ASTM D3957-90(2002)
(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
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 , 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 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.
General Information
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Standards Content (Sample)
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Designation: D 3957 – 90 (Reapproved 2002)
Standard Practices for
Establishing Stress Grades for Structural Members Used in
Log Buildings
This standard is issued under the fixed designation D 3957; 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 (e) 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 D 245 and D 2899, 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 meant to supplement the ASTM standards listed in Section 2,
which cover stress-grading of sawn lumber and round timbers.
1.1 These practices cover the visual stress-grading prin-
Pieces covered by these practices may also be used in building
ciples applicable to structural wood members of nonrectangu-
types other than log buildings.
lar shape, as typically used in log buildings. These practices are
1.2 The grading provisions used as illustrations herein are
not intended to establish grades for purchase, but rather to
These practices are under the jurisdiction of ASTM Committee D07 on Wood
show how stress-grading principles are applied to members
and are the direct responsibility of Subcommittee D07.02 on Lumber and Engi-
used in log buildings. Detailed grading rules for commercial
neered Wood Products.
Current edition approved April 27, 1990. Published May 1990. Originally
published as D 3957 – 80. Last previous edition D 3957 – 80 (1984).
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NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 3957 – 90 (2002)
stress grades which serve as purchase specifications are estab- 3.1.1 This section is intended to apply to wood members,
lished and published by agencies that formulate and maintain referred to as wall-logs, which are normally stacked horizon-
such rules and operate inspection facilities covering the various tally or laid-up vertically to form a load-bearing, solid-wood
species. wall, in any building. These structural members can vary
1.3 This standard does not purport to address all of the greatly in dimension and section profile, and therefore previ-
safety concerns, if any, associated with its use. It is the ously developed standards for solid sawn lumber are not
responsibility of the user of this standard to establish appro- readily applied to them (Fig. 1).
priate safety and health practices and determine the applica- 3.1.2 Wall-logs, as referred to in these practices, can also be
bility of regulatory limitations prior to use. used as beams, joists, and so forth, and do not have to be used
as wall components.
2. Referenced Documents
3.1.3 Unless they qualify as round-timber beams under
Section 4 of these practices, wall-logs must be considered as
2.1 ASTM Standards:
sawn lumber and therefore must respect the provisions of
D 9 Terminology Relating to Wood
stress-grading described in Practice D 245. The manner in
D 25 Specification for Round Timber Piles
which Practice D 245 is applied to wall-logs is described in
D 245 Practice for Establishing Structural Grades and Re-
3.2.
lated Allowable Properties for Visually Graded Lumber
3.2 Use of Rectangular Section Inscribed in Actual Sec-
D 2555 Test Methods for Establishing Clear Wood Strength
tions:
Values
3.2.1 A wall-log is to be graded as the largest piece of
D 2899 Practice for Establishing Allowable Stresses for
rectangular lumber that can be embedded in the wall-log
Round Timber Piles
without protrusion from any wall-log surface except that each
D 3200 Specification and Test Methods for Establishing
corner may protrude up to ⁄2 in. in either or both directions
Recommended Design Stresses for Round Timber Con-
(Fig. 2). All provisions of Practice D 245 that would apply to
struction Poles
a piece with the same cross section as the inscribed rectangle
shall apply to the wall-log.
3. Stress-Grading of Wall-Logs
3.2.2 Maximum knot sizes shall be determined by the wide
3.1 General:
and narrow face dimensions of the inscribed rectangle, using
knot tables in Practice D 245. Boundaries between portions of
the wall-log surface considered “wide-face” and portions
considered “narrow-face” shall be found by extending the
Annual Book of ASTM Standards, Vol 04.10.
NF—Narrow Face
WF—Wide Face
FIG. 1 Typical Wall-Log Sections Showing Wide and Narrow Faces as Determined by Inscribed Rectangle
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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D 3957 – 90 (2002)
(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 3 B inscribed rectangle. knot = D−d (top) and D (bottom). for A 3 B* inscribed rectangle.
FIG. 2 Determination of Inscribed Rectangle
diagonals of the inscribed rectangle to the wall-log surface between the knot size at the surface of the wall-log, and the
(Fig. 3). Just as the wide and narrow faces of the inscribed knot size at the inscribed rectangle is, for practical purposes,
rectangle belong to a quadrant between diagonals, so shall the disregarded.
wide and narrow faces of the wall-log surface belong to the 3.2.4 If the wall-log design has saw-cuts that penetrate
same quadrants. In general, then, the surface boundaries deeply into the piece, then any “inscribed rectangle” that
between wide and narrow faces may not coincide with actual remains unpenetrated by sawing may be too small to use for
edges on the wall-log. determining knot limitations. To accommodate wall-logs of
3.2.3 Knot size limitations, as determined by the dimensions this type, cuts into the inscribed rectangle may be treated as
of the inscribed rectangle, shall apply to knot measurements follows.
taken at the surface of the wall-log. The slight difference 3.2.4.1 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
1 1
saw cut ⁄2 in. wide could effectively increase a 2-in. knot to 2 ⁄2
in., as when the knot and the cut are tangent to each other.
