1、Designation: D 245 06Standard Practice forEstablishing Structural Grades and Related AllowableProperties for Visually Graded Lumber1This standard is issued under the fixed designation D 245; the number immediately following the designation indicates the year oforiginal adoption or, in the case of re
2、vision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This practice (1
3、,2)2covers the basic principles forestablishing related unit stresses and stiffness values for designwith visually-graded solid sawn structural lumber. This prac-tice starts with property values from clear wood specimens andincludes necessary procedures for the formulation of structuralgrades of any
4、 desired strength ratio.1.2 The grading provisions used as illustrations herein arenot intended to establish grades for purchase, but rather toshow how stress-grading principles are applied. Detailed grad-ing rules for commercial stress grades which serve as purchasespecifications are established an
5、d published by agencies whichformulate and maintain such rules and operate inspectionfacilities covering the various species.1.3 The material covered in this practice appears in thefollowing order:SectionScope 1Significance and Use 3Basic Principles of Strength Ratios 4Estimation and Limitation of G
6、rowth Characteristics 5Allowable Properties for Timber Design 6Modification of Allowable Properties for Design Use 7Example of Stress-Grade Development 81.4 The values given in parentheses are provided for infor-mation purposes only.1.5 This standard does not purport to address all of thesafety conc
7、erns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3D9 Terminology Relating to Wood and Woo
8、d-Based Prod-uctsD 143 Test Methods for Small Clear Specimens of TimberD 2555 Practice for Establishing Clear Wood Strength Val-uesE 105 Practice for Probability Sampling Of MaterialsIEEE/ASTM SI-10 Practice for Use of the InternationalSystem of Units (SI) (the Modernized Metric System)3. Significan
9、ce and Use3.1 Need for Lumber Grading:3.1.1 Individual pieces of lumber, as they come from thesaw, represent a wide range in quality and appearance withrespect to freedom from knots, cross grain, shakes, and othercharacteristics. Such random pieces likewise represent a widerange in strength, utility
10、, serviceability, and value. One of theobvious requirements for the orderly marketing of lumber isthe establishment of grades that permit the procurement of anyrequired quality of lumber in any desired quantity. Maximumeconomy of material is obtained when the range of quality-determining characteris
11、tics in a grade is limited and all piecesare utilized to their full potential. Many of the grades areestablished on the basis of appearance and physical character-istics of the piece, but without regard for mechanical proper-ties. Other grades, called structural or stress grades, areestablished on t
12、he basis of features that relate to mechanicalproperties. The latter designate near-minimum strength andnear-average stiffness properties on which to base structuraldesign.3.1.2 The development of this practice is based on extensiveresearch covering tests of small clear specimens and offull-sized st
13、ructural members. Detailed studies have includedthe strength and variability of clear wood, and the effect onstrength from various factors such as density, knots (See1This practice is under the jurisdiction of ASTM Committee D07 on Wood andis the direct responsibility of Subcommittee D07.02 on Lumbe
14、r and EngineeredWood Products.Current edition approved March 1, 2006. Published March 2006. Originallyapproved in 1926. Last previous edition approved in 2002 as D 245 00 (2002)e1.2The boldface numbers in parentheses refer to references at the end of thispractice.3For referenced ASTM standards, visi
15、t the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-29
16、59, United States.Terminology D9), and other defects, seasoning, duration ofstress, and temperature.3.2 How Visual Grading is Accomplished Visual gradingis accomplished from an examination of all four faces and theends of the piece, in which the location as well as the size andnature of the knots an
17、d other features appearing on the surfacesare evaluated over the entire length. Basic principles ofstructural grading have been established that permit the evalu-ation of any piece of stress-graded lumber in terms of astrength ratio for each property being evaluated. The strengthratio of stress-grad
18、ed lumber is the hypothetical ratio of thestrength property being considered compared to that for thematerial with no strength-reducing characteristic. Thus a pieceof stress-graded lumber with a strength ratio of 75 % inbending would be expected to have 75 % of the bendingstrength of the clear piece
