Wind braces for structures

There is no dispute on the need for wind braces on any structure. There is often confusion on how and when those braces need to be installed.

The sooner that some sort of bracing is installed on a building, the easier it is to maintain plumb or vertical. For this reason, we stress the importance of tying off a building, both ways, as soon as the first section has been erected.

One detail that many overlooks are the total surface area of the hoops. It does not take very long to have the combined surface of a 4’x8’ sheet of plywood. For this reason, it is important to not rely for an extended period on your initial tie-down ropes as your “bracing”. There have been instances where the combined surface area of the hoops is more than the entire gable end.

If you are doing a long building with long hoops, i.e. 30’ wide, it would be wise to install the bracing before the whole structure is assembled.

Wind bracing can be installed in two directions, one goes away from the end wall and one goes toward the end wall. Both ways are acceptable, provided that you do both. If you install them toward the end at one end, you must do the same at the other end. By doing the same concept at each end, you are essentially holding things in opposite directions.

When bracing goes away from the end, the load is referred to as “tensile load”. Cable is often used for this. The cable and the clamps used must be rated against stretching and breaking. This is a convenient system since you are not limited by a precise measurement. When using this system, it is important to end up at the ground, in the 4 corners.

When bracing goes toward the end, the load is referred to as “compression load”. Round tubing with flattened ends is most often used. It is important to make sure that the tubing is strong enough as to not bend as it is being compressed. This is the method which has been used the longest since the instinctive way to brace something it to “prop something against it”.

With either method of bracing, where you start and end is very important. For bracing to be effective, you must start at a point that is connected to the whole structure. This would be either the ridge or a row of purlins. Starting at a mid point of a hoop will give little reinforcement since the hoop can flex from side to side. The closer that bracing goes at 45 degree angle, the stronger it will be.

In the case of longer hoops with multiple rows of purlins, it is advisable to have a series of shorter braces than one long one. This means you would start at a certain hoop/purlin connection and go down and over for 3 or 4 hoops and anchor at that hoop/purlin connection. Go over to the same hoop you did the first brace on and repeat the process from the lower purlin. You would then be going down to the base or the next row of purlins. Remember that if at one end, if you are going left to right, on the other end you go right to left.

Where ever you end up, it is wise to have extra anchoring at that point.

For extra clarification, please watch our “installing wind braces” video on the website.

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.

Air Circulation & Humidity Control

Even though many people would consider air circulation and humidity control as totally separate functions, they are closely intertwined.

You may have the proper sized openings to create the proper amount of air changes.

You could still have hot or cold spots in your greenhouse if you do not have proper circulation.

The same can be said about removing humidity.

For a ventilation system to have optimum efficiency and benefit, there must be balance. Having proper air circulation allows you to achieve that balance.

Horizontal air flow (HAF) fans typically come with a cage around the blades, a hanger bracket and a cord with plug. This allows them to be attached or suspended from the frame at the proper location. The motors are rarely more than 1/3 hp.

HAF fans always are installed in pairs and blow in opposite directions. A short greenhouse will have one in the front right corner and in the back left corner. A longer greenhouse will still have one in the front right and back left but also two half way down the length. The one on the right will blow in the same direction as the front right. The one on the left will be blowing in the same direction as the back left.

HAF fans should never be mounted in such a way that allows them to be blowing directly at plants. This would create an uneven drying. Some people will aim the fans slightly in the direction of the cover to ensure maximum air flow along the cover to maintain dry covers.

These fans run continuously to ensure that the temperature and humidity are spread evenly throughout. It also ensures that your thermostat or humidistat are reacting to air or moisture that is representative of what is going on in the greenhouse.

Ventilation is a difficult area of greenhouse production to get perfect. The more attention you pay to the details and modify what you are doing, the greater your production. The tricky part is that with all the variables, no two years will be the same. Carefully consider all your options and the situation you’re dealing with and you will have success!

