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.

摘要

横臂是用于支撑电线杆上的电线的重型木材。对横臂材料的要求相当严格，尤其是对节的存在的限制，节代表木材中的间隙或孔洞，还会导致可能影响性能的纹理偏差。严格的分级规则有时会导致可接受材料的短缺。在 200 个被拒绝的花旗松横臂和 50 个符合当前标准的臂上探索了接受具有轻微但目前不可接受的缺陷的臂的可能性。对臂上的结直径和位置进行映射，并在模拟实际线负载的专门设计的设备中对臂进行故障测试。所得数据用于计算断裂模量 (MOR) 和弹性模量 (MOE)。几乎所有失效的臂都高于最低美国国家标准假定的 MOR 值为 53.78 MPa，并且许多被拒绝的臂的 MOR 值高于可接受的臂。虽然被拒绝的手臂群体的特性差异更大，但结直径或结位置与 MOR 之间的相关性都非常差。r ²  ≤ 0.25)。结果表明需要进一步评估分级规则中对结的重视程度。