When a business considers entering a new product market, there are many factors to be considered in making such an investment. Among these can be the need for a new or reconfigured physical plant to conduct the manufacturing, new pieces of equipment that perform different operations, a different raw material, employees with different competencies, and a revised marketing strategy, among others. This is essentially the situation that hardwood lumber manufacturers face as they contemplate entrance into the CLT (Cross-Laminated Timber) market. These factors all come into play for a hardwood producer considering manufacturing CLT’s for the commercial market. In this situation, the raw material, in the form of lumber, requires close consideration.
Softwood Lumber Grading
Currently, CLT manufacturers produce panels made from softwood species. This lumber is readily available in the marketplace and can be purchased and placed into the production process with relative ease. The American Softwood Lumber Standard (ASLS) was developed and approved by the American Lumber Standards Committee (ALSC) in accordance with the Procedures for the Development of Voluntary Product Standards of the U.S. Department of Commerce and is the basis upon which softwood lumber is graded. Softwood lumber that is 2 to 4 inches thick, for the most part, is stress graded and assigned structural properties according to the National Grading Rule as part of the ASLS (Wood Handbook, 1999).1 There are a number of independent organizations that write and publish grading rules, such as the Northeastern Lumber Manufacturers Association (NELMA) in the Northeastern US. There are also a number of independent agencies that provide grading and marking services to the industry. Most of these grading services use visual assessment of the lumber and the associated defects to assign a structural lumber grade to hardwoods.
Stress grading is another approach to the structural grading process and can be conducted either visually or mechanically.
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.
Results of the Study
Visual Assessment of Yellow-Poplar
The analysis of grades included initial grading of the yellow-poplar boards in the rough form (kiln-dried, 4/4,) by both NHLA and NELMA grading rules and regrading following processing (surface and rip) into uniform dimensions of 6 inches wide by 7/8 inches thick by 10 feet long. For the purposes of this article, the key comparison is how the NHLA grades of the rough lumber graded out as NELMA structural grades following processing.
Based on the American Panel Association (APA) standards provided in APA PRG 320 (2018), a softwood board should achieve at least a structural visual grade comparable to a No. 3 NELMA grade for use in the production of CLT panels. From the researched population of NHLA low-grade yellow-poplar, 54.6% received a structural visual grade above the standard requirements for CLT and GLULAM manufacturing. The percentages of boards that achieved a structural grade (at least No. 3) within their NHLA grades were: 2A (64.4%); 2B (66.3%); 3A (52.3%); and 3B (30.7%). The grade 3B, with over 69% of boards being Below Grade, presented the least amount of yield when used as structural grade material, based on NELMA rules; therefore this grade is considered least likely to result in a significant amount of suitable materials for structural lumber and might not be worth the cost of remanufacturing.
Table 1 summarizes the grading results by NHLA grade and NELMA structural grade. Of particular interest is the percentage of No. 2 and better structural grades, by NHLA grade. This is a very important consideration, given that a board must be at least a No. 2 structural grade in order to qualify for the parallel layers of a CLT panel. Both NHLA 2A and 2B grades had No. 2 and better structural proportions above 40%.
During the grading process, the structural grader identified the limiting defect on each board and those results are displayed in Table 2 and Figure 2. The most common limiting defects, overall, were knots and splits. The most common defects in grades No. 1, No. 2, and No. 3 were knots, while in Below Grade the most common defect was splits.
Most of the observed defects, such as knots, shake, wane and slope of grain, are inherent in boards following manufacture at the sawmill, while splits and cracks are more commonly associated with handling and processing of the boards. In fact, Lamb (1992)2 indicates that splits and cracks are associated with four factors: wood characteristics, processing, drying, and handling. There was no conclusive way to fully understand how many of these splits and cracks were caused by handling and processing the boards during the research. We were not able to track every board and determine the effects produced from each step of processing surface and rip the board. In this study, the primary objective was to estimate a final NELMA grade after processing surface and rip NHLA graded low-grade lumber.
Elasticity of Low-Grade Yellow-Poplar
While NELMA visual grading indicated that a large proportion of NHLA low-grade lumber met structural grades, nearly half of the lumber graded visually was Below Grade (45.4%). To determine if the Below Grade lumber would actually be unsuitable for CLT manufacturing, non-destructive proof loading was used in the study as an alternative grading approach. Table 3 summarizes the non-destructive test results for MOEb of the NHLA low-grade yellow-poplar lumber as a sample population (i.e., no breakdown by NHLA grade).
The MOEb design values of a population are usually calculated from their mean, which, for this distribution, was 1.66 x106 psi. The minimal MOEb value was 0.77 x106 psi and the 5th percentile was 1.25 x106 psi showing that a majority of boards (96.6%) presented an acceptable MOEb relative to the minimal requirement for CLTs as outlined in the American Panel Association (APA) CLT standards (ANSI/APA PRG 320)3 of 1.2 x106 psi for boards qualifying for the parallel layer of a CLT panel. Only 39 boards (3.4%) did not meet the minimum MOE requirement specified in ANSI/APA PRG 320.
Taking into account the average MOEb, lumber within any NELMA grade would meet the minimum requirements of 1.2e as set in the American Panel Association (APA) CLT standards.
This study focused on an evaluation of the structural characteristics of a large sample of low-grade yellow-poplar lumber for use in CLT panel production. The boards were evaluated before and after processing. The parameters used for board evaluation included NELMA visual structural grading and non-destructive testing to determine the MOEb value for each board.
The sampled population of yellow-poplar lumber with NHLA grades 2A and below produced a total of 651 acceptable boards out of a sample population of 1,192 boards. In other words, 54.6% of the tested boards, met visual NELMA structural grades. These findings support the argument that low-grade yellow-poplar lumber can be used for structural purposes, and in particular, as a raw material for manufacturing CLT panels. Results from non-destructive proof loading tests indicated that 96.6% of the boards met the minimum MOEb (1.2 x106 psi) specified for the production of CLT panels under ANSI/APA PRG 320 (2018). Finally, in terms of grading, the non-destructive test results showed a higher yield than visual grading and should be further evaluated as a more efficient and economical means for structurally grading low grade hardwood lumber for CLT applications. This can easily be accomplished through the incorporation of machine stress rating (MSR) technology in production settings, a common feature in many larger softwood manufacturing facilities.
Any large-scale change in grading methods by hardwood lumber manufacturers will depend on investment and a market able to justify the investment. Although, based on these results, re-grading low-grade hardwood lumber, currently used primarily in pallet manufacturing, would add considerable value to this lumber and allow it to be used in CLT panel manufacturing.