Yield of 2 by 4 red oak stress-graded dimension lumber from factory-grade logs

Transcription

1 Yield of 2 by 4 red oak stress-graded dimension lumber from factory-grade logs Kent A. McDonald James W. Whipple Abstract Efficient conversion of red oak logs to structural dimension lumber depends on knowledge about the expected yield from different grades of logs. Our results show that nominal 2 by 4 inch (standard 38 by 89 mm) dimension-lumber yields from USDA Forest Service construction-grade logs are greater than yields from USDA factory-grade F3 logs. Comparable yields of dimension lumber were obtained from factory-grade F2 and construction-grade log classes. Market competition for the higher valued factory-grade F2 log for factory lumber indicates that only logs from the construction-grade class should be considered by potential producers of dimension lumber and then considered only with caution. Hardwood sawmill operators have a vested interest in knowing how much dimension lumber (structural lumber <4 in. (<102 mm) thick) compared to factory lumber (used for remanufacture such as furniture) can be expected from hardwood logs of different qualities. Recent national attention on using hardwood lumber for structural purposes has resulted in an increased interest in improving standard ASTM procedures (1, 2) for assigning stress ratings to hardwood dimension lumber. Several factors are responsible for the increased interest in the stress rating of hardwood dimension lumber. These factors include 1) the perceived abundance of low quality hardwood timber that should be utilized; 2) the potential competition between the use of imported species and domestic species for softwood dimension lumber; 3) the use of a readily available supply of local material to meet the demand for timber bridges; and 4) the desire to reduce the demand for FOREST PRODUCTS JOURNAL Vol. 42, No. 6 western softwood dimension lumber. Redirecting any of the existing hardwood resource to another use could have a decisive effect on the raw material supply, the hardwood economy, and the processing methods used to manufacture dimension-lumber products. Obtaining the dimension-lumber yield by log quality classes using the USDA Forest Service (USDA FS) standard factory-log grades would help in making knowledgeable decisions about what raw material should be converted to dimension lumber. Sawmill operators must be able to select the log quality that will most efficiently produce an order of dimension lumber. They also will need special instructions for sawing hardwood logs into stress-graded dimension lumber. These decisions must be based on a rigorous financial analysis. Background Traditionally, hardwood sawmills produce factory lumber for markets that are largely dominated by furniture producers (12). Grading this lumber for remanufacture is based on National Hardwood Lumber Association (NHLA) grades (7), which are quite different from the dimension-lumber grades (8, 11) traditionally applied to softwood species. Dimensionlumber grades are for structural material destined to be used as-graded, not remanufactured. This material, referred to in this paper as dimension lumber, The authors are, respectively, Research Wood Scientist, USDA Forest Serv., Forest Prod. Lab., One Gifford Pinchot Dr., Madison, WI ; and Wood Utilization Specialist. Dept. of Natural Resources, 518 W. Somo Ave., Tomahawk, WI This paper was received for publication in July Forest Products Research Society Forest Prod. J. 42(6):

