[TowerTalk] Mixing Rohn 20G and 25G
Linux Mercedes
linuxmercedes at gmail.com
Thu Jul 3 13:28:50 EDT 2014
On Wed, Jul 2, 2014 at 10:26 PM, Roger (K8RI) on TT <
K8RI-on-TowerTalk at tm.net> wrote:
> 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.
In that case, I am not too worried about guying. I don't want to bracket
the tower (I am renting so I'm putting the tower somewhere far away from
buildings so it can't fall on them), but I will be borrowing your dirt base
design with some modifications.
>
>
>
>> 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,
>
Wait, so 3/16" EHS is heavier than 1/4" wire rope? I do want to avoid
permanent tower damage as I'd like to be able to take down and sell the
tower when I'm done with it.
>
> 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.
>
I'd love to use Phillystran but it is incredibly expensive compared to
3/16" EHS.
>
> 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.
>
That makes a lot of sense. The leg braces on 20G are thinner, too. If I use
it at all, it'll be at the very top of the tower.
>
> 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).
Fortunately I'm young and stupid so I'm not too worried about tower
climbing. I do need to pick up a guy bracket for the middle bay; I was
debating making my own since Rohn wants something outrageous for theirs,
but I'll probably pick up one from Norm's Fab as they're a bit cheaper and
I don't have to spend the time designing it myself. Fortunately, the tower
I was given came with a flat top that has loops for guy wires built into
it.
>
>
> 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)
>
Neat. 20G will collapse before the 3/16" EHS guys you'd use on it will
break. I guess that is why it's not recommended for much!
>
> 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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>>>>>>>
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>>>>>>
>>>>>>
>>>>>>
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>>>>>
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>>>>>
>>>>>
>>>>>
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>
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