Strength of small-diameter round and tapered bending members

An early focus on structural use of processed rather than round timber resulted in an underestimation of the structural advantages of retaining the natural form of small-diameter round timber. In the round and tapered form, timbers are not susceptible to the strength-reducing effects of diving grain and exposed juvenile wood. Fiber continuity around knots on the surface of a debarked log rarely exhibits the stress concentration and fracture propagation commonly seen in disrupted grain around knots in lumber. Symmetry of material properties about the centroidal axis in a round timber improves the efficacy of standard section property equations derived for uniform isotropic materials. Ignoring these benefits and comparing the strength of round and processed timbers solely on the basis of section property, the round section has from two to fourtimes the bending load design capacity of any standard-sized processed timber that could be sawn from it. D ense stands of small-diameter trees (< 9 in., 5 ft. [< 228 mm, 1.5 m] off the ground) present a threat to our National Forests. Low demand for these trees makes it difficult to encourage thinning by private companies and thinning is prohibitively expensive for the government. One solution appears to be to develop value-added markets for this material, and one potential market is timber frame structures. Structural use of small-diameter trees represents a major shift from tradition. This is especially true in the western United States, where the abundance of large trees encouraged rapid growth and evolution of the lumber industry. Larger trees yield structural lumber over a variety of standard widths. The mix of sizes results in £ 90 percent yield of usable material from these trees. Small trees can be processed into lumber, but the conversion efficiency is reduced with log size. In addition to the obvious size limitations on yield small trees containing a large proportion of juvenile wood often yield poor lumber quality. Juvenile wood is laid down within the active crown of the tree. It has characteristically lower strength and stiffness, greater longitudinal shrinkage with drying, and a larger proportion of knots than does mature wood that is produced below the active crown. Once the active crown moves higher in the tree to compete in the forest canopy, branches become less active and eventually die. In some cases, branches fall off, leaving a knot to be grown over by mature wood. In any case, knots rarely continue to grow in diameter in the mature wood of trees harvested for lumber production. Knots therefore take up a smaller portion of the circumference and cross section in mature wood than in juvenile wood. Reduced efficiency and quality associated with milling structural timbers from small-diameter trees that contain a large proportion of juvenile wood reduces the incentive to harvest these trees. There are, however, some advantages to using the stems from these trees in their natural form. Trees have evolved as structural elements efficiently designed to resist bending stresses. As the tree grows in height to compete for sunlight, it must either lay down stronger (mature wood) fiber or more fiber in the outer layers of the trunk to resist the higher bending moments caused by wind. Retaining this sheath of mature wood around the juvenile-wood core provides a bending element with strength and stiffness that has a higher mean value and lower variability than does lumber that is produced by sawing. Moreover, exposing juvenile wood by sawing can increase the tendency for warp. Because small-diameter trees are naturally suited to use as bending elements, it seems logical that greater effort be expended to develop opportunities for their structural application in a round form. The only processing required for the debarked log is drying (air or kiln). Production of lumber from small-diamThe authors are Research General Engineers, USDA Forest Serv., Forest Products Lab., One Gifford Pinchot Dr., Madison, WI 53726-2398. This paper was received for publication in November 2003. Article No. 9801. ©Forest Products Society 2005. Forest Prod. J. 55(3):50-55.