Effect of Holes Drilled Various Distances from the Edge of Douglas Fir Utility Poles Tested in Bending

Abstract

Abstract Wood utility poles are one of the most critical components of U.S. infrastructure. In the Pacific Northwest, Douglas fir is the species of wood predominantly used. Douglas fir has a relatively thin band of treatable sapwood surrounding an impermeable, but only moderately durable, heartwood core that is susceptible to internal decay. Historically, through boring has been the most commonly adopted practice for improving the performance of Douglas fir poles. Despite the inclusion of through boring in national utility standards, questions continue to rise regarding through boring. One important question regarding the process is how close to the edge holes should be drilled. The objective of this study was to assess the effects of through boring edge distance on the flexural strength of Douglas fir poles using a combination of full-scale flexural testing of poles as well as finite element analysis (FEA). Full-scale testing of 48 utility poles revealed that drilling holes within 50 or 75 mm of the outer edge of a pole had no significant effect on flexural properties. FEA consistently predicted failure modes and flexural properties after accounting for through boring and knot locations. These models could be improved through more accurate representation of grain around knots and then used to explore the effects of different sizes, species, quantities of knots and checks, and other drilling patterns on strength, and, therefore, would reduce the need for actual physical testing.