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

ROBERT G. ERIKSON [+]

THOMAS M. GORMAN [+]

DAVID W. GREEN [+]

DEAN GRAHAM [+]

ABSTRACT

Forest lands of the Inland Northwest have many timber stands consisting of overgrown, densely stocked trees that create a fire hazard and are prone to disease. These stands need to be thinned, but the cost of harvesting often exceeds the value of the timber produced. However, because of the dense stocking and the resulting slow growth these trees may produce lumber with desirable mechanical properties. One method for sawmills to more fully utilize the potential grade yield and realize greater economic return from such lumber may be to produce machine-stress-rated (MSR) lumber instead of visually graded dimension lumber. The purpose of this study was to determine the mechanical properties, and corresponding economic value, of lodgepole pine, grand fir, and ponderosa pine dimension lumber produced from typical overstocked forest stands in northern Idaho. The lumber was visually graded and tested for modulus of elasticity and modulus of rupture, and each piece was sorted into two types of grade categories: 1) vi sual Structural Light Framing; and 2) MSR. This study indicated that two of the three species we tested had good visual and mechanical characteristics. MSR grading of the lodgepole pine group produced a $27/MBF increase in value above visual grading, and MSR grading the grand fir group produced a $15/MBF increase in value above visual grading. The ponderosa pine samples were from poor quality trees "thinned from below." Because of the poor yield in the higher visual grades, ponderosa pine thinnings in this study were judged not to be a good candidate for production of MSR lumber. This study points out the potential value of lumber sawn from overstocked stands of timber, but demonstrates the need for an assessment process to estimate local resource capability.

The Inland Northwest has many dense forest stands consisting of predominately small-diameter, same-age trees. These stagnated forests were created by large stand replacement fires followed by natural regeneration and several decades of fire suppression and minimal self-thinning. A survey of the Colville National Forest [21] provides a good example of typical stagnated forests. The stands are characterized by high densities of trees less than 9 inches diameter at breast height (DBH). The species mix consists of lodgepole pine (Pinus contorta), ponderosa pine (Pinus ponderosa), western larch (Larix occidentals) and Douglas-fir (Pseudotsuga menziesii), often with a thick under study of grand fir (Abies grandis).

Thinning of dense, stagnated timber stands can increase the quality of future harvests and improve forest health [15]. However, thinning these stands produces mostly small-diameter (4 to 10 in. DBH) logs that do not have a high economic value, and may actually have negative value at the mill [19]. Traditionally, the logs from such thinnings have not been utilized for solid-sawn lumber because of their low value. These logs have low volume and a high cost of removal. However, as the average size of logs being harvested has declined, technology has been developed to process small-diameter logs more efficiently. Alternative mill strategies, such as producing mechanically graded lumber, may increase mill profits [17]. Although these logs are of small diameter, they may have slow growth rates that produce logs with less juvenile wood, higher density wood, and greater mechanical properties. A study of an evenage lodgepole pine forest in British Columbia demonstrated that lumber strength increased as tree DBH decre ased [16].

Two methods of structural grading were considered to evaluate the potential value of small-diameter timber from northern Idaho. The first, and most common, was structural grading by visual assessment. Visual grading employs the concept that pieces of lumber with small knots are usually stronger than lumber with larger knots. Procedures for visual grading of 2-inch-thick "dimension" lumber are governed by the National Grading Rule, and are the same across species [14,20]. While visual grading rules for dimension lumber are species independent, property assignment procedures depend upon species. Further, visually graded lumber is often marketed in species groupings, the assigned properties of which may be controlled by the weaker species in the grouping [5]. A producer having available a stronger species in the grouping, or trees having superior growth characteristics, may have to settle for assigned properties that are less than might be possible through more precise grading methods.

The second grading method used was for machine-stress-rated (MSR) lumber. Commercially produced since the early 1960s, MSR grading combines visual assessment of knots, and other defects, with nondestructive measurement of bending stiffness to more precisely estimate properties [7]. The most popular method for producing MSR lumber is to segregate lumber into grades by means of mechanical evaluation of bending stiffness combined with visual limitations. Because of the more precise grading methods used, yield of MSR lumber grades tend to be more dependent upon the local resource than are those of visually graded lumber. MSR grading procedures produce lumber having a lower coefficient of variation in modulus of elasticity (MOE) than does visual grading, and uses destructive quality control testing to verify property assignment. The reduced variability in MOE is of particular importance in engineered structural components such as metalplated wood trusses, structural gluelaminated beams, and fabricated I-joists [2 2]. The MSR process may also allow a producer to take advantage of a superior local resource and achieve design values not possible through visual grading (13).