Therefore, a cut ⁄2 in. wide will reduce the maximum allow-
able knot by ⁄2 in. (Fig. 3).
3.2.4.2 As an alternate to 3.2.4.1, reduce the maximum
allowable knot displacement D to D −2d where d is the
displacement of the sawcut(s) when the knot does not encom-
pass the sawcut. For example,a6by12in. with maximum
knot displacement of 50 %, and two grooves ⁄4 by 1 in. 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 %.
4. Stress-Grading of Sawn Round Timber Beams
4.1 General:
4.1.1 This section describes how the formulas of Practice
D 2899 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 D 3200.
4.1.2 Specification D 25 and Practice D 2899 set forth one
FIG. 3 Strength Ratio for Sawn Round Timber Beam structural grade. These practices supplement Specification
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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D 3957 – 90 (2002)
D 25 and Practice D 2899 so that a series of grades can be 4.5.1.2 Slope of Grain Strength Ratios—The exact relation-
constructed. This is accomplished by means of the strength ship between slope of grain and bending strength has not been
ratios defined in 4.5. determined for unsawn round timbers. These strength ratios,
listed below, are thought to be conservative estimates of the
4.2 Allowable Sawing:
effect of slope of grain on sawn round timber beams (Note 3):
4.2.1 The flat side of a sawn round timber beam shall not
Slope of Grain Bending Strength Ratio, %
penetrate more than 0.3 R into the piece, where R is the radius
1in4 27
of the beam (Fig. 2). This limits the reduction of the cross-
1in6 40
sectional area, by sawing or shaving, to less than 10 %. 1in8 53
1in10 61
4.2.2 A “form factor” equal to 1.18 is the factor by which
1in12 69
design-bending stresses of square-sawn pieces are multiplied in
1in14 74
1in15 76
order to derive design-bending stresses for beams with circular
cross-sections. Since sawn round timber beams do not have a
NOTE 3—Round timbers that are sawn within the limitations stated in
circular cross section, their form factor is set equal to 1.0 rather 4.2 will have hybrid strength characteristics that are between those of
sawn lumber and round timber. It can be assumed that the effect of a given
than 1.18. In order to apply the bending stress formula of
slope of grain on the bending strength of sawn round timber beams will
Practice D 2899 to sawn round timber beams, the form factor
not be as great as its effect on the bending strength of sawn lumber. This
included in that formula must be set equal to 1.0.
assumption, which is based on engineering judgment, allows for the
4.3 Knot Measurement—Knots on the sawn face of a sawn
application of the above strength ratios to sawn round timber beams.
round timber beam are measured by their smallest diameter.
4.5.2 In addition to factors for form and grade, the Practice
Other knots are measured in accordance with Specification
D 2899 formula for design bending stress includes factors to
D 25.
account for load duration, tip weakness, and variability. These
4.4 Slope of Grain Measurement—Slope of grain in sawn
factors are also applied to sawn round timber beams.
round timber beams is measured by the angle between the
4.5.3 A formal factor of safety of 1.3 shall be included in the
direction of the fibers and the axis of the piece. As for lumber,
formula for design bending stresses used for sawn round timber
this angle is expressed as a slope.
beams.
4.5 Design Bending Stress Values:
4.6 Other Design Stresses:
4.6.1 Tension-Parallel-to-Grain—Design values for
4.5.1 Bending strength ratios are determined by slope of
tension-parallel-to-grain in sawn round timber beams shall be
grain or knot size, whichever is most restrictive. The substitu-
taken to be 55 % of design bending stresses, in accordance with
tion of alternative strength ratios into the design stress formula
the convention for lumber recommended by Practice D 245.
of Practice D 2899 is not meant to result in higher allowable
4.6.2 Compression-Parallel-to-Grain—For sawn round
bending stresses than can be obtained when the bending
timber beams, the strength ratio for compression-parallel-to-
strength ratio equals 0.76, that is, bending strength ratios
grain shall be taken to be equal to the bending strength ratio
higher than 0.76 are not recommended for sawn round timber
described in 4.5.1. This strength ratio is to be used in the
beams.
Practice D 2899 formula for compression-parallel-to-grain, as
NOTE 2—The formula in Practice D 2899 for finding design bending
explained in Appendix X1.
stress values assumes that clear wood bending strength values should be
4.6.3 Shear—The Practice D 2899 shear formula can be
reduced by a factor of 0.9 to account for “form and grade.” Since the form
used for sawn round timber beams.
factor for round timbers is 1.18, the strength ratio for grade restrictions in
4.6.4 Compression-Perpendicular-to-Grain—The Practice
Specification D 25 is 0.76. To provide for other grade descriptions in sawn
D 2899 compression-perpendicular-to-grain formula can be
round timber beams, a strength ratio system
...
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