Agenda - Council - 04/11/2000 - Laserfiche WebLink

Home

cot~ Tl~ NO. Tf:~ P.E/~ ~ ~5 '~ 01:1~=F~ ,I I I I I I I I I I I I I ' Va/mont Industries, Inc. · We~ Highway 275. P.O. Box 358. Valley, Nebraska 68064-0358 U.S~, c~ges ~ w~d sp~ a heist ~e~oi~ m ~o~ for ~g ~d ~ ~ hei~ ~d ~ c~os~o coe~cie~ ~ acco~t (to some de~) ~ ~ cff~. The loads generated by this wind and'the weight of the members (along with any ice co~idered) are then used to size momars of the pole. There is at least a2~% factor of safety required under these conditions. This assumes that the wind blowing from the worst possible direction. Some directions are worse thrm others, depending on the equipment.attached to th~ pole, the arrangement, antl the orientation. The ~ 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 bueldlng" where a relatively small portion of the sha,~ distorts and"kinks" the steel. This does not cause a free failing pole. After the buckle, the cross section of tiao poIe is capable of carrying the entire vertical (weight) load ad a substantial portion of the load the. t caused the buckling. The pole is likely, however, to be Out of plumb. This may be somewhat dramatic and the buckled section shouId be replaced. There are 3 mecb_anSsrns which p~vent the pole from a flee fall type failure.. 1;L,~ as the pole distorts this dL,'tortion may relieve the load from Ge pole ei~er by.orlenti~g the pole more favorably in the wind or, if buck~Lng has occ~ by reducing the moment arm of the wind force. The second mechanL~m involves a redistribution of the stress in the polo after buckling toward the temalnlng portion or the cross section fiat b~ unused capacity. Th~ 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 buo -Ide 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 siztm'tah-e to "see' the impact loading. Even atk-r a local buckle, the pole has sign~cant capacity. It is this capacity along with the transitory nature of the loading that prevents a pole from "failing over". Pole design and testhzg 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 thinly, has been served well. Valmont has pro~idecl strnctures that have performed weft durinll the earthquakes in California, the hurricanes in the South, and a number of tornadoes. To my knowledge, Valmont has never experienced an in service failure ora communication pole due to weather induced overloading, even though, as in the cases of Hurricane Hugo and Hurricane Andrew, the wind speeds my have exceeded the design wind speed. i\