[TowerTalk] Mixing Rohn 20G and 25G

Roger (K8RI) on TT K8RI-on-TowerTalk at tm.net
Wed Jul 2 23:26:50 EDT 2014


On 7/2/2014 8:38 PM, Linux Mercedes wrote:
> Is there a significant difference between guying and bracketing a 
> tower? I'd expect them to behave about the same, but maybe house 
> bracketing is more rigid since guys are springy.

There are too many variables to say for sure, but if you have ever been 
on the roof of a single story home, next to a large chimney, you will be 
amazed at how much the house roof and walls move. On a house out in the 
open you will see the roof move a couple of inches in a 30-40 mph wind.  
That was a well built home.  Many brackets only guy in one axis and they 
usually guy around 12 feet.  The wall holding the bracket should be 
reinforced, but seldom is.  Typically, by themselves, the wall will not 
stand against much wind until the house is completely enclosed.  I've 
seen a number of them fold up when the frame and roof were completed.  
There was even a video on the net recently.. IIRC they are not insured 
while under construction.

Last photo on http://www.rogerhalstead.com/ham_files/Dirt_Base.htm is a 
house btacket that attaches to a gable end consisting of 2 X 6s.

At best the bracket only raises the base about 12 feet. Above that the 
tower is free standing.  So you only gain what ever height above ground, 
the bracket is located.  Brackets located on an wall not reinforced 
doesn't add much stability.

>
> Alternatively, I hear that for short (~100 foot or less) 25G towers, 
> 1/4" EHS can be used instead of 3/16" EHS for a more rigid tower 
> without putting too much weight on the legs. If flex is an issue, 
> maybe that's the way to go? 

For me, EHS is way to heavy and larger is much more so.  Figure the 
weight for two or three level EHS guying.
I replace the 3/16" guys with 1/4" wire rope (which is not as heavy as 
1/4" EHS) and the extra weight and tension caused the bottom two 
sections to "bell" out between the two bolts in each leg, requiring a 
jack to get them apart.  The stronger and heavier guys require more 
tension which translates to more down force,

I much prefer Phillystran which is far lighter and I've never, in 13 
years noted any stretch problems and I've bee "up there" in winds far 
higher than I should have been.  I figure 20G is about half the rating 
of 25G while others figure 75%.  To me the lighter construction with 
wider spacing on smaller braces along with thinner tubing for lega makes 
a substantial.

The legs of a 25 G by themselves are not strong.(25G=16ga, 20G=18ga)  
They depend on the braces for strength.I much prefer to err on the safe 
side.  I figure, by the time you get 20G engineered to satisfy the 
insurance company, or codes if applicable, you could have purchased and 
installed a 45G.

Had I known I wouldn't be able to continuing to climb through my 70s, 
I'd have found a way to install a heavy duty 100' crank up instead of 
the 100' 45G.  To me, going up needs to be safe, with a good safety 
margin.  I use guy brackets and do not tie guys to tower legs so the 
whole tower receives the sideways/lateral force, not just one leg. If 
you do not use a pier pin base, wing transfers a lateral force to the 
back side (side away from wind)  and an upward force to the front side 
(side on which the wind impinges).

> Although if leg weight loading is a concern, I guess I should work out 
> how much downforce a 20G section can support, since it'll be less than 
> 25G.
>

18Ga Vs 16ga  (3,900# for 18ga 20G.  I don't have a figure for 25G)

73

Roger (K8RI)


> --Nathan
>
> On 07/01/2014 10:32 PM, Wayne Kline wrote:
>> Two levels of guy is what my concern would be. I am in as now and be 
>> home in PA next week.
>> I have a library or Rohn catalogs and in the older ones I have the 
>> full spec on r 25 -G.
>> But if my gray matter serves me well  R 25- Gg is not suitable for 
>> guying let alone two levels .
>> And this was the old spec. So with out a house bracket your walking 
>> on thin ice.
>>
>> YMMV
>>
>> Wayne W3EA
>>
>> Sent from my iPad
>>
>> On Jul 1, 2014, at 6:24 PM, "Linux Mercedes" 
>> <linuxmercedes at gmail.com> wrote:
>>
>>> Wayne,
>>>
>>> I'll primarily be using it for wireless internet. Unfortunately, the 
>>> tech at the WISP I spoke to was pretty vague about the wind load of 
>>> the antenna they'll be using, but he assured me it would be 
>>> 'minimal' and didn't think it would be that big of a concern.
>>>
>>> I won't be house bracketing this install, but it will be guyed at 
>>> two levels (what Rohn recommends for 50' of 25G).
>>>
>>> --Nathan
>>>
>>> On 07/01/2014 09:59 AM, Wayne Kline wrote:
>>>> I also have experience with 20-G and 25-G towers... @ my station my 
>>>> 2mter/450 MHZ  and one 6 meter beam
>>>>   5 element 14' boom  are on at 36ft. an consist of  3 sections of 
>>>> 20-G and a top of 25-G  AG3 ?   it's house bracketed @ 18 ft.
>>>> Been in place for 24 years  BUT  extremely light wind load.
>>>>
>>>> IMHO  @ HF  a C-3. TA33  or A3 would be the BIGGEST  .
>>>>
>>>> So my question to you is WHAT are you planning to adorn the tower 
>>>> with ?
>>>>
>>>> Wayne W3EA   . .
>>>>
>>>>
>>>>> Date: Tue, 1 Jul 2014 07:42:14 -0500
>>>>> From: kkbroadcastengineering at gmail.com
>>>>> CC: towertalk at contesting.com
>>>>> Subject: Re: [TowerTalk] Mixing Rohn 20G and 25G
>>>>>
>>>>> Nathan,
>>>>>
>>>>> I would caution you to not mix the sections nor even use the 20 for
>>>>> anything more than VERY light duty use.
>>>>>
>>>>> With that said...  Back in my younger days when I did a lot of 
>>>>> tower work
>>>>> for CB and Ham operators, there were a number of mixed 20/25 towers
>>>>> supporting large CB antennas and they stayed up many years.  I do 
>>>>> recall
>>>>> working on one where the 20 top section physically broke during a 
>>>>> wind
>>>>> storm.
>>>>>
>>>>> Climbing a mixed 20/25 tower is a real pain as the steps are 
>>>>> different
>>>>> distances apart.
>>>>>
>>>>> Good luck,
>>>>>
>>>>> Kevin C. Kidd, CSRE/AMD
>>>>> AM Ground Systems Company  -  WD4RAT
>>>>> kkidd at kkbc.com  --  866-22-RADIO -- 866-227-2346
>>>>> www.amgroundsystems.com
>>>>>
>>>>>
>>>>> On Mon, Jun 30, 2014 at 7:42 PM, Linux Mercedes 
>>>>> <linuxmercedes at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> Whilst moving tower sections from the shed to the shop so I could 
>>>>>> clean
>>>>>> them up and make a couple of fixes before putting up my tower, I 
>>>>>> noticed
>>>>>> that some of them seemed lighter than others. Turns out I've got 
>>>>>> a mix of
>>>>>> 20G and 25G sections.
>>>>>>
>>>>>> What are your thoughts on putting up, say, 30 or 40 feet of 25G 
>>>>>> and then
>>>>>> another 20 or 10 feet of 20G? I'd like to use the best sections I 
>>>>>> have on
>>>>>> the tower, but if mixing is a no, then I'll fix up some of the other
>>>>>> sections I have and build a homogeneous tower.
>>>>>>
>>>>>> Thanks,
>>>>>> Nathan
>>>>>> _______________________________________________
>>>>>>
>>>>>>
>>>>>>
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