ISO 13912:2017
(Main)Structural timber — Machine strength grading — Basic principles
Structural timber — Machine strength grading — Basic principles
ISO 13912:2017 establishes the basic principles for rules and procedures governing the machine sorting of timber for use in structural applications. This applies to conventional bending type strength-grading machines as well as more recent machines that use other properties for strength-grading purposes.
Bois de structure — Classement par machine selon la résistance — Principes de base
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 13912
Second edition
2017-11
Structural timber — Machine strength
grading — Basic principles
Bois de structure — Classement par machine selon la résistance —
Principes de base
Reference number
ISO 13912:2017(E)
©
ISO 2017
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ISO 13912:2017(E)
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ISO 13912:2017(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 1
5 General . 1
5.1 Machine strength-graded timber . 1
5.2 Machine strength-grading operations . 1
5.3 Machine strength-grading quality procedures . 2
5.3.1 Machine-controlled system . 2
5.3.2 Output-controlled system . 2
6 Resource and sawn timber input requirements . 3
6.1 General . 3
6.2 Input requirements . 4
6.2.1 Resource . 4
6.2.2 Sawn timber . 4
6.3 Control of inputs . 4
6.4 Reprocessing of previously graded material . 4
7 Machine strength-grading requirements . 4
7.1 Machine checking requirements . 4
7.2 Machine acceptance criteria . 4
7.3 Machine strength-grading requirements . 5
7.4 Visual grading requirements . 5
7.4.1 General. 5
7.4.2 Automated visual grading . 5
8 Machine strength-graded timber structural properties . 5
8.1 General . 5
8.2 Initial evaluation . 5
8.3 Daily evaluation . 6
9 Product identification . 6
10 Documentation . 6
Annex A (informative) Example of a machine strength-grading timber standard .7
Annex B (informative) Use of the CUSUM procedure for daily evaluation.20
Annex C (informative) Strength-grading machine acceptance considerations .24
Annex D (informative) Training, supervision and oversight of grading .26
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ISO 13912:2017(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
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on the ISO list of patent declarations received (see www.iso.org/patents).
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URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 165, Timber structures.
This second edition cancels and replaces the first edition (ISO 13912:2005), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— Annexes C and D have been added;
— Clauses 5 and 7 have been technically revised.
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ISO 13912:2017(E)
Introduction
The general principle of this document is that any type of machine strength-grading procedure is
acceptable, provided it is defined, controlled, and documented to the extent required to reflect the
degree of reliability intended for the structural application of the product.
The body of this document specifies the essential features common to all machine strength-grading
operations. The requirements are minimal so as to ensure maximum scope and flexibility in the
application of this document to the machine strength-grading process as applied to timber.
Annex A provides an example of a conformance standard reflecting the requirements of this document.
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INTERNATIONAL STANDARD ISO 13912:2017(E)
Structural timber — Machine strength grading — Basic
principles
1 Scope
This document establishes the basic principles for rules and procedures governing the machine sorting
of timber for use in structural applications. This applies to conventional bending type strength-grading
machines as well as more recent machines that use other properties for strength-grading purposes.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 12122-1, Timber structures — Determination of characteristic values — Part 1: Basic requirements
ISO 12122-2, Timber structures — Determination of characteristic values — Part 2: Sawn timber
ISO 13910, Timber structures — Strength graded timber — Test methods for structural properties
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13910 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
4 Symbols and abbreviated terms
For the purposes of this document, the symbols and abbreviated terms given in ISO 13910 apply.
5 General
5.1 Machine strength-graded timber
Machine strength-graded timber is sawn wood that has been machine sorted according to selected
criteria allowing for classification into structural grades. The machine criteria shall identify mechanical
and/or physical properties that reflect the timber strength and can affect the utility of the product.
5.2 Machine strength-grading operations
A typical machine strength-grading operation shall be comprised of a grading machine that sorts an
input resource into one or more grades (see Figure 1).
The data recorded by the machine shall be processed so as to produce a sorting criterion, and this
sorting criterion shall be used as a basis for assigning a grade of machine strength timber.
