Ultimate Tensile Stress and Modulus of Elasticity of Fast-Grown Plantation Loblolly Pine Lumber

Abstract

The purpose of our study was to define the influence of juvenile wood on the tensile structural performance of dimension lumber from fast-grown plantation pine wood. Ultimate tensile stress and modulus of elasticity were measured on nominal 2- by 4-in. (standard 38- by 89-mm) lumber from a 28-year-old fast-grown loblolly pine plantation in North Carolina. We compared four grades of lumber and two lumber lengths. Strength and stiffness decreased with increasing amounts of juvenile wood. Average ultimate tensile stress and stiffness values of pieces composed entirely of juvenile wood were from 45 to 63% of those pieces composed entirely of mature wood, depending on grade and property. We used physical properties as grading criteria to conform our test material to 1988 National Design Specifications (NDS) allowable design values. For our test material, percentage of latewood, number of rings per inch (25.4 mm), specific gravity, and presence or absence of pith could not be used individually as successful criteria for assuring conformance to design values. We compared data on mechanical properties to 1988 NDS allowable design values and to In-Grade test values. The performance of the Select Structural lumber conformed closest to the NDS design values; in the three lower grades tested—No. 1, No. 2, and No. 3—about 20% of the lumber did not conform to the design values. Compared to In-Grade test values for southern pine, the 5th-percentile strength, average stiffness, and specific gravity values of our material fell below the In-Grade test values for all grades. For both the NDS and In-Grade comparisons, strength values of our material could be raised above the 5th-percentile cut-off level by removing a given proportion of juvenile material from the analysis. The correlation between ultimate tensile stress and modulus of elasticity of test material was about equivalent to that of southern pine lumber reported in the literature. Apparently, fast-grown pine plantation lumber with high proportions of juvenile wood can be machine stress-rated using techniques normally applied to traditionally used southern pine lumber. However, there does appear to be a pattern of decreasing slope in the strength-stiffness relationship with increasing juvenile wood content. The results reported here are not representative of the global population of southern pine lumber on the market. However, the significant differences between juvenile and mature wood indicate that as the industry moves to a faster grown, shorter rotation resource, grading rules and allowable design stress values will need to be modified to accommodate this changing resource.