1、Designation: D 3957 09Standard Practices forEstablishing Stress Grades for Structural Members Used inLog Buildings1This standard is issued under the fixed designation D 3957; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、 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.INTRODUCTIONThese practices are based on the assumption that structural members in log buildings can bestress-graded by method
3、s that derive from accepted standards for conventional solid sawn lumber andround timbers. It is assumed that the material to be graded bears enough similarity to either sawnlumber or round timber, both in dimensional properties and in use, to warrant application ofstress-grading standards written f
4、or sawn-lumber or round-timber, or both. These practices, such asPractices D 245 and D 2899, cannot be applied directly because the structural members used in logbuildings are generally neither perfectly rectangular nor perfectly round in section. These practices usecertain conventions regarding cro
5、ss-sectional dimensions that make it possible to extend establishedstress-grading methodologies to cover the members used in log buildings.Where log member characteristics deviate from sawn lumber or round timber standards, there maybe uncertainty as to the exact effect of the deviation on strength
6、properties. To compensate for thisuncertainty, some design stress values are herein derived with practices that are, by engineeringjudgment, conservative. The philosophy guiding this approach is that while the absence ofexperimental data may make a measure of conservatism unavoidable, the reliabilit
7、y of design stressvalues must not be achieved through wood quality or size requirements that are an unnecessary burdenon the wood resource.In general, the sawing, cutting, and shaving required to bring a piece to its final shape must becompleted before it can be visually graded using the principles
8、in these practices. Small cuts may beallowed after grading if it can be shown that either (1) the cuts do not affect the grade, or (2) the gradetakes the additional cuts into consideration.Both sawn lumber standards and round timber standards are herein referenced, because these twostress-grading me
9、thodologies can be assumed to apply to two different types of structural membersused in log buildings: wall-logs and round timber beams. Since wall-logs must be provided with ameans of joining together (for example, tongue-and-groove joints), they resemble sawn lumber and aretreated as such in the s
10、tandard. Rafters, purlins, and beams, on the other hand, are sometimes left asround logs, except for a small amount of sawing to provide a flat nailing surface. These practices thusdeal with stress-grading of wall-logs and round-timber beams separately.1. Scope1.1 These practices cover the visual st
11、ress-grading prin-ciples applicable to structural wood members of nonrectangu-lar shape, as typically used in log buildings. These practices aremeant to supplement the ASTM standards listed in Section 2,which cover stress-grading of sawn lumber and round timbers.Pieces covered by these practices may
12、 also be used in buildingtypes other than log buildings.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 to membersused in log buildings. Detailed grading rules for commercialstress g
13、rades which serve as purchase specifications are estab-lished and published by agencies that formulate and maintainsuch rules and operate inspection facilities covering the variousspecies.1These practices are under the jurisdiction of ASTM Committee D07 on Woodand are the direct responsibility of Su
14、bcommittee D07.02 on Lumber and Engi-neered Wood Products.Current edition approved May 1, 2009. Published June 2009. Originallyapproved in 1980. Last previous edition approved in 2006 as D 3957 06.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, Un
15、ited States.1.3 The values stated in inch-pound units are to be regardedas standard. The values given in parentheses are mathematicalconversions to SI units that are provided for information onlyand are not considered standard.1.4 This standard does not purport to address all of thesafety concerns,
16、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:2D25 Specification for Round Timber PilesD 245
17、 Practice for Establishing Structural Grades and Re-lated Allowable Properties for Visually Graded LumberD 2555 Practice for Establishing Clear Wood Strength Val-uesD 2899 Practice for Establishing Allowable Stresses forRound Timber PilesD 3200 Specification and Test Method for EstablishingRecommend
18、ed Design Stresses for Round Timber Con-struction Poles3. Significance and Use3.1 It is useful to grade logs to improve the consistency inperformance. Using the visual stress-grading principles appli-cable to rectangular and round shape structural wood members,these practices illustrate the developm
