United States Department of Agriculture Designated Fiber Stress Forest Service for Wood Poles Forest Products Laboratory General Ronald W. Wolfe Technical Robert O. Kluge Report FPL–GTR–158 Abstract Abbreviations Wood poles have been used to support utility distribution ANSI American National Standards Institute lines for well over 100 years. Over that time, specifications ASA American Standards Association for a “wood utility pole” have evolved from the closest ASC O5 Accredited Standards Committee O5 available tree stem more than 15 ft in length to straight, ASTM American Society for Testing and Materials durable timbers of lengths ranging up 125 ft and base diame- COV coefficient of variation ters of as much as 27 in. The continued success of wood CSA Canadian Standards Association poles in this application is due in part to the development of DFS designated fiber stress consensus standards. These standards define the phrase EPRI Electric Power Research Institute “minimum acceptable” to the satisfaction of both users and LRFD load-and-resistance-factor design producers. They also encourage more competitive pricing by MOR modulus of rupture relaxing species as well as quality limitations, opening the NESC National Electrical Safety Code market to a broader range of available timber resources. The REA Rural Electrification Administration American National Standards Institute (ANSI) standard ANSI O5.1 is an internationally recognized standard that has SI conversion factors served as a guide for selecting the quality and size of wood Conversion utility poles for more than 70 years. From its inception, this English unit factor SI unit standard has addressed issues of relative load capacity as inch (in.) 25.4 millimeters (mm) well as physical quality to allow for species substitutions. In square inch (in2) 6.45 square centimeter (cm2) 2002, the relative strength evaluations previously published as a designated fiber stress took on added meaning when foot (ft) 0.3048 meter (m) pound per square inch 6.895 kilopascal (kPa) they were defined to represent the mean of the distribution of 2 (lb/in ) pole groundline strength values for various species. The temperature (°F) [T − 32]/1.8 temperature (°C) change in meaning was accompanied by a more rigorous F evaluation recognizing a change in strength with height and notation that pole strength distributions have a coefficient of Contents variation of 20%. This paper reviews the history and phi- Page losophy of the ANSI designated fiber stress to help the Introduction.......................................................................... 1 reader more fully understand and appreciate the significance Objectives ............................................................................ 2 of changes adopted by the American Standards Committee Background.......................................................................... 2 O5 (ASC O5) in 2002. Pole Strength........................................................................ 5 Keywords: pole, ANSI standard, fiber stress, strength, First National Standard Specification for Wood Poles ... 6 history Subsequent Standards for Wood Poles ........................... 6 Addition of H-Class ...................................................... 10 June 2005 Wood Pole Test Data ......................................................... 10 ASTM Wood Pole Test Program.................................. 10 Wolfe, Ronald W.; Kluge, Robert O. 2005. Designated fiber stress for wood EPRI Wood Pole Test Program .................................... 11 poles. Gen. Tech. Rep. FPL-GTR-158. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 39 p. Nominal Strength............................................................... 11 A limited number of free copies of this publication are available to the Initial Estimates............................................................. 11 public from the Forest Products Laboratory, One Gifford Pinchot Drive, Test Data and In-Service Performance ......................... 13 Madison, WI 53726–2398. This publication is also available online at ANSI O5-02.................................................................. 19 www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries in the United States and elsewhere. Conclusions........................................................................ 20 Observations ...................................................................... 21 The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. Literature Cited .................................................................. 22 The United States Department of Agriculture (USDA) prohibits discrimina- Appendix A—Effect of 2002 Revision on Pole-Class tion in all its programs and activities on the basis of race, color, national Strength Distribution.......................................................... 24 origin, sex, religion, age, disability, political beliefs, sexual orientation, or Appendix B—Wood Pole Test Data Sources .................... 26 marital or familial status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication Appendix C—Data Used to Derive DFS for Long of program information (Braille, large print, audiotape, etc.) should contact Poles................................................................................... 27 the USDA’s TARGET Center at (202) 720–2600 (voice and TDD). To file a Appendix D—Class Oversize Adjustment......................... 39 complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Wash- ington, DC 20250–9410, or call (202) 720–5964 (voice and TDD). USDA is an equal opportunity provider and employer. Designated Fiber Stress for Wood Poles Ronald W. Wolfe,1 Research Engineer Forest Products Laboratory, Madison, Wisconsin Robert O. Kluge,2 Team Leader, Design Standards American Transmission Company, Madison, Wisconsin Introduction In June 1931, standard specifications for wood poles were approved by the ASA (Jones 1931). These standards incor- Round timbers have been used structurally for centuries. porated a pole classification system originally developed by Over the past 140 years, round timbers have been used the American Bell Telephone Company in a standard speci- primarily for utility structures. As in so many other areas of fication aimed at maintaining uniform reliability for pole timber use, increased demand and competition from other structures, despite a range of species. The standards quanti- materials have spurred development and efficiency in the fied pole capacity using existing cantilever test data to assess structural use of poles. There is little doubt that Alexander groundline stress at failure for each recognized pole species. Graham Bell and Thomas Alva Edison played key roles in This value was then used to determine pole groundline di- initiating demand, but it is the influence of consensus stan- mensions required for each pole length and species. While dards (preservative treatment, strength tests, and material changes have been made to the designated fiber stress (DFS) specification) that has enabled wood to retain a dominant values referenced by the ASA standard, this basic approach role in support structures for telecommunications and elec- to classifying poles remained unchanged for over 70 years. tric power utilities. As utility structure loads and importance increased, design Prior to 1930, there was little demand for standards that became more of an issue. Most utility companies began rigorously addressed issues of the “design” capacity of wood using larger poles by the early 1960s, but they attracted little poles. The National Bureau of Standards (1927) began de- attention in the American National Standards Institute velopment of the National Electrical Safety Code (NESC) (ANSI) standard until 1972 when “H-class” pole sizes were for this purpose in 1913. Early concerns focused on durabil- introduced. Shortly thereafter, greater attention focused on ity and cost. American chestnut and western redcedar were the fact that the largest poles tested to that point were 55 ft in preferred for their durability. The second edition of the length with a butt diameter of 19 in., while the standard NESC (1916) recognized southern pine as a viable pole implied a knowledge of capacities for poles up to 125 ft long species after the development of creosote treatment. In that with a 30-in. butt diameter. Research conducted over the edition, southern pine was listed along with western redcedar next 20 years pointed to the need for change to the tradi- and chestnut as having an ultimate bending strength of tional pole classification algorithm. As larger poles were 5,000 lb/in2. By 1927, northern white cedar was added, with shown to have a lower groundline stress at failure, the need an ultimate strength of 3,600 lb/in2, and the bending strength to incorporate a pole “size” adjustment in the algorithm of “dense” southern pine was rated at 6,500 lb/in2. By this became apparent. This issue was added as an annex to the time, accumulating knowledge and experience related to the ANSI standard in 1986, but no changes were made to the strength of wood poles indicated that somewhat higher pole size classification tables. values might be allowed (Wilson 1923). A Sectional Com- mittee on Wood Poles was formed by the American Stan-