04-27-1999 Council Agenda - Laserfiche WebLink

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"a 3.25.1998- 2 :27PM COMM 'RJR 402 359 58,56 VALMONT. MICROELECT COMM mill N0.760 P.2/3 AUG 25 '98 01:1EFM Valmont Industries, Inc. • West Highway 275 • P.O. Box 358 Valley, Nebraska 68054-0358 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 andthe weight ofthe members (along with any ice considered) are then used to size members of the pole. There is at least a 25% factor sof of ety requir dununder these conditions. This assumes that the wind blowing from the worst po dire the directions are worse than others, depending on the equipment attached to the p 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 budding" where a relatively small portion of the shaft distorts and "kinks" the steel. This does not cause entire free 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 and buckling. section hlcely, however, to be out of plumb. This may be somewhat dramatic should be replaced • There are 3 mechanisms which prevent the pole from a free fall type failure.. First, as the pole distorts this distortion may relieve the load from the pole either by.orienting the pole more favorably in the wind or, if buckling has occurred, by reducing the moment arm of the wind 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 native 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 after a local buckle, the pole has significant capacity. It is this capacity along with the transitory nature ofthe 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 South, and a number of tornadoes. To my knowledge, Valmont has never experienced an in service the of communication pole due to weather induced overloading, even though' Hurricane Hugo and Hurricane Andrew, the wind speeds may have exceeded the design wind speed. Page 38