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The fight to keep Polytunnels grounded in the face of storms is continuous – until it is solved. Here we explore how to keep your Polytunnel’s low size-to-weight ratio from being blown away by the powerful and often destructive force – of wind. We take a brief look at the science behind it and then at how to mitigate the risk factors.
- Wind Load
- A practical example of the calculations to stop your tunnel from blowing away.
- Mitigating Risk
- Anchoring your polytunnel to prevent it from blowing away
Changes in localized atmospheric pressures created by factors of air composition, density, temperature, and the earth’s rotation result in air movement, mostly in a horizontal direction, from a high-pressure system to a low-pressure system.
This air movement (wind) exerts a calculable force on any object in its way. The intensity of that force, known as the wind load, is a product of the speed of the wind, the size and shape of the object in its way, and the drag coefficient of the wind on the object’s surface.
A practical example of the calculations to stop your tunnel from blowing away.
For example, let us consider a 20 x 10-foot Polytunnel with a height of 8 feet and a mass of 68 pounds standing with its 20-foot side facing an oncoming wind of 15 miles per hour. Calculating the wind force uses the formula: F = A x P x Cd, where F is the wind force; A is the surface area; P is the wind pressure per square foot, and Cd is the drag coefficient of the wind on the surface.
- A is Area = Length x Height = 20 x 8-foot, equaling 160 square feet.
- P is the wind pressure per square foot, and this is a product of the wind velocity squared multiplied by a constant factor of 0,00256. Thus 15 x 15 x 0.00256 = 0.576 pounds of air pressure per square foot.
- Cd has no unit of measure and is a variation between 0.8 to 1.2. For a cylindrical shape with a taut, smooth surface, like our lovely Polytunnel, we shall use a conservative drag coefficient of 0.8
Thus, the wind load caused by the 15-mph wind on our 20 by 8-foot polytunnel is 160 x 0.576 x 0.8 = 73.728 pounds. This is almost 6 pounds more than the weight of the tunnel (68 pounds) and will result (if no countermeasures are taken) in the polytunnel becoming part of the moving air.
Wind 1: Polytunnel 0.
There are many ways in which purchased polytunnels can be anchored to the ground. One of my favorites is the Northern Polytunnel which uses corkscrew-like anchors, and you can see me building one of their tunnels in the video below.
Protecting our polytunnel from being blown away by wind will require us to reduce the controllable risk factors:
- Lifting Weight
- Size of exposure
- Air Pressure
- Drag-coefficient of our surface.
The lifting weight of your polytunnel
Because the mass of the Polytunnels is a given, the only solution is to increase the force required to lift it off the ground by weighing or anchoring it down. More on this, in some detail, under the topic Anchoring Your Polytunnel.
The effects of exposure on your polytunnel
The size of the surface directly exposed to the wind is critical. To reduce this, we: a) need to know the general wind direction; b) need to position the Polytunnel so that the directly exposed surface is the smallest possible size; c) consider exposing the strongest point of a Polytunnel (a braced corner) to the wind’s onslaught.
The slant of trees and bushes often betrays the general wind direction. Constant high winds will cause higher plants in the area to demonstrate a biased slant in the wind direction, resulting from prolonged yielding to wind load and the subsequent growth in that direction.
If possible, position your tunnel with the strongest point facing the wind. This would be the braced corner of the Polytunnel. A good approach is to face the back of the tunnel, slightly off-center, to the wind. From a bird’s eye, your tunnel will be aligned to a slant of local trees and bushes.
Air pressure on your polytunnel
Though the per square foot air pressure caused by wind is an immutable factor of wind velocity, there are steps that you can take to reduce the immediate impact on our Polytunnel. Windbreaks are a common way of how to protect your Polytunnel from being blown away.
