ISO 16598:2015

INTERNATIONAL ISO
STANDARD 16598
First edition
2015-09-01
Timber structures — Structural
classification for sawn timber
Structures en bois — Classification structurelle pour bois sciés
Reference number
ISO 16598:2015(E)
©
ISO 2015

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ISO 16598:2015(E)

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ISO 16598:2015(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Timber population and structural property data . 2
5 Primary properties . 2
5.1 Framework . 2
5.2 Bending or tension property evaluation . 3
5.2.1 General. 3
5.2.2 Exception . 3
5.3 Density evaluation . 3
5.4 Class boundaries. 3
6 Derived properties . 3
6.1 Framework . 3
6.2 Evaluation . 3
6.2.1 General. 3
6.2.2 Exception . 3
7 Allocation to a class . 4
7.1 Framework . 4
7.1.1 General. 4
7.1.2 Normalization of characteristic values . 4
7.2 Grading systems . 4
7.2.1 General. 4
7.2.2 Exception . 4
7.3 Design values . 5
8 Reporting . 5
Annex A (normative) Timber population and structural property data .6
Annex B (informative) Structural class example table . 9
Annex C (informative) Commentary .13
Bibliography .19
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ISO 16598:2015(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
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
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
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 165, Timber structures.
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ISO 16598:2015(E)