MSR lumber currently constitutes a small portion of the dimension lumber market, but it may have the greatest potential for growth of sawmill products [7]. MSR lumber production in the United States continues to grow; production increased by nearly 60 percent over the past 5 years (18). The production of MSR lumber from smaller-diameter logs may offer a higher value alternative to the production of visually graded lumber. However, before lumber manufacturers will invest in the equipment necessary for MSR grading, the mechanical properties of lumber from logs harvested from timber-stand thinnings must be determined. Grand fir and lodgepole pine were identified as two species that are likely to be produced from timber-stand thinnings and have high potential for increased value through MSR grading. Ponderosa pine was also selected for study because of its prevalence in stagnated forests in the intermountain west.

The first objective of this study was to determine the mechanical properties of grand fir, ponderosa pine, and lodgepole pine 2 by 4's manufactured from logs harvested from one site in northern Idaho. The second objective was to sort the sample into both visual and MSR grade categories and to compare the grade yield and economic value for the two grading methods.

METHODS

The trees for this study were obtained from two locations in the Inland Northwest. Grand fir and lodgepole pine logs were harvested from a second-growth, even-age, forest stand in the Idaho Panhandle National Forest north of Priest River, Idaho. There was no documented stand history, but the district forester was able to provide the following information based on a current inspection. [1] The stand was regenerated by a catastrophic fire, or a combination of logging and residual burning, approximately 45 years ago. The forester estimated a species mix of 30 perenct grand fir, 30 percent lodgepole pine, 20 percent western redcedar, and 20 percent western hemlock. The stand had an approximate basal area of 180 square feet per acre, an average height of 65 feet with a range of 47 to 85 feet, and an average DBH of 8.5 inches with a range of 7 to 11 inches. The ponderosa pine was harvested from a second-growth site planted in 1952 in the Nez Perce National Forest southeast of Grangeville, Idaho. The site consisted of 99 percent ponderosa pine and 1 percent Douglasfir. The logs for this study were obtained by "thinning from below," which removed the poorer quality trees.

A cooperating sawmill produced a maximum number of 12-foot 2 by 4's from each log. The 12-foot length was chosen because it is a commonly marketed length of MSR lumber and could be easily handled in the laboratory. The rough 2 by 4 lumber was dried in a commercial kiln to a moisture content (MC) no greater than 19 percent, planed to dimension size, and shipped to the University of Idaho for analysis. At the University, all 2 by 4's were assigned a visual grade by a Western Wood Products Association (WWPA) grader. To improve grade, a small quantity of lumber was "pencil trimmed" to a shorter length. The goal of this study was to evaluate approximately 300 12-foot 2 by 4's of each species for their potential as MSR lumber. To assist with assignment of MSR grades to the remaining 12-foot 2 by 4's, the WWPA inspector assigned a visual limitation class per WWPA rules [20].

The dynamic modulus of elasticity (MOE) for each 2 by 4 was then determined using a Metriguard Model 340 Transverse Vibration E-Computer. The E-Computer determines MOE based on resonant vibration frequency and density. A 10-foot-long aluminum bar, of known stiffness, was used in vibration to calibrate the E-computer. Each 2 by 4 was simply supported flatwise as a beam spanning the entire length of the board (12 ft. for this analysis), and the specimen was then set into vibration by gently tapping it near the center of the span. A load cell measured the frequency of vibration and board weight, and the E-Computer calculated MOE for each piece.

An Instron Model 1137 Universal Testing Machine was used to perform the static mechanical strength and stiffness tests; no tension tests were included in this study. Data were collected via a National Instruments model PC-LPM-16 data-acquisition board and Measure software. The testing was performed per ASTM Standard D 198 (2) with the 2 by 4's loaded on edge. Third-point loading was used to create constant moment in the center third of the span. Span length was 73.5 inches to achieve a span-to-depth ratio of 21:1. Pieces were tested such that maximum strength-reducing characteristics were randomly located. Each piece was loaded at a 2-in./min. rate of deflection and loading proceeded until ultimate failure. The time to failure averaged approximately 1 minute. Deflection was measured using a linear voltage differential transducer (LVDT), and force was measured with the load cell on the Instron machine.