ASTM D2555-17a(2024)
(Practice)Standard Practice for Establishing Clear Wood Strength Values
Standard Practice for Establishing Clear Wood Strength Values
ABSTRACT
This practice presents the two standard methods for determining the strength values of clear wood of different species in the unseasoned condition, unadjusted for end use, applicable to the establishment of working stresses for different solid wood products such as lumber, laminated wood, plywood, and round timbers. Method A provides for the use of the results of surveys of wood density involving extensive sampling of forest trees, in combination with the data obtained from standard strength tests. The average strength properties are obtained from wood density survey data through linear regression equations establishing the relation of specific gravity to the several strength properties. Method B, on the other hand, provides for the establishment of tables of strength values based on standard tests of small clear specimens in the unseasoned condition for use when data from density surveys are not available. Separate tables are employed to present the data on woods grown in the United States and on woods grown in Canada. Guidelines for the interpretation of the data in terms of assigned values, information basic to the translation of the clear wood values into working stresses, presently available data with appropriate provisions for their application and use, and methods for estimating some useful mechanical properties by relating them to other properties are presented herein as well.
SCOPE
1.1 This practice covers the determination of strength values for clear wood of different species in the unseasoned condition, unadjusted for end use, applicable to the establishment of design values for different solid wood products such as lumber, laminated wood, plywood, and round timbers. Presented are:
1.1.1 Procedures by which test values obtained on small clear specimens may be combined with density data from extensive forest surveys to make them more representative,
1.1.2 Guidelines for the interpretation of the data in terms of assigned values for combinations of species or regional divisions within a species to meet special marketing needs, and
1.1.3 Information basic to the translation of the clear wood values into design values for different solid wood products for different end uses.
1.1.4 For species where density survey data are not as yet available for the re-evaluation of average strength properties, the presently available data from tests made under the sampling methods and procedures of Test Methods D143 or Practice E105 are provided with appropriate provision for their application and use. Because of the comprehensive manner in which the density survey is undertaken, it follows that the re-evaluated strength data are intended to be representative of the forest stand, or rather large forest subdivisions.
1.1.5 Some useful mechanical properties (tensile strengths parallel and perpendicular to grain, modulus of rigidity for a longitudinal-transverse plane, and transverse modulus of elasticity) have not been extensively evaluated. Methods are described for estimating these properties by their relation to other properties.
1.2 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.
1.3 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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Designation: D2555 − 17a (Reapproved 2024)
Standard Practice for
Establishing Clear Wood Strength Values
This standard is issued under the fixed designation D2555; 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
The development of safe and efficient design values for lumber, laminated timber, plywood, round
timbers, and other solid wood products, each with its own special requirements has, as a common
starting point, the need for an authoritative compilation of clear wood strength values for the
commercially important species. Also required are procedures for establishing, from these data, values
applicable to groups of species or to regional groupings within a species where necessitated by
marketing conditions. This practice has been developed to meet these needs and to provide, in
addition, information on factors for consideration in the adjustment of the clear wood strength values
to design values for engineering. Since factors such as species preference, species groupings,
marketing practices, design techniques, and safety factors vary with each type of product and end use,
it is contemplated that this practice will be supplemented where necessary by other appropriate
standards relating to specific design values for each such product. Practice D245 is an example of such
a standard applicable to the interpretation of the clear wood strength values in terms of allowable
properties for visually graded lumber.
A primary feature of this practice is the establishment of tables presenting the most reliable basic
information developed on the strength of clear wood and its variability through many years of testing
and experience. The testing techniques employed are those presented in Test Methods D143. Among
the recognized limitations of such strength data are those resulting from the problems of sampling
material from forests extending over large regions, and the uneconomical feasibility of completely
testing an intensive sample. A practical approach to the improvement of strength data is through the
application of the results of density surveys in which the specific gravity of the entire forest stand for
each species is determined on a sound statistical basis. Through regression equations derived from
presently available strength data, revised strength values are established from the specific gravity-
strength relationship for clear wood. This procedure greatly extends current capabilities to develop
new estimates of strength and to improve or verify estimates made in the past.
