Agenda - Planning Commission - 01/04/2000

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' Valmont Industries, Inc,· West Highway 275 * P.O. Box 358. Valley, Nebraska 680640358 U.S.A. * (402) 359-2201 c~ges ~ w~d sp~ a heist ~e~cie~ m ~o~t for ~c~g ~d sp~ ~ hei~ ~d ~ e~os~e coe~ciont m ac~t (to some d~) ~ ~ eff~. The loads generated by this wind and'the weight of the members (along with any ice eo~idered) are then used to size members of the polo. Them is a~ least a25% factor of safety required under these conditions. This ~ssumes that tho wind blowing from the worst possible direction. Some directions are worse than others, depending on the equipment.at~aehed to the pole, the arrangement, and the orientation. Tho wind must exceed all our estimates for magnitu~, duration, be at tho worst orientation and overcome the factor of safety. Let us assume that a pole be~mmes overlo~ed. Tho typical consequence of this overl~ng is "local bueldlng" where a relatively small portion of the shaf~ distorts and "kinks" tho steel. This do~s not cause a free fatl/ng polo. Afar the buckle, the cross section of the polo is capable of carrying tlao entire vertical (weight) icad and a substantial portion of the load ttm caused the buckling. The pole is likely, however, to be ~ut ofplttmb. This may be somewhat dramatic and the buclded section should be replaced. Them are 3 mechanisms which prevent the polo from a free fall type failure.. Firs~ as the pole distorts this distortion may relieve the load from the pole either b¥.orien~g the pole more favorably in the w/nd or, if buckling has occurred, by reducing tho moment arm of the wind force. Tho second mechanism involves a redistribution of the stress in the pole after buckl/ng toward the rernain/ng 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, tho gust factor, and the factor of safety combined. A gust would soon dissipate and, after this peak w~d is gone, t~ stress in the pole would be reduced. Poles are flexible, forgiving structures which are not generally susceptible to damage by impact loads stroh as a wind gust or earthquake shocks. It takes some time for tile entire stmcttwe to "sce' the impact loading. Even a~,er a local buckle, tile pole ~ significant eapae/ty. It is this capacity along with th~ transitory nature of the load/rig that prevont~ a polo from "falling over". Pole design and te~ting have provided the public w/th 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 be~ ~orved well. Valmont ha~ provided structures that have performed well during the earthquakes in California, the hurricanes in the South, and a number of tornadoes. To my knowlexige, Valmont has never experienced an in service failure of a communication polo due to weather Induced overloading, even though, as fn the cases of Hurricane Hugo and Hurricane Andrew, the wind speeds ~_y have exceeded the design wind speed.