[TowerTalk] Mast Length to Wind Load Questions

[TexasRF at aol.com]

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