19、. In effect, the strength ratio systemof visual structural grading is thus designed to permit practi-cally unlimited choice in establishing grades of any desiredquality to best meet production and utilization requirements.3.3 Classification of Stress-Graded Lumber:3.3.1 The various factors affecting
20、 strength, such as knots,deviations of grain, shakes, and checks, differ in their effect,depending on the kind of loading and stress to which the pieceis subjected. Stress-graded lumber is often classified accordingto its size and use. Four classes are widely used, as follows:3.3.1.1 Dimension Lumbe
21、rPieces of rectangular crosssection, from nominal 2 to 4 in. thick and 2 or more in. wide,graded primarily for strength in bending edgewise or flatwise,but also frequently used where tensile or compressive strengthis important. Dimension lumber covers many sizes and enduses. Lumber graded for specif
22、ic end uses may dictate a specialemphasis in grading and require an identifying grade name.NOTE 1For example, in North American grading under the AmericanLumber Standards Committee, stress graded dimension lumber categoriesthat reflect end use include Light Framing, Structural Light Framing,Structur
23、al Joists and Planks, and Studs.3.3.1.2 Beams and StringersPieces of rectangular crosssection, 5 in. nominal and thicker, nominal width more than 2in. greater than nominal thickness, graded for strength inbending when loaded on the narrow face.3.3.1.3 Posts and TimbersPieces of square or nearlysquar
24、e cross section, 5 by 5 in., nominal dimensions and larger,nominal width not more than 2 in. greater than nominalthickness, graded primarily for use as posts or columns.3.3.1.4 Stress-Rated BoardsLumber less than 2 in. nomi-nal in thickness and 2 in. or wider nominal width, gradedprimarily for mecha
25、nical properties.3.3.2 The assignment of names indicating the uses for thevarious classes of stress-graded lumber does not preclude theiruse for other purposes. For example, posts and timbers maygive service as beams. The principles of stress grading permitthe assignment of any kind of allowable pro
26、perties to any of theclasses of stress-graded lumber, whether graded primarily forthat property or not. Recommendations for allowable proper-ties may include all properties for all grades or use classes.While such universal application may result in loss of effi-ciency in some particulars, it offers
27、 the advantage of a moresimple system of grades of stress-graded lumber.3.4 Essential Elements in a Stress-Grade Description:3.4.1 A stress grade formulated by this practice contains thefollowing essential elements:3.4.2 A grade name that identifies the use-class as describedin 3.3.3.4.3 A descripti
28、on of permissible growth characteristicsthat affect mechanical properties. Characteristics that do notaffect mechanical properties may also be included.3.4.4 One or more allowable properties for the grade relatedto its strength ratio.4. Basic Principles of Strength Ratios4.1 General Considerations:4
29、.1.1 Strength ratios associated with knots in bending mem-bers have been derived as the ratio of moment-carryingcapacity of a member with cross section reduced by the largestknot to the moment-carrying capacity of the member withoutdefect. This gives the anticipated reduction in bending strengthdue
30、to the knot. For simplicity, all knots on the wide face aretreated as being either knots along the edge of the piece (edgeknots) or knots along the centerline of the piece (centerlineknots).4.1.2 Strength ratios associated with slope of grain inbending members, and in members subjected to compressio
31、nparallel to grain, were obtained, experimentally (3).4.1.3 Strength ratios associated with shakes, checks, andsplits are assumed to affect only horizontal shear in bendingmembers. These strength ratios were derived, as for knots, byassuming that a critical cross section is reduced by the amountof t
32、he shake, or by an equivalent split or check.4.1.4 Strength ratios associated with knots in compressionmembers have been derived as the ratio of load-carryingcapacity of a member with cross section reduced by the largestknot to the load-carrying capacity of the member withoutdefect. No assumption of
33、 combined compression and bendingis made.4.1.5 Tensile strength of lumber has been related to bendingstrength and bending strength ratio from experimental results(4).4.1.6 Strength in compression perpendicular to grain is littleaffected in lumber by strength-reducing characteristics, andstrength rat
34、ios of 100 % are assumed for all grades.4.1.7 Modulus of elasticity of a piece of lumber is known tobe only approximately related to bending strength ratio. In thisstandard, the relationship between full-span, edgewise bendingmodulus of elasticity and strength ratio was obtained experi-mentally.4.1.
