MECHANICAL GRADING OF LUMBER SAWN FROM SMALL-DIAMETER LODGEPOLE PINE, PONDEROSA PINE, AND GRAND FIR TREES FROM NORTHERN IDAHO.

Immediately after breaking, a small sample (1 in. by 1-1/2 in. by 3-1/2 in.) was removed from each 2 by 4 near the point of failure to determine MC and specific gravity. The MC of each sample was measured using the ovendry method specified by ASTM Standard D 4442 [4]. Specific gravity, based on ovendry weight and volume at time of test was determined according to ASTM Standard D 2395 [6].

The MOE, modulus of rupture (MOR), and visual grade data for each piece were used to sort the lumber into MSR categories. WWPA rules allow sorting into any design bending strength (Fb) and modulus of elasticity (E) combination. However, there are a limited number of categories that are commonly marketed in the western United States, and for the species considered in this study they include 2400Fb - 2.0E, 2lOOFb- 1.8E, 1800Fb- 1.6E, 1650Fb - l.5E, and 1450Fb - l.3E (18). For this study, only 2400Fb - 2.0E, 2l00Fb -1.8E, and 1650Fb - l.5E categories were considered. The 1450Fb - 1.3E category was not considered because there is less of this grade produced. The MSR categories were compiled according to American Lumber Standards (ALS) Committee rules [1], which include the following requirements:

1) Average MOE for a grade group must be equal or greater than assigned E.

2) 95 percent of the pieces must have an MOE greater than 82 percent of assigned E value:

For 2.0E grade: 2.0 X [10.sup.6] psi X 0.82 = 1.64 X [10.sup.6] psi.

For 1.8E grade: 1.8 X [10.sup.6] psi X 0.82 = 1.48 X [10.sup.6] psi

For 1 .5E grade: 1.5 X [10.sup.6] psi X 0.82 = 1.23 x [10.sup.6] psi

3) 95 percent of pieces must have an MOR greater than 2.1 times the assigned Fb:

For 2400Fb grade: 2,400 psi X 2.1 = 5,040 psi

For 2l00Fb grade: 2,100 psi X 2.1 = 4,410 psi

For 1650Fb grade: 1,650 psi X 2.1 = 3,465 psi

The results of the static bending tests for each species group were sorted by static MOE to determine the highest-strength mix of MSR categories that could be produced for each species. Table 2 of ASTM D 2915 [3] provides the acceptable number of failed pieces as a function of sample size. A 75 percent confidence level was used to determine grade levels.

ALS rules also stipulate that after removing MSR boards there shall be no visual grades produced that have a design bending strength greater than the design bending strength of the lowest MSR category produced (1,650 psi for this study). A Select Structural grand fir (hem-fir species grouping) 2 by 4 has a size adjusted Fb of 2,100 psi. Therefore, this is not an acceptable visual grade after 1650 Eb material has been removed. However, the adjusted Fb for a No.1 hem-fir is 1,425 psi, which is less than the lowest MSR category of 1,650 psi; therefore, a No.1 visual grade can be produced. The same logic is also valid for lodgepole pine (spruce-pine-fir(s) species grouping); therefore one could produce a No. 1 visual grade lodgepole pine but not a Select Structural grade.

After the lumber was sorted into visual and MSR grades, the economic value was determined for each species group. The value comparison of visual grades versus machine grades was based on approximate current, random-length lumber prices as of November 14, 1997, as reported by a local mill.

RESULTS AND DISCUSSION

RESULTS BY VISUAL GRADING

The grand fir and lodgepole pine logs had an average small-end diameter of 5 to 6 inches, while that of the ponderosa pine was about 7 inches. The gross output of lumber from the sawmill green chain was 4,317 board feet (BF) of grand fir and 3,444 BF of lodgepole pine. The ponderosa pine logs were slightly larger, averaging 9 inches on the small-end. Complete gross output data were not obtained for ponderosa pine. After planing and removal of boards (1 by 4's), the remaining 2 by 4's were visually graded prior to further sorting. The results of visual grading are shown in Table 1.

As Table 1 demonstrates, the logs produced over 65 percent of the 2 by 4's in a visual grade of No. 1 or Select Structural, based on board foot volume, and less than 7 percent Economy for both grand fir and lodgepole pine. This high yield is attributed to the small knot size of the lumber. However, the ponderosa pine thinnings, with larger knots, yielded approximately 50 percent Economy grade lumber. A low yield of higher grade lumber is typical of kiln-dried ponderosa pine from "young-growth" trees [9]. One of the initial goals of the study was to produce approximately 300 12-foot 2 by 4's of each species, and since this goal was met, all of the 8-foot and 10-foot 2 by 4's were removed from further study. Since the Economy lumber would generally not make MSR lumber, it was also removed. In addition, a small number of grand fir pieces were used for pre-testing and therefore not included in the final analysis.

