Stan,
Perhaps the difference can be attributed to the weight of that portion of the
tower from the hinge
to the apex. I did a quickie calculation using 60 lbs per 10' section (I'm not
near my Rohn books)
and came up with 3675 ft-lbs for the tower part. Add that to the 135 lbs at
35' from the hinge
(4725 ft-lbs) and you get 8390 ft-lbs as the estimate. Not too far off.
Bob, W9UI.
-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of Stan or Patricia
Griffiths
Sent: Tuesday, January 16, 2001 6:00 AM
To: K7LXC@aol.com; towertalk@contesting.com
Subject: Re: [TowerTalk] Rohn45G tiltover Antenna capacity
K7LXC@aol.com wrote:
> In a message dated 1/15/01 1:24:32 PM Pacific Standard Time, brewer@flash.net
> writes:
>
> > I have the following that I want to put on my "new-to-me" 64 foot Rohn 45g
> > tiltover: 2 element Cushcraft 40 meter beam, TH3-Mk 4 tri band beam, 5
> > element 6 meter beam, and 13 element 2 meter beam. All of this on a 15
> foot
> > mast and turned with a TX2 rotor. Does this sound unreasonable?
>
> According to my current Rohn catalog, it'll take 17.2 square feet of load
> and a total weight of 135 pounds at 30 PSF wind pressure (86.6 MPH). It looks
> like you're below the max. How come this info isn't in your Rohn catalog?
> (They're available from Champion Radio Products - <A
> HREF="www.championradio.com">www.championradio.com</A> )
>
> A problem is that Rohn doesn't say where the the load is relative to the
> top of the tower. Is it 1-foot or what? By adding ten-feet of mast sticking
> out from the top of the tower with antennas up it, you're increasing the
> length of the lever arm and the resultant forces. I'm not an engineer but I
> suspect the forces would be significant.
Rohn USED to say where the load was to be located. I looked it up in my 1977
Rohn Catalog and it does indeed say 17.2 square feet and 135 pounds but it says
"safe load at the apex". I doubt that has changed since 1977 . . .
I don't think dead weight will be a problem. Rather, the problem will come
during the crank-up process when maximum moment is on the hinge. This occurs
when the top portion of the tower is in the horizontal position. My 1977 Rohn
Catalog says the maximum safe moment at the hinge is 8240 ft-lb. It also says
the distance from the hinge to the apex is 35 ft. If you divide the allowable
safe moment of 8240 ft-lb by the distance from the apex to the hinge, I think
you
should get the allowable apex load. When I divide those numbers, I get 235 lb .
. . not 135 lb so I guess I don't fully understand what I am reading. Maybe
this
is part of the "safety factor" Rohn has cranked into these specs.
Anyway, if you are going to distribute the load above and below the apex, you
need to calculate the contribution that each piece makes to the total moment at
the hinge and then add them together to see if it exceeds the safe moment of
8240
ft-lb.
As an example, lets calculate the moment contribution of the mast only. I
assume
about 13 feet of your 15 foot mast will be sticking out the top of your tower.
This means the center of your mast will be about 40.5 feet (35 ft from hinge to
apex plus 5.5 ft above the apex to get to the center of the mast) above the
hinge. I will further assume it is a fairly light mast of 50 lb (I think this
is
way too light for the antenna loads you are talking about, but that is another
engineering problem . . .) So . . . 40.5 ft times 50 lb equals 2025 ft-lb of
hinge moment contributed by the mast alone.
You did not say where you planned to place the antennas on the mast. I will
assume the 2 el 40 goes on top and weighs 30 lb. It will be located 48 ft above
the apex and contribute 30 lb times 48 ft equals 1440 ft-lb. The total from
just
the 40 meter beam and mast is now up to 3465 ft-lb, and that assumes they are
really as light as my guesses. You still have to add in the contributions from
the rotator, feedlines, control lines, tribander, 6 meter beam, and 2 meter
beam. When you add it all up, I think you may be over Rohn's recommendations .
.
.
It would certianly be worth the trouble to draw this all out on paper and take
your time doing the calculations.
Stan
w7ni@teleport.com
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