Jerry:
Well now, I am a structural engineer and you are going to get this for
free.....
Guy the 74' tower at 35' and the top. Rohn shows the top guy lower to allow
for horn or dish antennas at the top.
Guy it at 80' out if you wish no big problems with that and you are set for
the taller tower. This assumes you have
the room to do it.
The rule of thumb tension for guys is 10 percent of breaking strength, that
is why they are always the same for
the same guy diameter.
There don't you feel better for the free advice.
Hank Lonberg P.E. / KR7X
-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of Jerry W5KP
Sent: Wednesday, May 30, 2001 11:18 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Anchor placement - Rohn 45G
Ok, here's my current headscratcher: Putting up 7 sections plus a flat top
section of 45G. Book says guy at either 29' & 61' (70 ft) or 35' & 71' (80
ft). After sinking 4' or so of the first section in concrete, and adding the
8' flat top section, the thing should be about 74' tall at the thrust
bearing, so I guess I'll consider it a 70' tower for guying purposes. I'm
considering things I can do now that will make it easy someday to add 3
sections to this thing to get it up to 104'. Guys for a 100 footer are
listed at 31', 61', & 91'. Clearly, using the 70' guy pattern at 29' and 61'
would make adding 3 sections and a third guy at 91' a reasonable thing to
do, except for the distance from the base of the tower to the guy anchors,
which is what has me worried. The question is: Structurally, is it a bad
thing to put the guys for the 74 footer at the right distance from the tower
for a 100 footer now? If I did, that would put the guy anchors at 80'
instead of the 56' specified, which is a pretty hefty distance change.
Obviously I'd like to avoid putting in three more guy anchors at 80' later,
if possible. In line with this possibility, I am using Rohn anchors with
three-hole equalizer plates. I know putting guy anchors CLOSER than specs is
a very bad idea due to the added leverage and downforce they put on the
tower during high winds. Is there a bad effect from moving them further
away, say from 56' to 80'? Common sense tells me the stress on the tower
goes down as you move the guys further out, but I suppose there's a
practical limit due to sag and who knows what else. Also, I note that Rohn
always specs guy tension at 600# for the top 1/4" guy, and 400 for the
remaining 3/16" guys, no matter what the tower height (at least from 40' -
160'), so tensioning doesn't seem to be a problem either way. I'd just move
the 1/4" guy to the 91' point, add some length to it, and tension it at
660#.
Sorry for bothering you guys with questions like this but this is my first
"full-size" tower, and the Rohn data sometimes isn't all that helpful,
especially if you want to vary some parameter. BTW, this is NOT important
enough to me to spend $1500 for a PE to study it, so if it's a bad idea just
say so. Thirty feet ain't that big a deal to me as far as the yagi goes, it
would just help me out on the lower band wire antennas.
Thanks,
Jerry W5KP Mustang, Oklahoma
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