[TowerTalk] Windloading

[Kurt Andress]

K7LXC@aol.com wrote:

>       According to the Rohn engineer that I talked to, the rating is for the
> WHOLE tower and is regardless of where the load is. Remember these towers are
> designed/configured for commercial applications which many times means
> antennas and dishes all up and down the tower. It's my understanding that the
> capacity is more or less a constant.

Seasons greetings,

I don't doubt your presentation of what the Rohn people have advised. And we'll
not make an arguement out of this whole deal.
Rohn is telling you and everyone else that the total load is regardless of
location on the tower, because this is the simplest, and safest way to approach
the problem in the absence of a specific analytical solution.
If I didn't have the time or resources to figure it out properly, I'd say the same
thing.

This type of boilerplate instruction does not lead to any kind of enhanced
understanding about  the problem.

The Rohn people know what many of us know, that the plethora of configurations are
endless and need to be individually analyzed to figure out what is really going
on.
There are some aspects of the possible variations that don't make much difference.
There are others that can be critical.


>       I will always defer to your technical and engineering knowledge so am
> obviously interested in your comments. A caveat: Rohn engineers and specs are
> obviously meant to be useful to anyone so the specs have built-in engineering
> overhead and have worse-cased potential configurations with their liability
> and risk-exposure interests in mind. In other words, there is a good sized
> "fudge factor" built in already for everyone's protection.

This is the usual and expected position from those who are potential targets of
litigation.

The real guy who does the calculations figures it is ok with a safety factor of
>2.00. The corporate lawyer raises the standard to 4.0 because he doesn't
understand it and wants to maintain his account.

I'll say this again, I'm not a supreme expert on this subject! There are many
others that have gone before.
I have just spent time examining certain behaviors with tools that appear to
provide reasonable results in the physical realm.
Just like the NEC codes don't always handle all configurations, the FEA codes are
still a work in progress, good foundation, but still developing, and hence, always
suspect.

Everybody, models the heck out of antennas in the electrical  realm, and nobody
goes to court over antenna electrical performance. Have just one tower fall down,
and everybodies lawyer starts salivating.
What the heck! They put lots of men on the moon with all of these tools. We ought
to try and use them to help us work some obscure country in a contest.

> >
> >  > Distributing the loading differently could get you extra capability?
> >
> >  In most cases, Yes.
>
>       I understand how this is possible but I think this requires you (Kurt or
> any other installer) to make your own assumptions and calculations. This is
> not a course I would suggest anyone undertake (pun intended). If you're not a
> good engineer and can't do the calculations with a great deal of confidence, I
> would recommend going with the factory specs and not trying to second guess
> the manufacturers specs. (The LXC Prime Directive always applies - "DO what
> the manufacturer says".)

Agreed! I think I said that in my post.

Every guy who puts up a tower needs to remember that it is his (or her) tower,
money, and hard work that has been invested. One needs to seriously consider the
risks against the information at hand when contemplating "stepping outside of the
box!"

So, I'll restate my opinion about this subject, to make it perfectly clear.
"If one does not have the ability to perform adequate analyses or obtain the
assistance of one who can do this, DON'T guess at a solution to the problem. Just
do what the manufacturer has advised!" This is the best way to avoid problems that
can occur from the things you may not have thought about!

Now, with that clearly stated, let's talk about the real situation.

If one is inclined to "step outside the box" (the one defined by the mfgr) without
proper engineering, there is a better way to make a non-technical guess.
Follow the example of a successful existing structure. This option is not risk
free! But it is better than making a guess without any valid information.

Empirical evidence (real life experience) is a powerful tool in increasing our
knowledge and calibrating our analytical tools.
I see recurring comments on this reflector that come from those who have built
things and made measurements to define how they perform. These are the essense of
empirical experimentation to validate the analytical tools we use.
It seems to me that some people think that empirical experimentation only applies
to the electrical aspects of an antenna system design. I'll suggest that it also
applies to the physical realm.

There is a huge database of empirical experience on this reflector! I would expect
that many of the contributors have towers that are NOT exactly configured to the
Rohn specs. And have found that they are suitable for the conditions they
experience.

Unfortunately, there are many people that have tried physical configurations that
have failed. Since, no one wants to admit that they have made mistakes, they don't
share the valuable experience that would enrich us all. Such is life!

The Rohn catalogue clearly indicates that the specified antenna loads are applied
at the top of the tower. Other configurations that distribute the total allowable
antenna area along the length of the tower will usually result in higher safety
margins. So it is safe for Rohn to make this statement.

Let's consider an extreme example. We put the entire allowable area 10' above the
base of the tower. We all know we won't be able to work very many stations, but
does anyone think that an extra 5 Sqft at this location will make the tower fall
down?
Well, the previously stated Rohn boilerplate says it will.

I just answered the questions, as asked! And will continue to do so when I can and
percieve that such comment is appropriate.

I don't think I violated the precious "Prime Directive" when I stated

>Without doing a complete analysis of the tower, the safest thing to do is
>stick to the rated area regardless of placement. This is a very conservative
>approach as tower will have lower stresses etc with the distributed loads.
>But, it is better than just guessing and getting it wrong.

 AND

>The only way I trust to figure out how much antenna can go where is to model
>and analyse the situation.
>Otherwise stick to the published information or just guess! Which, BTW has
>been successful for many. Not my recommendation.

 It may be best for me not to answer these types of questions publicly. They seem
to generate too much controversy.

I thought that this venue was for open discussion of these things related to
towers and antennas. And was aimed at enhancing the knowledge of subscribers
regarding these issues.

Is there another reflector where it is more appropriate to discuss things beyond
"just doing what the Mfgr says?" Sometimes, what they say is not entirely correct
or complete.
This falls into the same category as antenna projected areas and safe wind speeds.

Someone please point me to the site where these discussions are appropriate.

73, Kurt

--
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/



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