Strategically grown tree lanes or hedges and special broad-ribbed netting are common solutions to reducing the impact of wind directly on your Polytunnel. Even single trees, positioned strategically to reduce the impact on areas where the poly-wrap will create a trough under pressure (around areas of cross-arches), will reduce the risk of damage.
Drag-coefficient on polytunnels
A loose surface folds and pleats under the force of a wind resulting in increased wind-flow resistance. By ensuring your Polytunnel is tautly wrapped, you will significantly reduce the drag-coefficient factor of your Polytunnel. Because it is impossible to create a glass finish, it is advised that the combination of tautness and strategic windbreaks be considered.
Anchoring your polytunnel to prevent it from blowing away
Ideas for anchoring your Polytunnel are as abundant as the number of enterprising salespeople. While some of them are excellent, others are less so. However, as the adage goes: “The right solution is the one that works.” We will consider three solutions and their variations.
- Build a wooden frame using 2 x 4 or 4 x 4 boards to the size of your Polytunnel’s base (so that the base frame rests in the center part of the wood throughout)
- Concrete corner and center stakes onto the ground, onto which you can fasten the frame.
- Fasten the Polytunnel frame to the wooden frame
Points to Note
- It is advisable to treat the wood pre-assembly
- Depending on the quality of the ground, you could opt not to concrete the stakes but to use pole stakes driven into the ground. If this is your preferred route, endure at least a 2-foot depth into the ground. Securely attach the wooden frame to the stakes.
- Use quality braces for joins and corners.
- Secure the Polytunnel frame to the wooden frame using conduit saddles to ensure the optimal fixture
- Using a fencing stake (usually with one sharpened end) driven into the ground so that the top of the stake aligns at the cross-point of the leg and the first crossbar of the polytunnel frame
- Drill a hole through the stake below the height of the first rung
- Using high-tensile cable ties, fasten the cross-point to the stake via the drilled hole.
Points to Note
- The tie must exert downward pressure on the frame; in other words, it is fastened to the stake at a point lower than the cross-section.
- Ensuring the stake is flush with the inside of the downward frame pole further strengthens the structure.
- Make sure that the stake is well-driven into the ground and is sturdy.
- Minimally stake corners and the center of the long side
- Dig a trench the size of your polytunnel base. The trench should be 12 inches deep and 18 inches wide.
- Cut four strips of the heavy-duty ground cover sheet 18 inches wide to fit the trench’s base.
- Attach the cover sheet strips to the base of the complete polytunnel using high-tensile cable ties backed with heavy wire so that the full length of the strip is sturdily attached to the frame
- Place the complete Polytunnel, with attached base strips, in the trench and, using U-shaped stakes, further anchor the bottom frame and cover sheet to the trench base.
- Cover the base and strips with the excavated soil, using a compactor to compact the soil around the base.
Points to Note
- Using the Pythagoras Theorem, a straightforward way to ensure a square corner is by:
- hammering a corner peg into the ground. This is your fixed corner point – Peg A.
- Take a string affixed to Peg A and draw an arch in the general direction of the desired long side of the rectangle – 4 feet from the fixed point. Insert a second peg (Peg B) on the arch in the exact representative direction of your long side. Extending a lightly tangent line to pegs A and B will give you your first long side.
- Now draw an arch with a string 3 feet long at a right angle to your long side.
- Using a 5-foot string, draw a further intersecting arch from PEG B. The 3-foot arch (from Peg A) intersects the 5-foot arch (from Peg B). Insert Peg C.
- This is your first right-angled corner. Remember, right sides 3 and 4, adjoining side 5.
With all this in mind, you might be thinking, Is a polytunnel worth the effort? Would I be better to buy a greenhouse instead? Well, check out my article on each of these pros and cons and which is better and why. You can view that here.
How to stop a Polytunnel from blowing away? It is a combination of science and the benefit of learning from those who have previously had to deal with the loss or damage. Just ensure that however you decide to build your polytunnel, anchoring it to the ground is of paramount importance if you want to keep it.
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