[TowerTalk] Tower Permitting Spec's
infomet at embarqmail.com
Mon Apr 30 14:24:17 EDT 2018
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.
How could they be "accurate" when the only evaluation is whether or not the tower falls over?
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.
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?
And the wind profile? It can change load by enormous amounts. And turbulence? Ever mentioned?
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!
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.
OK, rant over. I know I have few relevant qualifications, but I do have some.
I just hope to generate some semi quantitative discussion and shake out applicable test literature, if there is any.
Reports of actual wind induced failures especially.
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.
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