Graduate Thesis Or Dissertation


Reassessment of Grading Criteria with Respect to the Flexural Properties of Douglas-fir Crossarms Public Deposited

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  • Wood plays a key role in modern electrical infrastructure; crossarms and power poles utilize the high strength to weight ratio, availability and low cost of wood to facilitate the access of power to millions of Americans. However, wood presents a variety of challenges as a material. In structural applications, one important challenge is the presence of knots and other defects. Knots have long been recognized as crucial defects in structural timber and lumber-grades have consistently incorporated knot limitations into grading criteria. The following tests were performed on Douglas-fir crossarms to better understand how the presence of knots affects overall strength and, whether current grading criteria mitigate these effects. The full-scale tests simulated the complex loading conditions of a crossarm in the field. Two hundred fifty kiln dried Douglas-fir crossarms (3 ¾ in by 4 ¾ in by 8 ft.) were acquired, and knots and other defects were measured. Two hundred specimens had been rejected by the current ANSI standards while the remaining fifty were ANSI compliant. Specimens were placed in the test apparatus with connections utilizing pre-drilled holes from the manufacturer and subjected to a diagonal point load. Maximum bending moment and deflection were measured for each specimen. Statistical comparisons between the ANSI compliant and non-compliant arms were conducted and regressions analyses were performed on knot size, location and number versus bending strength and modulus of elasticity. Most of the rejected arms had flexural properties that were similar to those of the acceptable arms. Modulus of rupture or modulus of elasticity were poorly correlated with knot characteristics. The results suggest that reliance on knot position is a poor predictor of crossarm capacity and further trials are recommended to better understand how defects affect properties.
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