There are over 130 million wood poles supporting electrical transmission and distribution lines in the U.S. The vast majority of these poles are preservative treated to prolong their useful life. In some cases, however, the depth of treatment is relatively shallow, leaving a deep zone of moderately durable, untreated heartwood. This zone is susceptible to the development of internal decay that reduces pole capacity and shortens service life. Douglas-fir is one such species. Extensive efforts by Pacific Northwest utilities in the 1960’s led to the development of a number of methods for improving the treatment of heartwood in Douglas-fir. One of the most popular is through boring which involves drilling a series of holes through the cross section in critical decay areas such as the groundline, prior to treatment. The process results in nearly complete treatment of this zone.
Any process that drills holes in a pole removes cross sectional area, creating the potential for reduced flexural properties. Engineers have long been concerned about the effects of through boring. Prior studies; however, have shown that through boring produces only a minimal effect on modulus of rupture (MOR) or modulus of elasticity (MOE) and the process has been standardized through the American National Standards Institute (ANSI) with a 5 % reduction in properties compared to non-through bored poles. However, there are still lingering questions about the process. One of these questions is the possible effect of proximity of through bored holes to the edge of the pole. Current standard specify a minimum edge distance of two inches; however, some utilities extend this distance to three inches. This increases the risk of incomplete preservative penetration that might reduce pole properties.
In order to address the issue, finite element modeling was used to examine the effect of edge distances ranging from 1 to 3 inches on pole properties. The models suggested that the current 2 inch edge distance would have no significant effect on pole properties. Forty eight air-seasoned Douglas-fir poles were used to confirm the model results. The poles were divided into 4 groups of 12 poles each. Poles in a given group were left without holes or drilled so that the through boring holes were 1, 2, or 3 inches inward from the outer edge. The poles were then tested to failure in a four point bending test and the results were used to calculate MOR and MOE. While MOE and MOR were lower than those found in previous tests, there were no statistical differences between the treatments.
The Mean Stress Method was used to simulate wood anisotropy, as stress concentration theory over-predicts stress in wood products. This was implemented as a post processing tool and used the Hoffman failure criterion to determine strength.
The numerical model or Finite Element Model was able to predict the location of failure in 36% of poles, and failure criteria within 25% of the experimental load. The model predicted failure location for 40% of poles from a previous test using the same testing procedures, and failure criteria of 23% of the experimental load. The model predicted an observed significant difference within the previous data, but was not sensitive enough to predict a smaller difference. The model cannot predict the properties of individual pole, but may be useful for examining the relationships between anatomical and mechanical properties collected here to model changes in failure location and strength between different drilling patterns.