19、ent of stress gradingmethodologies for wall-logs and round timber beams, astypically used in log buildings. The clear wood strength valuesare used as the basis for deriving the design stress values inthese applications.4. Stress-Grading of Wall-Logs4.1 General:4.1.1 This section is intended to apply
20、 to wood members,referred to as wall-logs, which are normally stacked horizon-tally or laid-up vertically to form a load-bearing, solid-woodwall, in any building. These structural members can varygreatly in dimension and section profile, and therefore previ-ously developed standards for solid sawn l
21、umber are notreadily applied to them (Fig. 1).4.1.2 Wall-logs, as referred to in these practices, can also beused as beams, joists, and so forth, and do not have to be usedas wall components.4.1.3 Unless they qualify as round-timber beams underSection 5 of these practices, wall-logs must be consider
22、ed assawn lumber and therefore must respect the provisions ofstress-grading described in Practice D 245. The manner inwhich Practice D 245 is applied to wall-logs is described in4.2.4.2 Use of Rectangular Section Inscribed in Actual Sec-tions:4.2.1 A wall-log is to be graded as the largest piece ofr
23、ectangular lumber that can be embedded in the wall-logwithout protrusion from any wall-log surface except that eachcorner may protrude up to12 in. (12.7 mm) in either or both2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For
24、Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.NFNarrow FaceWFWide FaceFIG. 1 Typical Wall-Log Sections Showing Wide and Narrow Faces as Determined by Inscribed RectangleD3957092directions (Fig. 2).All provisions of Practice D 245 tha
25、t wouldapply to a piece with the same cross-section as the inscribedrectangle shall apply to the wall-log.4.2.2 Maximum knot sizes shall be determined by the wideand narrow face dimensions of the inscribed rectangle, usingknot tables in Practice D 245. Boundaries between portions ofthe wall-log surf
26、ace considered wide-face and portions consid-ered narrow-face shall be found by extending the diagonals ofthe inscribed rectangle to the wall-log surface (Fig. 2). Just asthe wide and narrow faces of the inscribed rectangle belong toa quadrant between diagonals, so shall the wide and narrowfaces of
27、the wall-log surface belong to the same quadrants. Ingeneral, then, the surface boundaries between wide and narrowfaces may not coincide with actual edges on the wall-log.4.2.3 Knot size limitations, as determined by the dimensionsof the inscribed rectangle, shall apply to knot measurementstaken at
28、the surface of the wall-log. The slight differencebetween the knot size at the surface of the wall-log, and theknot size at the inscribed rectangle is, for practical purposes,disregarded.4.2.4 If the wall-log design has saw-cuts that penetratedeeply into the piece, then any inscribed rectangle that
29、remainsunpenetrated by sawing may be too small to use for determin-ing knot limitations. To accommodate wall-logs of this type,cuts into the inscribed rectangle may be treated as follows.4.2.4.1 If a face of an inscribed rectangle has a maximumallowable knot size of D inches when the face is unpenet
30、ratedby any cuts, then the maximum allowable knot size for knotsthat do not encompass the sawcut is reduced to D d incheswhen the face is penetrated by a cut d inches wide. That is, asaw cut12 in. (12.7 mm) wide could effectively increase a 2-in.(50.8 mm) knot to 212 in. (63.5 mm), as when the knot
31、and thecut are tangent to each other. Therefore, a cut12 in. wide willreduce the maximum allowable knot by12 in. (Fig. 3).4.2.4.2 As an alternate to 4.2.4.1, reduce the maximumallowable knot displacement D to D 2d where d is thedisplacement of the sawcut(s) when the knot does not encom-pass the sawc
32、ut. For example, a 6 by 12 in. (152.4 by 304.8mm) with maximum knot displacement of 50 %, and twogrooves34 by 1 in. (19.1 by 25.4 mm) each on one narrow face,a groove displacement of 2 %, the allowable knot displacementfor knots that do not encompass the grooves would by reducedto502(2)=46%.5. Stres
33、s-Grading of Sawn Round Timber Beams5.1 General:5.1.1 This section describes how the formulas of PracticeD 2899 are applied to round timbers that are shaved or sawnalong one side (Note 1). Since these members are normally(a) Wall-Log Without Saw Kerf: Maximumallowable narrow face knot, D, deter-mine
34、d for A 3 B inscribed rectangle.(b) Same Wall-Log, With Saw Kerf: Maxi-mum allowable narrow faceknot = Dd(top) and D (bottom).(c) Alternative Method: Maximum allow-able narrow face knot, D*, determinedfor A 3 B* inscribed rectangle.FIG. 2 Determination of Inscribed RectangleFIG. 3 Strength Ratio for
