A New Look At Mechanical Lumber Grading

This article attempts to reappraise the whole question of machine stress-grading. The proposal is made that there should be a change in approach, with greater emphasis being placed on modulus of elasticity. Working stresses in flexure are of relatively less importance for most uses of dimension lumber and need not always be estimated with any degree of precision. Modulus of elasticity (E) is an extremely important property of dimension lumber used for joists and decking. In many cases it is the most important property. Machine grading for E would improve the accuracy of deflection calculations, would separate pieces with inherent differences in wood density and strength such as black and white spruce and western hemlock and amabilis fir, and would provide a new tool for calculating span tables for load-sharing systems. Accurate estimates of strength are not always needed but, when they are, accurate E values for every piece are a help rather than a hindrance to their determination. In short, the machine grading of lumber for E should materially improve the efficiency of utilization of construction lumber.

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