Korean standards of visual grading and establishing allowable properties of softwood structural lumber

Transcription

1 Korean standards of visual grading and establishing allowable properties of softwood structural lumber Park, Moon-Jae 1, Shim, Kug-Bo 1 ABSTRACT Korean standards related to wood products such as "Sizes of lumber", and "Glued-laminated lumber", etc. had been established several decades ago. Recently, light frame and glued laminated timber constructions were introduced to Korean housing industry from northern America. But Korean standards for strength properties of structural member were not established yet. Therefore, standards that related to size, quality, grading, and establishment of allowable properties for structural member would be necessary to improve Korean wood construction industries. As a first part of this effort, Korean Standards on Softwood structural lumber, and Structural glued laminated timber were established in the year of And additionally, Korean Standards on Visual grading for softwood structural lumber and Establishment of allowable properties for softwood structural lumber" were developed in In this report, major contents of these Korean Standards are summarized. But, these standards were established by limited information on experimental data of lumber strength properties. To improve these standards, more experimental data for lumber strength properties and relationship between characteristics and strength properties including measuring method of structural member were necessary. I. VISUAL GRADING FOR SOFTWOOD STRUCTURAL LUMBER Individual pieces of sawn lumber represent a wide range in quality and appearance with respect to knots, cross grain, shakes, and other characteristics. Those sawn lumbers represent a wide range in strength, utility, serviceability, and value. One of the obvious requirements for the orderly marketing of lumber is the establishment of grades that permit the procurement of any required quality of lumber in any desired quantity. Maximum economy of material is obtained when the range of quality-determining characteristics in a grade is limited and all pieces are utilized to their full potential. Many of the grades are established on the basis of appearance and physical characteristics of the piece, but without regard for mechanical properties. Other grades, called structural or stress grades, are established on the basis of features that relate to mechanical properties. The latter designate near-minimum strength and near-average stiffness properties on which to base structural design. 1. Definition The terms of visual grading, machine stress rating, structural grade or stress grade, and strength ratio is defined as follows; 1.1 Visual grading is accomplished from an examination of all four faces and the ends of the piece, in which the location as well as the size and nature of the knots and other features appearing in the surfaces are evaluated over the entire length. 1.2 Machine stress rating is nondestructive test to grade structural lumber. The nondestructive test is highly automated, and the process takes very little time. As lumber comes out of the mill, it passes through a series of rollers. In this process, a bending load is applied about the minor axis of the cross section, and the modulus of elasticity of each piece is measured. 1.3 Structural grade or stress grade is lumber grade, which is related to the mechanical properties based on the size and distribution of defects. The allowable stresses for structural design in each grade are represented in the minimum strength 1 Research Scientists, Korea Forest Research Institute, 207 Cheongnyangni-2 dong, Dongdaemun-gu, Seoul , Korea

2 and average stiffness. 1.4 Strength ratio After the clear-wood strength properties for the species have been determined, the effects of the natural growth characteristics that are permitted in the different grades of full-size members are taken into account. This is accomplished by multiplying the clear-wood values by a reduction factor known as a strength ratio. In other words, the strength ratio takes into account the various strength-reducing defects (e.g., knots, slope of grain etc.) that may be present. 2. Grading methods of structural lumber 2.1 Visual grading a) Classifications and grades are classified based on size and use of lumber and distributed defects in lumber. b) Allowable defects are determined by the kind of defects, size, and location in each lumber grade. c) Allowable properties are determined by the strength ratio in each lumber grade that calculated based on the defects, size and distribution. 2.2 Calculation of strength ratios Generals a) Knot (in bending members). Strength ratio associated with knots in bending members have been derived as the ratio of moment-carrying capacity of a member with cross section reduced by the largest knot to the moment-carrying capacity of the member without defect. This gives the anticipated reduction in bending strength due to knot. For simplicity, all knots on the wide face are treated as being either knots along the edge of the piece (edge knots) or knots along the centerline of the piece (centerline knots). b) Knots (in compression members) Strength ratio associated with knots in compression members have been derived as the ratio of load-carrying capacity of a member with cross section reduced by the largest knot to the load-carrying capacity of the member without defect. No assumption of combined compression and bending is made. c) Slope of grain Strength ratio associated with slope of grain in bending members, and in members subjected to compression parallel to grain, was obtained, experimentally. d) Fiber separation Strength ratios associated with shakes, checks, and splits are assumed to affect only horizontal shear in bending members. These strength ratios were derived, as for knots, by assuming that a critical cross section is reduced by the amount of the shake, or by an equivalent split or check. e) Tensile strength Tensile strength of lumber has been related to bending strength and bending strength ratio from experimental results. f) Compressive strength perpendicular to grain Strength in compression perpendicular to grain is little affected in lumber by strength-reducing characteristics, and strength ratios of 100% are assumed for all grades. g) Modulus of elasticity Modulus of elasticity of a piece of lumber is known to be only approximately related to bending strength ratio. In this standard, the relationship between full-span, edgewise bending modulus of elasticity and strength ratio was obtained experimentally Strength ratios a) Slopes of grain Strength ratios corresponding to various slopes of grain for stress in bending and compression parallel to grain is are given in ASTM D 245. b) Knot Strength ratios for various combinations of size and location of knot and width of face are given in ASTM D 245.

