At 04:04 AM 8/23/98 -0600, T A RUSSELL wrote:
>Thanks for your reply Steve. See comments and more questions
> intersperced below. N4KG
>
>On Sat, 22 Aug 1998 23:42 "Steven H. Sawyers n0yvy"
><sawyers@inav.net> writes:
>
>>By the time the tower rotates far enough to have the torque arms
>>do any good you have already used up most of the allowable lattice
>>strength. Commercial installations don't use "torque arms" as hams
>>do, they use things like the TA25 Torque Arm Stabilizer to get 6 guys
>>to 3 anchors. Only the ones they use fasten onto the tower a lot
>>better than the TA25 does and have more iron in them.
>>
> OK, I can follow that, but what if a base plate and pier
> pin is used to allow the bottom of the tower to rotate
> to accomodate the twisting movement?
I will use either a pier pin or base BOLTED down, but no section
in concrete. I have seen too many things go wrong with the poured
in place section. Things like freeze splits, and corrosion.
One installation had dirt on top of the base so the wife could grow
flowers and hide the "ugly" concrete. Two of the three legs of the
tower were corroded off - plant fertilizer is very corrosive - and the
only thing keeping the tower up was the house bracket and it
was lag screwed into the eave with whooping 3/8" x 4" lag screws.
The pier pin will let the tower tilt a little and twist a little on erection.
It doesn't really give much under a dynamic load like high winds.
Remember, the harder the wind blows, the more down load on the tower
base plate and the more friction between the base and the concrete
so things aren't going to go very far. Steel on concrete does have a
rather high coefficient of friction.
The pier pin does have the advantage allowing for repair/replacement
of the bottom tower section.with out getting out the jack hammer to
pour a new base.
>
> It has taken a long time, but I have finally come to
> appreciate that a rigidly mounted base is not a
> good thing, especially if side mounted antennas
> are installed, adding greatly to the torsional load.
>
> Still, I cannot help but recall that towers with
> torque arms FEEL more rigid when climbing.
> Is this an illusion? de Tom N4KG
It is hard to argue with perceptions. I find the a tower with proper
guy tensions "feels" a lot more rigid under foot than one that is
under tensioned. Perhaps this is what you are seeing.
>
>>Lots of commercial installations have 8 and 12 foot dishes side
>>mounted
>>to a tower, but they use the six guys to take up the torsional load
>>and not feed it into the whole tower length.
>
> Yes, I have seen some of those. NO question,
> they will handle a LOT more torsion than a single
> guy with a torque arm.
>
> Your revelation that guy wire strength is the limiting
> factor for towers of 100 ft or less is enlightening.
> Rather than going up a step in guy wire size, would
> using a torque arm stabilizer and 6 guys at the top
> be a viable alternative or would the torsional loads
> require heavier cable as well?
Six guys at the top would definitely increase your wind load
and torsionally stiffen the tower a bunch, so it is what
I recommend on a tower that is heavily loaded or has longer
booms on it.
Generally, I just like 1/4" guys. On Rohn 25 they give me
the extra margin for things like ice load and that extra
antenna I'll find at the flea market that will just fit when fixed
the Caribbean at 40-50 feet, or the six meter that I want to
add to the stack after everyone goes to HDTV and I won't
have the neighbors complaining about wiping out channel 2.
>
> Final questions, I ASSUME that a guy bracket,
> with or without torque arms, is a better / stronger /
> safer way of attaching guys than looping around
> a tower leg. Correct?
The guy bracket is much better and I strongly recommend
it, at least at the top, for HF installations or long boom
multi antenna VHF/UHF. The bracket will get the windward
guy loads distributed to all three of the tower legs without
requiring the load to go through the Z bracing.
> IF one does loop a tower leg,
> is it better to include the corners of the Z bracing in
> the loop? de Tom N4KG
Going through both Z braces is better as you are grabbing
more metal. The leg on Rohn 25 is only 16 gauge, so if
you are pulling on the leg alone, the load has to go from
this thin wall tube through the weld and then into the Z
brace to get to the other tower leg. When the load gets to
the other tower leg it still has to through the weld into the
the tower leg, but at least you have done what you can to
relieve some stress concentration.
I hope this helps.
A tower is not an end unto itself. It is there to support
antennas and antennas are there to get your signal out.
I think of them as the footing of a house. When you are
building it yourself, you go a little oversize, then you
don't have to worry and get to think about more important
and interesting things.
My general advice is get as heavy a tower as you can afford,
guy the heck out of it, oversize the anchors and the base,
make sure it won't corrode or freeze split, then quit worrying
about towers and think about antenna's and fun stuff.
73 de n0yvy steve
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