ASTM D3957-09(2015)
(Practice)Standard Practices for Establishing Stress Grades for Structural Members Used in Log Buildings
Standard Practices for Establishing Stress Grades for Structural Members Used in Log Buildings
ABSTRACT
These practices cover the visual stress-grading principles applicable to structural wood members of nonrectangular shape, as typically used in log buildings. The grading provisions presented herein are not intended to establish grades for purchase, but rather to show how stress-grading principles are applied to members used in log buildings.
SCOPE
1.1 These practices cover the visual stress-grading principles applicable to structural wood members of nonrectangular shape, as typically used in log buildings. These practices are meant 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 building types other than log buildings.
1.2 The grading provisions used as illustrations herein are not intended to establish grades for purchase, but rather to show how stress-grading principles are applied to members used in log buildings. Detailed grading rules for commercial stress grades which serve as purchase specifications are established and published by agencies that formulate and maintain such rules and operate inspection facilities covering the various species.
1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D3957 − 09 (Reapproved 2015)
Standard Practices for
Establishing Stress Grades for Structural Members Used in
Log Buildings
This standard is issued under the fixed designation D3957; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
These practices are based on the assumption that structural members in log buildings can be
stress-gradedbymethodsthatderivefromacceptedstandardsforconventionalsolidsawnlumberand
round timbers. It is assumed that the material to be graded bears enough similarity to either sawn
lumber or round timber, both in dimensional properties and in use, to warrant application of
stress-grading standards written for sawn-lumber or round-timber, or both. These practices, such as
Practices D245 and D2899, cannot be applied directly because the structural members used in log
buildingsaregenerallyneitherperfectlyrectangularnorperfectlyroundinsection.Thesepracticesuse
certain conventions regarding cross-sectional dimensions that make it possible to extend established
stress-grading methodologies to cover the members used in log buildings.
Where log member characteristics deviate from sawn lumber or round timber standards, there may
be uncertainty as to the exact effect of the deviation on strength properties. To compensate for this
uncertainty, some design stress values are herein derived with practices that are, by engineering
judgment, conservative. The philosophy guiding this approach is that while the absence of
experimental data may make a measure of conservatism unavoidable, the reliability of design stress
valuesmustnotbeachievedthroughwoodqualityorsizerequirementsthatareanunnecessaryburden
on the wood resource.
In general, the sawing, cutting, and shaving required to bring a piece to its final shape must be
completed before it can be visually graded using the principles in these practices. Small cuts may be
allowedaftergradingifitcanbeshownthateither(1)thecutsdonotaffectthegrade,or(2)thegrade
takes the additional cuts into consideration.
Both sawn lumber standards and round timber standards are herein referenced, because these two
stress-grading methodologies can be assumed to apply to two different types of structural members
used in log buildings: wall-logs and round timber beams. Since wall-logs must be provided with a
meansofjoiningtogether(forexample,tongue-and-groovejoints),theyresemblesawnlumberandare
treated as such in the standard. Rafters, purlins, and beams, on the other hand, are sometimes left as
roundlogs,exceptforasmallamountofsawingtoprovideaflatnailingsurface.Thesepracticesthus
deal with stress-grading of wall-logs and round-timber beams separately.
1. Scope Pieces covered by these practices may also be used in building
types other than log buildings.
1.1 These practices cover the visual stress-grading prin-
ciples applicable to structural wood members of nonrectangu-
1.2 The grading provisions used as illustrations herein are
larshape,astypicallyusedinlogbuildings.Thesepracticesare
not intended to establish grades for purchase, but rather to
meant to supplement the ASTM standards listed in Section 2,
show how stress-grading principles are applied to members
which cover stress-grading of sawn lumber and round timbers.
used in log buildings. Detailed grading rules for commercial
stress grades which serve as purchase specifications are estab-
lished and published by agencies that formulate and maintain
These practices are under the jurisdiction ofASTM Committee D07 on Wood
suchrulesandoperateinspectionfacilitiescoveringthevarious
and are the direct responsibility of Subcommittee D07.02 on Lumber and Engi-
species.
neered Wood Products.
