Hi All,
I have been biting my tongue for years about the theoretical load of Rohn
towers. It may be important for building code purposes and surely is for
Rohn's litigation exposure, but in the real world it is really unimportant.
A properly installed and guyed Rohn tower will hold MANY times more than
indicated in the catalogs. I look at the pictures of Rohn 25 with a single
beam at the top because of "loading' and I laugh. Here in the Northeast
there are dozens, if not hundreds of big contesters with super-loaded towers
for years without any mishaps, except for the ocassional 2 inches of ice and
50 mph winds together. My 100 foot Rohn 45 tower has a 15 foot, 3 inch moly
mast with a 4 element homebrew 15 meter beam on a 35 foot boom and a 4
element homebrew 20 meter beam on a 57 foot 3 inch boom. At 80 feet I have
a 2 element Cushcraft 40 meter beam, at 68 feet I have a TH7DXX, at 50 feet
I have a homebrew 6 element 10 meter beam on a 27 foot boom, and 34 feet I
have another TH7DXX, and at 70 feet I have a Ringo Ranger II vertical. You
calculate the wind loading!! It's been up for over 25 years without any
problems. So, If you need official documentation, follow the catalog values.
If you want to put up real antennas, just do it, but do it right, no corner
cutting or compromises in materials. Just my opinion. 73 Saul K2XA
----- Original Message -----
From: "Bill" <w7vp@comcast.net>
To: <NPAlex@aol.com>; <towertalk@contesting.com>; <k6xn@comcast.net>
Sent: Sunday, March 26, 2006 2:38 AM
Subject: Re: [TowerTalk] Mast wind loading
> Norm
> Replying to your post is difficult. There may be others on this forum who
> can reply more effectively than I can but let me pass on some thoughts.
>
> First of all the Rohn guyed tower specification indicates different
> allowable projected areas for round member antennas than it does for flat
> plate antennas, depending on the height of the tower. The specification
> provides details of the assumptions associated with the tower and the
> antenna. Insofar as the antenna is concerned note 4 states the assumption
> that the antenna and mounting hardware are place symmetrically at the apex
> of the tower.
> If you go above the apex of the tower with a vertical mast carrying the
> antenna you not only add to the effective projected area, but you can
> assume
> the wind loading figures will not be accurate. There will be a center of
> pressure associated with the wind loading of such a design that will
> include
> a moment at the top of the tower that is not present when the antenna is
> mounted at the apex., where the wind loading presents only a horizontal
> load
> on the top of the tower with very little moment. The implications of this
> could be considerable, particularly when considering the resistance to
> buckling presented by a tower structure with a vertical load and a moment
> at the top. The inherent stability of any vertical column is greatly
> reduced when the vertical loading is compounded by a moment at the top. At
> the very least I would be very wary of using the antenna area allowables
> in
> the Rohn catalog for such a design and would expect the allowable to be
> significantly less. How much less would be a subject for a structural
> engineer.
>
> As a practical matter many hams do this, including me. But the choice is
> to
> make the antenna much smaller than the limit stated in the catalog, in my
> case with a 45G tower.
>
> 73
> Bill W7VP.
> ----- Original Message -----
> From: <NPAlex@aol.com>
> To: <towertalk@contesting.com>; <k6xn@comcast.net>
> Sent: Saturday, March 25, 2006 7:33 PM
> Subject: [TowerTalk] Mast wind loading
>
>
>> In a message dated 3/25/2006 7:16:21 PM Eastern Standard Time,
>> towertalk-request@contesting.com writes:
>> If for example the mast section extending above the Rohn 25G tower to be
>> used is 2 inches in diameter and 10 feet long what is the value we
>> should
>> use for calculating the additional wind load "area" due to the 10 foot
>> mast?
>> Is it simply 2 times 10 times 12 for 240 square inches for 1.6 additional
>> square feet or ??? Moving forward would the total effective wind load
>> for
>> the ensemble to be used in checking the Rohn 25G "Allowable Antenna Area"
>> then be the wind load for the antennas plus the wind load for the mast?
>>
>> This is probably obvious to everyone but I wanted to double check before
>> I
>> put it up an additional tower. :-).
>> ---------------------------------------
>> You can apply the reduced drag factor for round surface of 2/3 in your
>> calculations. Don't forget the pipe below the top adds area as well.
>>
>> Regards,
>> Norm W4QN
>> _______________________________________________
>>
>>
>>
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