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## [TowerTalk] ROHN SPECIFICATIONS

 To: [TowerTalk] ROHN SPECIFICATIONS n4kg@juno.com (n4kg@juno.com) Thu, 23 Dec 1999 07:50:31 -0600
 ```The old Rohn catalogs only gave load ratings based on the full rated height of each tower type (200 ft for R25, 300 ft for R45). No deratings were called out for lower heights. All guying illustrations showed guys equally spaced along the full height of the tower (i.e. top guy attached to the top of the tower). The new Rohn catalog (mine is dated 1994) shows increasing loads for lower heights. This is a welcome clarification but the examples show the top guys for R25 placed 5 ft down from the top of the tower and the top guys for R45 placed 9 ft down from the top of the tower. The ratings are appropriate only if ALL of the antenna load is placed along the open tower above the top guy. If you extend a mast above the top of the tower, the bending moment applied to the tower can become excessive. To minimize the bending moment applied to the tower, the largest antenna should be placed as close to the guys as possible. With the guys 9 ft down from the top, the only way minimize the bending moment and rotate the antenna is to use an expensive ring rotor or side mount just above the guy wires. If the large antenna is placed one foot above the top of the tower the bending moment will be 10 (ft) X the antenna windload vs 1 or 2 (ft) times the windload if the guys are placed at the top of the tower. The bending moment for an antenna at the top of a 12 ft mast will be (12 + 9) X the antenna windload vs. (9 + 1 or 2) X the antenna windload if the guys are placed at the top of the tower. Example: Consider two antennas with 8 sq ft of windload placed 1 ft above the top of the tower and 12 ft above the tower top with a wind force of 30 psi (87 mph). The windload forces will be 240 lbs at each antenna. With the top guy 9 ft below the top of the tower, the bending moment is (9+1) X 240 + (9 + 12) X 240 = 7440 ft lbs. With the top guy 1 ft below the top of the tower, the bending moment is (1+1) X 240 + (1 + 12) X 240 = 3600 ft lbs. The bending moment contribution of the mast and tower was not included in these calculations. Notice that the guying configuration shown in the new Rohn catalog results in more than twice the bending moment applied to the tower for these moderate sized antennas (similar to TH6 for example). Full size 3L40's or 2L80's will be much worse. When using a rotating mast to support more than one antenna above the top of a tower the top guys should be placed as close to the top of the tower as possible. This is the ONLY way to safely realize the full capacity of a guyed tower with multiple antennas. The amateur community needs to encourage Rohn to provide load ratings for their guyed R25 and R45 towers with the guys placed at the top plate or first step down from the top. You may ask, "Is the bending moment at the top of a guyed tower a real consideration?" I'm glad you asked. The answer is a resounding YES! One of locals had a Force 12 6L20 (8.3 sq ft) mounted above a Force 12 2L40 / 2L80 (13 sq. ft.) on 130 ft of Rohn 45 with the top guy 9 ft below the top of the tower. The rotor was approximately 5 ft down from the top of the tower. A storm ripped the top plate off the top section causing the mast and antennas to plummet to the ground. He suspects a cold weld may have contributed to the accident but the above calculations show that the load born by the tower is considerably higher when the guys are placed below the top of the tower. de Tom N4KG ___________________________________________________________________ Why pay more to get Web access? Try Juno for FREE -- then it's just \$9.95/month if you act NOW! Get your free software today: http://dl.www.juno.com/dynoget/tagj. -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
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