[TowerTalk] Downforce calculation

Tim Cotton, N4UM N4UM at bellsouth.net
Sun Feb 12 10:20:17 EST 2006

I'm certainly not an engineer or any kind of expert on such calculations so take what I say with a very very large grain of salt and realize that these are very very crude calculations based upon a number of simplifying assumptions.

There are several factors to be considered in determining the down force...

    1. The weight of the tower
    2. The weight of the antennas & feedlines
    3. The weight of the guy wires & insulators plus pretensioning etc.
    4.  The compressive forces generated by the effects of the wind.

Rohn 25 weighs about 40 pounds per section.  The tower weight would be about 230 pounds.  Assume the antennas and feedline weigh another 120 pounds.  Assume three sets of guys to the top of the tower spaced out 30 feet from the base... 22 pounds.  The downward force in dead calm air would be about 370 pounds - not counting anything due to guy wire tensioning.  Tensioning at 10 percent of breaking strength would add 400 pounds per guy wire (assuming 3/16 ehs wire with a breaking strength of about 4000 pounds).  This would result in additional compressive forces of approximately 1040 pounds).  The total downward force in calm air would be about 1410 pounds.

Horizontal forces in 20 psf winds (approximately 70 mph) would be about 690 pounds on the tower plus 280 pounds on the 14 square feet of antennas for a total of 970 pounds.  This translates into roughly 1940 pounds of additional tensioning in the guy wires and an additional downward force on the tower of about 1680 pounds.  

Horizontal forces in 50 mph winds would be about half of those in 70 mph winds and would be about 840 pounds.

Total downward force on the tower would be approximately 3100 pounds in 70 mph winds and about 2250 pounds in 50 mph winds.  

The tower certainly should be able to handle this much compression.  The real issue boils down to the ability of the roof to handle this sort of force.  The base that the tower is attached to should be able to distribute these forces over a large area - otherwise the tower is likely to poke its way right thru the roof!

Note that none of these calculations take into account the possiblity of the tower's buckling since you failed to proved any information on guying aside from the anchor point distance from the tower base.

Also note that the tower installer has violated a fundamental principle of failing to follow the manufacturer's directions by placing the guy anchor points 30 feet from the base of the tower instead of the recommended 45 feet.  

If you really want an answer to your question, spend the money and hire a professional engineer to do a detailed analysis.  You get what you pay for - if you're lucky!!

Tim Cotton, N4UM
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