Stress-grading of wood utility poles

Superposition of the longitudinal stress concentrations associated with knots was used to determine the interaction distances between different knot sizes. However, only 55 out of 4,374 knots measured in 44 Douglas-fir poles showed significant (10% increase in the maximum stress concentration) interactions with their adjacent knots. The assumption that stress interaction between knots is a major factor contributing to the failure of poles under bending was not proved significant.; A procedure to develop a nondestructive evaluation (NDE) grading system for wood utility poles was described and proposed under the consideration of economic and design aspects. The proposed NDE grading system was calibrated to provide at least the same economic benefit to the producers and at least the same structural reliability as currently exists with visually graded-poles. The proposed NDE grading system retains the size classification currently used by American National Standard Institute (ANSI 05.1), but would upgrade or downgrade poles according to their strength. The horizontal load capacities at two feet from the pole tip for different classes of poles in a given NDE grade can be obtained by multiplying the horizontal load capacities currently provided by ANSI 05.1 by NDE-grade dependent strength weighting factors.; Additionally, a method is developed to estimate the standard deviation of any NDE grade with a known strength interval, and the associated standard error of estimate from the relationship between NDE predicted strength and actual strength. With the estimated NDE grade standard deviation, a new set of overload capacity factors (OCF) were therefore computed. The OCF for NDE graded poles was designed in such a way that the pole structure will possess the same reliability as currently exists with visually graded poles.; The data used to derive the NDE grading system were the strength of new, green transmission size Douglas-fir poles. The NDE-based grading system can be easily adapted to other species provided that their relationship between the NDE parameters and actual strength is known.