Date: Tue, 13 Aug 2013 07:09:25 -0700
From: Jim Lux <jimlux@earthlink.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Adding guys to self supporting towers
On 8/13/13 6:47 AM, Jim Thomson wrote:
> ## The stress on the cross bracing is actually reduced when guys are added
> to a
> free standing tower, not an issue.
How so?
In a no wind static case, the guys add down force, some of which will
appear on the diagonal cross braces. How much depends on the relative
compression stiffness of the brace vs the verticals.
In the wind loaded case, the guys transform some of the horizontal
bending load into down force. That could be a wash in terms of loads
but it's hard to tell, depending again on the relative stiffnesses. E.g.
downwind side has less load due to bending, but more load due to guy
induced downforce, but the guy downforce is distributed among all
three/four vertical legs.
looking at a BX manual (the only one I have handy).. the diagonal braces
have maximum allowable compressive loads around 1000-1200 lbs (sections
1-4) or twice that (sections 5-8). The verticals have max loads around
5000-6000 lb (sec 1-2), gradually increasing, up to around 20,000 lbs
for section 8.
the loads analysis shows vertical loads in the 19000 lb range for the
bottom (BX-8) and 2400 lb for the top (BX-1). That's not a huge margin
for the bottom section. The diagonal brace loads are a few hundred
pounds, which is roughly 25-30% of the allowable. The question I would
have is whether increasing the down load (some of which transfers to the
diagonal braces.. Rohn uses cos(angle) for the factor) would change things.
The other question is (particularly in the case of BX, which has a very
low allowed antenna area vs the tower area) whether the guying is being
done not to "help" the existing design and usage, but to allow putting
more antenna on the tower. Or, is it to handle "extreme events" (in
which case the guys could actually be slack in normal use with normal
winds).
I think the big take home is that while something like Rohn 25 is fairly
straightforward to analyze, being of constant cross section and
construction, any sort of tapered self supporting tower gets a lot more
complex. A cursory glance at the BX analysis shows that the ratio of
"expected load" vs "allowable load" varies a lot up and down the tower
(mostly because there are standard sized parts used in various places).
Add in the non-uniform wind speed over the height of the tower, and I
think any sort of casual analysis is doomed to failure.
## I think the BX towers are similar to the delhi towers in that you can buy
#5 straight sections. Nobody up here uses sections 1-4. Typ it goes
8-7-6-5S-5S-5S-5S-5S-5S-5T..... or 8-7-6-5S-5S-5S-5S-5S-5-4T.
In free standing mode, one leg will be in compression, while the other 2 legs
are in tension. Or the other way around, one leg in tension, and 2 legs in
compression. The one leg in compression and 2 in tension is the worse
case of the two. Now add in guy wires, and all 3 legs are in compression.
## Steve is correct though, you don’t crank the guy wires up super tight.
No need for 600 lbs of pre-tension in all 3 x guy wires. 100-200 lbs max.
I have seen 3-4 cases of free standing towers folding over 2-3 sections
down from the top. Never seen a guyed freestanding tower failure, that’s
going back as far as 1969. I have seen mast failures on guyed freestanding
towers
however. Nothing new there, the owners went cheap on the mast. Not thick
enough,
and or too low a yield strength.
Jim VE7RF
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