[TowerTalk] Universal Freestanding Tower
Don Havlicek
n8de@thepoint.net
Thu, 14 Feb 2002 01:09:14 -0500
Comments inserted below:
Steve Maki wrote:
>
> I wrote:
>
> >However, aluminum IS much more subject to work hardening
> >than steel, and that fact must be taken into account during
> >the design process.
>
> I just want to expand on this a little - comparing the
> aging properties of flexing steel vs aluminum.
>
> BTW, I'm not a materials (or any other kind of) engineer
> - my thoughts on this subject are formed mainly by 25 years
> in the tower business, and some informal reading. I HAVE
> written many insurance estimates for broken towers and antennas,
> and have what I consider a pretty good feel for what various
> structures can handle over the long term. I invite anyone to
> correct my misconceptions, especially if you know my off-the-cuff
> estimates are WAY off.
25 years in the tower business? ...and you're just using suppositions?
Where's the evidence?
>
> My little thought experiment:
>
> Let's say we erect two self supporting towers - one aluminum
> & one steel - and load them to the hilt. Let's say they each
> initially will JUST barely handle a 20 sq' long boom yagi at
> the top in 80 MPH winds. No, neither is 'engineered' for 20
> sq', but we know that each will handle it on the day of
> installation, and 25 sq' would topple them in the first 80 MPH
> gust, or the first 90 MPH gust would topple them with the 20 sq'.
You use the phrases: 'load them to the hilt' .. and 'know that' ...
Suppositions are NOT evidence!
>
> So we load them with 20 sq', and buffet them with 80 MPH gusts.
> and stand back, and watch to see what happens.
Let's see .. both towers will fail because of compression failure in the
leg/legs that are opposite the direction from which the wind strikes
them. THAT is the greatest failure for any self-supporting tower.
>
> In my experiment, I find that the steel tower gradually de-rates
> due to work hardening or stress fracture, or a combination of
> the two - and comes down in just a few years time - I think it
> was five years :-). The aluminum tower however is on the ground
> in just a few weeks due to the same processes.
You're assumption that the steel tower will derate at a slower rate than
the aluminum tower is just that .. a supposition. Perhaps you should go
to a metals 'manual' and look at the coefficient of 'flexation' for each
metal. The aluminum tower will bend and return to its original position
many times before ANY work hardening begins to appear. The steel tower?
.. unless it was tempered before being erected, it will work harden
QUICKER than the aluminum .. it's a property of the two metals. In
fact, the aluminum used in towers IS high-grade and designed to flex
without work-hardening UNLESS it's installed incorrectly [poor section
connections .. people who use come-alongs to force sections
apart/together, thereby stressing the welds .. etc.]
>
> Then, in an effort to see what corrections need to be made
> so that each tower would last 50 years with a 20 sq' load in
> 80 MPH gusts every day (very nasty weather here but no CC&Rs),
> we install many many different sized towers and then wait out the
> 50 years. There is no other kind of weather here - no ice, no rain
> - only 80 MPH gusts every day. Somehow, the 20 sq' yagis survive.
>
> Results:
> First of all, it's apparent that each material has a threshold
> of flexing if you will, below which there is virtually no work
> hardening or deterioration going on, because there are no failures
> after 20 years.
>
> The smallest steel tower that makes it is one that would handle
> 35 sq' on Day One. The smallest aluminum tower that survives the
> onslaught is one that could initially handle 50 sq'. Interesting,
> I think - both materials can survive equally well if designed
> for the application.
Where did those numbers come from [35 sq' .. 50 sq' ??]... another
supposition?
>
> I then look at prices, and see that for the same 50 year performance,
> the aluminum tower costs many times what the steel tower would
> cost, and I think - hmmm. So I pick steel for all my future towers.
>
Let's remember that we're talking about SELF-SUPPORTING structures .. a
steel tower of the same height/load will cost MORE than an aluminum
tower with the same specs.
> --
> As always, YMMV, and I've got no problems with those who choose
> aluminum towers for special reasons - but it seems clear to many
> that in most cases, aluminum is less appropiate than steel for
> ground mounted support structures.
If I were to put up a guyed tower ... of course, it would be steel.
But, in my situation, I can only put up self-supporting structures, so,
after much research and 'price-shopping' .. I put up a Universal
Aluminum tower [instead of a Heights - 10' sections instead of 8']. Now
have 4 Universal towers [two up, two being readied for erection] .. and
will have a total of at least FIVE when I'm finished with all my antenna
s this spring/summer/fall.
My experiences with steel towers is limited, but not zero.
Don
N8DE
>
> 73
> --
> Steve K8LX
>
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