My standard guying for Rohn 25 under 100' towers is 1/4" EHS. I even suggest a
spread mount and 6 guys for anti-torque at the top.
Increasing the guying does wonders for increasing the allowable antenna wind
load if you increase the concrete for the anchors.
If you look at the types of antennas that most commercial tower users install,
you will find the smaller towers - Rohn 25, 45, and 55 - are generally
supporting one or two antennas in a business application. So this is what the
Rohn catalog has chosen to show to allow the largest group of potential
customers to select what will probably work for them. The standardized designs
are also specified to be the most economical for the specified wind speed
antenna height and antenna load.
The typical ham application has about 6 antenna and fourteen feedlines and
wires hanging from it, and some of the antennas have 20 - 30 foot booms on
them. Long booms are not generally found in the commercial world, unless it is
HF or military. This is a significantly different application than getting a
small antenna up high as cheaply as possible. Different design requirements
direct you to take different approaches. Lots of antennas point me to shorter
towers and heavier guys on those towers.
I would never think of using rohn 25 in 190 foot ham installation. It is a
matter of finding a good balance.
There are only about 500 ways to guy a given tower at a given height. Rohn has
chosen to show one that fits the larger customer base, unfortunately the chosen
loading is not what a typical ham wants to put up.
Tower engineering is not that complicated, you just need to do a little
research and thinking about it.
Steven H Sawyers PE (n0yvy)
ARRL Volunteer Consulting Engineer and Life Member
At 05:13 PM 8/25/99 +0000, alsopb wrote:
>
>Steven,
>
>Lets look at the 190' tower you quote.
>A 190 footer has a guy load of about 4000 pounds
>in no wind. Using the 9800 pounds you quote from
>Rohn the wind/no wind is 2.45
>
>Now follow the usual thinking. A 190 foot tower
>is a big tower. It really needs stronger guys
>than Rohn uses. I want a strong tower. Thus guys
>tensioned to 1200 pounds are installed replacing
>the nominal Rohn guys. The no wind load for these
>would be about 12000 pounds. The load with wind
>would be about 30000 pounds. This exceeds the
>25350 pounds the three legs can carry.
>
>Bottom line: If you are not going to do what the
>manufacturer says, the you better be prepared look
>at the numbers and determine things are OK. This
>is consistent with Steve's advice to always do
>what the manufacturer says.
>
>Your conclusion "for short towers" is probably
>valid but again, check or have some PE check the
>numbers.
>
>73 de Brian/K3KO
>
>Steven H Sawyers (n0yvy) wrote:
>>
>> The Rohn book says 25G will handle 8,450 pounds on each leg. I am assuming
>> this means properly
>guyed with the guy spacing under 30'.
>>
>> The 90 MPH chart for a 190 foot tower shows a base reaction of 9,870 pounds.
>> This is the base
>load under full wind load. A 190 foot tower has 6
>guy levels for a total of 18 guys 3/16 dia.
>>
>> The section is designed to take compressive loads and appears to do a pretty
>> good job of it. So over
>sized guying on shorter towers (under 100 feet)
>should not be a problem.
>>
>> de n0yvy steve
>>
>> At 09:15 PM 8/24/99 +0000, alsopb wrote:
>> >
>> >By the way, these 6 guys put a downward force of
>> >1500# on the tower.
>> >Those considering heavier guys (hence higher guy
>> >tensions) ought to consider the impact. Doubling
>> >guy tension will double downward force.
>> >Do you really want to do that?
>> >
>> >73 de Brian/K3KO
>> >
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