On 4/30/18 11:24 AM, Wilson Lamb wrote:
It's painful to read this stuff.
The spec's are regularly revised and interpreted.
They are based on model(s), history, tradition, and probably rules of thumb,
backed up by calculations to three decimal places, then applied at the whim of
local authorities and planners.
I would say that over the last 20 years, the standards and analysis are
closer to actual measured behavior than in the past. The analytical
tools have gotten better.
How could they be "accurate" when the only evaluation is whether or not the
tower falls over?
A failure isn't necessarily a collapse - if the tower "bent", that would
be a failure, in a design sense.
But not necessarily in an application sense, as you point out below..
You could get the 100 yr wind tomorrow, the 500 yr wind next week, and failures
would occur! Just ask the people in New Orleans or Houston.
They were "protected" by those same planners and zoners. Every criterion has
to start with a definition of failure and risk of failure.
One good microburst puts them all to shame.
One reg I like is that the tower has to fall on your own property. There is NO
acceptable level of risk when it comes to harming innocent neighbors.
But "must fall on your own property" is actually a fairly tricky one to
assess - most tower failures do not fall out in a straight line (even
for an unguyed tower). And it is certainly possible to design so that
if it fails, it fails "in place"
There's an awful lot of stuff (including and especially large trees)
which are perfectly acceptable to most people which cannot meet the
"must fall within property"
If you are putting up YOUR tower on YOUR back 40, "I'll replace it if it
falls." is a perfectly reasonable design basis.
And how good are the towers? Are there articles describing destructive pull
over tests of strain gaged towers, to see what their failure levels and modes
actually are? Realistic instrumented tests in labs?
Yes- they do. They run them in wind tunnels too.
And the wind profile? It can change load by enormous amounts. And turbulence?
Ever mentioned?
That's been studied a lot recently, because the wireless industry is
putting a lot of shapes in the air that look quite different from what
we have 50 years of empirical experience with.
The area of interference drag (drag due to air flow of component 1
interacting with component 2) is a pretty hot topic.
Fastest mile, three second gust; what's the difference? How long does the load
have to be present to cause failure? A mile at 60mi/hr takes a minute, long
enough for turbulence to affect real load.
Three second gust; which one? Which three seconds?
Then there are people who lump all the point loads together, without saying
where they are!
A load at 70' creates 40% more overturning moment than it would at 50', without
even considering the wind profile!
Well, I think what the standards attempt to do is to create a
standardized way to approach the analysis, and provide some simple "safe
harbor" worst case analyses.
Compare it to RF exposure safety - most folks don't go out with a field
probe and measure their fields. They use some deliberately conservative
approximations which can be done quickly and easily.
Have you noticed the WIDE variation in foundation requirements for similar
installations?
Compare a Rohn spec'd base with a magic earth pole standing in sand.
Have you noticed the marginal towers that don't fail in storms. Same with trees. A
magnificent oak will go over when some nearby species of much less "strength"
survives.
Sometimes the oak is strong enough to fail the foundation entirely. That's the
usual mode here in my woods.
I have asked a couple times for reports of actual wind induced failures of ham
towers, but I don't remember getting anything except trees falling on guys, not
really a failure.
Putting a dozen strain gages on a tower, in the right pllaces, could tell us a
lot, especially if we have a few anemometers to give us a wind profile, but I
haven't seen those papers sited here.
There's quite a bit of literature (although I can't lay my hands on it
right now) that I ran across about 10-15 years ago on just this sort of
thing. A lot of it was in connection with structures to measure winds,
support wind generators, etc., but also interesting applications like
temporary towers used to guide crop dusting.
I'll bet if you go to the committee reports that lead to specs like
TIA-222, you'll find plenty of references.
My guess is that most ham towers are built with large margins of safety,
because the calculations to do otherwise are so difficult.
Otherwise, we'd see more of them go over. Heck, some have been up through several
"revisions" and some had no engineering at all!
Remember, we all have "delete" buttons.
I think that's very much the case (significant inherent design margin)
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