Current edition approved Nov. 1, 2015. Published December 2015. Originally
1.3 Thevaluesstatedininch-poundunitsaretoberegarded
approved in 1980. Last previous edition approved in 2009 as D3957–09. DOI:
10.1520/D3957-09R15. as standard. The values given in parentheses are mathematical
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3957 − 09 (2015)
conversions to SI units that are provided for information only methodologies for wall-logs and round timber beams, as
and are not considered standard. typically used in log buildings.The clear wood strength values
are used as the basis for deriving the design stress values in
1.4 This standard does not purport to address all of the
these applications.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Stress-Grading of Wall-Logs
priate safety and health practices and determine the applica-
4.1 General:
bility of regulatory limitations prior to use.
4.1.1 This section is intended to apply to wood members,
2. Referenced Documents
referred to as wall-logs, which are normally stacked horizon-
tally or laid-up vertically to form a load-bearing, solid-wood
2.1 ASTM Standards:
wall, in any building. These structural members can vary
D25Specification for Round Timber Piles
greatly in dimension and section profile, and therefore previ-
D245Practice for Establishing Structural Grades and Re-
ously developed standards for solid sawn lumber are not
lated Allowable Properties for Visually Graded Lumber
readily applied to them (Fig. 1).
D2555PracticeforEstablishingClearWoodStrengthValues
4.1.2 Wall-logs,asreferredtointhesepractices,canalsobe
D2899Practice for Establishing Allowable Stresses for
used as beams, joists, and so forth, and do not have to be used
Round Timber Piles
as wall components.
D3200Specification and Test Method for Establishing Rec-
4.1.3 Unless they qualify as round-timber beams under
ommended Design Stresses for Round Timber Construc-
Section 5 of these practices, wall-logs must be considered as
tion Poles
sawn lumber and therefore must respect the provisions of
stress-grading described in Practice D245. The manner in
3. Significance and Use
whichPracticeD245isappliedtowall-logsisdescribedin4.2.
3.1 It is useful to grade logs to improve the consistency in
performance. Using the visual stress-grading principles appli-
cabletorectangularandroundshapestructuralwoodmembers,
these practices illustrate the development of stress grading
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
NF—Narrow Face
WF—Wide Face
FIG. 1 Typical Wall-Log Sections Showing Wide and Narrow Faces as Determined by Inscribed Rectangle
D3957 − 09 (2015)
4.2 Use of Rectangular Section Inscribed in Actual Sec- cut are tangent to each other. Therefore, a cut ⁄2 in. wide will
tions: reduce the maximum allowable knot by ⁄2 in. (Fig. 3).
4.2.1 A wall-log is to be graded as the largest piece of 4.2.4.2 As an alternate to 4.2.4.1, reduce the maximum
rectangular lumber that can be embedded in the wall-log allowable knot displacement D to D−2d where d is the
without protrusion from any wall-log surface except that each displacement of the sawcut(s) when the knot does not encom-
corner may protrude up to ⁄2 in. (12.7 mm) in either or both pass the sawcut. For example, a 6 by 12 in. (152.4 by 304.8
directions (Fig. 2).All provisions of Practice D245 that would mm) with maximum knot displacement of 50%, and two
apply to a piece with the same cross-section as the inscribed grooves ⁄4by1in.(19.1by25.4mm)eachononenarrowface,
rectangle shall apply to the wall-log. agroovedisplacementof2%,theallowableknotdisplacement
4.2.2 Maximum knot sizes shall be determined by the wide for knots that do not encompass the grooves would by reduced
and narrow face dimensions of the inscribed rectangle, using to50−2(2)=46%.
knot tables in Practice D245. Boundaries between portions of
5. Stress-Grading of Sawn Round Timber Beams
thewall-logsurfaceconsideredwide-faceandportionsconsid-
ered narrow-face shall be found by extending the diagonals of
5.1 General:
the inscribed rectangle to the wall-log surface (Fig. 2). Just as
5.1.1 This section describes how the formulas of Practice
the wide and narrow faces of the inscribed rectangle belong to
D2899 are applied to round timbers that are shaved or sawn
a quadrant between diagonals, so shall the wide and narrow
along one side (Note 1). Since these members are normally
faces of the wall-log surface belong to the same quadrants. In
loaded on their flat surface, they are stressed primarily in
general,then,thesurfaceboundariesbetweenwideandnarrow
bendingandarehereinreferredtoassawnroundtimberbeams.
faces may not coincide with actual edges on the wall-log.
