To: |
"Roger \(K8RI\)" <K8RI-on-TowerTalk@tm.net>, "Steve Maki" <StevesLists1@gmail.com> |
---|---|

Subject: |
Re: [TowerTalk] Guying a self-supporter |

From: |
"Bill" <w7vp@comcast.net> |

Date: |
Fri, 7 Nov 2008 08:09:39 -0800 |

List-post: |
<towertalk@contesting.com">mailto:towertalk@contesting.com> |

Hi Roger Let me summarize the loads on the tower. Remember that in the equilibrium condition the sum of the forces and the sum of the moments have to equal zero. 1. Compressive load from the weight of the tower itself plus the structures attached to the tower. These create compressive stresses in the legs of the tower and are applied at the point where the thrust bearing transfers the load to the tower. 2. Compressive load induced by the guys. These add to the compressive stresses in the tower legs. They are applied at each location where the guys are attached to the tower. 3. The twisting moment due to uneven wind loading of the antenna system or due to rotational inertia in starting and stopping the rotation applied at the base attachment point of the rotator to the tower. These loads are partially transferred to the guys through torsion brackets but the actual moment is also transferred partly to the tower since there must be some tensional deflection of the tower for the guys to pick up some of the load. These create shear stresses in the legs of the tower. 4. The bending moment from wind on the antennas and mast. These create bending stresses in the tower legs that depend on the section modulus of the tower and the worst case is usually the compressive part of the bending stresses since they add to the tower compressive stresses. These may be reduced to some extent by the proper location of the upper guys though I doubt the reduction is very much in most cases. This moment is applied to the tower where the radial bearing point is located. 5. The bending moment of the wind acting on the tower itself. Again this creates a tensile and a compressive stress with the latter probably being more significant. 6. The bending moment from the column loading induced by (a) the weight of the tower and everything attached to it, (b) the bending moment of the wind on the mast and antenna, and (c) the bending moment of the wind on the tower itself. This load is non-linear since it results from the deflection profile of the tower and creates a moment based on the other loadings being applied at a deviation from the centerline of the column when the deflection occurs. It is significantly affected by the relationship between the section modulus and the length of each section of the column, taking into account the distance between guys, and can significantly increase the stresses. All the stresses must be combined together in a way that determines the magnitude of the principle stress to assure that the principle stress does not exceed the yield strength of the tower material. How these are affected by wind loading, rotation and placement of the guys is the trick and are probably not much of a challenge to a qualified structural engineer using computerized structural analysis methods these days. One thing I have done with my own tower is to build a triangular bracket which is attached at the base plate of the rotator. The bracket is attached to the tower legs so that each leg of the bracket is attached to two legs of the tower. This give a longer moment arm for the guys to pick up the rotational moments of the antenna system. Even better would be to create a "Y" in the guys near the top so that the moment from the rotator is picked up at an angle on the base thus increasing the effectiveness of the guys in resisting rotational moments. 73 Bill W7VP ----- Original Message ----- From: "Roger (K8RI)" <K8RI-on-TowerTalk@tm.net> To: "Steve Maki" <StevesLists1@gmail.com> Cc: "towertalk" <towertalk@contesting.com> Sent: Thursday, November 06, 2008 9:55 PM Subject: Re: [TowerTalk] Guying a self-supporter > |

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