Bending strength and stiffness of wide dimension southern pine No. 2 lumber

Southern pine (Pinus spp.) 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 can negatively impact the strength and stiffness. This study was undertaken to assess the bending strength and stiffness properties of No. 2 grade lumber in the 2 × 6 (n = 86), 2 × 8 (n = 112), 2 × 10 (n = 87), and 2 × 12 (n = 53) sizes collected across the geographical range. In total 338 pieces were collected and tested. All material was re-graded as No. 2 (on-grade) prior to testing to confirm the grade. The lumber was tested in four-point bending according to ASTM D198. The study suggests that as lumber size (width) increases (2 × 6, 2 × 8, 2 × 10, and 2 × 12) the stiffness (MOE₁₅) increases (10.1, 10.3, 11.0, 11.8 GPA, respectively). The bending strength (F_b) values for 2 × 6, 2 × 8, 2 × 10, and 2 × 12 lumber were 8.8, 8.2, 5.1, and 8.1 MPa, respectively. Overall lumber graded as No. 2 because of wane and skip had the largest stiffness, strength, and specific gravity compared to lumber graded as No. 2 due to other factors such as knots. No. 2 Prime lumber was significantly less stiff (8.5 vs. 10.9 GPa), less strong (28.3 vs. 42.1 MPa), and had lower specific gravity (0.44 vs. 0.49) than No. 2 lumber (Common). Lumber with pith was generally less stiff and strong in the 2 × 6 and 2 × 8 sizes but in 2 × 10 and 2 × 12 lumber there were no significant differences compared to lumber without pith.