Orientation & Structural Considerations

Light

• 

  Latitude and longitude of the contiguous United States.

  Shadows are cast by the structure’s frame

  • Orientation to optimize for light is not as critical for a high tunnel as it is for a greenhouse structure that will be used for winter production and has a more substantial frame
• Above 40°N latitude, orient the length of a single high tunnel east to west
  • This orientation allows low angle light from the winter sun to enter from the side where it will not be blocked by ribs of the frame
• Below 40°N latitude, orient the length of a single high tunnel north to south
  • This orientation is acceptable because the angle of the winter sun is much higher at these latitudes
• Gutter-connected greenhouses at all latitudes should be oriented north to south
  • This orientation avoids the shadow that would occur from the greenhouse just to the south of it in an east-west layout

Wind Direction

• Air flow is used to modulate temperature and relative humidity in the high tunnel through ventilation of the end and side walls
  • Single bay high tunnels can be oriented perpendicular to prevailing winds to accommodate maximum air flow if structure has roll-up sides
  • Multi-bay high tunnels should be oriented parallel to prevailing winds
• Prevailing winds are generally from the south-west in the summer and from the north in the winter, but specifics must be determined for each site
• Windbreaks can be used to moderate or redirect wind
• Sites located within 15,000 feet of large open bodies of water are exposed to very severe winds:

Structural Considerations and Loads

• Loads are stresses to the structure from external or internal forces
  • Commercial greenhouses are required to meet structural standards to withstand various loads
  • In contrast, high tunnels are generally not required to meet structural standards, which is one reason why they are inexpensive
  • Specifications are in place for high tunnels in some countries such as Holland
  • However, high tunnel structures should be built strong enough to withstand severe weather
• Dead loads are gravity loads that are constant throughout the structure’s life
  • These include fixed equipment such as fans, suspended heaters, and water pipes, however, these are typically not found in high tunnels
  • Long term crops that are suspended from the frame, such as trellised tomatoes, are often considered dead loads
• Live loads are temporaryorshort term
  • These include wind loads, snow loads or any other environmental stress
  • Short term crops such as hanging baskets (though not usually produced in high tunnels) are considered live loads
• Wind loads can come from any direction but usually act against the side walls by hitting them at a perpendicular angle
  • Basic wind speeds for greenhouse structural designs are 90 mph for most of the US; high tunnel structures are generally not framed to withstand gusts of this magnitude
    • 44 mph wind creates a live load of 5 psf (pounds per square foot)where as 62 mph wind creates a 10 psf live load
  • Strong seasonal winds are a concern
    • High tunnels placed perpendicular to prevailing winds typically receive the most damage
• Wind uplift could result in the high tunnel structure being lifted out of the ground
  • The lightweight hoop structure acts like an airplane wing that creates uplift when air passes over it
  • Values for wind uplift should ideally not exceed the structure’s dead load
  • The best protection against uplift is to ensure that posts are driven at least 24 inches into the ground
• Snow loads are determined by factors influencing snow and ice accumulation on the structure
  • Many high tunnel structures are not under production during winter
  • Snow loads vary considerably by geographic location
  • Snow with a one inch rain equivalent will load a structure with 5.2 psf (pounds per square foot)
    • One inch rain may equal 12 inches of light, fluffy snow or 2 to 4 inches of heavy, wet snow
  • A high tunnel built from ribs of 20 foot pipe bent to create a 14 foot wide X 6 foot high structure will withstand only about 10 psf
  • Other factors influencing snow accumulation include building exposure, roof slope, and heat loss (from the heating system) through the roof
  • If snow loads are heavy, various strategies can be used to minimize damage
    • Orient the structure so prevailing winds blow the snow off the high tunnel
    • Remove the plastic in the winter if not under production
    • Utilize portable heaters to melt the snow load
    • Manually remove snow, for example, brush from the inside using a long-handled broom
• Combination loads from more than one loading factor are common
  • For example, high winds may be associated with a snowstorm