# [TowerTalk] Windload calculation and freestanding versus guyed

wa4dou@juno.com wa4dou@juno.com
Sun, 12 Mar 2000 06:53:43 -0500

```Hi Trond,
I would guess that you are a candidate for a study in tower
engineering.
Probably beyond this reflector. However it can be interesting to take
data,
and synthesize new info from it.
For instance, Rohn specifies that a Rohn 25G tower, sitting in 2.4
cu.yds. of
concrete to make it self supporting, when 10 ft. high, no ice, can
support
mph.
and 10.5 sq. ft. at 90 mph.
For a 25G at 20 ft. high, the figures are 14.2, 9 and 6.9 for 70,80 and
90 mph
respectively.
For a 25G at 30 ft. high, the figures are 6.4,3.7 and 1.7 for 70,80 and
90 mph
respectively.
For 25G at 30 ft. high, the figure is 3.6 and 1.4 for 70 and 80 mph
only.
For 25G at 40 ft. high, the figure is 1.5 sq. ft. for 70 mph only.

Rohn 25G is a 3 tube equi-lateral triangle design with continuous
steel
Zig-Zag cross bracing, entirely electrically welded. The tubes are 1-1/4"
steel.
Everything is hot dipped galvanized. The spacing of the steel tubes is 12
or 12-1/2"
apart.

The same tower, sitting in 1/2 cu.yd. of concrete, with appropriate
guying, can
withstand wind up to 110 mph, supporting 9.3 , 8.3,8.2,8.0,7.8,7.7 and
7.4 sq. ft.

The fact that your  proposed tower is aluminum and that the
manufacturer
recommends only guying, is your tip off that what you get and what it
does, is
in your hands, if you don't follow the recommendation. I doubt that it
can be as
strong as this example above.
If you're not an engineer, fully capable of making your own decisions,
unaided,
in this matter, you're probably going to overload the towers capability
and invite
failure.
Better to under rate the tower. I'm sure the manufacturer knows his
product well.

73 Roy Lincoln WA4DOU Elm city, N.C.
-------------------------------------------------------------------------
---------------------------------------

On Sun, 12 Mar 2000 19:56:26 -0000 "Trond Thorman" <trond@radius.co.uk>
writes:
>
> Gentlemen,
> Many of you are very experienced in tower design, erection and use
> in
> various conditions and I'm appealing for your input.
>
> I'm curious on how to calculate the windload of an aluminium tower
> design.
>
> I'm contemplating a purchase of a 12m (40') + top tube of 3m (9')
> lattice
> tower and although I can get hold of manufacturers guy calculations,
> however
> I cannot access any information to the tower free-stand
> capabilities. The
> manufacturer states that it has been well tested as a free-standing
> tower in
> harsh climates but does not want to commit himself to any data
> related to
> free-standing deployment. He recommends guying above 9m (30').
>
> The statement puzzles me and asked him what actually brakes or what
> is the
> weakest point of a free-standing mast. His answer was the junction
> between
> the bottom of the mast and the foot. So I guess here's my questions:
>
> - How do I calculate the wind resistance of a free-standing tower,
> knowing
> all the mechanical dimensions? (no aerials)
> - What decided the strength of a mast bottom section and its
> connection to
> the foot?
> - How do I calculate the foot dimensions (amount of concrete) for
> free-standing use?
>
> Are there any good books available on the subjects? How does tower
> manufacturer attack and solve the problem? I'm extremely keen to go
> find out
> and to get to the bottom of my curiosity.
>
> Best Regards and 73 from a very pleasant and mild London (UK)
> Trond (G0TRD/SM7DJX)
>
>
>
>
>
>
> --
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>

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```