2 should not to be confused with factory lumber marketed for hardwood dimension products. (Seesidebar.) Factory lumber and dimension lumber differ in many ways. For example, thicknesses for factory lumber are roughsawn by quarter-inch classes from nominal 4/4 up to nominal 16/4; dimension lumber is usually 2 inch (38 mm standard) nominal or 1-1/2 inch (38 mm) actual when dried and planed. Widths are random for factory lumber and 2-inch (38-mm) nominal multiples for dimension lumber. Defect limitations allowed on graded products are also quite different for factory lumber and dimension lumber. More wane and edge knots are acceptable in factory lumber than in dimension lumber. Such differences reflect the end uses of factory and dimension lumber, and they pose some significant problems to the sawmill decision makers who produce both types of lumber. These problems are further aggravated by different target sizes for green thicknesses and widths. decisions on lumber quality that face the head saw. edger. and trimmer operators, and cost/benefit ratios that must be considered when converting hardwood logs into a mixture of products. Therefore, getting a stress-graded product like hardwood dimension lumber on the market involves more than simply providing the means to grade the product. Hardwood logs will have to be sawn like softwood logs to produce a product similar to softwood dimension lumber. However, different sawing methods will be needed than those conventionally used for hardwood factory lumber. In addition, producers will be faced with difficult decisions when planning which hardwood logs should be sawn into dimension lumber and which logs should be sawn into conventional factory lumber. Decisions will need to be made about setting the saws to cut the dimension lumber (nominal 7/41 as opposed to factory lumber (nominal 4/4, 5/4, and 6/4), because the target sizes (sets) will be different for the different products. Other management decisions will include selecting the drying schedules of the different products (40 to 90 days of kiln-drying for 7/4 red oak compared to 1 to 2 days for 8/4 southern pine), possible installation of a predryer system, a lumber-handling system within the mill based on two very different products, the dual grading and marketing decisions associated with the two products, and the possibilityof dealingwith more than one agency for grading and inspection services. The simple approach of having one mill devoted to the production of stress-graded hardwood dimension lumber is not a likely solution because of the relatively higher value of factory lumber compared to the lower value of dimension lumber, and the efficiency of getting both products from the same logs. The logical solution to the problem is that a portion of the log should be sawn into factory lumber and a portion into dimension lumber. The USDAFS standard factory-log grades (3, 10, 13) were developed for the production of factory lumber. Surface characteristics on logs that affect the underlying factory lumber are considered when deciding the Dimension defined The word dimension is used by both the hardwood and softwood industries to define wood that is sawn or surfaced to a specific size. Dimension is a noun but is also used as a noun modifier. The National Dimension ManufacturersAssociation defines both hardwood and softwood dimension components as being cut to a specific size from kiln-dried rough lumber, bolts, cants, or logs. Dimension components include Flat stock (solid and edge glued) and Squares (solid and laminated) for furniture, cabinet, and specialty manufacturers. Components are in specified thicknesses, widths, and lengths, or multiples thereofand are classified as Rough dimension. Semi-machined dimension, or Fully machined dimension. ByASTM D-09 definition, theword dimension is used as an adjective to modify lumber that is sawn or surfaced 'from nominal 2 through 4 inches thick and 2 or more inches wide." This defines all lumber that is produced to a specific size, which is typically most of the softwood lumber production and a portion of the hardwood lumber production. The National Hardwood Lumber Association lists grades of lumber for construction use that are titled No. 1 Dimension, No. 2 Dimension, andselect Dimension. Lumber that is sawn and surfaced to nominal 2 by 4 size is correctly referred to as dimension lumber whether it is softwood or hardwood. Some of the total volume of dimension lumber is stress graded, and some is used in construction. All dimension lumber is not used in construction and all Is not stress graded. Hardwood lumber of random width to be used for dimension parts should not be referred to as 'dimension lumber" but rather 'lumber for dimension." log grade. Determining nonfactory-lumber product yields from factory-grade logs may not produce the clear-cut differences in product yield between the log grades as normally happens with factory-lumber products. Three USDA FS factory-log grades, F1, F2, and F3, are generally accepted by the industry as standard for sorting hardwood logs. Each grade admits only those logs with similar standard factory-lumber yields and also similar values. In today's market, sound grade F1 logs of basswood and certain species of red oak are often marketed for veneer production and therefore are not discussed in this paper. Grade F2 requires 8-foot- (2.4-m-) long (withouttrim) logs tobe 12 inches (305 mm) in scaling diameter on the small end, and under special conditions. logs as small as 10 inches (254 mm) in scaling diameter can be admitted. Although 8- and 9-foot (2.4- and 2.7-m) logs must yield 75 percent of each of the three best faces clear in not more than two 3-foot (0.9-m) clear lengths, longer logs needyield only 66 percent. Sweep or crook deductions can be up to 30 percent of the gross scale, and total scaling deduction (including sweep or crook) can be 6 JUNE 1992