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ISO 13912:2017(E)
Where applicable, visual requirements for the total piece of timber, the unscanned end portions, or
both shall be specified to supplement the information obtained by the machine.
NOTE 1 The “grading machine” comprises one or more devices that can measure properties along the length
of a piece of timber as it passes through the machine. For some machines, properties along the length of a piece
of timber are not measured, but rather properties are measured that relate to the piece of timber considered in
total as a single unit.
NOTE 2 Within the context of this document, the term “scanner” is used to denote the device used to measure
a property of the timber, and the term “scanned length” is used to denote that portion of the timber for which
measurements are made by the scanning device if it measures properties along the length of a piece of timber.
For conventional bending type strength-grading machines, end portions of the timber are not scanned [see
Figure 2 a)].
NOTE 3 In the use of the conventional bending type of strength-grading machine, the only parameter that is
measured (mechanically) along the length of each piece of timber (except for the unscanned end lengths) is the
local modulus of elasticity on flat.
NOTE 4 For a machine using single or multiple scanners, the data obtained is commonly used to produce a
prediction of strength along the length of a piece of timber [see Figure 2 b)]. For this case, the minimum predicted
strength value within the piece is usually taken to be the grade control criterion.
5.3 Machine strength-grading quality procedures
Machine strength-grading quality procedures include both qualification and ongoing quality control
operations. The quality procedures related to the machine grading operation shall be undertaken by
placing checks on the four components of the strength-grading operation: 1) the resource and sawn
timber inputs; 2) the machine operation; 3) the visual requirements (when specified); and 4) the graded
timber output (see Figure 1).
The qualification and quality control procedures shall establish that the production is in compliance
with expected performance, taking into account the normal temporal variation of the input, as well as
1)
the machine, the machine settings, and any other procedure added to define the population.
5.3.1 Machine-controlled system
Where quality is controlled through the resource inputs and machine operation, monitoring shall be
done to ensure that the resource is similar to that initially used to establish the machine settings.
5.3.2 Output-controlled system
Where quality is controlled through output, the initial evaluation (see 8.2) shall involve sample sizes
larger than those normally used for daily evaluation (see 8.3) to ensure that the 5th-percentile strength
requirements are met.
Figure 1 — Schematic of machine strength-grading operation
1) Quality control for products conforming to this document will include programmes for training, supervision
and oversight of grading (see Annex D for further explanation).
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ISO 13912:2017(E)
a) Typical output of a scanner
b) Typical form of processed output
Key
X distance from start of piece
Y scanner output
Y′ parameter used to predict strength
1 unscanned length (if applicable)
2 value used to classify piece (e.g. low-point parameter as shown in figure)
3 end of piece
Figure 2 — Measurements made by a strength-grading machine
6 Resource and sawn timber input requirements
6.1 General
The input resources shall be identified in terms of all parameters that can affect the output of the
machine grade sorting operation.
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ISO 13912:2017(E)
6.2 Input requirements
6.2.1 Resource
The parameter that shall be identified is the timber species or mixture of species.
Other parameters that shall be considered are
a) silvicultural practices used,
b) log source,
c) log size,
d) cutting pattern used to manufacture sawn timber from logs, and
e) any other parameters deemed to be important.
6.2.2 Sawn timber
Parameters that shall be specified are
a) condition (such as seasoned, unseasoned, etc.),
b) moisture content, and
c) any other parameters deemed to be important.
6.3 Control of inputs
A periodic check on the resource and sawn timber inputs shall be defined and specified.
6.4 Reprocessing of previously graded material
If reprocessing of previously graded material is permitted, then any requirements for re-grading of the
material shall be specified.
7 Machine strength-grading requirements
7.1 Machine checking requirements
In addition to verification of the calibration of the scanner, all machine checks specified by the machine
manufacturer shall be undertaken.
