At 10:56 AM 4/17/2006, ROBERT CARROLL wrote:
>I have a question more or less related to this thread. I have a MonstIR
>which is about to go up on 70' of Rohn 55. The system has been designed by
>a couple of real pros and I have had a PE look over the design. Although I
>am on two acres, the lot is not level and I could not put the tower in the
>best location because it turned out a septic field is in the way. So I am
>left in a situation where the guys are going to be elevated and attached to
>posts, 2/3 of which are in cement per the PE's guidance. They are equally
>spaced in azimuth angle and all meet the Rohn minimum distance from the base
>of the tower guidance.
>
>I am by no means a structural engineer, but--and I see traces of the idea in
>the "small lot" thread--had assumed the guys should also be anchored the
>same distance out from the tower. That sounds pretty from a symmetry
>standpoint, but it is just not possible in my case. The PE and the tower
>guru say there are fairly frequent commercial installations where the same
>problem exists--and in fact where one guy has to be almost horizontal due to
>tilted earth. They say the important thing is maintaining the minimum
>attachment distance from the base and the 120 degree spacing. They also say
>the guys, though of different lengths of Phillystran, should be set to the
>same static tension.
It's symmetry of forces that's the important thing.. If the guys aren't 120
degrees apart, then you're not pulling equally in all directions, and, more
importantly, with a wind load from some directions, the guy force won't
necessarily be in the right directions. Consider a weird arrangement..
Say, one guy to the North, and the other two guys at SSE and SSW. Winds
loads from north and south would be nicely carried by the guys, but
windloads from the east or west wouldn't be resisted much, and the tower
would tend to fall over. You could compensate by really cranking up the
tension (the analog would be holding up a weight with two strings, and
trying to keep the included angle close to 180), but that buys a bunch of
other problems, eh?
I doubt that going to 110 degrees on one pair (and 130 on another) would
instantly result in collapse, but getting too far away, and you start to
get serious differences in the loads depending on what direction you consider.
Then, you have a tradeoff between the static loads on the guys and the wind
induced loads, and those all change with the angles. (not to mention
seismic loads.. 80 ft of Rohn 25 weighs about 400 lbs, so with a nominal
0.6g earthquake load, it's like having 200 pounds of windload at the top of
the tower)
>It is too late to change any of this, and I trust the two experts, but I
>would appreciate some educational feedback on the subject of guys of
>different lengths.
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