Introduction
The production of many different combinations of species and grades complicates timber specification
and structural design. Structural classification groups together species and grades of similar properties,
to make them interchangeable for structural purposes. The development of this International Standard
is intended to benefit industry, consumers, governments and distributors, by balancing the principles
of simplicity, product utility and structural reliability.
One of the key reasons for developing this International Standard is to provide a framework for
understanding and working on compatibility between approaches used in different regions, through
standardization of the basic elements of structural classes, including underlying assumptions and the
general method for setting up classes. An example table is presented in Annex B for illustration purposes.
This International Standard provides a model or template that may be modified before adoption
and it does not present final design values and adjustment factors. The scope includes evaluation of
structural properties; it does not cover suitability with regard to durability, fire resistance and other
timber properties.
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INTERNATIONAL STANDARD ISO 16598:2015(E)
Timber structures — Structural classification for sawn
timber
1 Scope
This International Standard provides a basic international framework for establishing structural
classes for sawn timber.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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
3 Terms and definitions
For the purpose of this document, the following terms and definitions apply.
3.1
characteristic value
standard estimate of a structural property of a timber population (3.11) corresponding to a fractile,
tolerance limit or mean of sample test data after being adjusted to accepted reference conditions
3.2
derived property
structural property for which characteristic values are determined by correlation with one or more
primary properties, and not typically used in deciding how to classify a population
3.3
design value
numerical quantity assigned to a timber population for use in structural design, calculated from the
characteristic value and modified to suit provisions in the appropriate building code and standards
Note 1 to entry: Design values are not used directly in structural classes.
3.4
full-size specimen
timber test piece that is similar in size and characteristics to typical structural timbers and is prepared
and tested in a way similar to use in construction
Note 1 to entry: See also small clear specimen (3.7).
3.5
normalization
adjustment of data, beyond basic data breakdown and moisture and size corrections, to be on a
compatible basis with other data for evaluation in the same structural class system
3.6
primary property
structural property that is used to set criteria for allocating a timber population to a structural class
system, based on the characteristic value for that property
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3.7
small clear specimen
timber test piece that is much smaller than typical structural timbers and normally prepared to be free
of visible defects
Note 1 to entry: See also full-size specimen (3.4).
3.8
species combination
two or more species or multiple data sets of the same species from different growth or production
regions, combined into a single marketing group on the basis of similar properties and property
relationships, or other applicable criteria
3.9
grade
solid sawn timber population produced with standardized rules that maintain well-defined limits on
strength-controlling characteristics or properties
3.10
structural classification
system for assigning shared structural properties to timber populations of similar capacities, for the
purpose of structural design and specification
3.11
timber population
solid sawn timber product of a grade and species or species combination intended for use in
structural applications
4 Timber population and structural property data
This International Standard is based on the timber population and structural property data estimates
conforming to those identified in ISO 12122-1 and ISO 12122-2.
NOTE See Annex A for additional information.
5 Primary properties
5.1 Framework
Primary properties shall be established on a standardized basis to ensure compatibility between
different sets of test data from accepted sampling and testing programs. Primary characteristic
properties include the following:
— bending strength: the modulus of rupture at the 5th percentile, with specified level of confidence;
— bending stiffness: the modulus of elasticity at the mean and 5th percentile levels;
— density: the density at the mean and 5th percentile levels.
NOTE 1 Strength properties are typically estimated as tolerance limits at the 5th percentile level with
confidence specified at 75 %. Stiffness and density properties are typically estimated as either means or
percentiles without a confidence statement, or as tolerance limits with confidence specified at 75 %.
NOTE 2 An alternative approach is to use axial (tension) strength instead of bending strength as a primary
property. Some representative testing standards for primary properties are identified in the Bibliography.
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5.2 Bending or tension property evaluation
5.2.1 General
Primary bending property data or, where applicable, tension property data shall be evaluated as
characteristic values determined on the basis of standardized full-size specimen testing, except as
permitted in 5.2.2.
5.2.2 Exception
Where it is necessary to evaluate bending or tension property data on a basis other than full-size
testing, the data shall be supported by but not combined or interchanged with full-size testing results.
NOTE Although full-size timber testing is preferred, small clear specimen testing is used in cases where
full-size bending or tension testing is not feasible or practical, with modification factors to partially calibrate
the data in accordance with characteristic value standards, and subject to a review of consistency in the level of
safety between small clear and full-size testing approaches (see Annex C).
5.3 Density evaluation
Density property data shall be evaluated as characteristic values determined on the basis of
standardized small clear wood specimen testing.
NOTE ISO 13910 provides a standardized approach to evaluating density properties.
5.4 Class boundaries
Class boundaries for primary bending or tension strength properties shall be set at intervals of no
less than 2 MPa.
6 Derived properties
6.1 Framework
Derived properties shall be established on the basis of standardized relationships to one or more of
the primary properties provided that it can be demonstrated that these relationships are generally
applicable to the timber population.
Derived strength and stiffness properties for structural classes shall include properties other than
the primary strength and stiffness properties and shall be expressed on the same basis as primary
properties (See NOTES to 5.1).
6.2 Evaluation
6.2.1 General
Derived properties shall be assigned as characteristic values determined on the basis of standardized
relationships identified in 6.1 or, where further evidence of compliance is required, evaluated in
accordance with the alternative method in 6.2.2.
6.2.2 Exception
Where derived properties are to be evaluated or corroborated by testing, they shall be evaluated on a
standardized basis to ensure compatibility between different sets of test data from accepted sampling
and test programs. Bending or tension property data used to evaluate derived properties shall be
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ISO 16598:2015(E)

