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Weekly UpdatesLaminating lumber assigned to the Softwood Species grouping has relatively low values for mechanical properties. Thus ponderosa pine in this species grouping was historically restricted to homogeneous L3 combinations. Other species, such as Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and southern pine (Pinus spp.), employ measurement of MOE (E-rating) of the outer laminations to make efficient use of the timber resource. Recent research has demonstrated that it is possible to produce glulam beams with only ponderosa pine lumber using a combination of E-rated lumber and visual characteristics (Hernandez et al. 2005). These grades are expressed in terms of MOE followed by the limiting knot size. Thus, a 2.0E-1/6 grade has a MOE of 2.0 x 106 psi (13.8 GPa) and a maximum edge knot size of 1/6 the lumber width. The L-grades for our lumber were determined by the WWPA lumber inspector.
Results
Visual grading
Grade yield. -- In this paper, grade yield is based on the total volume of 2 by 4's produced from all the logs, rather than being relative to the volume of wood in each individual log as was done in our previous paper (Lowell and Green 2001). This was necessary because only the 2 by 4 lumber was evaluated for structural products. This decision also removes the sawing efficiency of a given sawmill as a limitation and makes the results more applicable to other mills that might have different equipment configurations to process the logs.
Under the Structural Light Framing system, most of the 2 by 4's were either grades No. 2 or No. 3, with only a little more than 7 percent making the higher grades of Select Structural No. 1 (Table 1). These results are similar to those we previously found for lumber cut from small-diameter ponderosa pine from Grangeville, Idaho (Table 2). Here the lumber that did not make at least No. 3 grade is termed "No. 4", to avoid confusion with the lumber that did not make at least Utility grade in the Light Framing grading system, which is called "Economy." The primary characteristics that limited grade were warp (43%), wane (17%), and knots (7%). Likewise, when graded as Light Framing (Table 1), only about 13 percent of the lumber made Construction grade. Grade limiting characteristics were essentially the same as those for Structural Light Framing. Thus, as has been shown in previous studies, warp is the primary grade limiting problem. Prevention of warp could significantly increase utilization of lumber from small-diameter ponderosa pine trees (Simpson and Green 2001, Simpson 2004).
Mechanical properties. -- To reduce the amount of lumber that was tested, only the 2 by 4's that were broken in this study had potential for mechanical grading and glulam. Generally, these were at least a No. 3 visual grade and met other required visual characteristics, as determined by the WWPA lumber inspector. Only a small proportion (< 10%) of the No. 3 visual grade qualified for MSR and was tested, and only a few pieces (< 5%) of the No. 2 grade were not tested. In the Light Framing system, only one piece of Utility grade lumber qualified for MSR, whereas only a few pieces of Standard grade did not qualify.
Table 3 shows the flexural properties when the lumber is graded as Structural Light Framing. The lumber had a MC of about 10 percent. The SG of all the lumber averaged about 0.41 based on ovendry weight and volume. This is slightly lower than the value of 0.43 given in the National Design Specifications (NDS 2005). When adjusted to 15 percent MC as per ASTM Standard D1990 (ASTM 2005, Evans et al. 1989), the allowable bending strength, Fb, and MOEs of the Structural Light Framing and Light Framing grades were at least as high as those specified in the NDS (Table 4). Property values are not shown for the No. 3 and Utility grades because too many pieces in these grades were not tested. Light framing grades for ponderosa pine were not tested in the in-grade program (Green and Evans 1987).
Mechanical grading
There is a good relationship between MOE and Etv and between MOR and MOE (Table 5, Fig. 1). Although the [r.sup.2] values are a little lower than expected, our previous studies on ponderosa pine from small-diameter trees found higher [r.sup.2] values; thus, there is no reason to expect a problem with mechanically grading this lumber. The lower 90 percent confidence interval on the MOE-MOR relationship is shown in Figure 1. Five percent of the data would be expected to be below this line and is the basis for establishing mechanical grades. The equation that we used for the lower confidence interval is:
[MOR.sub.0.9LCI] = 7.093 x MOE - 5.786
This lumber was evaluated for the production of a number of MSR grading alternatives (Cisternas 2000). While a significant amount of the lumber would make a grade of 900Fb-1.0E, there is currently no market for such a grade. In fact, the lowest MSR grade with much of an established market is 1450f-1.3E (James 2001). Only about 7 percent of the volume (425 BF) of these 2 by 4's would make 1450f. With no established market for ponderosa pine (or Western Woods) MSR, it would not be economical to consider such production. This conclusion is consistent with what we found in our previous study for ponderosa pine from near Grangeville, Idaho (Erickson et al. 2000).