In the 1960s, a Western Wood Density Survey was conducted to determine the distribution of specific gravity for 15 western species [10,11]. The specific gravity values for Idaho reported in the Survey were converted from a green basis to the approximate MCs of our test samples [6]. The average specific gravity for grand fir from northern Idaho was converted to 0.36 at 11.5 percent MC according to the Survey, and it was 0.34 in our study (Table 2). For lodgepole pine, the Survey value for northern Idaho converted to 13.5 percent MC was 0.40, and the Table 2 value is 0.38. The Survey value for ponderosa pine from northern Idaho is 0.39 at 11 percent MC versus the value of 0.35 that we found. Thus, the values in our study are slightly lower than those of grand fir and lodgepole pine from the Western Density Survey. Our values for ponderosa pine are 0.04 lower than those from the Survey, but this would seem logical because our material came from a ponderosa pine plantation.

The mechanical properties, by visual grade, of the 2 by 4's tested are summarized in Table 2. It is of interest to compare these results against values from a more representative sample. The properties for the northern Idaho data were adjusted to an MC of 12 percent [8] and compared against the data collected on about 80 2 by 4's collected during the In-Grade Testing Program [12] (Table 3). Grand fir was sampled as part of the hem-fir species grouping in the In-Grade Program, and not as an individual species. Therefore, grand fir is not included in this comparison. For MOE, the median value (50th percentile) of the lodgepole pine data from northern Idaho was above the upper 75 percent confidence interval of the median In-Grade data for both Select Structural and No. 2 grades. The same is true for the 5th percentile value for lodgepole pine. For No. 2 ponderosa pine thinnings, the median MOE value of the northern Idaho data is slightly less than the 75 percent lower confidence limit of the In-Grade value. For MOR, the 5th percentile of the northern Idaho data is within the confidence interval of the In-Grade data. Although of a limited sample size, it would appear that the mechanical properties of the 45-year-old, small-diameter lodgepole pine sample is higher than might be expected. For the ponderosa pine thinnings, the MOR is within the confidence interval and therefore appears to be typical, but the MOE is low. This would be expected for trees that probably contain a higher percentage of juvenile wood.

MECHANICAL GRADING OPTIONS

Because of the low yield of visually graded lumber obtained from the ponderosa pine thinnings, no MSR yield simulations were conducted with this lumber. Of all the combined grand fir and lodgepole pine pieces tested, only three lodgepole pine 2 by 4's were removed from MSR grade due to visual characteristics and no pieces in either species were removed due to low MOR. Although three pieces of grand fir in the 1650Fb - 1.5E grade had an MOR less than 2.1 times design Fb, this amounted to less than 5 percent of that category. Therefore, none were reduced in grade. As shown in Table 4 all categories in both species had an actual minimum MOE greater than the minimum allowable MOE.

A considerable portion of both grand fir and lodgepole pine had mechanical properties high enough to be graded as MSR lumber Table 5. Sixty-six percent of the grand fir 12-foot 2 by 4's made MSR grade and 83 percent of the lodgepole pine 12-foot 2 by 4's qualified for an MSR grade. Figure 1 shows that an almost equal proportion of No. 1, No. 2, and No. 3 visual grades made MSR grade. ALS rules required that no Select Structural 2 by 4's could be produced as MSR grade residuals for this case. However, all visually graded Select Structural grand fir and lodgepole pine pieces made MSR grade, therefore, the ALS rule had no effect on this study sample.

Table 6 shows the comparison of economic value for the grand fir and lodgepole pine groups when graded visually and graded with MSR with visual grade residuals. There was a $15/MBF increase in the value of grand fir when MSR grading was added to the value of random-length hem-fir lumber, and a value increase for MSR lodgepole pine over random-length SPF(s) lumber was $27/MBF. The increase in value was largely due to the high percentage of No. 3 visual grades that moved into MSR categories. Sixty-four percent of the No. 3 grand fir 2 by 4's and 77 percent of the No. 3 lodgepole pine 2 by 4's moved into an MSR category. Producing MSR lumber in categories below the l650Fb 1 5E level had diminishing economic return. As shown in Table 4 there was a large quantity of No. 1 and No. 2 visual grades remaining after removal of MSR grades. The value of No. 1 and No. 2 is greater than that of the 1450Fb - 1 .3E. Therefore, grades of MSR lower than 1 650Fb - 1.5E produced lower values than the visual grades.

Using portable equipment, such as the transverse vibration tester used in this study, provides an economical method of evaluating a local resource at a particular mill. However, allowable MOE values are based on static tests of lumber tested on edge. Therefore, equations are needed to estimate static MOE from the MOE determined using portable equipment. This information, coupled with visual grade requirements, will allow a mill to evaluate the potential for MSR lumber yields from their resource. The linear relationship obtained in this study between static edgewise MOE and longspan, flatwise MOE determined in transverse vibration is shown in Figures 2 and 3. The coefficient of determination of the two species is greater than 0.85.

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