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@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@gmail.com
CC: towertalk@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@kkbc.com -- 866-22-RADIO -- 866-227-2346
www.amgroundsystems.com
On Mon, Jun 30, 2014 at 7:42 PM, Linux Mercedes
<linuxmercedes@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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