Strength Ratios of Knots in Bending for Two Alaskan Softwood Species

Abstract

The effects of knots on the bending strength of dimension softwood lumber were investigated for two Alaska-grown species, yellow cedar (Chamaecyparis nootkatensis) and Sitka spruce (Picea sitchensis). An empirical measure of the bending strength ratio for knots was determined by comparison of full-size in-grade test results to corresponding test results from matching small clear specimens. The empirical strength ratios were compared with theoretical strength ratios calculated using existing formulas given in ASTM D245 (ASTM International 2005). It was determined that existing formulas do not provide accurate estimates of bending strength ratio for the two Alaska-grown species under study. Additional predictor variables were used to develop a new model to estimate the influence of knots on the bending strength ratio. Comparison of new formulas to existing formulas demonstrates that this new model is more effective at accounting for variations in bending strength ratio caused by the presence of a knot. It was concluded that the use of the models presented in the 2005 version of the ASTM standards for knot bending strength ratios should be reevaluated. This is especially important for any new timber species being added to the design tables. Species-specific models for estimated bending strength ratios, utilizing additional predictor variables, would help to better predict the effect of knots on bending strength of structural lumber.