WVFA Mountain State Forestry Fall 2021

F E A T U R E D N E W S www.wvfa.org Fall 2021 | West Virginia Forestry Association Mountain State Forestry 9 In the case of visual grading, it is assumed that the mechanical properties of lumber differ from clear wood due to the variety of characteristics associated with lumber. That is, visual grading considers knots (and particularly the location of the knot on the board), slope of grain, checks, splits, shake, density, decay, heartwood, sapwood, pitch pockets, and wane in establishing a structural grade. For lumber being used for CLT manufacture, there are four visual grades that can be assigned: Select Structural, No. 1, No. 2, and No. 3. The bending strength in relation to clear wood for these grades is maximized with Select Structural (~65%) and minimized with No. 3 (~26%). For purposes of CLT manufacture, the operating standard PRG-320 requires No. 2 and better visually- graded lumber in the parallel layers and No. 3 or better in the perpendicular layers of CLT panels. Machine stress rating (MSR) is performed on lumber in a non-destructive test procedure, with a follow up visual grading to identify any characteristics that the machine cannot properly evaluate. The most common measure for mechanically sorting lumber is modulus of elasticity (E). Each piece of softwood MSR lumber is generally stamped to reflect the evaluated properties of that piece. Hardwood Lumber Grading Hardwood lumber is primarily graded for appearance purposes with rules adopted and promulgated by the National Hardwood Lumber Association (NHLA). The objective in hardwood lumber grading is to determine, using specific rules, the proportion of the surface area that is clear (free of defects). Grades are assigned based on the proportion of the surface area that is clear. Hardwood lumber grades are designated as FAS, F1F, Selects, No. 1, No. 2A, No. 2B, 3A, and 3B Common. In this case, the lumber is not being graded for structural purposes, which places the use of hardwood lumber for structural products such as CLT, in a difficult market position. There is no established, recognized correlation between the structural grades and the appearance grades. In an attempt to determine any correlations between the grading systems, the Appalachian Hardwood Center (AHC) conducted a study to compare both visual and mechanical grades to the appearance grades of yellow-poplar lumber. The focus population was low grade yellow- poplar lumber typically used in industrial applications such as wooden pallets and graded by NHLA rules. Initially, 8 MBF of kiln-dried, rough-cut, 4/4 yellow-poplar classified as NHLA 2A and below was obtained from a mill in northern West Virginia. The boards were kiln dried to a target moisture content that ranged between 6% and 8% (the normal moisture content target of the mill supplying the lumber; the target moisture content for CLT lumber is 12% ±3%). While hardwood lumber is usually sold in random widths and multiple fixed lengths, a specific board dimension was available from the sawmill, with the average dimensions of 6.88 inches wide x 1.06 inches thick x 121.2 inches long. Initially the boards were numbered, measured, and graded to NHLA and NELMA visual grades; NELMA rules for structural light framing were used to establish a visual structural grade. The grades were assigned by professional graders, with certification from their respective associations/agencies. The boards were then surfaced on both sides and sent through a gang rip saw to achieve the final dimension of 6 inches wide x 7/8 inches thick x 121 inches long. After this dimensioning, the same professional graders regraded the boards. During this second grading, the determinant defect that limited the board from achieving a higher NELMA grade was recorded. The boards were then shipped to West Virginia University research laboratories where they were tested non-destructively to determine the flatwise bending modulus of elasticity (MOE) using a center point loading configuration, according to established standards for this type of non-destructive testing. The boards were evaluated flatwise to better simulate the stresses of a board in a CLT panel used as a floor or roof type panel, where bending properties are more relevant. Figure 1: Typical lumber lay-up when producing a Cross Laminated Timber (CLT) Panel. The parallel layers in the figure are those that extend lengthwise into the background, while the perpendicular layers are those that extend from left to right across the face of the figure.