Hi Jim,
Actually, there are at least 3 moments to be concerned about.
The bending moment on the mast where the mast enters the tower.
The bending moment applied to the top and bottom tower mast
supports.
The bending moment applied to the tower at the top guy,
which is also coupled throughout the rest of the tower.
With 18 ft out of the top, you need to calculate all of these
moments and make sure none of the ratings (mast and tower)
are exceeded. The more mast inside the tower, the better.
You should seriously consider placing the top guys at the
very top of your tower. If you plan to have large antennas,
it would be wise to add another set of guys at or just below
the bottom mast support which is probably your rotor.
de Tom N4KG
On Thu, 23 Dec 1999 07:40:20 -0600 froemke@attglobal.net writes:
> Tom, very interesting note.
>
> Can you advise me what difference it makes by having a long mast
> (27'
> total) 18' above the tower extend down into the tower the height of
> the
> top section (about 9') with the rotor then mounted between the top
> and
> next lower section?
> It would seem that the tower bending moment (from several antennas
> on
> the mast) would be almost completely moved down to the base of the
> mast
> in this case. Then, having a set of guys 4-5' above the bottom of
> the
> mast should solve the problem. (Yes, the mast may be the first to
> go.)
>
> Comments?
>
> Thanks,
>
> Jim, K0MHC
>
> n4kg@juno.com wrote:
> >
> > The old Rohn catalogs only gave load ratings based on the
> > full rated height of each tower type (200 ft for R25, 300 ft for
> R45).
> > No deratings were called out for lower heights. All guying
> > illustrations showed guys equally spaced along the full height
> > of the tower (i.e. top guy attached to the top of the tower).
> >
> > The new Rohn catalog (mine is dated 1994) shows increasing
> > loads for lower heights. This is a welcome clarification but the
> > examples show the top guys for R25 placed 5 ft down from the
> > top of the tower and the top guys for R45 placed 9 ft down from
> > the top of the tower.
> >
> > The ratings are appropriate only if ALL of the antenna load is
> > placed along the open tower above the top guy. If you extend
> > a mast above the top of the tower, the bending moment applied
> > to the tower can become excessive.
> >
> > To minimize the bending moment applied to the tower, the
> > largest antenna should be placed as close to the guys as
> > possible. With the guys 9 ft down from the top, the only way
> > minimize the bending moment and rotate the antenna is to
> > use an expensive ring rotor or side mount just above the
> > guy wires. If the large antenna is placed one foot above
> > the top of the tower the bending moment will be 10 (ft) X
> > the antenna windload vs 1 or 2 (ft) times the windload if
> > the guys are placed at the top of the tower.
> >
> > The bending moment for an antenna at the top of a 12 ft mast
> > will be (12 + 9) X the antenna windload vs. (9 + 1 or 2) X the
> > antenna windload if the guys are placed at the top of the
> > tower.
> >
> > Example: Consider two antennas with 8 sq ft of windload
> > placed 1 ft above the top of the tower and 12 ft above the tower
> > top with a wind force of 30 psi (87 mph). The windload forces
> > will be 240 lbs at each antenna.
> >
> > With the top guy 9 ft below the top of the tower, the bending
> > moment is (9+1) X 240 + (9 + 12) X 240 = 7440 ft lbs.
> >
> > With the top guy 1 ft below the top of the tower, the bending
> > moment is (1+1) X 240 + (1 + 12) X 240 = 3600 ft lbs.
> >
> > The bending moment contribution of the mast and tower
> > was not included in these calculations.
> >
> > Notice that the guying configuration shown in the new Rohn
> > catalog results in more than twice the bending moment
> > applied to the tower for these moderate sized antennas
> > (similar to TH6 for example). Full size 3L40's or 2L80's
> > will be much worse.
> >
> > When using a rotating mast to support more than one
> > antenna above the top of a tower the top guys should
> > be placed as close to the top of the tower as possible.
> >
> > This is the ONLY way to safely realize the full capacity
> > of a guyed tower with multiple antennas. The amateur
> > community needs to encourage Rohn to provide load
> > ratings for their guyed R25 and R45 towers with the
> > guys placed at the top plate or first step down from
> > the top.
> >
> > You may ask, "Is the bending moment at the top of
> > a guyed tower a real consideration?"
> >
> > I'm glad you asked. The answer is a resounding YES!
> >
> > One of locals had a Force 12 6L20 (8.3 sq ft) mounted
> > above a Force 12 2L40 / 2L80 (13 sq. ft.) on 130 ft of
> > Rohn 45 with the top guy 9 ft below the top of the tower.
> > The rotor was approximately 5 ft down from the top of the
> > tower.
> >
> > A storm ripped the top plate off the top section causing
> > the mast and antennas to plummet to the ground. He
> > suspects a cold weld may have contributed to the
> > accident but the above calculations show that the load
> > born by the tower is considerably higher when the guys
> > are placed below the top of the tower.
> >
> > de Tom N4KG
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