BENDING STRENGTH AND STIFFNESS OF NO. 2 GRADE SOUTHERN PINE LUMBER
Keywords:visual grading, mechanical properties, growth characteristics.
Southern pine is the most important species group planted and used for lumber products in the United States. Most southern pine trees come from managed forests, with relatively short rotations and excellent growth yields. The accelerated growth volume allows trees to reach merchantable size in 16- 22 yr. However, these trees may contain large amounts of juvenile wood which can negatively impact the bending properties of lumber. In 2010, the Southern Pine Inspection Bureau (SPIB) began to reevaluate the mechanical properties of southern pine lumber, which resulted in changes in design values. The objective of the study herein was to summarize the growth characteristics and bending properties of No. 2 grade 24, 26, 28, and 210 samples collected from across the geographical growing range (southern United States). Each piece met the requirements for No. 2 grade southern pine lumber. Overall, 34.5% of the sample contained pith, averaged 4.6 rings per inch, and contained 43.8% latewood. The sample’s average specific gravity,MOE, and MOR were 0.54, 10.1 GPa, and 41.7 MPa, respectively. The mean MOE found in this study was higher than the current design value required for No. 2 southern pine lumber. For allowable design bending strength (Fb), the results showed that, as dimension stock size increased, the Fb decreased from 11.2 MPa for 2 4 s to 7.1MPa for 2 10 s. The Fb values determined herein exceeded the new published design value and also met the previous SPIB design values. These results suggest that the timber resource quality might have increased since the housing crisis of 2008-2010.
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