Bending strength and stiffness of in-grade Douglas-fir and southern pine No. 2 2 × 4 lumber

Abstract

Douglas-fir ( Psuedotsuga menziesii (Mirb.) Franco) (DF) and southern pine ( Pinus spp.) (SP) trees are increasingly grown on intensively managed plantation forests that yield excellent growth. Lumber cut from these trees often contains a large percentage of juvenile wood, which negatively impacts its strength and stiffness. Design values of lumber must accurately reflect the available forest resource, and because the design values were determined over 25 years ago, questions exist whether wood quality has declined. To help address this, 1488 samples of commercial-grade No. 2 2 × 4 DF and SP lumber were destructively tested in edgewise bending. Mean stiffness of DF and SP was 11.51 and 10.89 GPa, respectively, comparing favorably with the 11.02 GPa mean design values; however, variation for SP was higher than in previous studies. The nonparametric 5th percentile bending strengths for DF and SP were 8.30 and 9.06 MPa, respectively. Both DF and SP tested at less than their design values, 9.3 and 10.34 MPa, respectively. Stiffness explained 66% and 52%, respectively, of the variability in strength for DF and SP. Because this relationship seems only moderately predictive, it may be prudent to couple stiffness with additional measures of quality when predicting bending strength.