03-24-1999 Council Agenda - Laserfiche WebLink

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- 7-7,—'-fUG.25.1998— 2 :27PM COMM TWR 402 359 5956 VALMONT. MICROFLECT COMM TWR NO.760 P.2/3 AUG 25 '98 01:16PM Valmont InduaLiee, Inc. • West Highway 275 • P.O. Box 358 Valley, Nebraska 68084-0358 U.S.A. • (402) 359.2201 changes in wind speed, a height coefficient to account for increasing wind speed with height, and an exposure coefficient to account (to some degree) the terrain effects. The loads generated by this wind and•the weight of the members (along with any ice considered) are then used to size members of the pole. There is at least a 25% factor of safety required under these conditions. This assumes that the wind blowing from the �hpossible odirection. Some directions are worse than others, depending on the equipment arrangement, and the orientation. The wind must exceed all our estimates for magnitude, duration, be at the worst orientation and overcome the factor of safety. Let us assume that a pole becomes overloaded. The typical consequence of this overloading is "local buckling" where a relatively small portion of the shaft distorts and "kinks" the steel. This does not cause entire free me falling pole. After the buckle, the cross section of the pole is capable of carrying The pole is vertical (weight) load and a substantial portion of the load that caused the buckling, likely, however, to be out of plumb. This may be somewhat dramatic and the buckled section should be replaced. There are 3 meehaniama which prevent the pole from a free fall type failure.. First, as the pole distorts this distortion may relieve the load from the pole b etienn�g� polio more favorably in the wind or, if buckling has occurred, by force. The second mechanism involves a redistribution of the stress in the pole after buckling toward the remaining portion or the cross section that has unused capacity. The third phenomenon and more important, is the nature of the force being applied. We expect the wind to produce this force. A wind that would cause a buckle would be larger than the basic wind speed, the gust factor, and the factor of safety combined. A gust would soon dissipate and, after this peak wind is gone, the stress in the pole would be reduced. Poles are flexible, forgiving structures which are not generally susceptible to damage by impact loads such as a wind gust or earthquake shocks. It takes some time for the entire structure to "see" the impact loading. Even alter a local buckle, the pole has significant capacity. It is this capacity along with the transitory nature of the loading that prevents a pole from "failing over". Pole design and testing have provided the public with a very reliable product. Poles have gone through extensive full scale testing, resulting in a history of being extremely reliable. The public I think, has been served well. Valmont has provided structures that have performed well during the earthquakes in California, the hurricanes in the an is service a nu nber of of a tornadoes. To my knowledge, Valmont has never vi a the cases of of communication pole due to weather induced overloading, even though, as Hurricane Hugo and Hurricane Andrew, the wind speeds may have exceeded the design wind speed. Page 42