Aerodynamics Considerations on Greenhouses

Aerodynamics is a very complex concept in how it applies to greenhouses and similarly constructed buildings. The better that you understand these dynamics and how to deal with them, the better your chances of having long-term success with the structural integrity of your building.

In many of Norm’s presentations, he has compared the similar shape of a greenhouse to that of an airplane wing. The basic challenge is that you want the opposite result. An airplane needs to efficiently get off the ground while a greenhouse needs to stay where it is.

The greater the distance over a surface is (wing or building), the greater that the lift is. When height is combined with distance, there is a multiplier effect. That is why a pilot will extend the flaps on a wing when taking off and landing.

It is the vacuum on the backside of a structure or a wing that causes the lift. The upward force of the vacuum on the backside is actually double what the leading edge force is.

This effect happens exponentially even when a small change is made. Many people have suggested, “I am only lifting the building two feet (or some other relatively small amount)”. They do not realize that doing that small lift is sometimes adding 50% or even doubling the wind load on the building.

These forces must especially be considered when a structure is oriented so that the prevailing winds hit the sides of the building. Structurally, it is preferable to have the structure facing into the prevailing winds. If the lay of the land or the logistics of how you work in the structure do not allow this, there are ways that the stress on the building can be minimized.

The least expensive way to stiffen a building is to add crossties (also known as collar ties). Crossties tie the left and right sides of the structure together so that the load is more balanced in how it affects the structure. Reducing the hoop spacing will automatically make the structure stronger with a greater ability to resist the effects of external forces.

Ultimately, however, a structure’s ability to deal with the aerodynamics of a situation is only as good as the anchoring provided. Anchors provide stability for resisting up, down and lateral forces. There is no such thing as too many anchors.

These things add to the structural ability to handle things but it does not help the cover. Extra steps must be implemented to secure the sides of the cover. This includes but is not limited to adding extra and longer screws for the cover fastening system.

A discussion on aerodynamics is not complete without discussing windbreaks. The only effective windbreak is trees. Trees restrict the speed of the wind coming through and therefore minimize aerodynamic stresses on a building. There is an optimal distance from a windbreak. This is determined by the size, density and texture of the trees involved. Since this is something that changes with age and season, there is not a perfect formula.

One last note on windbreaks. Another building, especially a bigger building is not an effective barrier. If anything, this building will significantly increase the aerodynamic stresses on your building. Wind will be swirling in one direction and then a slight shift in wind direction will reverse the direction of the swirling.

Please note: We are not engineers and as such can not give structural opinions. The above points are simply items that should be considered and come from 40+ years of experience of watching what wind does to buildings.

Anchoring: Base Brackets vs Anchor Posts

edit-Base Bracketanchor post photo

We offer two main types of anchoring for our structures: Base Brackets (left pic) and Anchor Posts (right pic).

Which one you decide to go with largely depends on your application and location. They are not to be used together, it is a one or the other option. No matter which option you choose, please be aware, there is no such thing as too many anchors!

Although the building can be anchored directly into the ground with Anchor Posts, it can also sit on a slab, curb or beam or it can be elevated on some sort of a wall. Base brackets with lag bolts are supplied standard to fasten the building to the chosen form of foundation. Anchor Posts are available at an additional cost.

Anchor Posts must be set into concrete when:

  • the soil has been recently excavated (within the last 5 years)
  • it is required by the building code (use of concrete usually classifies the building as permanent)
  • extremely windy and exposed areas exist (at least use on the corner posts)
  • more than 10% of the anchor post will be out of the ground (upgrading anchor post size may be needed)
  • there are areas where erosion has been a problem in the past

Anchor Posts SHOULD NOT be used (and base brackets used instead) when:

  • the soil is a very heavy clay (heaving would be a constant problem)
  • there is a shallow rock layer
  • there are major amounts of rocks interfering with the accuracy of anchor post setting
  • the structure will be moved shortly (anchor posts must be cleaned out before reusing)

**Recommendations are based on years of experience. Ultimately the customer is responsible to properly anchor a structure**
Please see our installation pages for a more detailed breakdown of this topic Base Brackets vs Anchor Posts

Anchoring: Base Brackets vs Anchor Posts

edit-Base Bracketanchor post photo

We offer two main types of anchoring for our structures: Base Brackets (left pic) and Anchor Posts (right pic).

Which one you decide to go with largely depends on your application and location. They are not to be used together, it is a one or the other option. No matter which option you choose, please be aware, there is no such thing as too many anchors!

Although the building can be anchored directly into the ground with Anchor Posts, it can also sit on a slab, curb or beam or it can be elevated on some sort of a wall. Base brackets with lag bolts are supplied standard to fasten the building to the chosen form of foundation. Anchor Posts are available at an additional cost.

Anchor Posts must be set into concrete when:

  • the soil has been recently excavated (within the last 5 years)
  • it is required by the building code (use of concrete usually classifies the building as permanent)
  • extremely windy and exposed areas exist (at least use on the corner posts)
  • more than 10% of the anchor post will be out of the ground (upgrading anchor post size may be needed)
  • there are areas where erosion has been a problem in the past

Anchor Posts SHOULD NOT be used (and base brackets used instead) when:

  • the soil is a very heavy clay (heaving would be a constant problem)
  • there is a shallow rock layer
  • there are major amounts of rocks interfering with the accuracy of anchor post setting
  • the structure will be moved shortly (anchor posts must be cleaned out before reusing)

**Recommendations are based on years of experience. Ultimately the customer is responsible to properly anchor a structure**
Please see our installation pages for a more detailed breakdown of this topic Base Brackets vs Anchor Posts

Video: Installing Anchors into a Base Beam

The main function of anchors is to prevent uplift. Do not forget about the “foundation function” as well which is meant to prevent settling or lateral shifting.

Improperly anchoring a building from up lift, down force and lateral movement will all, equally, cause problems and expense.

There are many different ways of anchoring a building because there are so many circumstances which people are dealing with.

When anchors can be installed at opposing angles, they work against each other and therefore will multiply their  holding power.

  • Care must be taken to stay away from anchors that will bend (i.e. re-bar).
  • When the anchors are going straight into the ground, care must be taken to ensure proper holding power. This is most often done with plugs of concrete.
  • When anchors have been extended out of the ground, care must be exercised to eliminate the possibility of outward lean.
  • It is important to consider the total amount of square inches of contact area between anchors and soil. Many time fewer big anchors is less holding power.

One thing which simply can not be stressed enough is that there simply is no such thing as too many anchors. Anchors are generally very inexpensive, especially when you are looking with hind site at some damage.

For more details and to watch an illustration, please see our YouTube Video below