3 TABLE 1. - Scaling diameter classes and log grades. TABLE 2. - Net board-foot volume of study logs (international 1/4-in. log scale). TABLE 3. - Board foot dimension-lumber tally. Figure 1. - Log sawing pattern (1 in. = 25.4 mm). up to 50 percent of the gross scale. Grade F3 logs must be at least 8 inches (203 mm) in scaling diameter and 8 feet (2.4 m) long (without trim), and each of the three best faces must be half clear in not less than 2-foot (0.6-m-) long clear lengths. Sweep or crook deductions can be up to 50 percent of the gross scale; however, total scaling deductions can not exceed 50 percent of the gross scale. These grade specifications are only partially listed to point out that considerable sweep, crook, and other scaling deductions are permitted in the factory-grade logs. Logs that do not meet the surface clearness requirements of the factory-grade logs may meet the specifications for USDA FS construction-grade log class. The major factors that determine admission to this class are log straightness and soundness, and size and condition of log defects. Minimum small-end diameter is 8 inches (203 mm), minimum length is 8 feet (2.4 m) (without trim), and sweep must be less than one-fourth of the small-end diameter for each 8 feet (2.4 m) of length. Sound surface defects are limited to not more than one-third the diameter of the log at the point of their occurrence, and unsound defects are generally not permitted (10). Note that portions of the factory lumber produced from USDA FS standard factory-grade logs with sweep and crook are in short lengths and random widths, and can be obtained without substantial reductions in lumber yield. In contrast, a similar yield in fulllength dimension lumber from logs with up to a 50 percent deduction for sweep is not possible. Construction-grade logs, as specified. are reasonably straight and sound and yield higher volumes of dimension lumber than factory-grade logs of equivalent quality. The economic information needed to assist sawmill operators must include comparative values of the factory lumber and the stress-graded hardwood dimension lumber, and should be based on the previously stated considerations. The common denominator for the initial consideration is the grade of the logs that are most available to the producer. Objective The objective of this study was to determine the 2 by4 stress-graded dimension-lumber yields from graded red oak logs by scaling diameter classes and to compare those yields to the expected factory-lumber yields reported for logs of the same size and quality (13). Procedure For this study, 95 red oak logs that were 12 feet long (3.7 m) were obtained from 1 sawmill and were sorted into USDA FS standard hardwood grades F2 and F3 and the hardwood construction grade. The logs were either northern red oak (Quercus rubra) or black oak (Quercus velutina), coming from central Wisconsin. Only logs of 10-inch (254-mm) scaling diameter and larger that could be classified into three log grades and three scaling diameter classes were selected (Table 1). Logs were not available for every log grade and diameter class. Logs that were reduced in grade for scaling deduction only were not accepted in this FOREST PRODUCTS JOURNAL Vol. 42. No. 6 7

4 study. The net board-foot (BF) volume of study logs (International 1/4-in. (6.35-mm) log scale) for the 95 logs by factory-grade log and scaling diameter class is given in Table 2 (4). Logs were identified and sawn by grade and scaling diameter class. The sawing pattern specified a 4-inch (102-mm) cant through the center of each log and 7/4 flitches on each side of the center cant. Another 7/4 flitch was taken outside of these flitches if possible; otherwise 4/4 side lumber was sawn (Fig. 1). During the sawing process, the nominal 2- by 4-inch (standard 38- by 89-mm) material was end-trimmed only enough to remove excessive wane. Although all lumber was tallied on the green chain, only the 7/4 lumber was retained for further study. The 7/4 lumber was kiln-dried to 15 (±3) percent moisture content in 37 days using a mild industry schedule developed for oak to avoid severe internal TABLE 4. - Percentage of 2 by 4 dimension lumber produced TABLE 5. - Dimension lumber tally and overrun by log grade and diameter class. TABLE 6. - Volume and value of dimension lumber by log grade and diameter class. a 8 JUNE 1992

5 honeycomb and checking. All lumber was planed to 1.5 by 3.5 inches (38.1 by 88.9 mm) in a four-head moulder. The lumber was graded by a Southern Pine Inspection Bureau inspector, who assigned the structural light-framing grade of Select Structural (SS), No. 1, No. 2. No. 3, or below grade (BG) to each piece and designated its length (either 8, 10, or 12 ft. (2.4, 3, or 3.7 m)) as graded. These lengths were selected to maximize dimension-lumber yield and to meet the length required for market acceptance. Boards were trimmed and numbered after grading. Results All 7/4 and 4/4 lumber was tallied by log grade and scaling diameter class. Nominal thickness and width dimensions of 2 by 4 inches (38 by 89 mm) were used for calculating board feet of lumber (Table 3). The surfaced lumber was graded and cut to length before tallying by log grade, dimension-lumber grade, and board length. Dimension-lumber yield expressed as a percentage of the total sawn 2 by 4 volume by log grade and log scaling diameter is given in Table 4. The log F2 grade and construction grade yielded about half the sawn 2 by 4 volume in merchantable (No. 3 and better) dimension lumber. Log F3 grade yielded only about one-fourth of its 2 by 4 volume in merchantable dimension lumber. Little difference was found between the yield of merchantable oak and the two scaling diameter classes of grades F2 and F3. A greater difference was found between the 10- to 1 1-inch (254 to 279-mm) scaling diameter class and the > 14-inch (>356-mm) class for the construction-grade logs, although for the number of logs sawn, the difference was not significant. There is an apparent trend toward the smaller diameter logs producing higher yields of No. 2 and better dimension lumber (Table 4). Total yield of 2 by 4 and 1 by 4 dimension lumber expressed as a percentage of the net International 1/4-inch scale is the overrun (negative values are underrun) by log grade and scaling diameter class (Table 5). This overrun is not consistent with that of factory lumber, which is likely due to sawing every log to a 2 by 4 size. Discussion The value per 1,000 BF (MBF) (2.36 m 3 ) of dimension-lumber yield was used to express the relative difference in the quality of lumber from the log grades and scaling diameter classes studied. These values also provided a basis for comparison between the production of 2 by 4 dimension lumber from the study logs and the conventional production of factory lumber normally produced from this red oak resource. Because red oak dimension-lumber market values have not been established, estimated values using prices (9) for southern pine dimension lumber were used, with values per MBF. The values were: $300 for SS. $280 for No. 1, $260 for No. 2, and $150 for No. 3 (Table 6). Applying these contrived values to log grade FOREST PRODUCTS JOURNAL Vol. 42, No. 6 9

