Tower Loading

[K7LXC@aol.com]

In a message dated 97-02-05 11:57:14 EST, W7NI and WE9V write:

>>Who here has actually calculated the wind loading of their antenans on
>>guyed towers and determined if they are within the Rohn specified limit?

     Uh, everyone I hope!?!
>
>No me.  Being Rohn dealer, I don't really know what their limits are.  Rohn
>publishes a few, very specific, configurations and tells you how many square
>feet you can put at the top of these specific configurations.  To my
>knowledge, Rohn has never published any configuration with a mast sticking
>out the top of the tower and when Rohn talks about side mounts, I believe
>they mean commercial communications verticals a couple of feet away from the
>tower or a dish right on the tower.  I don't believe they ever talk about
>anything like an HF Yagi sidemounted.

      Actually the figure in the catalog is the total TOWER loading with the
normal configuration being antennas mounted at the top of the tower.  Most
communications and many amateur towers are populated up and down the tower
with antennas, appurtenances and cables and the Rohn spec is for the WHOLE
tower.
>
>>I would like to correspond with anyone who is familiar with
>>said calculations, including calculating the allowable loads for side
>>mounted antennas (yagis).  (Please, no hand-waving.)

     I'm waving and holding the TIA-222-F Tower Structure Standards.
>
>>The way I read it, a simple tribander (TH7DX) (9 sq.ft.) and short 
>>2-el 40 (402CD) (6.4 sq.ft.) are beyond the capabilities of Rohn 45, at
>>100', 90MPH wind (~13 sq.ft capable).  Yes, in the fine print, there is
>>another 8 sq.ft. allocated for equally spaced side mounts, but is one
>>to assume that that additional 8 sqft can be placed at the top along
>>with the 13sqft?

      I reiterate: YES.  The net square foot capacity in the scenario you
 mention is ~21 square feet.

>>I would like to know this so I can determine if this can be overcome by
>>larger anchors, larger (diameter) guys, strategic guy placement, increased
>>guying radius, etc.

     Generally, no.  The capacity of these towers is limited by the leg
strength.  That's why 55G is rated at more than 45G; same dimensions but
thicker legs.

    If this whole capacity calculation thing bothers you, why don't you just
put up a bunch of 55G and quit trying to outguess Rohn specifications and
other practical advice you have received from TowerTalk?  

73,  Steve  K7LXC

     TOWER TECH -- professional tower supplies and services for amateurs



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