NOTE1—Unsawnroundtimbersusedinthesuperstructureofbuildings
4.2.3 Knotsizelimitations,asdeterminedbythedimensions
are covered by Specification and Test Methods D3200.
of the inscribed rectangle, shall apply to knot measurements
5.1.2 Specification D25 and Practice D2899 set forth one
taken at the surface of the wall-log. The slight difference
structuralgrade.ThesepracticessupplementSpecificationD25
between the knot size at the surface of the wall-log, and the
and Practice D2899 so that a series of grades can be con-
knot size at the inscribed rectangle is, for practical purposes,
structed. This is accomplished by means of the strength ratios
disregarded.
defined in 5.5.
4.2.4 If the wall-log design has saw-cuts that penetrate
deeplyintothepiece,thenanyinscribedrectanglethatremains 5.2 Allowable Sawing:
unpenetrated by sawing may be too small to use for determin- 5.2.1 The flat side of a sawn round timber beam shall not
ing knot limitations. To accommodate wall-logs of this type, penetrate more than 0.3 R into the piece, where R is the radius
cuts into the inscribed rectangle may be treated as follows. of the beam (Fig. 3). This limits the reduction of the cross-
4.2.4.1 If a face of an inscribed rectangle has a maximum sectional area, by sawing or shaving, to less than 10%.
allowable knot size of D inches when the face is unpenetrated 5.2.2 A form factor equal to 1.18 is the factor by which
by any cuts, then the maximum allowable knot size for knots design-bendingstressesofsquare-sawnpiecesaremultipliedin
that do not encompass the sawcut is reduced to D− d inches ordertoderivedesign-bendingstressesforbeamswithcircular
when the face is penetrated by a cut d inches wide. That is, a cross-sections. Since sawn round timber beams do not have a
sawcut ⁄2in.(12.7mm)widecouldeffectivelyincreasea2-in. circularcrosssection,theirformfactorissetequalto1.0rather
(50.8mm)knotto2 ⁄2in.(63.5mm),aswhentheknotandthe than 1.18. In order to apply the bending stress formula of
(a) Wall-Log Without Saw Kerf: Maximum (b) Same Wall-Log, With Saw Kerf: Maxi- (c) Alternative Method: Maximum allow-
allowable narrow face knot, D, deter- mum allowable narrow face able narrow face knot, D*, determined
mined for A × B inscribed rectangle. knot =D−d (top) and D (bottom). for A × B* inscribed rectangle.
FIG. 2 Determination of Inscribed Rectangle
D3957 − 09 (2015)
5.5.1.1 Knot Strength Ratios—Strength ratios for sawn
round timber beams shall be determined assuming that knots
effectively reduce the cross-sectional area by a pie-shaped
sector that radiates from the center of the beam to the
outermostboundariesoftheknot(Fig.3).Itisfurtherassumed
thatthesectorofarealosttoaknotliesoppositethesawnface,
since this will most reduce the beam’s section modulus.
(1) Given (1) the section modulus, S, of a beam sawn to
the limit of 5.2.1, and (2) the section modulus, Sʹ, obtained
when S is reduced to account for a knot, the bending strength
ratio associated with the knot is that number that when
multiplied by S gives Sʹ.
(2) By substituting the above strength ratios into the
Practice D2899 bending stress formula as explained in Appen-
dix X1 and by the application of the other adjustments to this
formula described in 5.2.2 and 5.5.3, design bending stress
values 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 been
determined for unsawn-round timbers. These strength ratios,
listed below, are thought to be conservative estimates of the
effect of slope of grain on sawn-round timber beams (Note 3):
Slope of Grain Bending Strength Ratio, %
1in4 27
1in6 40
1in8 53
1in10 61
1in12 69
1in14 74
FIG. 3 Strength Ratio for Sawn Round Timber Beam
1in15 76
NOTE 3—Round timbers that are sawn within the limitations stated in
Practice D2899 to sawn round timber beams, the form factor
5.2 will have hybrid strength characteristics that are between those of
included in that formula must be set equal to 1.0. sawnlumberandroundtimber.Itcanbeassumedthattheeffectofagiven
slope of grain on the bending strength of sawn round timber beams will
5.3 Knot Measurement—Knots on the sawn face of a sawn
not be as great as its effect on the bending strength of sawn lumber. This
round timber beam are measured by their smallest diameter.
assumption, which is based on engineering judgment, allows for the
application of the above strength ratios to sawn round timber beams.
