Tower Loading

[Stan Griffiths]

>Reader poll:
>Those of you with towers larger than 70':  What tower are you using?
>How much windloading do you have at the top?  How much additional side
>mount? 

Tower #1:  11 sections of Rohn 25.  5 el 10 meter beam 10 feet above top of
tower.  6 el 15 meter beam at top of tower.   4 el 10 at 70 feet.  4el 10 at
55 feet.  4 el 15 at 50 feet.  Never actually calculated wind area.  Guyed
at four levels.

Tower #2:  40 feet of Rohn 25 on top of 80 feet of Rohn JJ (like 45 only
bolted construction rather than welded).  3 el 20 10 feet above top of
tower.  3 el shorty forty (Hygain Discoverer) at top of tower.  Small
tribander at 50 feet.
Tower is guyed at four levels.

Tower #3:  107 feet if AB105.  Large 5 el 20 at the top.  Tower is guyed at
three levels.

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

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.

>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 don't know what you mean by "no hand-waving" but I have never run into
such a person in 44 years of ham experience.  I think most people run SOME
calculations . . . the ones they understand how to do.  It is always risky
to not do a full set of calculations.  Everybody wants exactly what you
want:  a fully designed antenna system with predictable performance during
heavy winds.  Nobody gets it.

I am absolutely certain Rohn knows how to make these exact calculations and
they will do it for you for several thousand dollars per tower.  (This is
why nobody has them do it . . . )  They need to charge a lot of money
because they are responsible for what they tell you and they have to buy
expensive insurance to cover the cost of any lawsuit they may get as a
result of giving you some advice that does not work out (your tower crashes).

>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?

Not safe to assume any of this stuff.

>I know there are people with much larger (or similar) loads on a Rohn 25,
>which is rated for less load, and does not have the extra 8sqft in fine
>print.
>
>How did you deem this safe?  Are you playing with fire (or specifically,
>wind) or am I missing something here?

I think you have it correct.  We are playing with wind here and there is
some risk involved.  There are ways to minimize the risk.

>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.

Once again, we would all like to know those answers.  You can have them
directly from Rohn, but I already told you the cost.  Most guys will take
some risk rather than pay the price.

One thing you CAN do is look closely at some installations in your area that
have been up for 10 or more years.  They are probably strong enough and it
will give an idea of what works over the long haul.  My towers have been up
for more than 23 years.

It will be interesting to read the other answers you get.

Stan  w7ni@teleport.com


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