Ability to predict flexural properties of Douglas-fir crossarms

ABSTRACT

Crossarms are heavy timbers used to support electrical lines on utility poles. The requirements for crossarm material are fairly strict especially with respect to limitations on the presence of knots, which represent a gap or hole in the timber and also result in grain deviations that can affect properties. The strict grading rules can sometimes result in shortages of acceptable material. The potential for accepting arms with minor, but currently unacceptable defects was explored on a population of 200 rejected Douglas-fir crossarms and 50 arms that met current standards. Knot diameter and location on the arms were mapped and the arms were tested to failure in a specially designed apparatus that simulated an actual line load. The resulting data were used to calculate Modulus of Rupture (MOR) and Modulus of Elasticity (MOE). Nearly all arms failed above the minimum American National Standards assumed MOR value of 53.78 MPa and many of the rejected arms had higher MOR values than acceptable arms. While there was more variability in properties for the rejected arm population, correlations between knot diameter or knot location and MOR were all exceeding poor (r² ≤ 0.25). The results suggest the need for further evaluation of the emphasis on knots in grading rules.

KEYWORDS:

• Crossarms
• flexural properties
• knots
• Douglas-fir
• MOR
• MOE

Disclosure statement

No potential conflict of interest was reported by the author(s).