>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.
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'.
So we load them with 20 sq', and buffet them with 80 MPH gusts.
and stand back, and watch to see what happens.
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.
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.
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.
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.
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.
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