Don,
Thanks for your comments.
N8DE wrote:
>> 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!
I didn't represent anything I said as evidence. As a thought
experiment, it's legal to "know" something like that.
>> 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.
Are you suggesting that the failure mode is exactly the same when
a tower fails due to initial overload as it is when the cause is long
term stress? Most of the aluminum failures (on older towers) I've seen
had jagged breaks in the strangest places.
>> 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.
Everything I've read indicates otherwise. Have you got a reference?
>> 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?
Well, yes. Just like I said.
>> 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.
Absolutely wrong; if that were true, virtually all commercial self
supporters would be aluminum. Make sure you compare apples to apples
(same wind speed, same ice loading, and an engineer's seal).
73
--
Steve K8LX
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