[TowerTalk] Fwd: Tower Permitting Spec's
hanslg at aol.com
Mon Apr 30 18:54:49 EDT 2018
So, I will add an other penny that come from an actual failure:
I had put up 30' aluminum mast in Brooklyn, NY. It was made of 1 1/4 " aluminum conduit pipes. As I didn't know much about support etc. I supported the mast with two sets of guy wires, one at 15' and one at the top. Everything went OK until one day (night actually), about two years after I erected the mast. We had a storm with a lot of strong wind gusts. I noticed the mast as it was bending in the wind. There were a very distinct sinusoidal curb with the "peaks" between the support points and the nulls of the sinusoidal curve at the bottom the mast, at the middle guy support and the top guy support. The mast was vibrating like a guitar string in between the support point. Needless to say the mast collapsed by breaking half way between the support points.
Later, I put up a new mast, the same size pipes but the middle guy support now higher up. My reasoning was that this would not allow a resonant vibration to develop the same way as the last time. This mast has been up ever since and ridden through several similar storms. I have observed the mast in several of these storms and can conclude that the type of vibration I saw the first time, when the mast failed, never happened again. That mast has been up for over 30 years. (I have replaced the guy wires once due to excessive corrosion of the guy wires.)
It is not only the force on the tower from the wind that counts. It is also resonance build-ups due to wind gusts that we have to worry about. My believe is that a gusty wind can cause a failure at much lower wind speed than what a steady wind would.
My 85' tower in Vermont is only guyed in one place and I am a little worried that it could experienced a similar excitation. Maybe I should add an extra guy support in between the present guy point and the bottom just to make sure.
N2JFS - Hans
From: jimlux <jimlux at earthlink.net>
To: towertalk <towertalk at contesting.com>
Sent: Mon, Apr 30, 2018 2:58 pm
Subject: Re: [TowerTalk] Tower Permitting Spec's
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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