1. Scope 1.1.2 Guidelines for the interpretation of the data in terms of
assigned values for combinations of species or regional divi-
1.1 This practice covers the determination of strength values
sions within a species to meet special marketing needs, and
for clear wood of different species in the unseasoned condition,
1.1.3 Information basic to the translation of the clear wood
unadjusted for end use, applicable to the establishment of
values into design values for different solid wood products for
design values for different solid wood products such as lumber,
different end uses.
laminated wood, plywood, and round timbers. Presented are:
1.1.4 For species where density survey data are not as yet
1.1.1 Procedures by which test values obtained on small
available for the re-evaluation of average strength properties,
clear specimens may be combined with density data from
the presently available data from tests made under the sampling
extensive forest surveys to make them more representative,
methods and procedures of Test Methods D143 or Practice
E105 are provided with appropriate provision for their appli-
cation and use. Because of the comprehensive manner in which
1 the density survey is undertaken, it follows that the re-
This practice is under the jurisdiction of ASTM Committee D07 on Wood and
are the direct responsibility of Subcommittee D07.02 on Lumber and Engineered evaluated strength data are intended to be representative of the
Wood Products.
forest stand, or rather large forest subdivisions.
Current edition approved Feb. 1, 2024. Published February 2024. Originally
1.1.5 Some useful mechanical properties (tensile strengths
approved in 1966. Last previous edition approved in 2017 as D2555 – 17a. DOI:
10.1520/D2555-17AR24. parallel and perpendicular to grain, modulus of rigidity for a
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2555 − 17a (2024)
longitudinal-transverse plane, and transverse modulus of elas- 4. Procedure for Establishing Clear Wood Strength
ticity) have not been extensively evaluated. Methods are Values
described for estimating these properties by their relation to
4.1 Method A—Six steps are involved in establishing
other properties.
strength values by the wood density survey procedure. These
1.2 The values stated in inch-pound units are to be regarded are: conducting the wood density survey, development of unit
as standard. The values given in parentheses are mathematical areas, determination of average specific gravity for a unit area,
conversions to SI units that are provided for information only determination of strength-specific gravity relations, estimation
and are not considered standard. of average strength properties for a unit area, and combining
values for unit areas into basic groups and establishing average
1.3 This standard does not purport to address all of the
strength properties and estimates of variance for the groups. In
safety concerns, if any, associated with its use. It is the
these methods a basic group is a combination of unit areas
responsibility of the user of this standard to establish appro-
representing a species or a regional division thereof.
priate safety, health, and environmental practices and deter-
4.1.1 Conducting Wood Density Survey—A well-designed
mine the applicability of regulatory limitations prior to use.
and thorough wood density survey is required to provide
1.4 This international standard was developed in accor-
needed data on specific gravity for the reevaluation of strength
dance with internationally recognized principles on standard-
properties. Such a survey requires consideration of the geo-
ization established in the Decision on Principles for the
graphic range to be covered, the representativeness of the
Development of International Standards, Guides and Recom-
sample, the techniques of density evaluation, and adequate data
mendations issued by the World Trade Organization Technical
analysis.
Barriers to Trade (TBT) Committee.
NOTE 2—Detailed information on an acceptable method of conducting
2. Referenced Documents wood density surveys, together with survey data, are presented in the U.S.
Forest Service Research Paper FPL 27 (1).
2.1 ASTM Standards:
4.1.2 Development of Unit Areas—Subdivide the geographi-
D143 Test Methods for Small Clear Specimens of Timber
cal growth range of each species into unit areas that contain
D245 Practice for Establishing Structural Grades and Re-
1 % or more of the estimated cubic foot volume of standing
lated Allowable Properties for Visually Graded Lumber
timber of the species and are represented by reliable estimates
D2915 Practice for Sampling and Data-Analysis for Struc-
of specific gravity of at least 20 trees. Make up unit areas of
tural Wood and Wood-Based Products
U.S. Forest Service Survey Units, or similar units or subdivi-
E105 Guide for Probability Sampling of Materials
sions of units, for which reliable estimates of timber volume
are available. Develop unit areas objectively by means of the
3. Summary of Methods
following steps:
3.1 Two methods are presented for establishing tables of
4.1.2.1 Select a base survey unit or subdivision of a survey
clear wood strength properties for different species and re-
unit to be grouped with others,
gional subdivisions thereof in the unseasoned condition and
4.1.2.2 Group with similar adjacent areas to make up a unit
unadjusted for end use. These are designated Method A and
area on the basis of a timber volume, and
Method B.
4.1.2.3 Determine the number of tree specific gravity
3.1.1 Method A provides for the use of the results of surveys
samples available in the proposed unit area.
of wood density involving extensive sampling of forest trees,
NOTE 3—The rules for developing unit areas should represent an effort
in combination with the data obtained from standard strength
to subdivide objectively and uniquely the range of a species into small
tests made in accordance with Test Methods D143. The
geographic areas, which are assumed to be considerably more homoge-
average strength properties are obtained from wood density
neous with respect to the mechanical properties of the species than is the
survey data through linear regression equations establishing the
entire range itself. The number of unit areas associated with a species is
relation of specific gravity to the several strength properties.
a function of the volume of timber on the smallest usable areas and the
number of tree specific gravity samples taken. In general, the larger the
NOTE 1—Density surveys have been completed for only a limited
range and the greater the commercial importance of the species, the
number of species. Data are thus not currently available for the use of
greater are the number of unfit areas. One acceptable procedure for
Method A on all commercial species. As such data become available they
establishing unit areas is presented in Appendix C of U.S. Forest Service
will be incorporated in revisions of this practice.