7.2 Machine acceptance criteria
The introduction of new machine strength-grading processes shall be supported by testing of samples
from at least two reference grades containing a random selection of the permitted features, resulting
in data on the mean modulus of elasticity and the 5th percentile of at least one strength property. This
data shall be permitted to be used to formulate the structural grade model including other grades and
sizes, provided it shows a consistent relationship to the reference grades.
The framework for the grading system shall be supported by evidence that it can provide a stable basis
for delivering graded products that achieve key characteristic properties.
NOTE The intent is to select reference grades and sample sizes that will serve best to represent the grades in
the system, reflecting both the range and frequency of features that can appear in these grades and demonstrating
that the grading system is robust enough to differentiate properties for the timber population. Annex C provides
an example of grading machine acceptance considerations.
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ISO 13912:2017(E)
7.3 Machine strength-grading requirements
The machine grading process shall specify, at regular intervals, checks to ensure:
a) repeatability – within prescribed limits, the scanners shall provide the same readings for repeated
passes of the same piece of timber;
b) calibration – within prescribed limits, scanners shall reproduce the original readings made on
special pieces of timber set aside for calibration purposes or on non-timber calibration bars;
c) consistency – within prescribed limits and where scanners provide readings along the length of
timber, the data from scanners shall indicate a consistent calibration all along the length of a piece
of timber.
Grading machines that use the same types of scanners shall be calibrated against the same check
procedure.
NOTE Grading machines that are not identical but use the same types of scanners include a number of
conventional strength-grading machines. This clause refers to the use of a common standard test method (e.g.
ISO 13910) in such cases for calibration purposes.
7.4 Visual grading requirements
7.4.1 General
Where visual grading requirements are deemed to be important then rules to satisfy the strength
requirements shall be specified and rules to satisfy relevant utility requirements shall be specified.
For the visual grading requirements that are specified, then a periodic check shall be made to assess the
accuracy of the grading process. If a check indicates that the process is inadequate, then appropriate
measures shall be specified to modify the process so that the process is adequate.
7.4.2 Automated visual grading
For visual-grading machines that are used to replace visual grading people, the repeatability and
reproducibility identified in 7.3 shall be met.
NOTE An example of visual strength-grading requirements, as applied to rectangular timber for structural
applications, is given in Annex A.
8 Machine strength-graded timber structural properties
8.1 General
The structural properties shall be as defined and measured as specified in the test methods defined in
ISO 13910.
The structural design properties shall be determined from tests on timber having a defined moisture
content. If the tests are conducted on timber having a moisture content that differs from that specified
by the procedure conforming to this document, the properties resulting from the tests shall be adjusted
(using sound engineering principles) so that the structural design properties reflect the intent of these
basic requirements and/or the applicable associated design codes.
8.2 Initial evaluation
Once the grading operation has been selected, evidence shall be provided that the resultant output
grades have the structural properties stated for the material.
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ISO 13912:2017(E)
For cases where such evidence is not available or it is not appropriate to link the evidence to other
mills, an initial test programme shall be specified. The requirements for this test programme shall be
based on sound sampling principles and the tests for the structural properties shall be based on the
test procedures specified in 8.1.
NOTE The objective is to involve multiple independent data sets when qualifying a machine for machine
controlled operations. Alternatively, the test sample can be a sample representative of the global population that
will be graded using the machine; in such cases, the evidence can be linked to other mills carrying out equivalent
sorting procedures.
8.3 Daily evaluation
When specified in the quality control documentation, one or more structural properties shall be
measured for each production shift on a limited sample of timber. The data obtained shall be applied
to a statistical process control procedure and the results used to monitor the control of the strength-
grading operation.
9 Product identification
A product identification mark shall be placed on the timber to indicate the standard on which the sorting
is based, the grade and/or strength class, and the producer responsible. The product identification
mark shall also include any other relevant information required to show suitability for end use.
Each piece of timber shall be marked except for high quality strength-graded timber intended for
structural and appearance purposes. For this high quality appearance timber, each shipment shall be
accompanied by documentation containing the product identification requirements specified in the
standard.
