If a tower manufacturer tells you that a 15 sq ft antenna load can be placed
at one foot above the top of the tower when the wind blows 70mph then some
conclusions can be drawn:
The tower is strong enough to handle it's own wind load (never specified but
it has one) plus an added load generated by the 15 sq ft antenna. If the
tower is 72 ft tall then the load is 73 ft above the concrete foundation
where the total over turning moment reaches a maximum (feet X lbs of force).
At 70mph each square foot of load presents approx 20 lbs of force to the
structure. If the height of the antenna is 73 ft then the total available
moment generated by the antenna is 15ft sq X 20 lbs X 73 ft for a total of
21,900 ft lbs.
When you move the load higher than specified you basically use the same
proccess to assure that the 21,900 ft lbs in this example is not exceeded.
The wind load of the mast has to be included as well as transmission lines
and any other hardware that can be "seen" by the wind.
The loading for the mast is determined by square footage of the mast, pounds
of force per square foot and the height above ground to the center of the
mast (same as average height above ground).
If more than one antenna is used it is accounted for in the same manner; sq
ft X lbs/sqft X height above ground and added to the total.
We would like to scale the calculations to other wind speeds but we can not
do that because of the missing windload data of the tower structure mentioned
above. This is where the professional engineering calcs come into play; they
know how to calculate the missing data, we don't!
Hope all this makes sense.
73 de Gerald/K5GW/Texas Towers