Establishing a through-boring pattern and method of test for utility poles

Graduation date: 2006The goal of this study was to optimize a through-boring pattern that will provide adequate preservative penetration while minimizing the reduction in overall pole bending strength. Hole spacing was developed using longitudinal and transverse chemical penetration data from previous studies. Finite element analysis was used as a preliminary assessment tool to examine the effects of hole size, hole spacing, and loading direction on the peak stresses in a through-bored pole section. The through boring pattern was a replicated staggered pattern with 5-in. longitudinal spacing and 1-1/2 in. transverse spacing, and 2-in minimum clear edge distance. The holes were parallel to each other and perpendicular to the longitudinal axis of the pole. The hole pattern and was applied to the poles in a zone 36 in. to +24 in. from the groundline. Through-bored poles were tested in a modified 4-point bending test to assess the effect of hole size on the bending strength of the lower half of the pole. The experimental design was a completely randomized design with 140 poles assigned to five treatment groups of four different hole sizes (1/4-in., 1/2-in., 3/4-in, and 1-in.) and a control group. Douglas-fir poles, green and untreated, were the experimental units for the study. The class 4, 40-ft poles were cross-cut into two parts: the top 20-ft section was set aside for another study while the butt ends with the through-bore treatments were tested. The pole was oriented for testing so that the load was applied perpendicular to the longitudinal axis of the drilled holes. The poles were ramp-loaded until failure in a modified 4-point bending test method to simulate the stresses on a utility pole embedded in soil. This setup facilitated an examination of the critical, through-boring effects on the pole and included stresses below the groundline. Average mean bending strengths of the five different groups showed a highly linear trend with small decreases in mean bending strength with hole size increases. The 1/4-in, and 1/2-in, hole groups had bending strengths not significantly different than those of the control group at the 5-percent significance level. This suggests that the smaller holes did not significantly affect the overall bending strength of the poles. However, the presence and size of holes did affect the variation in bending strengths among hole-size treatments. The control group, whose strength was largely dependent on knots and other strength reducing defects, had a coefficient of variation of 18 percent while the four hole treatment groups had coefficients of variation between 11 percent and 13 percent. The lower variability of the hole treatment groups may have a positive influence on bending strength design values if pole design values were based on the 5-percent parametric tolerance limit (PTL). When the PTL of the treatments with holes are compared to the control group, all treatment groups but that with the largest holes had higher PTL values than the controls. This could result in potentially higher design values even though the trend of lower mean strengths existed for poles with larger holes. In general, the results showed that through-boring using holes less than or equal to 1/2-in, does not negatively affect pole bending strength when used in the specified pattern