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.
Category Archives: structure information
Preparing for a Hurricane
To our east coast customers, please note, with Hurricane Fiona approaching, (and any in the future, as we’ve posted previously with Teddy and Dorian) there are some key things you need to do to prepare and give your structures the best chance possible of surviving.
- It is critically important to inspect all the greenhouses and their anchoring systems.
- If you have anything extra which can be used as an anchor … use it in the direction of the prevailing winds.
- Make sure that there are no loose ends of wirelock inserts or any loose plastic coverings
- Make sure that there aren’t any frayed parts to any ropes on the tunnels and that the ropes are TIGHT.
- If you see things coming apart during the storm and it is still safe to go out,
CUT THE COVER.
It is easier to replace the cover then the whole structure.
If you have any questions or concerns, please email or phone us this week, we will respond to every email and please, most of all be safe, our thoughts are with you
We had previously shared this ahead of hurricane Teddy and Dorian (as many of you have found on our website already!) but it is all still very relevant ahead of any hurricane, as Fiona approaches the east coast. Please be safe and prepare what you can. We at Multi Shelters will hold you in our thoughts this week
Season Extension: Ventilation & Heating
To make your greenhouse more of a year round functioning entity, there are two main areas in which you have to make the structure more efficient-Heating and Ventilation.
1. Heating is your biggest expense for the winter so retaining heat is a priority.
The easiest way to retain heat in your greenhouse is to install a double poly cover with air in between. A small squirrel cage blower attached to the inside layer of plastic aids in maintaining air between layers.
The more dead air space between these layers you can create, the closer you will be to achieving a 30% reduction in heat loss. Holes will result in air movement and therefore less efficiency. 3″-5″ consistent space is ideal. It is a given that you would have less than that around the edges and over the ridge.
Double plastic will have a considerably longer life span. It is important to realize that as plastic gets older, the light transmission will be reduced which will reduce production.
Infrared plastic (IR poly) does further reduce heat loss and increase light diffusion so it can also be a consideration when looking for ways to reduce heating costs.
2. Ventilation is also one of your biggest considerations for the warmer times of the year. Ventilation can be done through forced or mechanical methods or passive through vents or roll up sides.
Vents are extremely effective since they can be placed higher up where the heat needs to be expelled. Mechanical ventilation is more costly both up front and to operate but it is easier to control since it is attached to a thermostat. For mechanical ventilation to be effective, it needs to be sized and located properly.
Roll up sides are less costly and simpler to install but are restricted by the fact that you have to be there to open and to close.
Climate control is especially challenging in the spring and the fall since most days you will have the need for both ventilating and heating.
One area that you need to be especially aware of is stagnant air. Without proper air movement, circulation and exchanging, stagnant air can cause many different types of diseases. It is important to understand what your plants require
Season Extension: Moving Your Structure
A significant part of season extension involves moving an intact structure.
This basically allows you to get two (or possibly three) plots of production from one investment.
The idea is to start a relatively cold tolerant crop very early in the season (the timing will be different in different locations).
- Once the crop is firmly established in location A, (and it has warmed up) you will move the structure to location B and start another crop.
- You will harvest the crop in location A and then after working the soil, plant another crop in location A which is intended for fall harvesting.
- After location B is harvested and before frost you will move the structure back to A.
- Instead of doing twice in location A you could also choose location C.
A structure can be equipped with wheels which will run over the soil. There is quite a bit of flexibility where you go and the terrain you navigate.
The structure can be equipped with rollers on a track. This will determine where you go and this is usually intended for moving a bigger structure with fewer people.
The most common method of moving is sliding the structure on the soil. The base rail can be wood or steel.
It is critically import to understand the logistics of moving on a structure before you start. It is not hard to move a structure but it is also not hard to do damage.
Having a plan for proper anchoring is very important for a movable structure. Your structure is at a vulnerable state when you release the anchors. Once you start, the job must be completed quickly. You have to be aware that the anchors may not come out or go back in easily so you may need to give yourself some extra time.
One other area of consideration on a movable structure is the ends. There must be some sort of a flap or vent along the bottom so that when a structure is being moved, the ends will not uproot plant material. Generally speaking to have this ability in the ends takes away from the structural integrity, so some extra anchoring may be required.
You can see more information and photos on our movable information page. Please don’t hesitate to call us with ANY questions you may have. This can be a very useful addition to your structure, but must be understood correctly.
Season Extension: Hanley Caterpillar Tunnels
The first area to discuss regarding season extension are those structures that are simple enough to disassemble and relocate to another spot-primarily referring to Hanley Caterpillar Tunnels.
The real lure of these buildings is their low cost and simplicity to move. Generally the area is prepared in advance and then the tunnel is moved over the area when the planting is to be done.
Since the original design specified 6′ hoop spacing, the intent was always to remove the cover before risk of heavier snow fall. More often now, growers are going to 5′ spacing (and sometimes 4′) to allow the covers to remain on longer or even permanently.
By having a smaller hoop spacing, it allows the cover to stay on permanently, which allows for a much earlier start in the spring.
The key to the concept of the Hanley working is in the rope.
- The back and forth pattern of the rope resembles the way a person would lace up a boot.
- Typically there is a spring loaded clip at the base where the rope passes through.
- Once the rope is fully installed, it is important to tighten it through three passes from end to end.
- If the rope is not tight enough, the wind will create wiggle and movement sideways.
- When the plastic is tight enough, it also allows the grower to slide the plastic up the hoop to allow ventilation.
Generally the plastic is about 25′ longer then the tunnel.
This allows the installer to bunch up (pig tail style) the extra poly at both ends between a pair of posts.
It is important to get the plastic as tight as possible lengthwise first.
There must also be a rope from the top of the last hoop to the tie off post.
There are a couple of challenges to consider with the hanley tunnels.
- One of these challenges is to create a proper entrance. Typically entrance is achieved by slightly lifting the plastic and ducking underneath.
- The other are of challenge is the low shoulder height of the structure. This restricts the use to a couple rows of taller items in the middle and a couple rows of shorter plants along the side. The width of the tunnel does lend itself to three regular size beds.
Once you have learned about the nuances of the hanley tunnels, and asked any questions that come up, you will be better prepared to have these efficient season extenders work for you and your application.
Feel free to look at our Hanley Application page for more information and photos, and give us a call with anything else you require.
We are pleased to have many happy customers these structures are working for, and look forward to you being next!
Check out our book if you want to know more “So You Want to Buy a Greenhouse…Your Guide to Planning a Greenhouse Purchase”
Livestock Shelter or Greenhouse?
One of the questions we are often presented with has to do with customers wanting a structure for dual purpose. In itself, this could be considered as a wise strategy to get multiple uses for a building so that it can be used closer to year round.
The question which needs to be asked is “Are these complimentary applications?
One of these dual applications is for greenhouses and livestock shelters. There are a lot of similarities between the two with the most prominent ones being a double roof cover with air between and that they often have roll up sides.
Where the problem arises is that when you have a greenhouse, there MUST be a clear cover to allow the proper spectrum of light to come through to allow plant growth to occur. This level of light comes with heat which can easily be a problem for animals.
For a livestock shelter 90% of the battle is keeping them dry and out of the wind, cool is better then warm. If your animals will only be in the shelter during inclement weather, you will likely have more leeway in dealing with heat.
We have had customers who cover with single clear during growing season and the put an extra layer of white during the animal housing.
If you have any questions or concerns on how you plan to use your shelter, please call or email for extra input.
Multi Shelter Solutions 