[FIGURE 1 OMITTED]
Glulam
Table 6 shows the distribution of flatwise Etv values for our data by edge knot size classes. Most of the pieces were either in the classes with 1/6- or 1/2-in. edge knots, and had Etv values below 1.4 million pounds per square inch (9.8 GPa). When graded by the visual glulam grades, only about 20 percent of the lumber would make at least an L3 grade (Table 7). The largest single grade-limiting factor for glulam with this lumber was warp (36%). Because laminations are pressed during the manufacturing process, bow is a less critical form of warp than are cup and twist. About 22 percent of the lumber was limited by cup, 7 percent by bow, and 6 percent by twist. These percentages are based on the primary limiting characteristic listed by the WWPA lumber inspector. It is common for more than one type of warp to occur simultaneously. About 1 percent of the limitations were just listed as "warp" (kind of warp unspecified). About 23 percent of the pieces had wane listed as the grade limiting characteristic. Had this lumber been sawn knowing that it was to be used for glulam production, it could have been sawn a little over size so that when planed to standard dimension there would have been less grade loss due to wane.
Discussion
As has been found in previous studies, warp is the biggest factor limiting the utilization of lumber cut from small-diameter (less than about 16-inches (406 mm) DBH), young growth (less than 150 years old) ponderosa pine. The amount of warp in this study could have been reduced slightly if a top load of 150- to 200-pounds per [ft.sup.2] (psf) had been used during kiln-drying. Unfortunately, only enough weights were available to achieve a top load of about 75 psf. It is estimated that the increase in grade recovery might have been up to 17 percent had sufficient top load been applied (Simpson and Green 2001). Additional improvements in warp control could be obtained by employing kiln temperatures of 240 [degrees]F, or higher (Simpson 2004). Excess wane was another characteristic that limited grade in this study, especially for production of laminating grades. Sawing the 2 by 4's oversized could have reduced this problem, at the expense of overall yield.
This study supports previous conclusions that structural lumber can be produced from small-diameter trees if careful attention is paid to kiln-drying procedures. However, yields will not likely be as good as those expected from small-diameter trees of other species (Green et al. 2005, Gorman and Green 2000, Willits et al. 1997). For mills already cutting small-diameter trees for structural lumber, visual grading in the Light Framing or Structural Light Framing grading systems would provide the highest value. Although not evaluated in this study, production of Stud grade 2 by 4's should also be attractive provided that wane and warp are controlled. Production of MSR lumber is not recommended for this resource. Grade yields would likely be quite low, and no established market currently exists for mechanically graded ponderosa pine or Western Woods. Glulam remains a possible market for ponderosa pine from suppressed-growth ponderosa pine (Hernandez et al. 2005), but potential producers are well advised to investigate the needs of specific glulam buyers before trying to compete in this market.
For this resource, sawing for appearance grades offers a higher value alternative for lumber production from this suppressed-growth ponderosa pine than does structural dimension lumber (Lowell and Green 2001). Had these logs been sawn for appearance-grade products, about 25 percent would have made No. 2 Common or Better with 66 percent grading as No. 3 Common (Table 8). The estimated premium for appearance products over dimension lumber at that time was $53 per 1000 cubic feet (gross log scale). Other previous research (Lowell et al. 2000) showed that there are opportunities to increase the value of appearance lumber through further processing into cut-stock material. The Flagstaff resource had a high yield of No. 3 Common appearance lumber, and about 60 percent of the boards were 6 inches (152 mm) wide or wider. While not evaluated in our study, there may be an opportunity to recover additional value by further processing into secondary products.
Conclusions
For the production of 2 by 4 structural lumber from 80- to 100-year-old suppressed-growth ponderosa pine 16 inches and less in diameter, we found that:
1. 34 percent of the 2 by 4's graded as No.2 and better (43% as Standard and Better). Warp (43%) and wane (17%) were the primary grade limiting defects, with only 7 percent of the grades being limited by knots.
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