Preliminary evaluation of a density-based lumber grading method for hem-fir CLT manufacturing

In the manufacturing process of cross-laminated timber (CLT), lumber selection and grading are essential steps for the quality control of CLT. In this work, a density-based lumber grading method was preliminarily evaluated for hem-fir CLT manufacturing. Based on the univariate regression model of hem-fir lumber modulus of elasticity (MOE) and its density, a density-based grading model was developed. To identify the effectiveness of the model, 470 pieces of thin hem-fir lumber, were segregated into four grades: E1, E2, E3, and E4 based on their density values. Two 3-ply thin-type customized CLT panels were also fabricated with E2 grade lumber as the parallel layer and E3 grade lumber as the transverse layer. Then, five CLT beam specimens were cut from each panel for flatwise bending test. Comparing the experimental and the theoretical values of the MOE, the relative errors were found to be + 11.95% (using shear analogy theory) and − 7.21% (using gamma theory), respectively. By comparison, for this hem-fir lumber, the accuracy of this density-based grading method is close to that obtained by other means of non-destructive testing. However, it seems to be simpler and more cost-effective for the potential hem-fir CLT manufacturing in some circumstances.