3 c) Split Strength ratios for various lumber thickness and shake, check, or split size are given in ASTM D 245. Strength ratios below 50% are not used, because a bending member that is split completely through lengthwise will still hold one half the shear load of an unsplit member. Splits are given special treatment in 2-in. nominal dimension lumber to be used in light building construction. d) Modulus of elasticity Modulus of elasticity is modified by a quality factor that is related to bending strength ratio, as given in ASTM D 245. e) Tensile parallel to grain Strength ratios in tension parallel to grain are 55% if the corresponding bending strength ratios. 3. Estimation and limitation of growth characteristics 3.1 General quality of lumber Finishing All lumber should be well manufactured Decay Only sound wood, free from any form of decay, shall be permitted, unless otherwise specified. Unsound knots and limited amounts of decay in its early stages are permitted in some of the lower stress-rated grades of lumber intended for light frame construction Fiber separation In stress grading, all knot, wane, split, check, shake, average ring width, slope of grain, decay, and twist in four faces and the ends shall be considered. 3.2 Knots Knots in dimension lumber Knots in dimension lumber may be measured by displacement method, in which the proportion of the cross section of the knot to the cross section of the piece is multiplied by actual face width to establish the equivalent knot size. The ratio of knot diameter was percent of knot diameter to the surface width. Alternatively, Knots in dimension lumber may be measured on the surface of the piece. The size of a knot on a narrow face is its width between lines enclosing the knot and parallel to the edges of the piece. A narrow-face knot that appears also in the wide face of a side-cut piece (but does not contain the intersection of those faces) is measured and graded on the wide face. A corner knot is measured on its end between lines parallel to the edges of the piece and is graded with respect to the face on which it is measured. A corner knot in a piece containing the pith is measured either by its width on the narrow face between lines parallel to the edge, or by its smallest diameter on the wide face, whichever is more restrictive. If a corner knot appears also on an opposite face, its limitation there as well. When the long diameter of knot was longer as 2.5 times than short diameter of knot was decided to a half of measured diameter as follows; Knot contacted in two or three surfaces was measured the diameter of knot in the cross-section. The location of knot was decided to the location of knot center (fig. 5). The location of knot was classified to the wide surface knot and narrow surface knot. In the structural lumber, the wide surface knot was classified in the edge and center knot. Knot cluster was defined as clustered within 15-cm more than two knots. The ratio of diameter of knot cluster was summed of these knots.

4 3.3 Wane Wane consists of bark remaining on the surface of the finished lumber. Obviously, the strength is reduced by wane, so wane is taken into account in the allowable design stresses assigned to a given grade of lumber. The largest wane length to surface length was measured to the wane ratio in all surfaces. 3.4 Fiber separation Shakes are measured at the ends of the piece. The size of a shake is the distance between lines enclosing the shake and parallel to the wide face of the piece. Splits and checks are treated as "equivalent shakes", but are measured differently. The size of a side check is its average depth of penetration into the piece, measured from and perpendicular to the surface of the wide face on which appears. The size of an end split or end-check is one third of its average length measured along the length of a piece Where 2-in. nominal dimension is to be used in light building construction in which the shear stress is not critical, a more liberal provision on end splits may be made. The size of the split, measured its average length along the length of the piece. Strength ratios are given for this special case ASTM D Average ring width Average ring width was measured in the end surface in the lumber. 3.6 Slope of grain Slope of grain resulting from either diagonal sawing or from spiral or twisted grain in the tree is measured by the angle between the direction of the fibers and the edge of the piece. The angle is expressed as a slope. Slope of grain is measured and limited at the zone in the length of a structural timber that shows the greatest slope. It shall be measured over a distance sufficiently great to define the general slope, disregarding such short local deviations as those around knots II. ESTABLISHING ALLOWABLE PROPERTIES OF SOFTWOOD STRUCTURAL LUMBER The allowable properties are based on normal loading duration. The allowable modulus of elasticity and compression perpendicular-to-grain stress is intended to be average values for the species group and stress grade. The other stresses are intended to be less than the stress permissible for 95% of the pieces in a species group and stress grade, that is, most allowable properties are based on the concept of a 5% exclusion limit. The allowable stresses may be determined by establishing procedure either by strength values of small clear wood or by full-sized structural lumber as following procedure. 1. Establishing allowable properties by small clear wood strength values 1.1 Procedure for establishing clear wood strength values Method A Method A uses the results of surveys of wood density involving extensive sampling of trees, in combination with the data obtained from standard strength tests made in accordance with Korean Standards. Six steps are involved in establishing strength values by the wood density survey procedure. These steps are as follows. a) Conducting wood density survey b) Development of unit areas c) Determination of average specific gravity for a unit area d) Determination of strength-specific gravity relations e) Estimation of average strength properties for a unit area. f) Combining unit areas into basic groups and development of average strength properties and estimates of variance for the group: 1) Determine average strength properties for a group of unit areas 2) Compute the variability index 3) Estimate a standard deviation 4) Average compression perpendicular to the grain Method B Method B bases average strength properties for clear wood of species for which density survey data are not available on

