Timber sawed or split into planks, boards, and similar products. Lumber can come in many forms, species, and types from a wide variety of commercial sources. Because most lumber is manufactured similarly and graded by standardized rules, it is fairly uniform throughout the United States. Lumber is manufactured from round logs primarily in rectangular shapes of different dimensions. Lumber length is recorded in actual dimensions. Width and thickness are traditionally recorded in nominal dimensions, which are somewhat more than actual dimensions. Lumber is classified by thickness into three categories: (1) board, lumber less than 38 mm (nominally 2 in.) thick; (2) dimension, lumber from 38 mm to, but not including, 114 mm (nominally 5 in.) thick; and (3) timber, lumber 114 mm (nominally 5 in.) or more in thickness in the least dimension. Logging Lumber can be produced with either a rough or surfaced (dressed) finish. Rough-sawn lumber has surface imperfections caused by the primary sawing operations. Surfaced lumber is smoothed on either one or both sides and one or both edges. In general, the grade of a piece of lumber is based on the number, character, and location of features that may lower the strength, durability, or utility value of the wood. Lumber grading can be divided into two main categories: remanufacture “shop grade” and structural “stress grade.” Sorting of lumber for remanufacture is based on visual inspection. The wood is designated shop grade on the proportion of defect-free or clear cuttings of a certain size that can be made from a piece of lumber. The larger volume and more frequent number of clear cuttings, the higher the grade. Pieces of lumber graded for structural uses are put into classes with similar mechanical properties called stress grades. Stress grades are characterized by (1) one or more sorting criteria, (2) a set of allowable properties for engineering design, and (3) a unique grade name. The allowable properties are inferred through visual grading criteria or are determined nondestructively by machine-grading criteria. Visual grading is the oldest stress-grading method. It is based on the premise that mechanical properties of lumber differ from mechanical properties of clear wood. Growth characteristics, which affect properties and can be seen and judged by eye, are used to sort the lumber into stress grades. Typical visual sorting criteria include density, decay, proportion of heartwood and sapwood, slope of grain, knots, shake, checks and splits, wane, and pitch pockets. Machine-graded lumber is evaluated by a machine using a nondestructive test followed by visual grading to evaluate certain characteristics that the machine cannot or may not properly evaluate. Machine-stress-rated (MSR), machine-evaluated (MEL), and E-rated lumber are three types of machine-graded lumber. Machine-graded lumber allows for better sorting of material for specific applications in engineered structures. Clear, straight-grained lumber can be about 50% stronger when dry than when wet. For lumber containing knots, the increase in strength with decreasing moisture content is dependent on lumber quality. For timber, often no adjustment for moisture content is made because properties are assigned on the basis of wood in the green condition.Wood properties |
