Dennis,
Use the concrete anchors that are recommended by Rohn for your tower and
wind exposure. The screw in anchors can't handle the load rating of the
recommended guys.
I have been working on FEA models of the towers I plan on putting up here
and can say that the cantilevered top section is NOT the way to go for most
ham installations. We usually want to put a lot of stuff above the top of
the tower on a long mast. This can generate some huge bending moments in
the top tower section. The tower bending loads, with the guys at the top
are about 1/2 of what they are when the guys are 5' below the top. This is
just ballparking from my memory. But the difference is tremendous.
Additionally, the attachment of the top stub to the tower is not
particularly suited for high bending loads. I know of several stations that
had problems in this area. The problem is not that Rohn didn't design and
build the tapered section right. The problem is that we (hams) put more
stuff on the mast, at a higher distance above the tower top than Rohn
designed for. Rohn wants us to put the entire wind load at the top of the
tower not on a long mast above the top of the tower.
What happens in the amateur scenario is the loads of all the antennas bend
the mast significantly. The bent mast fetches up in Rohn top section stub
and transfers all of the bending load into the rather compact structure
that holds the stub in the tower. When enough bending load occurs, pieces
of the stub attachment deform or crack, or break.
Experience tells us that many installations have worked with the tapered
top section. I do also know that if the wind is blowing, you may need a
very large rotator to turn your fetched up mast in the top stub.
I prefer the flat top section with a pillow block bearing. It allows the
mast bend to occur without inducing any moment into the top of the tower.
The tower reactions that counteract the mast bending loads are purely
horizontal and occur at the top and at the rotor.
What defines what will work for your installation is how many square tons
of gain will be how far above the top of the tower.
73, Kurt
YagiStress - The Ultimate Mechanical Design Software for Yagi's
Visit - http://freeyellow.com/members3/yagistress
----------
> From: Dennis M Allen <n9tzl@juno.com>
> To: towertalk@contesting.com
> Subject: [TowerTalk] Tower Questions
To: <towertalk@contesting.com>
> Date: Tuesday, May 12, 1998 7:07 AM
>
> Hello to All:
>
> I am planning to put up 80-90 feet of Rohn 25 this summer....if all goes
> well! This will be used for multiple VHF and UHF antennas, sometimes
> multiples. So I will eventually use up the full square footage limit
> recommend for this tower at this height. I will use three sets of guys
> set out at approximately the 80% distance of height and use the proper
> cement base. At present my thoughts are to end up with at least 15ft. of
> mast for mounting antennas.
> The location is Northern Illinois...about 15 miles south of the Illnois/
> Wisconsin border. I am pretty sure the underneath soil is clay.
>
> There are some things I need to make decisions on before this can happen.
>
> 1. Whether to use screw in anchors or regular cement anchors? If screw
> anchors are used I would think they should be at least 6ft. long?
> Sources?
> 2. What to use for a top? I do have a 25AG3 but at present I do not
> understand the purpose/function of the pointy tops?
> 3. Should I use a thrust bearing?
> 4. Why does Rohn always place the top guys 5ft. down from the top? Sure
> seems like it would be better to have them closer to the top??
>
> If anybody can share some wisdom on the above questions it would surely
> be appreciated.
>
> Thanks,
> 73, Dennis N9TZL
> EN52ri
>
>
>
> _____________________________________________________________________
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