6 F2, the 12- to 13-inch (305- to 330-mm) diameter logs are valued at $121/MBF, whereas for log grade F-3, the 12- to 13-inch diameter logs are valued at $53/MBF. This spread, a factor of 2.28, reflects the difference in value of 2 by 4 dimension lumber between F2 and F3 logs. Values of $116 and $57/MBF for >14-inch diameter logs in grades F2 and F3, respectively, show a similar spread with a factor of The value computed for the >14-inch construction grade, a class of log that is lower in quality for factory-lumber production than is the F3 log, is $110/MBF. This is comparable to the value for the F2 log in the same scaling diameter class, indicating similar yields of 2 by 4 dimension lumber. All the 10- to 11-inch (254- to 279-mm) diameter logs were in the construction-grade class with a computed value of $145/MBF. This value reflects the higher conversion efficiency of logs of this size and quality into dimension-lumber products when compared to other logs. This information provides evidence that dimension lumber should be produced from the logs in the construction-grade class. which basically are sound and straight with knots only limited by size and not by frequency or spacing. To compare logs converted to dimension lumber and the same logs converted to factory lumber, we computed the value per MBF for factory lumber. Using published yield tables (5, 13) developed for the factorygrade logs, the percentage yield of factory lumber was obtained for the F2 and F3 red oak logs. The results (Table 7) for these grades are based on the published market prices for dry, red oak factory lumber obtained from the Hardwood Market Report for June 9, 1990 (6). Prices per MBF are $1,015 for FAS (First and Second), $1,005 for FlF (First and Second One Face). $995 for Sel (Select), $845 for No. 1C (Common). $395 for No. 2C, $280 for No. 3A, and $150 for No. 3B. Using this information, the values per MBF for factory lumber in the 12- to 13-inch diameter class that could be expected from the study logs are $568 for F2 and $440 for F3 logs. No logs from the construction-grade class were sawn in this scaling diameter class. The factory lumber values per MBF in the >14-inch diameter class are $631 for F2 logs and $479 for F3 logs. The dimension-lumber value for the logs in the construction-grade class of this scaling diameter was estimated to be $110/MBF. The value per MBF of factory lumber is considerably higher when compared to that of dimension lumber sawn from the same logs. Construction-grade class logs yield low volumes of factory-grade lumber; therefore, their yield and values have not been reported for comparison. Conclusions Log quality was found to have an effect on the yield of 2 by 4 dimension lumber from red oak logs. The yield of 2 by 4 s from USDA FS standard factory grade F2 was comparable to the yield from the USDA FS construction grade. with about 50 percent yield in graded dimension lumber coming from both classifications. The yield of 2 by 4's from USDA FS standard factorygrade F3 was only 25 percent in graded dimension lumber. Log size may have had some effect on the 2 by 4 yield, with higher values per MBF coming from the group of 10- to 1 1-inch diameter logs. Market competition for the higher valued F2 logs for factory lumber indicates that only logs from the construction grade should be considered by potential producers of dimension lumber and then considered only with caution. This study provides evidence that critical processing decisions need to be made by sawmill operators for the efficient production of dimension lumber and factory lumber from red oak logs. Further investigation of smaller diameter logs in grade F3 and medium-diameter construction logs would provide additional information, but will not likely deviate from what has been reported or from what would be expected. Literature cited 10 Printed on Recycled Paper JUNE 1992