5 standard strength test data obtained in accordance with Korean Standards Tensile strength parallel and perpendicular to grain Tensile strength parallel and perpendicular to grain and modulus of rigidity associated with a longitudinal transverse plane are sometimes needed for design considerations. These properties have not been evaluated extensively. However, They may be estimated from the clear wood properties of any combination of species, as described in the following criteria: a) Tensile parallel to grain For clear wood strength in tension parallel to grain, the clear wood strength value for modulus of rupture may be used. b) Tension perpendicular to grain For clear wood strength in tension perpendicular to grain, 0.33 times the clear wood strength value for shear may be used. a) Modulus of rigidity For clear wood modulus of rigidity, times the modulus of elasticity may be used Determination the allowable properties from small clear specimen The 5 % exclusion limit for bending strength, tensile strength parallel to grain, compressive strength parallel to grain, and horizontal shear strength for clear straight-grained wood in the green condition shall be obtained for any species or group of species from proper statistics. These properties when divided by the reduction factors provided in the Standard. a) The average green modulus of elasticity, proportional limit in compression perpendicular to grain, and stress in compression perpendicular to grain at 1-mm deformation shall be obtained for any species or group of species. b) The bending stress is based on an assumed 38-mm depth. To adjust the stress to other sizes, multiply it by the factor, F, taken from follows. 1 d 9 F = ( 1 ) d where F = size factor d 1 = net surfaced depth of clear small specimen d = net surfaced depth 2. Evaluating allowable properties for grades of structural lumber 2.1 Sample size Selection of a sample size depends upon the properties to be estimated. To determine the sample size for near-minimum properties, the nonparametric tolerance limit concept or parametric approach may be used. Sample size and order statistic for estimating the 5 % nonparametric tolerance limit is determined by the method of ASTM D Analysis for evaluating allowable properties Allowable properties are evaluated by adapting strength reduction factor to the 5% exclusion limit of strength data from the actual-sized structural lumber. Properties shall be adjusted to single moisture content appropriate for the objective of the testing program. Although, test results can be adjusted for moisture content, these adjustments decrease in accuracy with increasing change in moisture content. For this reason, it is suggested that the specimens be conditioned as closely as possible to the target moisture content prior to test, and that adjustments for more than five percentage points of moisture content are to be avoided. K factors for adjusting apparent modulus of elasticity of simply supported beams described in ASTM D 2915 was adapted. 2.3 Modification of allowable properties for design use The principal modifications made in design properties are assumed to be the final responsibility of the designing engineer to relate design assumptions and allowable properties and to make modifications of the allowable properties for seasoning and duration of load to fit a particular use. The modifications are often subject to the requirements of a building code. CONCLUDING REMARKS Korean Standards on Visual grading for softwood structural lumber and Establishing allowable properties of softwood structural lumber" were established in Major contents of these Korean Standards are summarized in this report. By developing standards, such as visual grading and establishing allowable properties of structural member, quality of building will be improved and developing standards, such as visual grading and establishing allowable properties of structural member will expand wood construction market expanded in Korea.

6 But, these standards were established by limited information on experimental data of lumber strength properties. To improve these standards, more experimental data for lumber strength properties and relationship between characteristics and strength properties including measuring method of structural member were necessary REFERENCES American Society for Testing and Materials Standard practice for establishing structural grades and related allowable properties for visually graded lumber. ASTM D 245. American Society for Testing and Materials Standard practice for evaluating allowable properties for grades of structural lumber. ASTM D American Society for Testing and Materials Standard test methods for establishing clear wood strength values. ASTM D Japanese Agricultural Standard Association Grading rules for softwood structural lumber. Korean Standard Association Establishing allowable properties of softwood structural lumber. KS F Korean Standard Association Visual grading for softwood structural lumber. KS F 2151 KugBo Shim, MoonJae Park Establishing system of allowable stresses for structural lumber. FRI journal of forest science No. 57. Pp Ministry of Agriculture and Forestry Development of new uses for domestic wood resources. MoonJae Park, KugBo Shim, ChoonTaek Lee Mechanical properties of Korean pine and larch structural lumber. Proceedings of 5 th World Conference of Timber Engineering. Vol. 2. Pp