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A thought exercise about guy wires
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? Thanks for your help. |
wrote in message ups.com... 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? Thanks for your help. AS you said , not recoomeded for only one set at that heigth but the top set of guy wires always go near the top of the tower. |
I would put them at the center
as I see buckling as the worst danger. I have also done this with a 60 foot boom antenna with no problem. I was forced into this position in the first place because I have a fold over tower and eventualy stopped using the top guy wires thru lazyness. Art "Ralph Mowery" wrote in message ink.net... wrote in message ups.com... 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? Thanks for your help. AS you said , not recoomeded for only one set at that heigth but the top set of guy wires always go near the top of the tower. |
wrote in message ups.com... 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? If the tower is 4 feet across at the base, you may not have a problem, otherwise - If you do that, be sure the tower is more than 60 feet from your house - so it will miss the house when it falls. Most of the water towers you see are a hundred feet or higher, but have no guy wires, because the base is pretty wide. guying at the top only may cause it to bow in the middle during a windstorm to such a degree as to collapse. I would guess the tower manufacturers would have wind and static loading data available. You have wind loading and the static loading due to gravity - which begs to become dynamic. try practicing with half-inch pvc water pipe - see where the guy wires do the most good. |
wrote:
I can only put one set of guy wires up, should they go near the top of the structure or near the middle? According to Rohn specs, a single set of guy wires limits your tower to 40 feet maximum height - sorry about that. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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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. |
That wasn't the question.
"Cecil Moore" wrote in message ... wrote: I can only put one set of guy wires up, should they go near the top of the structure or near the middle? According to Rohn specs, a single set of guy wires limits your tower to 40 feet maximum height - sorry about that. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
CW wrote:
That wasn't the question. No, but that was the answer AFAIAC. "Cecil Moore" wrote: According to Rohn specs, a single set of guy wires limits your tower to 40 feet maximum height - sorry about that. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
"(A)What are the forces that the wind exerts on a tower and
(B)are these forces evenly distributed?" A. More than you might think! B. No. Depends on the particular configuration of the tower's load, and the strength/speed of the wind. Infinitely variable. You already know that there's no 'good' solution for one set of guywires. The 'best' solution I've heard so far is an excess of insurance and a gulible claims adjuster. 'Doc |
Thank you all for the information. I knew I could find good"stuff" out
there. The wind generator (not an antenna) I was planning has limited information available as to loads. ie. No Flat Plate or FPE numbers. The only statement is that the tower must take a 200 pound lateral thrust and not deflect more than six inches with winds of 200 mph. The Trylon T-200 tower specs. for my maximum wind speed area (from the chart at their web site), shows that the T-200 will take a 42 pound lateral thrust at winds of 85 mph. If 120 mph winds cause the wind generator to exert 200 pounds of lateral thrust, what speed of the wind will yield the 42 pound maximum lateral thrust that the tower will hold? Please show your work. |
fcrumpler,
Figured as a simple ratio, about 25-30 mph. No idea if that's accurate... 'Doc |
Dear "fcrumpler" no call, no location:
Theoretically, the force produced by moving air impacting on a flat plate is proportionally to the square of the air's velocity. However, in the real world where one should appropriately deal with wind gusts and variability of materials, things are not so simple. Safety factors should be used. Accommodation for the significant vibration to be caused by your proposed "wind generator" must be made. Numbers supplied by some vendors involve no safety factors. You need to retain a Professional Engineer licensed in your state or province who is experienced with vertical wind structures. This is certainly the case if the proposed structure is able to damage other property or endanger living things. It is not safe for you to scale the numbers that you have. Too little information. Too much risk. Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: |
Cecil, !5DXP wrote:
"According to Rohn, a single set of guy wires limits the tower to 40 - feet maximum height - sorry about that." Well, it depends on the Model No. and how you will load the tower. It is best to follow thw manufacturer`s recommendations to the letter. There is a length that you can stack which will endure without guys in most cases. Towers usually come in 20-ft. sections. Often one or two sections is all it is safe to use without guys. Windloading the tower is expected to withstand, often determines space between guy points. Wind exerts force on the tower and its appurtences, often expressed in a sum of pounds per square foot on the area facing the wind.. Resistance (drag) produces a differential pressure on items in the wind. Force is proportional to the projected area in the wind, and varies with the shape and roughness of the item in the wind. Windload on a tower is not uniform. The wind usually blows harder at higher altitudes. Wind tries to turn the tower over but until something yields, it is in equilibrium and the summation of of forces about the base of the tower is zero. The summation of torques is zero. Guywires exert force in yje only way they can. They have tension in the direction of the wire. This may be resolved into vertical and horizontal components. This prevents movement and increases base loading when the wind blows. Pressure on items in the wind (pounds per square feet) is a function of the square of the wind velocity (miles per jour). One formula considered useful is named "Ensewiler" which is said to give: 22.5 psf = 85 MPH 45 psf = 134MPH 60 psf = 155MPH 75 psf = 173MPH Best regards, Richard Harrison, KB5WZI |
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