Research Paper FPL 27 (1).
3.1.2 Method B provides for the establishment of tables of
4.1.3 Determination of Average Specific Gravity for a Unit
strength values based on standard tests of small clear speci-
Area—Calculate the average specific gravity of trees in each
mens in the unseasoned condition for use when data from
unit area as the simple average of individual estimates of
density surveys are not available. Separate tables are employed
specific gravity of trees within the unit area.
to present the data on woods grown in the United States and on
4.1.4 Determination of Strength-Specific Gravity
woods grown in Canada.
Relations—From matched specific gravity and strength data on
small clear specimens of wood, establish relationships of the
form:
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 boldface numbers in parentheses refer to a list of references at the end of
the ASTM website. this standard.
D2555 − 17a (2024)
y 5 a1bx (1) where:
= weighted average strength property for the group of
where: Y
unit areas,
y = estimated strength value,
¯
Y = average strength property for the ith unit area,
i
a, b = constants for the species, and
V = percentage of standing timber volume of the species for
I
x = specific gravity of the species.
the ith unit area, and
V = total percentage of standing timber volume of the
for each species, using standard statistical methods of
species in the group of unit areas being combined.
regression analysis. Equations for modulus of rupture, modulus
of elasticity, maximum crushing strength, and maximum shear-
4.1.6.2 Compute the variability index, which is a measure of
ing strength are established in this manner. The distribution of
the homogeneity among average values for unit areas within a
specific gravity in the samples used to compute regressions
group, by dividing the group average by the lowest unit area
should be representative of the species and, in particular, shall
average included in the group.
represent the full specific gravity range. The nature of the true
4.1.6.3 Estimate a standard deviation, providing a measure
distribution of specific gravity can be obtained from results of
of the dispersion of individual strength values about the group
wood density surveys. Obtain the data from specimens tested
average, for each basic group of unit areas using information
in accordance with Test Methods D143.
on variance obtained from density survey and standard strength
4.1.4.1 Several methods are available for securing suitable
data. Compute estimates of standard deviation for each prop-
samples for obtaining data to compute strength-specific gravity
erty as:
relationships, as follows: strength and specific gravity values
2 2 2
s 5 =b s 1s 1RMS (3)
~ !
w a
from samples obtained in conformance with Test Methods
D143 may be employed solely or in combination with data where:
secured by sampling techniques described below or test
s = standard deviation
samples may be obtained from the forest resource in the form
b = slope of the strength-specific gravity relation,
of trees, logs, or lumber. Select samples that are representative s = within-tree variance in specific gravity esti-
w
of all growing stock from each of at least five different mated from data used to obtain strength-
locations within the growth range of a species that include the specific gravity relations,
s = among-tree variance in specific gravity ob-
scope of environmental conditions of the range. This implies
a
tained from density survey data,
that the sample from a single location must be such that all of
2 2
(s + s ) = estimate of total variance in specific gravity,
the growing stock from that location is represented. w a
and
4.1.4.2 Where relationships between strength and specific
RMS = residual mean square from the strength-specific
gravity are shown to have a statistically significant difference at
gravity relation.
the 5 % level within a species growth range, subdivide the
NOTE 4—When a sampling technique is used that ensures only one
range to permit the development of more accurate estimating
specimen will be taken per tree (such as a suitably designed mill sample),
2 2
equations for each subdivision. Develop equations for subdi-
the quantity (s + s ) is automatically obtained as a total variance of
w a
specific gravity.
visions of a species growth range only if specimens from at
NOTE 5—An alternative procedure for developing average strength
least five distinctly different places in the proposed subdivision
values where all unit areas are contained within a single species or
are available and if the correlation coefficients from the
regional subdivision thereof consists of combining the
...