The Variations of Passive Venting

In a nutshell, passive venting is creating an opening and letting the warm / hot air escape. The simplest form of this is opening a door or window. Just because it is simple does not mean that this method will be effective in cooling your greenhouse.

If your ends face into the prevailing winds, if you can make your doors big enough and if your greenhouse is short enough. That is a lot of “if’s” and you will still be doing a lot of running back and forth to have the correct amount of opening for the amount of ventilating you need to do.

We have already reviewed the most common form of passive ventilation in the roll up sides. It was noted that roll up sides work much better when paired with a high opening to create a chimney effect. It is a fact that hot air rises so the higher you can create an opening, the more effective it will be.

If you are relying solely on gable end windows, either motorized or manual, they will need to be quite large. You will also need to have the benefit of a regularly strong prevailing wind.

The most effective form of passive venting is a continuous roof vent. This will provide a continuous opening in the precise area where the air is the hottest. A roof vent should always be mounted down wind of the prevailing wind. The benefit of being down wind is that the wind creates a vacuum as it goes over the greenhouse and sucks the warm air out.  The air intake for a roof vent is often a roll up side.

A significant down side of a roof vent is the up-front cost. The cost is the same for a narrow and a wider structure. This is the reason they are almost exclusively put on wider buildings. The cost is simply spread out over a bigger area. The effectiveness of a roof vent still makes it appealing in spite of the cost.

Roof vents can be controlled manually with a chain fall opener or with a motorized gearbox. A motorized system can be a simple open / close controller that you set the limit switches or it can have a proportional controller that allows for pre-set stages. It is with an automatic controller that the benefit of the roof vent will really become apparent. Every couple of minutes it will react and adjust to the inside condition of the greenhouse.

All of the expenditures involved with your greenhouse need to be weighed as a cost versus benefit or cost versus return. This process is especially challenging when considering the options of ventilation. Many of the expenses are subtle and hard to measure. The returns are equally hard to quantify since weather is an additional variable. This makes it even more important to keep accurate records and be aware of your options for improvements.

The benefits and drawbacks of forced ventilation

By definition, forced ventilation included an electrically operated exhaust fan at one end of a building and intake louvre at the opposite end.

The biggest advantage of this system is how simply it can be automated. Running your power supply through a thermostat will allow the system to come on and shut off at the set point.

There are numerous things which need to be considered for forced ventilation to be effective.

The prime consideration is proper sizing of the system. Most greenhouses require one air change per minute to keep the temperature within a tolerable range for the plants. Fans are rated for the cubic feet of air they can move per minute (cfm). By calculating the air volume in a greenhouse, you can then get a fan that is appropriately sized.

There also is a formula to match the air intake to the exhaust. When the intake is too small, the capacity of the fan will be restricted but if the intake is too large, it also is undesirable. A properly sized intake will create some turbulence so that incoming air is mixed with the existing air before it moves through.

Consideration must also be given if the intake has netting over it to control incoming weed seeds or insects. This will restrict more air flow than is usually expected.

The orientation of fan / louvre combination is important. If the intake is on a south facing wall, the incoming air will always be warmer than average. This means you will be moving warm air rather than cooling.

If a fan is blowing into a prevailing wind, the fan capacity can be significantly decreased as well. If you do not have a choice but point that way, it would be wise to consider a hood or other deflector over the fan.

When you are deciding on what size fan to get, it is often suggested to get two fans with that combined capacity. It will cost more up front, but the extra payback will be on extra flexibility or options for ventilating. This gives you the option of “half capacity”.

When planning for a forced ventilation system, it is important to understand the requirements and cost of getting the electrical service to that area and also what the on going operating costs will be. In both cases, these expenses can be significant.

One last thing which needs to be pointed out for consideration has to do with irrigation. In a forced ventilation structure, things dry out more quickly and they will also dry out unevenly. Just something to be aware of when you are thinking everything will get watered the same.

In our next article we will explore the opposite end of this spectrum … some of the options of passive venting.

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