Thursday, April 16, 2015

High Tunnels

There are many companies out there selling high tunnels. Some are worthwhile to buy and build. Others are just made too light and not worth the effort putting up. Hopefully I can help you out in this. I will point out, that my company (Morgan County Seeds LLC) does sell high tunnels and greenhouse.

First of all let’s look at steel gauges. This is the thickness of the steel. In high tunnels, it is the wall thickness of the tubing. Steel gauges for high tunnels usually range from 17 gauges all the way to a 12 gauge. No that is not a mistake; the numbers are in the correct order.

Now let’s look at what the numbers mean. When you see steel tubing or sheeting gauge, make a fraction out of it. For instance if you have a 13 gauge it would become 1/13 of an inch. In other words, the gauge means the number of sheets or tubing wall thickness it takes to make an inch in thickness. The thicker the tubing is, the more strength it has. In tubing, 13-gauge wall thickness has more strength then a 17-gauge tube. Plus if you are putting in a screw, there is more meat there to hold unto.

The next thing we need to look at is the tubing diameter. In all of the high tunnels out there, tubing sizes range from 1.315” all the way up to 2 3/8” diameter. What does this mean? Talking with Neal Zimmerman from Zimmerman’s Welding I found out that each time you increase the tubing diameter by 1/3, the strength increases by 66%. That can make a big difference. If the tubing size from one company is 1.7” diameter and the other is 2.27” diameter, there is 66% more strength in the 2.27” tubing. This one area that some high tunnel manufactures are cutting corners on cost. It also cuts down on the strength of the tunnel.

Now let’s look at bracing. Some have lots of bracing and others have almost no bracing or none. For strength of a building you need bracing to help it stay square or upright. On buildings with a sheet metal cover, the sheet metal can work as bracing. On a high tunnel you do not have any sheet metal on the outside for bracing. Greenhouse plastic does not brace. In fact it will stretch if pulled to hard.

Here are some of the common types of bracing used in high tunnel construction. Corner bracing. This brace is usually placed in between the first and 2nd sidewall post at all 4 corners of the high tunnel. This helps keep the corners of the tunnel in line. Some manufactures use them and others cut corners here and do not use them to save a little money on the cost of the high tunnel. It also leaves a weak spot in the tunnel.

Roof bracing, is another spot some cut the cost of a tunnel. These cheap tunnels rely on the purlins to help brace the roof. A very poor choice. Other tunnel manufactures use several braces in the roof. Usually the bracing starts at the peak of the tunnel on the first bow and goes at angle to the 2nd bow. Some also continue this bracing from the 2nd bow to the third bow. This is done on both sides of the center purlin. When standing under the tunnel and looking up, it will make V. Both ends of the tunnel roof should have this bracing.

Side to side bow bracing. Some provide this option and others do not. There are several types used for this. One is just a long brace from one side of the bow to the other side. This makes a small truss in the peak of the tunnel. It will help with wind and snow loads.

The other popular one is called a W truss. On the bottom of the W truss, there is a long pipe. This pipe fastens to the bow at each end. Several feet in, at a angle a short brace goes from the bow to the lower pipe of the W truss. At this point, another brace takes off and goes to the center of the peak of the bow. This is done on both sides of the bow. When looking at it, it makes capital W. This is by far the strongest bracing you can get for your bows. If each bow has one of these W trusses, it will hold a lot more snow or ice.

(By Norman Kilmer, Morgan County Seed)

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