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Re: [TowerTalk] sorry for multiple mails, but had another idea

To: Donald Hofmann <electroubleshooter@hotmail.com>
Subject: Re: [TowerTalk] sorry for multiple mails, but had another idea
From: jimlux <jimlux@earthlink.net>
Date: Sun, 14 Dec 2008 11:56:37 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
jimlux wrote:
> Donald Hofmann wrote:
>>  
>>
>>
>> *The point is: who wants their tower to fall down?  Not me. If I can guy 
>> it as well as make it self supporting then why not?*
>>
>> * *
> 
> Towers are engineered structures: that is, they're designed to balance 
> the various loads among the various components in a particular way, 
> generally with the objective of doing it in a cost effective manner.  If 
> you start changing the design, you change the distribution of loads. It 
> might be better, and it might be worse.
> 
>
To elaborate on this..

Consider someone modifying their car (e.g. building a hot-rod)..  You 
can modify the engine, and it might have better performance, however, 
you've also potentially changed the overall balance of things.  Maybe 
you now have more horsepower than the differential can take (yep, been 
there, turned the ring gear into steel gravel)..

Or maybe you've added a turbo, so you get twice the HP.. the rest of the 
vehicle takes it just fine, but you've shortened the life of some 
component, or made the thing much less tolerant of, say, the timing 
being wrong or the mixture going lean, so you get detonation and break 
the rings.

However, there's a reason the original manufacturer didn't do these 
things. Sometimes it's a fitting the design to the market, so maybe, 
another manufacturer does go for the performance. And the user pays for 
it, one way or another (e.g. Ferrari runs much closer to the mechanical 
limits than GM, so Ferraris require more maintenance to keep them 
running.  I leave comments about English cars and their elecrical 
systems to others)


This is sort of the distinction between "craft" and "engineering"... The 
craftsman builds something, drawing on experience (their own, and that 
of the craftsmen before) to design it, but doesn't necessarily know why 
it works or what the limits are, except by direct experience.  They 
might make a good guess as to what might happen with something 
different, but they can't "know".   The engineer understands the "why" 
and can "know" the limits, without having built it before.


The difference between the two becomes important when risk and 
consequences of failure are evaluated.

Depending on your environment, craft might be just fine.  Build it by 
leveraging experience, and if it fails, oh well, just weld on another 
brace. This is the way much of the stuff for special effects in the 
movie business gets done.  Speed is of the essence, long term durability 
is not, and you can accept a fair amount of risk that it might not work, 
because you can fix it right there and then.  We used to build all sorts 
  of structures, essentially by eye, and you'd throw a proof load on it 
to see if it would work, and if it didn't, you'd just modify on the fly. 
Rarely would you do any "engineering", in the sense of actually 
calculating what loads there would be and how big to build the 
structural members, etc.  There's a reason they talk about the "craft of 
film".. everything is done this way, from makeup, to costumes, to 
lighting, to set building, to special effects.

In another environment, though, craft might not hack it.  If you're 
building airplanes to carry paying passengers, there is regulatory 
oversight to make sure that you KNOW that the plane will survive the 
loads, and, indeed, specified margins for testing, etc.  Those who want 
to build planes by craft can do so, but they're registered as 
experimental, have restrictions on their operation and use, etc.


Amateur radio is in a funny junction between the craft and engineering. 
  We buy engineered stuff, sold for use in things other than amateur 
radio.  But we also have enormous regulatory flexibility, at least as 
far as RF goes.  You can build your own transmitter however you like, 
and as long as you adhere to a few simple spectrum constraints, you're 
free to do what you like.  And this is totally cool and wonderful.

However, when it comes to structures like towers, it changes, depending 
on where you are.  If you're sticking up a tower down on the lower 40 of 
your section, nobody is going to care whether you use craft or 
engineering (or, at least, nobody should care..), or for that matter, 
whether you assemble your tower from white glue and pasta. (it's been 
done... There's a picture on the web somewhere of a 40 foot tower made 
from spaghetti).

However, if you're putting up your tower in a suburban backyard, and 
there's some legitimate regulatory concern from a safety standpoint, 
then, a more engineering approach is probably called for.  A failure 
might have severe consequences for other folks, and society will want to 
make sure you have an awareness of and accommodate those consequences. 
Craft approaches often won't work in this environment, unless you are 
making a copy of a "known good design". The same sort of argument for 
why we have drivers licenses or pilot certificates.... clearly one can 
drive a car or fly a plane without the paperwork.

Where it gets even stickier is in forums like this one.. There's no way 
to tell what the risk acceptance environment is for the person asking 
the question, so folks weigh in with suggestions, typically based on 
their own experience (and their own personal risk acceptance posture). 
Someone like me, who make his living being an engineer, will tend to go 
with the more conservative (in an engineering sense) approach, trying to 
explain what happens, or why things are done the way they are.  I figure 
the reader can ignore it as they see fit, and hopefully, they have more 
information to make an informed decision (after all.. if I tell you that 
you need both belt and suspenders, you can decide that the consequences 
of your pants falling down is small enough that you're willing to go 
without either, and just trust in waistband friction.)


Jim
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