determined as characteristic values on the basis of standardized full-size specimen testing, except as
permitted in 5.2.2.
NOTE Ongoing monitoring or quality control testing is sometimes used to demonstrate suitability for cases
involving new or modified products or grading systems.
7 Allocation to a class
7.1 Framework
7.1.1 General
A timber population shall be qualified to be allocated to a class if the characteristic values for the
primary properties equal or exceed the tabulated class values. In addition, where derived properties
are to be evaluated or corroborated by testing, the population shall be qualified to be allocated to a
class only if its characteristic values for such properties also equal or exceed the tabulated class values,
except as provided in 7.1.2.
7.1.2 Normalization of characteristic values
Prior to allocation to a class, characteristic values for data sets derived following standard methods
for sampling the timber population and evaluating the primary properties shall be permitted to be
normalized to account for documented differences in property estimation as well as the dispersion in
data sets around the characteristic value.
NOTE 1 This requirement is based on the assumption that a uniform approach will be applied to structural
property modification for each or all structural classes to achieve a desired level of safety. Timber from
grading systems or processes that are very different can require the use of normalization factors to calibrate
characteristic values (see Annex C) to ensure that products allocated to a class demonstrate a consistent level of
structural performance in end use.
NOTE 2 A different basis for sampling and deriving characteristic values (such as the use of small clear
specimens versus samples of full-size commercial production) will yield different levels of dispersion in the data
set as well as characteristic values. These differences require the development of different data set adjustments;
for example, see Annex C for a comparison between the small clear and full-size specimen approach.
7.2 Grading systems
7.2.1 General
Structural classes shall be permitted to include timber populations from different grading systems
provided that these populations fit within the established framework of structural property
relationships.
NOTE An example of a structural classification framework appears in Annex B.
7.2.2 Exception
Where timber populations from different grading systems have significantly different structural
property relationships or levels of variability, they shall be allocated either under separate classification
systems, or in accordance with rules addressing these differences through appropriate adjustments to
properties or grading processes.
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ISO 16598:2015(E)

7.3 Design values
This International Standard is based on the assumption that any structural property modifications
made to establish design values in building codes and design standards will be consistent with the data
and assumptions used to allocate populations to structural classes.
NOTE The body adopting a structural class system can also decide to incorporate individual species-grade
design values in appropriate codes or standards as an alternative approach to structural class design values.
8 Reporting
Timber population and data estimates, sampling, test procedures and adjustments to data and
structural class allocation shall be recorded and included in a report.
NOTE Other factors to be considered in setting up and implementing a structural classification system are
identified in Annex C.
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ISO 16598:2015(E)

Annex A
(normative)

Timber population and structural property data
A.1 Resource
This International Standard is applicable only to timber populations with the following characteristics.
— The population is a defined forest region managed with a goal of maintaining similar or improved
characteristics in the future. Where properties or property relationships differ substantively
from the resource in other regions, i.e. resource of the same species or from other members of the
classification system, this is noted and represented appropriately in the sample data.
— The population is defined in terms of species or combination of species, geographical origin and
other relevant traits. Geographical origin means the overall growth region supplying all included
mills in the broadest sense, except where the source is explicitly restricted. Where subdivisions of
the same species or species combination have been identified, all relevant boundaries or differences
between these subdivisions are defined.
— Long experience of a grading system is desired for the population, and any significant changes in
the resource or production process will be reviewed prior to data evaluation. Grading systems may
produce significantly different property relationships or variability in relationships, requiring special
review: to show them to be compatible or to be treated differently from other grading systems.
— Test samples are representative of the population following the selected sampling strategy that
either a) includes as much variation as possible or b) focuses on the lower end of the structural
property distribution, and the selected approach is carried forward to analysis.
A.2 Test data
This International Standard is applicable only to data sets with the following characteristics.
— All major property data are based on standardized full-size testing of graded timber, with exceptions
for cases where consideration of small clear wood test data are necessary and supported by evidence
from experience in relationship to structural performance, and this data are kept separate from
data from full-size in-grade sampling and testing programs.
— Data sets for size-grade categories are made as stable and consistent as possible by means of a
comprehensive strategy considering natural variation, grading provisions, the sampling plan and
quality control procedures.
— A representative sample matrix of sizes and grades, unless shown to be unnecessary, is used to
ensure a stable data set (an exception may be ongoing testing through quality control procedures).
Stratified sampling plans are another way of approaching data stability. Sample sizes are selected
to match the data strategy.
— Each individual sample (e.g. of a timber size) is adequate to achieve precision targets. Pooling of
data are done following a standardized approach, not to make up for deficiencies in individual
cell sample sizes. Improper representation at the cell level is not compensated for by pooling from
different test programs.
— Data are adjusted to a reference moisture content, size and other appropriate conditions using
standardized procedures (see Bibliography).
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ISO 16598:2015(E)

