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VALMONT • Valmont Industries, Inc. •West Highway 275• P.O. Box 358 Valley, Nebraska 68064-0358 U.S.A. • (402) 359-2201 9/11/96 City of Elk River Planning Department City Hall 13065 Orono Road Elk River, MN 55330 ATTENTION: Mr. Scott Harlicker SUBJECT: Failure Modes for Poles and a Description of the Design Criteria A T and T Wireless Elk River, MN Pole Valmont Order 12867-96 Dear Mr. Harlicker: I have been asked to write to you about the mode of failure for pole type structures used to support cellular antennas and equipment, and give some information to allow you to judge how remote the chances of that failure occurring would be. I think it would be appropriate to start off by a brief description of the design criteria that is typically used. •The poles Valmont supplies are designed in accordance with the E.I.A. / T.I.A. 222 Revision F Standard entitled "Structural Standards for Steel Antenna Towers and Antenna Supporting Structures." This is an approved A.N.S.I. standard that has dealt with the design of lattice type structures for a number of years. Revision E was the first version that went into extensive detail about the design of pole type structures. The provisions of the standard have, as their foundation, provisions of other nationally known specifications and standards that have a long history of reliability. This history should be of interest to you considering the hundreds of thousands of sites throughout the United States which have structures designed with concepts spelled out in publications like "The Design of Steel Transmission Pole Structures" (published by A.S.C.E.), and "Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals" (published by A.A.S.H.T.O.). Many of the concepts used to design poles in these two publications are included in the E.I.A./ T.I.A. 222 Revision F Standard. The E.I.A. / T.I.A. 222 Revision F requirements would dictate in Hennepin County Minnesota a basic design wind speed of 80 mph and / or 69 mph with 0.5 inches of radial ice. The wind speed to be used depends on the location of the site within the state. Valmont's policy is to use the wind loading listed in E.I.A. / T.I.A. 222 as a minimum loading. A T and T gave us a wind speed of 90 mph with 0.5 inches of radial ice for this project. This represents and increase in loading of at least 27% more than the E.I.A. Standard would dictate. Statistically, the wind speed listed in E.I.A. / T.I.A. 222 has been determined to be that wind which has an average reoccurrence of 50 years. This wind is also a "fastest mile" wind which means that it is the average velocity of a mile of wind passing a point. For example, a 70 mph average fastest mile wind would take 51 seconds to pass a point. This standard "fastest mile" wind is customized with factors that apply to the particular installation. There is a 1.69 gust response factor imposed in order to account for sudden changes in wind speed, a height coefficient to account for increasing wind speed with height, and an exposure coefficient to account (to some degree) the terrain effects.