Mechanical Grading of Oak Timbers

For many wood species, the grading of timbers [>76 mm (>4-in.) thick] has not changed in decades. Most timbers are still visually graded by methods that originated in the 1930s. Mechanical grading procedures used to accurately grade 38-mm (2-in. nominal) dimension lumber have not been adapted for use with timbers. Furthermore, the only reliable timber test data available for most softwood species were obtained near the turn of the 20th century, and no test data are available for hardwood timber. The lack of better grading procedures and test data makes it likely that current property assignments for timbers are wasteful. Given that the reduced availability of lumber is expected to continue well into the 21st century, the structural engineer must have the widest possible options for structural timbers. Therefore, a study was conducted by the USDA Forest Service, Forest Products Laboratory, to examine use of a pulse echo stress wave technique along the length of timbers to estimate modulus of elasticity (MOE) of green timbers and the development of a machine stress rated grade for oak timbers. Two hundred and ninety-five untreated 178- by 229-mm (7- by 9-in.) mixed oak timbers, 4.9 m (16 ft) in length, were measured, weighted, and nondestructively evaluated to determine their pulse echo MOE. Two hundred twenty were destructively tested in bending; 75 were tested to failure in compression parallel to the grain. The results of these tests determined relationships between pulse echo MOE and static bending MOE and between bending and compressive strength. These tests suggests a clear potential of establishing a machine stress rated grade for oak timbers. This article discusses the implication of these results, potential commercial grades, and a follow-up study to qualify the suggested grades.