Designated Fiber Stress for Wood Poles

摘要

Wood poles have been used to support utility distribution lines for well over 100 years. Over that time, specifications for a 'wood utility pole' have evolved from the closest available tree stem more than 15 ft in length to straight, durable timbers of lengths ranging up 125 ft and base diameters of as much as 27 in. The continued success of wood poles in this application is due in part to the development of consensus standards. These standards define the phrase 'minimum acceptable' to the satisfaction of both users and producers. They also encourage more competitive pricing by relaxing species as well as quality limitations, opening the market to a broader range of available timber resources. The American National Standards Institute (ANSI) standard ANSI 05.1 is an internationally recognized standard that has served as a guide for selecting the quality and size of wood utility poles for more than 70 years. From its inception, this standard as addressed issues of relative load capacity as well as physical quality to allow for species substitutions. In 2002, the relative strength evaluations previously published as a designated fiber stress took on added meaning when they were defined to represent the mean of the distribution of pole ground-line strength values for various species. The change in meaning was accompanied by a more rigorous evaluation recognizing a change in strength with height and notation that pole strength distributions have a coefficient of variation of 20%. This paper reviews the history and philosophy of the ANSI designated fiber stress to help the reader more fully understand and appreciate the significance of changes adopted by the American Standards Committee 05 (ASC 05) in 2002.