6.1. & 6.2. SR 10-21-1996 - Laserfiche WebLink

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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 is blowing from the worst possible direction. Some directions are worse than 'others, depending on the equipment attached to the pole, the 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 a free falling pole. After the buckle, the cross section of the pole is capable of carrying the entire vertical (weight) load and a substantial portion of the load that caused buckling. The pole is likely, however, to be out of plumb. This may be somewhat dramatic and the buckled section 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 of the cross section that has unused capacity. The third phenomenon and most 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 after 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 "falling 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 well served. Valmont has provided structures that have performed well during the earthquakes in California, the hurricanes in the South including Hugo, Andrew and Opal, (we have no word yet on Fran) and a number of tornadoes. To my knowledge, Valmont has never experienced an in service failure of a communication pole due to weather induced overloading, even though, as in the cases of Hurricane Hugo and Hurricane Andrew, the wind speeds may have exceeded the design wind speed. I hope this has helped. Please feel free to call with any comments you may have. I can be reached at 1- 800-345-6825- ext. 3739 and will be glad to discuss any concerns you may have. Sincerely, Anthony J. Hansen PE Manager of Communication Structure Engineering Valmont Industries Inc.