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A PIRO inc. <br />November 29, 1989 <br />Mr. Tim Malloy <br />D.S.U. Inc. <br />300 First Ave. North <br />Suite 210 <br />Minneapolis, MN. 55401 <br />Dear Mr. Malloy: <br />1200 N. OAK RD. <br />P.O. BOX 128 <br />PLYMOUTH, INDIANA 46563-0128 <br />(219) 936 -4221 <br />FAX (219) 936-6796 <br />Thank you for your inquiry regarding self - supporting tower design <br />ractices and predicted failures. <br />While failure is extremely rare in any king of tower, it is <br />especially so for self- supporting towers. In fact, only if a <br />tower were subjected to a direct hit from a tornado or the <br />severest of hurricanes would failure be predicated. <br />We aL aware of only a very few documented instances of self - <br />supporting tower failure. In each of these instances, the <br />failure was in the upper middle region of the tower, with the <br />upper portion of the tower remaining connected and "bowing over" <br />against the base of the tower. The fact that the wind is <br />normally greater on the upper portion of the structure <br />contributes to the likelihood of this type of failure. In <br />addition, the factors of safety in the tower in the upper middle <br />region are kept somewhat below the factors of safety in the rest <br />of the tower. Thus, if a failure condition is reached, it should <br />be'reached in the upper middle region of the tower first. <br />Needless to say, the engineering codes which govern tower designs <br />are extremely conservative. Towers are designed for extreme wind <br />conditions, and even under these extreme conditions, stringent <br />factors of safety are required. <br />Should you have any questions, on the foregoing, please do not <br />hesitate to contact us. <br />Myron C. Noble P.E. <br />President I hereby certify that this elan, specifi- <br />cation, or report w,,s piep,,red by me or <br />under my direct supervic.i..n and that I am <br />a duly Registered I r9 si nzl Etigneei,- <br />nder the laws of�ttie St to f Mirline•'sota) <br />MCN /lsc <br />Registered No.�3T- a-S- -� <br />Page 54 <br />