02/12/2001 Council Packet - Laserfiche WebLink

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1u /u.ivu LUr, 15:1$ FiL JUJ JUJ 401J The loads generated by this wind speed, along with weight of the monopole sections and any ice loading being considered, are used to design the structural system. To fail monopole sections, the wind must exceed all of our estimates for magnitude, duration, as well as overcome the factor of safety determined from the design. Say for instance, a 150 ft. monopole is designed for an 90 mph Basic Wind Speed. This Basic Wind Speed translates into a sustained, gusted wind speed of 145 mph at the top of the monopole. Considering a survivable wind speed which considers the ultimate yield strength of the steel, failure of the monopole would not be expected until wind speeds exceeded 162 mph. Beyond 162 mph we can only make an educated guess as to the failure mechanisms that would take place. The term Failure used above only suggests that "local buckling" has occurred at this point. "Local Buckling" is the result of a relatively small portion of the shaft distorting by "kinking" the steel. Local Buckling does not cause a free falling pole, rather it relieves the stresses from the pole at this location. If wind speeds continue to increase, the pole may continue to buckle and eventually crumple/fold over upon itself. Once this takes place, a sig aincant reduction in wind load will cause the pole not to experience further deformations. In the above scenerio, we are assuming sustained, gusted winds. As we all know, 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 darnage by impact loads such as a wind gust or earthquake shocks. It may take 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." It should also be noted that typical antennas are designed to survive 125 mph. With the absence of antenna loading the pole would have additional capacity. Valmont/Microflect's Pole design and full -scale testing have provided the public with a very reliable product. Poles have gone through extensiv&full scale testing, resulting in a history of being extremely reliable. This public, in my opinion, has been well served. Valmont/MViicrol'lect has provided structures that have performed well during the earthquakes in California, the hurricanes in the South (including Hugo, Andrew, and Opal), and a number of tornadoes. ValmontfMicroflect 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. Valrnont/Microflect has no direct experience with monopole failures. However, from what evidence we have seen from other vendor's experiences, I can state that it is rare for a monopole to fall in a radius larger than, say, half it's height, and that in most cases it will collapse in a small confined area as it comes down upon itself. 10 -03 -00 17:21 TO:US WEST WIRELESS Page 2 of 3 FROM:503 363 4613 P52