Other knots are measured in accordance with Specification
D25.
5.5.2 In addition to factors for form and grade, the Practice
5.4 Slope of Grain Measurement—Slope of grain in sawn D2899 formula for design bending stress includes factors to
round timber beams is measured by the angle between the
account for load duration, tip weakness, and variability. These
direction of the fibers and the axis of the piece.As for lumber, factors are also applied to sawn round timber beams.
this angle is expressed as a slope.
5.5.3 Aformalfactorofsafetyof1.4shallbeincludedinthe
formulafordesignbendingstressesusedforsawnroundtimber
5.5 Design Bending Stress Values:
beams.
5.5.1 Bending strength ratios are determined by slope of
grain or knot size, whichever is most restrictive. The substitu-
5.5.4 Sawnroundtimberbeamsmaybeselectedasdenseby
tion of alternative strength ratios into the design stress formula
grain characteristics for Douglas fir and southern pine. To be
of Practice D2899 is not meant to result in higher allowable
classifieddense,thewoodshallaverageonthetipnotlessthan
bending stresses than can be obtained when the bending
six annual rings per inch (25.4 mm) and one third or more
strength ratio equals 0.76, that is, bending strength ratios
summe
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D3957 − 09 D3957 − 09 (Reapproved 2015)
Standard Practices for
Establishing Stress Grades for Structural Members Used in
Log Buildings
This standard is issued under the fixed designation D3957; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
These practices are based on the assumption that structural members in log buildings can be
stress-graded by methods that derive from accepted standards for conventional solid sawn lumber and
round timbers. It is assumed that the material to be graded bears enough similarity to either sawn
lumber or round timber, both in dimensional properties and in use, to warrant application of
stress-grading standards written for sawn-lumber or round-timber, or both. These practices, such as
Practices D245 and D2899, cannot be applied directly because the structural members used in log
buildings are generally neither perfectly rectangular nor perfectly round in section. These practices use
certain conventions regarding cross-sectional dimensions that make it possible to extend established
stress-grading methodologies to cover the members used in log buildings.
Where log member characteristics deviate from sawn lumber or round timber standards, there may
be uncertainty as to the exact effect of the deviation on strength properties. To compensate for this
uncertainty, some design stress values are herein derived with practices that are, by engineering
judgment, conservative. The philosophy guiding this approach is that while the absence of
experimental data may make a measure of conservatism unavoidable, the reliability of design stress
values must not be achieved through wood quality or size requirements that are an unnecessary burden
on the wood resource.
In general, the sawing, cutting, and shaving required to bring a piece to its final shape must be
completed before it can be visually graded using the principles in these practices. Small cuts may be
allowed after grading if it can be shown that either (1) the cuts do not affect the grade, or (2) the grade
takes the additional cuts into consideration.
Both sawn lumber standards and round timber standards are herein referenced, because these two
stress-grading methodologies can be assumed to apply to two different types of structural members
used in log buildings: wall-logs and round timber beams. Since wall-logs must be provided with a
means of joining together (for example, tongue-and-groove joints), they resemble sawn lumber and are
treated as such in the standard. Rafters, purlins, and beams, on the other hand, are sometimes left as
round logs, except for a small amount of sawing to provide a flat nailing surface. These practices thus
deal with stress-grading of wall-logs and round-timber beams separately.
1. Scope
1.1 These practices cover the visual stress-grading principles applicable to structural wood members of nonrectangular shape,
as typically used in log buildings. These practices are meant 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 building types other than
log buildings.
1.2 The grading provisions used as illustrations herein are not intended to establish grades for purchase, but rather to show how
stress-grading principles are applied to members used in log buildings. Detailed grading rules for commercial stress grades which
These practices are under the jurisdiction of ASTM Committee D07 on Wood and are the direct responsibility of Subcommittee D07.02 on Lumber and Engineered Wood
Products.