35、8 In developing a stress-grade rule, economy may beserved by specifying strength ratios such that the allowablestresses for shear and for extreme fiber in bending will be inbalance, under the loading for which the members are de-signed.4.1.9 A strength ratio can also be associated with specificgravi
36、ty. Three selection classes called dense, close grain, andmedium grain are described herein, based on experimentalfindings (5).4.2 Strength Ratios:D2450624.2.1 Table 1 gives strength ratios, corresponding to variousslopes of grain for stress in bending and compression parallelto grain.4.2.2 Strength
37、 ratios for various combinations of size andlocation of knot and width of face are given in Table 2, Table3, and Table 4. Since interpolation is often required in thedevelopment of grading rules, the use of formulas in Table 2,Table 3 and Table 4 is acceptable. These formulas are found inthe Appendi
38、x.4.2.2.1 Use of the tables is illustrated by the followingexample: The sizes of knots permitted in a 712 by 1512-in. (190by 394-mm) (actual) beam in a grade having a strength ratio of70 % in bending are desired. The smallest ratio in the columnfora712-in. (190-mm) face in Table 2 that equals or exc
39、eeds70 % is opposite 218 in. (54 mm) in the size-of-knot column.Asimilar ratio in the column for 1512-in. (394-mm) face in Table3 is opposite 414 in. (108 mm). Hence, the permissible sizes are218 in. (54 mm) on the 712-in. (190-mm) face and at the edgeof the wide face (see 5.3.5.2) and 414 in. (108
40、mm) on thecenterline of the 1512-in. (394-mm) face.4.2.3 For all lumber thicknesses, a strength ratio of 50 %shall be used for all sizes of shakes, checks and splits. A 50 %strength ratio is the maximum effect a shake, check or split canhave on the load-carrying capacity of a bending member.Limitati
41、ons in grading rules placed on the characteristics attime of manufacture are for appearance and general utilitypurposes, and these characteristics shall not be used as a basisfor increasing lumber shear design values.NOTE 2The factor of 0.5 (50 %) is not strictly a “strength ratio” forhorizontal she
42、ar, since the factor represents more than just the effects ofshakes, checks and splits. The factor also includes differences between testvalues obtained in Methods D 143 shear block tests and full-size solid-sawn beam shear tests. The strength ratio terminology is retained forcompatibility with prio
43、r versions of Practice D 245, but prior provisionspermitting design increases for members with lesser-size cracks have beendeleted since the factor is related to more than shakes, checks and splits.4.2.4 Modulus of elasticity is modified by a quality factorthat is related to bending strength ratio,
44、as given in Table 5.4.2.5 Strength ratios in tension parallel to grain are 55 % ofthe corresponding bending strength ratios.4.2.6 Table 6 gives strength ratios and quality factors for thespecial specific gravity classes described in 4.1.9.5. Estimation and Limitation of Growth Characteristics5.1 Gen
45、eral Quality of Lumber:5.1.1 All lumber should be well manufactured.5.1.2 Only sound wood, free from any form of decay, shallbe permitted, unless otherwise specified. Unsound knots andlimited amounts of decay in its early stages are permitted insome of the lower stress-rated grades of lumber intende
46、d forlight frame construction.5.1.3 In stress-grading, all four faces and the ends shall beconsidered.5.2 Slope of Grain:5.2.1 Slope of grain resulting from either diagonal sawingor from spiral or twisted grain in the tree is measured by theangle between the direction of the fibers and the edge of t
47、hepiece. The angle is expressed as a slope. For instance, a slopeof grain of 1 in 15 means that the grain deviates 1 in. (2.5 mm)from the edge in 15 in. (381 mm) of length.5.2.2 When both diagonal and spiral grain are present, thecombined slope of grain is taken as the effective slope.5.2.3 Slope of
48、 grain is measured and limited at the zone inthe length of a structural timber that shows the greatest slope.It shall be measured over a distance sufficiently great to definethe general slope, disregarding such short local deviations asthose around knots except as indicated in 5.2.5.5.2.4 In 1-in. n
49、ominal boards (See Terminology D9), orsimilar small sizes of lumber, a general slope of grain any-where in the length shall not pass completely through thethickness of the piece in a longitudinal distance in inches lessthan the number expressing the specified permissible slope.Where such a slope varies across the width of the board, itsaverage may be taken.5.2.5 Local deviations must be considered in small sizes,and if a local deviation occurs in a piece less than 4 in. nominalin width or on the narrow face of a piece less than 2 in. nominalin thickness, and is not as
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