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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