>I've just completed the transit work for a Rohn 55 rotating tower. I want
>to start pouring concrete for the base and anchors very soon. I have a
>couple of unique situations inherent to my property and wonder if anyone
>could offer me some advice. i.e. has anyone "Been there, done that."?
>
>
>First issue:
>
>I'd like to use a regular 55G straight section as the base. The bottom of
>the legs would extend below the level of the concrete and into some gravel
>for drainage. However, the area where I MUST locate the tower is relatively
>low with a high water table. Bedrock is probably at about seven to eight
>feet beneath the surface so there is stable support down there.
>
>I am concerned that water may enter the legs from beneath, seek its own
>level in the legs, freeze inside the legs in the winter, and do structural
>damage to the concrete base or legs themselves.
I agree that you should not use a tower section or short tower base buried
in concrete if you think water can get into it from below and rise to the
freezing level.
>The alternative to the would be to utilized the Rohn base plate for concrete
>(BPC-55G), a flat plate with stubs to mount the first section on and a
>central pier pin.
A flat base with a pier pin means tha that the flat base will not take any
of the tower torque. The base is free to rotate on the pier pin. Some
people have used the flat base with "J" bolts into the concrete (you have to
drill holes in the base plate for the "J" bolts) and not used a pier pin.
This makes the section about as sturdy and solid as an embedded base but it
won't rotate at all so it will have to take the tower torque transmitted to
the base.
This makes the above situation a non-issue, but I loose
>the solid support an embedded section would afford. Temporary guying would
>be more critical during erection, AND, I wonder about the stresses tower
>rotation or wind torque would impose on the stubs of the BPC-55G with the
>rotating tower scenario.
Hmmm . . . You did not say it was ROTATING tower until now. I assume it
rotates above some point like the first set of guy wires. Are you using guy
rings? How many?
Anyway, I think the best way to absorb torque in a tower is to use the
torque arm stabilizer assemblies (TA55). In your case, I would install one
right at the first set of guys where (I assume) the tower rotating bearing
and rotator motor will be. This will remove most, if not all, the torque
from the tower base.
This means your will need six guy wires at this point but it is certainly
the best way to go.
>Second issue:
>
>This also relates to the terrain where I have to put up the tower. I plan a
>tower height of approximately 180 feet. My guy points will be 220 feet from
>the tower's center. This will give me additional room to swing the beams
>under the guy lines, but are there any precautions I need to take when
>guying this far from the tower base? I cannot move the guy points in closer
>because of the terrain.
>
>Should the tensioning be adjusted differently since forces on the tower will
>be different than if guyed at the conventional 80% of tower height distance?
Different from WHAT? I have never gotten the feeling that there was a
general consensus on how tight guy wires should be in the first place. Rohn
says tighen to 10 percent of the breaking strength. But I think that
assumes you are using the exact type of guy wire they recommend in their
drawings. What if you deviate from their drawings (like you are doing by
moving your guy points out farther)? Who knows? What if you use larger guy
wire like some people I know? If you still tighten to 10 percent of
breaking strength, it might be too tight.
In an extreme case, if you used 1 inch guy wire on a Rohn 25 and tightened
to 10 percent, you would surely pull the tower down. So what are you
supposed to do if you use 5/16 wire where Rohn specifies 3/16? Should you
tighten to 10 percent of the breaking strength of 3/16? I would guess yes,
but it is only a guess.
>
>I'd appreciate comments on the above. You can e-mail me directly.
Yes, I could have emailed you directly but I think your situation deserves
some more exposure so we call all learn a little more.
>Thanks,
>
>Ralph - K0IR
>rfedor@cloudnet.com
Your welcome,
Stan w7ni@teleport.com
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