[Towertalk] Tower Buidling 201
n4kg@juno.com
n4kg@juno.com
Tue, 3 Sep 2002 22:17:08 -0600
QSL Roger. I believe the REAL reason Rohn was able to specify
CLOSER SPACING for the Tiltover Tower Guys is the fact that Tiltovers
were limited to 68 ft MAXIMUM Height and modest antenna weights
which also translates to modest windloads.
The 80% guying distance recommendation is a *compromise* between
COST, LOAD CAPACITY, and SPACE REQUIREMENTS at the MAXIMUM RATED HEIGHT
for the tower (200 ft for R25, 300 ft for R45). Lower
heights and / or lighter loads can be safely handled at closer spacings.
This requires a thorough ANALYSIS to verify that the load ratings of
the Tower and GUYS are not exceeded.
As you noted, 4 way guying was needed for TILTOVERS to provide
clearance when tilted. To my mind, the additional 15% guy load reduction
going from 3 to 4 guy directions is not terribly significant compared to
the other issues addressed above.
Tom N4KG
On Tue, 3 Sep 2002 "Roger Borowski" <K9RB@bellsouth.net> writes:
> Tom, I don't doubt the calculations that you and Hank have provided
> on 3 vs. 4
> guys. I only base my information on the reason that a Rohn engineer
> in Peoria
> had told me some years back when I inquired if the specs for guying
> the fold
> over tower I had purchased were correct and considerably closer in
> than their
> specs for conventional 3 way guying of the standard tower. Rohn
> seemed to think
> that 4 guys provided enough of an increased safety factor to move in
> the
> minimum guy spacing specification. This has been interpreted
> incorrectly by
> many who have used this spec. for their 3 way guyed, conventional
> towers.
> I have seen, as I'm sure you all have, many installations that have
> steeper
> guying than even this, and they still stay up. Most installations
> are done that
> way due to inadequate property to do it right, or a complete lack of
> knowledge
> of the guying specifications. Is it OK to do this? Its done every
> day. Will it
> stay up? Its impossible to know when a severe enough wind will cause
> it to
> fail, maybe never....Is it done according to the manufacturers
> specifications?
> Definitely not. That, and that alone, is my only point. Debating 3
> vs. 4 way
> guying is not a tread I cared to start once more. I'm not even a
> proponent of 4
> way guying, but it is an absolute requirement on the Rohn 25 and 45
> fold over
> series due to the required clearance for the backbone strut when
> folded over.
> BTW, the 4 way guying of my fold over is nowhere near the minimum
> distances
> provided in the specs. for this tower, but considerably greater.
> 73 all, -=Rog-K9RB=-
>
>
> ----- Original Message -----
> From: <n4kg@juno.com>
> To: <TOWERTALK@CONTESTING.COM>; <K9RB@arrl.net>
> Sent: Tuesday, September 03, 2002 10:07 AM
> Subject: Re: [Towertalk] Tower Buidling 201
>
>
> > Roger,
> >
> > I politely suggest you do the CALCULATIONS to determine
> > EXACTLY how much benefit there is in 4 guys vs 3 guys.
> > At 5 degrees off line, the IN-LINE wire carries 99.6% of the
> > load. NOT much help from the side wires IMHO). (I assume
> > you do know how to resolve a force vector into orthogonal
> > equivalents as it is rotated).
> >
> > Further, I suggest you do the calulations for a 70 ft tower
> > guyed 3 ways at 50% of the tower height. I'm sure you will
> > find that the loads placed on the tower are WELL within
> > the R25 ratings and the guy wire ratings assuming a
> > nominal antenna load under 12 sq ft.
> >
> > There are many thousands of towers that have withstood the
> > rigors of weather for many decades, all using 3 WAY guying,
> > testifying to the soundness of this approach. If 4 way guying
> > was so much better, then why don't ANY of the BIG tower
> > designers use it? I expect that if all the guys in ONE direction
> > were removed from a guyed tower, the tower would still FALL
> > DOWN, whether it was guyed in 3 or 4 directions.
