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Shortening guy lines on Rohn 25G tower
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by KC0ZRX on October 28, 2009
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I have a 70 foot Rohn 25G tower installed on my property. It is guyed in accordance with Rohn specifications with 3 sets of two guys extending 56 feet out from the tower base. Due to some easement issues that have unexpectedly come up it appears that I might have to move two of the guy anchor points back about 5 to 10 feet toward the tower but maintain the 120 degree spacing. My thought was to put in two concrete pads with a metal beam inserted in them that would reach up to a height that would intercept the guys lines at the same angle they now come down from the tower toward the original anchor points. I would then install eye bolts in the beams and shorten the guy lines so that they would connect to the beams at this point. I know that there will be a sizable moment on the new anchor points and will utilize a structural engineer to design the new points. Does this seem feasible? The only other option is to move the whole tower, which would involve much more expense and effort.
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RE: Shortening guy lines on Rohn 25G tower
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by W5CPT on October 28, 2009
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I would first check the geometry to see how much the guy angle changes. If it does change to an unacceptable level then go with elevated guy points. I however would not use I-Bolts. I would try to find a sufficient I-Beam and have the upper end cut at a 45 degree angle. Place the I-Beam in the ground with concrete with the angle facing the tower. Then drill (torch ?) a hole through the center web and use an Eye to Jaw turnbuckle with the Jaw on the elevated guy point. I hope that make sense.
Clint - W5CPT
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RE: Shortening guy lines on Rohn 25G tower
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by W8JI on October 28, 2009
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I can't even count how many towers I've seen blow over from using elevated guy points and eye-bolts.
The stress on the bottom of an I beam and the bending moment on the base is unreal. You can easily have several thousand pounds of bending moment of an elevated guy post without very much wind.
The only way I would ever do an elevated guy post here would be to use a BIG deep footer on the post and a forward brace to the top that is under compression.
If you use eyebolts, make sure they are forged steel closed eye and large enough to not strip threads or break. Also remember the leverage you will have near the ground on the vertical post. It can be huge! You have four foot long level trying to roll the base or bend the pole. It better be a damn big cross section to reduce the stresses to safe levels, and pretty deep in the earth. The safest thing is a forward brace pole, or better yet two of them, to keep the pole vertical. Then it would try to suck the main post up out of the erath while tring to dribe the forward braces into the ground.
I remember a 400-500 foot tower that had HUGE posts, and they rolled right over by pushing the dirt right out of the way after week of rain and down she came.
Tom
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RE: Shortening guy lines on Rohn 25G tower
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by WX7G on October 28, 2009
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Let's do some math. Others please check my trigonometry.
If I understand correctly the top guys are at the 70' level and attach at ground level 56' from the tower. This forms a 51 degree angle to the horizontal. For 1 lb of horizontal force on the guy (wind force on tower) the stress on the guy is 1.59 lb.
Moving the guy ground attach point to 46' from the tower forms an angle of 57 degrees. The stress on the guy is now 1.84 lb.
This is equivalent to increasing the wind velocity on the original installation by 8%. If the tower is subjected to a maximum wind velocity of 8% less than the design velocity it should be good.
A licenced structural engineer should be consulted of course.
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RE: Shortening guy lines on Rohn 25G tower
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by K9KJM on October 28, 2009
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I have run in to the problem you have, And have used elevated guy anchor points, However they are not the I beam freestanding type. I used 2" I.D. schedule 40 pipe along with a PAIR of back guy anchors, Spaced about 6 feet apart, And about 6 feet behind the elevated pipe. (Looking down on the anchor would look like a "Y") The in ground concrete anchors behind the pipe are both standard spec anchors (For the added load of closer space) (The only load on the pipe is downpressure- The point of the pipe is to elevate the guy wire close to it's original angle, And to keep the yard clear of "low" guy wires.
(About 7 feet of the pipe above ground level, About 3 feet underground in concrete)
As W8JI pointed out, Do NOT use hardware quality eye bolts, etc.
We have used a number of this type of anchor around here, Some have been up for over 30 years now with no problems. But they are not exactly cheap. You are actually pouring three concrete anchors for each guy, Plus the hardware, Extra turnbuckles, etc.
I also agree with JI about the I beam freestanding being a poor idea. There are a few high voltage power poles in this area with such guys, But because of the very extreme leverage forces involved, I would not trust such an anchor for a tower.
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RE: Shortening guy lines on Rohn 25G tower
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by AA4PB on October 29, 2009
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I think it is possible to design an elevated guy attachment that will withstand the forces. The design goal should be based on the specified maximum load for the guy wire being used such that the guy wire would break before it would tip over the anchor. It all depends on how strong the I beam is, how deep in the ground it is and how much concrete you have in the hole. For those who don't think its possible, think about free standing towers that have no guy wires at all.
Certainly use hardware designed for the load. Rohn makes eye bolts intended to be used as guy anchors. Your structural engineer should certainly be able to ensure that the structure is stong enough.
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RE: Shortening guy lines on Rohn 25G tower
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by W8JI on October 29, 2009
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It's always possible to use self-supporting guy posts if they are strong enough, but the cost of the posts and labor would start to approach the cost of the tower. :-)
Tower installers have a way of doing things wrong because they don't think through what they are doing. This isn't limited to Hams.
Back staying the pole is a good idea if the stuff is heavy enough, but then you move the guy pole closer still to the tower. Short-guying, as WX7G suggests, is a good option if you don't change angles too much.
It would be much better than a typically installed pole, at least those I've usually run across.
It's all about the triangles.
Tom
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RE: Shortening guy lines on Rohn 25G tower
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by AA4PB on October 29, 2009
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I don't think the cost of a suitable supporting I-beam would come anywhere near the cost of the tower. Certainly some people just "guess" at it and wind up with a tower on the ground but given that he said he was going to get a structural engineer I anticipate that it will be done correctly in his case. I've seen quite a few ham and TV tower installations that are unsafe and some of them have ended up on the ground too. That doesn't mean that nobody should ever use a tower :-)
I calculate that if he sets the pole 10-feet back then it will need to be 12-feet tall to maintain the angle. He said 5-10 feet back so I'd pick 6-feet which would put the height at 7.4 feet so you could walk under it but still be easy to work on.
On the other hand, in this particlar case the short guy is also a reasonable and less expensive option as it doesn't change the guy angle all that much.
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RE: Shortening guy lines on Rohn 25G tower
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by AA4PB on October 29, 2009
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Here's a link to elevated guy anchors that were designed by a professional engineer in Maryland:
http://users.erols.com/n3rr/guyanchors/index.htm
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RE: Shortening guy lines on Rohn 25G tower
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by K9KJM on October 30, 2009
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I suspect that the elevated I beam anchors of N3RR cost more than many hams spent on an a small entire tower project....... The present cost of that size steel I beam will shock you! Plus the large excavation and how much concrete?
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