[TowerTalk] Elevated Guys ??

[Jim Lux]

At 10:23 AM 1/8/2006, Alton J Drummond Jr wrote:


>Hello All,
>I am a Amateur Radio operator and I am installing a 65ft Rohn 25G. I dug 
>the founation 4ft deep by 2.5 ft sq. This hole took 35 bag of Ready Mix. 
>One ten foot section of tower is in the ground, 4.5 ft. Now, the guys, I 
>want to raise the guys so they don't "Clothes Line" anyone, and I can 
>still walk around the yard. What are some suggestions on doing this. I was 
>thinking a 9' or 12'steel pole 3 ft in the ground with cement, How large 
>or thick should it be, 2 or 3 or maybe 4 inch round??? I am putting a 11 
>ele 70cm yagi, 13 ele 2m yagi and a TH6DX on this tower. I live in 
>southeast Ala., average winds is 85 mph when the season is right, I am 
>within 100 miles of Gulf of Mexico, Hurricanes and High winds during the 
>Fall seasons. I want to do this right, any and all suggestion, I Thank You 
>now for them.
>Alton J Drummond Jr
>KA7RWT / AAV4WL

Do you care if the pole permanently bends under extreme load, as long as 
the tower doesn't fall down? I assume you're talking about using steel pipe?

First off... if you expect the pole to take the wind load of the tower 
without failing, you'll need a much larger post, and a much sturdier anchor 
into the ground.  You're thinking of putting hundreds, if not thousands, of 
pounds of force on the end of a 9 ft lever arm, which will almost certainly 
bend the pipe.  It's something worth finding an engineer to analyze.

Just for grins, a 9 foot length of 2" OD pipe with 1/8" walls will fail in 
bending with a side load at one end of 85 lbs.  A 4" OD pipe with 1/4" 
walls will fail at a load of 670 lbs) Think of sign posts.. they're pretty 
easy to bend.  And, if you've ever used a piece of pipe as a "torque 
enhancer" to get the crank pulley nut or an axle nut off, you'll note the same.

However, if the goal is to just get the guy wire up high enough so you 
don't get hung up, and that the post takes the run of the mill tensions 
without failing, you might have a strategy.  When the force gets too high, 
the pipe will bend, but it's pretty strong in tension, so it will just bend 
so that it's in line with the guys.  Won't be pretty, you'll need to fix 
it, but it might keep the tower from collapsing.  The operative word here 
is "might", and to be sure, you need to do some analysis or tests (as in 
hooking up a cable to the trailer hitch on a truck and pulling a test pipe 
over to see how it fails).  I'd be a bit nervous about the pipe cracking as 
it bends over, and the crack propagating around.  Maybe with a backup cable 
inside the pipe to take the tension loads if the pipe fails in tension 
after bending.


There's a world of difference between "probably will survive" and 
"guaranteed will survive", too.  We've had some fairly high winds recently, 
and I have a couple of Hustler 6BTVs tied to a standard fencing T-stake 
that I would have thought would fail, but have survived (even with 10 ft 
plus deflection at the top). But, being happy it didn't fail is a long way 
from being sure it won't fail.

By the way, that's a pretty small hunk o'concrete for your base.  The 
drawings I've seen call for a 4x4x4 ft cube for Rohn 25.  Here's an 
example: http://www.anwireless.com/nello103337.pdf

>_______________________________________________