10 Documentation
Documentation requirements shall include:
a) the standard on which the machine strength-grading process is based;
b) specifications for the timber grade criteria;
c) specifications and control checks of the resource input where applicable to machine control
operations;
d) specifications and control checks for the machine grade sorting process;
e) where applicable, specifications and control checks for the visual requirements;
f) specifications and control checks of the structural properties;
g) specifications for the identification of the product;
h) methods for assigning and confirming a grade; and
i) any other specifications or materials deemed to be important.
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ISO 13912:2017(E)
Annex A
(informative)
Example of a machine strength-grading timber standard
This is a practical implementation of the machine strength-grading principles defined in the main body
of ISO 13912, as applied to the following case:
— conventional bending type strength-grading machine;
— rectangular cross-section timber;
— for structural applications;
— requiring strength characteristic values with a moderate to high degree of engineering reliability.
The layout corresponds to that of an International Standard (not a normal annex) to clearly show how
an International Standard in this domain should look. It includes
— elements mandatory in all International Standards (Scope, Normative references, and Terms and
definitions),
— conditional elements, i.e. those present depending on the provisions of a particular document
(Symbols and abbreviated terms, Requirements, Sampling, Test methods and Bibliography).
Informative annexes (like this annex) and normative annexes can also form part of an International
Standard. Although some of this annex is written in mandatory language for illustration purposes, it is
not a normative part of this document.
NOTE The Scope does not usually form part of an annex, but is included in this example for completeness
purposes.
A.1 Scope
This annex specifies the grading procedures for producing machine-sorted strength and stiffness
graded rectangular timber for structural applications where a moderate to high degree of engineering
reliability is required.
It is applicable for timber that is graded in the seasoned state.
For timber that is graded in the unseasoned state, there is greater uncertainty about effects on the
grading evaluation, and so the structural design properties for the timber might need to be modified to
reflect the intent of this standard and/or the associated design codes.
A.2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 12122-1, Timber structures — Determination of characteristic values — Part 1: Basic requirements
ISO 12122-2, Timber structures — Determination of characteristic values — Part 2: Sawn timber
ISO 13910, Timber structures — Strength graded timber — Test methods for structural properties
ISO 13912, Structural timber — Machine strength grading — Basic principles
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ISO 13912:2017(E)
A.3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
A.3.1
grade
population of timber derived from a specified resource and by applying a specified sorting procedure
A.3.2
CUSUM
cumulative summation procedure for assessing structural quality trends from data sampled at inter-
vals of time
A.3.3
knot
portion of a branch or limb that has become incorporated into a piece of sawn timber
A.3.4
machine strength-grading
strength-grading through mechanically measuring the modulus of elasticity on flat along a piece
of timber
A.3.5
seasoned timber
timber with moisture content of 19 % or less
A.3.6
split
separation of the wood at the end of the piece that runs from one surface to an opposite or adjacent face
A.3.7
structural requirements
grade requirements that affect the structural properties of the timber
A.3.8
shake
lengthwise separation of the wood which occurs between or through the annual growth rings
A.3.9
thickness
lesser dimension perpendicular to the longitudinal axis of a piece of timber
A.3.10
unseasoned timber
timber with moisture content greater than 19 %
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ISO 13912:2017(E)
A.3.11
utility requirements
grade requirements that do not affect the structural properties of the timber and/or to an amount
which threatens the serviceability of the piece
A.3.12
visual requirements
visual grading requirements that are additional to machine sorting necessary to attain required struc-
tural and utility properties within a grade of timber
A.3.13
width
greater dimension perpendicular to the longitudinal axis of a piece of timber
Other features such as bow, cup, spring (or crook), twist and wane should be defined in the appropriate
subclauses and by referencing the feature to an illustration.
A.4 Symbols and abbreviated terms
A.4.1 General notation
E modulus of elasticity
f strength
N sample size
CV coefficient of variation
A.4.2 Subscripts
0,05 5-percentile value
data value measured on data sample
mean mean value
m bending
target target or specified value for the grade
A.5 General
A.5.1 Machine strength-grading operations
The machine strength-grading operation shall be compri
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