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'QED 10: 16FAX 503 316 2040 )MICROFLECT l002 Amtr--,sek.4‘1, 411 SUBJECT: Failure Modes and Design Criteria Information for Self- Supperting and Guyed Towers Design Criteria Communications cowers designed by Microflect are typically sized in accordance with the latest revision of the EIA/TTA 222 Standard entitled "Structural Standards for Steel Antenna Towers and Antenna Supporting Structures. " This standard has been approved by ANSI, who has generated the standard for ''Minimum Design Loads for Buildings and Other Structures" . This standard, which is the basis for design loading for practically every building code and standard in the country, has dealt with the design of lattice type structures for a large number of years . This standard is based on provisions of this nationally kncwn specification that has a long history of reliability. At it' s core root is it's first and foremost priority is to safeguard and maintain the health and welfare of the public. The standard dictates minimum wind loading the predominant loading on a tower structure) for each county in the United States. It is Microflect' s policy to use the wind loading listed in the latest • ETA standard as a minimum loading unless the customer specifies a larger value. Statistically, the wind speed _fisted in EIA-E has been determined to be that wind which has an average reoccurrence of 50 years (ie. the magnitude has a 2% chance of occurring in any one year; . This wind is also a "fastest mile" wind which, by definition, is the average velocity of a mile of wind passing a fixed point. This standard "fastest mile" wind is customized with height , exposure, and gust factors that apply to the particular Thstallations. The loads generated by this wind, along with weight of the tower members and any ice being considered, are used to size members of the tower. There is a minimum of a 25% safety factor required under these conditions (ie. each and every member has a reserve capacity which is 25% higher than the maximum load it will experience during the extreme 50-year wind) . Besides magnitude of the wind, the orientation to the tower is also critical. Some directions are worse than others, depending on placement of antennas, appurtenances, etc. To fail tower members the wind must exceed all of our estimates for magnitude, duration, be at worst orientation and overcome the factor of safety. Failure Modes Let us assume that a tower member force has exceeded all rational design criteria and has becomed overloaded. The typical • PLANT ADDRESS TELECOMMUNICATIONS MAILING ADDRESS 35'5 25" Street SE Telephone (503) 363-9267 P.O. Box 12985 Colum ;'bonen R73C•2.1190 FAX (5C3) 353-4513 Sa em OR USA 97309-0985