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: D2555 − 17a D2555 − 17a (Reapproved 2024)
Standard Practice for
Establishing Clear Wood Strength Values
This standard is issued under the fixed designation D2555; 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
The development of safe and efficient design values for lumber, laminated timber, plywood, round
timbers, and other solid wood products, each with its own special requirements has, as a common
starting point, the need for an authoritative compilation of clear wood strength values for the
commercially important species. Also required are procedures for establishing, from these data, values
applicable to groups of species or to regional groupings within a species where necessitated by
marketing conditions. This practice has been developed to meet these needs and to provide, in
addition, information on factors for consideration in the adjustment of the clear wood strength values
to design values for engineering. Since factors such as species preference, species groupings,
marketing practices, design techniques, and safety factors vary with each type of product and end use,
it is contemplated that this practice will be supplemented where necessary by other appropriate
standards relating to specific design values for each such product. Practice D245 is an example of such
a standard applicable to the interpretation of the clear wood strength values in terms of allowable
properties for visually graded lumber.
A primary feature of this practice is the establishment of tables presenting the most reliable basic
information developed on the strength of clear wood and its variability through many years of testing
and experience. The testing techniques employed are those presented in Test Methods D143. Among
the recognized limitations of such strength data are those resulting from the problems of sampling
material from forests extending over large regions, and the uneconomical feasibility of completely
testing an intensive sample. A practical approach to the improvement of strength data is through the
application of the results of density surveys in which the specific gravity of the entire forest stand for
each species is determined on a sound statistical basis. Through regression equations derived from
presently available strength data, revised strength values are established from the specific gravity-
strength relationship for clear wood. This procedure greatly extends current capabilities to develop
new estimates of strength and to improve or verify estimates made in the past.
1. Scope
1.1 This practice covers the determination of strength values for clear wood of different species in the unseasoned condition,
unadjusted for end use, applicable to the establishment of design values for different solid wood products such as lumber, laminated
wood, plywood, and round timbers. Presented are:
1.1.1 Procedures by which test values obtained on small clear specimens may be combined with density data from extensive forest
surveys to make them more representative,
This practice is 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 Aug. 1, 2017Feb. 1, 2024. Published October 2017February 2024. Originally approved in 1966. Last previous edition approved in 2017 as
D2555 – 17.D2555 – 17a. DOI: 10.1520/D2555-17A.10.1520/D2555-17AR24.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2555 − 17a (2024)
1.1.2 Guidelines for the interpretation of the data in terms of assigned values for combinations of species or regional divisions
within a species to meet special marketing needs, and
1.1.3 Information basic to the translation of the clear wood values into design values for different solid wood products for different
end uses.
1.1.4 For species where density survey data are not as yet available for the re-evaluation of average strength properties, the
presently available data from tests made under the sampling methods and procedures of Test Methods D143 or Practice E105 are
provided with appropriate provision for their application and use. Because of the comprehensive manner in which the density
survey is undertaken, it follows that the re-evaluated strength data are intended to be representative of the forest stand, or rather
large forest subdivisions.
1.1.5 Some useful mechanical properties (tensile strengths parallel and perpendicular to grain, modulus of rigidity for a
longitudinal-transverse plane, and transverse modulus of elasticity) have not been extensively evaluated. Methods are described
for estimating these properties by their relation to other properties.
1.2 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.
1.3 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D143 Test Methods for Small Clear Specimens of Timber
D245 Practice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber
D2915 Practice for Sampling and Data-Analysis for Structural Wood and Wood-Based Products
E105 Guide for Probability Sampling of Materials
3. Summary of Methods
3.1 Two methods are presented for establishing tables of clear wood strength properties for different species and regional
subdivisions thereof in the unseasoned condition and unadjusted for end use. These are designated Method A and Method B.
3.1.1 Method A provides for the use of the results of surveys of wood density involving extensive sampling of forest trees, in
combination with the data obtained from standard strength tests made in accordance with Test Methods D143. The average strength
properties are obtained from wood density survey data through linear regression equations establishing the relation of specific
gravity to the several strength properties.
NOTE 1—Density surveys have been completed for only a limited number of species. Data are thus not currently available for the use of Method A on
all commercial species. As such data become available they will be incorporated in revisions of this practice.
3.1.2 Method B provides for the establishment of tables of strength values based on standard tests of small clear specimens in the
unseasoned condition for use when data from density surveys are not available. Separate tables are employed to present the data
on woods grown in the United States and on woods grown in Canada.
4. Procedure for Establishing Clear Wood Strength Values
4.1 Method A—Six steps are involved in establishing strength values by the wood density survey procedure. These are: conducting
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.
D2555 − 17a (2024)
the wood density survey, development of unit areas, determination of average specific gravity for a unit area, determination of
strength-specific gravity relations, estimation of average strength properties for a unit area, and combining values for unit areas
into basic groups and establishing average strength properties and estimates of variance for the groups. In these methods a basic
group is a combination of unit areas representing a species or a regional division thereof.
