# ASTM D3957-09 (Reapproved 2015)

Designation: D3957 − 09 (Reapproved 2015)Standard Practices forEstablishing Stress Grades for Structural Members Used inLog Buildings1This standard is issued under the fixed designation D3957; the number immediately following the designation indicates the year oforiginal adoption or, in 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.INTRODUCTIONThese practices are based on the assumption that structural members in log buildings can bestress-graded by methods 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 for sawn-lumber or round-timber, or both. These practices, such asPractices D245 and D2899, 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 cross-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 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 reliability 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 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 methodologies 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 standard. 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 stress-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 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 grades which serve as purchase specifications are estab-lished and published by agencies that formulate and maintainsuch rules and operate inspection facilities covering the variousspecies.1.3 The values stated in inch-pound units are to be regardedas standard. The values given in parentheses are mathematical1These practices are under the jurisdiction of ASTM Committee D07 on Woodand are the direct responsibility of Subcommittee D07.02 on Lumber and Engi-neered Wood Products.Current edition approved Nov. 1, 2015. Published December 2015. Originallyapproved in 1980. Last previous edition approved in 2009 as D3957 – 09. DOI:10.1520/D3957-09R15.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1conversions 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, 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 PilesD245 Practice for Establishing Structural Grades and Re-lated Allowable Properties for Visually Graded LumberD2555 Practice for Establishing Clear Wood Strength ValuesD2899 Practice for Establishing Allowable Stresses forRound Timber PilesD3200 Specification and Test Method for Establishing Rec-ommended Design Stresses for Round Timber Construc-tion 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 development 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 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 lumber 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 considered assawn lumber and therefore must respect the provisions ofstress-grading described in Practice D245. The manner inwhich Practice D245 is applied to wall-logs is described in 4.2.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.NF—Narrow FaceWF—Wide FaceFIG. 1 Typical Wall-Log Sections Showing Wide and Narrow Faces as Determined by Inscribed RectangleD3957 − 09 (2015)24.2 Use of Rectangular Section Inscribed in Actual Sec-tions:4.2.1 A wall-log is to be graded as the largest piece ofrectangular lumber that can be embedded in the wall-logwithout protrusion from any wall-log surface except that eachcorner may protrude up to1⁄2 in. (12.7 mm) in either or bothdirections (Fig. 2). All provisions of Practice D245 that 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 D245. Boundaries between portions ofthe wall-log surface 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 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 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 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 unpenetratedby 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 cut1⁄2 in. (12.7 mm) wide could effectively increase a 2-in.(50.8 mm) knot to 21⁄2 in. (63.5 mm), as when the knot and thecut are tangent to each other. Therefore, a cut1⁄2 in. wide willreduce the maximum allowable knot by1⁄2 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 sawcut. For example, a 6 by 12 in. (152.4 by 304.8mm) with maximum knot displacement of 50 %, and twogrooves3⁄4 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 reducedto50−2(2)=46%.5. Stress-Grading of Sawn Round Timber Beams5.1 General:5.1.1 This section describes how the formulas of PracticeD2899 are applied to round timbers that are shaved or sawnalong one side (Note 1). Since these members are normallyloaded on their flat surface, they are stressed primarily inbending and are herein referred to as sawn round timber beams.NOTE 1—Unsawn round timbers used in the superstructure of buildingsare covered by Specification and Test Methods D3200.5.1.2 Specification D25 and Practice D2899 set forth onestructural grade. These practices supplement Specification D25and Practice D2899 so that a series of grades can be con-structed. This is accomplished by means of the strength ratiosdefined in 5.5.5.2 Allowable Sawing:5.2.1 The flat side of a sawn round timber beam shall notpenetrate 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 inorder 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 of(a) Wall-Log Without Saw Kerf: Maximumallowable narrow face knot, D, deter-mined for A × B inscribed rectangle.(b) Same Wall-Log, With Saw Kerf: Maxi-mum allowable narrow faceknot = D−d(top) and D (bottom).(c) Alternative Method: Maximum allow-able narrow face knot, D*, determinedfor A × B* inscribed rectangle.FIG. 2 Determination of Inscribed RectangleD3957 − 09 (2015)3Practice D2899 to sawn round timber beams, the form factorincluded in that formula must be set equal to 1.0.5.3 Knot Measurement—Knots 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 Measurement—Slope of grain in sawnround timber beams is 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 alternative strength ratios into the design stress formulaof Practice D2899 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 2—The formula in Practice D2899 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 D2899 and the sizefactor is based on a (2/d)1/9adjustment. The grade reduction is based onthe grade description of the particular product using a strength ratiosystem similar to Practice D245 for sawn lumber. With the wide range insizes used in the log home industry, Practice D2899, Section 10.3, may beapplicable when the diameter of the sawn 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 Ratios—Strength 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 to 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 beam’s section modulus.(1) Given (1) the section modulus, S, of a beam sawn tothe limit of 5.2.1, and (2) the section modulus, Sʹ, obtainedwhen S is reduced to account for a knot, the bending strengthratio associated with the knot is that number that whenmultiplied by S gives Sʹ.(2) By substituting the above strength ratios into thePractice D2899 bending stress formula as explained in Appen-dix X1 and by the application of the other 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 Ratios—The exact relation-ship between slope of grain and bending strength has not beendetermined for unsawn-round timbers. These strength 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 3—Round timbers that are sawn within the limitations stated in5.2 will have 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 PracticeD2899 formula for design bending stress includes factors toaccount for load duration, tip weak