35、 Sawn Round Timber BeamD3957093loaded on their flat surface, they are stressed primarily inbending and are herein referred to as sawn round timber beams.NOTE 1Unsawn round timbers used in the superstructure of buildingsare covered by Specification and Test Methods D 3200.5.1.2 Specification D25and P
36、ractice D 2899 set forth onestructural grade. These practices supplement SpecificationD25and Practice D 2899 so that a series of grades can beconstructed. This is accomplished by means of the strengthratios defined in 5.5.5.2 Allowable Sawing:5.2.1 The flat side of a sawn round timber beam shall not
37、penetrate more than 0.3 R into the piece, where R is the radiusof the beam (Fig. 3). This limits the reduction of the cross-sectional area, by sawing or shaving, to less than 10 %.5.2.2 A form factor equal to 1.18 is the factor by whichdesign-bending stresses of square-sawn pieces are multiplied ino
38、rder to derive design-bending stresses for beams with circularcross-sections. Since sawn round timber beams do not have acircular cross section, their form factor is set equal to 1.0 ratherthan 1.18. In order to apply the bending stress formula ofPractice D 2899 to sawn round timber beams, the form
39、factorincluded in that formula must be set equal to 1.0.5.3 Knot MeasurementKnots on the sawn face of a sawnround timber beam are measured by their smallest diameter.Other knots are measured in accordance with SpecificationD25.5.4 Slope of Grain MeasurementSlope of grain in sawnround timber beams is
40、 measured by the angle between thedirection of the fibers and the axis of the piece. As for lumber,this angle is expressed as a slope.5.5 Design Bending Stress Values:5.5.1 Bending strength ratios are determined by slope ofgrain or knot size, whichever is most restrictive. The substitu-tion of alter
41、native strength ratios into the design stress formulaof Practice D 2899 is not meant to result in higher allowablebending stresses than can be obtained when the bendingstrength ratio equals 0.76, that is, bending strength ratioshigher than 0.76 are not recommended for sawn-round timberbeams.NOTE 2Th
42、e formula in Practice D 2899 for finding design bendingstress values assumes that clear wood bending strength values should bereduced by factors to account for form, size, and grade. The form factorfor round timber is found in Section 10.1 of Practice D 2899 and the sizefactor is based on a (2/d)1/9
43、adjustment. The grade reduction is based onthe grade description of the particular product using a strength ratiosystem similar to Practice D 245 for sawn lumber. With the wide range insizes used in the log home industry, Practice D 2899, Section 10.3, may beapplicable when the diameter of the sawn
44、round timber beam exceeds 13.5in. (342.9 mm) at a point 10 ft (3 m) from its tip.5.5.1.1 Knot Strength RatiosStrength ratios for sawnround timber beams shall be determined assuming that knotseffectively reduce the cross-sectional area by a pie-shapedsector that radiates from the center of the beam t
45、o theoutermost boundaries of the knot (Fig. 3). It is further assumedthat the sector of area lost to a knot lies opposite the sawn face,since this will most reduce the beams section modulus.(1) Given (1) the section modulus, S, of a beam sawn tothe limit of 5.2.1, and (2) the section modulus, S8, ob
46、tainedwhen S is reduced to account for a knot, the bending strengthratio associated with the knot is that number that whenmultiplied by S gives S8.(2) By substituting the above strength ratios into thePractice D 2899 bending stress formula as explained inAppen-dix X1 and by the application of the ot
47、her adjustments to thisformula described in 5.2.2 and 5.5.3, design bending stressvalues for specific knot sizes can be determined.5.5.1.2 Slope of Grain Strength RatiosThe exact relation-ship between slope of grain and bending strength has not beendetermined for unsawn-round timbers. These strength
48、 ratios,listed below, are thought to be conservative estimates of theeffect of slope of grain on sawn-round timber beams (Note 3):Slope of Grain Bending Strength Ratio, %1in4 271in6 401in8 531in10 611in12 691in14 741in15 76NOTE 3Round timbers that are sawn within the limitations stated in5.2 will ha
49、ve hybrid strength characteristics that are between those ofsawn lumber and round timber. It can be assumed that the effect of a givenslope of grain on the bending strength of sawn round timber beams willnot be as great as its effect on the bending strength of sawn lumber. Thisassumption, which is based on engineering judgment, allows for theapplication of the above strength ratios to sawn round timber beams.5.5.2 In addition to factors for form and grade, the PracticeD 2899 formula for design bending stress includes factors toaccount for load duration, tip weak
