ASTM D245-2006(2011) Standard Practice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber《确定目检分级木材的结构等级和相关允许性能的标准操作规程》.pdf

1、Designation: D245 06 (Reapproved 2011)Standard Practice forEstablishing Structural Grades and Related AllowableProperties for Visually Graded Lumber1This standard is issued under the fixed designation D245; the number immediately following the designation indicates the year oforiginal adoption or, i

2、n the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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 T

3、his practice (1,2)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 structur

4、algrades of any 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

5、 established and 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

6、Limitation of Growth 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

7、 thesafety concerns, 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

8、to Wood and Wood-Based Prod-uctsD143 Test Methods for Small Clear Specimens of TimberD2555 Practice for Establishing Clear Wood Strength Val-uesE105 Practice for Probability Sampling of MaterialsIEEE/ASTM SI-10 Practice for Use of the InternationalSystem of Units (SI) (the Modernized Metric System)3

9、. Significance 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 stren

10、gth, utility, 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

11、 characteristics 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, areesta

12、blished on the 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 of1

13、This practice is under the jurisdiction of ASTM Committee D07 on Wood andis the direct responsibility of Subcommittee D07.02 on Lumber and EngineeredWood Products.Current edition approved Oct. 1, 2011. Published October 2011. Originallyapproved in 1926. Last previous edition approved in 2006 as D245

14、 06. DOI:10.1520/D0245-06R11.2The boldface numbers in parentheses refer to references at the end of thispractice.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the

15、 standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.full-sized structural members. Detailed studies have includedthe strength and variability of clear wood, and the effect onstrength fr

16、om various factors such as density, knots (SeeTerminology 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 si

17、ze andnature of the knots and 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

18、strengthratio of stress-graded 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 bendin

19、gstrength of the clear piece. 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 T

20、he various factors affecting 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 fol

21、lows:3.3.1.1 Dimension LumberPieces 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 endus

22、es. Lumber graded for specific 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, Struc

23、tural Light Framing,Structural 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 TimbersPie

24、ces of square or nearlysquare 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 widt

25、h, gradedprimarily for mechanical 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

26、of any kind of allowable properties 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

27、 some particulars, it offers 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 descr

28、ibedin 3.3.3.4.3 A description 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 Ratios

29、4.1 General Considerations:4.1.1 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 reduc

30、tion in bending strengthdue 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 mem

31、bers subjected to compressionparallel 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

32、is reduced by the amountof the 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 wit

33、houtdefect. No assumption of 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 char

34、acteristics, andstrength ratios 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 o

35、btained experi-mentally.4.1.8 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 a

36、ssociated with specificgravity. Three selection classes called dense, close grain, andmedium grain are described herein, based on experimentalfindings (5).D245 06 (2011)24.2 Strength Ratios:4.2.1 Table 1 gives strength ratios, corresponding to variousslopes of grain for stress in bending and compres

37、sion parallelto grain.4.2.2 Strength 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. Th

38、ese formulas are found inthe Appendix.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

39、) face in Table 2 that equals or exceeds70 % 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

40、face (see 5.3.5.2) and 414 in. (108 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. A50 %strength ratio is the maximum effect a shake, check or split canhave on the load-carrying ca

41、pacity of a bending member.Limitations 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

42、 “strength ratio” forhorizontal shear, since the factor represents more than just the effects ofshakes, checks and splits. The factor also includes differences between testvalues obtained in Methods D143 shear block tests and full-size solid-sawn beam shear tests. The strength ratio terminology is r

43、etained forcompatibility with prior versions of Practice D143, 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 re

44、lated to bending strength ratio, 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

45、 of Growth Characteristics5.1 General 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 stre

46、ss-rated grades of lumber intended 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 directio

47、n of the fibers and the edge of thepiece. 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 t

48、he effective slope.5.2.3 Slope of 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 in

49、dicated in 5.2.5.5.2.4 In 1-in. nominal 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.