I beams should be easy to obtain in most larger cities. If the top is cut on
an angle you should be able to connect a turnbuckle directly to the center
web without have to weld a bracket. K5IU, who designed the Rotating Tower
System, apparently did a presentation on elevated tower anchors. I am not
sure if it exists on the interweb but below is a summary of it written by
W7XU:
GL, John KK9A
The following information was from a talk K5IU (a mechanical
engineer and owner of Rotating Tower Systems) gave at (I
believe) the New Orleans DX convention a few years back:
For posts 7-8 ft above ground:
Installation Width and Ht. Length
Small 2 ft 5 ft
Med 3 9
Big 4 10
The above dimensions are for the concrete bunker. The
post sits in the middle of the bunker. The top of the
bunker is 1 ft below ground level. (Yes, that's right,
almost 6 yards of concrete per anchor for a large
installation).
Post size:
Round post, with guys attaching at 7-8 ft above ground
Installation Post diameter Wall thickness
Small 5" or larger 0.25" or more
Med 7" or larger 0.38" or more
Big 9" or larger 0.50" or more
Wide flange I-beams, with guys at 7-8 ft above ground
Installation Wide flange I-beam size, A36 steel
Small W 6x12 (6" X 4" - 12 lbs/ft)
Med W 8x15 (8" X 4" - 15 lbs/ft)
Big W 10x33 (10" X 8" - 33 lbs/ft)
What is a medium or big installation? I have a rotating
Rohn 25 tower approximately 135 ft high. It has eight
17-el 2 meter antennas on it, stacked one above the
other from 30 to 135 ft. The windload of each antenna
is 2.7 sq. ft., plus phasing lines, etc. That qualifies
as a medium size installation (per K5IU).
I have another rotating tower, 140 ft high. Rohn 55.
It's designed to hold a 10 ft dia. dish, a 6 ft. dia.
dish, a 4 ft dia. dish, 8 long boom (~22 ft) yagis on 432,
4 long boom (~28 ft) yagis on 222, and a couple of
vertical beams for 2m FM and 440 FM. That is a big
installation.
I have elevated guy posts for both of the above
towers, and followed K5IU's guidelines for the
concrete bunkers. I also did some reading on my
own and discussed it with a friend who is a civil
engineer -- it takes a lot of concrete and steel to
make proper elevated guys. A friend of mine skimped
when he put up what is probably a small system (a
tribander and 2m beam on 70 ft of Rohn 25) and now
his posts are pulled out of plumb.
Hope that gives you some ideas.
-- Arliss W7XU
Rick Harrington N0VT wrote:
My thanks to all who took the time to reply. I have a much better grasp
now on what I should (and in some cases, shouldn't) do. I want to use an
I-beam but the final choice will likely be determined by availability,
and it's good to know I have options.
73, Rick N0VT
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