Agenda - Planning Commission - 03/06/2000

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' · Valmont Industries, Inc. · We~ Highway 275, RO. Box 358. Valley, Nebraska 680640358 U.S.A. · (402)359-2201 changes in wind spe~i, a hcight coefficient to account for ina, rasing wind spe~l with heist, and an cxposure coefficient to account (to some degree) the terrain effects. The loads generated by this wind and'the weight of the m0mb~rs (along with any ice co~id~red) are then used to size members oftha pole. There is at least a2~% fa~tor of safety required under these conditions. This assumes that the wind blowing from tho worst possible direc~on. Some directions are worse than o~ers, depending on the equipment..a!Iached to the pole, the arrangement, and the orientation. Tho wind must exceed all our eaimates for magnitude, duration, be at the worst orientation and overcome tho factor of safety. ~ 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 sloeI. This does not cause a free falling pole. After the buckle, the cross section of the pole is capable of carrying tho entire vertical (weight) load and a substantial portion of the load that canseA the buckling. The pole is likely, however, to be Out of plumb. This.maY be somewhat dramatlo and the buckled section should bo replaced. There are 3 mechanisms which prevent the pole Dom 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 buckl_ing has occurred, by reducing the moment ann of the wind force. The second mechani,~m involves a redistribution of the stress in the pole affex buckling toward the remaining portion or the cross ltoCtiOU that h~ UlRlsed capacity. The third phenomenon and more important, is the nature of the force being applied. We exper 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 o£sdety combined, A gust would soon dissipate and, after this peak wind is gone, the stress in the pole wouM be reduced. POles are/lera"vl~, forgiving structures which are not generally susceptible to damage by impact loads such as a wind gust or earthquake sh~cks. It takes some time for the entire smmure to "see" the impact loading, Even at%or a local buckle, the pole has significant capacity. It t~ this capacity along with the transitory nature of the loading that prevents a pole from eT-ailing over~. Pole design and testing have provided the public with a very reliable product. Polos have gone through extensive full scale ~esting~ resulting in a history ofbelng extremely reliable. The public I think, has bi.al served well. raiment has provided structures that have performed weft during the earthquakes in California, the hurricanes in the South, and a number of tornadoes. To my knowledge, Valmout has never experienced an in service f~tlure eta communication pole due to weather induced overloading, even though, as iu the cases of Hurricane Hugo and Hurricane Andrew, the wind speeds may have exceeded the design wind speed.