[TowerTalk] Earth Anchors

[N3AE]

Patrick's comments on a vector sum of forces is spot on. The real world is never as simple as some of the drawings in catalogs. My terrain is uneven. Plus for a variety of reasons I was not able to locate my anchors to keep all the guy angles the same (for the set of guys at the same height on the tower). But if you lay out your installation for each guy to-scale on some graph paper (side-looking view) along with your terrain, you can figure out the angle with respect to the horizontal that you need to place each anchor. The average of the guy angles going to the same anchor (again, with respect to the horizontal) will get you close to the angle you need to install the anchor at. 

The recommended guy tension is 10% of the breaking strength. So for 3/16" EHS, that would be about 400 lbs. But if the anchor placements are not equidistant from the tower base, or the terrain is not flat, you'll want to adjust each set (i.e. all guys at the same height on the tower) so that the horizontal component of force pulling on the tower are all equal. That neutralizes the net tipping moment at the base of the tower, although it does result in different downward compression forces on the legs (can't have it all without complete symmetry). 

The horizontal pull force a guy produces is the guy tension (along the line of the guy) times the cosine of the angle of the guy with respect to the horizontal. That angle is easily measured with a protractor and plumb bob, or an inclinometer placed on the guy. So in the end, if the guy angles are not equal for a set at the same height on the tower, you have to adjust each guy tension to whatever is needed to make the horizontal pull forces balance (no net tipping moment at the tower base). 

As an example, consider a bottom set of guys with angles (respect to the horizontal) of 37, 45 and 50 degrees. In the same order, the guy tension is set to 322 lbs, 363 lbs and 400 lbs respectively. The resulting horizontal pull forces are all 257 lbs, for a net tipping moment on the tower of zero (each guy pulling equally in the horizontal). 400*cos(50) = 257, etc. Even with a good Loos guy tension gauge, it's hard to adjust to this precision anyway. 

Do the same for the next higher set of guys. Once you graph out the installation, average the guy angles for all the guys going to the same anchor and set the screw anchor at that angle. You'll be reasonably close to having the net force along the axis of the screw anchor. 

If you have uneven terrain but not other constraints (like stuff in the way), the best way to place the anchor points is one which keeps the guy angles the same (for a set at the same height on the tower). That requires moving the anchor point either towards the tower or away from it based on the terrain (dip or rise). In general, best keep the closest top guy at 80% of the height of the tower away from the base. If the terrain is highly irregular, you could need a dedicated anchor for each guy to keep all the guy angles the same (for the set at the same height on the tower). In really hilly terrain, you can get the anchor too far from the base, and have to start worrying about the length of the guy itself and any mechanical resonances it might have (think guitar string). 

Hope this helps. 

N3AE 

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