> >
> > BTW, Percentages of wind direction is MEANINGLESS.
> > Any guying system MUST be capable of supporting the
> > tower 100% of the time. RUN the NUMBERS. It's the
> > RATINGS that count, not the number of directions the
> > tower is guyed.
> >
> > Tom N4KG
> >
> >
> > On Mon, 2 Sep 2002 Roger Borowski" <K9RB@bellsouth.net> writes:
> > > With 4 guys at 90 degrees apart there are only 4 degrees out of
> 360
> > > degrees
> > > when one guy is holding the whole system up. With a 3 guy system
> at
> > > 120 degrees
> > > apart, there are 90 degrees out of 360 degrees when one guy has
> all
> > > the tension
> > > to support the whole system. This is the engineering reason for
> > > closer in
> > > spacing with a four guy system. The three guy system has each
> guy
> > > individually
> > > capable of providing for supporting the entire system 25% of the
> > > time the wind
> > > is blowing whereas the four guy system has each individual guy
> > > providing for
> > > support of the entire system less than 1% of the possible wind
> > > directions. All
> > > other than these 4 degrees out of 360 degrees there are two guys
> > > doing the wind
> > > load support work. This was quite an extensive tread about 4-5
> years
> > > back and I
> > > guess some non-believers then still think its OK to use Rohn's
> > > guying standard
> > > for the fold over series of towers, which needed four guys to
> clear
> > > the
> > > backbone strut, on a conventional system using three guys. NOT!
> > > -=Rog-K9RB=-
> > >
> > > ----- Original Message -----
> > > From: <n4kg@juno.com>
> > > To: <TOWERTALK@contesting.com>; <K9RB@arrl.net>
> > > Sent: Monday, September 02, 2002 5:26 PM
> > > Subject: Re: [Towertalk] Tower Buidling 201
> > >
> > >
> > > > Do you consider 15% to be a SIGNIFICANT increase in strength?
> > > > I don't.
> > > >
> > > > Worst case for a 3 way guy arrangement is at 30 degrees
> > > > to any one of the guys. Force in the guy is equal to the
> > > > in line force divided by COS 30 degrees (= 0.866) = 1.155
> > > > or 15.5% more tenstion that an inline force which is the worst
> > > > case for 4 way guying.
> > > >
> > > > The old 80% guying guideline applied to the FULL rated height
> > > > of the towers (200 ft for R25). At 70 ft or less, the tower
> can
> > > take
> > > > more of the load from close in guys since it does NOT have to
> > > > support the weight, guy tension, and resolved wind load
> > > > of a tower extended to the full rated height.
> > > >
> > > > Tom N4KG
> > > >
> > > > On Mon, 2 Sep 2002 08:53:46 -0400 "Roger Borowski"
> > > <K9RB@bellsouth.net>
> > > > writes:
> > > > >
> > > > > ----- Original Message -----
> > > > > From: <n4kg@juno.com>
> > > > > To: <TOWERTALK@contesting.com>
> > > > > Sent: Monday, September 02, 2002 3:01 AM
> > > > > Subject: Re: [Towertalk] Tower Buidling 201
> > > > >
> > > > >
> > > > > >
> > > > > > Note that for tower heights under 70 ft, Rohn shows a 35
> ft
> > > (50%)
> > > > > > guying radius for their (discontinued) fold over tower
> > > > > intallations.
> > > > > >
> > > > > > Tom N4KG
> > > > > >
> > > > > That is because the fold over series must be guyed with FOUR
> > > guys, 90 degrees
> > > > > apart, which is a significantly stronger method of guying
> vs.
> > > three > > guys at 120
> > > > > degrees apart. There was a tread about this about 4-5 years
> ago.
> > > > > 73, -=Rog-K9RB=-
> > > >
> >
> > ________________________________________________________________
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