Stanley Fastening Systems

The practice of through-boring extends the service life of poles by protecting the pole from internal decay, but little is known about the effects of through-boring on the bending strength of utility poles. A preferred through-boring pattern was developed using current knowledge of preservative penetration, finite-element modeling, as well as the current literature on stress concentrations in circular members with diametral holes. Poles were subsequently through-bored and tested to determine if the boring hole size affected the bending strength of the poles when compared to a control group without any holes. The preferred pattern was a repeated staggered pattern with hole spacing 5 in. by 1 1/2 in. in the longitudinal and transverse direction, respectively, with a 2-in. edge distance. This pattern was applied to poles with 1/4-in, 1/2-in., 3/4-in., and 1-in. diameter holes in a zone −36 in. to +24 in. about the groundline of Class 4 poles. Results showed a slight decrease in average bending strengths, defined as the modulus of rupture (MOR) values, with increasing hole size. However, lower parametric tolerance limits for the through-bored poles were equivalent to or greater than that for the control (no holes) for all but the largest hole size group. The through-boring pattern tested with the smaller hole sizes did not significantly reduce the strength of the pole in bending when used in the proposed through-boring pattern. The most practical hole size for production would be a 1/2 in.-diameter hole with the 5 by 1 1/2 in. staggered pattern with a 2-in. edge distance