— Where data are fit to parametric distributions, the test data show a reasonable fit in the distributional
region of interest.
A.3 Species and species combinations
This International Standard is applicable only to structural classes with the following provisions for
species.
— The population consists of species and grades of designated products with structural applications.
Separate structural class tables are permitted to be used for species that differ significantly in
properties or property relationships, such as softwoods—hardwoods. Species with similar properties
and property relationships are permitted to be grouped into combinations for marketing purposes.
— All relevant boundaries or differences (e.g. geographic, topological, climatic or by provenance) are
defined and recorded, particularly where subdivisions of the same species or species combination
have been identified.
— In cases where there is limited experience with species, species varieties or specific resource
subgroups, evidence is gathered on compatibility of properties and property relationships with the
classification framework.
— Where the resource consists largely of plantation timber, or where the resource is immature or
substantively different from other members of the class system or from other resource of the same
species, this is noted as a minimum and represented appropriately in the sample data. Alternatively,
if the database is limited, the application of the structural class is also limited.
A.4 Size
This International Standard is applicable only to structural classes with the following provisions
for member size.
— The system is formulated on the basis of a specific characteristic value size.
— Adjustment factors are used to modify values for different timber sizes. These adjustment factors
are comparable to factors used to evaluate raw data in the development of characteristic values, or
steps are taken to ensure compatibility.
— The effects of moisture content on timber size and capacity are accommodated in a manner
comparable to how they are accommodated with raw data in the development of characteristic
values, or steps are taken to ensure compatibility.
— In cases where the sample matrix does not incorporate sizes that could be produced or that are
implied in the structural class system, further review is undertaken before the population is
allocated to a class.
— Ongoing quality control testing is permitted to be used to demonstrate consistency for a given size-
grade, particularly for machine grading systems.
A.5 Grading systems
This International Standard is applicable only to structural classes with the following provisions for
grading systems.
— The population is produced using a well-defined and stable grading system.
— For visual grading systems, grade characteristics are standardized and interpreted in a consistent
manner. Any significant change in grading provisions or interpretations is examined as to potential
impacts on properties or property relationships.
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— For other cases, such as machine-graded timber systems, the properties and property relationships
may be maintained through a standardized process and quality system, or there are specified steps
to ensure that the critical properties stay in control.
— For cases involving new or modified grading systems, further review is undertaken before the
population is allocated to a class. Ongoing monitoring or quality control testing may be used to
demonstrate suitability for a given population.
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ISO 16598:2015(E)

Annex B
(informative)

Structural class example table
B.1 Structural class example 1: Softwood species (150 mm width basis)
Table B.1 — Structural class example for softwood species (150 mm width basis)
S10 S16 S20 S24 S28 S32 S36 S40
Bending (MPa) 10 16 20 24 28 32 36 40
F
m,k
Tension parallel (MPa) 5 8 11 13 15 17 19 21
F
t,0,k
Tension perp. (MPa) 0,5 0,5 0,5 0,5 0,5 0,5 0,5 0,5
F
t,90,k
Compression parallel (MPa) 13 17 19 21 22 22 23 24
F
c,0,k
Compression perp. (MPa) 2 2,5 3 3,5 3,5 4 4,5 5
F
c,90,k
Shear parallel (MPa) 1,5 2,5 2,5 3 3 3,5 3,5 3,5
F
v,k
Mean MOE parallel (GPa) 7 8,5 9,5 10,5 11,5 12,5 13,5 14,5
E
0,mean
3
Density (g/cm ) 0,33 0,36 0,39 0,41 0,44 0,46 0,49 0,51
ρ
mean, oven-dry
NOTE This softwood timber example is for illustration purposes only, and structural classes can be added or removed to
adjust to standardization requirements.
B.2 Derivation assumptions
Any example will be based on some method of establishing properties, and the selected method
influences the relationship between properties as well as the allocation of species and grades to classes.
Properties in Table B.1 are based lar
...