At 11:22 AM 9/14/2006, Jim Jarvis wrote:
>The argument for the MA550 is completely aesthetic, in my mind.
>The real question is whether, if motorized, one could convince
>zoning to rate safety based on a retracted tower. It would be
>easy to have windspeed monitored, and store the antenna nested,
>when not in use.
This would be fine for safety, if safety is defined as "minimizing
probability of expensive damage to the equipment being protected",
where the maximum downside exposure is the cost of the equipment.
It probably wouldn't be appropriate for life safety, or the property
of others, unless the windspeed threshold were set ridicuously low
and the "automatic lowering" system were suitably redundant and safeguarded.
Here's a scenario. Light breeze of 10-15 mi/hr at ground level with
occasional 25 mi/hr gusts as a thunderstorm approaches.. tower stays
up because the windspeed threshold is set at, say, 35
mi/hr. Downburst with 70 mi/hr gust comes, folds tower over. This
might be fairly unlikely, but you'd have to go through a bunch of
work to prove it.
This is hardly a one in a million probability, by the way. What
you're really proposing is that the likely time between the wind
reaching one speed (your threshold) and it reaching the failure load
is less than the time to retract the antenna. As it happens, there
is a fairly large body of research on this, so you could do an
analysis that would give you a probability of failure. Say it takes 2
minutes to lower and stow your tower. You'd want to know what the
probability is that a gust of, say, 90 mi/hr would occur less than 2
minutes after a gust of, say, 30 mi/hr.
Then, the question would be what's an acceptable failure
probability.. I'd venture that the city might be happy with something
like a 1 failure in 100 years kind of probability. You'd combine the
odds of a 90 mi/hr gust occuring (which is fairly low) with the odds
of it not being preceded by a trigger gust.
Other questions that would be asked:
What happens if anemometer fails?
How would know if anemometer is failed, vs just no wind?
Can the tower be retracted under load?
After being triggered to automatically lower, how long will it stay down?
This could all be done, and a system designed to do it
reliably. It's not a lot different than the kinds of things used for
rocket launches and to assist air traffic controllers. However, it
would almost certainly cost more than just buying a stronger tower.
For more reading, just google for "extreme wind gust statistics"
http://arxiv.org/PS_cache/physics/pdf/0112/0112063.pdf is a paper
talking about just this sort of application (for auto feathering of
wind generators, among other things)
http://www.physik.uni-oldenburg.de/hydro/BLM2003.pdf is a better
version of the same thing. The author proposes that the "waiting
time distribution" is a power law.
This one might be interesting:
> I would anticipate a topmast, placing a 3 el
>steppIR @ 65'. Height would be 'tuned' appropriate to the band
>in use. So nesting storage could be a normal event.
>You'd still need a PE evaluation to get the permit, but it would
>be a significantly more robust result.
and, once someone bit the bullet and paid for the analysis, it
wouldn't be too hard to extend it to other towers and locations, if
the regulatory environment would accept it. This is a pretty
standard sort of thing for the design of tall buildings and bridges
(and construction cranes and ski lifts) which all deal with
windloads. Some bridges and buildings use active damping of wind
induced vibration, and they have redundancy systems, etc. I suspect
that these things aren't build to a "standard code", but make use of
the "or if it can be shown safe by engineering analysis" kinds of
exception. Ski lifts are typically regulated under different rules,
for instance. Forking out the massive bucks for the engineering and
the systems probably isn't a problem with a $100M bridge, but might
be a bit of a problem for a $10K ham antenna.
However, maybe you could get someone to do it as a PhD
thesis. Colorado State has a program that deals with wind
effects(http://www.windlab.colostate.edu/index.htm), and there's a
very well known firm in the field (Cermak Peterka Petersen) in that
area (Ft. Collins) too.
TowerTalk mailing list