Tips on Installing A Concrete Base

For a Self Supporting Tower

 

(Recommendations contained in this article are based
upon personal experience. Use them at your own risk)

With 40 years in business and considerable training and experience over those years in tower installation the following information might help the layman do a professional job installing a suitable concrete base for the various self supporting towers on the market today.

First you will need an appropriate tower which will depend on the desired height and windload of the antenna or antennas you wish to install.

Some municipalities have height and siting restrictions so it is best to check for any limitations or bylaws before you purchase a tower. The location of the tower should be thought out carefully. You should not have your antennas turning over the neighbour’s air space. The tower manufacturer can provide a pamphlet stating the size of the hole required. If your soil is clay it is ideal. If your soil either sandy or loose backfill, you will need to put in at least an extra 20 % more cement into the foundation. The best way to excavate a hole is with a pick and shovel A backhoe is not desirable unless it is something like the small Kaboda. A backhoe with the large bucket cannot dig a nice square hole if the hole is five feet square or less. However if a good operator is careful, he should be able to dig a five foot square hole or larger with the standard backhoe. If the hole is dug properly, the sides of the hole can be used as the form and the building of a form for the cement is not needed.

For example, to erect something like a Trylon 'Titan" T-500-72 tower, you will need a hole at least six feet square and four and a half feet deep. When finished, it will require approximately 6 yards of concrete for the base which will weigh about 12 tons. Self-supporting towers usually come with four-foot concrete base stubs that are to be mounted on the bottom tower section. Although these "stubs" are four feet long, they overlap on the bottom section about six inches. In other words, the stubs will extend 42 inches below the bottom section. As we do not want to bury part of the tower into the concrete, we will have to raise the overall structure off the bottom of the excavated hole. To do this in a five- foot- deep hole, you will lower the tower base with stubs attached into the hole. To raise the structure you can use cement blocks or similar material placed under each of the three stub legs.

In a four-foot-deep hole, you will need to raise it off the bottom about six inches and by about 12 inches for a four-foot-six inch hole etc. Before you raise it off the bottom, first decide what direction you wish the legs pointed. It is generally agreed that you should point one of the legs against the prevailing wind. After you decide in which direction to anchor the base section you will require three (preferably metal) stakes, and about 60 feet of about #12 gauge solid wire or a length of "farmers" black wire. Do not use stranded wire or rope as it has a tendency to stretch.

To make the necessary adjustments, three turnbuckles about a foot long will make the adjustment much easier To start drive the three stakes into the ground about 15 feet directly out from each leg. If you are using turnbuckles attach one to each stake and run a length of the guy wire from each turnbuckle to the top of each leg of the tower. These guy wires are simply to keep the tower in the correct position until the concrete has set.

One of the most important aspects of the tower installation is getting the tower plumbed properly With the tower sitting on concrete blocks, you can make the necessary adjustment to get the tower straight. Each leg of the tower is tapered, typically about 3/4 inch of taper every four feet. Now take a good four-foot-level and lay it against each leg.

 

 

 

 

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If you find that a leg is too low, have someone steady the tower while you remove the cement block under that leg. Once it is removed, just put some dirt where the cement block was and replace the block. Conversely, if the leg is too high, remove some dirt. When you get each leg measuring about the same, adjust the turnbuckles to mid- range and then tighten the guy wire by hand, reasonably secure.   You can get the tower perfectly straight by doing some fine tuning. Take the four-foot level in the left hand and a 1/2' drill bit or 1/2" bolt or similar material in the right hand. As an added precaution do not turn the level over if it is not of the professional type. If the vials holding the fluid are not perfect you will get a different reading if you turn the level over.

With the level in the left hand and the 1/2" stock in the other hand, lay the level against the side of the leg with the bottom part touching the leg. If the tower section is reasonably straight you should have a gap of about 3/4" at the top of the level when the bubble is in the middle of the vial. Now slide the 1/2" bolt from top of the level downwards against the body of the level and at same time watch the bubble in the top of the level. At this time, pay particular attention where the 1/2" stock touches both the level and the tower leg simultaneously when the bubble is in the center of the vial. Go to the next two legs and do the same. By observing where the 1/2" stock is on each leg, you will find which leg or legs needs to be adjusted. Have a friend loosen or tighten each turnbuckle as required.

If considerable adjustment has to be made, you might have to shore up one or more of the legs as you will find one or more not firmly in the base. When you get it straight make sure the guy wires are fairly snug. For added strength for the concrete base you may add "rebar" and be sure that the reinforced steel rods are covered by at least three inches of concrete when finished.

It is no problem to get the tower plumb within 1 inch at 64 feet. On a four foot level you should be able to adjust the taper within a 1/16" which is equivalent to 1/8" for an 8 foot tower section If the tower is 64 feet consisting of eight sections, you have an accuracy of one inch in 64 feet. (1/8" X 8 = 1 inch). Remember that if each leg measures the same, the tower is STRAIGHT UP AND DOWN.  If you wish to do a bit of practicing, stand one of the sections on a level floor and take the above measurements with the level. That way, you will get an idea what it should be like when you stand a section in the excavated hole. When you pour the concrete, be careful not to disturb the tower within the excavation. It is best to pour the concrete between the base stubs.

Periodically check as you are pouring that the pressure of the concrete has not moved the base structure If it does get moved, adjust it accordingly before the cement sets. I recommend 3500-pound concrete mix which means that its strength should be 3500 pounds per square inch in 28 days. Do not let the concrete truck operator add any additional water. The 3500 pound mix is only guaranteed if water is not added. You may find this statement written in small print on your invoice from the concrete company.

Lastly, after you have done a good job and you know the tower is good and straight and plumb and someone suggests that maybe it is a bit crooked, just say you leaned it against the prevailing wind.