Current edition approved May 1, 2009Nov. 1, 2015. Published June 2009December 2015. Originally approved in 1980. Last previous edition approved in 20062009 as
D3957 – 06.D3957 – 09. DOI: 10.1520/D3957-09.10.1520/D3957-09R15.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3957 − 09 (2015)
serve as purchase specifications are established and published by agencies that formulate and maintain such rules and operate
inspection facilities covering the various species.
1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D25 Specification for Round Timber Piles
D245 Practice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber
D2555 Practice for Establishing Clear Wood Strength Values
D2899 Practice for Establishing Allowable Stresses for Round Timber Piles
D3200 Specification and Test Method for Establishing Recommended Design Stresses for Round Timber Construction Poles
3. Significance and Use
3.1 It is useful to grade logs to improve the consistency in performance. Using the visual stress-grading principles applicable
to rectangular and round shape structural wood members, these practices illustrate the development of stress grading
methodologies for wall-logs and round timber beams, as typically used in log buildings. The clear wood strength values are used
as the basis for deriving the design stress values in these applications.
4. Stress-Grading of Wall-Logs
4.1 General:
4.1.1 This section is intended to apply to wood members, referred to as wall-logs, which are normally stacked horizontally or
laid-up vertically to form a load-bearing, solid-wood wall, in any building. These structural members can vary greatly in dimension
and section profile, and therefore previously developed standards for solid sawn lumber are not readily applied to them (Fig. 1).
4.1.2 Wall-logs, as referred to in these practices, can also be used as beams, joists, and so forth, and do not have to be used as
wall components.
4.1.3 Unless they qualify as round-timber beams under Section 5 of these practices, wall-logs must be considered as sawn
lumber and therefore must respect the provisions of stress-grading described in Practice D245. The manner in which Practice D245
is applied to wall-logs is described in 4.2.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
D3957 − 09 (2015)
NF—Narrow Face
WF—Wide Face
FIG. 1 Typical Wall-Log Sections Showing Wide and Narrow Faces as Determined by Inscribed Rectangle
4.2 Use of Rectangular Section Inscribed in Actual Sections:
4.2.1 A wall-log is to be graded as the largest piece of rectangular lumber that can be embedded in the wall-log without
protrusion from any wall-log surface except that each corner may protrude up to ⁄2 in. (12.7 mm) in either or both directions (Fig.
2). All provisions of Practice D245 that would apply to a piece with the same cross-section as the inscribed rectangle shall apply
to the wall-log.
4.2.2 Maximum knot sizes shall be determined by the wide and narrow face dimensions of the inscribed rectangle, using knot
tables in Practice D245. Boundaries between portions of the wall-log surface considered wide-face and portions considered
narrow-face shall be found by extending the diagonals of the inscribed rectangle to the wall-log surface (Fig. 2). Just as the wide
(a) Wall-Log Without Saw Kerf: Maximum (b) Same Wall-Log, With Saw Kerf: Maxi- (c) Alternative Method: Maximum allow-
allowable narrow face knot, D, deter- mum allowable narrow face able narrow face knot, D*, determined
mined for A × B inscribed rectangle. knot = D − d (top) and D (bottom). for A × B* inscribed rectangle.
FIG. 2 Determination of Inscribed Rectangle
D3957 − 09 (2015)
and narrow faces of the inscribed rectangle belong to a quadrant between diagonals, so shall the wide and narrow faces of the
wall-log surface belong to the same quadrants. In general, then, the surface boundaries between wide and narrow faces may not
coincide with actual edges on the wall-log.
4.2.3 Knot size limitations, as determined by the dimensions of the inscribed rectangle, shall apply to knot measurements taken
at the surface of the wall-log. The slight difference between the knot size at the surface of the wall-log, and the knot size at the
inscribed rectangle is, for practical purposes, disregarded.
