7.6. & 7.7. SR 08-20-2001 - Laserfiche WebLink

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-AU~.~5.199B~ ~:~TPM COMM TWR ~0~ 359 5856 COMM TWR M0.760 P.~3 AU~ ~5 ~98 Oi:16PM Valmont Industries, Inc. ° West Highway 275 · P.O. E&ox 358 Valley, Nebraska 68064-0358 U.5.A. · (402) 359-2201 clu~ngcs in wind speed, a height coefficient tn ~ccount for increasing wind speed with heist, and an exposure coefficient to account (to some degree) tim terrain e~ecrs. The loads generated by this wind and'the weight of the members (along with any ice cor[sidered) are then used to siz~ members of the pole. There is at least a 25% factor of s~ety required under these con Jitions. This assumes that the wind 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 the buckling. The pole is likely, however, to be out of plumb. This may be somewMt dramatic and the buclded section should be replaced. Them are 3 mechanisms which prevent the pole from a free fall type failure., l~irst~ 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 buclding has occurred, by reducing the moment ann of the wind force. The second mechanism involves a redistribution of the stress in the pole after buckling toward the rem,inlng portion or the cross section that has unused capacity. The third phenomenon and more important, is the nature of the force being applied. Wc expect the wind to produce this force. A wind tha~ would ca, se a buckle would be larger than the basic wind speed, the gust factor, and thc factor of safety c. ombined. A gust would soon dissipa~ 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 damag~ by impact lo,_d~s 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 tlm 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 web during the earthquakes in California, the hurricanes in the South, and a number of tornadoes. To my knowiedie~ Valmont has never experienced an in service failure of a eommuniemfion ole due to weather induced overloadinll~ even though, as in the cases of Hurricane Hult~P:nd Hurricane Andrew, thc wind speeds ma), have exceeded the design wind speed.