Tower Loading

Stan Griffiths
Fri, 7 Feb 1997 14:57:12 -0800 (PST)

>It is not my intention to "outguess" Rohn, but to do several, iterative
>calculations myself.  Rohn tower applications as drawn/described in many
>pages, are over engineered for safety.  How much overengineered is one
>of the questions I would like to figure out, along with WHAT is the
>limiting factor?  Moment?  Compression?  Guy strength?  Anchor holding?

Again, we would ALL like to know that stuff.  Believe me, we are not
intentionally keeping secrets from you . . .

>(Of course the real failure mode for Rohn/ham installations is TORQUE,

How do you know this?  It certainly is not obvious to me . . . 

>but can be combated with torque compensated antennas and tower torque
>bars.  Which then leads us back to the windload issue.)
>Also, if you would perform some of the calculations yourself, you would
>see that some examples are much, much more overengineered as compared to
>the next example 10' taller, but list almost the same windloading capability.
>The reason being that the top guypoint didn't change, and that the
>moment increased by THREE times.  (I am referring to to the examples
>that have the top guy point at 5' below the top versus 15' below the top.)
>In other words, you could put three times the square footage of antenna
>on top if you keep the guy point close to the top.  But then, if you'd
>know how to figure this stuff out for youself, you would be able to
>figure out that the moment isn't the biggest concern now, but now your
>attention needs to turn to vertical compression of the tower and the
>reactive force of the guy.  Maybe that was this limiting factor the whole
>time.  But who here can tell me this?

This is what you learn when you take a four year course in Structural
Engineering at an accredited engineering university.  You can't do it
properly by getting a few pages of advice on the internet.  It takes YEARS
of study and experience.  Certainly there are guys who know this stuff and
some of them are no doubt listening to all of this.  They are probably
professional engineers who spent a significant part of their life learning
this stuff the hard way and are not about to give it away for free.  Even if
they are willing to give it away for free, they incur some liability for
what they say and it just isn't smart to take such risks without compensation.

>And is one to hire a PE every time he changes an antenna??  Wouldn't it
>be a lot better if you could figure it out for yourself?  This is the reason
>I do not want to get a PE involved at this point.

Then YOU need to become a PE!

  I would like to try
>on a few antenna configurations for size, see if they work, and then
>when I change my mind, and experiment with antennas like a normal ham,
>I will be able to figure out for myself if it is within the capabilities
>of the tower, guys, and guy anchors.

Yes, so would I, but alas, I am not a PE . . . so I must take some risks.

>And Steve, n0yvy, says:
>>If you want professional advice, i.e. no handwaving...
>No, I do not want professional advice at this stage.  I want to correspond
>with someone here on this reflector, that would know how to do this.

Well, actually, you DO want professional advice.  You just don't want to pay
for it.  I can understand that.  Good advice of this kind is darned
expensive and part of ham radio is getting the job done for minimum bucks . . .

>I thought hams like to do things for themself.  Experiment.  Did Hiram
>hire someone else to do his work?  Marconi?

There was a lot less litigation back in those days . . .  The question is,
what would those guys do in today's legal atmosphere?  They might hire a PE
. . .

  If you cannot do the
>calculations, that's fine.  I would like to speak TO ANOTHER HAM, who
>does know how.  Because I would like to learn how to do it myself.  And
>then I can pass the knowledge on to someone else as eager as I.

Another ham who knows how will undoubtedly be a PE.  You want him to take
off his PE hat and put on his ham hat and give you free PE advice as a ham.
I hope you find one who will do this for you.

>>I'm waving and holding the TIA-222-F Tower Structure Standards.
>Okay, does that doccument show the calculations??

I have not seen TIA-222-F but I would bet it is only a document that tells
what the structure has to stand in the way of winds, etc.  University
engineering textbooks show the calculations . . .

Chad, I commend you for wanting to "do it right" but you have to realize
that to eliminate all the risk (as we would all like to do) takes either an
engineering degree or a lot of money to hire someone who has one.  The rest
of us take some risk and one way to minimize that risk is study all the
tower failures you can find data on and make sure you don't make the same
mistakes.  Another way is to look at tower installations that have been up a
long time and see how they are done.  I know you poo-pooed this idea as the
"blind leading the blind" but to extend this idea a little, if I were blind
and the blind guy leading me had been down this particular path a couple of
dozen times, I would have faith that he knows the way pretty well . . . of
course a tree can always fall across the path . . .


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