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Re: [TowerTalk] "Wind Load" "Wind Area"

Subject: Re: [TowerTalk] "Wind Load" "Wind Area"
Date: Sat, 16 Dec 2006 11:05:22 EST
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In a message dated 12/10/2006 7:11:24 A.M. Pacific Standard Time, writes:

>       The windload rating of a tower  usually stipulates that the specified 
load is applied at some distance above  the tower top, like maybe one or two f
eet.  If your multiple antenna  installation spreads out your antennas over 
let's say 15 feet of mast (triband  Yagi a foot above the tower, 2L 40M Yagi 
ten feet above that and a longboom  VHF Yagi five feet above that), then you 
can't simply add the antennas'  respective areas.  You have to take into 
the moment (leverage)  that these various antennas at these various distances 
above the tower top  impart to the tower.  If you do this (spread out your 
antennas) you could  very easily exceed the tower's maximum windload.   

>   Another way of saying this is if your tower is rated  at 20 sqft of load 
at one foot above the tower top, it is then rated for LESS  than 20 sqft if 
that one antenna is MORE than a foot above the tower  top.  

>   There is an article by W7NI that was  published in the NCJ for Sep/Oct 
1982 and Mar/Apr 1983 and republished in  Jan/Feb 1992 NCJ titled Antenna Mast 
Design.  Stan goes over the fairly  simple math involved in calculating the 
moment that is imparted on towers by  antenna wind loads at various distances 
above the tower  top. 
    That was the article that got everyone aware of  the increased windload 
bending moment with masts but unfortunately according  to Dick Weber, PE, K5IU, 
his calculations and formulas were wrong. A much  better and more accurate 
discussion of mast forces is in Dave Leeson's,  ex-W6QHS, W6NL, book "Physical 
Design of Yagi Antennas". It's out of print but  you can probably find a copy 
somewhere. It's got LOTS of interesting  information about lots of tower and 
antenna system topics.
Steve     K7LXC


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