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#11
April 10th 05, 01:30 AM
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"Wes Stewart" wrote in message ... On 9 Apr 2005 14:22:06 -0700, wrote: Please don't get upset! I don't intend to actually do this! All you antenna experts out there. If I have a tower say 60 feet tall and I can only put one set of guy wires up, should they go near the top of the structure or near the middle? What are the forces that the wind exerts on a tower and are these forces evenly distributed? This is really a complicated subject and off the cuff remarks and supposition are worthless. The forces *can* be analyzed as uniformly distributed but usually, the wind pressure is stronger near the top, particularly if there is an antenna up there, which is the normal reason for a tower. You might be inclined to say aha, then the top is the place for the guys. But, guys turn horizontal force into vertical force that tries to drive the tower into the ground. With the top and bottom restrained, then the middle might be inclinded to deflect horizontally under wind load. With it so moved, then the down force from the weight of the tower and the additional force from the guys can cause the tower to buckle. I seem to remember from "structures" classes that the rule of thumb was that if the solid diameter of a member was one tenth or more of the height then the member will break in normal compression mode. (Top loaded) If the ratio of length to diameter was more then the member will buckle. Seems like a good starting point to me. But then that is the easy part since one would have to tension the guy wires to a specific force with multiple variables ! Note Tower manufacturers do not have to introduce safety factors in their calculations as would a crane or pulley manufacturer where personel are usualy in the area of use.Thus strength of material used must be on the mark. This is why I would guy at 30 feet and accept that movement at the top will still happen but with less moment of forces (wind loads) that would be around in the event of no guys. Art Another concern is mechanical resonance. Top guying is going to make the top resemble a string on a musical instrument with the wind trying to pluck it. It has been my (sad) experience with a Cushcraft AV-80 tubular vertical that guying half way up, is equally bad. I had a resonance set up and the tubing bent to yield and broke in two right *under* the guys. When I installed it I didn't have the data sheet but knew that it needed guys. So I guessed at 50% up. Since then I've acquired the data sheet and the recommendation is to guy about 2/3 up from the bottom. The bottom line is to listen to the manufacturer. |
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