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.
Tag Archives: Aerodynamics
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.
How do I make sure it stays where it is supposed to?
Pointers on anchoring, what kind of things will impact holding power of anchors,
anchoring after moving a structure
A phrase Norm has used thousands of times in his career is, “There is no such thing as too many anchors.” We consider it important to help customers understand the aerodynamics involved with a greenhouse shape.
The relatively small cost of the anchors is a short-lived savings after a building has blown away or shifted. By understanding and implementing what is required you can ensure a long term usage of your structure.
The job of your anchoring system is to counter the three aerodynamic forces working on your building.
- Down force is usually seen as snow load
- Up lift and lateral shift are forces typically associated with wind load
- Wind going over a structure creates lift similar to wind going over an airplane wing
The first thing you will need to assess is what you are actually working with. If you have bedrock close to the surface, you have large rocks, or you have straight sand, it will impact your options for anchoring.
The actual holding power of your soil is an important consideration. Soil that has been recently excavated has considerably less holding power than soil that has not been moved in a long time.
The total surface area of an anchor post in contact with the soil is an important consideration. Large and short may have more contact area than long and slender.
When anchors can be used in alternating directions, it will multiply their individual holding power.
Wet soil usually does not have the same holding capacity as well drained soil. An important consideration here is rainwater which is shed by the building. As an example, with a 16’ wide structure, 8’ of rainwater goes each way. This means that the narrow area next to your building get 8 times the rain that the rest of the property gets. If you have poorly draining soil, this could be a potential problem.
If you are planning to use the movable structure option, proper anchoring has some additional challenges. Since your building is vulnerable to sudden increases in wind during this process, your anchoring protocol needs to be done quickly. Always do a test in the new area to confirm what you will be working with. With the quickness that is required, there is also a reminder that taking shortcuts will come back to haunt you.
We would like to reiterate, “If you are not 110% sure, please ask” There is no situation out there that is so unusual that we have not already been through it.
Your long term success is very important to us!
How do I make sure it stays where it is supposed to?
Pointers on anchoring, what kind of things will impact holding power of anchors,
anchoring after moving a structure
A phrase Norm has used thousands of times in his career is, “There is no such thing as too many anchors.” We consider it important to help customers understand the aerodynamics involved with a greenhouse shape.
The relatively small cost of the anchors is a short-lived savings after a building has blown away or shifted. By understanding and implementing what is required you can ensure a long term usage of your structure.
The job of your anchoring system is to counter the three aerodynamic forces working on your building.
- Down force is usually seen as snow load
- Up lift and lateral shift are forces typically associated with wind load
- Wind going over a structure creates lift similar to wind going over an airplane wing
The first thing you will need to assess is what you are actually working with. If you have bedrock close to the surface, you have large rocks, or you have straight sand, it will impact your options for anchoring.
The actual holding power of your soil is an important consideration. Soil that has been recently excavated has considerably less holding power than soil that has not been moved in a long time.
The total surface area of an anchor post in contact with the soil is an important consideration. Large and short may have more contact area than long and slender.
When anchors can be used in alternating directions, it will multiply their individual holding power.
Wet soil usually does not have the same holding capacity as well drained soil. An important consideration here is rainwater which is shed by the building. As an example, with a 16’ wide structure, 8’ of rainwater goes each way. This means that the narrow area next to your building get 8 times the rain that the rest of the property gets. If you have poorly draining soil, this could be a potential problem.
If you are planning to use the movable structure option, proper anchoring has some additional challenges. Since your building is vulnerable to sudden increases in wind during this process, your anchoring protocol needs to be done quickly. Always do a test in the new area to confirm what you will be working with. With the quickness that is required, there is also a reminder that taking shortcuts will come back to haunt you.
We would like to reiterate, “If you are not 110% sure, please ask” There is no situation out there that is so unusual that we have not already been through it.
Your long term success is very important to us!
We hope you are finding value in this series of posts, helping you prepare for your new growing adventure! Reminder that the deadline is May 31st for Fall Delivery. We look forward to working with you for your projects!
Winter Storms and Maintenance
With the upcoming storms in both Ontario and the east coast forecasted, we wanted to remind everyone: it’s okay, and even preferred that the buildings have some snow around them.
This prevents the wind from getting under them and it also means that the height above the ground is less. This significantly decreases the aerodynamic lift.
As we’ve stated in our winter care articles and in the installation guide, there are things you can do to prepare your building to withstand the storms as best as possible. Taking extra steps during the ‘worst case scenario’ situations can often make all the difference and help your building stay standing.
Please don’t hesitate to contact us if you have any questions about any of these situations. Good luck and stay warm!
Greenhouses 202: Making Sure Your Structure Survives the Elements
Norm did a presentation at the Guelph Organic Conference on Greenhouses 202: Making sure your structure survives the elements. The presentation is broken into three parts for easy viewing, the last section of which is the Q&A. These tips apply for greenhouses, storage buildings, livestock shelters, really anything we sell. Key Points Covered in the presentation:
1. Some basic principles of engineering so that the forces exerted on the buildings could be better understood.
2. The many components of anchoring. Anchoring prevents a structure from settling under snow load, prevents lifting under aerodynamic forces and prevents shifting with wind forces.
3. The similarities of an airplane wing to the shape of a structure. What happens when surfaces become bigger, wider. lower and higher.
4. How uneven loads can happen and how to prevent them.
5. The proper procedure to removing excessive snow load
Please don’t hesitate to contact us with any questions that may arise. You can see the rest of our how to collection of videos on our channel here: https://www.youtube.com/channel/UCcLxDBMpNEv4qn_Nua9lg5Q
Multi Shelter Solutions 