in designing a greenhouse structure is to maximize light transmittance (i.e. minimize obstructions to light entry) while providing adequate support. In many cases minimizing heat loss is important, while in others, allowing maximum air exchange for cooling is desired.
Greenhouse engineers often refer to design loads. The design load includes the dead load and the live load. The dead load includes the weight of the structure, framing, glazing, permanent equipment, heating and cooling units, vents, etc. The live load includes the weight of people working on the roof, hanging plants, snow loads and wind loads. Greenhouse structures must be designed so as to be able to support the maximum combined dead and live loads that they will experience. Most often permanent greenhouses are required to support an 80-mph wind (higher in some locations). The required snow load is based upon the expected accumulation, the roof slope and whether the greenhouse is a gutter-connected structure or a stand-alone greenhouse.
In gutter-connected greenhouses, the gutters should slope slightly to encourage drainage of runoff from the roof. The gutters or eaves should be high enough to allow for automation with 12' to 14' being recommended (also some newer Venlo types may be even higher). At least one entrance into the greenhouse should be large enough for carts, trucks or other equipment.
Greenhouse structures should be designed to allow for automation. This requires that width of walkways and driveways accommodate carts and equipment. Width of greenhouse bays may need to be designed to be compatible with irrigation systems such as irrigation booms.
The foundation must support the structure and transfer loads to the ground. In some cases, the structure may be set on an intact concrete foundation or slab. Supports may be bolted onto the foundation. In other cases, whether or not a concrete foundation is present, the structure may be supported by vertical beams placed on concrete footings. Footings should extend below the frost line or at least 24 inches into the ground.
Quonset greenhouses using metal tubes for the structure may be anchored into the ground by inserting the tubes into slightly larger tubes driven into the ground.
Electrical conduit or pipe may work well for a small polyethylene covered quonset house. However, it is not strong enough if the diameter of the quonset becomes too great or if the loads are too great The gothic arch increases the strength of the standard arch by more effectively directing the load to the ground. This increases the potential span and the strength of the structure and reduces the need for internal structural supports which in turn allows for a larger unobstructed space. In an A-frame greenhouse, the structural support is derived from the supporting trusses and rafters. The strength and number of rafters and trusses required depends upon the dead and live loads expected. However, as the support required increases, there is a reduction in light availability to the plants.
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