[TowerTalk] Tower Questions

[Kurt Andress]

Al Williams wrote:
> 
> I would like to read a mini-tutorial on tower strength and wind loading > from some of our tower experts.

Hi Al,
Here's a go at answering some of your questions.

> 1.  Is it true that the wind speed at say 100' is greater than at 50'?
> always?

The EIA-222-F and UBC 1997 spec's both produce higher wind loads (not
necessarily wind speeds) as the height is increased. Remember that these
spec's use a "basic wind speed" which is for the fastest mile of wind
measured at 33 feet. That remains constant, what increases with height
is the windload. "Is it really true?", they think so.

> 2. For a free-standing tower, is the failure mode most likely at the very
> bottom of tower?,

It all depends on what the designer did. The bending moment in a
freestanding tower is always greatest at the base, but what the designer
did to configure the structure to carry the loads determines where the
first failure will occur. We might assume that a freestanding tower was
designed to have the same stress (not bending moment) at the bottom of
each change in section size. This would mean that all sections would
fail simultaneously. But, we don't know that this is true for all tower
designs.
We'd have to analyze each one to really know what is going on. 
For these kinds of questions, ask the tower designer (usually not the
sales guy), I'd bet he knows.


> 3. Tri-ex in FirstCalls nice website states that the designer should add > up wind surface area of all of the installations on the tower and that a > tower should be selected that exceeds the total. But they go on to state > that for stacked antenna installations, the tower should be derated.
> 

Ok, if that's what they say....sounds pretty vague to me.

> Intuitively, it seems to me that lower beams of the stack would not  cause as much stress on the tower as the higher beams and that the towers rating could be actually be increased.  Or more correctly, the square   footage of the lower installations could be derated. To illustrate my   point, surely a 70' tower rated at 10' sq ft   would withstand a much   greater wind speed if the beam were placed at say the extreme 2' from
> ground.
> 

Your intuition is correct.
Most of the freestanding tower designs I have seen are rated for the
entire antenna load applied at the top of the tower. If the same total
antenna area is split up into several separate loads at and below the
tower top, then the net load at the tower base will be lower, allowing
the total of the areas to be increased to generate the same overall
tower loads as the original rating. What is happening along the tower
with the new point loads is another thing, might be ok, might not, 
When the antennas are stacked on a mast extending above the tower top,
the areas need to be reduced to create a load equivalent to the original
one. That's probably what they meant about derating for stacked
antennas.

I've made some posts regarding this subject in the past, the stuff is
floating about in the TT archive. Exercise the search engine and you'll
find a lot of stuff from many sources.

> Although these questions refer to free-standing towers, I would like to 
> also read a tutorial on guyed towers.  I suspect that many other   towertalk readers would enjoy, appreciate, and benefit from our towertalk  talkers writings as I have not seen this kind of material in my 20 or so  antenna books!
> 

You can go to the URL below and read some stuff about guyed towers.

Heights Towers has some interesting information on their website:
www.heightstowers.com/engspecs.htm

Since you raised questions about antenna loads and towers, I'll take the
opportunity to post my bi-annual reminder that the antenna area ratings
cited by most tower designers is not necessarily the same antenna area
that one is likely to get from the antenna folks.
As with most things, it is important to understand what a number really
means!

Someday, information on this topic will likely find its way into an
amateur antenna publication. Till then, keep searching and asking the
questions. Someone around here is likely to know the answer, if not, at
least have an opinion about it.

-- 
73, Kurt, K7NV 

Visit http://www.freeyellow.com/members3/yagistress/

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