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[TowerTalk] Mast Length to Wind Load Questions

To: <>
Subject: [TowerTalk] Mast Length to Wind Load Questions
Date: Sat Feb 8 15:05:06 2003
I am not shore if I understand your comments.
If a tower has been rated for a load at its top and this is known.
Then to move the load, why would the description of the tower be needed?

In case you missed it I included the summery of replies I posted.

Hello to all,
A very hardy thanks to all that replied to my question.
I have summarized some of the messages and included below.
I hope this information will be as useful to others as is has been to me.

Rich AA2MF

If you can get an answer from them, try the manufacturer. Years ago a Triex
engineer told me at what point above the tower the allowed wind load went to
zero (can't remember his name now).

He said to do a linear derating from the top of the tower to zero. Sounds
reasonable to me.

73 Don  K5AQ

Jim Lux

IF you assume identical mast and identical connection to the top of the
AND If you assume that the wind speed is the same at both places (not
necessarily valid.. average wind speed goes up with height, which is
factored into "real" wind loading calculations)

If the actual force due to wind remains the same, the bending force at the
mast/tower attachment will be 15 times greater, because the same force is
acting on a lever arm that is 15 times longer. Superficially, then, the
limit would be 1 square foot (based on keeping the bending load the same).

However, the mast's equivalent cross section will start to be a significant
fraction of the total cross section (a 2" pipe 12 feet long has a 2 square
foot cross section)

The real question, in a practical situation, will be what's the strength
limiting component?  Is it the mast strength in flexure? Is it the load on
the bearings?  In this case how much of the mast sticks into the tower
becomes quite important, although... if the moment is 15 times greater, the
loads are 15 times greater, assuming the geometry doesn't change.

The first thing you need to know is your county windspeed rating. You can
look it up at  <A HREF="";>
</A> under Tech Notes.
I offer a little program called MARC - the Mast, Antenna, and Rotator
Calculator. you can get it at the aforementioned website for ten bucks.

Steve    K7LXC


----- Original Message -----
From: <>
To: <>
Sent: Saturday, February 08, 2003 5:56 PM
Subject: Re: [TowerTalk] Mast Length to Wind Load Questions

> When asking for advice it helps to include a
> of the problem.
> One would need to know a LOT more information
> to answer your question, such as TYPE of tower,
> i.e. guyed, free standing, crankup?  and,
> HEIGHT of tower?
> If the tower is something other than a Rohn guyed
> tower, then we would also need to know a LOT more
> about the strength of the legs and the maximum
> allowable load.
> Tom  N4KG
> On Fri, 7 Feb 2003  "RCARIELLO" <> writes:
> > Mast Length to Wind Loading Questions.
> >
> > Hello to all,
> >
> > Is there a rule of thumb to follow?
> >
> > Tower rated:
> > 15 Square feet 1 foot above tower top.
> >
> > Moving the antenna to 15 feet above tower top.
> >
> > How large in square feet can the antenna be without exceeding the
> > tower
> > limits?
> >
> > Thanks,
> > Rich AA2MF
> >
> > _______________________________________________
> >
> > See:  for "Self Supporting Towers",
> > "Wireless Weather Stations", and lot's more.  Call Toll Free,
> > 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
> >
> > _______________________________________________
> > TowerTalk mailing list
> >
> >
> >
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