4.1.1 Conducting Wood Density Survey—A well-designed and thorough wood density survey is required to provide needed data
on specific gravity for the reevaluation of strength properties. Such a survey requires consideration of the geographic range to be
covered, the representativeness of the sample, the techniques of density evaluation, and adequate data analysis.
NOTE 2—Detailed information on an acceptable method of conducting wood density surveys, together with survey data, are presented in the U.S. Forest
Service Research Paper FPL 27 (1).
4.1.2 Development of Unit Areas—Subdivide the geographical growth range of each species into unit areas that contain 1 % or
more of the estimated cubic foot volume of standing timber of the species and are represented by reliable estimates of specific
gravity of at least 20 trees. Make up unit areas of U.S. Forest Service Survey Units, or similar units or subdivisions of units, for
which reliable estimates of timber volume are available. Develop unit areas objectively by means of the following steps:
4.1.2.1 Select a base survey unit or subdivision of a survey unit to be grouped with others,
4.1.2.2 Group with similar adjacent areas to make up a unit area on the basis of a timber volume, and
4.1.2.3 Determine the number of tree specific gravity samples available in the proposed unit area.
NOTE 3—The rules for developing unit areas should represent an effort to subdivide objectively and uniquely the range of a species into small geographic
areas, which are assumed to be considerably more homogeneous with respect to the mechanical properties of the species than is the entire range itself.
The number of unit areas associated with a species is a function of the volume of timber on the smallest usable areas and the number of tree specific
gravity samples taken. In general, the larger the range and the greater the commercial importance of the species, the greater are the number of unfit areas.
One acceptable procedure for establishing unit areas is presented in Appendix C of U.S. Forest Service Research Paper FPL 27 (1).
4.1.3 Determination of Average Specific Gravity for a Unit Area—Calculate the average specific gravity of trees in each unit area
as the simple average of individual estimates of specific gravity of trees within the unit area.
4.1.4 Determination of Strength-Specific Gravity Relations—From matched specific gravity and strength data on small clear
specimens of wood, establish relationships of the form:
y 5 a1bx (1)
where:
y = estimated strength value,
a, b = constants for the species, and
x = specific gravity of the species.
for each species, using standard statistical methods of regression analysis. Equations for modulus of rupture, modulus of
elasticity, maximum crushing strength, and maximum shearing strength are established in this manner. The distribution of specific
gravity in the samples used to compute regressions should be representative of the species and, in particular, shall represent the
full specific gravity range. The nature of the true distribution of specific gravity can be obtained from results of wood density
surveys. Obtain the data from specimens tested in accordance with Test Methods D143.
4.1.4.1 Several methods are available for securing suitable samples for obtaining data to compute strength-specific gravity
relationships, as follows: strength and specific gravity values from samples obtained in conformance with Test Methods D143 may
be employed solely or in combination with data secured by sampling techniques described below or test samples may be obtained
from the forest resource in the form of trees, logs, or lumber. Select samples that are representative of all growing stock from each
of at least five different locations within the growth range of a species that include the scope of environmental conditions of the
range. This implies that the sample from a single location must be such that all of the growing stock from that location is
represented.
4.1.4.2 Where relationships between strength and specific gravity are shown to have a statistically significant difference at the 5 %
The boldface numbers in parentheses refer to a list of references at the end of this standard.
D2555 − 17a (2024)
level within a species growth range, subdivide the range to permit the development of more accurate estimating equations for each
subdivision. Develop equations for subdivisions of a species growth range only if specimens from at least five distinctly different
places in the proposed subdivision are available and if the correlation coefficients from the strength-specific gravity regressions are
0.50 or greater.
4.1.5 Estimation of the Average Strength Properties for a Unit Area—Given a set of strength-specific gravity estimating equations
for each species or subdivision thereof, compute average strength properties for each unit area using these equations and the
average specific gravity for the unit area.
4.1.6 Combining Unit Areas into Basic Groups and Development of Average Strength Properties and Estimates of Variance for
the Groups—Combine all unit areas containing timber whose properties are described by the same strength-specific gravity
relationships to produce a basic group of unit areas. Develop the following information for these basic groups:
4.1.6.1 For each unit area, obtain, from reliable volume data, the volume of the species being considered and estimate strength
properties from appropriate equations. Determine average strength properties for a group of unit areas for a species or a subdivision
thereof by the following equation:
% ¯
Y 5 ~Y V /V! (2)
(i i i
where:
= weighted average strength property for the group of unit areas,
Y
Y¯ = average strength property for the ith unit area,
i
V = percentage of standing timber volume of the species for the ith unit area, and
I
V = total percentage of standing timber volume of the species in the group of unit areas being combined.
4.1.6.2 Compute the variability
...
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