1、Designation: D 3957 06Standard 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 (e) 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 metho
3、ds 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
4、for 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 cr
5、oss-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 reliabili
7、ty 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 m
9、ethodologies 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
10、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 s
11、tress-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 ma
12、y 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
13、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.1These practices are under the jurisdiction of ASTM Committee D07 on Woodand are the direct responsibility of S
14、ubcommittee D07.02 on Lumber and Engi-neered Wood Products.Current edition approved May 15, 2006. Published May 2006. Originallyapproved in 1980. Last previous edition approved in 2003 as D 3957 03.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, U
15、nited States.1.3 The values stated in inch-pound units are to be regardedas 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 an
16、d determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D25 Specification for Round Timber PilesD 245 Practice for Establishing Structural Grades and Re-lated Allowable Properties for Visually Graded LumberD 2555 Practice for Establishing Clea
17、r Wood Strength Val-uesD 2899 Practice for Establishing Allowable Stresses forRound Timber PilesD 3200 Specification and Test Method for EstablishingRecommended Design Stresses for Round Timber Con-struction Poles3. Stress-Grading of Wall-Logs3.1 General:3.1.1 This section is intended to apply to wo
18、od 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
19、are notreadily applied to them (Fig. 1).3.1.2 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.3.1.3 Unless they qualify as round-timber beams underSection 4 of these practices, wall-logs must be considered ass
20、awn 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 in3.2.3.2 Use of Rectangular Section Inscribed in Actual Sec-tions:3.2.1 A wall-log is to be graded as the largest piece ofrectang
21、ular lumber that can be embedded in the wall-logwithout protrusion from any wall-log surface except that eachcorner may protrude up to12 in. in either or both directions(Fig. 2). All provisions of Practice D 245 that would apply toa piece with the same cross-section as the inscribed rectangleshall a
22、pply to the wall-log.3.2.2 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 surface considered wide-face and portions consid-ered narrow-face shall be found by exten
23、ding the diagonals ofthe inscribed rectangle to the wall-log surface (Fig. 2). Just asthe wide and narrow faces of the inscribed rectangle belong to2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandar
24、ds 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 RectangleD3957062a quadrant between diagonals, so shall the wide and narrowfaces of the wall-log su
25、rface belong to the same quadrants. Ingeneral, then, the surface boundaries between wide and narrowfaces may not coincide with actual edges on the wall-log.3.2.3 Knot size limitations, as determined by the dimensionsof the inscribed rectangle, shall apply to knot measurementstaken at the surface of
26、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.3.2.4 If the wall-log design has saw-cuts that penetratedeeply into the piece, then any inscribed rectangle that remainsunpenetr
27、ated 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.3.2.4.1 If a face of an inscribed rectangle has a maximumallowable knot size of D inches when the face is unpenetratedby any cut
28、s, 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. wide could effectively increase a 2-in. knot to 212in., as when the knot and the cut are tangent to each other.Therefo
29、re, a cut12 in. wide will reduce the maximum allow-able knot by12 in. (Fig. 3).3.2.4.2 As an alternate to 3.2.4.1, 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,a6by12in.with maximumknot d
30、isplacement of 50 %, and two grooves34 by 1 in. eachon one narrow face, a groove displacement of 2 %, theallowable knot displacement for knots that do not encompassthe grooves would by reduced to 50 2(2) = 46 %.4. Stress-Grading of Sawn Round Timber Beams4.1 General:4.1.1 This section describes how
31、the formulas of PracticeD 2899 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 1Unsawn round timbers used in the
32、 superstructure of buildingsare covered by Specification and Test Methods D 3200.4.1.2 Specification D25and Practice 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
33、 strengthratios defined in 4.5.(a) Wall-Log Without Saw Kerf: Maximumallowable narrow face knot, D, deter-mined 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*,
34、 determinedfor A 3 B* inscribed rectangle.FIG. 2 Determination of Inscribed RectangleFIG. 3 Strength Ratio for Sawn Round Timber BeamD39570634.2 Allowable Sawing:4.2.1 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.
35、3). This limits the reduction of the cross-sectional area, by sawing or shaving, to less than 10 %.4.2.2 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. Sin
36、ce 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 factorincluded in that formula must be set equal to 1.0.4.3 Knot MeasurementKnots
37、 on the sawn face of a sawnround timber beam are measured by their smallest diameter.Other knots are measured in accordance with SpecificationD25.4.4 Slope of Grain MeasurementSlope of grain in sawnround timber beams is measured by the angle between thedirection of the fibers and the axis of the pie
38、ce. As for lumber,this angle is expressed as a slope.4.5 Design Bending Stress Values:4.5.1 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 D 2899 is not me
39、ant 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 2The formula in Practice D 2899 for finding design bendingstress values assumes that
40、 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)1/9adjustment. The grade reduction is based onthe grade description of the particula
41、r 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 round timber beam exceeds 13.5in. at a point 10 feet from its tip.4.5.1.1 Knot St
42、rength 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 to theoutermost boundaries of the knot (Fig. 3). It is further assumedthat the sector of area los
43、t 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 4.2.1, and (2) the section modulus, S8, obtainedwhen S is reduced to account for a knot, the bending strengthratio associated with the kno
44、t 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 other adjustments to thisformula described in 4.2.2 and 4.5.3, design bending stressvalues for spe
45、cific knot sizes can be determined.4.5.1.2 Slope of Grain Strength RatiosThe exact relation-ship between slope of grain and bending strength has not beendetermined for unsawn-round timbers. These strength ratios,listed below, are thought to be conservative estimates of theeffect of slope of grain on
46、 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 in4.2 will have hybrid strength characteristics that are between those ofsawn lumber and round timber. It can
47、 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 rou
48、nd timber beams.4.5.2 In addition to factors for form and grade, the PracticeD 2899 formula for design bending stress includes factors toaccount for load duration, tip weakness, and variability. Thesefactors are also applied to sawn round timber beams.4.5.3 Aformal factor of safety of 1.4 shall be i
49、ncluded in theformula for design bending stresses used for sawn round timberbeams.4.5.4 Sawn round timber beams may be selected as dense bygrain characteristics for Douglas fir and southern pine. To beclassified dense, the wood shall average on the tip not less thansix (6) annual rings per inch (25 mm) and one third or moresummerwood on a representative radial line. Pieces thataverage between four (4) and six (6) annual rings per inch (25mm) shall be accepted as dense if they average one half ormore summerwood. The contrast in color between springwoodand summerw
