Sorting Lumber by Pith and its Effect on Stiffness and Strength in Southern Pine No. 2 2x4 Lumber
Keywords:Southern pine, juvenile wood, pith, strength, stiffness, specific gravity, grading rules
AbstractSouthern pine (SP) lumber is visually graded based on knots, slope of grain, and amount of wane. Today, SP lumber contains more juvenile wood than in the past because of decreased rotation ages and the combined effect of mill and forestry practices. The presence or absence of pith is one method to identify lumber that contains a high percentage of low-stiffness and low-strength juvenile wood. However, it is not included in the visual grading system. In this study, we examined the effect that pith had on specific gravity, stiffness (modulus of elasticity [MOE]), modulus of rupture (MOR), and bending strength (Fb) in 744 samples of No. 2 2x4 SP. Lumber without pith had 14% greater specific gravity (15% MC) (0.50 vs 0.44), 35% greater stiffness (11.9 vs 8.8 GPa), and 49% greater MOR (53.4 vs 35.8 MPa) than lumber with pith. Lumber without pith met the 2011 design values for Fb (10.3 MPa) as well as MOE (11.0 GPa), whereas lumber with pith did not. These results show that if the presence of pith was included in the visual grading system, it could improve lumber properties and thus should be considered when grading SP lumber in the No. 2 2x4 grade and size.
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