4.2.4 If the wall-log design has saw-cuts that penetrate deeply into the piece, then any inscribed rectangle that remains
unpenetrated by sawing may be too small to use for determining 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 maximum allowable knot size of D inches when the face is unpenetrated by any
cuts, then the maximum allowable knot size for knots that do not encompass the sawcut is reduced to D − d inches when the face
is penetrated by a cut d inches wide. That is, a saw cut ⁄2 in. (12.7 mm) wide could effectively increase a 2-in. (50.8 mm) knot
1 1
to 2 ⁄2 in. (63.5 mm), as when the knot and the cut are tangent to each other. Therefore, a cut ⁄2 in. wide will reduce the maximum
allowable knot by ⁄2 in. (Fig. 3).
4.2.4.2 As an alternate to 4.2.4.1, reduce the maximum allowable knot displacement D to D − 2d where d is the displacement
of the sawcut(s) when the knot does not encompass the sawcut. For example, a 6 by 12 in. (152.4 by 304.8 mm) with maximum
knot displacement of 50 %, and two grooves ⁄4 by 1 in. (19.1 by 25.4 mm) each on one narrow face, a groove displacement of
2 %, the allowable knot displacement for knots that do not encompass the grooves would by reduced to 50 − 2(2) = 46 %.
5. Stress-Grading of Sawn Round Timber Beams
5.1 General:
5.1.1 This section describes how the formulas of Practice D2899 are applied to round timbers that are shaved or sawn along
one side (Note 1). Since these members are normally loaded on their flat surface, they are stressed primarily in bending and are
herein referred to as sawn round timber beams.
NOTE 1—Unsawn round timbers used in the superstructure of buildings are covered by Specification and Test Methods D3200.
5.1.2 Specification D25 and Practice D2899 set forth one structural grade. These practices supplement Specification D25 and
Practice D2899 so that a series of grades can be constructed. This is accomplished by means of the strength ratios defined in 5.5.
5.2 Allowable Sawing:
FIG. 3 Strength Ratio for Sawn Round Timber Beam
D3957 − 09 (2015)
5.2.1 The flat side of a sawn round timber beam shall not penetrate more than 0.3 R into the piece, where R is the radius of 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 which design-bending stresses of square-sawn pieces are multiplied in order
to derive design-bending stresses for beams with circular cross-sections. Since sawn round timber beams do not have a circular
cross section, their form factor is set equal to 1.0 rather than 1.18. In order to apply the bending stress formula of Practice D2899
to sawn round timber beams, the form factor included in that formula must be set equal to 1.0.
5.3 Knot Measurement—Knots on the sawn face of a sawn round timber beam are measured by their smallest diameter. Other
knots are measured in accordance with Specification D25.
5.4 Slope of Grain Measurement—Slope of grain in sawn round timber beams is measured by the angle between the direction
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 of grain or knot size, whichever is most restrictive. The substitution of
alternative strength ratios into the design stress formula of Practice D2899 is not meant to result in higher allowable bending
stresses than can be obtained when the bending strength ratio equals 0.76, that is, bending strength ratios higher than 0.76 are not
recommended for sawn-round timber beams.
NOTE 2—The formula in Practice D2899 for finding design bending stress values assumes that clear wood bending strength values should be reduced
by factors to account for form, size, and grade. The form factor for round timber is found in Section 10.1 of Practice D2899 and the size factor is based
1/9
on a (2/d) adjustment. The grade reduction is based on the grade description of the particular product using a strength ratio system similar to Practice
D245 for sawn lumber. With the wide range in sizes used in the log home industry, Practice D2899, Section 10.3, may be applicable when the diameter
of the sawn round timber beam exceeds 13.5 in. (342.9 mm) at a point 10 ft (3 m) from its tip.
5.5.1.1 Knot Strength Ratios—Strength ratios for sawn round timber beams shall be determined assuming that knots effectively
reduce the cross-sectional area by a pie-shaped sector that radiates from the center of the beam to the outermost boundaries of the
knot (Fig. 3). It is further assumed that 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 to the limit of 5.2.1, and (2) the section modulus, Sʹ, obtained when
S is reduced to account for a knot, the bending strength ratio associated with the knot is that number that when multiplied by S
gives Sʹ.
(2) By substituting the above strength ratios into the Practice D2899 bending stress formula as explained in Appendix X1 and
by the application of the other adjustments to this formula described in 5.2.2 and 5.5.3, design bending stress values for specific
knot sizes can be determine
...
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